Marine Micropaleontology, 7 (1982): 237--281 237 Elsevier Scientific Publishing Company, Amsterdam --Pr inted in The Netherlands
RADIOLARIAN BIOGEOGRAPHY IN SURFACE SEDIMENTS OF THE EASTERN INDIAN OCEAN 1
DAVID A. JOHNSON and CATHERINE NIGRINI
Woods Hole Oceanographic Institution, Woods Hole, MA 02543 (U.S.A.) 510 Papyrus Drive, La Habra Heights, CA 90631 (U.S.A.)
(Revised version accepted January 26, 1982)
Abstract
Johnson, D.A. and Nigrini, C., 1982. Radiolarian b[ogeography in surface sediments of the eastern Indian Ocean. Mar. Micropaleontol., 7: 237--281.
We analyzed recurrent groups of Radiolaria in 74 core top samples from a transect through the eastern Indian Ocean in order to supplement our previous results from the western Indian Ocean (Johnson and Nigrini, 1980). We now identify six distinct recurrent groups and nine radiolarian assemblages in the combined data set of 120 samples; this extended sample coverage has led to several re-interpretations of the oceanographic significance of the radiolarian distribution patterns. Assemblage boundaries closely reflect the presence of major oceanographic fronts and surface currents including the South Equatorial Divergence, Subtropical Gyre, Subtropical Conver- gence, and Antarctic Convergence. At least four major aspects of the assemblages in the eastern transect are notably different from those in the western transect, leading to a marked east--west asymmetry in faunal distribu- tion patterns across the Indian Ocean. The assemblage formerly associated with strong upweUing near the Arabian Peninsula is present throughout the Bay of Bengal as well, and is interpreted to reflect high salinity and low oxygen in the subsurface waters of the Indian Ocean north of the Equator. A new assemblage has been identified associated with the westward-flowing Pacific water into the eastern Indian Ocean in low latitudes, and may be a potential stratigraphic and paleoclimatic marker for times of low sea level when this westward near-surface flow was shut off (i.e., glacial maxima). An extensive region in the core of the subtropical gyre between 25°S and 35°S is relatively barren of Radiolaria, yet is marked by a characteristic assemblage distributed asymmetrically, perhaps reflecting the lack of a strong boundary current off the west coast of Australia. Assemblage boundaries in the vicinity of the eastward circumpolar flow are not strictly zonal, and may indicate significant deviations from the mean eastward flow as a necessary condition for conservation of potential vorticity when the flow en- counters topographic irregularities.
Introduction
Radiolaria are important constituents of sediments over much of the world's oceans, and have been applied successfully in numer- ous stratigraphic, biogeographic, and paleo- oceanographic investigations. Recent review articles summarizing the use of these methods
include those of Riedel and Sanfilippo (1977, 1978), Moore (1978), Kling (1978), and Moore et al. (1980). Radiolaria have proven to be equal to, and in some instances superior to, other major microfossil groups in provid- ing a body of data from which one may derive paleo-oceanographic and paleo~ecologic in- formation (e.g. Sancetta, 1978, 1979a).
i Contribution No. 5030 of the Woods Hole Oceanographic Institution.
Part of the reason for this success lies in the widespread global distribution of Radiolaria (Lisitzin, 1967), the lack of a "lysocline" comparable to that for calcareous microfossils (e.g. Kolla et al., 1976a), and the high divers- ity of Radiolaria in tropical and temperate latitudes (e.g. Nigrini and Moore, 1979; Takahashi, 1981).
In the Indian Ocean, radiolarian-bearing sediments extend from the Antarctic con- tinental margin northward to the southern margin of Asia (Lisitzin et al., 1967; Lisitzin, 1972; Joust , 1977; Caulet, 1977, 1978). However, there have been relatively few attempts to interpret the spatial distribution patterns of modern radiolarian assemblages in terms of the properties and flow patterns of the major surface and sub-surface water masses. The previous studies of Nigrini (1967), Petrushevskaya (1967, 1971), Lozano and Hays (1976), and Dow (1978) are par- ticularly notewor thy because of the number of samples considered and the degree of success in identifying distinct radiolarian assemblages and their geographic distribution patterns in modern sediments.
During the decade since the initial studies of Nigrini (1967) and Petrushevskaya (1967), extensive drilling (e.g. Heirtzler et al., 1977) and conventional piston coring have added enormously to the available sample control from the Indian Ocean. There are presently over 350 piston and gravity cores (north of 50°S) in the collections of Lamont-Doher- ty, Scripps, Florida State, and Woods Hole. Moreover, with the completion of several investigations of radiolarian biogeography (e.g. Nigrini, 1970; Goll and BjCrklund, 1971, 1974; Sachs, 1973; Lozano, 1974; Robertson, 1975; Morley, 1977) and radiol- arian distribution in the water column (e.g. Renz, 1976; Kling, 1979; Takahashi and Honjo, 1981), a comprehensive synthesis has been prepared incorporating the taxon- omy and synonymy of many of the ex- tant forms of Radiolaria (Nigrini and Moore, 1979). This updated taxonomic monograph has served as the basis for iden- tifying the 74 radiolarian taxa and their geographic distribution patterns in the present study.
In our recently completed report on modern radiolarian biogeography in the western Indian Ocean (Johnson and Nigrini, 1980), we applied recurrent group analysis to 46 samples in a north--south transect between 50°E and 70°E extending from the Arabian margin to the Antarctic Con- vergence. From this analysis we identified five distinct recurrent groups and eight geo- graphic assemblages of Radiolaria, and sug- gested some possible relationships between the faunal assemblage boundaries and the first-order oceanographic characteristics of the region. Upon complet ion of this study it became evident to us that the significant east--west asymmetry in the oceanographic characteristics of the Indian Ocean (e.g. Wyrtki, 1971, 1973) is most probably re- flected in the sediments, and that in order to interpret satisfactorily the radiolarian assemblage patterns, we need to incorporate sample coverage from the eastern half of the Indian Ocean as well.
The objective of the current s tudy was to select a longitudinal transect of core top samples from the eastern Indian Ocean (80°E--120°E), extending from the Bay of Bengal to the Antarctic Convergence near 50°S, and to apply the procedures of re- current group analysis to identify radiolarian assemblage distribution patterns. We initially intended to treat the data from the eastern transect separately from the data of our previous study farther west (Johnson and Nigrini, 1980), but in the course of the study we determined that combining the two data sets was the only satisfactory procedure for identifying recurrent groups and assemblages which could be mapped and interpreted on an ocean-wide basis.
Oceanographic setting
In our previous report (Johnson and Nigrini, 1980) we summarized the first-order aspects of the hydrography and circulation patterns of surface and abyssal waters in the western Indian Ocean. In general, our previous description of the three major circulation systems (i.e., the monsoon gyre,
239
the subtropical gyre, and the eastward cir- cumpolar flow) is applicable to the eastern Indian Ocean. Nevertheless, there are a number of important geographic and oceano- graphic factors which may contribute to producing a significant east--west asym- metry in the major flow patterns. We shall list these factors and briefly comment on their role in controlling the overall circula- tion pattern, particularly insofar as these factors may be reflected in the microfossil distributions.
(a) Upwelling regions. The strong south- west monsoon during the Northern Hemi- sphere summer produces strong upwelling off the coasts of Somalia and the Arabian Peninsula (Zeitzschel, 1973). There are no comparable regions of such strong upwelling along the eastern margin of the Indian Ocean.
(b) Northern Indian Ocean source waters. The eastern and western sides of the north- ern Indian Ocean have strikingly different water source regions. The hypersaline out- flows from the Persian Gulf and the Red Sea enter the Arabian Sea and appear as strong salinity maxima in the shallow subsurface. In the Bay of Bengal, by contrast, the enorm- ous fresh water inflow via the Ganges and Brahmaputra rivers provides a low-salinity cover which extends far to the south (Wyrtki, 1971). The oceanographic contrast between the Arabian Sea and Bay of Bengal is un- doubtedly blurred by the monsoonal reversals of surface water flow south of India (Wyrtki, 1973), yet these two major subdivisions of the northern Indian Ocean retain marked contrasts in many of their characteristic properties.
(c) Pacific "through f low". There is a major influx of Pacific surface waters into the eastern Indian Ocean via the straits and pas- sages through the Indonesian archipelago. Recent surveys indicate that this influx is quite substantial, with a volume transport on the order of 10 " 10 ~ m3/sec (Godfrey and Golding, 1981). It is clear that such a current would contribute to the properties and the intensity of the southern subtropical gyre, yet the exact nature of this contribu- tion has not been determined. We shall
present microfossil data in this report which may allow one to determine the extent of Pacific flow into the Indian Ocean during the past.
(d) Western intensification of flow. All ocean basins exhibit some degree of east-- west asymmetry in the flow of both surface and deep waters. The western intensification is a necessary consequence of the earth's rotation, and leads to a narrower and more intense flow along the western limb of major circulation gyres than on the corresponding eastern limbs (Defant, 1961). Thus, the southern Indian Ocean exhibits a well-defined boundary current flowing southward near Madagascar, but the corresponding northward flow along western Australia is imperceptible (Wyrtki, 1971).
(e) Topographic effects. When a major current system (e.g., the circumpolar flow) encounters a topographic feature which causes a change in the thickness of the flow along a substantial portion of the flow's trajectory, the current will necessarily veer to the left or to the right, depending on whether the flow depth is shallowing or deepening. This effect is particularly im- portant as a possible explanation of zonal asymmetries in microfossil distribution pat- terns. Even if the mean current flow is strict- ly zonal, topographic effects will cause major perturbations in flow direction in order that the potential vorticity of the flow be con- served (Defant, 1961). In the Southern Hemisphere, this effect causes a veering toward the left as a flow thins (e.g., passing over a plateau), and veering toward the right as the flow thickens (e.g., passing into deeper water). The latitudinal fluctuations in the mean position of the Subtropical Convergence (S.T.C.) and the Antarctic Convergence (A.A.C.) are in many instances a consequence of this "topographic-beta" effect (N. Hogg, pers. commun., 1981). Thus, we might anticipate that microfossil assemblages will reflect these flow perturbations in regions where the flow is sufficiently thick to inter- sect the sea floor.
(f) Effects of bottom currents. Deep ther- mohaline currents can in some instances play
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Fig.1. Bathymetric and oceanographic setting of the eastern Indian Ocean, modified after Defant (1961) and Wyrtki (1971, 1973). Bathymetric contours are in km, modified from Udintsev (1975). Heavy arrows designate the principal surface currents during the northeast monsoon; open arrows indicate the pathway of deep thermo- haline currents (after Kolla et al., 1976b; Corliss, 1979; Johnson and Warren, 1979). Major frontal zones shown include the South Equatorial Divergence, Subtropical Convergence, and Antarctic Convergence.
