RADIOLARIAN BIOGEOGRAPHY IN SURFACE SEDIMENTS OF THE WESTERN INDIAN O C E A N 1
DAVID A. JOHNSON and CATHERINE NIGRINI
Woods Hole Oceanographic Institution, Woods Hole, Mass 02543 (U.S.A.) 510 Papyrus Drive, La Habra Heights, Calif. 90631 (U.S.A.)
(Revised version received and accepted September 18, 1979)
Abstract
Johnson, D.A. and Nigrini, C., 1980. Radiolarian biogeography in surface sediments of the western Indian Ocean. Mar. Micropaleontol., 5: 111--152.
Recurrent group analysis of Radiolaria in 46 core top samples from a north--south transect in the western Indian Ocean has allowed the discrimination of eight distinctive radiolarian assemblages. Most of the designated assemblages have distribution patterns which closely reflect the major oceanographic fronts and first-order patterns of surface water circulation including the South Equatorial Divergence, Subtropical Gyre, Subtropical Convergence, and Antarctic Convergence. An exotic assemblage consisting of four taxa was found off the Arabian coast. These same taxa were previously reported only in the eastern equatorial Pacific, and may be representative of upwelling with relatively restrictive salinities. Radiolarian abundance drops abruptly near 48°S at the Antarctic Convergence, south of which the sediment is a radiolarian-poor diatomaceous ooze.
I n t r o d u c t i o n
Biogenic silica is k n o w n to be an i m p o r t a n t c o n s t i t u e n t o f the sed imen t s o f the Ind ian Ocean , ranging in la t i tud ina l e x t e n t f r o m the An ta rc t i c c o n t i n e n t a l marg in to the s o u t h e r n marg in o f Asia (Lisi tzin, 1967 , 1972 ; Lisi tzin e t al., 1967; Jous6, 1977; Caulet , 1977, 1978) . To date , however , there have been re la t ively few a t t e m p t s to i n t e r p r e t spat ia l var ia t ions in si l iceous microfoss i l assemblages o f the Ind ian Ocean in t e rms o f the f i r s t -order oceanograph- ic p rope r t i e s o f sur face and subsur face waters . A m o n g the prev ious r epo r t s on rad io lar ian d i s t r ibu t ion p a t t e r n s in the Ind ian Ocean surface s e d i m e n t samples , t hose o f Nigrini
1 Contrib, No. 4441 of the Woods Hole Oceanogr. Inst.
{1967), P e t r u s h e v s k a y a (1967 , 1971, 1972a , b, 1973) , L o z a n o and Hays (1976) and D o w (1978) are especial ly n o t e w o r t h y .
Nigrini (1967) ident i f ied and c o u n t e d 45 species o f Radio la r ia in 32 sur face s ed imen t samples which spanned the ent i re Ind ian Ocean and e x t e n d e d to 45°S. She was able to d iscr imina te a low- la t i tude assemblage, com- posed o f twelve t a x a and e x t e n d i n g f r o m 10°N to 20°S; and a midd le - l a t i tude assemblage, c o m p o s e d o f seven t axa and e x t e n d i n g f r o m 35°S to 45°S. Nigrini suggested t h a t these t w o regions are sepa ra ted by the core of the sub- t rop ica l an t i cyc lon ic gyre, which is essent ial ly bar ren of radiolar ians.
P e t r u s h e v s k a y a (1967 , 1971) e x a m i n e d rad io lar ian assemblages in a p p r o x i m a t e l y 70 core t o p samples o b t a i n e d on the Sovie t Ant-
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arctic Expedit ion (1955--1958). Sample coverage extended from the Bay of Bengal to the Antarctic continental shelf, but the majority of the samples were from south of the Subtropical Convergence near 40°S (Petrushevskaya, 1967, fig. 4). From observa- tions of the distribution patterns of approx- imately 75 taxa, she was able to discriminate five principal zoogeographical groups of Radiolaria:
(1) A tropical group, confined to latitudes north of the Subtropical Convergence (near 40°S).
(2) A subtropical group, found not only in the tropical region but also extending south- ward into the "nota l" zone between the Sub- tropical Convergence and the Antarctic Con- vergence or Antarctic Polar Front (near 50°S).
(3) A cosmopoli tan group, widely distrib- uted in tropical, temperate, and high-latitude regions of all oceans.
(4) A group restricted to Antarctic regions, south of the Antarctic Convergence, Petrushev- skaya (1971) further subdivided this group into "high-antarctic species", which are rela- tively more abundant south of 60°S; and "low-antarctic species", which are most abun- dant near 60°S and become relatively rare near the Antarctic continent.
(5) A bipolar group, which occurs in the North Atlantic boreal zone as well as in middle to high latitudes of the southern Indian Ocean.
Petrushevskaya ( 1971) suggested that only two of these groups (the "tropical group" and the "high-antarctic group") can be used as indicators of specific water masses. She also pointed out that radiolarian preservation is usually good to excellent in all regions except the "nota l" zone, where shell preservation is usually very poor.
Lozano and Hays (1976) examined core top samples from the South Atlantic and western Indian Ocean sectors of the Antarctic Ocean (east of 80°E: south of 35°S). Using Q-mode factor analysis (Imbrie and Kipp, 1971) on 72 samples which they considered to represent non-reworked Recent sediments,
they were able to resolve three factors or as- semblages (Antarctic, subantarctic and sub- tropical) with distributions closely corre- sponding to the principal surface water masses in the area. Dow {1978) carried out similar procedures on 36 core top samples from the southeastern Indian Ocean between 40°S and 65°S. After counting 52 radiolarian species or species groups in each sample, 35 of the species were grouped by Q-mode factor analysis into four distinct factors or assem- blages. One of these assemblages (the "Ant- arctic Factor") was found to characterize the region south of the Antarctic Polar Front; a second assemblage ( t h e " Su bantarctic Factor") was dominant in northern samples close to the Subtropical Convergence; and a third as- semblage (the "Transitional Factor") reached its highest abundance in the intermediate region near the Antarctic Polar Front. A fourth assemblage was found to be indicative of lateral advection by bo t tom water activity. Dow (1978) was able to formulate regression equations which related the distribution of the first three factors to summer and winter values of sea-surface temperature.
During the decade since the initial studies of Nigrini (1967) and Petrushevskaya {1967), the number of available cores from the Indian Ocean north of the Antarctic Convergence has increased enormously. There are now well over 300 piston and gravity cores from the Indian Ocean (north of 50°S) in the collec- tions of Lamont-Doherty, Scripps, and Woods Hole. Moreover, the extensive studies of Holocene radiolarian biogeography by CLIMAP workers (e.g., Sachs, 1973a, b; Moore, 1973; Lozano, 1974; Robertson, 1975; Molina-Cruz, 1977; Morley, 1977; Moore, 1978) and by others (e.g. Petrushev- skaya, 1967; Nigrini, 1967, 1968, 1970; Goll and Bj~brklund, 1971, 1974) have allowed Nigrini and Moore (1979) to synthesize the t axonomy and synonymy of those extant Radiolaria which have commonly been used for biogeographic studies. This updated tax- onomic "encyclopedia" has served as a basis for identifying the radiolarian taxa whose
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geographic distribution patterns were analyzed in the present study.
In addition, several compilations and syn- theses of extensive oceanographic data col- lected during the International Indian Ocean Expedition have been completed during recent years (e.g., Wyrtki, 1971; Zeitzschel, 1973). Thus, the geographic distribution pat- terns and seasonal variability of the major oceanographic parameters have become rela- tively well identified, and there is, therefore, an increased potential for interpreting micro- fossil distribution patterns in terms of proper- ties of surface and subsurface waters.
The objective of our investigation was to select a longitudinal transect of core top samples in the western Indian Ocean, ex- tending from the Arabian Sea to the Ant- arctic Convergence near 50°S, to identify radiolarian assemblage distribution patterns, and to interpret their significance in terms of known oceanographic parameters. We selected a western transect in order to avoid the rela- tively unproductive core of the anticyclonic subtropical gyre, and selected a southern limit near 50°S, south of which extensive studies of radiolarian biogeography have already been carried out (e.g., Hays, 1965; Petrushevskaya, 1967; Lozano and Hays, 1976). In this report we consider only the distribution patterns of Radiolaria in surface sediment samples, and their possible oceanographic significance. Sub- sequent reports will treat (1) biogeographic distribution pattern of Radiolaria in the east- ern Indian Ocean; and (2) paleo-oceanographic interpretation of the late Pleistocene record, using precisely dated core material (Johnson and Nigrini, in prep.).
