Encrustation and trace element composition of Neogloboquadrina dutertrei assessed from single chamber analyses – implications for paleotemperature estimates

[摘要] Crust formation is a common phenomenon in planktonic foraminifera. Becauseof their different formation mechanism and hence composition, crusts affectthe overall test composition and therefore complicate the use ofcrust-bearing foraminifera in paleoceanography. Such species are often usedto estimate subsurface paleotemperatures, and although the influence of encrustation on the trace element/Ca ratio is recognised, it has not beensystematically explored between and within tests. Here we use laser ablationICP-MS to assess the variability in trace element composition of the crustof Neogloboquadrina dutertrei within individual chambers, as well as the effect of compositionalheterogeneity of the crust on whole test chemistry. Compositionally, theouter crust differs from inner layer by lower Mg/Ca and Mn/Ca, but isindistinguishable in Sr/Ca. Crust thickness decreases towards the youngerchambers, and it may be entirely absent from the last chamber. In contrast toMn/Ca and Sr/Ca, crustal Mg/Ca ratios show a tendency towards higher valueson the younger chambers. These patterns in crust thickness and in crustMg/Ca indicate that temperature is not the dominant factor controlling crustcomposition. Temperature estimates based on N. dutertrei, and presumably othercrust-forming species too, are therefore biased towards too low values.Through comparison of modern and glacial tests, we show that this bias is notconstant and that changes in crust thickness and/or in Mg/Ca values canspuriously suggest temperature changes.

[学科分类] 地球化学与岩石