Absolute ages of Quaternary radiolarian datum levels in the equatorial Pacific

QUATERNARY RESEARCH 5, 99-l 10 (1975)

Absolute Ages of Quaternary Radiolarian Datum Levels in the

Equatorial Pacific

DAVID A. JOHNSON’ AND ANDREW H. KNOLL’

Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543’ and Department of Geological Sciences, Harvard University

Cambridge, Massachusetts 02138* Received November 25,1974

Radiolarian assemblages were examined in two Quaternarye>($24-58; RCll- 209) from the tropical Padific Ocean. Eight radiolarian datum levels were identified in each core, and “absolute” ages were estimated for these levels by interpolation between paleomagnetic reversal boundaries previously established for the cores. The tropical radiolarian zonation for the Quaternary proposed by Nigrini (1971) appears to be most useful in terms of the reliability and ease of identification of the proposed zonal boundaries. Our estimated ages for the base of each of these zones are: Buccinosphaera invaginata Zone (Zone 1) : 210,000 yr BP; Collosphaera tuberosa Zone (Zone 2) : 370,000 yr BP; Amphirhopalum ypsilon Zone (Zone 3) : 940,000 yr BP;Anthocyrtidium angulare Zone (Zone 4) : 1,700,OOO yr BP.

A comparison of our age estimates with those of Quaternary radiolarian datum levels in cores from other regions suggests that significant diachroneity on a scale of up to several hundred thousand years may exist for some (and perhaps all) of these “events.” Diachroneity is most readily studied and documented in late Neogene cores where the absolute ages of the magnetic polarity reversals are known most precisely, but may also exist (though difficult to resolve) in earlier Cenozoic sediments. The existence of such diachroneity, if demonstrated through further studies, would have significant implica- tions for our understanding of evolutionary patterns of planktonic communities in different biogeographic regions.

INTRODUCTION

The value of fossil radiolarian assemblages has been demonstrated in numer- ous recent investigations of Quaternary paleoclimatology and biostratigraphy. Radio&a are of particular importance in the extensive oceanic regions below the carbonate compensation depth where siliceous microfossils provide the only available means for stratigraphic correlation. With the increasing interest in de- veloping models of oceanic and atmos- pheric circulation during the Quatemary (e.g., Gates, 1974), the use of siliceous microfossils will become increasingly im- portant in order that sample materials from widespread geographic areas can be used to form the necessary data base.

Paleoclimatic studies using Radiolaria (e.g., Hays, 1967; Nigrini, 1970; Keany

and Kennett, 1972; Moore, 1973; Sachs, 1973a, 1973b; Johnson and Knoll, 1974) have interpreted changing species assemblages within the Quatemary to be a re- flection of climatic changes and migrat- ing biogeographic provinces. The result- ing variability in the depositional record, expressed as a systematic variation in faunal composition with depth in a single core, carries great potential for reliable regional correlation. Unfortunately, climatic fluctuations in and of themselves are insufficient for uniquely establishing biostratigraphic control. An indepen- dent stratigraphic method, such as isotopic, paleomagnetic, or paleontological techniques, is required to determine an approximate “absolute” age for a given core interval before one can attempt more precise correlations within that

99 Copyright 0 1975 University of Washington. All rights of reproduction in any form reserved. Printed in the United States.

100 JOHNSON AND KNOLL

interval. This fact is perhaps best illustrated by analogy to a sine curve. Any given location on the curve is equivalent to all others with the same “phase,” un- less the location of the point relative to some origin can also be specified. Be- cause of the widespread occurrence of unconformities in shallow piston cores (e.g., Riedel, 1971), reliable correlation within Quaternary sediments requires the use of identifiable datum planes before precise correlations can be reliably attempted using climatic data.