241
a major role in the lateral displacement of microfossils over distances of hundreds to thousands of kilometers (e.g., Burckle, 1981). Thus, one should a~oid regions where strong bo t tom currents are present, because these currents are sufficiently strong (at least locally) to cause significant reworking of both coarse and fine fractions of the sedi- ment. The deep circulation of the Indian Ocean is reasonably well defined as a result of hydrographic transects (e.g., Jacobs and Georgi, 1977; Warren, 1974, 1981), the recently completed GEOSECS observations (e.g., Chung and Kim, 1980), and geological studies (e.g., Kolla et al., 1976b, 1978; Johnson and Damuth, 1979). Moreove r , benthic foraminifera may serve as diagnostic indicators of deep water masses and, there- fore, give some clues about their trajectories {e.g., Lohmann, 1978; Corliss, 1979). In the eastern Indian Ocean, Antarctic Bot tom Water (AABW) crosses the Southeast Indian Ridge and flows clockwise around the western margin of the South Australian Basin and Wharton Basin (Fig.l) . The AABW continues northward along the base of the Ninety-East Ridge (Johnson and Warren, 1979), and the upper portions of the current pass westward through the deeper saddles in the Ninety-East Ridge and enter the Central Indian Basin (Fig.l) . This current system may, to some extent, be responsible for the abnormally poor preservation of radiolarians in many samples from the Wharton Basin south of ~25°S (Fig.2). Yet the microfossil preser- vation is equally poor in numerous cores from relatively shallow depths in this region (see Table II). We have found no direct evidence in our samples that AABW is re- sponsible for lateral displacement of radiolar- ian assemblages over significant distances (hundreds to thousands of kilometers). Yet the possibility of mixing must be kept in mind for any sample obtained within the "core" of one of these deep current systems.
Material studied
We examined 157 samples from the tops (or as near to the top as possible) of gravity,
piston, and pilot (trigger) cores in a transect of stations in the eastern Indian Ocean, ex- tending from the central Bay of Bengal
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Fig .2 . L o c a t i o n s o f all s a m p l e s e x a m i n e d in e a s t e r n I n d i a n O c e a n t r a n s e c t . N u m b e r e d c i rc les i n d i c a t e t h e 74 c o r e s in w h i c h r a d i o l a r i a n a b u n d a n c e a n d p r e s e r v a t i o n were s u f f i c i e n t t o a l l ow t h e i n c l u s i o n o f t h e s e c o r e s in o u r s t u d y . U n - n u m b e r e d a s t e r i s k s i n d i c a t e t h e l o c a t i o n s o f 83 a d d i t i o n a l c o r e s w h i c h we re e x a m i n e d b u t e x c l u d e d f r o m t h e s t u d y b e c a u s e o f p o o r m i c r o f o s s i l p r e s e r v a t i o n . All c o r e l o c a t i o n s a re l i s t ed in T a b l e s I a n d II.
242
(~ 10°N) to the Polar Front (~ 50°S). Sam- ples in the transect were selected so as to avoid regions of high rates of supply of terrigenous sediments, rugged topography, and the axis of deep thermohaline currents.
All cores from the Lamont-Doherty and Scripps collections were examined and sampled by the senior author. Core samples from the Eltanin cruises were supplied by D. Cassidy (Florida State University). We selected gravity and trigger weight cores whenever possible. Smear slides were prepared and examined for the presence of reworked Tertiary discoasters. In some instances the uppermost sediment in the split cores was several centimeters below the top of the core liner. We have used these measured sample depths in our tabulations (Tables I and II), but recognize that settling or shrink- age of the core material has occurred within the core liner since the cores were split, there- by producing in some instances a void inter- val at the top of the liner; thus the actual sample depths below the original zero-level in the core may be somewhat less than our tabulated values.
We were forced to examine a large number of samples in this eastern transect due to notably poor preservation of radiolarians in a large number of cores from middle latitude regions (Fig.2). We eliminated 83 samples due to poor specimen preservation (Table II), and focused upon the 74 remaining sam- ples for microfossil analysis (Table I).
Radiolarian assemblages were separated from each sample, using standard procedures described by Riedel and Sanfilippo (1977). In most cases two microscope slides of pre- pared residue yielded a sufficient number of Radiolaria for examination, although addi- tional sample material and/or cleaning pro- cedures were occasionally required. As in our previous work on the western transect, we eliminated from consideration samples from south of ~ 50°S, since diatoms outnumber radiolarians by several orders of magnitude as one crosses the Polar Front.
Regional distribution of taxa
The presence or absence of 74 taxa (37 Spumel- laria and 37 Nassellaria) was recorded in each of the
TABLE I
List of 74 samples used in biogeographic study of radiolarian assemblages in eastern Indian Ocean (locations of samples are shown as numbered open circles in Fig.2)
Sample Cruise Core Level Latitude Longitude Water Rad. Rad. No. No. (cm) depth (m) abund, pres.
101 TSDY 16-PG 0-3 09°57'N 92°23'E 1262 A G 102 CIRCE 39-G 10-12 09°48'N 83°30'E 3644 C G 103 CIRCE 41-PG 0-2 08°00'N 85°39'E 3745 C G 104 ANTIPO 159-PG 0-2 07°25'N 86°11'E 3833 A G 105 CIRCE 42-G 0-3 06°55'N 89°02'E 3741 C G 106 CIRCE 44-G 1-3 04o11 'N 88°35'E 4057 C G 107 DODO 201-G 0-2 02°59'N 91°41'E 4131 C G 108 RC17 128-TW 8-10 02°57'N 80°07'E 4366 C M 109 ANTIPO 170-PG 3-5 01°30'N 93°21'E 4326 C G 110 RC17 132-TW 7-10 00°52'N 84°33'E 4496 C M 111 ERDC 34-PG 5-7 00°38'N 96°15'E 4650 C G 112 INDP 41-PG 0-2 00°14'N 97°21'E 2649 C G 113 INDP 39-PG 0-2 00°14'N 97°20'E 2938 C G 114 ANTIPO 155-P 0-2 00°22'S 82°07'E 4643 A G 115 RC17 130-TW 7-10 00°56'S 80°09'E 4541 F M 116 ERDC 27-P 5-7 01°38'S 92°30'E 4335 A G 117 DODO 226-G 0-2 02°55'S 96°24'E 4743 C G 118 DODO 236-G 0-2 04°04'S 98°37'E 4875 C M 119 RC17 136-TW 5-8 04°47'S 88°44'E 4201 C M 120 V34 56-P 5-8 06°29'S 89°05'E 2769 F M
C C C C C C A C C C C F C C A C C F C F C C C C F A A C C A F F C F F F F F C F F F F C F C C F C C C C C C
G M G G G G G G G G G M G G G M G M M G M M G M M G G G G G M M G G G G G G G M M P P M M M M M M G M G G G
244
TABLE II
List of 83 cores examined from eastern Indian Ocean in which radiolarian abundance and preservation were not sufficient to allow species identification (these core locations are shown as un-uumbered asterisks in Fig.2)
V16 97-P 33°31'S 101°08'E 5967 R C l l 125-P 33°38'S 91°56'E 4305 LSDA 132-PG 33°47'S 96°00'E 4328 ELT45 14-PC 34°12'S 101°38'E 6030 V16 90-P 34°30'S 88°18'E 4123 ELT55 41-TC 34°32'S 109°44'E 2862 ELT48 31-TC 34°49'S 84°07'E 3721 ELT45 16-PC 35°07'S 101°58'E 4494 R C l l 135-PC 35°15'S 111°19'E 4823 ELT55 33-TC 35°27'S l l 0 ° 0 0 ' E 4448 ELT48 30-TC 35°49'S 81°38'E 4392 ELT49 54-PC 35°55'S 99°59'E 4595 R C l l 124-P 36°06'S 90°13'E 3775 V16 98-TW 36°07'S 106°13'E 6077 MSN 67-P 36°19'S 98°40'E 4400 ELT48 29-TC 36°30'S 80°01'E 2380 ELT55 30-TC 36°30'S 110°14'E 3998 ELT45 17-PC 36°80'S 102°36'E 4744 ELT45 20-TC 37°39'S 103°05'E 4753 ELT49 53-PC 37°51'S 100°02'E 4426 ELT55 23-TC 37°51'S 114°36'E 4632 R C l l 122-PC 38°02'S 83°29'E 3490 ELT55 21-TC 38°11'S 118°40'E 4126 ELT48 28-TC 38°33'S 79°55'E 3290 ELT49 52-PC 39°00'S 99°57'E 4299 RC8 53-P 39°23'S 104°22'E 4429 ELT45 26-PC 41°46'S 105°01'E 3904 RC8 51-P 44°02'S 93°53'E 2736 ELT45 29-TC 44°53'S 106°31'E 3910
74 samples selected from the transect. No estimates of abundance or counts of total assemblages were made. Species included in this study are the same as those used and illustrated by us (Johnson and Nigrini, 1980) in our western Indian Ocean study. One sub- species (Spongaster tetras irregularis) is essentially absent from our eastern Indian Ocean samples and was, therefore, excluded from subsequent recurrent group analysis.
The generic assignments of 2 species were changed subsequent to the time when Johnson and Nigrini (1980) went to press. The species are: (a)Buceino- sphaera invaginata (Haeckel), reassigned to the genus Collosphaera by Bj~rklund and Goll (1979); and (2) Ommatartus tetrathalamus tetrathalamus (Haeckel), reassigned to the genus Didymocyrtis by Sanfilippo and Riedel (1980).
However, in order to aid comparison between Johnson and Nigrini (1980) and this paper, we have chosen herein to retain the previously used generic assignments.
In the following section, we discuss the distribu- tion pattern of each taxon in the eastern Indian
Ocean transect (see Figs.3 through 15) and compare it with the pattern previously observed by us in the western Indian Ocean. Taxa are arranged according to the family level taxonomy of Riedel (1967).
Acrosphaera flammabunda (Haeckel) (Fig.3a). Pres- ent in most samples north of about 20°S, with scattered occurrences at 25°S and 32°S. Distribution pattern in the eastern Indian Ocean is similar to that found in the western Indian Ocean, except that oc- currences are rare between 20°S and 32°S in the eastern sector.
Acrosphaera lappacea (Haeckel) (Fig.3b). Present in most samples north of about 32°S. Distribution pattern in the eastern Indian Ocean is similar to that found in the western Indian Ocean, except for very rare occurrences south of 35°S in the western sector.
Acrosphaera spinosa (Haeckel) (Fig.3c). Present in all samples north of about 33°S. Between 33°S and 46°S there are rare occurrences of a similar, but
246
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larger (over 200 p in d i ame te r ) fo rm. Di s t r ibu t ion p a t t e r n in the eas tern Ind ian Ocean differs f rom t h a t f o u n d in the wes te rn Ind ian Ocean where the smaller fo rm of the species is cons i s t en t ly p resen t as far s o u t h as 48°S.
