The B/Ca proxy for deep-water carbonate ion concentration: A new species-specific calibration for Nuttallides umbonifera

Reconstructions of deep-ocean paleocarbonate ion concentration ([CO32-]) can offer significant constraints on pCO2 by elucidating the CaCO3 cycle’s contribution to pCO2 variations. Recently, a potential proxy for paleo-[CO32-] was established; Yu and Elderfield (2007) show that the boron calcium ratio (B/Ca) in individual species of modern benthic foraminifera tracks linearly with deep water Δ[CO32-]. While this proxy has thus far been used to reconstruct Δ[CO32-] of North Atlantic deep water on glacial/interglacial timescales (Yu and Elderfield, 2008), extending our ability to reconstruct Δ[CO32-] deeper into the Cenozoic would open many further opportunities for constraining the relationships between ocean and atmosphere in the global carbon cycle. To that end, we are working to complete a new core-top calibration for Nuttallides umbonifera, the closest extant relative to Nuttallides truempyi, a long-lived and fairly common Paleogene species. We measured B/Ca ratios of both Cibicidoides wuellerstorfi and N. umbonifera from several Atlantic and Pacific core-tops. The values for C. wuellerstorfi (~130 - 225 μmol/mol) are in perfect agreement with the calibration by Yu and Elderfield (2007) (Fig. 1). Preliminary data for N. umbonifera, however, are consistently offset toward lower concentrations (~85 - 130 μmol/mol). While the absolute B concentrations differ, it appears that the overall sensitivity to Δ[CO32-] is similar (Fig. 1). This result should facilitate the calibration of N. truempyi B/Ca ratios to constrain [CO32-] of past oceans, particularly during periods characterized by major shifts in climate and the carbon cycle, such as the Eocene-Oligocene and Paleocene-Eocene boundaries. Yu, J. and Elderfield, H. (2007) EPSL, 73-86. Yu, J. and Elderfield, H. (2008) EPSL, 209-220. Figure 1. B/Ca ratios of modern C. wuellerstorfi plotted against carbonate ion concentration (Yu and Elderfield, 2007). We have added our own data for core-top C. wuellerstorfi and N. umbonifera analyzed by ICP-MS.