Oceanographic setting of western Indian Ocean
With the recent compilation of extensive oceanographic data collected during the Inter- national Indian Ocean Expedition (Wyrtki, 1971), the geographic distribution and season- al variability of the major oceanographic parameters and water masses have become reasonably well identified. The surface cur-
30°E 40 ° 50 ° 60 ° 70 ' 80°E
Fig. 1. Bathymetric and oceanographic setting of the western Indian Ocean, modified after Defant (1961) and Wyrtki (1973). Bathymetric contours are in kilo- meters. Heavy arrows designate principal surface cur- rents during the northeast monsoon; open arrows indicate the principal pathway of the deep western boundary current (Warren, 1974, 1978; Johnson and Damuth, 1979). Major frontal zones shown include the South Equatorial Divergence, Subtropical Conver- gence, and Antarctic Convergence.
rents of the Indian Ocean can conveniently be discussed in terms of three major circulation systems (see Fig. 1) and the subsurface distri- bution patterns of nutrients (Wyrtki, 1973):
A. T h e m o n s o o n gyre
This circulation system is marked by high nutr ient values in surface and subsurface waters, with a sharp hydrochemical front near
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10°S marking the southern boundary of the gyre. During the NE monsoon {November to April), surface water flow north of the equator is from east to west. Off the coast of Somalia most of the water turns south, crosses the equator, and forms the equatorial countercur- rent which extends south to near 10°S. The circulation during the NE monsoon is only moderately developed, with no striking up- welling zones. During the SW monsoon (May to October) water flows eastward everywhere north of the equator, and the countercurrent shifts north to join the eastward-flowing mon- soon current. Strong upwelling develops along the coast of Somalia between 5°N and l l ° N (Swallow and Bruce, 1966; Warren et al., 1966), and is terminated at the north by a flow of warm surface water out of the Gulf of Aden, forming a strong temperature front (Wyrtki, 1973). Intense upwelling also devel- ops off the coast of Arabia during the SW monsoon. This upwelling is different from the Somali upwelling in that no strong current develops parallel to the coast. However, in volume it may be even stronger than the Somali upwelling region, showing maximum values of (PO4) -3 in excess of 1,5 ugm-atoms per liter, and a larger area affected by higher concentrations of (PO4) -3 and (NO.~)-' (Wyrtki, 1973, p. 22).
High-salinity surface water is formed by the strong excess of evaporation over precipitation in the central and northern Arabian Sea. Two other sources of high-salinity water, the out- flow from the Persian Gulf and from the Red Sea, contribute to the formation of a thick layer extending between 150 and 900 m which is of almost uniform salinity. Tongues of water from this layer, which Wyrtki (1973) refers to as the North Indian high-salinity intermediate water, can be recognized hun- dreds of kilometers "downst ream" from the Arabian Sea source area in several branches of the monsoon current system.
Of particular interest for studies of faunal biogeography are the anomalously high values of salinity and nutrients in the Arabian Sea, particularly near the coast of Arabia during
the SW monsoon. B~ and Hutson {1977) recently reported that the northern Indian Ocean is apparently a refuge for relict species of foraminifera (Globoquadrina hexagona, G. conglomerata, and Globigerinella adamsi) which disappeared from the Atlantic during the Pleistocene. These authors suggest (p. 380) that this region "deserves special at tention of zoogeographers and paleoecologists, because these relict species may reflect unique ancient oceanic environments". Our work on radio- larian biogeography (this report) documents in the existence of an equally unique radio- larian assemblage in the Arabian Sea, and indi- cates that the special oceanographic conditions in the area do indeed deserve special attenti on.
Thp monsoon gyre is separated from the subtropical gyre by a strong hydrochemical front near 10°S. The front is marked by a salinity minimum in the surface waters ex- tending from Sumatra to Africa, and caused by the advection of low-salinity waters by the South Equatorial Current from the Australian/ Indonesian region. The front is also marked by sharp horizontal gradients in the distribu- tion of chemical properties, particularly oxygen, phosphate, and silica. The front is inclined and slopes from about 100 m depth at 10--12°S to 800 m depth at 16 18°S (Wyrtki, 1973, p. 24).
B. The subtropical gyre
This anticyclonic circulation system consists of the South Equatorial Current, the Agulhas Current, and the portions of the west wind drift lying north of the Subtropical Conver- gence near 40°S (see Fig. 1). There is no strong eastern boundary current off Australia; in- stead, slow equatorward flow extends across much of the width of the ocean. The South Equatorial Current, drawing water both from the Timor Sea and from the equatorial coun- tercurrent, flows westward toward the north- ern tip of Madagascar where it divides, with approximately one-third of the water in the current turning south. The Agulhas Current system off southeast Africa is fed by t h e
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South Equatorial Current, both from the Madagascar Channel and from flow around the southern tip of Madagascar. When the Agulhas Current reaches the longitude of Cape Agulhas, the current turns south and then east to form an elongated eddy which is permanently situated approximately 300 km offshore from southeast Africa. The Sub- tropical Convergence extends eastward from the Agulhas eddy in a zonal strip between 40°S and 41°S, and separates warm subtropi- cal water of high salinity from cooler tem- perate water of lower salinity. The Subtropi- cal Convergence is narrower and more sharply defined in the western Indian Ocean (west of approximately 65°E) than in the eastern Indian Ocean (Prell et al., 1979). It coincides approximately with the northern limit of strong westerly winds, which approximates the northern boundary of the Circumpolar Current.
C. Circumpolar flow
Between about 40 and 50°S the tempera- ture and salinity decreases from greater than 15°S and 35% 0 to less than 5°C and 34% 0, indicating the surfacing of the main thermo- cline. This inclined front is associated with intense geostrophic flow in the eastward- flowing Antarctic Circumpolar Current, and is kept in position by strong west winds over the whole area. Cold, low-salinity surface water from Antarctica drifts north under the pre- vailing westerly winds, where it meets the warmer waters of higher salinity at the Ant- arctic Polar Front or Antarctic Convergence (see Fig. 1). Mixing along the front forms Ant- arctic Intermediate Water, which spreads northward as a subsurface salinity minimum. The front is near 48°S in the western Indian Ocean, and appears to undergo little seasonal fluctuation. The sou them boundary of Circumpolar Flow is the Antarctic Divergence near 65°S, south of which there is a slow westward flow along the Antarctic continen- tal margin.
The abyssal circulation of the western
Indian Ocean is an important factor in con- trolling microfossil preservation, and is a potentially complicating factor producing post-depositional lateral advection of skeletal material "downs t ream" from its initial accumulation site. Abyssal flow is well devel- oped in the form of a deep western boundary current (DWBC) in the Crozet, Madagascar, and Mascarene Basins (Fig. 1). This current, consisting largely of Antarctic Bot tom Water formed in the Weddell Sea, is denoted by east- ward-dipping contours of potential tempera- ture and dissolved oxygen along the western margins of the basins (Warren, 1974; Jacobs and Georgi, 1977), and by T-S characteristics which are traceable from basin to basin (Warren, 1974, 1978). From the known path of the DWBC (Fig. 1), selection of samples in the western Indian Ocean transect was carried out so as to minimize the number of samples obtained from regions of DWBC activity.
Material studied
Fifty-three samples from the tops of gravity, piston, and pilot (trigger) cores were prepared from a transect of stations in the western Indian Ocean, extending from the Arabian Sea to south of the Antarctic Polar Front (Table I, Fig. 2). The transect was selected so as to avoid regions of high terri- genous sediment supply, rugged topography (e.g., fracture zones and spreading centers), and possible complications of lateral advec- tion in the deep western boundary current. All cores were examined and sampled by the senior author. Gravity and trigger weight cores were used wherever possible. Smear slides were prepared and examined for the presence of reworked Tertiary discoasters. In some instances (e.g., samples nos. 27, 37, 40, 41; see Table I) the uppermost core mate- rial available was several centimeters below the top of the core liner. For these samples we have assumed that the sampled material does in fact represent the uppermost sediment recovered, and that settling or shrinkage of
Fig. 2. Locations of 51 s a m p l e s e x a m i n e d in w e s t e r n I n d i a n Ocean t ransec t . Logis t ical i n f o r m a t i o n for each core is given in Tab le I. T w o add i t iona l s a m p l e s examined from south of 50°S had insufficient num- bers of Radiolaria for inclusion in the study (Table I).
the sediment has occurred within the core liner since the cores were split, thereby pro- ducing the void interval at the top of the liner.
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 material yielded a sufficient number of Radiolaria for examination, although in several instances additional sample material and/or cleaning procedures were required. From the initial group of 53 samples prepared
and examined, we eliminated from considera- tion those south of 50°S, since diatoms out- number radiolarians by several orders of mag- nitude in this region. Moreover, Radiolaria from south of the Polar Front have been studied by previous workers (e.g., Hays, 1965; Petrushevskaya, 1967; Riedel, 1958). We also eliminated 5 samples from the Mascarene Basin in which radiolarian preservation was especially poor, leaving us with 46 samples in the transect (Table I, Fig. 2).
One or two specimens of older Radiolaria were observed in a few of the radiolarian samples (Table I); otherwise there is no evi- dence for significant reworking in any of the material examined.