Buccinosphaera invaginata ( H a e c k e l ) ( F i g . 3 d ) . R a t h e r rare, bu t p resen t in mos t samples n o r t h of a b o u t 16°S. D i s t r i bu t ion p a t t e r n in the eas tern Ind ian Ocean is cons i s t en t wi th t h a t f o u n d in the wes te rn Ind ian Ocean.
Collosphaera huxleyi (Muller) (Fig.3e). P resen t in all samples be tween a b o u t 14°S and 33°S, wi th rare occur rences as far n o r t h as 5°S. D i s t r i bu t ion p a t t e r n in the eas te rn Ind ian Ocean is s imilar to t h a t f o u n d in the wes te rn Ind ian Ocean, e x c e p t t h a t the geo- graphic range is sh i f t ed a l i t t le to the sou th (18°S - 37°S) in the wes te rn sector . ( E r r a t u m to J o h n s o n and Nigrini (1980 , p .1147) : desc r ip t ion of C. huxleyi should read " . . . 9 - -13 on a ha l f e q u a t o r . . . " )
Collosphaera sp. aff. C. huxleyi (Mi]ller) Fig.3f) . Present in all samples n o r t h of a b o u t 0 °, wi th scat ter- ed occur rences (usually small , impover i shed speci- mens ) as far s o u t h as a b o u t 14°S. D i s t r i bu t ion pat- t e rn in the eas te rn Ind ian Ocean is similar to t h a t f o u n d in the wes te rn Ind ian Ocean, excep t t h a t the s o u t h e r n l imi ts of b o t h the c o n s i s t e n t and sca t t e red occur rences are sh i f ted a b o u t 5 ° to the sou th in the eas te rn sector .
Collosphaera macropora ( P o p o f s k y ) ( F i g . 4 a ) . Present in mos t samples b e t w e e n a b o u t 7°N and 21°S, w i th rare occur rences as far s o u t h as 32°S. Very rare t owards the n o r t h e r n l imi t of its geographic range; spec imens are small t ow ar ds the s o u t h e r n l imi t of its geographic range. D i s t r i bu t i on p a t t e r n in the eas te rn Ind ian Ocean is s imilar to t h a t f o u n d in the wes te rn Ind ian Ocean, excep t t h a t the species do'es no t occur as far s o u t h (on ly to 25°S) in the wes te rn sector .
Collosphaera tuberosa (Haeckel ) (Fig.4b) . Present in all samples n o r t h of a b o u t 25°S, wi th two oc- cur rences at a b o u t 32°S. D i s t r i bu t i on p a t t e r n in the eas te rn Ind ian Ocean is s imilar to t h a t f o u n d in the wes te rn Ind ian Ocean. excep t for the two occur- rences at 32°S in the eas te rn sector .
Disolenia quadrata (Eh renbe rg ) (Fig.4c). P resen t in all samples n o r t h of a b o u t 25°S; increas ingly rare t owards the s o u t h e r n e x t r e m i t y of its geographic range. D i s t r i bu t i on p a t t e r n in the eas te rn Ind ian Ocean is cons i s t en t wi th t h a t f o u n d in the wes t e rn Ind ian Ocean.
Disolenia zanguebarica (Eh renbe rg ) (Fig .4d) . P resen t in all samples n o r t h o f a b o u t 33°S; increasingly rare t owards the s o u t h e r n e x t r e m i t y of i ts geographic range. D i s t r i bu t ion p a t t e r n in the eas te rn Ind ian Ocean is cons i s t en t wi th t h a t f ound in the wes t e rn Ind ian Ocean.
Otosphaera auriculata (Haeckel ) (Fig.4e) . P resen t in m o s t samples n o r t h of a b o u t 25°S; increasingly rare t owards the s o u t h e r n e x t r e m i t y of i ts geo- graphic range. D i s t r i bu t i on p a t t e r n in the eas te rn Ind ian Ocean is cons i s t en t wi th t h a t f ound in the wes te rn Ind ian Ocean.
Siphonosphaera polysiphonia (Haeckel ) (Fig.4f) . P resen t in m o s t samples n o r t h of a b o u t 32°S. Species ranges m u c h fa r the r s o u t h in the wes t e rn Ind ian Ocean ( to a b o u t 46°S). This d i sc repancy in the d i s t r i bu t i on p a t t e r n s m a y be due in pa r t to the general ly p o o r f a u n a f o u n d b e t w e e n 25°S and 37°S in the eas te rn sector .
Actinomma antarcticum (Haeckel ) (Fig.5a). P resen t in all b u t one sample s o u t h of a b o u t 45°S, w i th a single o c c u r r e n c e at a b o u t 40°S. D i s t r i bu t ion p a t t e r n in the eas te rn Ind ian Ocean is s imilar to , b u t b r o a d e r (poss ibly due to b e t t e r sample coverage) t h a n t h a t found in the wes t e rn Ind ian Ocean.
Actinomma arcadophorum (Haeckel ) (Fig .5b) . Pres- en t in mos t samples n o r t h of a b o u t 32°S. Dis t r ibu- t ion p a t t e r n in the eas te rn Ind ian Ocean is con- s is tent wi th t h a t f o u n d in the wes te rn Ind ian Ocean.
Actinomma medianum (Nigrini) (Fig.5c) . B imoda l d i s t r ibu t ion . Very rare in samples n o r t h of a b o u t 20°S, t end ing to be f o u n d mos t o f t e n in samples closest to the I n d o n e s i a n Arch ipe lago; m o r e abun- dan t and cons i s t en t ly p re sen t in samples b e t w e e n 38°S and 48°S. D i s t r i bu t ion p a t t e r n in the eas te rn Ind ian Ocean is s imilar to t h a t f o u n d in the wes t e rn Ind ian Ocean, excep t t h a t b o t h the n o r t h e r n and
Fig.3. D i s t r i bu t i on p a t t e r n s of rad io la r ian taxa. Maps in Figs.3 t h r o u g h 15 c o m p l e m e n t the maps of J o h n s o n and Nigrini ( 1 9 8 0 ) for the wes te rn Ind ian Ocean t ransec t , wi th the same sequence of 74 t axa in this paper and in our previous paper . Open circles r ep re sen t samples in which the t a x o n was s o u g h t b u t n o t f o u n d a f te r examina- t ion of two or more s t r ewn slides. Half-fi l led circles r ep resen t samples in which on ly a single spec imen or ques- t ionab le f r agmen t s of a given t a x o n were observed. In Figs.3 t h r o u g h 15, maps showing d i s t r ibu t ion p a t t e r n s are a r ranged in the same order as t h a t l is ted in the t ex t u n d e r "Reg iona l d i s t r i bu t ion o f t axa" .
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Fig.5. Distribution patterns of radiolarian taxa (see Fig.3 caption for explanation of symbols).
250
s o u t h e r n l imi ts of the area in which the species does n o t occur in the eas te rn sec to r are sh i f t ed a b o u t 5 ° to the sou th .
Anomalacantha dentata (Mast) (Fig.5d). B imoda l d i s t r ibu t ion . Sporad ic occur rences n o r t h of a b o u t 20°S; p resen t in greater n u m b e r s and in m o s t sam- ples b e t w e e n a b o u t 38°S and 48°S. D i s t r ibu t ion p a t t e r n in the eas te rn Ind ian Ocean is similar to t h a t f o u n d in the wes te rn Ind ian Ocean, excep t t h a t b o t h the n o r t h e r n and s o u t h e r n l imi ts of the area in which the species does no t occur in the eas tern sec tor are sh i f ted a b o u t 8 ° to the sou th .
Ommatartus tetrathalamus tetrathalamus (Haeckel ) (Fig.5e) . P resen t in all samples n o r t h of a b o u t 33°S. In the eas te rn Ind ian Ocean the species does no t occur as far sou th as it does in the wes te rn Ind ian
-Ocean where it is a b u n d a n t to 37°S and occurs rarely as far s o u t h as 46°S. This d i sc repancy m ay be due in pa r t to the general ly p o o r f auna f o u n d be t w een 25°S and 37°S in the eas te rn sector .
Cypassis irregularis (Nigrini) (Fig.5f) . Present in six samples b e t w e e n a b o u t 8°S and 16°S near the coast of Java; very rare. D i s t r i bu t ion p a t t e r n in the eas tern Ind ian Ocean is similar to t ha t f o u n d in the wes te rn Ind ian Ocean on ly in t ha t in b o t h sectors it occurs rarely and in " c o a s t a l " samples .
Saturnalis circularis (Haeckel ) (Fig.6a). Bimodal d i s t r ibu t ion . Present in mos t samples n o r t h of a b o u t 15°S, wi th sporadic occur rences to a b o u t 20°S and isolated occur rences at 25°S and 32°S; p resen t in every sample s o u t h of a b o u t 40°S. D i s t r ibu t ion pat- t e rn in the eas te rn Ind ian Ocean is similar to t ha t f ound in the wes te rn Ind ian Ocean, excep t t h a t the n o r t h e r n l imi t of the h igh- la t i tude occur rences in the wes te rn sec tor is a t a b o u t 30°S. Rare , bu t dis t inc- tive.
Spongurus cf. elliptica (Ehrenbe rg ) (Fig .6b) . Present in all samples n o r t h of a b o u t 32°S. D i s t r ibu t ion p a t t e r n in the eas te rn Ind ian Ocean is cons i s t en t wi th t h a t f o u n d in the wes te rn Ind ian Ocean.
Spongurus pylomaticus (Riedel) (Fig.6c). Present in 4 of 7 samples sou th of a b o u t 45°S. Dis t r ibu t ion p a t t e r n in the eas te rn Ind ian Ocean is s imilar to t ha t f ound in the wes te rn Ind ian Ocean, e x c e p t t ha t the species is f ound as far n o r t h as 37°S in the wes te rn sector .
Spongocore puella (Haeckel ) (Fig.6d) . Present in m o s t samples t h r o u g h o u t the s tudy area; absen t f rom s o u t h e r n m o s t sample (a t 51°S). D i s t r ibu t ion p a t t e r n in the eas te rn Ind ian Ocean is cons i s t en t wi th t ha t f ound in the wes t e rn Ind ian Ocean.
Styptosphaera ? spumacea (Haeckel ) (Fig.6e) . P resen t in every sample be tween a b o u t 38°S and 49°S; absen t f rom s o u t h e r n m o s t sample (at 51°S). D i s t r i bu t ion p a t t e r n in the eas te rn Ind ian Ocean is cons i s t en t wi th t h a t f o u n d in the wes te rn Ind ian Ocean.
Heliodiscus asteriscus (Haeckel ) (Fig.6f) . Present in every sample e x c e p t for th ree in the s o u t h e r n m o s t par t of the s tudy area. D i s t r ibu t ion p a t t e r n in the eas te rn Ind ian Ocean is cons i s t en t wi th t ha t f o u n d in the wes te rn Ind ian Ocean.
