Paleo-oceanographic utility of oxygen isotopic measurements on planktic foraminifera: Indian Ocean core-top evidence

Palaeogeography, Palaeoclimatology, Palaeoecology, 33 ( 1981 ): 173--191 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

173

PALEO-OCEANOGRAPHIC UTILITY OF OXYGEN ISOTOPIC MEASUREM E N T S ON P L A N K T I C F O R A M I N I F E R A : I N D I A N O C E A N C O R E - T O P EVIDENCE

W. B. CURRY and R. K. MATTHEWS Department of Geological Sciences, Brown University, Providence, R.I. 02912 (U.S.A.) (Received and accepted July 28, 1980) ABSTRACT Curry, W. B. and Matthews, R. K., 1981. Paleo-oceanographic utility of oxygen isotopic measurements on planktic foraminifera: Indian Ocean core-top evidence. Palaeogeogr., Palaeoclimatol., Palaeoecol., 33 : 173--191. Isotopic analyses have been performed on narrow size ranges of seven species of planktic foraminifera picked from 43 core-top locations in the Indian Ocean. Comparison of observed ~ '80 for each species with calculated profiles of equilibrium 8 '80 at each location yield three distinct depth habitat preferences. For surface-dwelling species, Globigerinoides ruber and Globigerina bulloides, observed oxygen isotopic compositions are in close agreement with the seasonal range of calculated 6 'aO for calcite precipitated in surface waters. Observed 5 '80 values for Orbulina universa, Neogloboquadrina dutertrei, and Pulleniatina obliquiloculata are consistently enriched relative to calculated 5 '80 for surface conditions. In the northern Indian Ocean, test precipitation for O. universa, N. dutertrei, and P. obliquiloculata occurs over a narrow range of temperature (22°C 24°C), with no systematic relationship with sea-water o t. In contrast, Globorotalia inflata and G. truncatulinoides observed ~ '80 apparently reflect temperature and salinity variation along the 26.8 ot isopycnal. INTRODUCTION T h e o x y g e n isotopic analysis o f tests o f microfossils plays a m a j o r role in studies of p a l e o - o c e a n o g r a p h y and p a l e o c l i m a t o l o g y . Variations in 5 ' 8 0 in b e n t h i c and p l a n k t i c f o r a m i n i f e r a have been used to infer Pleistocene ice v o l u m e h i s t o r y ( S h a c k l e t o n , 1 9 6 7 ; S h a c k l e t o n and O p d y k e , 1973, 1976) a n d sea-surface t e m p e r a t u r e (Emiliani, 1 9 5 5 , 1966). A n a s s u m p t i o n implicit in all p a l e o - o c e a n o g r a p h i c isotopic studies is t h a t the o x y g e n i s o t o p i c c o m p o s i t i o n of the microfossil a c c u r a t e l y reflects the t e m p e r a t u r e and o x y g e n isotopic c o m p o s i t i o n o f the sea w a t e r in w h i c h it grew. Thus, the precise c a l i b r a t i o n o f variations in 5 ' s O o f R e c e n t samples with m o d e r n o c e a n o graphic c o n d i t i o n s is a necessary p r e c o n d i t i o n for the q u a n t i t a t i v e interpret a t i o n o f d o w n - c o r e isotopic records. Analysis o f c o r e - t o p samples o f p l a n k t i c f o r a m i n i f e r a is the m o s t straightf o r w a r d a p p r o a c h t o evaluating the paleOclimatic u t i l i t y o f isotopic measure0031-0182/81/0000--0000/$02.50 © 1981 Elsevier Scientific Publishing Company