Biostratigraphic studies (e.g., Nigrini, 1971; Hays, 1970; Kling, 1973; San- filippo and Riedel, 1974, Knoll and Johnson, in press) have documented radiolarian speciation during the Quater- nary, with certain datum levels existing as useful stratigraphic indicators for correlation purposes. Several previous at- tempts have been made to subdivide the Quaternary on the basis of these radiolarian datum levels. Zonations have been proposed for high latitudes (Hays, 1965; Hays and Opdyke, 1967) and middle latitudes (Hays, 1970; Kling, 1971, 1973), and age estimates for these zonal boundaries have been made by correlat- ing with the known paleomagnetic time scale for the late Neogene (e.g., Hays and Berggren, 1971). Nigrini (1971) has proposed a fourfold Quaternary zonation for tropical radiolarian assemblages; however, to date there have been no correlations proposed between Nigrini’s (1971) zonation and the other biostratigraphic and paleomagnetic zonations for the Quaternary. Some workers (e.g., Dinkle- man, 1973; Johnson, 1974) have had difficulty in applying Nigrini’s zonation scheme because of low abundance or poor preservation of the diagnostic taxa. This difficulty prompted us to undertake a thorough reexamination of well-preserved assemblages of tropical Quater- nary Radiolaria to identify and date all potentially useful stratigraphic datum levels.

We have investigated two cores from

the equatorial Pacific in which the radiolarian assemblages are very well preserved, and for which the magnetic stratigraphy has been previously established (Hays et al., 1969). Our principal objec- tive was to identify Nigrini’s (1971) zonal boundaries and other stratigraphically useful Late Neogene radio&an events (Sanfilippo and Riedel, 1974), and to estimate absolute ages of these events. We anticipate that many of these events may prove useful in subsequent studies for local and for regional correlation. Our results have also allowed us to make some preliminary observations con- cerning the presence and significance of diachroneity in Late Neogene sediments.

MATERIAL STUDIED

Two piston cores from the equatorial Pacific were selected for this study: V24- 58 (02” 16’N; 141”4O’W) and RCll-209 (03”39’N; 140”04’W). Each of these cores apparently contains a complete record of Quaternary sedimentation in the equatorial Pacific. Radiolarian assemblages throughout the cores are very well preserved (Johnson and Knoll, 1974), and the paleomagnetic stratigraphy of the cores has been determined (Hays et al., 1969). Because of the thick- ness of the Quaternary section in these cores and the reliable paleomagnetic age control, excellent resolution of identifiable events can be achieved.

For estimating absolute ages of radiolarian datum levels, we used as reference points three magnetic reversal boundaries: the base of the Brunhes, the base of the Jaramillo, and the base of the Olduvai. The assumed ages of these boundaries (after Opdyke, 1972) and their locations within the cores studied (after Hays et al., 1969) are summarized in Table 1 and in Fig. 1.

DATUM LEVELS

Eight Quaternary radiolarian datum levels were identified in the cores, includ- ing those events which define the zonal

101 QUATERNARY RADIOLARIANS IN THE PACIFIC

TABLE 1 Magnetic Stratigraphy used for Estimating Ages of

Radiolarian Datum Levels in Cores V24-58 and RCll-209

Locationb

Event Age (m.y.)’ V24-58 (cm) RCll-209 (cm)

Base of Brunhes 0.69 550 680 Base of Jaramillo 0.92 720 920 Base of Olduvai 1.86 1020 1410

aAfter Opdyke (1972). bAfter Hays et al. (1969).

boundaries proposed by Nigrini (1971). The locations of these levels within the cores, along with our estimated ages for these levels, are presented in Table 2. Be- low we discuss the reliability of each datum level, from youngest to oldest:

(1) Collosphaera sp. A - Buccinosphaera invaginata

Nigrini (1971) defined the base of the youngest Quaternary zone (Zone 1) by the first appearance of B. inuagina ta. The use of this species has heretofore been limited by the fact that the test is very fragile, and has been identified only in

EPOCH f&E7 DEPTH IN CORE (meters1

2 4 6 8 10 12 14

FIG. 1. Paleomagnetic stratigraphy of two cores (V24-58; RCll-209) which were used in this study, modified after Hays et al. (1969). Assumed ages of magnetic boundaries (after Opdyke, 1972), and exact locations of these boundaries in the two cores studied, are indicated in Table 1. Numbers in parentheses are average sedimentation rates, in mm/lo3 yr, for each of the core intervals indicated.

well-preserved assemblages. Recently Knoll and Johnson (in press) have identified the precursor of B. inuaginata, and have designated this form as Collosphaera sp. A. Recognition of this evolutionary event has made the Zone l/Zone 2 boundary considerably easier to identify.

