QUATERNARY RESEARCH 4, 206216 (1974) Radiolaria as Paleoclimatic Indicators: Pleistocene Climatic Fluctuations in the Equatorial Pacific Ocean DAVID A. JOHNSON~ AND ANDREW H. KNOLLS Received January 28, 1974 Late Pleistocene radiolarian assemblages were examined in two cores (V24-58 and RCll-209) from the eastern equatorial Pacific Ocean, using Nigrini’s (1970) method of recurrent group analysis. The radiolarian “climatic index” curves obtained by this analysis were in close agreement with fluctuations in carbonate levels established previously for the same cores. This correspondence suggests that equatorial radiolarian assemblages changed significantly in response to Pleistocene climatic oscillations. The radiolarian “climatic indices” of the two cores showed close agreement with each other. Our data indicate that equatorial radiolarian assemblages may prove useful for regional correlations between cores, and for paleoclimatic interpretations. INTRODUCTION It has long been recognized that assem- blages of microfossils in Quaternary sedi- ments may reflect the series of glacial/in- terglacial climatic cycles which have oc- curred during the past few million years. Particular attention has been devoted to paleoclimatological studies using calcareous microfossils: foraminifera and nannofos- sils. Early investigators (e.g., Phleger et al., 1953) examined foraminiferal assemblages in Pleistocene cores from the North Atlan- tic and determined that biogeographic fau- nal provinces migrated in response to changing climatic conditions. Subsequent investigators have successfully utilized Quaternary foraminiferal assemblages for biostratigraphic correlations (e.g., Ericson et al., 1961; Ericson and Wollin, 1968)) paleooceanographic interpretations (e.g., Berger, 1968; Ruddiman et al., 1969; Rud- diman, 1971)) and quantitative approaches ’ Woods Hole Oceanographic Institution, Woods Holes, Massachusetts 02543. * Department of Geological Sciences, Harvard University, Cambridge, Massachusetts 02138. to Quaternary paleoclimatology (e.g., Emiliani, 1970; Imbrie and Kipp, 1971). Quaternary nannofossil sequences have also been used successfully for biostratigraphic correlations and paleoclimatic interpreta- tions (e.g., McIntyre et al., 1970; McIntyre et al., 1972). By contrast relatively little attention has been devoted to the biogeography and paleobiogeography of siliceous microfossil groups-diatoms, radiolarians, and sili- coflagellates. Mandra (1969)) Mandra and Mandra (1970)) and Jendrzejewski and Zarillo (1972) have made some progress using silicoflagellates; Jouse et al. (1963)) Kanaya and Koizumi (1966), Donahue (1970), and Burckle (1972) have utilized diatoms ; and Casey (1971)) Nigrini (1967, 1970, 1971), Sachs 1973a, 1973b), and Moore (1973) have examined the potential use of radiolaria as paleoclimatic indicators. The relative lack of information on the biogeography of siliceous organisms is un- fortunate, inasmuch as a large fraction (-60%) of the sea floor lies below 4000 m (Menard and Smith, 1966), which may be assumed to be the average depth of cal- Copyright o 1974 by University of Washington. P dll rigbta o reproduction in any form reserved.
Transcript
QUATERNARY RESEARCH 4, 206216 (1974)
Radiolaria as Paleoclimatic Indicators: Pleistocene Climatic Fluctuations in the Equatorial Pacific Ocean
DAVID A. JOHNSON~ AND ANDREW H. KNOLLS
Received January 28, 1974
Late Pleistocene radiolarian assemblages were examined in two cores (V24-58 and RCll-209) from the eastern equatorial Pacific Ocean, using Nigrini’s (1970) method of recurrent group analysis. The radiolarian “climatic index” curves obtained by this analysis were in close agreement with fluctuations in carbonate levels established previously for the same cores. This correspondence suggests that equatorial radiolarian assemblages changed significantly in response to Pleistocene climatic oscillations. The radiolarian “climatic indices” of the two cores showed close agreement with each other. Our data indicate that equatorial radiolarian assemblages may prove useful for regional correlations between cores, and for paleoclimatic interpretations.
