Controls on boron incorporation in cultured tests of the planktic foraminifer Orbulina universa

Culture experiments with living planktic foraminifers reveal that the ratio of boron to calcium (B/Ca) in Orbulina universa increases from 56 to 92 mu mol mol(-1) when pH is raised from 7.61 +/-0.02 to 8.67 +/-0.03 (total scale). Across this pH range, the abundances of carbonate, bicarbonate, and borate ions also change ( + 530, -500, and + 170 mu mol kg(-1), respectively). Thus specific carbonate system control(s) on B/Ca remain unclear, complicating interpretation of paleorecords. B/Ca in cultured O. universa also increases with salinity (55-72 mu mol mol(-1) from 29.9-35.4 parts per thousand) and seawater boron concentration (62-899 mu mol mol(-1) from 4-40 ppm B), suggesting that these parameters may need to be taken into account for paleorecords spanning large salinity changes (similar to 2 parts per thousand) and for samples grown in seawater whose boron concentration ([B](SW)) differs from modern by more than 0.25 ppm. While our results are consistent with the predominant incorporation of the charged borate species B(OH)(4) - into foraminiferal calcite, the behavior of the partition coefficient K-D. (defined as [B/Ca](calcite)/[B(OH)(4)- / HCO3-](seawater)) cannot be explained by borate incorporation alone, and suggests the involvement of other pH-sensitive ions such as CO32- For a given increase in seawater B(OH)(4) -, the corresponding increase in B/Ca is stronger when B(OH)(4) - is raised by increasing [B](sw) than when it is raised by increasing pH. These results suggest that B incorporation controls should be reconsidered. Additional insight is gained from laser-ablation ICP-MS profiles, which reveal variable B/Ca distributions within individual shells. (C) 2011 Elsevier B.V. All rights reserved.