The age of this evolutionary transition is estimated to be 210,000 yr BP, with an uncertainty of about 20,000 yr. Both Collosphaera sp. A and B. inuaginata have been identified only in tropical assemblages, and are relatively rare taxa.

(2) Last Occurrence of Axoprunum angelinum

This cosmopolitan species, more commonly known as Stylatractus universus, was previously recognized to be a stratigraphically useful form in high latitude regions (Hays, 1967; Hays and Opdyke, 1967). It is fairly common and easily identified in the Neogene, and appears to decrease in abundance toward the end of the Neogene. The last appearance of this species has generally been easy to recog- nize, and has been dated at approximately 400,000 yr BP in the Antarctic (Hays, 1967) and in the North Pacific (Hays, 1970). In the equatorial Pacific, however, the last appearance of this form may be significantly younger. Our investigation (see Table 2), as well as the previous study by Hays et al. (1969, Table 3), identified the extinction of Axoprunum angelinum (= Stylatractus sp.) in core V24-58 at a level of approximately 250 cm. This level would corre- spond to an age of 320,000 yr BP, as-


TABLE 2 Estimated Ages of Quaternary Radiolarian Datum Levels in Cores V24-58 and RCll-209.

Age Estimates are Based on Paleomagnetic Stratigraphy Summarized in Table 1

Depth (cm) Age (m.y.)

Event V24-58 RCll-209 V24-58 RCll-209

Collosphaera sp. A + B. invaginata

T Axoprunum angelinum B Collosphaera tuberosa B Collosphaem sp. A T Anthocyrtidium angulare T Lamprocyrtis neoheteroporos T. vetulum ----) T. tmchelium T Pterocanium prismatium

150-180 200-230 0.19-0.23 0.20-0.23 240-250 320-340 0.31-0.32 0.32-0.34 272-297 362-380 0.35-0.38 0.36-0.38 476-504 598-618 0.61-0.65 0.60-0.62 730-741 920-930 0.95-0.98 0.92-0.94 730-753 980-1015 0.95-1.02 1.03-1.10 882-928 1162-1200 1.42-1.57 1.39-1.46 960-990 1281-1321 1.68-1.76 1.62-1.70

suming a constant sedimentation rate during the Brunhes (see Fig. 1).

Since the average sedimentation rates apparently did change in both cores studied (Fig. l), we should consider the possibility that deposition rates within the Brunhes portions of the cores were also nonuniform. One interpretation of the average sedimentation rate data (see Fig. 1) would be .of an increasing rate during the Brunhes, since the average rate for the underlying Matuyama sediment was substantially lower. In this case, the age of the A. angelinum extinction would be younger than our 320,000 yr age estimate. Alternatively one could interpret the lower carbonate minima in the upper Brunhes intervals of the cores (Hays et al., 1969, Fig. 12) as indicative of de- creasing sedimentation rates, in which case the age of the A. angelinum extinction in these cores would be older than 320,000 yr, and probably indistinguish- able from the 400,000 yr age estimate

for the A. angelinum. extinction in higher latitudes. The issue of synchroneity vs diachroneity in the A. angelinum extinction can be resolved only with more precise means of core-to-core correlation, such as oxygen isotope techniques.

