THE DISTRIBUTION OF MG IN PLANKTONIC FORAMINIFERA TESTS: IMPLICATIONS FOR MG/CA PALEOSEAWATER THERMOMETRY AND HABITAT MIGRATION

The distribution of Mg and other trace elements within planktonic foraminifera tests have been determined using a high resolution, laser ablation ICPMS technique. Using this technique, Mg/Ca ratios can be profiled through test walls of sequentially precipitated chambers that form each test. Simultaneous analysis of Mg and other trace elements (e.g. Mn, Zn, Sr, Ba, Cd, U) and their isotopes, with detection limits to low ng/g levels, is undertaken in <60 seconds. The mass of material consumed by each analysis is ~20-30 ng (cf. typical test mass of ~10-30 µg) and replicate analyses can be made on individual chambers. Mg/Ca ratios can be reproducibly measured within particular chamber wall layers to ±2-5 % and more uniformly distributed Sr/Ca ratios to ±1 %. Studies on key planktonic species used in paleoclimate reconstruction (e.g. Globigerinoides sacculifer, Globigerinoides ruber, Neogloboquadrina sp.) define the capabilities of our technique, highlight the inherent limitations of conventional bulk analysis methods, and provide new insights into biomineralisation processes and habitat migration during adult stages of foraminifera life-cycles. The ability to measure Mg/Ca in shell parts grown under specific seawater conditions, rather than bulk compositions integrated over the life-cycles of multiple whole shells, promises the ability to calibrate more accurate and reliable Mg/Ca thermometers than is currently possible, to assess the variability and the mean of seawater temperature within a deep-sea core sample interval, and to reconstruct changes in the thermocline. The tiny amount of sample consumed further allows for subsequent d¹⁸O microanalysis of the same shell material, and presents the potential to more reliably reconstruct paleoseawater salinity.