TEMPERATURE CONTROL ON THE INCORPORATION OF MAGNESIUM, STRONTIUM, FLUORINE, AND CADMIUM INTO BENTHIC FORAMINIFERAL SHELLS FROM LITTLE BAHAMA BANK - PROSPECTS FOR THERMOCLINE PALEOCEANOGRAPHY

Surface sediments from Little Bahama Bank(LBB), intersecting the subtr opical thermocline, were used to assess the influence of temperature o n the incorporation of Mg, Sr, F, and Cd into shells of benthic forami nifera. Samples were obtained from twelve box cores along the southern slope of LBB, covering a temperature range of 18-4.5 degrees C betwee n 301 and 1585 m. We studied the composition of ten calcitic and one a ragonitic species, which are often used in paleochemical reconstructio ns. Mg/Ca ratios decrease with increasing water depth in all benthic s pecies, both with calcitic and aragonitic mineralogy, showing a strong correlation with water temperature. Similar decrease is seen in Sr/Ca but with no correlation with temperature. None of the benthic species studied here exhibits a depth or temperature related change in F/Ca. Similar trends are observed when using an ocean-wide dataset, which in cludes shallow and deep core tops (300-5000 m). We suggest that temper ature is the primary control on the Mg content of benthic foraminifera . Based on inorganic precipitation experiments and thermodynamic consi derations, presented here, a 30-40% decrease in the Mg distribution co efficient in calcite may be expected as a result of a temperature chan ge from 25 degrees C to 5 degrees C, which is about half the observed change in LBB. A calibration curve applied to C. pachyderma data from core tops along the slope of Little Bahama Bank suggests that water te mperature may be inferred from Mg/Ca ratios with an uncertainty of abo ut +/-0.8 degrees C. Therefore, the Mg content of benthic foraminifera may provide a new, independent temperature proxy for studying shallow waters paleoceanography. The linear decrease in Sr/Ca with increasing depth is not correlated with temperature; the trend is constant from the ocean surface down to 5 km, suggesting that pressure related effec ts on the calcification process are a more likely explanation than pos t-depositional dissolution. Mg/Ca ratios in aragonitic shells of H. el egans covary with temperature, in accord with recent observations from corals. In contrast, the Sr and F chemistry of H. elegans is very dif ferent than that of corals and inorganically precipitated aragonites. The disparities between the elemental composition of biogenic and inor ganic phases and the large intergeneric and interspecific differences observed both in planktonic and benthic foraminifera implicate tempera ture related physiological processes in regulating the coprecipitation of elements in foraminiferal shells. Our work demonstrates that Cd/Ca ratios of shallow calcitic species reflect the vertical distribution of nutrients; no significant influence of temperature on the partition ing of Cd into the shells was found. Our data extend the previous deep water calibration (Boyle, 1992), thereby allowing for the reconstruct ion of the nutrient chemistry of shallow thermocline waters. Copyright (C) 1997 Elsevier Science Ltd.