Reconstructing the stable isotope geochemistry and paleotemperatures of the equatorial Atlantic during the last 150,000 years: Results from individual foraminifera

Abstract

This study represents an attempt to extract paleoclimatic data from the deep‐sea record by analyzing foraminiferal shells individually. Using the oxygen (δ18O) and carbon (δ13C) isotopic composition of individual Orbulina universa and Neogloboquadrina dutertrei, we present an approach to reconstruct the δ18O of seawater (δ18Ow), the δ13C of ΣCO2, and seasonal maximum sea surface temperatures (SST) in the western and eastern equatorial Atlantic. We examine the glacial and interglacial extremes of the last 150,000 years (isotope stages 1, 2, 5e, and 6). Comparison of recent water column hydrography with reconstructions from core top assemblages shows that O. universa and N. dutertrei δ18O and δ13C values accurately record hydrographic conditions in the mixed layer and upper thermocline at both sites. By analyzing shells individually, we can evaluate the effect of bioturbation on the range of δ18O and δ13C values in each interval and take it into consideration in our data interpretations. Downcore results show that N. dutertrei δ18O values in the western equatorial Atlantic reflect glacial to interglacial changes in δ18Ow due to continental ice formation (Δδ18O=1.30 ‰). We use changes in N. dutertrei δ18O values between core intervals to estimate the ice‐volume effect in paleotemperature calculations for the mixed layer. To validate the use of O. universa for mixed layer reconstructions, we have added individual Globigerinoides sacculifer data for stages 1 and 2 at both sites. Paleotemperature reconstructions from O. universa δ18O values indicate that maximum seasonal mixed layer temperatures in the equatorial Atlantic decreased by at most 2.6°C between isotope stages 1 and 2 and by no more than 3.4°C between stages 1 and 6. Individual shell data from G. sacculifer yield similar results indicating that maximum O. universa δ18O values reflect the mixed layer environment. In agreement with Climate: Long‐Range Investigation, Mapping, and Prediction (CLIMAP) [1981] SST reconstructions for stage 2, these data indicate little change between glacial and interglacial paleotemperatures in the equatorial Atlantic.An electronic supplement of this material may be obtainedon a diskette or Anonymous FTP from KOSMOS.AGU.ORG.(LOGIN to AGU's FTP account using ANONYMOUS as theusername and GUEST as the password. Go to the rightdirectory by typing CD APEND. Type LS to see what files areavailable. Type GET and the name of the file to get it.Finally, type EXIT to leave the system.) (Paper 95PA03773,Reconstructing the stable isotope geochemistry andpaleotemperatures of the equatorial Atlantic during the last150,000 years: Results from individual foraminifera,Katharina Billups and Howard J. Spero). Diskette may beordered from American Geophysical union, 2000 FloridaAvenue, N. W., Washington, DC 20009; $15.00. Paymentmust accompany order.