Some preliminary investigations of In- dian Ocean core material suggest to us that diachroneity in the A. angelinum extinction remains a likely possibility. In core CHAIN 100-26 (07”48’N; 56”12’E) we have identified the last appearance of A. angelinum well above the first occurrence of Buccinosphaera invaginata, in the upper third of Nigrini’s Zone 1. Con- sequently, it appears that diachroneity on the order of lo5 yr is required for one or both of these radiolarian events.

(3) First Appearance of Collosphaera tuberosa

Nigrini (1971) defined the second youngest Quaternary zone (Zone 2) by the first appearance of this species,

TABLE 3 Location and Age of Radiolarian Events at DSDP Site 173

Event Location, Site 173’ Depth (m) Age (m.y.)b

T Axoprunum angelinum 2-212-3 7-9 0.40 T Lamprocyrtis neoheteroporos 2-~~13-1 15-16 0.54 L. neoheteroporos -+ L. haysi 4-l/4-3 25-29 0.76 T Eucyrtidium matuyamai 4-cc/5-1 34-35 0.90

‘After Kling (1973, Tables 3 and 9). Numbers designate the core and section numbers between which each event occurs.

bWe have used the ages given by Kling (1973) for the top and bottom of the Axoprunum angelinum Zone of 0.4 and 0.9 m.y., respectively. We then assume a constant sedimentation rate within the A. angelinum Zone to date the intermediate events.

QUATERNARY RADIOLARIANS IN THE PACIFIC 103

herein dated as approximately 370,000 yr BP f - 10,000 yr. Collosphaera tuberosa is larger, more robust, and more common than either Buccinosphaera invaginata or Collosphaera sp. A; hence, its first appearance is relatively easy to rec- ognize. To date this species has been identified only in tropical assemblages.

(4) First Appearance of Collosphaera sp. A

This taxon, which Knoll and Johnson (in press) have identified as the ancestor of Buccinosphaera invaginata, entends substantially below the base of Nigrini’s (1971) Zone 2, and appears to have evolved from a smooth-shelled collosphaerid by the increasing development of outward protuberances from the test surface. The first appearance of Collo- sphaera sp. A is here estimated to occur at around 610,000 yr BP (Table 2). However, this event is not as easily recognized as the others discussed in this section because Collosphaera sp. A was present only in rare amounts (one to four individuals on a strewn slide) in the intervals immediately above its first appearance in the two cores examined. Further work is required to identify more precisely the ancestral form of Collosphaera sp. A and the age of the evolutionary transition, using core material containing better preserved assemblages.

(5) Last Occurrence of Anthocyrtidium angulare

This species is rare to common in early Pleistocene sediments in tropical latitudes. Iti last appearance, here dated as approximately 940,000 yr BP, defines the base of Nigrini’s (1971) Zone 3. This datum level is easily recognized because of the common occurrence of A. angulare below the level, and because of the apparently abrupt extinction of this species. To date this form has not been identified in middle and high latitudes.

(6) Last Occurrence of Lamprocyrtis neoheteroporos

This species was first described by Kling (1973) in sediments from DSDP Sites 173 and 175 in the North Pacific, and was believed to be part of an evolutionary lineage beginning with Lumpro- cyrtis he teroporos and terminating with Lamprocyrtis haysi, the extant form. Sanfilippo and Riedel (1974) subse- quently identified this lineage in the tropical Indian and Pacific oceans, and Hays (1965) identified L. heteroporos in high southern latitudes, Consequently it appears that some, and perhaps all, of the species within this late Neogene Lamprocyrtis lineage are cosmopolitan in distribution.

We initially sought to identify the levels in cores V24-58 and RCll-209 where evolutionary transitions occur within the Lamprocyrtis lineage. How- ever, we found that the ancestral and descendant morphotypes commonly had long overlapping ranges, and that morphotypes of a transitional nature or of uncertain relation to the principal lineage were uncomfortably common. Con- sequently, we were able to reliably identify only the last appearance of the morphotype of L. neoheteroporos, which we interpret to occur at approximately 1.03 m.y. BP (Table 2).