174

ments on this microfossil group. Two such studies have made progress toward calibrating planktic foraminiferal isotopic measurements with modern oceanographic conditions. Savin and Douglas (1973) analyzed the isotopic variation in planktic foraminifera from a broad geographic array of South Pacific core-top samples. However, this data set included analyses of multispecific samples of planktic foraminifera. Many investigations have demonstrated the need to analyze only monospecific samples, as many species of the same genus have very different isotopic compositions (Williams, 1976; Shackleton and Vincent, 1978; Berger et al., 1978). Williams (1976) investigated the isotopic variation in monospecific samples of planktic foraminifera from core-tops in the Indian Ocean. This data set provides broad geographic coverage across a wide range of oceanographic conditions. However, little attention is paid to size variations in the planktic foraminiferal species analyzed. Recent investigations (Berger et al., 1978; Curry and Matthews, 1977) have demonstrated large variations in isotopic composition between size fractions of the same species. Our approach in this study has been to analyze the patterns of isotopic variation in narrow size ranges of planktic foraminiferal species picked from core-top samples and relate the variations to overlying physical oceanographic conditions. The core-top array in this data set provides the broad geographic coverage necessary to assess the various questions of isotopic fractionation in planktic foraminifera. Thus, the relationship of isotopic variation to variations in sea water temperature and salinity can be reasonably achieved. In addition, the conditions of post-depositional alteration (i.e., burrowing and dissolution) will be similar to the conditions encountered in marine sediments of any age. Therefore, the relationship of isotopic variation in planktic foraminifera in this data set with modern physical oceanographic parameters can provide a realistic framework within which down-core isotopic records can be interpreted. STRATEGY

We have analyzed the isotopic compositions of seven species of planktic foraminifera picked from 43 core-top locations. The core-tops represent a wide range of oceanographic conditions including sea-surface temperature variation of 5°C--28°C and sea surface salinity variation of 33.6%o--36.5%o. At each core-top location, vertical profiles of temperature and salinity in the upper water column (Wyrtki, 1971) have been used to construct vertical profiles of 5180 composition of calcite precipitated in equilibrium with sea water. The Epstein et al. (1953) paleotemperature equation was used for all calculations. Approximations of the isotopic composition of sea water were calculated using this salinity--518Ow~e~ empirical relationship for Indian Ocean surface waters: ' 8 0 ~ o w = 0.48 (S°/oo) -- 16.53

175 (Williams, 1976 -- from Craig and Gordon, 1965 data). Comparison of observed oxygen isotopic compositions with the calculated 5~sO profiles for equilibrium calcite precipitation allows us to address the various problems of isotopic fractionation in the planktic foraminifera ecosystem. METHODS R a w sediment samples were disaggregated in distilled water for 24 h and dried at 50°C. Each sample was dry-sieved into ten narrow size ranges. Planktic foraminifera samples were picked from narrow size fractions (Table I presents the size ranges used for each species). Previous studies (Berger et al., 1978; Curry and Matthews, 1977) have demonstrated large variations (1%o) in isotopic composition between size fractions of the same species. Minimizing the isotopic variability in 5~sO because of size variation (particularly for globorotalid species) is an important prerequisite for the precise interpretation of the results. Samples for isotopic analysis were ultrasonically cleaned in distilled water to remove fine-fraction contamination. The samples were roasted in a vacuum at 370°C for I h CO2 and H 2 0 resulting from the carbonate reaction with H3PO4 at 50°C were separated by a series of three freezing/transfer steps. Resultant CO2 was analyzed in an on-line VG Micromass 602D mass spectrometer at the Benedum Stable Isotope Laboratory of Brown University. All isotopic data are referred to the Chicago PDB reference in the standard 5 notation (Craig, 1957). Calibration to PDB is achieved through intercalibration of three intermediate standards. Agreement among all standards is +0.2%o. Analytical precision of carbonate standards run before and after groups of planktic foraminifera samples is +0.12 (1 o) for 51sO and _+0.11 (1 a) for 513C. In addition, duplication of foraminifera sample splits yielded results which differed by an average of 0.22 for 5 ~sO and 0.22 for 5 ~3C. DATA All isotopic data for planktic foraminifera species from Indian Ocean coretop locations are presented in Table I. Included in this table are calculated equilibrium 51sO values for summer and winter surface conditions (Epstein et al., 1953; Wyrtki, 1971) at each core location. Profiles of 51sO for calcite precipitated in equilibrium with sea-water for each core-top location are presented in Curry (1980). We have sought to classify the oxygen isotopic variability of each species under consideration by comparing the observed 51sO values for each species with calculated equilibrium 51sO for summer and winter surface conditions (temperature and salinity) at each location. The oxygen isotopic variation observed in the spinose species, Globigerina bulloides and Globigerinoides ruber, is presented in Fig.1. Note that virtually all of the oxygen isotopic variation seen in these species can be a c c o m m o d a t e d