INTRODUCTION
It has long been recognized that assem- blages of microfossils in Quaternary sedi- ments may reflect the series of glacial/in- terglacial climatic cycles which have oc- curred during the past few million years. Particular attention has been devoted to paleoclimatological studies using calcareous microfossils: foraminifera and nannofos- sils. Early investigators (e.g., Phleger et al., 1953) examined foraminiferal assemblages in Pleistocene cores from the North Atlan- tic and determined that biogeographic fau- nal provinces migrated in response to changing climatic conditions. Subsequent investigators have successfully utilized Quaternary foraminiferal assemblages for biostratigraphic correlations (e.g., Ericson et al., 1961; Ericson and Wollin, 1968)) paleooceanographic interpretations (e.g., Berger, 1968; Ruddiman et al., 1969; Rud- diman, 1971)) and quantitative approaches
* Department of Geological Sciences, Harvard University, Cambridge, Massachusetts 02138.
to Quaternary paleoclimatology (e.g., Emiliani, 1970; Imbrie and Kipp, 1971). Quaternary nannofossil sequences have also been used successfully for biostratigraphic correlations and paleoclimatic interpreta- tions (e.g., McIntyre et al., 1970; McIntyre et al., 1972).
By contrast relatively little attention has been devoted to the biogeography and paleobiogeography of siliceous microfossil groups-diatoms, radiolarians, and sili- coflagellates. Mandra (1969)) Mandra and Mandra (1970)) and Jendrzejewski and Zarillo (1972) have made some progress using silicoflagellates; Jouse et al. (1963)) Kanaya and Koizumi (1966), Donahue (1970), and Burckle (1972) have utilized diatoms ; and Casey (1971)) Nigrini (1967, 1970, 1971), Sachs 1973a, 1973b), and Moore (1973) have examined the potential use of radiolaria as paleoclimatic indicators.
The relative lack of information on the biogeography of siliceous organisms is un- fortunate, inasmuch as a large fraction (-60%) of the sea floor lies below 4000 m (Menard and Smith, 1966), which may be assumed to be the average depth of cal-
Copyright o 1974 by University of Washington. P dll rigbta o reproduction in any form reserved.
PLEISTOCENE RADIOLARIA 207
cite compensation (J. Edmond, personal communication). Consequently, paleocli- matic analyses using the more familiar calcareous microfossil groups are impossi- ble in these areas. With the recent increased interest in Late Cenozoic paleoclimatology (e.g., I.D.O.E.‘s CLIMAP Project), numer- ous techniques for evaluating Pleistocene glacial history will be required. The devel- opment of methods for detailed paleocli- matologic analysis of siliceous deposits will be critical for the analysis of sediments from deeper regions.
Radiolarian assemblages now appear to be promising as reliable paleoclimatic indi- cators. Nigrini (1971) investigated SDSE cores 61 and 62 from the east-central Pacific, and attempted to correlate varia- tions in the abundance of characteristic low latitude species with fluctuations in carbon- ate content as determined by Arrhenius (1952). She concluded that, at least for the cores examined, the radiolarian fauna did not reflect Pleistocene climatic oscillations. In a later study, Nigrini (1970) applied recurrent group analysis (Fager, 1957) to radiolarian assemblages from 68 surface samples taken from cores located through- out the North Pacific Ocean. She was able to construct three recurrent groups roughly corresponding to “warm,” “transitional,” and LLcold” water. Then, following a method developed by Kanaya and Koizumi (1966)) she defined a “radiolarian temperature” number, and using samples from core V20- 130 (35O.57 N, 152O36’ E), she was able to construct a paleotemperature curve which showed good agreement with a simi- lar curve determined from diatom assem- blages for the same core (Kanaya and Koizumi, 1966).
The success of Nigrini’s (1970) investi- gation prompted the present authors to re- examine equatorial radiolarian assemblages using recurrent-group analysis, and to ask two basic questions :
1. Did equatorial radiolarian assem- blages fluctuate in response to Pleistocene glacial episodes?
2. Is recurrent group analysis of radio- larian assemblages a useful paleoclimatic tool? How reliable is the method for ob- taining correlations over wide geographic areas?
Cores from equatorial latitudes contain well-preserved calcareous and siliceous microfossils; such materials therefore pro- vide an opportunity to compare radiolarian paleoclimat’ic data with data obtained from calcareous microfossils in the same cores.
MATERIALS STIJDIED
Two cores from the east-central Pacific Ocean (V24-58 at 02O16’N, 141O4O’W; and RCll-209 at 03O3YN, 14OOO4’ W) were chosen for this study. These cores were examined extensively by previous in- vestigators (Hays et al., 1969) for their paleomagnetic, biostratigraphic, and tli- matic record. The sediment types prewnt are calcareous and calcareous-siliceous oozes with common and well-preserrecl radiolaria. The Quaternary sequence is quite thick (>lO m) in each core; there are no apparent discontinuities in sedi.- mentation, and virtually no evidence of re- working. The calcium carbonate content in these cores has been determined by Hays et al. (1969)) and shows approximately 13 oscillations during the past 2 million yr. Arrhenius (1952) argued persuasively that high carbonate levels in sediments of the equatorial Pacific correspond to glacial cpi- sodes, and this interpretation is reinforced by the data of Hays et al. (1969). IJnder this assumption, climatically controlled variations in fauna1 assemblages in the cores should correspond to variations in the car- bonate content. We undertook this investi- gation to determine if such variations could be detected in the radiolarian assemblages.
