Mg/Ca variation in planktonic foraminifera tests: implications for reconstructing palaeo-seawater temperature and habitat migration

• Published in: Earth and Planetary Science Letters
• Main Authors: Eggins, Stephen, De Deckker, Patrick, Marshall, John
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: Elsevier
• Subjects: Keywords: calcium; chemical composition; magnesium; paleotemperature; planktonic foraminifera; seawater; shell Laser ablation ICP-MS; Mg/Ca seawater thermometry; Palaeoceanography; Trace elements; Neogloboquadrina pachyderma; Planktonic foraminifera
• Online Access: http://hdl.handle.net/1885/87700; https://doi.org/10.1016/S0012-821X(03)00283-8

The nature of compositional variability within the tiny calcitic shells (tests) that are precipitated by planktonic foraminifera has been investigated using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Systematic large and correlated variation of Mg/Ca, Mn/Ca, Ba/Ca and Zn/Ca but relatively uniform Sr/Ca are observed through the test walls of analysed species (Globigerinoides sacculifer, Globigerinoides ruber, Neogloboquadrina pachyderma and Neogloboquadrina dutertrei). Distinct chamber and chamber-wall layer compositions can be resolved within individual tests, and Mg/Ca compositional differences observed in sequentially precipitated test components of the different species analysed are consistent with seawater temperature changes occurring with habitat migration during their adult life-cycle stages. Estimated test calcification temperatures are in keeping with available seawater temperature constraints, indicating the potential for accurate seawater temperature reconstruction using LA-ICP-MS. Mg-rich (<1-6 mol% Mg) surface veneers that are also enriched in Mn, Ba, and Zn have been found on all species and all fossil tests, as well as on live-sampled tests of G. ruber, with the latter suggesting a possible biogenic origin. These Mg-rich surfaces bias bulk test compositions toward higher Mg/Ca values by between 5 and 35%.