Heliodiscus echiniscus (Haeckel ) (Fig.7a). Present in all samples n o r t h of a b o u t 25°S. D i s t r i bu t ion p a t t e r n in the eas tern Ind ian Ocean is cons i s t en t wi th t ha t f o u n d in the wes te rn Indian Ocean.
Amphirhopalum cf. Tessarastrum straussii (Haeckel ) (Fig .7b) . Present in mos t samples be tween a b o u t 10°S and 32°S. Add i t iona l samples which were examined , bu t s u b s e q u e n t l y re jec ted because of p o o r fauna, indica te t ha t the species may be f o u n d as far s o u t h as 40°S in the eas te rn Indian Ocean. In the wes te rn Ind ian Ocean the species is f ound as far sou th as 46°S. ( E r r a t u m to J o h n s o n and Nigrini (1980 , Fig .7b) : sample n u m b e r 33 shou ld be a half- filled circle.)
Amphirhopalum ypsilon (Haeckel ) (Fig.7c) . Present in all samples n o r t h of a b o u t 20°S, wi th sca t t e red occur rences as far s o u t h as 32°S. Dis t r ibu t ion pat- te rn in the eas te rn Ind ian Ocean is s imilar to t ha t f o u n d in the wes te rn Ind ian Ocean, e x c e p t for the two occur rences at 32°S in the eas tern sector . Sou th - e r n m o s t occur rences in the wes te rn sec to r are at a b o u t 25°S ( E r r a t u m to J o h n s o n and Nigrini ( 1 9 8 0 ) for this species (p. 121) : " . . . b e tween a b o u t 20°S and 25°S. ' '
Trigonastrum sp. (Fig .7d) . Present in mos t samples be tween a b o u t 32°S and 43°S, wi th sca t t e red oc- cur rences as far n o r t h as 18°S. D i s t r ibu t ion p a t t e r n in the eas te rn Indian Ocean is cons i s t en t wi th t h a t f o u n d in the western Ind ian Ocean.
Euchitonia elegans (Ehrenbe rg ) (Fig.7e) . Present in mos t samples n o r t h of a b o u t 25°S. F r e q u e n t l y spec imens are i n c o m p l e t e and c a n n o t be d is t inguished f rom E. furcata. Dis t r ibu t ion p a t t e r n in the eas tern Ind ian Ocean is cons i s t en t wi th tha t f o u n d in the wes te rn Ind ian Ocean. Note t h a t in the desc r ip t ion of the d i s t r ibu t ion of this species in J o h n s o n and Nigrini ( 1 9 8 0 ) the s o u t h e r n l imi t of rare occur rences shou ld read 27°S, no t 37°S.
Dictyocoryne profunda (Ehrenbe rg ) (Fig.8a). Present in m o s t samples n o r t h of a b o u t 32°S, wi th one oc- cu r rence at a b o u t 40°S. D i s t r i bu t ion p a t t e r n in the
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254
eastern Indian Ocean is consistent with that found in the western Indian Ocean.
Dictyocoryne truncatum (Ehrenberg) (Fig.8b). Pres- ent in all samples north of about 32°S. Distribution pattern in the eastern Indian Ocean is consistent with that found in the western Indian Ocean.
Hymeniastrum euclidis (Haeckel) (Fig.8c). Present in all but two samples throughout the study area; absent from southernmost samples (at 51°S). Dis- tribution pattern in the eastern Indian Ocean is con- sistent with that found in the western Indian Ocean.
Spongaster tetras tetras (Ehrenberg) (Fig.8d). Pres- ent in all but one sample north of about 32°S. Dis- tribution pattern in the eastern Indian Ocean is consistent with that found in the western Indian Ocean.
Spongaster tetras (Ehrenberg) irregularis Nigrini (Fig.8e). Only two specimens of this subspecies were found in the entire study area: one at about 16°S and one at about 39°S, and both in the western part of the study area. In the western Indian Ocean the subspecies occurs in most samples between 30°S and 40°S, with rare occurrences extending its geo- graphic range to between 25°S and 45°S.
Spongobrachium sp. (Fig.8f). Present in most sam- ples between about 8°S and 28°S. Distribution pat- tern in the eastern Indian Ocean is consistent with that found in the western Indian Ocean. (Erratum to Johnson and Nigrini, 1980, fig.8f: sample num- ber 30 should be a half-filled circle.)
Larcospira quadrangula (Haeckel) (Fig.9a). Pres- ent in all but two samples north of about 32°S. Species extends much farther south in the western Indian Ocean (to about 46°S). This discrepancy may be due in part to the generally poor fauna between 25°S and 37°S in the eastern sector.
Antarctissa spp. (Fig.9b). Present in all but one sample south of about 40°S. Distribution pattern in the eastern Indian Ocean is consistent with that found in the western Indian Ocean.
Liriospyris reticulata (Ehrenberg) (Fig.9c). Present in all samples north of about 28°S, with a single occurrence at about 32°S. Distribution pattern in the eastern Indian Ocean is consistent with that found in the western Indian Ocean.
Lophospyris pentagona pentagona (Ehrenberg) (Fig. 9d). Present in all but one sample north of about 25°S. Distribution pattern in the eastern Indian Ocean is consistent with that found in the western Indian Ocean.
Phormospyris stabilis (Goll) antarctica (Haecker) (Fig.9e). Present in all samples south of about 45°S, with a single occurrence at about 40°S. In the western Indian Ocean the subspecies ranges as far north as 37°S.
Carpocanistrum spp. (Fig.9f). Present in all but three samples north of about 45°S. Distribution pattern in the eastern Indian Ocean is consistent with that found in the western Indian Ocean.
Carpocanarium papillosum (Ehrenberg) group (Fig. 10a). Bimodal distribution. Present in most samples north of about 18°S, with isolated occurrences at about 25°S and 32°S. Less abundant in the high- latitude portion (south of about 38°S) of its geo- graphic range. Distribution pattern in the eastern Indian Ocean is consistent with that found in the western Indian Ocean.
Cornutella profunda (Ehrenberg) (Fig.10b). Present in most samples throughout the study area; however, between about 25°S and 40°S it is frequently absent or very rare. Distribution pattern in the eastern Indian Ocean is consistent with that found in the western Indian Ocean.
Lithopera bacca (Ehrenherg) (Fig.10c). Present in most samples north of about 32°S; particularly abundant in a cluster of samples between about 105°E to 110°E and 8°S to 12°S. Distribution pattern in the eastern Indian Ocean is consistent with that found in the western Indian Ocean, but the area of high abundance is farther south (22°S--35°S) in the western sector.
Dictyophimus crisiae (Ehrenberg) (Fig.10d). Present in most samples throughout the study area. Distribu- tion pattern in the eastern Indian Ocean is consistent with that found in the western Indian Ocean.
Pterocanium praetextum praetextum (Ehrenberg) (Fig.10e). Present in all samples north of about 32°S. Distribution pattern in the eastern Indian Ocean is consistent with that found in the western Indian Ocean.
Pterocanium praetextum (Ehrenberg) eucolpum (Haeckei) (Fig.10f). Present, but rare, in most sam- ples between about 32°S and 48°S. Distribution pattern in the eastern Indian Ocean is similar to that found in the western Indian Ocean, except that the subspecies ranges a little farther north (to 27°S) in the western sector.
Pterocanium trilobum (Haeckel) (Fig . l la ) . Present in all samples north of about 32°S, with scattered occurrences as far south as 50°S. Distribution pat- tern in the eastern Indian Ocean is consistent with that found in the western Indian Ocean.
255
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257
258
Pterocanium sp. (F ig . l ib ) . Bimodal distribution. Scattered occurrences north of about 15°S and south of about 38°S; always very rare. Distribution pattern in the eastern Indian Ocean is similar to that found in the western Indian Ocean, but with different limits (north of 0 ° and between 30°S and 46°S) in the western sector.
Theocalyptra bicornis (Popofsky) s.s. (Fig.11c). Present in all but one sample south of about 40°S. Distribution pattern in the eastern Indian Ocean is consistent with that found in the western Indian Ocean.
Eucyrtidium acuminatum (Ehrenberg) (Fig . l id) . Present in most samples between about 10°S and 47°S. Species is rare in samples between about 10°S and 20°S and many specimens have a morphology which is transitional between E. acuminatum and E. hexagonatum. Distribution pattern in the eastern Indian Ocean is consistent with that found in the western Indian Ocean.
Eucyrtidium hexagonatum Haeckel (F ig . l i e ) . Present in most samples north of about 25°S, but generally rare or transitional to E. acuminatum in samples between about 20°S and 25°S. Distribution pattern in the eastern Indian Ocean is consistent with that found in the western Indian Ocean.
Lithocampe sp. (Fig.11f). Present in most samples between about 5°S and 47°S. Distribution pattern in the eastern Indian Ocean is similar to that found in the western Indian Ocean, except that the species does not range as far north (only to 10°S) in the western sector.
Anthocyrtidium ophirense (Ehrenberg) (Fig.12a). Present in all samples north of about 32°S. Distribu- tion pattern in the eastern Indian Ocean is similar to that found in the western Indian Ocean, except that the southern limit of the geographic range is a little farther south (to 40°S) in the western sector.
Anthocyrtidium zanguebaricum (Ehrenberg) (Fig. 12b). Present in most samples north of about 25°S. The geographic range of this species extends much farther south (to 45°S) in the western Indian Ocean. This discrepancy may be due in part to the generally poor fauna found between 25°S and 37°S in the eastern sector.
Androcyclas gamphonycha (JSrgensen) (Fig.12c). Present in most samples between about 38°S and 49°S. Distribution pattern in the eastern Indian Ocean is similar to that found in the western Indian Ocean, except that the species ranges farther north (to 30°S) in the western sector.
Lamprocyclas maritalis maritalis (Haeckel) (Fig. 12d). Present in all samples between about 19°S and 47°S, with scattered occurrences as far north as 0 °. Distribution pattern in the eastern Indian Ocean is similar to that found in the western Indian Ocean, except that the scattered occurrences extend farther north (to 18°N) in the western section.
Lamprocyclas maritalis (Haecke l )po lypora Nigrini (Fig.12e). Present in all samples north of about 43°S; most abundant in low latitudes. Distribution pattern in the eastern Indian Ocean is consistent with that found in the western Indian Ocean.
Lamprocyclas maritalis (Haeckel) ventricosa Nigrini (Fig.12f). Present in most samples north of about 5°S, but very rare (1 specimen per slide) in about half of those samples. Distribution pattern in the eastern Indian Ocean is similar to that found in the western Indian Ocean, except that the subspecies does not occur as far south (only to 10°N) in the western sector.
Lamprocyrtis nigriniae (Caulet) (Fig.13a). Present in all but one sample north of about 15°S. Distribu- tion pattern in the eastern Indian Ocean is similar to that found in the western Indian Ocean, except that the species does not occur as far south (only to 5°S) in the western sector.