When our data are compared with those of Kling (1973) from the North Pacific, it appears that the extinction of L. neoheteroporos is significantly dia- chronous between low and high latitudes. Kling (1973, Fig. 1; Table 2A) indicates that at Site 173 the evolutionary transition from L. neoheteroporos to L. haysi occurs within the A. angelinum Zone, and that the last morphotype of L. neoheteroporos occurs in the upper part of this zone. We have estimated the ages of the morphotypic and evolutionary upper limits of L. neoheteroporos using Kling’s (1973) data from site 173; our estimates for the ages of these events in the North Pacific are 0.54 and 0.76 m:y.,


respectively (see Table 3). In the tropical Pacific, however, we have identified the last morphotype of L. neoheteroporos well below the base of the Jaramillo at around 1.03 m.y. BP (Table 2). Thus, a diachroneity of several hundred thousand years in the extinction of L. neo- he teroporos is suggested.

Substantially more work will be required on the Late Neogene genus Lam- procyrtis before the species within this genus can be used for precise stratigraphic correlations.

(7) Theocorythium vetulum+ Theocorythium trachelium

Nigrini (1971) first described the species T. vet&urn in Late Pliocene and Early Pleistocene material from the tropical Pacific, and suggested that this species was the ancestor of the common Pleistocene form T. tmchelium. Al- though we can tentatively identify the evolutionary transition from T. vet&urn to T. trachelium in cores V24-58 and RCll-209 (Table 2), we have some doubts that this event will prove easy to identify for purposes of correlation between cores.

Theocorythium trachelium has been reported to extend substantially below the Pliocene-Pleistocene boundary in both the tropical Pacific (Dinkelman, 1973) and tropical Indian Ocean (John- son, 1974). Nigrini (1971) found that in several cores both T. vetulum and T. trachelium occur throughout Zone 4. In other cores studied, Nigrini (1971) found no specimens of T. vetulum in either the early Pleistocene or late Plio- cene intervals. Consequently, it appears that the relatively low abundance of T. uetulum may inhibit the reliable identification of the evolutionary transition to its descendant form.

(8) Last Occurrence of Pterocanium prismatium

This extinction has long been recognized as a useful stratigraphic indicator.

It occurs near the top of the Olduvai magnetic event and is here dated as approximately 1.70 m.y. BP. Go11 (1972a) has suggested that this event is diachron- ous in the equatorial Pacific; verification of such diachroneity will require more extensive biostratigraphic and paleomagnetic investigations.

DISCUSSION

There are three principal sources of uncertainty in estimating absolute ages for the datum levels which we have discussed: (a) the reliability of the ages assigned to the paleomagnetic time scale; (b) possible effects of postdepositional reworking; (c) the validity of the assumption of constant sedimentation rates for given core intervals. The first of these factors is not likely to be a major source of error. The paleomagnetic time scale for the Late Neogene has been refined considerably during the past decade (Watkins, 1972; Opdyke, 1972), and the Quaternary portion in particular has been extensively documented in a large number of cores and terrestrial samples. Con- sequently, the estimated ages of the Quaternary polarity reversals are likely to be in error by no more than a few percent (C. Denham, personal communi- cation).

Postdepositional reworking can fie- quently cause the blurring of biostratigraphic boundaries, particularly extinctions; one should therefore evaluate the possible influence of reworking in any material studied. We examined radiolarian and nannofossil assemblages at lo- to 30-cm intervals in both cores studied, and found no evidence for con- tamination of any of the assemblages by Tertiary species. Consequently we feel that reworking processes were not significant in blurring any of the datum levels which were used in this study.