METHODS
Each of the two cores was sampled at 10 cm intervals from the top to the B9 carbon- ate minimum of Hays et al. (1969). Strewn slides of radiolaria were prepared following standard procedures described by Riedel
20s JOHNSON AND KNOLL
PLATE 1
PLEISTOCENE RADIOLARIA “09
TABLE 1
RECURRENT GROUP MEMBERS CH.~RACTF;RISTIC OF Low LATITUDE (“WARMS’) R~DIOL~RI~N ASSEMBLAGES, AFTER NIGRINI (1967, 1!)70)a
a Those species marked by an asterisk (*) were not counted because their geographic range extends beyond equatorial latitudes. Collosphaera tuberosa was not counted because of its limited stratigraphic range in the Pleistocene. For each taxon a reference is given to a recently published description or ilhrstra- tion of the species.
(1957) and Riedel and Sanfilippo (in used in this study. For a discussion of re- press). current group analysis and the procedures
Tables 1 through 3 list the recurrent used in assigning species of radiolaria to re- group radiolarian assemblages established current groups, the reader is referred to the by Nigrini (1970) ; this same grouping was work of Fager (1957) and Nigrini ( 1970).
EXPLANATION OF PLATES. These plates have been designed to supplement Nigrini’s (1967, 1970) illustrations of the same species. Only those species found in the cores studied are illustrated here. Specimens illustrated are located according to core number, dept,h in core, slide number, and England Finder coordinates (slide label to viewer’s right).
PLATE 1. (All figures x 80.) 1-Polysobnia spinosa (Haeckel), V24-58, 30 cm, sl. A, W34/3; 2-Polysoleniu Zuppacea (Haeckel), RCll-209, 70 cm, sl. A, M44/0; 3-Polysoleniu arktios Nigrini, V24-58, 90 cm, sl. A, V31/2; A--Siphonosphaera polysiphonia Haeckel, RCll- 209, 110 cm, sl. A, B58/2; 5-Actinomma arcadophorum Haeckel, V24-58, 159 cm, sl. A, V46/0; 6-Actinomma medianum Nigrini, V24-58, 426 cm, sl. B, C52/3; 7-Spongnster tetrus tetras Ehrenberg, V24-58, 504 cm, sl. A, T43/0; 8-Ommatartus tetrathabzmus tetrathulamns Haeckel, RCll-209, 50 cm, sl. A, 035/O; 9-Ommaturtus tetruthalumus tetrathntamus Haeckel, RCll-209, 20 cm, sl. A, F49/0; lO--Ommaturtus tetruthulumus coronatus Haeckel, RCll-209, 20 cm, sl. A, M57/0; 11-Euchitonia furcata (Ehrenberg), RCll-209, 50 cm, sl. .4, U41,/0; 12--Euchitonicl elegans (Ehrenberg), V24-58, 504 cm, sl. A, U58/1.
210 JOHNSON AND KNOLL
PLATE 2
2 3 4
6 7
11 12
13 l4 I
PLEISTOCENE RADIOLARIA 211
TABLE 2 TABLE 3
RECURRENT GROUP MEMBERS CHARACTERISTIC OF MIDLATITUDE (“TRANSITIONAL") R~DIOLARIAN ASSEMBLAGES, AFTER
NIGRINI (1967, 1970)a
I~ECURRENT GROUP MEMBERS CHAR.~CTRRISTIC OF HIGH-LSTITUDE ("COLD") RADIOLARL4N ASSEMBLAGES, AFTER NIGRINI (1967, 1970)a
Lithocampe sp. *Heteracantha dentata Mast N&K 1970 tion. Counts from different slides prepared
from the same sample were also quite a Those species marked by an asterisk (*) were consistent.