Lamprocyrtis (?) hannai (Campbell and Clark) (Fig.13b). Present in most samples north of about 15°S, with scattered and rare occurrences as far south as 43°S. Distribution pattern in the eastern Indian Ocean is similar to that found in the western Indian Ocean, except that in the eastern sector the southern limit of the area of consistent occurrence is 5 ° farther south and the southern limit of scattered occurrences is about 13 ° farther south.
Pterocorys hertwigii (Haeckel) (Fig.13c). Present in most samples north of about 20°S; less abundant in the western part of the study area. Distribution pattern in the eastern Indian Ocean is consistent with that found in the western Indian Ocean.
Pterocorys sabae (Ehrenberg) (Fig.13d). Present in all samples north of about 32°S. Distribution pattern in the eastern Indian Ocean is similar to that found in the western Indian Ocean, except that the species ranges farther south (to about 40°S) in the western sector.
Theocorythium trachelium trachelium (Ehrenberg) (Fig.13e). Present in all samples north of about 32°S. Distribution pattern in the eastern Indian Ocean is consistent with that found in the western Indian Ocean. The expected area of transition be-
"~ • • . Z ~ ~ to ° , • • e • ~ Io°- I • 1 • "l ' , O 0
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o PTEROCORYS SABAE 0 TR~EHELIU M •
J ( d ) 0 o . . . . . . . . . . o 0 •
o 9 0 ° i 0 0 o 0 i10o E ()e o 9 0 ° I 0 0 ° o I IO°E ( f ) • 9 0 ° iO0O o i10o E I I Q I I I I I I I Q I I I I I I I i l I i I
0 0 0
Fig.13. Distribution patterns of tad•olaf•an taxa (see Fig.3 caption for explanation of symbols).
261
tween this subspecies and T. trachelium dianae has an impoverished fauna and so transit ional forms were rarely observed.
Theocorythium trachelium (Ehrenberg) dianae (Haeckel) (Fig.13f). Present in all but two samples be tween about 32°S and 50°S. Distr ibut ion pat tern in the eastern Indian Ocean is similar to that found in the western Indian Ocean, excep t that in the western sector the subspecies occurs as far nor th as 25°S.
Botryostrobus aquilonaris (Bailey) (Fig.14a). Bimodal distr ibut ion, but no t markedly so. Present in most samples th roughou t the s tudy area, but absent or rare in samples be tween about 15°S and 30°S. Dis- t r ibut ion pat tern in the eastern Indian Ocean is similar to that found in the western Indian Ocean, excep t that the species is much more common , especially in low lati tudes, and is more widely dis- t r ibuted in the eastern sector.
Botryostrobus auritus/australis (Ehrenberg) (Fig. 14b). Present in all but one sample th roughou t the study area. Distr ibut ion pa t te rn in the eastern Indian Ocean is consis tent with that found in the western Indian Ocean.
Phormostichoartus corbula (Harting) (Fig.14c). Pres- ent in all but one sample north of about 20°S, with very rare occurrences at about 25°S and 32°S. Dis- t r ibut ion pat tern in the eastern Indian Ocean is similar to that found in the western Indian Ocean, excep t that the species ranges farther south ( to 33°S) in the western sector. A similar form has been ob- served in high lat i tudes; it differs f rom P. corbula in that the cephalis protrudes f rom the thorax and the shell is less broad.
Siphocampe lineata (Ehrenberg) group (Fig.14d). Present in most samples nor th of about 20°S, with isolated occurrences at about 32°S and 47°S. Dis- t r ibut ion pat tern in the eastern Indian Ocean is similar to that found in the western Indian Ocean, except that there are more , rare, high la t i tude (37°S to 46°S) occurrences in the western sector.
Spirocyrtis scalaris (Haeckel) (Fig.14e). Present in most samples nor th of abOut 18°S; less c o m m o n in the western part of the study area. Distr ibut ion pat- tern in the eastern Indian Ocean is consis tent with that found in the western Indian Ocean.
Botryocyrtis scutum (Harting) (Fig.14f). Present in all but one sample nor th of about 32°S. Distr ibut ion pat tern in the eastern Indian Ocean is consis tent with that found in the western Indian Ocean.
Centrobotrys thermophila (Petrushevskaya) (Fig. 15a). Present in most samples nor th of about 20°S,
with sporadic occurrences as far south as about 32°S. Distr ibut ion pat tern in the eastern Indian Ocean is consistent with that found in the western Indian Ocean.
Saccospyris conithorax (Petrushevskaya) (Fig.15b). Present in most samples south of about 40°S. Dis- t r ibut ion pat tern in the eastern Indian Ocean is consis tent with that found in the western Indian Ocean.
Recurrent group analysis
A number of quantitative and semi-quan- titative techniques have been applied to the interpretation of biogeographic distribu- tion patterns of microfossil assemblages (e.g., Imbrie and Kipp, 1971; Haq and Loh- mann, 1976; Sancetta, 1978; Moore et al., 1980). In our previous s tudy of modern radiolarian assemblages in the western Indian Ocean (Johnson and Nigrini, 1980), we used recurrent group analysis of species presence-- absence data, and from this analysis sub- divided 74 radiolarian taxa into five recurrent groups and eight geographic assemblages. Since the complet ion of that study, Sancetta (1979b) has reported a comparative s tudy of quantitative and semi-quantitative approaches to the t reatment of microfossil census data. Her results indicate that presence--absence data alone are generally sufficient to describe the essential characteristics of microfossil distribution patterns. On this basis we are confident that our statistical t reatment of the western Indian Ocean data set was appro- priate, and have, therefore, followed the same approach herein.
The theory of recurrent group analysis and the assumptions involved have been discussed elsewhere (e.g., Fager, 1957; Fager and McGowan, 1963; Renz, 1976), and will be briefly summarized here. The procedure uses the concept of affinity, based on the co- occurrence of species pairs, as a means of identifying those species which consistently co-occur and thus may be indicative of a particular biological environment. Those species which often co-occur have strong affinity; those that never do, have no affinity. Recurrent group analysis selects the largest
"o " • " ... II • " ." • ] o. ,oo- 0~9 0 • • • O 0 • • • •
o o o , 2@ o o , 20 • • . 20
0 0 0 ", ~ • • ', 0 0 '+ ~i" 0 ~ " 0 ~ T
e e I 30 ~ '~ 30~ P '~, 30~
o + o o o• if• o ':I o • o 4[00 .j o o o ° o 4 0 o. o o o ° o 4 0 o o ) o 4 0
o o , o o t o o o , j; qn:}~pf o , aiRi}ii*Ri [ : O pq.~O(yRT [
d) ,o o o o ~po ~ , , o o o , , ,O .E , ( e ) o ~ o o , o o o , ,o°E ( f ) o ~ o - , o o ° , O ° E l ~ I {DI I ~ I a / l I I Q I 1 I [ I /
0 0 0
Fig.14. Dis t r ibut ion pa t t e rns of radiolarian taxa (see Fig.3 capt ion for exp lana t ion of symbols ) .
263
I D I A BURM,~
I ~;i/~T ~'~) • • " :ik ~ ' ~ ,oo-
- Oo .%~\ ..... ,, T.o . . . - ~ , ~ ~ C~oo.
:'.. ~ • • • 41 I0 °
• • • • •
• 0 0 0 0 • • Q
• 0 20~
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o o
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o o
o CENTROBOTRYS
0 THERMOPH I LA 0
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I I Qi I n I I 0
%THA'LANO ::
)oooO oo
o °o . ~ \ \ 0 ~ o o o o o ~ ~ : ~ ~ 0 o
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o
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o o
o o o
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(b).
D o
0 • 0 I0 ° 0
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0
. 20 Q
o ~
o o)
0
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0 0 40 °-
SACCOSPYRIS
• CONITHORAX
tO0 ° tlO°E I L I t
Fig.15. Distribution patterns of radiolarian taxa (see Fig.3 caption for explanation of symbols).
separate uni ts wi th in which each species shows s t rong af f in i ty fo r all o the r species in the group.
For any two species A and B, the index o f thei r a f f in i ty (I.A.) is given by
I.A. = J / x / ( g A " N B ) - - 1/~,,,'2N B
where J = n u m b e r of jo in t occur rences o f A and B; N A = to ta l n u m b e r of occur rences
o f A; N B = to ta l n u m b e r o f occur rences o f B; and N B t> N A . Fager (1957) has de f ined a r ecu r ren t g roup as one tha t satisfies the fol lowing requ i rements : (1) the index o f a f f in i ty is /> 0.50 for all pairs o f species wi thin the group; (2) the group includes the greatest possible n u m b e r o f species; (3) if two or more groups wi th the same n u m b e r o f species and wi th members in c o m m o n are
264
possible, the one for which the sum of af- finities is greatest is chosen.
Selection o f recurrent groups
To implement this procedure, we used the computer program REGROUP (designed by E.W. Fager), in the same manner as de- scribed in our report on the western transect. The number of occurrences and the number of joint occurrences of the 74 species iden- tified in our transect of 74 samples in the eastern Indian Ocean (Fig.2; Table I) were computed. The index of affinity (I.A.) for each pair of species was calculated and compared to the assigned cutoff value (0.50). If this index is equal to or greater than the assigned cutoff valve, the pair of species is considered to have affinity. Recurrent groups were selected according to the three criteria of Fager (1957) summarized above.
We first applied recurrent group analysis procedures to the 74 samples in the eastern transect alone (Fig.2), and identified six recurrent groups of Radiolaria. Each of these groups met the required criteria for species affinities and exhibited a coherent geographic distribution pattern within the transect. Five of these six recurrent groups bore a very close, though not exact, correspondence with the five groups previously identified in the western transect (Johnson and Nigrini, 1980, table II). It became evident that the inclusion of a more extensive geographic sample cover- age allowed us to identify species groups which did not emerge with more limited sample control. Moreover, since our overall objective is to understand radiolarian distribu- tion patterns throughout the entire Indian Ocean, the identification of recurrent groups in the two transects combined is clearly preferable to the analysis of a single transect alone. Therefore, we applied recurrent group analysis to the original set of 46 samples from the western transect (Johnson and Nigrini, 1980, table I), plus the 74 samples in the eastern transect (this report , Table I).
We identified six recurrent groups of Radiolaria in the combined data set of 120 samples which represented both the eastern
and western transects. We have designated these groups A' through F' since their com- positions correspond approximately with those of the five groups which we previously identified in the western transect alone (groups A through E; see Johnson and Nigrini, 1980, table II)*. The revised recurrent groups are listed in Table III, and may be compared with our previous grouping as follows:
Recurrent Group A' (46 species) contains each of the 43 species assigned to Group A in the western transect (Johnson and Nigrini, 1980, table II), plus three additional species (Buccinosphaera invaginata, Collosphaera macropora, Lithopera bacca) which were not previously grouped, but which met the required criteria for inclusion in Group A' based on the expanded data set of 120 sam- ples. These three species are generally rare; thus we might anticipate that their actual affinities with other taxa would become clearer as more samples are included.