The assumption of constant sedimentation rates between identified polarity reversals is far more uncertain. Episodic depositional processes and erosional


events have been well documented in deep ocean sediments; therefore, criteria are needed for evaluating the extent to which either of these processes has af- fected a given sedimentary unit to a de- gree which would yield a nonuniform average accumulation rate. In the two cores studied for this report, additional data indicate that the assumption of uniform deposition rates may indeed be justified for the intervals considered. The close similarity between the two cores in their carbonate content (Hays et al., 1969) and in their radiolarian paleoclimatic indices (Johnson and Knoll, 1974) suggests that there are no missing intervals on the order of 10’ yr or longer. The paleoclimatic data for cores V24-58 and RCll-209 also show a striking periodicity on the order of 90,000 yr during the Brunhes (Hays et al., 1969; Johnson and Knoll, 1974). This periodicity, and the close core-to-core corre- spondence, suggests either uniform deposition rates during the Brunhes, or rates which varied similarly in the two cores. In the upper Matuyama epoch there is good core-to-core agreement in the carbonate content (Hays et al., 1969, Fig. 12), but the periodicity of the carbonate data is less obvious than in the overlying Brunhes material. Conse- quently the assumption of uniform sedimentation below the Brunhes in the two cores studied is more open to question.

A more precise method of identifying and quantifying nonuniform sedimentation rates during the Brunhes has been proposed by Shackleton and Opdyke (1973). They argue persuasively that isotopic changes in the ocean should occur essentially synchronously, and have proposed a chronolo,q for oxygen isotope stages 1 through 22 by assuming a constant sedimentation rate in core V28- 238 (Shackleton and Opdyke, 1973, Fig. 9, Table 3). If one then assumes a one- to-one relationship between the oxygen isotope stages and the Pacific carbonate variations of Hays et al. (1969), one can

then estimate an age for Brunhes sediments within which the carbonate cycles have been identified. For example, the A. angelinum extinction in cores V24- 58 and RCll-209 occurs approximately at the B9/BlO carbonate transition of Hays et al. (1969). If the B9/BlO carbonate transition is equivalent to the stage ll/stage 12 transition on the oxygen isotope curve, then the age of the A. angelinum extinction in the two cores studied would be around 440,000 yr BP. This age is virtually indistinguish- able from the 400,000 yr BP age estimate for the A. angelinum extinction in higher latitudes. One would therefore interpret synchroneity in the A. ange- hum extinction by assuming nonuniform Brunhes sedimentation rates in V24-58 and RCIl-209. Clearly the problem of synchroneity vs diachroneity in microfossil datum levels can be resolved only with the use of a precise correlation technique, such as oxygen isotopic analysis, in all material studied.

Of the eight datum levels which we have discussed in this report (Fig. 2), six

+ T Anthocyrtidium an(lulare - T Lomprocyrtis neoheteroporos

- Theocorythium vetulum + trachalium T .

+ T Pterocanium prismatium ___~

FIG. 2. Estimated absolute ages of Quater- nary radiolarian zonal boundaries, and other identifiable radiolarian events in the two cores studied. Uncertainties associated with dating these events are discussed in the text.


appear to be sufficiently reliable and easily recognizable to be presently useful as stratigraphic indicators, and two should be regarded as tentative and re- quiring further work. The evolution of the form which we have designated Collosphaera sp. A needs to be documented more precisely, and further work is also required in establishing the ranges of the morphotypes of Theocorythium vetulum and T. trachelium. During this study we attempted without success to identify and use the Tholospyris taxa which Go11 (1972b) recognized as useful Late Neogene stratigraphic indicators. We were able to identify in several samples many of the extant taxa recently investigated by Renz (1973), but were unsuccessful in attempting to establish stratigraphic ranges for any of these forms.

The Quatemary radio&an zonation of Nigrini (1971) appears to have utilized the most reliable and easily recognized stratigraphic indicators, and consequently we see no need to recommend any changes or further subdivisions at this time. Two principal disadvantages re- main in the application of this zonation. One is that Zone 3 is defined totally on the basis of negative evidence, with a first appearance defining the top of the zone and a last occurrence defining the base of the zone. The second is that Zone 4 is nearly as long as the remaining zones combined, and consequently a further subdivision of Zone 4 would be desirable. Both of these difficulties can perhaps be overcome through a more thorough investigation of the Lampro- cyrtis lineage to identify more precisely the morphotypic and evolutionary limits of the various taxa.