Regional distribution of taxa
The presence or absence of 74 taxa (37 SpumeUaria and 37 Nassellaria) was recorded in each of the 46 samples selected from the transect. No estimates of abundance or counts of total assemblages were made. We believe that presence--absence data alone are suffi- cient to delineate major water masses , and recent quantitative work by Sancetta (1978) documents that this is indeed the case for three of the four major microfossil groups (including Radiolaria) in North Pacific sedi- ments.
Species were chosen for inclusion in the study if they satisfied at least one of the fol- lowing conditions: (1) known to have some latitudinal restrictions on their distribution; (2) relatively common; and (3) easily recog- nizable. Most of the species used have been previously described, but a few required new or expanded descriptions (see Appendix). All 74 taxa are illustrated in Plates I through V.
In the following section, we discuss the distribution pattern of each taxon in the western Indian Ocean transect (see Figs. 3 through 15). Taxa are arranged according to the family level t axonomy of Riedel (1967).
Acrosphaera [lammabunda (ttaeckel) (Fig. 3a; Plate I, 1). Present in most samples north of
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TABLE I
List o f samples used in biogeographic s tudy
Sample Cruise Core No. Level Lat i tude Longi tude Water Rad Rad No. (cm) dep th abund, pres.
(m)
Remarks ÷
1 RC 9 161-TW top 19°34'N 59°36 'E 3332 C G 2 AII 15 596-FF top 18°56'N 61°23 'E 3694 C G 1
AII 15 597-FFA top 17°26 'N 57° l l ' E 1805 F G 4 AII 15 602-FF top 14°56 'N 57°21 'E 3357 A G 1 5 RC9 160-P top 12°03 'N 63°09 'E 4268 A G 6 V 14 103-P top 11°26 'N 56°14 'E 4232 A G 7 AII 15 8-PG top 10°15 'N 53° 10'E 4173 A G 8 AII 15 14-PG top 09°02 'N 53°40'E 4852 A G 9 CHN 100 26-PG top 07°48 'N 56°12 'E 4680 A G 10 CHN 100 29-PG top 06°53 'N 43°41 'E 5106 A G 11 CHN 100 40-PG top 01°37 'N 59°40'E 5426 A G 12 ANTP 150-G 0--3 01°43 'S 57°32 'E 4456 A G 1 13 LSD-H 12-G 0 - 5 05 ° 23'S 60°02 'E 4100 A G 14 DODO 173-G 5--7 08 ° 19'S 69°01 'E 3977 A G 15 AII 93 l l - P C top 09 ° 28'S 52°10'E 4154 A G 1 16 ANTP 142-PG 0 - 5 10 ° 17'S 58°58'E 2763 A G 17 AII 93 5-PC top l l ° 0 1 ' S 54°32 'E 4599 A G 18 DODO 121-PG 4--5 12° 16'S 62°50 'E 3990 A G 19 AII 93 4-PC top 15 ° 17'S 53°31'E 4641 A G 1 20 DODO 163-G 5--7 15 ° 18'S 63°42 'E 3301 A G 21 DODO 157-G 5--7 17°40'S 63°23 'E 3196 A G 22 CHN 43 16-PG top 18°04'S 58°24 'E 3869 A M 23 DODO l l 7 - P G 3--5 18°21'S 62°04 'E 3398 A G 24 AII 15 735-HC top 20°02'S 52°29 'E 4936 A G 25 V 18 199-P top 20°31'S 61°55 'E 3299 C G 26 DODO 151-G 5--7 21°14'S 69°26 'E 3103 C G 27 LSD-A 119-G 13--14 22°02'S 57°33 'E 4770 A G 28 DODO 149-V top 22°28'S 68°03 'E 3141 C G 29 V 14 86-TW 5 cm 23°40'S 53°09 'E 4687 R P 2 30 RC 17 93-P top 23°41'S 69°41 'E 2665 C G 31 V 29 53-TW top 24° 16'S 61°54 'E 4973 A M 32 LSD-A 120-G(b) top 24°30'S 57°29 'E 4990 A G 33 RC 17 91-P top 25°21'S 69° 29'E 3444 A G 1 34 V 20 184-TW 5 cm 25°48'S 53°41 'E 5031 R P 2 35 RC 14 17-TW top 25°49'S 52° 16'E 5013 F P 36 RC 11 109-TW top 26°36'S 56°43 'E 5075 R P 2 37 LSD-A 121-G 9--10 26°51'S 58°14'E 5335 -- - - 2 38 DODO 130-G 3--5 26°56'S 61°49 'E 4700 A G 39 RC 14 16-TW top 29°05'S 52°48 'E 4819 R P 2 40 LSD-A 122-G 10--12 29°54'S 61°53 'E 4400 A G 41 DODO 132-PG 8--10 31°02 'S 64°52 'E 4805 C G 42 AII 15 766-HC top 32°00'S 55°07 'E 4417 A G 43 LSD-A 125-G top 33°14 'S 61°43 'E 4800 A G 44 RC 11 116-P top 34°55 'S 67°35 'E 4548 F G 45 RC 14 13-TW top 37°23 'S 59°19 'E 5128 A G 46 LSD-A 126-G top 39°46'S 64°00 'E 4980 A G 47 RC 11 104-TW top 40°55 'S 57°39 'E 4885 A G 48 RC 11 102-TW top 43°42 'S 58°48 'E 4709 A G 49 RC 11 100-P top 44°50 'S 60°52 'E 4742 A G 50 RC 11 99-P top 46°31 'S 61°02 'E 4449 A G 51 RC 17 62-TW top 47°35 'S 57°52 'E 4426 F G D 52 RC 17 61-P top 52°12'S 54°27 'E 3947 -- - - D, 2 53 RC 17 52-P top 56°22'S 51°58'E 5379 -- - - D, 2
÷Remarks: 1 = rare r eworked radiolarian species present ; 2 = insuff ic ient radiolaria present for biogeograpbic censu, D = mos t ly d ia toms and sponge spicules.
about 33°S; very rare or absent in Arabian Sea samples.
Acrosphaera lappacea (Haeckel) (Fig. 3b; Plate I, 2). Present in most samples north of about 33°S, scattered occurrences between 33°S and 46°S; more abundant in low latitudes.
Acrosphaera spinosa (Haeckel) (Fig. 3c; Plate I, 3). Present in all samples except southern- most (about 48°S) and nor thernmost (about 19°N); particularly abundant at about 20°S.
Buccinosphaera invaginata Haeckel (Fig. 3d; Plate I, 4). Consistently present, but rather rare, in samples north of about 18°S.
Collosphaera huxleyi Muller (Fig. 3e; Plate I, 5; Plate IV, 13). Present in most samples be- tween about 18°S and 37°S; see Appendix.
Collosphaera sp. aff. C. huxleyi Muller (Fig. 3f; Plate 1,6; Plate IV, 14). Consistently present in samples north of about 5°N; scat- tered occurrences of small, impoverished forms between 0 ° and 10°S; see Appendix.
Collosphaera macropora Popofsky (Fig. 4a; Plate I, 7; Plate IV, 15). Present in most samples between about 5°S and 20°S with very rare occurrences extending its geographic range to about 10°N and 25°S; see Appendix.
Collosphaera tuberosa Haeckel (Fig. 4b; Plate I, 8). Consistently present in samples north of about 25°S.
Disolenia quadrata (Ehrenberg) (Fig. 4c; Plate I, 9). Present in most samples north of about 22°S, but rare or absent near the Arabian coast; increasingly rare towards the southern extremity of its geographic range.
Disolenia zanguebarica (Ehrenberg) (Fig. 4d; Plate I, 10). Present, except near the Arabian coast, in samples north of about 35°S; increasingly rare towards the southern extrem- ity of its geographic range.
Otosphaera auriculata Haeckel (Fig. 4e; Plate I, 11). Consistently present in samples north of about 25°S; increasingly rare towards the southern extremity of its geographic range.
Siphonosphaera polysiphonia Haeckel (Fig. 4f; Plate I, 12). Present in most samples; absent from sample at 48°S; very rare or ab- sent near the Arabian coast; most abundant in low latitudes.
Actinomma antarcticum (Haeckel) (Fig. 5a; Plate I, 13). Present in four samples between about 40°S and 46°S.
Actinomma arcadophorum Haeckel (Fig. 5b; Plate I, 14). Present in most samples north of about 30°S.
Actinomma medianum Nigrini (Fig. 5c; Plate I, 15). Bimodal distribution, very rare in samples between about 5°N and 10°S (about 15°S in the western part of the study area), more abundant in samples between about 30°S and 46°S.