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Fig.1. Observed oxygen isotopic compositions of G. bulloides (212--250 #m) and O. ruber (212--250 urn) plotted against calculated seasonal surface 81sOc (calcite). Surface 61sOc calculated using temperature and salinity data (Wyrtki, 1971), the Epstein et al. (1953) paleotemperature equation and a salinity: 51SOwater approximation (Williams, 1976; from Craig and Gordon, 1965). Note Chat species observed 6 tsO generally fall within ~ 0.5%o of calculated seasonal surface 5 t80c" Data presented in Table I. within the limits of seasonal variation in equilibrium surface 51so. This is particularly apparent in G. bulloides from high latitude (6180 of 1.2°/oo2.0%0 PDB). While observed values are much lighter than expected winter 5180, agreement with summer 51SO is excellent. Similarly, for G. ruber, observed ~180 values in the range of --0.5%o to +1.0%o fall within the range of seasonal variation in surface equilibrium 5180. For tropical regions (5 lSO<--1.2°/oo) where seasonal variations in temperature are small, agreem e n t with calculated surface equilibrium 5 lSO for each species is excellent. The observed oxygen isotopic compositions for Orbulina universa, Pulleniatina obliquiloculata, and Neogloboquadrina dutertrei, when compared to surface equilibrium 6180 values, present a pattern distinctly different from G. ruber and G. bulloides. Fig.2 presents these data. In general, observed oxygen isotopic compositions for these species are consistently enriched relative to expected surface equilibrium 6180 compositions. Only a small number of samples fall within the range of seasonal variation of calculated surface 5180 for calcite. This pattern of foraminifera oxygen isotopic composition is best described as " c o n s t a n t e n r i c h m e n t " from calculated equilibrium surface conditions. Such a pattern likely represents calcification at some depth below the surface. The third pattern of oxygen isotopic variation for planktic foraminifera species in this data set is presented in Fig.3. The globorotalid species, G. truncatulinoides (L and R) and G. inflata, have patterns of oxygen isotopic

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Calculated Surface B~s0c (%o,PD3) Fig.2. Observed o x y g e n isotopic c o m p o s i t i o n s of O. universa ( 5 0 0 - - 6 0 0 urn), N. dutertrei ( 3 5 5 - - 4 2 5 /~m), and P. obliquiloculata ( 3 5 5 - - 4 2 5 urn) plotted against calculated seasonal surface a ' 8 0 c (calcite). Surface 8 ' 8 0 c calculated as discussed in text. N o t e that species observed a ' 8 0 are generally enriched relative to calculated surface a'8Oc . Observed pattern m o s t likely represents calcification b e l o w surface conditions (cooler water). All data presented in Table I.

variation that are distinctly different from the species previously discussed. In high-latitude regions (calculated surface 5 ' s O >1.0% o), the observed values for these species fall within 1%o of expected surface conditions. However, in regions where equilibrium 6 ' s O values for surface conditions are less than 1%o PDB, the observed oxygen isotopic compositions for G. truncatulinoides and G. inflata systematically deviate from surface values. This is particularly dramatic for G. truncatulinoides, where differences between calculated surface equilibrium 61sO and observed 6 ' 8 0 are as large as 3%0. This is equivalent to calcification in water 12°C cooler than surface water. For G. inflata, deviations as large as 2.5%o occur. Comparison of observed oxygen isotopic compositions in the planktic foraminifera with expected equilibrium surface 5180 values is a useful m e t h o d of presenting the various patterns of oxygen isotopic compositions for these species. However, it is unrealistic to believe that all planktic foraminifera should be at equilibrium with surface conditions. Planktic foraminifera can be found distributed over at least the upper 300 m of the water column (Berger, 1969; Fairbanks et al., 1980). In order to precisely determine