It would be inappropriate to end this discussion without at least briefly men- tioning the subject of diachroneity in biostratigraphic correlation. No microfossil “event,” be it extinction, evolutionary transition, or morphologic first appearance, occurs simultaneously

throughout the entirety of a species’ geographic range. Thus, it would seem that radio&an biostratigraphy is built upon a shaky framework of inherently dia- chronous phenomena. Fortunately, most datum levels may be considered iso- chronous in a practical sense because the time differences involved are in many cases too small to be resolvable in the sedimentary record. In the Late Neo- gene, however, our greater chronological resolution demands that we consider diachroneity, its causes, and its implica- tions.

Two causes for time-transgressive biostratigraphic events seem likely. One factor is the climatic deterioration and subsequent glacial/interglacial climatic oscillations during the Late Neogene. Extinction and migration due to changing environmental conditions would be time-transgressive along the oceanic climatic gradient. Such diachroniety has been well documented in North Atlantic foraminiferal assemblages (Ruddiman and McIntyre, 1973). Also, within a given latitudinal zone, local populations might persist in some areas after members of a species had vanished in other areas (Blow, 1970). This is apparently the case with A. angelinum which can be found in equatorial Indian Ocean sediments containing I?. inuaginata, a species which did not evolve until after A. angelinum had become extinct in equatorial Pacific waters. The second causa- tive factor involved in diachroniety is migration of adaptive mutations through oceans. The time required for a newly evolved species to migrate from its place of origin to another favorable region is dependent on both biological factors such as Darwinian fitness (Dobzhansky, 1970, p. 101) and on physical parameters, principally the motion of ocean cur- rents.

Synchroneity, or the lack thereof, can be investigated via several lines of evidence. One is a comparison of absolute age estimates based on interpolation


from paleomagnetic data. A legitimate argument against this approach is that sedimentation rates during a given polarity interval may have varied in one core relative to another. A second, more de- finitive line of reasoning is to compare the sequence of useful levels among different cores (Sanfilippo and Riedel, 1974). Thus the fact that the extinction of A. angelinum is stratigraphically below the evolution of B. inuaginata in the Pacific Ocean but above it in the Indian Ocean demands that at least one of these events be time-transgressive. Sanfilippo and Riedel (1974, Table 12, Fig. 2) have illustrated graphically a number of other sequence differences between Late Ceno- zoic radiolarian events in Indian Ocean and Pacific Ocean cores.

Recognition of diachroneity in Quater- nary microfossil datum levels has impli- cations for both radiolarian biostratigraphy and evolutionary biology. Care must be exercised in the use of fauna1 events for precise long-range correlation in the Late Neogene. In such correlations, maximum advantage should be taken of paleomagnetic stratigraphy, of sequences of events, and of climatically induced variations in cores recognizable as changes in carbonate content, isotopic composition, and fauna1 composition.

Knowledge of the rates of migration of evolutionary transitions will add substantially to a growing understanding of plankton evolution. Detailed stratigraphic investigations of many well-dated Late Neogene cores, coupled with infor- mation on the movement of ocean cur- rents, will provide a picture of the rate of migration of phenotypic adaptations through the oceans. Such studies are less practical in plankton groups for which relatively few Late Neogene extinctions and speciations have been recognized (e.g., the planktonic Foraminifera). Nev- ertheless, advances in the evolutionary biology of the Radiolaria will be applica- able to the larger problem of oceanic plankton evolution as a whole.

SPECIES LIST

The following is an abbreviated systematics section, listing all radiolarian species which have been used in this report for defining datum levels. For each species, two or three recent references are given which contain appropriate species descriptions and illustrations, thereby enabling the reader to determine the criteria which we have applied in identifying the taxa used in this study.

Anthocyrtidium angulare Nigrini (Plate 1, Fig. 3) Anthocyrtidium angulare Nigrini, 1971,

p. 445, pl. 34.1, Figs. 3a, 3b; Dinkel- man, 1973, pl. 10, Fig. 5.