Fig. 3. Distribution patterns of radiolarian taxa. Open circles represent samples in which the taxon was sought but not found after examination of two or more strewn slides. Half-filled circles represent samples in which only one specimen or questionable fragments of a given taxon were observed. Filled circles represent samples in which two or more clearly identified specimens of the taxon were observed. Asterisks designate five samples in which severe corrosion of the radiolarian skeletons yielded very poor preservation and insufficient skeletal material to reliably determine the presence or absence of a given taxon. In Figs. 3 through 15, maps showing the distribution patterns of the 74 taxa in this study are presented in the same order as that listed in the text, under "Regional distribution of taxa".
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PLATE I
121
Anomalacantha dentata (Mast) (Fig. 5, d; Plate I, 16). Bimodal distribution north of about 12°S and between about 30°S and 45°S.
Omrnatartus tetrathalamus tetrathalamus (Haeckel) (Fig. 5e; Plate I, 17). Present in all samples except southernmost (about 48°S); abundant in low latitudes but very rare be- tween about 37°S and 46°S.
Cypassis irregularis Nigrini (Fig. 5f; Plate I, 18). Present in four samples near the Arabian coast; very rare.
Saturnalis circularis Haeckel (Fig. 6a; Plate I, 19). Bimodal distribution, north of about 15°S and between about 30°S and 46°S; very rare, but distinctive,
Spongurus cf. elliptica (Ehrenberg) (Fig. 6b, Plate I, 20). Present in most samples north of about 30°S.
Spongurus pylomaticus Riedel (Fig. 6c; Plate I, 21). Present in most samples between about 37°S and 46°S.
Spongocore puella Haeckel (Fig. 6d; Plate I, 22). Present in most samples except southern- most (about 48°S) and three samples between about 30°S and 23°S in the eastern part of the study area.
Styptosphaera ? spumacea Haeckel (Fig. 6e; Plate II, 1). Present in all samples between about 37°S and 46°S.
Heliodiscus asteriscus Haeckel (Fig. 6f; Plate II, 2). Present in all samples except southern- most (about 48°S).
Heliodiscus echiniscus Haeckel (Fig. 7a; Plate II, 3). Present in samples north of about 20°S in the eastern part of the study area, but ranges as far south as 25°S in the western part.
Amphirhopalum cf. Tessarastrum straussii Haeckel (Fig. 7b; Plate II, 4; Plate IV, 1, 2). Present in most samples between about 10°S and 46°S; most abundant between 20°S and 46°S; see Appendix.
Amphirhopalum ypsilon Haeckel (Fig. 7c; Plate II, 5). Present in samples north of about 25°S, except for four samples in the eastern part of the study area between about 30°S and 25°S.
Trigonastrum sp. (Fig. 7d; Plate II, 6; Plate IV, 16, 17). Consistently present in samples between about 27°S and 46°S with a northward extension of its geographic range to about 18°S in the western part of the study area; see Appendix.
PLATE I (× 140)
1. Acrosphaera flammabunda (Haeckel), LSDA 120G(b), D26/0. 2. Acrosphaera lappacea (Haeckel), A I I 15- 766HC, E37/2. 3. Acrosphaera spinosa (Haeckel), A I I 93-11PC, L48/4. 4. Buccinosphaera invaginata Haeckel, A I I 93-11PC, W32/2. 5. Collosphaera huxleyi Muller, A I I 15-766HC, G31/4. 6. Collosphaera sp. aff. C. huxleyi Muller, RC9-161 TW, R16/1. 7. Collosphaera macropora Popofsky, A I I 93-11PC, F37/2. 8. Collosphaera tuberosa Haeckel, A I I 93-11PC, G29/0. 9. Disolenia quadrata (Ehrenberg), A I I 93-11PC, $39/0. 10. Disolenia zanguebarica (Ehrenberg), A I I 93-11PC, K42/2. 11. Otosphaera auriculata Haeckel, RC 9-161 TW, M31/1. 12. Siphonosphaera polysiphonia Haeckel, A I I 93-4PC, X19/0. 13. Actinomma antarcticum (Haeckel), RCll- 102TW, M35/4. 14. Actinomma arcadophorum Haeckel, V14-103P, P25/4. 15. Actinomma medianum Nigrini, RC11-100P, N14/1. 16. Anomalacantha dentata (Mast), LSDA 125G, G18/2. 17. Ommatartus tetrathalamus tetrathalamus (Haeckel), RC9-161TW, W45/1. 18. Cypassis irregularis Nigrini, A II 15-596FF, M20/3. 19. Saturnalis circularis Haeckel, RCll-102TW, U40/0. 20. Spongurus cf. elliptica (Ehrenberg), A I I 93-4PC, L48/4. 21. Spongurus pylomaticus Riedel, RC ll-100P, G43/0. 22. Spongocore puella Haeckel, A I I 93-4PC, C29/0.
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127
Euchitonia elegans (Ehrenberg) (Fig. 7e; Plate II, 7). Present in most samples north of about 20°S; very rare occurrences as far south as 37°S; frequently specimens are incomplete and cannot be distinguished from E. furcata.
Euchitonia furcata Ehrenberg (Fig. 7f; Plate II, 8). Consistently present in samples north of about 25°S with occurrences as far south as about 30°S in the central part of the study area; frequently specimens are incomplete and cannot be distinguished from E. elegans.
Dictyocoryne profunda Ehrenberg (Fig. 8a; Plate II, 9). Present in most samples north of about 25°S; very rare occurrences as far south as about 33°S.
Dictyocoryne truncatum (Ehrenberg) (Fig. 8b; Plate II, 10). Present in every sample, except one, north of about 32°S.
Hymeniastrum euclidis Haeckel (Fig. 8c; Plate II, 11). Present in all samples except southern- most (about 48°S) and one impoverished sample at about 30°S.
Spongaster tetras tetras Ehrenberg (Fig. 8d; Plate II, 13). Consistently present in samples north of about 30°S.
Spongaster tetras Ehrenberg irregularis Nigrini (Fig. 8e; Plate II, 14). Present in most samples between about 30°S and 40°S with rare occur- rences extending its geographic range to be- tween 25°S and 45°S.
Spongobrachium sp. (Fig. 8f; Plate II, 13; Plate V, 3). Present in most samples between about 10°S and 30°S; see Appendix.
Larcospira quadrangula Haeckel (Fig. 9a; Plate II, 15). Present in most samples except south- ernmost (about 48°S) and three northernmost (about 19°N); very rare north of about 10°N.
Antarctissa spp. (Fig. 9b; Plate III, 1). Present in all samples south of about 37°S.
Liriospyris reticulata (Ehrenberg) (Fig. 9c; Plate III, 2). Present in most samples between about 15°N and 33°S.
Lophospyris pentagona pentagona {Ehrenberg) emend. Goll (Fig. 9d; Plate III, 3). Present in most samples between about 18°N and 25°S.
Phormospyris stabilis (Goll) antarctica (Haecker) (Fig. 9e; Plate III, 4). Present in most samples between about 37°S and 46°S.
Carpocanistrum spp. (Fig. 9f; Plate III, 5). Present in all samples between about 19°N and 45°S.
Carpocanarium papillosum (Ehrenberg) group (Fig. 10a; Plate III, 6). Bimodal distribution; present in most samples north of about 15°S and very rare in three samples between about 37°S and 41°S.
Cornutella profunda Ehrenberg (Fig. 10b; Plate III, 7). Present in most samples north of about 46°S; however, between about 20°S and 35°S it is frequently absent or very rare.
Lithopera bacca Ehrenberg (Fig. 10c; Plate III, 8). Present in most samples between about 10°N and 35°S; particularly abundant in samples between about 22°S and 35°S; very rare in two samples between 40°S and 44°S.
Dictyophimus crisiae Ehrenberg (Fig. 10d; Plate III, 9). Present in most samples north of about 46°S; however, between about 15°S and 25°S it is frequently absent or very rare.
Pterocanium praetextum praetexturn (Ehren- berg) (Fig. 10e; Plate III, 10). Present in all samples north of about 20°S and in most samples between about 20°S and 33°S.
Pterocanium praetextum (Ehrenberg) eucolpum Haeckel (Fig. 10f; Plate III, 11). Present in all samples between about 27°S and 46°S except for one impoverished sample at about 30°S.
128
P L A T E II
129
Pterocanium trilobum (Haeckel) (Fig. l l a ; Plate III, 13). Present in most samples north of about 46°S; less abundant south of about 35°S.
Pterocanium sp. (Fig. 11b; Plate III, 13). Bi- modal distribution; present in all samples north of the equator; present, but very rare, in samples between abou t 30°S and 46°S.
Theocalyptra bicornis (Popofsky) sensu stricto (Fig. 11c; Plate III, 14). Present in all samples south of about 37°S; there is a similar, but diminutive, form in lower latitudes.
Eucyrtidium acuminatum (Ehrenberg) (Fig. l l d ; Plate III, 15). Present in all samples be- tween about 18°S and 46°S, except for two impoverished samples near 28°S; very rare occurrences extend its geographic range as far north as about 10°S; around 20°S many speci- mens are transitional to E. hexagonatum.