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Fig.3. Observed oxygen isotopic compositions of G. inflata (300--355 #m) and G. truncatulinoides (355--425 #m) plotted against calculated seasonal surface 81sO c (calcite). For G. truncatulinoides, the symbols "R and L denote right and left coiling varieties. Surface 618Oc calculated as discussed in text. Note that, in high latitudes (observed 180 >2%o PDB), there is close agreement of observed and calculated surface values. However, in tropical regions, observed values may be as much as 3%0 enriched relative to calculated surface 818 O c. All data presented in Table I. the f a c t o r s governing o x y g e n isotopic variation in p l a n k t i c f o r a m i n i f e r a , c o m p a r i s o n o f observed 6180 w i t h c o n s t r u c t e d vertical profiles o f equilibr i u m 5180 provides a m o r e realistic a p p r o a c h . DISCUSSION Equilibrium test precipitation T h e isotopic variation seen in G. ruber and G. bulloides suggests t h a t test p r e c i p i t a t i o n f o r each is substantially at o x y g e n isotopic equilibrium with surface w a t e r c o n d i t i o n s . T h e o b s e r v e d 5180 values f o r each species fall within 0.5%o o f the seasonal range o f calculated 5 ~sO for surface conditions. In tropical regions w h e r e seasonal variation is small, all observed values are n e a r t o e q u i l i b r i u m with surface c o n d i t i o n s . A t high latitudes, o b s e r v e d values generally fall within 0.5%o o f calculated 5180 f o r s u m m e r surface w a t e r c o n d i t i o n s . T h e seasonal a c c o m m o d a t i o n o f o b s e r v e d values with e x p e c t e d surface 6 ~sO values at m i d d l e and high l a t i t u d e s is likely a true r e f l e c t i o n o f seasonal variations in f o r a m i n i f e r a p r o d u c t i o n . Shallow ( 0 - - 1 0 m) p l a n k t o n t o w

181

observations have d o c u m e n t e d the presence of each species in near-surface water (B~ et al., 1971). Similarly, plankton t o w observations have documented seasonal variation in the abundance of each species (B~, 1960). For our purposes, we must conclude that G. bulloides and G. ruber are at equilibrium and are recording extremely valuable paleo-oceanographic information over a wide range of sea surface conditions. Our conclusions a b o u t equilibrium test precipitation are substantially in agreement with recent studies of oxygen isotopic variations of living planktic foraminifera. Williams et al. (1979) demonstrated that the oxygen isotopic composition of living planktic foraminifera collected in seasonal surface tows closely paralleled calculated values of equilibrium 5180. Their leastsquares regression of species observed (51SOc--518Ow) with temperature was slightly offset from values expected using the Epstein et ah (1953) paleotemperature equation. However, uncertainties in calculating equilibrium 5180 suggest that the difference was n o t significant. Similarly, Fairbanks et ah (1980) analyzed living planktic foraminifera collected in the western North Atlantic. They concluded that, while non-spinose species appeared to calcify in oxygen isotopic equilibrium, coexisting spinose species were on the average 0.35°/°° depleted relative to expected equilibrium. We note that small deviations from equilibrium such as these are within the scatter of our data set. However, it is important to note that observed 5180 values for G. bulloides (this data set) fall within 0.5°/°0 of calculated seasonal surface equilibrium 5180 over a temperature range of 5--28°C. Small errors because of disequilibrium isotopic fractionation are equivalent to the scatter seen in isotopic measurements of planktic foraminifera from sediment samples. The oxygen isotopic compositions of surface-dwelling species are, however, recording the major variations in sea surface-temperature and salinity. We believe that this is sufficient documentation to suggest that the 5180 values for other planktic foraminifera species are reflecting changing temperature and isotopic composition of the sea water in which test secretion occurred. We note that surface-dwelling, spinose planktic foraminifera are most likely to be o u t of oxygen isotopic equilibrium. Shallow-dwellers are often symbiont-bearing individuals (Lee et al., 1965; Anderson and B~, 1976); symbiotic algae have been shown to alter oxygen isotopic composition in other calcium carbonate secreting organisms (Weber and Woodhead, 1970; Buchardt and Hansen, 1977; Erez, 1978a). Likewise, surface-dwelling individuals are likely to have greatest metabolic activity. In contrast, deepdwelling species lack symbiotic algae and are likely t o h a v e slower metabolic rates. Thus, these species are most likely to be fractionating in oxygen isotopic equilibrium with sea water temperature and 5180. Furthermore, many species under investigation in this study (N. dutertrei, P. obliquiloculata, G. inflata, and G. truncatulinoides, in particular) have been shown in plankton tow samples to apparently be fractionating in oxygen isotopic equilibrium with sea water (Fairbanks et al., 1980). We, therefore, consider these

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184 species to be fractionating in oxygen isotopic equilibrium and proceed to relate their observed ~ 180 variations to observed patterns of physical oceanographic parameters.

