Calibrating Mg/Ca ratios of multiple planktonic foraminiferal species with δ~(18)O-calcification temperatures: Paleothermometry for the upper water column

摘要

In order to consistently approximate the thermal vertical structure of past upper water columns, Mg/Ca ratios of eight planktonic foraminiferal species with different preferential calcification depths selected from 76 tropical Atlantic and Caribbean sediment-surface samples were calibrated with δ~(18)O-derived calcification temperatures with an overall range of ≈ 8-28 °C. Extending the broad number of species-specific calibrations, which agree well especially with our shallow-dweller calibrations, this study presents new bulk calcite Mg/Ca vs. calcification temperature relationships for shallow-dwelling Globigerinoides ruber pink, thermocline-dwelling Globorotalia menardii, and deep-dwelling Globorotalia truncatulinoides dextral and Globorotalia crassaformis not separately calibrated before. The species-specific temperature sensitivities are relatively similar (≈ 7-11% increase in Mg/Ca per 1 °C), yet y-axis intercepts vary from 0.23-0.65 for the shallow and thermocline dwellers to 0.83-1.32 for the deep dwellers. Based on these differences, we established a 'warm water' calibration for temperatures > 19 °C (Mg/Ca = 0.29·exp(0.101·T); r = 0.90; shallow and thermocline dwellers) and a 'cold water' calibration for temperatures < 15 °C (Mg/Ca = 0.84·exp(0.083·T); r = 0.85; deep dwellers). These calibrations are offset by ≈ 8 °C. This may be significant for paleotemperature reconstructions, which are afflicted with the problem that similar Mg/Ca offsets are probably characteristic of extinct species used to calculate past temperatures.