Eucyrtidium hexagonatum Haeckel (Fig. 11e; Plate III, 16). Present in all samples north of about 25°S, but generally rare between about 20°S and 25°S; very rare specimens in samples as far south as 37°S.
Lithocampe sp. (Fig. 11f; Plate III, 17). Present in most samples south of about 10°S, but consistently present and most abundant south of about 30°S; some very rare occur- rences between about 0 ° and 10°S.
Anthocyrt idium ophirense (Ehrenberg) (Fig. 12a; Plate III, 18). Present in most samples north of about 40°S; less abundant in middle latitudes.
Anthocyrt idium zanguebaricum (Ehrenberg) (Fig. 12b; Plate III, 19). Present in most samples north of about 45°S; rare or absent in several samples between about 20°S and 25°S in the eastern part of the s tudy area.
Androcyclas gamphonycha (Jorgensen) (Fig. 12c; Plate III, 20). Present in most samples between about 30°S and 46°S.
Lamprocyclas maritalis maritalis Haeckel (Fig. 12d; Plate III, 21). Present in most samples north of about 46°S, but more abundant in middle latitudes.
Lamprocyclas maritalis Haeckel polypora Nigrini (Fig. 12e; Plate III, 22). Present in most samples north of about 40°S, but fre- quently rare or absent between about 20°S and 30°S; most abundant in low latitudes.
Lamprocyclas maritalis Haeckel ventricosa Nigrini (Fig. 12f; Plate III, 23). Present in all samples north of about 10°N.
Lamprocyrtis nigriniae (Caulet) (Fig. 13a; Plate III, 24). Present in all samples north of about 5°S.
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Lamprocyrtis (?) hannai (Campbell and Clark) (Fig. 13b; Plate III, 25). Consistently present in samples north of about 10°S with scattered occurrences as far south as 30°S.
Pterocorys hertwigii (Haeckel) (Fig. 13c; Plate IV, 1). Consistently present north of about 18°S and in one sample at about 21°S.
Pterocorys sabae (Ehrenberg) (Fig. 13d; Plate IV, 2; Plate V, 4, 5). Present in most samples between about 12°N and 40°S; abdomen rare- ly present in high-latitude specimens; see Ap- pendix.
Theocorythium trachelium trachelium (Ehrenberg) (Fig. 13e; Plate IV, 3). Present in all samples between about l l °N and 25°S; from about 25°S to 33°S specimens are rare and transitional to T. trachelium dianae.
Theocorythium trachelium (Ehrenberg) dianae (Haeckel) (Fig. 13f; Plate IV, 4). Present in all samples between about 25°S and 46°S except for one impoverished sample near 30°S; be- tween about 24°S and 30°S specimens are transitional to T. trachelium trachelium.
ly present between about 35°S and 46°S; scattered occurrences between about 0 ° and 20°S.
Botryostrobus auritus/australis (Ehrenberg) group (Fig. 14b; Plate IV, 6). Present in all samples except for two impoverished samples near 28°S.
Phormostichoartus corbula (Harting) (Fig. 14c; Plate IV, 7). Present in most samples north of about 33°S except for a group of five samples between about 20°S and 25°S in the eastern part of the study area; very rare occur- rences as far south as about 45°S.
Siphocampe lineata (Ehrenberg) group (Fig. 14d; Plate IV, 8). Present in most samples north of about 15°S; scattered occurrences between about 15°S and 26°S and very rare occurrences between about 37°S and 46°S.
Spirocyrtis scalaris Haeckel (Fig. 14e; Plate IV, 9). Present in most samples north of about 18°S; absent from two northernmost samples (about 19°N).
Botryocyrtis scutum (Harting) (Fig. 14f; Plate IV, 10). Present in most samples north of about 33°S.
PLATE III (× 140)
1. Antarctissa spp., RC11-102TW, X22/1. 2. Liriospyris reticulata (Ehrenberg), A I I 15-766HC, 019/0. 3. Lophospyris pentagona pentagona (Ehrenberg) emend. Goll, RC9-161TW, J29/2. 4. Phormospyris stabilis (Goll) antarctica (Haecker), RC11-102TW, D27/3. 5. Carpocanistrum spp., A II 93-11PC, 040/2. 6. Carpo- canarium papillosum (Ehrenberg) group, RC9-161TW, H32/0. 7. Cornutella profunda Ehrenberg, RC9-161TW, C25/0. 8. Lithopera bacca Ehrenberg, A I I 93-11PC, C44/2. 9. Dictyophimus crisiae Ehrenberg, RC11-100P, H43/0. 10. Pterocanium praetextum praetextum (Ehrenberg), A I I 93-11PC, U46/2. 11. Pterocanium praetextum (Ehrenberg) eucolpum Haeckel, RC11-102TW, X19/2. 12. Pterocanium trilobum (Haeckel), A I I 15-766HC, C13/2. 13. Pterocanium sp., CHN 100-40PG, G17/1. 14. Theocalyptra bicornis (Popofsky) sensu stricto, RC11- 100P, D31/0. 15. Eucyrtidium acuminatum (Ehrenberg), A II 15-766HC, P44/3. 16. Eucyrtidium hexagonatum Haeckel, A II 93-4PC, X31/2. 17. Lithocampe sp., A I I 15-766HC, F27/3. 18. Anthocyrtidium ophirense (Ehrenberg), RC9-161TW, N43/2. 19. Anthocyrtidium zanguebaricum (Ehrenberg), A I I 93-11PC, R38/4. 20. Androcyclas gamphonycha (Jorgensen), RCll-102TW, 020/0. 21. Lamprocyclas maritalis maritalis Haeckel, A II 15-766HC, Q45/2. 22. Larnprocyclas maritalis Haeckel polypora Nigrini, A II 93-4PC, T16/0. 23. Larnpro- cyclas maritalis Haeckel ventricosa Nigrini, RC9-161TW, G28/4. 24. Lamprocyrtis nigriniae (Caulet), RC9- 161TW, G47/4. 25. Lamprocyrtis (?) hannai (Campbell and Clark), RC9-161TW, R47/0.
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Centrobotrys thermophi la Petrushevskaya (Fig. 15a; Plate IV, 11). Consistently present in samples between about 10°N and 18°S; very rare occurrences extend its geographic range to about 30°S.
Saccospyris coni thorax Petrushevskaya (Fig. 15b; Plate IV, 12). Present in all samples south of about 37°S.
Recurrent group analysis
Following the procedures of recurrent group analysis which were used previously by Nigrini (1970) for North Pacific Radiolaria, recurrent groups were identified among the 74 taxa used in our study. The theory of re- current group analysis and the assumptions involved have been discussed elsewhere (Fager, 1957; Fager and McGowan, 1963; Renz, 1976), and will be briefly summarized here.
Recurrent group analysis uses the concept of affinity, based on co-occurrence of species (presence or absence only) as a means of identifying those species that are considered to be a nearly constant part of each others" biological environment. Those species which often co-occur have a strong affinity; those that never do, have no affinity. Recurrent group analysis leads to the largest, most fre.- quent, separate units within which each species shows a strong affinity for all others.
For any two species A and B, the index of
their affinity is given by:
J 1 I . A . -
x/NA X N B x/-2 NB
where J = number of joint occurrences of A and B; NA = total number of occurrences of A; NB = total number of occurrences of B; and N B ~> N A. Fager (1957) defines a recur- rent group as one that satisfies the following requirements:
(1) The index of affinity is ~> 0.50 for all pairs of species within the group; (2) the group includes the greatest possible number of species; (3) if two or more groups with the same number of species and with members in common are possible, the one for which the sum of affinities is greatest is chosen.
To implement this procedure, the compute r program REGROUP (designed by E.W. Fager) was used. The program computed the number of occurrences and joint occurrences of the 74 species identified in our transect of 46 samples. It then calculated an index of affinity for each pair of species, and compared this number to the assigned cutoff value (0.50). If this index is equal to or greater than the assigned cu tof f value, the pair of species is considered to have affinity. The program then determines the recurrent groups according to the three criteria of Fager (1957) summarized above.
