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 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 BOH4−into foraminiferal calcite, the behavior of the partition coefficient KD (defined as [B/Ca]calcite/BOH4−/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 BOH4−, the corresponding increase in B/Ca is stronger when BOH4− 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.

► A culture calibration with live Orbulina universa reveals new controls on B/Ca. ► B/Ca in cultured planktic foraminiferal calcite is sensitive to the carbonate system. ► B/Ca is also influenced by salinity and seawater boron concentration. ► We do not detect a significant temperature effect on B/Ca across 17.7–26.5 °C. ► Laser-ablation profiles reveal heterogeneous B/Ca distributions within single shells.