Recurrent Group B' (8 species) is identical to Recurrent Group B previously identified in the western transect. Species belonging to this group have very limited geographic distribution patterns, centered just to the south of the Subtropical Convergence, and thus these species are present in a relatively small proport ion of the total data set of 120 samples. The affinity indices associated with species pairs in this group range from 0.47 to 0.85, and 26 of the 28 indices are above the arbitrarily chosen cutoff value of 0.50. Two of the seven indices for Spongurus pylomaticus are 0.47, or slightly below the arbitrary cu tof f of 0.50. Nevertheless we feel justified in retaining S. pylomaticus with- in Group B' due to the relatively small num- ber of samples in which species of Group B' are present; this situation leads to significant changes in affinity indices if a species is not detected in a single sample due to poor sam- ple preservation. The eight taxa assigned to
*Note t ha t a typograph ica l e r ror appears in table 2, p. 141, of our original r epo r t on wes te rn Ind ian Ocean assemblages. Collosphaera sp. cf. C. huxleyi shou ld have been l is ted wi th R e c u r r e n t G r o u p D, and C. huxleyi was ung rouped .
265
Group B' are thus appropriate choices as diagnostic of the assemblages near and south of the Subtropical Convergence.
Recurrent Group C' (6 species)represents a substantial re-arrangement from the com- position of the original Recurrent Group C in our previous report (Johnson and Nigrini, 1980, table II). The revised Group C' contains four of the original taxa from Group C, plus two taxa not previously grouped (C. huxleyi and Spongobrachium sp.). Of the remaining taxa formerly assigned to Group C, three have been re-assigned to a sixth recurrent group not previously identified (Recurrent Group F'), and the final species from the original Group C (Spongaster tetras irregularis) is now ungrouped as a consequence of its virtual absence from the entire eastern transect (this report, Fig.8e).
Recurrent Group D' (3 species) consists of three of the four species assigned to Group D in the western transec£. The fourth species (Cypassis irregularis) had no affinities at all in the combined data set of 120 samples, and therefore was excluded from Group D'. We previously labeled Recurrent Group D as being associated with the Arabian Margin, and suggested that the distribution of these species in the northwestern Indian Ocean (Johnson and Nigrini, 1980) and in the eastern Pacific (Nigrini, 1968) may be in- dicative of an association with upwelling regions. However, the three taxa in the re- designated Group D' are clearly abundant throughout the northeastern Indian Ocean as well (this report, Figs.3f; 12f; 13a). There- fore, we have re-labelled Recurrent Group D' as a northern Indian Ocean group for the purposes of describing its approximate geographic extent (Table III).
Recurrent Group E' (5 species) contains four of the five taxa previously assigned to Group E in the western transect; Botryos- trobus aquilonaris now replaces Pterocanium sp. as the fifth taxon in Group E' (Table III). Four of the five species assigned to Group E' were selected by the computer program REGROUP; the fifth species, Anomalacantha dentata, was assigned by us to Group E' after inspecting the affinity indices of A. dentata
and ascertaining that all indices were above or sufficiently close to the cutoff value to warrant the inclusion of this species in Group E'. A similar test of the affinity indices of Pterocanium sp., however, revealed that three of the five indices were significantly below the minimum value required for in- clusion in Group E', and therefore Ptero- canium sp. is ungrouped in this report.
Recurrent Group F' (3 species) consists of 3 taxa formerly assigned to Group C (Ptero- canium praetextum eucolpum; Theocoryth- ium trachelium dianae; Trigonastrum sp.). The increased sample coverage with the addi- tion of the eastern transect has apparently allowed a finer discrimination of radiolarian distribution patterns in mid-latitude regions.
Geographic distribution patterns o f recurrent groups
After re<lesignating the six recurrent groups which were derived using samples from both the eastern and western Indian Ocean transects (Table III), we identified the samples in which each recurrent group appears. Our criterion is that a group is considered "present" in a given sample if at least 80% of the taxa within the group are present. Accordingly, Group A' is considered to be present in a sample if 37 of its 46 taxa are present; Group B' requires 7 of its 8 species; Group C' required 5 of its 6 species; Group D' requires 3 of its 3 species, Group E' requires 4 of its 5 species; and Group F' requires 3 of its 3 species. Following these criteria, we have prepared six sets of com- bined maps indicating the geographic dis- tribution patterns of the six re-designated recurrent groups (Figs.16--21).
Recurrent Group A' (Fig.16) shows a coherent distribution pattern in tropical latitudes, extending to ~ 20°S in the eastern half of the Indian Ocean, and somewhat farther south in the Madagascar Basin (~ 25°S). The southward transport of near- surface waters in the subtropical gyre east of Madagascar (Wyrtki, 1973) is most prob- ably responsible for the southward advec- tion of this species group into the Madagascar
266
TABLE III
Listing of recurrent groups of Radiolaria in eastern Indian Ocean transect
Carpocanarium papillosum group Saturnalis circularis
Trigonastrum sp.
Spongaster tetras irregularis
267
X, J ", '\ i )U~..~'-~ v I u I v -,~- I h,, "2,\ ~1 India / X BURMA ~ !
"' / °\ 5\ ~-JgN o M - Arab ia ~, : b 0 , ,f,~i THAILAND
...... / O0 ' " " / ' O 0 0 • • "~ b - - . ' ~ " ~ I O°-
.o • \ o o_
• - •• • " ~ - } " ~ _ ~ - . ~ - 4 ~ .
• • • • • O O ~ ~ ~ •
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~.~. • • - • o • ° o O o° • • o • . • • •
.. w o ~ d , ~ , ~ , ~ , ~ , ~ 0 2 c , ~ : O - O O / ~ ' ~ " ' ~ . . , ~ , ,
_ • o j o - o o o
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(46spp.) o
0 J O 0 0 _0 0 0
0 0 4 0 °- 0
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o o SO~L 60 o 70 ° 0 9 0 ° IIOOE
0 Fig.16. Distribution pattern of Recurrent Group A'. Filled circles denote samples in which at least 37 of the 46 species of Group A' are present.
Basin. In the northern Indian Ocean, Group A' is present in all samples from the Bay of Bengal, but is notably missing in three sam- ples from nearest the Arabian upwelling zone (Fig.16). Many of the species from Group A' (approximately 25--30) a r e in fact present in these three samples, but not a sufficient number to meet our 80% criterion
for "presence" of the group. The total species diversity is substantially less in these samples than farther to the south in the Arabian Sea, even though the specimens are very well preserved. The low species diversity and resulting absence of Group A' in these 3 samples may be a consequence of abnormal- ly low surface water temperatures which re-
0 Fig.17. Distribution pattern of Recurrent Group B'. Filled circles denote samples in which at least 7 of the 8 species of Group B' are present.
cur regularly during the southwest monsoon. If this explanation is correct, we would ex- pect a similar decrease in species diversity along the Somali coast to the south; unfor- tunately our present sample coverage is not adequate to test this possibility.
Recurrent Group B' is well~lefined in temperate latitudes lying between the Sub- tropical Convergence and the Antarctic Con-
vergence (Figs. l , 17). Both its northern and southern limits appear to be several degrees farther north in the western transect than in the eastern one (Fig.17). Our present sample coverage, however, does not give us particularly good control. Since there is a close correspondence between the northern limit of Group B' and the position of the Subtropical Convergence, our faunal data may
269
Arabia
Africa
c I ¸ \ X I
", ~ India
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O
~ 0 0 / % "2\./~
0 o
0
0
0
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, / ~ . 0
• o° e: • " O 0 •
0 • • O 0 • •
• •
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GROUP C' o
( 6 s p p )
O
0 0 0
0 50: E 6,9 ° 70 +
i , i
,
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- o 0
o o o " \ ~.'>~% ~ " ' , - ~ " O 00(~ _\ % ~ ~:~ / ~' -o - - , ~ \ . ~ ~. \ o o_
o o ~A ~ \<~ 2 o o ~ , , . . '~ .~ ~ - ~ _
o o " 6 ~ - ~ - " - o o o oo---~ ~ , ~ , ~
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Fig.18. Distribution pattern of Recurrent Group C'. Filled circles denote samples in which at least 5 of the 6 species of Group C' are present.
be indicative of several degrees of east--west asymmetry in the mean position of the S.T.C.
Recurrent Group C' is well-defined in subtropical latitudes extending between ap- proximately 15°S and 32°S (Fig.18). Its geographic extent corresponds very closely with the interior of the quasi-stationary southern subtropical gyre (Wyrtki, 1971; 1973). We note that three species which
were previously grouped with Group C in the western transect of samples (P. praetex- rum eucolpum; T. trachelium dianae; Tri- gonastrurn sp.) have now been re-assigned to a new group, Group F', whose latitudinal limits are substantially farther to the south than those of Group C' (see discussion be- low). The revised species group now con- stituting Group C' (Table III) appears to be
270
a clear marker for a major oceanographic feature, the core of the anticyclonic sub- tropical gyre. We were particularly pleased by the emergence of this group since many of the samples which we examined from these latitudes were notably poor in radiolar- ian abundance and preservation (Fig.2). Yet the pattern which emerges for Group C' (Fig.18) demonstrates that, in spite of low total diversity and/or poor specimen preser- vation in these latitudes, at least 5 of the 6 species in Group C' are generally present.
Recurrent Group D' now excludes Cypassis irregularis which we previously assigned to Group D in the western transect of samples (Johnson and Nigrini, 1980), but which is virtually absent from the eastern transect (Fig.5f). Using the combined data set of 120 samples, C. irregularis showed no affini- ties at all, and therefore is now ungrouped (Table III). The revised Group D' now ap- pears to be diagnostic not only of the Arabian upwelling zone, but of the entire Arabian Sea and Bay of Bengal as well (Fig.19). This distribution pattern is somewhat perplexing, since the Arabian Sea and Bay of Bengal are oceanographically quite distinct in many respects. Both regions, however, do exhibit high salinities and a strong oxygen minimum in the sub-surface waters above the main thermocline (Wyrtki, 1971). High near- surface salinities and low oxygen values are also present in the eastern tropical Pacific region where the species in Group D' have been reported previously (Nigrini, 1968). At present there is relatively little informa- tion regarding the depth habitats of the taxa in Recurrent Group D'; thus the oceano- graphic factors which control the geographic extent of Group D' are at present unclear.