Temperature and salinity of test precipitation We have compared the observed oxygen isotopic compositions of five nonsurface-dwelling species of planktic foraminifera (see Table IT) to vertical profiles of calculated equilibrium 6 ~80 at each core location. As stated previously, vertical profiles of seasonal temperature and salinity at each site (Wyrtki, 1971) were converted to equilibrium 5 ~80 values for calcite using the Epstein et al. (1953) paleotemperature equation and an equation relating salinity and 5~8Ow (Williams, 1976). By comparing observed 6~80 with calculated profiles of equilibrium 5 ~80, it is possible to determine a unique temperature and salinity of test precipitation for each species at each location. The calculated temperatures and salinities of test precipitation are presented in Table II. Observed oxygen isotopic compositions were compared to profiles of equilibrium 5 ~80 at each location for both winter and summer conditions. For locations where seasonal variations in the structure of the upper water column result in two possible temperatures and salinities, each pair is presented. At locations where observed 5180 values are lighter than winter surface conditions, t f e deduced summer temperature and salinity is presented. Seasonal variation in temperature and salinity is low for species living below 100 m. Hence, 6~80 values of m a n y deep-dwelling planktic species can be adequately represented by one temperature and one salinity of test preciptiation.

Constant temperature taxa. The pattern of temperature-salinity variation exhibited by P. obliquiloculata, N. dutertrei, and O. universa from samples in the northern Indian Ocean suggests that these species are calcifying at constant temperature (see Figs.4--6). Note that these species are generally found only in sediment samples from tropical regions (B~ and Tolderlund, 1971; B6 and Hutson, 1977). Typical tropical sea surface-temperatures in the northern Indian Ocean range from 26 to 28°C (Wyrtki, 1971). The average temperature of calcification for these species is 22--24°C, suggesting a subsurface depth habitat. There is an extremely wide range of salinities over which test calcification occurs. The northern Indian Ocean is unique in that a geographic salinity gradient in surface waters of 6%o results from monsoonal circulation (Wyrtki, 1971). A wide range of salinity is available for test calcification. However, the apparent temperature of test precipitation is relatively constant (_+2°C, see Table II). We note that relative abundances of these species in core-top samples in the northern Indian Ocean vary as a function of salinity (Cullen and Prell, 1978); salinity remains a factor governing the species occurrence and relative abundance. However, when present, the species appear to be calcifying over a narrow temperature range.

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Fig.4. Temperature and salinity of test precipitation for O. universa (500--600 um) for Indian Ocean core-top sample locations north of ~ 1 0 ° S latitude. Temperature and salinity of test precipitation deduced from comparison of observed species 5180 with calculated vertical profiles of 818 Oc (calcite) at each location as discussed in text. Plotted values represent averages of seasonal temperature and salinity data from Table II. Note that the pattern of temperature--salinity variation suggests that calcification occurs at constant temperature.

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Fig.5. Temperature and salinity of test precipitation for P. obliquiloculata (355--425 urn) for Indian Ocean core-top sample locations north of ~ 1 0 °S latitude. Temperature and salinity of test precipitation deduced from comparison of observed species 8180 with calculated vertical profiles of 8180 c (calcite) at each location as discussed in text. Plotted values represent averages of seasonal temperature and salinity data from Table II. Note that the pattern of temperature--salinity variation suggests that calcification occurs at constant temperature. Note also that the temperature and salinity variation does not parallel a t isopycnals.

