Calibration of the B/Ca Proxy in Cultured O. Universa Tests Under Paleogene Seawater Conditions

Abstract:

The B/Ca ratio of fossil planktic foraminiferal calcite, a proxy for surface ocean carbonate chemistry, holds promise for reconstructing carbonate system changes due to ocean acidification across the Paleocene-Eocene Thermal Maximum (PETM) at 56 Ma. However, uncertainties in how the proxy behaved under Paleogene seawater chemistry (low [B], low [Mg], high [Ca]) limit the application of modern calibrations during this critical period.

To extend the application of the B/Ca proxy to the PETM, we have undertaken culturing experiments with the symbiotic planktic foraminifer Orbulina universa and simulated changes in pH and total DIC appropriate to Paleogene seawater conditions. In addition, we have further investigated the effects of variable [B]_seawater(SW) on the proxy. Results confirm previous findings that boron incorporation into the calcite test decreases with increasing DIC, decreasing pH, and decreasing [B]_SW [1]. However, B/Ca values in Paleogene experiments are elevated above values predicted from boron concentration experiments at low [B]_SW. This may be due to elevated [Ca]_SW, which has been shown to raise B/Ca in inorganic precipitate experiments [2], and must be further investigated as [Ca]_SWhas varied significantly over geological time [3].

Most importantly, the proxy appears to be more sensitive to changes in pH under Paleogene conditions than in modern seawater, which helps to explain the large B/Ca decrease observed over the PETM [4]. However, assuming a pH drop from 7.8 to 7.53 as estimated from boron isotope analyses (Penman et al. 2014), our calibrations suggest that a large fraction of the observed B/Ca decrease cannot be explained by a pH decrease alone. The remaining B/Ca decrease can likely be attributed to an increase in DIC, but calibrations with additional species will be needed to further evaluate the sensitivity of the proxy to changes in carbonate system parameters across the PETM.

[1] Allen et al. (2012). EPSL 351-352. [2] Uchikawa et al., submitted to GCA. [3] Lowenstein et al. (2003). Geology 31. [4] Penman et al. (2014). Paleoceanography 29.