Exploring the potential for time-variable vital effects in planktic foraminifera, Globigerinoides Ruber

Abstract

The boron isotopic composition (δ¹¹B) of planktic foraminifera is a powerful proxy for reconstructing past seawater pH and thus atmospheric CO₂. Globigerinoides ruber has been used extensively to generate high resolution δ¹¹B-CO₂ records for the last 3.5 million years. However, these records rely on the assumption of stability in the empirical δ¹¹B_ruber-to-δ¹¹B_borate calibration, which was established using culture and core-top data obtained from modern specimens. It remains unclear whether this modern calibration is applicable on geological timescales. Here, we investigate this in a number of ways. Firstly, we examine the depth distribution of G. ruber during the Plio-Pleistocene Transition. We then report a suite of geochemical proxies including δ¹¹B for G. ruber across a range of test sizes and morphotypes and finally compare G. ruber δ11B reconstructions of seawater borate ion, pH and CO₂ to those from Trilobatus trilobus, another surface dwelling planktic foraminifera species. Taking these data together, we find no evidence for a change in G. ruber vital effects or depth habitat during the Plio-Pleistocene Transition with respect to those of the Holocene. We conclude that the utilization of the present δ¹¹B_ruber-to-δ¹¹B_borate calibration is suitable for at least the last 3 million years.

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Identifiers

ORCID for Megan R. Wilding: orcid.org/0000-0003-0063-2238

ORCID for Bastian Hambach: orcid.org/0000-0003-4546-5672

ORCID for G.L. Foster: orcid.org/0000-0003-3688-9668

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