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

Abstract

Culture experiments with living planktic foraminifers reveal that the ratio of boron to calcium (B/Ca) in Orbulina universa increases from 56 to 92 μmol 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 μmol 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 μmol mol − 1 from 29.9–35.4‰) and seawater boron concentration (62–899 μmol mol − 1 from 4–40 ppm B), suggesting that these parameters may need to be taken into account for paleorecords spanning large salinity changes (~ 2‰) 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 − / H C O 3 − seawater ) cannot be explained by borate incorporation alone, and suggests the involvement of other pH-sensitive ions such as CO 3 2 − 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.