It is important to note that the T - - S variation for these species exhibits n o systematic relationship with the at of sea water. For all samples occurring in the northern Indian Ocean, ot values range from 22.7 to 25.1. This result is in strong contrast to the notion t h a t density governs the vertical distribution of planktic foraminifera in the water column (Emiliani, 1954; Hecht and Savin, 1972; Savin and Douglas, 1973}. The patterns of oxygen isotopic variation and of deduced temperature and salinity variation are strong evidence that density or a, has little effect in the vertical distribution of these particular species. Our results suggest t h a t test secretion occurs strictly at constant temperature.

186 Salinity (%o) 3C i 2C ~

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Fig,6. Temperature and salinity of test precipitation for N. dutertrei (355--425 urn) for Indian Ocean core-top sample locations north of ~ 10°S latitude. Temperature and salinity

of test precipitation deduced from comparison of observed species 8 '8 O with calculated vertical profiles of 5 '80 c (calcite) at each location as discussed in text. Plotted values represent averages of seasonal temperature and salinity data from Table II. Note that the pattern of temperature-salinity variation suggests that calcification occurs at constant temperature. Note also that temperature and salinity variation does not parallel ot isopycnals. There are at least two hypotheses that may explain the observation of constant temperature of calcification for P. obliquiloculata, N. dutertrei, and O. universa. First, o p t i m u m conditions for calcification and reproduction (Hecht, 1976) for these species may occur in the temperature range of 22°C--24°C. Alternatively, the isotherms of 22°C to 24°C in the northern Indian Ocean may be coincident with the location of greatest food abundance and thus be associated with greatest species production (Berger et al., 1978; Fairbanks et al., 1980). It is impossible to distinguish between these hypotheses from our data. It is interesting to note, however, that greatest relative abundances of these species in surface plankton tows (0--10 m) are coincident with these temperatures (B~ and Tolderlund, 1971), suggesting that 22 ° C - 2 4 ° C m a y represent an o p t i m u m temperature range for these species. We look forward to continued investigation into these possibilities. Constant at taxa. The pattern of temperature and salinity variation for G. inflata and G. truncatulinoides (Figs.7, 8) parallel the 26.8 at isopycnal. Test precipitation for these species occurs over a wide range of temperature ( ~ 5 ° C - 1 5 ° C ) and salinity (33.9%o--35.5°/oo). However, test precipitation apparently occurs over a very narrow range of at (at = 26.8 + 0.3). Oxygen isotopic analysis of collected living Globorotalia sp. from nearsurface depths often have isotopic values in equilibrium with near-surface conditions (Williams et al., 1979) or at the top of the thermocline (Fairbanks et al., 1980). In core-top samples from the tropical Indian Ocean (this data set), observed ~ ~sO values for these species are significantly enriched relative to equilibrium surface 8 ~sO. The difference, often as high as 3%°, is equivalent to calcification in water ~ 12°C cooler than surface temperature. Com-

187

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Fig.7. Temperature and salinity of test precipitation for G. truncatulinoides (355--425 #m) for Indian Ocean core-top sample locations. Right coiling and left coiling varieties noted by squares and circles, respectively. Temperature and salinity of test precipitation deduced from comparison of observed species 6~80 with calculated vertical profiles of ~ ~80c (ealcite) at each location as discussed in text. Plotted values represent averages of seasonal

temperature and salinity data from Table II. Note that the pattern of temperature-salinity variation closely parallels at isopycnals. Salin,ty

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o $

l0

E

G inflato (300- 355/~)

Fig.8. Temperature and salinity of test precipitation for G. inflata (300--355 ~m) for Indian Ocean core-top sample locations. Temperature and salinity of test precipitation deduced from comparison of observed species 6 ~80 with calculated vertical profiles of 6 ~80 c (calcite) at each location as discussed in text. Plotted values represent averages of seasonal temperature and salinity data from Table II. Note that the pattern of temperature-salinity variation closely parallels a t isopycnals. parison o f c o r e - t o p results with p l a n k t o n - t o w isotopic results suggests t h a t significant s e c o n d a r y calcification m u s t o c c u r at greater d e p t h . I s o t o p i c analyses o n p l a n k t i c f o r a m i n i f e r a f r o m s e d i m e n t t r a p samples (Erez, 1 9 7 8 b ) suggests t h a t s e c o n d a r y calcification is occurring. O u r results are consist e n t with a m o d e l involving significant calcification at d e p t h . F u r t h e r m o r e , o u r results suggest t h a t if vertical m i g r a t i o n s o c c u r , the r e s u l t a n t d e p t h o f calcification is s t r o n g l y related to sea w a t e r Or. T h e r e a p p e a r to be various m e c h a n i s m s governing the vertical d i s t r i b u t i o n o f species in the planktic f o r a m i n i f e r a

Paleo-oceanographic

implications.