In our study, five recurrent groups of Radiolaria were selected (Table II). Groups A,
PLATE IV (x 140 unless otherwise noted)
1. Pterocorys hertwigii (Haeckel), ANTP 142PG, U20/3. 2. Pterocorys sabae (Ehrenberg), AII 93-11PC, K42/0. 3. Theocorythium trachelium tracheliurn (Ehrenberg), A I I 93-11PC, T33/2. 4. Theocorythium trachelium (Ehrenberg) dianae (Haeckel), RCll-102TW, L30/2. 5. Botryostrobus aquilonaris (Bailey), A II 93-4PC, J42/0. 6. Botryostrobus auritus/australis (Ehrenberg) group, A II 93-4PC, G34/0. 7. Phormostichoartus corbula (Harting), AI I 93-11PC, P47/4. 8. Siphocampe lineata (Ehrenberg) group, AII 93-11PC, F29/1. 9. Spirocyrtis scalaris Haeckel, CHN 100-26PG, X22/3. 10. Botryocyrtis scutum (Hatting), AII 93-11PC, C22/2. 11. Centro- botrys thermophila Petrushevskaya, AI I 93-11PC, N14/3. 12. Saccospyris conithorax Petrushevskaya, RCll- 102TW, U20/3. 13. Collosphaera huxleyi Muller, RC17-93P, R18/0; x 275. 14. Collosphaera sp. aff. C. huxleyi Muller, AI I 15-597FFA, C12-4; x 275. 15. Collosphaera rnacropora Popofsky, CHN 100-29PG, T40/1; x 275. 16, Trigonastrumsp.,RCll-lOOP, P30/3. 17. Trigonastrum sp.,RC11-102TW, Dll/0;X 275.
Fig. 1 5 . Distribution patterns of radiolarian taxa. (See Fig. 3 caption for explanation of symbols . )
B, and C met all the criteria specified for selection by the REGROUP program. Groups D and E were recognized by the authors, even though some affinity indices for the pairs in each group did not meet the 0 .50 cutof f criterion. Group D was selected because of the unusually limited geographic distribution pattern of each of the four taxa in the group (Figs. 3e, 5f, 12f, and 13a). Affinity indices for the six affinity pairs present in Group D range from 0.42 to 0.74, and average 0.60. Group E was selected because of the intriguing bimodal distribution pattern of its five taxa (Figs. 5c, 5d, 6a, 10a, l l b ) . The taxain Group E have affinity indices ranging from 0.49 to 0.67, and average 0.57. Six species remain ungrouped (Table II). These taxa had affinities
with some members of groups, but not with all the members of any one group.
After the designation of recurrent groups, we identified the samples in which each recur- rent group appears, using the criterion 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 36 of its 43 taxa are present; Group B requires 7 of its 8 species; Group C requires 7 of its 8 species; Group D requires 4 of its 4 species; and Group E requires 4 of its 5 species. The distribution patterns of the five recurrent groups are shown in Fig. 16.
From the geographic distribution of the recurrent groups, we designated eight radio-
141
TABLE II
Listing of recurrent groups in western Indian Ocean transect
larian assemblages for the western Indian Ocean transect (Fig. 17a). Although the num- ber of samples examined is no t sufficient to establish the zonal ex ten t of each assemblage, it is likely that the assemblage boundaries do in fact extend in directions which parallel the major oceanographic frontal zones (see Fig. 1). The designated assemblages are:
Arabian assemblage -- Recurrent group D only. Assemblage is characteristic of the upwelling zone east of the Arabian Peninsula. There is no evidence that it continues eastward into the northeastern Arabian Sea, although its ex ten t is difficult to establish because of poor preservation of radiolarians in sediments f rom the Indus Cone (Nigrini, 1974).
South Arabian assemblage -- Recurrent groups A, D and E. Since it is based on only one sample, this assemblage is obviously no t well established. More extensive sampling in the transition zone between the distinctive Arabi- an and Tropical assemblages is required to as- certain the geographic extent of this assem- blage.
Tropical assemblage -- Recurrent groups A and E. Assemblage is diverse with a southern limit corresponding to the sharp hydrochem- ical f ront at 10°S.
Subtropical assemblage -- Recurrent group A only. Samples containing this assemblage lie within the nor thern limb of the subtropical gyre. It is possible that its areal ex ten t includes the Madagascar and Mascarene Basins, but preservation of skeletal material in these regions is very poor (Fig. 17a).
Central assemblage -- No recurrent groups present. Although Radiolaria are common and generally well preserved in the central port ion
of the subtropical gyre (see Table I), none of our recurrent groups was found to be present (using the 80% cu tof f criterion). We presume that this assemblage extends eastward from our transect into the eastern Indian Ocean (Fig. 17a).
Tempera te assemblage -- Recurrent group C only. Assemblage appears to be characteristic of the southern, eastward-flowing limb of the subtropical gyre. The transition between the subtropical and temperate assemblages south- east of Madagascar (Fig. 17a) is not sharply defined due to poor sample preservation.
Transitional assemblage -- Recurrent groups C and E. Assemblage is partially defined by the southern por t ion of bimodally distributed Group E. It extends from about 35°S to the Subtropical Convergence.
Subpolar assemblage -- Recurrent Groups B, C and E. Assemblage can be identified with the eastward flow between the Subtropical Convergence and Antarctic Convergence. It is the only assemblage in which all eight taxa of Group B are found. Sediments to the south of this region (between ~48°S and 56°S) are predominant ly diatomaceous with insufficient Radiolaria for reliable identifications.
Discussion
The identification of eight distinctive radiolarian assemblages across a north- sou th transect (Fig. 17a) represents a significant extension of the previous investigations of radiolarian biogeography of Nigrini (1967) and Petrushevskaya (1967, 1971, 1973). In- creased availability of samples, recent refine- ments in the t axonomy of modern Radiolaria (Nigrini and Moore, 1979), and the use of re- current group analysis have aided significantly
Fig. 16. Distribution patterns of the five selected recurrent groups of Radiolaria. A recurrent group is considered to be "present" (filled circles on map) if at least 80% of the taxa within the group were present in a given sample. These five recurrent groups account for 68 of the 74 taxa examined during this study.
Fig. 17. a. Radiolarian assemblages in the western Indian Ocean, chosen on the basis of the presence of one or more recurrent groups: "Arabian" assemblage: Group D only. "South Arabian" assemblage: Groups A+D+E. "Tropical" assemblage: Groups A+E. "Subtropical" assemblage: Group A only. "Central" assemblage: No recur- rent groups. "Tempera te" assemblage: Group C only. "Transitional" assemblage: Groups C + E. "Subpolar" assem- blage: Groups, B, C and E. Poor preservation of radiolarians in the region southeast of Madagascar, and low abun- dances between 47°S and 56'~S (Table I), did not allow the designation of assemblages in these regions, b. Distri- bution pattern of planktonic foraminiferal assemblages in surface sediments, modified after B@ and Hutson (] 977, fig. 49).
in the selection of unique assemblages. It is no tewor thy that four principal oceano-
graphic fronts in the western Indian Ocean are all reflected by boundaries between char- acteristic radiolarian assemblages. Strong up- welling zones are present off the Somali and Arabian coasts (Swallow and Bruce, 1966; Warren et al., 1966; Wyrtki, 1973). The Arabian upwelling zone is clearly marked by the four taxa in recurrent group D (Fig, 16), each of which has also been reported from a relatively small region of upwelling in the
eastern equatorial Pacific (Nigrini, 1968). From the present s tudy it appears that the Arabian assemblage does not extend south- ward to the Somalian upwelling zone, perhaps as a consequence of the strong differences in the salinity characteristics of the two upwel- ling areas.
The South Equatorial Divergence near 10°S is a pronounced hydrochemical front (Wyrtki, 1973), and is marked by the Tropical/Sub- tropical Assemblage boundary. The Transi- tional Assemblage appears to correspond
145
approximately with the Subtropical Conver- gence between 38°S and 40°S, although this relationship is imprecise because of seasonal migrations of the S.T.C. (Prell et al., 1979) and the lack of good core top samples from this region. The Antarctic Convergence near 48°S is well marked by the boundary between the Subpolar Assemblage to the north and the radiolarian-poor diatomaceous ooze assem- blage to the south. The central portion of the subtropical gyre appears to be well delineated by a radiolarian assemblage in which none of the designated recurrent groups are present. The southward-flowing limb of the subtropical gyre to the east of the Madagascar includes the transition from the Subtropical to Temper- ate Assemblages (Fig. 17a). However, poor sample preservation, probably resulting from the highly corrosive polar bot tom water with- in the Madagascar and Mascarene Basins {Warren, 1974, 1978), makes it impossible to locate precisely the position of this transition.
B~ and Hutson (1977) analyzed foraminifera in the plankton and surface sediments of the western Indian Ocean, and selected five assem- blages of foraminifera characteristic of the surface sediments. In examining their assem- blage boundaries (Fig. 17b) together with those selected from radiolarian distributions (Fig. 17a), the following comparisons can be made:
(1) B~ and Hutson (1977) were not able to discriminate the upwelling zone off Arabia. More recent work by Prell (1978) and Hutson and Prell (in press), however, suggests that foraminifera in this region may in fact have distinguishing characteristics. Using an im- proved data base of sea surface temperatures, an expanded number of core top samples, and two new paleoecological transfer functions, these authors were able to select two new foraminiferal assemblages characteristic of up- welling within the Benguela Current (off Southwest Africa) and along the Somali- Arabian coasts. We have not found a unique assemblage characteristic of the Benguela Cur- rent, and our Arabian Margin assemblage does not appear to extend as far south as the Somali coast.
