High precision glacial-interglacial benthic foraminiferal Sr/Ca records from the eastern equatorial Atlantic Ocean and Caribbean Sea

Glacial-interglacial variation in the marine Sr/Ca ratio has important impl ications for coral Sr thermometry [J.W. Beck et al., Science 257 (1992) 644 -647]. A possible variation of 1-3% was proposed based on ocean models [H.M . Stoll and D.P. Schrag, Geochim. Cosmochim. Acta 62 (1998) 1107-1118]. Sub sequently, studies have used fossil foraminifera to test this prediction [P .A. Martin et al., Geochem. Geophys. Geosyst. 1 (1999); H.M. Stoll et al., Geochim. Cosmochim. Acta 63 (1999) 3535-3547; H. Elderfield et al., Geochem . Geophys. Geosyst. 1 (2000)]. But whether some component of foraminiferal Sr/Ca variation can be uniquely ascribed to seawater Sr variation is still not clear. To address this question, we developed cleaning and analysis tec hniques and measured Sr/Ca ratios on individual shells of the modern benthi c foraminifer Cibicidoides wuellerstorfi. We showed that different size she lls have different Sr/Ca ratios; however, samples with shell sizes of 355-5 00 mum appear to have normally distributed Sr/Ca ratios (1 sigma = 1.8%). F or multi-shell measurements (with estimated errors of 0.12-0.39%), the rati o varied by as much as 7.2 +/- 0.5% during the last glaciation for two Cari bbean records at the same site and by 3.7 +/- 0.5% over the past 40,000 yr for one record from the Sierra Leone Rise in the eastern equatorial Atlanti c. The two Caribbean records are very similar indicating that the behavior of shell Sr uptake was identical locally and that the shell Sr/Ca ratio fai thfully reflects the local environment. The Atlantic record differs from th e Caribbean records by as much as several percent. Thus, the foraminiferal Sr/Ca changes cannot be solely due to changes in seawater Sr/Ca unless the glacial deep ocean had spatial variation in Sr/Ca well in excess of the mod ern ocean. Certain similarities between the three records do exist. Notably , the rate of change of Sr/Ca is similar between 9 and 0 ka (-0.25%/kyr) an d between 25 and 16 ka (+0.16%/kyr). This suggests that during these interv als, benthic foraminiferal Sr/Ca was affected by similar large-scale variab les. One of these variables may be the average marine Sr/Ca ratio; however, comparison with model predictions [H.M. Stoll and D.P. Schrag, Geochim. Co smochim. Acta 62 (1998) 1107-1118] suggests other factors must also be cons idered. The discrepancies between the two sites may be related to the diffe rent water mass histories for the Caribbean and eastern Atlantic. Our resul ts suggest that variation of the seawater Sr budget only partially contribu ted to C wuellerstorfi Sr/Ca records, while other significant factors still need to be quantified. At present we cannot confidently determine past sea water Sr/Ca variation from our foraminiferal records. (C) 2001 Elsevier Sci ence B.V. All rights reserved.