188 ecosystem. Furthermore, the various depth-habitat preferences of species implies that different species will record different paleo-oceanographic information. For instance, the surface-dwelling species, G. ruber and G. bulloides, appear constrained to surface habitats over a wide range of oceanographic conditions. The surface habitat is independent of the vertical distribution of temperature, salinity and density in the upper water column. Thus, time-series variations in 6 '80 for these species record a combination of sea surface-temperature and sea-water oxygen isotopic composition. In contrast to the surface-dwellers, depth habitats of deeper-dwelling planktic foraminifera appear strongly related to the physical properties of the upper water column. P. obliquiloculata, N. dutertrei, and O. universa have oxygen isotopic compositions suggesting calcification occurs at constant temperature. These species exhibit no systematic relationship of depth habitat with sea-water density. Core-top variations in 5 'sO of these constanttemperature taxa are reflecting variations in sea-water oxygen isotopic composition. Stratigraphic variations in 5'80 of these species may thus provide an alternative monitor of changes in sea-water 5 '80 resulting from ice volume or local evaporation/precipitation changes. The oxygen isotopic variation in G. truncatulinoides and G. inflata suggests that test precipitation is strongly related to sea-water ot. In the modern ocean, depth habitats of these species vary significantly with variations in physical oceanographic setting. In time-series isotopic studies, depth habitats of these species will likewise change as oceanographic conditions change. Vertical migrations to warmer water in response to increases in density during glacial stages will tend to dampen glacial--interglacial 4 5 ' 8 0 . In addition, change in 5 '80 for density-controlled deep-dwellers should result from mean oceanic density changes not of glacial origin, thereby recording important paleo-oceanographic information not recorded in surface-dwelling planktic and benthic foraminifera. CONCLUSIONS (1) Oxygen isotopic variation in seven species of planktic foraminifera from core-top samples in the Indian Ocean yield three distinct patterns. G. ruber and G. bulloides 6 '80 are within _+0.5%0 of calculated equilibrium 5 '80 for seasonal sea surface conditions. P. obliquiloculata, N. dutertrei, and O. universa 5 'sO are consistently enriched relative to calculated equilibrium 5'80 for sea surface conditions. G. inflata and G. truncatulinoides 5 ' 8 0 are within 1%o of calculated equilibrium for high-latitude surface conditions. In tropical regions, observed 5 '80 for G. inflata and G. truncatulinoides are enriched relative to calculated equilibrium surface 5'80 by as much as

3%o. (2) Deduced temperature and salinity of test precipitation for nonsurfacedwelling planktics suggest that two different factors govern depth habitat. P. obliquiloculata, N. dutertrei, and O. universa precipitate tests at nearly

189

constant temperature (22°--24°C). There is no apparent relationship between depth habitat of these species and salinity or ot of sea water. The oxygen isotopic compositions of G. inflata and G. truncatulinoides are strongly related to T--S gradients along the 26.8-ot isopycnal. (3) Stratigraphic isotopic variations in species with different depth-habitat preferences record different aspects of the climate system. Surface-dwelling species record variations in both surface temperature and salinity. Species calcifying at constant temperature record variations in the oxygen isotopic composition of sea water. Oxygen isotopic variations in species with depth habitats governed by seawater ot will be modified by vertical migrations in response to sea-water density change. ACKNOWLEDGEMENTS

We thank W. Prell and J. Cullen for valuable discussions. We also thank W. H. Berger for criticism of the manuscript. Research was supported by the Climate Dynamics Research Section, National Science Foundation, grant ATM77-07755, and a grant from the Claude Worthington Benedum Foundation. APPENDIX