not found in the two cores studied. For each taxon a reference is given to a recently published descrip-
Nigrini (1970) defined a radiolarian
tion or illustration of the species. temperature index using the following equation :
In each sample examined, approximately 2-300 specimens belonging to the recurrent
TR = x, + g + X,) x loo,
groups were counted. Results of these counts are shown in Tables 4 and 5. Slide identification labels were covered and the slides for counting were selected at random to avoid unconscious biases introduced by the knowledge of the depth of each sample. A number of counts were repeated to test the reliability and reproducibility of the re- sults. The percentages of “warm” group specimens found in these recounts were al- ways within 1.5% of the initial determina-
where X, = number of “warm water” specimens; X, = number of “transitional water” specimens; and X, = number of “cold water” specimens. We here use the term “radiolarian climatic index” defined by the same equation, in order to avoid misleading connotations of the term “radiolarian temperature.” Al- though the calculated climatic index Tn is probably not related to sea-surface temper-
PLATE 2. (All figures x 80.) 1-Amphirhopalum ypsilon Haeckel, V24-58, 531 cm, sl. B, &57/4; 2-Ampirhopalum ypsilon Haeckel, V2458, 30 cm, sl. A, L32/3; 3-Pterocanium praetextum (Ehrenberg) eucolpum Haeckel, RCll-209, 250 cm, sl. B, V26/2; 4-Pterocanium praetextum praetextum (Ehrenberg), RCll-209, 20 cm, sl. A, K40/0; 5-Eucyrtidium hexa- gonatum Haeckel, V24-58, 627 cm, sl. A, P54/4; 6-Eucyrtidium acuminatum (Ehrenberg), RCll-209, 20 cm, sl. A, G49/3; 7-Lithocampe sp., V24-58, 210 cm, sl. A, 033/O; P-Ptero- canium trilobum (Haeckel), V24-58, 730 cm, sl. A, K27/0; 9-Lamprocyclas maritalis maritalis Haeckel, V24-58, 230 cm, sl. A, V58/4; lO--Lamprocyclas maritalis Haeckel polypora Nigrini, RCll-209, 20 cm, sl. A, G31/0; 11-Anthocyrtidium ophirense (Ehrenberg), V24-58, 30 cm, sl. A, Y45/2; 1%Anthocyrtidium ophirense (Ehrenberg), V24-58, 531 cm, sl. B, D51/2; 13-Botryocyrtis scutum (Harting), RCll-209, 210 cm, sl. A, M51/0; 14--Siphocampe corbula (Harting), RCll-209, 210 cm, sl. A, A39/2; 15--Theocorythium trachelium trachelium (Ehren- berg), V24-58, 777 cm, sl. A, 047/3; 16Theocorythium trachelium trachelium (Ehrenberg), V24-58, 627 cm, sl. A, P54/4.
212 JOHNSON AND KNOLL
TABLE 4
ABUNDANCE OF "WARM" AND “COLD" SPECIES IN CORE V2P58
ature in a simple way, it does present a reli- able indication of climatically induced changes in the radiolarian community. In addition, it may give an indication of the magnitude of the actual temperature fluc-
tuations. For example, Nigrini’s (1970) curve for the North Pacific core V20-130 showed a fourfold greater amplitude be- tween “temperature” extremes than did the equatorial cores utilized in the present study. This difference may be expected in view of the latitudinal difference between the cores.
The statistical significance of the calcu- lated “radiolarian temperature” fluctua- tions may be determined in the following way. If N specimens are counted such that Y% are classified in one group (warm water) and the remainder, (100 - Y) $% are in another group (cold water), then the accuracy of estimating Y, at the 95% confi-
dence level, is given by the following equa- tion (Simpson et al., 1960) :
For N = 300 and Y = 80, the 95% confi- dence interval is approximately 4.4%. Con- sequently, it appears that the variations in the calculated “radiolarian temperature” are significant at the 95% confidence level.
RESULTS AND DISCUSSION
Figure 1 shows the climatic index curve calculated for core V24-58, and the carbon- ate levels determined by Hays et al. (1969) for the same core. The radiolarian curve is highlighted by a profound cooling from a very warm interglacial period approxi- mately 110,000 yr BP to the coldest glacial interval recorded, occurring approximately 60,000 yr BP. Other features include a very warm interglacial peak at approximately
TABLE 5
AHINDANCE OF "WARM" AND “COLD" SPECIES IN CORE RCll-209
FIG. 1. Calcium carbonate content (after Hays et nl., 1969) and radiolarian “climatic index” (TR) in core V24-58. The climatic index is the percentage of warm-water species in a givrn assemblage, and therefore may be a direct reflection of surface water temperature. Note that the calcium carbonate content has been plotted inversely, in order that the t,wo curves will appear in phase. Estimated sediment age is indicated, using a uniform sedimentation rate (after Hays et al., 1969) of 7.8 mm/lo3 yr.