(2) The major oceanographic front at 10°S and 40°S are apparently reflected in the distributions of both microfossil groups. The differences in how the assemblage boundaries are drawn (Fig. 17) may reflect sample cover- age and not real differences in the position of faunal gradients. An extension of the radio- larian assemblage boundaries into the eastern Indian Ocean will aid in determining if there are significant differences in location between foraminiferal and radiolarian assemblage boundaries.
(3) The foraminiferal assemblage boundary near 30°S, southeast of Madagascar, is not well defined by Radiolaria because of poor sample preservation.
The oceanographic significance of the exotic Arabian assemblage will require more extensive investigation. The four radiolarian taxa characteristic of this region were also reported in the eastern tropical Pacific (Nigrini, 1968), and therefore seem to have an affinity for zones of strong upwelling (though they are apparently not present in the Somalian upwelling zone). B~ and Hutson (1977, p. 380) reported that the northern Indian Ocean is characterized by the presence of three extant foraminiferal taxa (Globo- quadrina hexagona, G. conglomerata, Globigerinella adamsi) which are no longer living in the Atlantic, and suggested that the northern Indian Ocean may be a refuge for certain "rel ict" species. We see no evidence that this is the case for radiolarians. In our examination of Pleistocene and pre-Pleistocene cores from the Indian and Pacific Oceans, we have seen no evidence that the four taxa in our Arabian assemblage became extinct at an earlier time elsewhere. Clearly the Arabian Sea represents an exotic environment for faunal biogeography, and a more extensive study on the characteristics and oceanograph- ic significance of the fauna in the region would be interesting.
The bimodal distribution pattern of recur- rent group E (Fig. 16) is particularly intriguing. Although the same taxa are characteristic of both the northern and the southern regions of
146
occurrence of the group, they are noticeably more common in the southern portion. The boundary for the group near 10°S corresponds closely with the hydrochemical f ront (Wyrtki, 1973), and the other boundaries for the group in the Arabian Sea, near 35°S, and near 48°S closely approximate other first-order oceano- graphic transitions. However, the reasons for a bimodal distribution pat tern are not at all ob- vious. From our sample control , we are reasom ably conf ident that the two areas of occur- rence of Group E are in fact geographically separate, at least in the western Indian Ocean (Fig. 16). We suspect that the same is true for the eastern Indian Ocean, since meridional current flow tends to be more intensified on the western and weakened on the eastern sides of the ocean basins; hence a "connec t ion" be- tween the two areas of Recurrent Group E should be more obvious on the west, but we see no such connection. It is possible that a connect ion exists between Madagascar and the African continent , but our present sample coverage does not permit us to examine this possibility.
One possible, perhaps fanciful, interpreta- tion of the bimodal pat tern of Group E is that it represents the recent separation of a former- ly contiguous distribution. The greater abun- dance of the species of Group E in the south- ern port ion of the pattern suggests that the taxa in the group may be most representative of more southerly (i.e., 35°S to 48°S) latitudes today, but at some earlier time the group may have extended more or less cont inuously into equatorial waters. At some time, the develop- ment of the southern Subtropical Gyre may have geographically divided the assemblage. The late Cenozoic development of the mon- soonal circulation could have re-enforced the separation by driving the equatorial water nor thward (during the more intense summer monsoon), and thereby inhibiting north .... south exchange across the f ront at 10°S.
A final point worth noting is the sharp drop in tad}olaf}an abundance near 48°S, cor- responding with the Antarctic Convergence. Numerous investigators (e.g. Hays, 1965;
Petrushevskaya, 1973; Lozano and Hays, 1976; Morley, 1977; Dow, 1978) have studied radiolarian assemblages in the circumpolar current south of the Antarctic Convergence, but to our knowledge there have been no reports of dramatic decreases in radiolarian abundance at the Antarctic Convergence. A~ extension of our investigation into the eastern Indian Ocean will allow us to document more extensively the extent of this phenomena. We do not know whether this represents a real difference in the phy top lank ton /zoop lank ton ratio, or whether it represents selective dis- solution within the water column prior to final deposition of the skeletal remains.
Appendix: t axonomy
The following species are described and illustrated in Nigrini and Moore (1979):
The following species are described and illustrated in the publications cited:
Buccinosphaera invaginata Haeckel in Nigrini, 1971 (see also Knoll and Johnson, 1975)
Cypassis irregularis Nigrini in Nigrini, 1968 Saccospyris conithorax Petrushevskaya in Petrushev-
skaya, 1965 Siphocampe lineata (Ehrenberg) group in Nigrini,
1977
The following species, used in the present study, have not previously been described or require some remarks in addition to already published descriptions.
FAMILY COLLOSPHAERIDAE Muller 1858
Genus Acrosphaera ttaeckel 1881
The generic name Polysolenia Ehrenberg has been used incorrectly in a number of publicatons (Camp- bell, 1954; Nigrini, 1967, 1968, 1970; Nigrini and Moore, 1979, etc.). The correct generic name for collosphaerids with irregularly scattered spines is Acrosphaera (cf. Strelkov and Reshetnyak, 1971).
Genus Collosphaera Muller 1855
Collosphaera huxleyi Muller (Plate I, 5; Plate IV, 13). Collosphaera huxleyi Muller, 1855, p. 238; 1858, p. 55, pl. 8, figs. 6--9; Strelkov and Reshetnyak, 1971, p. 332, text-figs. 19--21, pl. 4, figs. 21, 23.
Numerous variants of this species have been described by several authors (cf. Strelkov and Reshetnyak, 1971). The form here recognized is restricted to a simple sphere 120--160 um in diameter with small, irregularly shaped pores, 98--113 on a half equator. The shell may be slightly misshapen, but noL as pro- nouncedly as C. tuberosa. Thus defined C. huxleyi has a very restricted latitudinal range (about 18-- 3T~S (see Fig. 3e) in the Indian Ocean.
Collosphaera sp. aff. C. huxleyi Muller (Plate I, 6; Plate IV, 14). Description: Shell smooth, a simple sphere 120--160 pm in diameter, with 5--8 pores on a half equator; pores vary in size, but most are larger than those of typical C. huxleyi. Shell is more perfectly spherical than C. huxleyi. Remarks: This species is common in samples close to the Arabian coast (Fig. 3f). Rarely, in samples from about 0 to 10°S, a form transitional between C. sp. aff. C. huxleyi and C. macropora has been observed.
Collosphaera macropora Popofsky (Plate I, 7; Plate IV, 15).
Collosphaera macropora Popofsky, 1917, p. 247, text-figs. 5,6, pl. 14, figs. 2a--c; Strelkov and Reshetnyak, 1971, p. 337, pl. 4, figs. 30, 31.
Description: Shell smooth, usually spherical, 92--120 ttm in diameter, with large pores, 3--4 on a half equator. Small pores may be present between the larger ones. Remarks: In the study area this species has been found to have a very restricted latitudinal range (about5 S t o 2 O S ) .
FAMILY SPONGODISCIDAE Haeckel 1862, emend. Riedel 1967
The next three species are probably closely related. However, Haeckelian taxonomy requires that one of
148
them, a t least, be p laced in a d i f f e ren t genus. Unti l a c o m p r e h e n s i v e s tudy can be made , we will re ta in the c lass i f icat ion of Haeckel.
Genus Amphirhopalum Haeckel 1881, emend . Nigrini 1974.
Amphirhopalum ypsilon Haeckel (Plate II, 5). Amphirhopalum ypsilon Haeckel , 1887, p. 522; Nigrini, 1967, p. 35, pl. 3, figs. 3a- -d ; Nigrini, 1971, p. 447, pl. 34.1, figs. 7a--c; Nigrini, 1974, p. 1065, pl. 6, fig. 3.
Previous ly pub l i shed desc r ip t ions of this species are adequa te . The re is a P l iocene species, Amphirhopatum virchowii (Haeckel) , descr ibed by Dumi t r i ca (1973 , p. 835, pl. 9, figs. 2 ,4 , pl. 11, fig. 6, pl. 21, figs. 3 13), which may be the ances to r of A. ypsilon (cf. Nigrini, 1974, pl. 1065) .
Amphirhopalum cf. Tessarastrum straussii Haeckel (Plate II, 4; Pla te V, 1,2).
Tessarastrum straussii Haeekel , 1887, p. 547, pt. 45, fig. 8.
Description: Shell bi la teral ly symmet r i ca l wi th two oppos i te c h a m b e r e d arms, one or b o t h of which m a y be fo rked distal ly (usual ly on ly one) . Arms arise f rom a cen t ra l s t r uc tu r e c o m p o s e d o f two concen t r i c spheres and one ou t e r ob la te sphero id , all qui te s m o o t h and c o n n e c t e d by n u m e r o u s d i s con t inuous radial beams and p e r f o r a t e d by n u m e r o u s subc i rcu la r pores.