Table o f core l o c a t i o n s and water d e p t h s Core

Latitude

Longitude

Depth (m)

A15-585HC A15-586PG A15-591HC A15-592FF A15-596 A15-597AHC A15-612HC E45-27A E45-70-1 E45-73A E45-77-1 E48-11 E48-22A E48-27A MD76-132 MD77-185 RC9-139 RC9-161 RC12-328 RC 12-339 RC12-340 RC12-343 R C 14-7

20 ° 09.0'N 20 ° 07.5'N 21 ° 0 0 . 0 ' N 20 ° 50.0'N 18 ° 56.0'N 17 ° 2 6 . 0 ' N 13 ° 3 5 . 0 ' N 43 ° 18.5'S 48 ° 30.0'S 48 ° 33.2'S 46 ° 26.9'S 29 ° 39.6'S 39 ° 53.7'S 38 ° 32.5'S 16 ° 59.4'N 12 ° 24.0'N 47°46.2'S 19 ° 34.1'N 03 ° 5 7 . 0 ' 8 09 ° 08.0'N 12 ° 42.0'N 15 ° 10.0'N 35 ° 31.0'S

69 ° 26.0'E 67 ° 5 6 . 0 ' E 59 ° 33.0'E 61 ° 0 1 . 0 ' E 61 ° 2 3 . 0 ' E 57 ° 1 1 . 0 ' E 71 ° 3 3 . 5 ' E 105 ° 32.9'E 114 ° 28.6'E 114 ° 26.4'E 114 ° 25.0'E 93 ° 31.6'E 85 ° 42.6'E 79 ° 54.0'E 7 1 ~. 3 0 . 8 ' E 92° 0 3 . 7 ' E 123°06.2'E 59 ° 3 5 . 7 ' E 60 ° 36.0'E 90 ° 02.0'E 90 ° 01.0'E 90 ° 34.0'E 44 ° 45.0'E

216 3049 1297 2628 3694 1805 1697 3737 3639 3690 3740 3424 3324 3231 1430 1461 4158 3332 3087 3010 3012 2666 3288

190 APPENDIX I

(continued)

Core

Latitude

Longitude

Depth (m)

RC 14-9 RC14-29 RC14-36 RC17-98 V14-103 V14-104 V16-64 V16-65 V16-113 V19-178 V19-185 V20-170 V20-175 V29-14 V29-29 V34-80 V34-83 V34-85 V34-87 V34-88

39 ° 0 1 . 0 ' S 10 ° 5 5 . 4 ' S 00 ° 27.9'S 13 ° 1 3 . 0 ' S 11 ° 2 6 . 5 ' N 13 ° 2 5 . 5 ' N 46 ° 01.0'S 45 ° 0 0 . 0 ' S 48 ° 05.0'S 08 ° 0 7 . 0 ' N 06 ° 4 2 . 0 ' N 21 ° 4 8 . 0 ' S 22 ° 1 8 . 0 ' S 06° 4 1 . 0 ' N 05 ° 0 7 . 0 ' N 06 ° 0 7 . 0 ' N 10 ° 2 4 . 2 ' N 11 ° 4 8 . 2 ' N 16 ° 2 8 . 8 ' N 16 ° 3 1 . 0 ' N

47 ° 5 3 . 0 ' E 88 ° 18.9'E 89 ° 5 9 . 7 ' E 65 ° 3 7 . 0 ' E 56 ° 1 4 . 0 ' E 53 ° 2 7 . 0 ' E 44 ° 20.0'E 45 ° 4 6 . 0 ' E 137 ° 3 9 . 0 ' E 73 ° 1 5 . 0 ' E 59 ° 2 0 . 0 ' E 69 ° 1 4 . 0 ' E 68 ° 0 0 . 0 ' E 85 ° 5 8 . 0 ' E 77 ° 3 5 . 0 ' E 59 ° 2 6 . 0 ' E 57 ° 5 7 . 3 ' E 57 ° 3 6 . 6 ' E 59 ° 4 5 . 6 ' E 59 ° 3 2 . 0 ' E

2692 2869 3076 3409 4232 2670 2202 1618 3599 2188 2867 2479 3526 3713 2673 3292 1880 3190 2080 2120

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