Recurrent Group E'. In our previous analysis of Radiolaria in the western transect, we identified a recurrent group (designated Group E) with an intriguing bimodal distribu- tion pattern {Johnson and Nigrini, 1980, fig.16). Using the combined data set of 120 samples, we have identified a recurrent group {designated Group E') which has four of the five original species from Group E {Table III), and which retains the same puzzling bimodal
geographic distribution pattern (Fig.20). The southern half of the distribution pattern for Group E' is well<lefined lying approxi- mately between the Subtropical Convergence and Antarctic Convergence, and is similar {though not identical) to the distribution pattern of Group B' (Fig.17). The northern region of occurrence of Group E', however, is somewhat more irregular. Rather than being strictly zonal in its extent, Group E' is apparently most characteristic of the oceanic region nearest the Indonesian Ar- chipelago, and is irregularly present west of about 90°E. Two of the species in Group E' (Actinomma medianum; Anomalacantha dentata) are clearly missing from the region extending southeastward from Ceylon {Fig. 5c, d), thereby producing a somewhat ar- tificial separation between the group's oc- currence in the Bay of Bengal from that in the Arabian Sea (Fig.20).
The reasons for the peculiar bimodal distribution pattern of Group E' remain puzzling, since it is clear that the two prin- cipal regions of its occurrence are not con- nected along the margins of the Indian Ocean near Madagascar and western Australia (Fig.20). In our previous report, we suggested that the bimodal distribution pattern for Group E might represent the recent separa- tion of a formerly contiguous distribution, and that at some point " the development of the southern Subtropical Gyre may have geographically divided the assemblage" {John- son and Nigrini, 1980, p.146). We would like to expand upon this discussion in view of the additional sample control documenting the extent of Group E' (Fig.20) as well as some recent oceanographic observations.
The anticlockwise circulation of the southern Subtropical Gyre is presently fed to a large extent by a major westward trans- port {approximately 10-106 m3/sec) of Pacific water via the Indonesian--Australian seaway {Godfrey and Golding, 1981). The Coriolis effect upon this westward flow tends to direct the flow southward toward the Sub- tropical Gyre, rather than northward toward the Bay of Bengal. Thus we might anticipate that the oceanographic properties and the
271
' L J " , \ : ' , "" I n d i a
) \7 ../~/
Arabia . , I O O ", , J ~ ' -
• ; ' • •
Africa ' ~
0
0 0 0 /',< 0
-/@ S o o O O
,' ~ / 0 ,0 o 0
;' O 0 0
0
0
0
O 0
0 0
O
R E C U R R E N T GROUP D' o
( 3 spp.) o
5C,°E
0
0
0
0
0 0
60 °
0
0
0
70 o
I O < CAMBODIA "
. . ,oo
• _ 0
• O o ~:~
_ o ° o o " <
k o o o %0 .... <~<~:~
o o o
o o
0 ~ 0 .0 I0~ 0 w 0 0
0 0 o o o
o o o
o o
o o
o
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o o o o o
o
o
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A I Q i
o
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I t
Fig.19. Distribution pattern of Recurrent Group D' (3 species).
30 °.- 0 O \
t J 40 °-
IIO°E I i
intensity of the Subtropical Gyre are con- trolled, at least in part, by the influx of the Pacific throughflow, which at present is volumetrically quite substantial (Godfrey and Golding, 1981). During the glacial stages of the Late Pleistocene, however, sea level was on the order of 130 m lower than at present during quasi-periodic intervals (e.g. Milliman and Emery, 1968; Shackleton and
Opdyke, 1976; Imbrie and Imbrie, 1980). These recurring lowerings of sea-level were sufficient to establish a land bridge between Australia and much of Indonesia, thereby reducing considerably (and perhaps closing off completely) the westward near-surface f low from the Pacific. Recent studies of the oceanography of the subtropical Indian Ocean during the glacial Pleistocene (e.g. Prell et
Fig.20. Distribution pattern of Recurrent Group E'. Filled circles denote samples in which at least 4 of the 5 species of Group E' are present; half-filled circles represent occurrences of three of the five species in the group.
al., 1979, 1980) indicate that there was a well<tefined anticyclonic gyre comparable to that of today. Nevertheless, the termina- tion of the westward flow between 10°S and 20°S during the glacial Pleistocene most probably altered the oceanographic proper- ties, and perhaps the intensity, of the Sub- tropical Gyre. This effect may have been
sufficiently important to allow the radiolarian species of Recurrent Group E' to become established as essentially cosmopolitan species, and extend through the interior region of the modern subtropical gyre.
If this explanation for the modern distribu- tion pattern of Group E' is correct, then there must be one or more oceanographic
273
properties of the modern Pacific inflow which not only are advected into the sub- tropical gyre, but which are particularly un- favorable to the presence of the species in Group E' (and perhaps especially favorable to the subtropical species in Group C'; see Fig.18). An obvious test of this hypothesis would be to examine glacial-age (~18 000 yr./BP) sediments from a longitudinal tran- sect through the modern subtropical gyre, and determine if the species of Group E' re-appear in the core of the gyre, perhaps (though not necessarily) at the expense of one or more of the species in Group C'. Diatomaceous sediments are not present in glacial-age sediments from the northern and central Indian Ocean (L. Burckle, pers. com- mun., 1981). Thus we might anticipate a similar dramatic shift in the radiolarian assemblages, and that the explanation may be related to the relative contribution of Pacific " throughf low" to the subtropical Indian Ocean.
Recurrent Group F' consists of three taxa which were assigned to the subtropical Group C in the western transect (Johnson and Nigrini, 1980), and which now, with the additional sample control, emerge as a distinct group. Group F' is clearly defined in the western transect between ~25°S and 45°S, yet its distribution in the eastern tran- sect is notably more restricted (Fig.21).
We have reexamined samples in the eastern transect to ascertain whether the apparent east--west asymmetry may be an artifact of poor specimen preservation, but we are con- vinced that the asymmetry of at least the southern limit of Group F' is a real feature in modern sediments. The flow of both sur- face and subsurface waters in these latitudes is zonal and relatively strong. If this flow encounters the bot tom such that the flow depth changes significantly, the flow will necessarily veer to the left or to the right so as to conserve potential vorticity. We might anticipate that this effect is present in the circumpolar flow, and may account for some of the zonal asymmetry in the faunal distribution patterns for Recurrent Group F:.
Designation of radiolarian assemblages
In examining the geographic distribution patterns of the six recurrent groups (Figs. 16--21), we are able to identify nine radiolar- ian assemblages which characterize the en- tire Indian Ocean north of the Antarctic Convergence, near 50°S. Each of these assem- blages is characterized by the presence or absence of one or more of the recurrent groups. In Table IV we have listed these nine radiolarian assemblages; for each assem- blage we have indicated those recurrent
TABLE IV
Radiolarian assemblages in the Indian Ocean and the recurrent groups used to define each assemblage. A cross,
+, indicates that the recurrent group is an essential component of the assemblage. An open circle indicates that this recurrent group is necessarily absent from the assemblage. A cross in parentheses, (+), indicates that the
recurrent group may be present, but its presence is not essential to define the assemblage.
Assemblages Recurrent Groups
A' B' C' D' E' F '
I. Arabian Upwelling o o o + o o
If. Northern Indian Ocean + o o + (+) o III. Tropical + o o o (+) o
• India I ' / ~ \ ' =u='=, ' ? . . y 7 ,, ± < \ I I / k ~ THAILAND "% I _
' ~ \ ; ~. CAMBODIA
,~ . = o o . 7L "D ~ io °-
~ " } °oo "L ~ - " - O
o o - o o ~ ; ~ ,:~. t - 0,4
° : ° o o 0 0 .0 I 0 °
0 0 0
0 0 0 o o o o o
o o o o o 0 20'~
o o
o o i ~ i,
~ 30 ~" ~ - - 0 OC
50OE 600 700 i , I h I ,
• 0
~ 0
0 4 0 °-
0 '
0 IIO°E
0
o 9 0 ° i 00 o i I Q A I i
0 Fig.21. Distribution pattern of Recurrent Group F' (3 species). Filled circles represent occurrences of all three species in the group; half-filled circles denote presence of two of the three species.
g roups which def ine the assemblage. The des igna t ion o f "non , e s s en t i a l " r ecu r r en t g roups is r equ i red to t ake in to a c c o u n t the a p p a r e n t l y p a t c h y d i s t r ibu t ion p a t t e r n o f s o m e o f the g roups in la t i tudes where radiolar- ian a b u n d a n c e and p rese rva t ion are n o t a b l y poo r . Fig.22 shows the geographic dis t r ibu- t ion o f the nine des igna ted radio lar ian assem- blages.
Arabian Upwelling Assemblage This assemblage is wel l<lef ined in th ree
samples neares t the Arab ian margin . The assemblage was ident i f ied p rev ious ly ( John- son and Nigrini, 1980 , p .143 ) , b u t here we n o t e t ha t the p resence o f G r o u p D ' a lone is n o t suf f ic ient to def ine the assemblage; in add i t ion , G r o u p A' (as well as the r emain ing r ecu r r en t g roups) m u s t be absent . We can
275
Arabia
Africa
- ,-;} 0 0 ~ o ~ . ' - O 0 ~ T ~
. : ~ , o . . o f o ~ ;i ~ ~ " o / ,1o - " " -
0 0 0
R A D I O L A R I A N A S S E M B L A G E S ~..J \ "\ I I /U.~'!. t I v I . . l .2 ~-
}) T :• ;" India I / \ ~ u ~ ~ / . ~ ~ '0 Z .... i $ / / /'1 ) ~ ~ ~ 2 0 0 N -
o~ o . . . . , ~ ~ o o ~ "t& % ~ ,0o- ~ A ' . / ~ o ", %k
Fig.22. Distribution of the nine radiolarian assemblages. Recurrent groups defining each assemblage are listed in Table IV, and discussed in the text.
no longer use the presence of the three species of Recurrent Group D' as the only criterion to identify this upwelling zone; an additional requirement is that there be sufficiently low species diversity as to exclude Recurrent Group A'. The identification of the assem- blage provides an additional criterion to those
of Prell and Curry (1981) for identifying past climatic fluctuations in this upwelling region.
Northern Indian Ocean Assemblage The southern limit of Group D', near the
Equator, identifies the southern extent of this assemblage (Fig.22). The assemblage
276
necessarily includes Groups A' and D'; Group E' is present in some instances (e.g., the eastern Bay of Bengal), but is not an essential component. In the modern Indian Ocean there is strong current shear near the equator during the winter monsoon, though this contrast disappears during the summer mon- soon when eastward flow predominates both north and south of the equator (Wyrtki, 1973; Prell et al., 1980, fig.l). Therefore the preservation of this trans-equatorial con- trast in radiolarian assemblages may serve as a useful index of northeast monsoonal circulation patterns at least seasonally during the past.
Tropical Assemblage This assemblage extends over a broad
region from near the equator to ~16°S (Fig.22). It is defined by the presence of Group A' and the absence of Groups C' and D'. One might wish to subdivide this tropical assemblage even further on the basis of Recurrent Group E', which is general- ly present north of ~10°S (Fig.20). Such a subdivision of the assemblage at 10°S is particularly appealing from an oceanographic standpoint, due to the presence of a major hydrochemical "front" in the subsurface waters at this latitude (Wyrtki, 1973). We have elected not to make this subdivision because of the apparently non,contiguous distribution of the northern "mode" of Group E' (Fig.20). Nevertheless, it is clear that some or all of the species in Group E' may eventually prove to be reliable indices of the position of the hydrochemical front during the Late Cenozoic.
