Calibration of the B/Ca proxy in symbiont-bearing planktonic foraminifera for application to the Paleocene-Eocene Thermal Maximum

Abstract:

During the Paleocene-Eocene Thermal Maximum (PETM), rapid surface ocean acidification is indicated by a large decrease in the B/Ca ratios of planktic foraminiferal calcite, which is a proxy for the surface ocean carbonate system [1]. However, due to uncertainty in the effects of past seawater chemistry (e.g, different [Mg], [Ca], and [B]) on B/Ca, modern calibrations cannot be used to estimate the magnitude of acidification during this critical period. In addition, recent inorganic and sediment trap studies have respectively documented the controls of growth rate and light levels on B/Ca [2,3]. To extend the application of the B/Ca proxy to the PETM, we have conducted culturing experiments in O. universa, G. ruber, and G. sacculifer in which we simulated changes in pH and total DIC under Paleogene seawater conditions- high [Ca], low [Mg], and low [B]. We have further investigated the effects of variable light intensity (a control on symbiont activity), [Ca]_seawater, and [B]_seawater on the proxy. Results from O. universa confirm that B/Ca decreases with increasing DIC, decreasing pH, and decreasing [B]_seawater, supporting a [B(OH)₄^-]/DIC control on the proxy [4]. In contrast, neither low light nor [Ca]_seawater have a measurable effect on B/Ca, implying that influences of these parameters over the PETM were likely negligible. Critically, B/Ca appears to be more sensitive to pH at very low [B(OH)₄^-]/DIC in comparison to modern calibrations. Using estimates of surface ocean pH from boron isotopes, new calibrations can explain a larger proportion of the observed B/Ca excursion over the PETM. However, simulation of a large DIC pulse is necessary to explain the full excursion. New data will be presented from species that are more sensitive to pH, such as G. ruber and G. sacculifer, which will illuminate the range of responses of B/Ca to ocean acidification during the Paleogene.

[1] Penman et al. 2014. Paleoceanography 29. [2] Uchikawa et al. 2015. GCA 150. [3] Babila et al. 2014. EPSL 404. [4] Allen et al. 2012. EPSL 351-352.