Axoprunum angelinum (Campbell and Clark) (Plate 1, Fig. 5) Stylatractus uniuersus Hays, 1970, p.

215, pl. 1, Figs. 1, 2; Kling, 1971, p. 1086, pl. 1, Fig. 7; Dinkelman, 1973, pl. 10, Figs. 6,7.

Axoprunum angelinum (Campbell and Clark); Kling, 1973, p. 634, pl. 1, Figs. 13-16; pl. 6, Figs. 14-18.

Buccinosphaera invagina ta Haeckel (Plate 1, Fig. 2) Buccinosphaera invaginata Haeckel,

1887, p. 99, pl. 5, Fig. 11; Nigrini, 1971, p. 445, pl. 34.1, Fig. 2; Dinkel- man, 1973, pl. 10, Fig. 3; Knoll and Johnson, in press, pl. 1, Figs. 3-6.

Collosphaera tuberosa Haeckel (Plate 1, Fig. 4) Collosphaera tuberosa Haeckel, 1887,

p. 97; Nigrini, 1971, p. 445, pl. 34.1, Fig. 1; Dinkelman, 1973, pl. 10, Figs. 1, 2; Knoll and Johnson, in press, pl. 2, Figs. l-3.

Collosphaera sp. A. (Plate 1, Fig. 1) Collosphaera (?), Riedel and Sanfilippo,

1971, pl. lA, Fig. 1. Collosphaera irregularis Haeckel; Knoll

and Johnson, 1973, p. 11, pl. 1, Fig. 1; pl. 2, Figs. 4-7.

Collosphaeru sp. A, Knoll and Johnson, in press, pl. 1, Figs. 1, 2; pl. 2, Figs. 4-6.



Lamprocyrtis neoheteroporos Kling (Plate 1, Fig. 6) Lamprocyrtis neoheteroporos Kling,

1973, p. 639, pl. 5, Figs. 17, 19; pl. 15, Figs. 4, 5; Sanfilippo and Riedel, 1974, pl. 3, Fig. 11.

Pterocanium prismatium Riedel (Plate 1, Fig. 9) Pterocanium prismatium Riedel, 1957,

p. 87, pl. 3, Figs. 4, 5; Riedel and Sanfilippo, 1970, p. 529; Riedel and Sanfilippo, 1971, pl. 8, Fig. 1; Nigrini, 1971, pl. 34.1, Fig. 4.

Theocorythium trachelium (Ehrenberg) (Plate 1, Fig. 7) Theocory thium trachelium (Ehren-

berg); Nigrini, 1967, p. 79, pl. 8, Fig. 2, pl. 9, Fig. 2; Nigrini, 1971, pl. 34.1, Fig. 5; Dinkelman, 1973, pl. 10, Fig. 9.

Theocorythium uetulum Nigrini (Plate 1, Fig. 8) Theocorythium vetulum Nigrini, 1971,

p. 447, pl. 34.1, Fig. 6a, 6b; Dinkel- man, 1973, pl. 10, Figs. 11,12; San- filippo and Riedel, 1974, pl. 4, Figs. 6, 7.

ACKNOWLEDGMENTS

We thank Mr. Roy Capo of Lamont-Doherty Geological Observatory for enabling us to sample the cores used in this study. The Lamont core laboratory is supported under ONR Con- tract N00014-16-A-0108-0004 and NSF Grant GA-29460. We thank W. Riedel, C. Nigrini, J. Hays, and N. Shackleton for profitable dis- cussions during the course of this investigation. The manuscript was critically reviewed by G. Lohmann and B. Haq. This research was supported under NSF Grant GA-36825. Knoll is currently supported under an NSF Graduate Fellowship at Harvard University. This is con- tribution No. 3466 of the Woods Hole Oceano- graphic Institution.

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Absolute ages of Quaternary radiolarian datum levels in the equatorial Pacific

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