A6E (dyrsl 100 200 300 I I I
RC 11-209
DEPTH (cm)
FIG. 2. Calcium carbonate content (after Hays et al., 1969) and radiolarian “climatic index” (TH) in core RCll-209. As in Fig. 1, the calcium carbonate content has been plotted inversely, in order that the two curves will appear in phase. Estimated sediment age is indicated, using a uniform sedimentation rate (after Hays et al., 1969) of 9.9 mm/108 yr.
180,000 BP; a lesser, more rounded peak data provided by the carbonate curve. at approximately 300,000 BP; recognizable Similarly, the radiolarian climatic index glacial intervals peaking at 280,000 BP and curve for core RCll-209 (see Fig. 2) is in 150,000 BP ; and a recent warming toward close agreement with the corresponding car- the present climatic condition. These oscil- bonate curve of Hays et al. (1969,!. It, lations are in excellent agreement with the shows major glacial periods at 290,000 yr
214 JOHNSON AND KNOLL
mpth in RC 11-209
a loo cm 200 loo I”“““.I”““..‘I.’
I
55 - I
I ,...I.._. 1 ..,.I,.. 0 100
Depth in V&i 200
FIQ. 3. Radiolarian climatic indices in cores RCll-209 and V24-58. The agreement between the curves suggests that paleoclimatic analysis of radiolarian assemblages may be a valid method of obtaining reliabIe correlations between other cores with we&preserved radiolarian assemblages.
BP, 220,000 BP, and 70,000 BP, as well as distinct interglacial peaks at 180,000 BP, 110,000 BP, and the Late Pleistocene-Holo- cene climatic warming.
Figure 3 illustrates the similarity be- tween radiolarian climatic index curves for V24-58 and RCll-209. The depth scales for the two cores were correlated using sedi- mentation rates previously determined for these cores (Hays et al., 1969). The agree- ment between the variations in climatic index of the two cores is remarkably good.
The paleoclimatic curves obtained for the two cores used in this study show similar- ities to a curve calculated by Q-mode fac- tor analysis of radiolarian assemblages in a North Pacific core (Sachs, 1973b). Par- ticularly striking is the identification in all three cores of an unusuahy warm intergla- cial interval at approximately 110,000 yr BP.
Thus, it appears that the questions posed at the outset of this study can indeed be satisfactorily answered. Equatorial radio- larian assemblages did change significantly in response to Pleistocene climactic Auctua- tions. This change is observed as a shift in
the number of midlatitude radiolarians present relative to the number of warm water specimens present. In equatorial waters these shifts are not large, but they are measurable and the measurements can be repeated. The higher proportion of mid- latitude forms during glacial periods may be a result of the increased chances for suc- cessful competition offered by intensified upwelling, a more abundant food supply, and slightly lower temperature. Alterna- tively, vertical biogeographic zones (Casey, 1971) may have been drawn upward to- ward the nutrient-rich equatorial surface waters. Probably both of these factors were involved in producing the variability ob- served in the microfossil assemblages.
SUMMARY
Pleistocene radiolarian assemblages in equatorial regions appear to be useful paleoclimatic indicators, inasmuch as paleoclimatic curves obtained for different cores can be correlated with each other and with the glacial/interglacial carbonate re- cord in the same cores. Radiolaria therefore appear to be potentially useful for estab-
PLEISTOCENE RADIOLARIA 215
lishing stratigraphic correlations in equa- paleomagnetic, biostratigraphic and climatic
torial cores and for regional paleoclimatic record. Geological Society of America Bulletin
studies. 80, 1481-1514. IMBRIE, J. AND KIPP, N. G. (1971). A new micro-
ACKNOWLEDGMENTS
We thank Mr. Roy R. Capo of the Lamont- Doherty Geological Observatory for enabling us to sample the cores used in this study. Operations of the Lamont Core Laboratory are supported under O.N.R. Contract NO9014-67-A-0108-9004 and NSF Contract GA-29460. We thank C. Nigrini for providing samples of transitional radiolarian assemblages which were used in comparative studies, and for helpful discussions throughout the course of this project. This research was sup- ported under NSF Contract no. GA-36825, and a Woods Hole Oceanographic Institution Summer Student Fellowship to A. H. Knoll. Knoll is cur- rently supported under an NSF Graduate Fellow- ship at Harvard University. This is Contribution no. 3275 of the Woods Hole Oceanographic Insti- tution.
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