Data corresponding to article "Laboratory-grown crust in planktic foraminifera Neogloboquadrina pachyderma; insights into resolving inaccuracies in polar palaeotemperature estimates"
by: A. Westgård, M. M. Ezat, F. E. Sykes, G. L. Foster, J. Meilland, 4, T. B. Chalk, J. A. Milton
Abstract:
Proxy based records of past changes in the polar ocean-cryosphere-climate system can provide invaluable constraints on ongoing and future climate change. However, the dominant planktic foraminiferal species in polar regions, Neogloboquadrina pachyderma, exhibits considerable intra- and inter-test heterogeneity in Mg/Ca and potentially other elemental ratios, compromising their utility as palaeothermometers and tracers of other environmental variables such as carbonate chemistry. In particular, N. pachyderma often exhibits a thick “crust” characterised by low Mg/Ca calcite outside its higher Mg/Ca internal “lamellar calcite”. The cause of this heterogeneity is currently uncertain, but it is variably ascribed to environmental change via depth migration or to a “vital effect” related to variations in biomineralisation mechanism/foraminiferal physiology.
To help resolve this issue, we performed large scale culturing experiments of N. pachyderma under controlled environmental conditions and used Laser Ablation ICP-MS to study the intra-shell variability in trace element ratios. We document, for the first time, that in laboratory-grown N. pachyderma the calcite crust has lower Mg/Ca, Na/Ca, and B/Ca than the inner lamellar calcite even when both components are grown in equivalent environmental conditions. This implies that the crust and lamellar calcite of N. pachyderma are deposited via contrasting biomineralisation strategies, not only resulting in distinct geochemical compositions for multiple elements, but also in different sensitivities to changing environmental conditions. Given the crust constitutes a large, but variable percentage of a specimen’s final shell weight, this complicates the application of proxies using this and other similar species. It is therefore recommended that future studies either apply in situ techniques, such as applied here, to analytically separate the crust from lamellar calcite, or target specimens with only one component to avoid the complicating influence of this “vital effect” on environmental reconstruction.