Sou th Equatorial Current Assemblage To the south of ~16°S is a distinctive
assemblage which closely follows the mean position of the South Equatorial Current (Wyrtki, 1971, 1973). This assemblage ex- tends to ~ 20°S over the eastern three~luarters of the Indian Ocean (Fig.22), and to near 25°S near Madagascar. The assemblage con- sists exclusively of Recurrent Groups A' and C' (Table IV). The southern limit of the as- semblage is not well defined in the Madagas-
car Basin due to poor specimen preservation in this region (Johnson and Nigrini, 1980}, but it is clear that the southward-flowing limb of the South Equatorial Current is responsible for displacing this assemblage several degrees toward the south.
Central Assemblage The eastern three-quarters of the Indian
Ocean between ~20°S and ~35°S is marked by relatively poor preservation of radiolarians (Fig.l; Table II). Nevertheless, Recurrent Group C' occurs in a relatively coherent pattern through this region (Fig.18). Accord- ingly we have designated the Central Assem- blage (Fig.22) on the basis of the absence of all recurrent groups except Group C'. This assemblage corresponds closely in its distribu- tion pattern with the core of the subtropical gyre, and its east--west asymmetry is con- sistent with the relatively weak boundary current off western Australia compared to the more intense southward limb of the gyre near Madagascar. In our analysis of the western transect we identified a Central Assemblage based on the absence of all re- current groups (Johnson and Nigrini, 1980, Fig.17, a). We now modify the definition of the Central Assemblage such that all groups except Group C' are excluded (Table IV).
Temperate Assemblage This assemblage denotes the region of
strong eastward circumpolar flow north of the Subtropical Convergence (STC}. Its north- ern limit (Fig.22) is identified by the north- ern extent of Group F', which is well<lefined in the western transect, but less distinct due to poor specimen preservation in the eastern transect (Fig.21). Its southern limit is well defined by the northern boundary of Group E' near the S.T.C. (Fig.20). We identified a Temperate Assemblage in our western tran- sect, as well as Transitional and Subpolar Assemblages (Johnson and Nigrini, 1980, Fig.17, a). With the revised recurrent group- ings of Groups C', E', and F' (Table IV), however, we now are re<tefining the Temper- ate Assemblage based on the presence of Group F' and the absence of Group E'.
277
Transitional Assemblage We have designated this assemblage on the
basis of only four samples in the eastern transect (Fig.22); thus its significance is presently quite uncertain. Its presence reflects a peculiar east--west asymmetry in the re- current group distributions south of the S.T.C. Groups B' and E' have identical north- ern limits west of ~70°E, but there is ap- parently a difference of ~ 4 ° between their northern limits in the eastern transect (Figs. 17 and 20). We have designated the fauna in this 4 ° latitudinal segment as a Transitional Assemblage (Fig.22). The strong zonal flow at these latitudes would argue against the east--west asymmetry represented by this configuration; nevertheless, we present these results with the hope that additional sample control may allow us to ascertain whether this apparent faunal asymmetry is a real feature.
Subpolar Assemblage This assemblage is defined by the north-
ern limit of Group B' and the southern limit of Group E', and extends to ~48°S (Fig.22). One might just as conveniently define the assemblage on the basis of the northern and southern limits of Group B' alone, in which case its extent on the east would be widened somewhat. We have elected, how- ever, to retain both Recurrent Groups B' and E' in the defining criteria for this assem- blage (Table IV), since both groups are con- sistently present between the S.T.C. and A.A.C. (Figs.17 and 20).
Polar Front Assemblage This assemblage is defined by the absence
of Recurrent Groups E' and F' , with Group B' as a non,essential component (Table IV). Our present sample control is very limited south of 45°S, and radiolarian abundance decreases markedly as one approaches the Antarctic Convergence (Polar Front). Thus, the designation of this assemblage is not particularly useful, except to point out the abrupt disappearance of Groups E' and F' near the Polar Front (Figs.20 and 21), and the possible extent of Group B' slightly
closer to the Front than the other two groups (Fig.17).
Discussion
Importance o f eastern transect in providing sample control
Our initial supposition in beginning this Indian Ocean s tudy was that radiolarian pre- servation would be substantially more favor- able in the western half of the ocean, and that the essential biogeographic information would emerge from a single transect in the western region alone. The present s tudy con- firmed that radiolarian preservation is indeed poor in middle latitudes of the eastern half of the ocean (Fig.2), and that considerable sample processing was required to select a suitable group of samples to consti tute an eastern transect (Tables I and II). Therefore, we were particularly pleased when the recur- rent group analysis of the combined data set yielded a much more satisfactory grouping (this report, Table III) than that which emerged from the western transect alone (Johnson and Nigrini, 1980, table 2). In particular, the number of recurrent groups increased from 5 to 6, and the number of ungrouped taxa decreased from 6 to 3. This modification enabled substantially more of the species occurrences to be taken into ac- count in the final recurrent grouping. In our previous analysis of the western transect, ungrouped taxa accounted for 6.2% of the total occurrences (1918), whereas with the combined data set, the ungrouped taxa re- presented only 4.1% of the total occurrences (5167).
Revised recurrent groupings
At least four significant refinements in interpretation have emerged from the revised recurrent groupings (Table III) based on the combined data set of 120 samples:
(1) Three species which were previously ungrouped are now added to Recurrent Group A', characteristic of tropical latitudes (Fig.16).
278
(2) We have re<iefined Group C' and separated three species from it to form a new group (designated Group F'); these two groups now appear with clearly distinct latitudinal boundaries (Figs.18, 21 ).
(3) We now re-interpret the three taxa comprising Recurrent Group D' (Table III) as representative not only of the Arabian upwelling zone, but of the entire Indian Ocean north of the Equator in both the Bay of Bengal and in the Arabian Sea. The Arabian upwelling region is still distinguish- able using radiolarians, but the presence of the species of Group D' is not a sufficient condit ion. Also required is relatively low species diversity such that species from Group A' are absent (i.e., below the arbitrary cut-off criterion of 80%).
(4) The bimodally distributed Recurrent Group E' retains the same distinct separation between a southern " m o d e " (40°S--50°S) and a northern " m o d e " north of ~10°S (Fig.20). With our additional sample control, the northern " m o d e " now appears to have a patchy distribution, with a zone of separation south of India between its presence in the Arabian Sea and that in the eastern Bay of Bengal. We interpret the distribution pattern of Group E' (Fig.20) to be an artifact of the physical separation of a formerly contiguous assemblage. We have suggested that the post- glacial rise in sea-level has allowed the re- initiation of westward flow of Pacific surface waters into the southern subtropical gyre of the Indian Ocean, and that the prop- erties of this Pacific inflow are for some reason not favorable to the species in Re- current Group E'. If this interpretation is correct, it represents a substantial departure from the traditional interpretation of faunal assemblages migrating toward the pole or the equator in response to glacial/interglacial climatic changes. We believe this example of Recurrent Group E' may prove to be among the first examples of the separation of a for- merly contiguous assemblage distribution into two or more unconnected regions in response to a climatic change. We have further suggest- ed that the analysis of Radiolaria in precisely dated cores in a longitudinal transect through
the southern subtropical gyre should provide definitive evidence in support of, or in con- tradiction of, this hypothesis.
Oceanographic significance o f radiolarian as- semblages
We have identified nine distinct assem- blages which we believe fairly represent the radiolarian faunal patterns of the entire Indian Ocean north of the Antarctic Conver- gence (Fig.22). Studies of planktonic Forami- nifera have yielded a comparable number of assemblages (e.g., B~ and Hutson, 1977; Prell et al., 1980). Most, though not all, of the radiolarian assemblage boundaries closely cor- respond with one or more oceanographic gradients, and may therefore have some in- terpretative significance in Pleistocene pale- oceanographic reconstructions of the region. We have discussed these possible interpreta- tions in the preceding sections. The follow- ing aspects of the radiolarian assemblage distribution (Fig.22) remain enigmatic, and will require more extensive sample control, more precise definitions of the morphotypic limits of certain species, and a better under- standing of their t axonomy in order to un- derstand their oceanographic significance:
(1) The boundary near the Equator be- tween the Northern Indian and the Tropical Assemblages is difficult to explain. Both the Arabian Sea and the Bay of Bengal are charac- terized by high nutrients, high salinities, and low oxygen in the subsurface above the main thermocline (Wyrtki, 1971, 1973}; yet there does not appear to be a sharp gradient in any of these properties near the equator. Moreover, there is relatively little informa- tion regarding the depth stratification in the water column of the three species which comprise the Northern Indian Ocean re- current group (Group D'). Consequently, the oceanographic factors which control the geographic extent of the species in Recurrent Group D' remain unclear.
(2) The major hydrochemical front at 10°S does not correspond with an assemblage boundary (Fig.22), although it is clear that
279
one could propose subdividing the Tropical Assemblage at 10°S based on the scattered occurrences of Group E' north of this latitude (Fig.20). Group E' remains the single most intriguing of the recurrent groups whose distribution requires further documentat ion before its significance can be interpreted.
(3) The core of the subtropical gyre, which we previously indicated to be lacking any diagnostic recurrent groups, is now denoted by a recurrent group {designated Group C') which is present in virtually all of the sam- ples from the region. The Central Assemblage therefore appears as a major marker for the core of the gyre when it is present, to the exclusion of all other g r o u p s {Table IV).
(4) The east--west asymmetry of the re- current groups and assemblages south of the subtropical gyre appears to be a real feature from the limited sample control (Fig.22), but clearly requires more transects of sam- ples before it can be interpreted. We have suggested that the "topographic be ta" effect upon the circumpolar current necessarily causes the nearly-zonal flow to be diverted to the left or to the right if the flow thickens or thins over irregular topography, and that this may in part explain the asymmetrical faunal assemblage boundaries. Testing this possibility requires additional sample con- trol in the middle latitude regions (~35°S - 50°S).
Acknowledgments
Core samples for this s tudy were provided through the assistance of W. Riedel and T. Walsh {Scripps Institution of Oceanography); F. McCoy, D. Cooke and K. Thompson (Lamont-Doherty Geological Observatory); and D. Cassidy (Florida State University). Curatorial services at these sample repositories are supported under grants from the National Science Foundat ion (Submarine Geology and Geophysics Program). This project is sup- ported by N.S.F. Grant OCE 76-20154. We thank L. Peirson and A. Tricca for as- sistance in preparation of the figures and typing the manuscript. F. Theyer, K.
Takahashi, and S. Kling critically reviewed the manuscript.
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