Arms are each c o m p o s e d of up to six s m o o t h , d i s t inc t ly r o u n d e d c h a m b e r s bear ing r a the r few sub- c i rcular pores of d i f fer ing sizes. S t ruc tu re s be t w een chamber s are dis t inct . Arms e x p a n d only sl ightly distal ly usual ly e n d i n g wi th a c o m p l e t e chamber , bu t s o m e t i m e s t e r m i n a t i n g in a spongy m e s h w o r k which tapers distally.
Pa tag ium m a y or m ay n o t be present . Small thorn- like p ro jec t ions a long shell marg ins are o f t e n present , ind ica t ive of an inc ip i en t pa tag ium. Dimensions: Tota l shell l ength up to 285 t~m; maxi- m u m b read th of u n b r a n c h e d a rm 4 0 - - 6 5 urn. Remarks: T. straussii is descr ibed by Haeckel as having four s imple arms: two pr inc ipa l arms as in the species here descr ibed and two smaller la teral arms. Haeckel ' s f o rm also has more c h a m b e r s or jo in t s on each a rm than the species f o u n d by us in Ind ian Ocean sediments . Renz (1974 and 1976) used the species name T. straussii for a f o rm similar to t ha t here descr ibed, bu t n o t e d (1974) t h a t the cross arms were " r u d i m e n t a r y or c o m p l e t e l y lacking" . No speci- m e n wi th four a rms was f o u n d in the p r e s en t s tudy , bu t they have been observed, rarely, by one of us (C.N.) in N o r t h Pacific sed iments . I t is the a u t h o r s ' o p i n i o n t ha t Haeckel ' s spec imen was s imply an un- usual ly comple t e example of the r a t h e r c o m m o n spe-
cies f o u n d by us in the Ind ian Ocean (and wev ious ly in the N o r t h Pacif ic) and t ha t the lateral arms are n o t t a x o n o m i c a l l y i m p o r t a n t . The species might , there-- fore, be more p roper ly p laced in the genus Amphi- rhopalum. A few of our spec imens are similar ~o A. virchowii, suggest ing a re la t ionship be tween the two species.
Genus Trigonastrum Haeekel 1887
Trigonastrum sp. (Plate II, 6; Plate iV, 16 , ]7 ) . ?Trigonastrum regulare Haeckel , ] 887, p: 539, pl. 43, fig. 16; Dumi t r i ca , 1973, p. 835~ p l !0, figs. 1--4, pl. 11, figs. 1, 3, 5, 8.
Description: The spec imens e n c o u n t e r e d m the. p re sen t s tudy are s imilar in m a n y respects ~o those descr ibed by Dumi t r i c a (1973) . However , comple t e spec imens are very rare and usually on ly the cent ra l s t r uc tu r e and p rox ima l par ts of the arms (Plate IV, 17) were observed. No spec imen wi th a pa tag ium was ob- served, whereas Dumi t r i ca n o t e d a pa tag ium on a lmos t all of his Med i t e r r anean spec imens : The pres- ence or absence of a pa tag ium has previously been found (Nigrini, 1967) to be t axonomica l l y un impor - tan t , bu t it is in te res t ing in this case to no te tha t al- mos t all spec imens in one area have a pa tag ium while a lmos t all spec imens in a n o t h e r area do noL For the t ime be ing we have chosen to re ta in the generic n a m e Trigonastrum for the spec imens found in Indian Ocean sediments . However , a t a x o n o m i c revision of these forms would p r o b a b l y require t ha t Trigonastrum be made a j un io r s y n o n y m of Chitonastrum Haecket 1881. Dimensions: Radius of arms 65 - -145 ~rn. M a x i m u m bread th of arms 65 - -105 urn. Fo r fully deve loped spec imen radius is 160 urn, b read th 210 urn.
Genus Spongobrachium Haeckel 1881
Spongobrachium sp. (Plate II, 12; Plate V, 3)~ Description: Shell c o m p o s e d of a flat, lozenge-shaped spongy m e s h w o r k wi th a cen t ra l s t r uc tu r e of 2--.-5 concen t r i c spheres or obla te spheroids . Usually two " a r m s " of denser m e s h w o r k can be seen a long the long axis of the shell. The ex t r emi t i e s of these a rms are denser still. Of ten the pe r iphe ry of tho shell appears denser t h a n the cent ra l por t ion . Dimensions: Tota l shell l ength 2 5 0 - - 3 5 5 ~ m / u s u a l l y 250 - -305 urn) ; m a x i m u m b read th 120 - -265 u m (usual ly 120 - -185 urn). Remarks: No specific name has been given t~o this species because, as Riedel and Sanf i l ippo (1978 , pl. 73) have p o i n t e d out , the re are several poo r ly under- s t o o d forms of this general type which appa ren t ly have s t ra t ig raphic as well as geographic significance. The fo rm descr ibed here in appea r s to be the same as t h a t i l lus t ra ted by Renz (1974 , pl. 15, fig. 10) and
149
PLATE V (× 275)
1. Amphirhopalum cf. Tessarastum straussii Haeckel, A I I 15-735HC, X20/2. 2. Amphirhopalum cf. Tessarastrum straussii Haeckel, LSDA l19G, D33/1. 3. Spongobrachium sp., A I I 15-766HC, J27/3. 4. Pterocorys sabae (Ehrenberg), ANTP 142PG, 015/2. 5. Pterocorys sabae (Ehrenberg), LSDA 12G, M32/2.
150
called by her Spongobrachium sp. aff. S. ellipticum Haecket (1862, pl. 28, fig. 2).
FAMILY PTEROCORYTHIDAE Haeckel 1881 emend. Riedel 1967 emend. Moore 1972
Genus Pterocorys Haeckel 1881
Pterocorys sabae (Ehrenberg) (Plate IV, 2; Plate V, 4,5).
Pterocanium sabae Ehrenberg, 1972a, p. 319, 1872b, p. 299, pl. 10, fig. 17, Pterocorys sabae (Ehrenberg) Haeckel, 1887, p. 1317.
Description: Shell conical to ovate, quite smooth and rather thin-walled. Cephalis trilocular, the two secon- dary lobes beneath and somewhat lateral to the larger primary lobe; numerous subcircular pores; cephalis rather heavier than rest of shell. Stout three-bladed apical horn up to twice as long as cephalis. Collar stricture distinct.
Thorax basically conical, but shape strongly in- fluenced by three strong ribs which project as short wings about halfway along thoracic length. Pores sub- circular, longitudinally aligned. Pores immediately adjacent to cephalis often enlarged. Lumbar stricture distinct.
Abdomen broader than thorax, but with similar pores in more complete specimens. However, abdo- men usually rudimentary and may have irregular pore arrangement. Termination always ragged. In high latitudes abdomen rarely present. Dimensions: Length of cephalis and thorax 75--105 urn; of abdomen up to 127 urn, Maximum breadth of thorax 75--92 urn; of abdomen up to 127 urn. Remarks: This species differs from P. hertwigii by the absence of longitudinal ridges, and from P. zancleus by being generally larger with larger, more irregular pores and pore alignment.
A c k n o w l e d g m e n t s
Core samples fo r this s t u d y were p r o v i d e d t h r o u g h the ass is tance o f W. R iede l and T. Walsh (Sc r ipps I n s t i t u t i o n o f Oceanogra - p h y ) ; F. McCoy , D. C o o k e , and K. T h o m p s o n ( L a m o n t - D o h e r t y Geo log ica l O b s e r v a t o r y ) ; and J. Broda , H. F a r m e r and D. Ke i th (Woods Hole O c e a n o g r a p h i c I n s t i t u t i o n ) . Cura to r i a l services a t these sample r epos i t o r i e s are sup- p o r t e d t h rough c o n t r a c t s f rom the Off ice o f Naval Resea rch and grants f r o m the Na t iona l Sc ience F o u n d a t i o n ( S u b m a r i n e G e o l o g y and G e o p h y s i c s P rogram) . This p r o j e c t is sup- p o r t e d by N.S .F . G r a n t O C E 7 6 - 2 0 1 5 4 . We
t h a n k R. G r o m a n , T. Schu l t z , and A. Nigrini for ass is tance in m o d i f y i n g the R E G R O U P p r o g r a m fo r c o m p a t i b i l i t y wi th the Woods Hole Sigma-7 c o m p u t e r . T h e K e n n e c o t t Devel- o p m e n t Cen t e r p r o v i d e d faci l i t ies for p h o t o - m i c r o s c o p y . G. L o h m a n n , T. Moore , and B. Corliss c r i t i ca l ly r ev iewed the manusc r ip t . C o n t r i b u t i o n No. 4441 o f the Woods Hole O c e a n o g r a p h i c I n s t i t u t i o n .
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