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 92μmolmol^-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 +170μmolkg^-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 (55-72μmolmol^-1 from 29.9-35.4‰) and seawater boron concentration (62-899μmolmol^-1 from 4-40ppm 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 0.25ppm. While our results are consistent with the predominant incorporation of the charged borate species BOH4-into foraminiferal calcite, the behavior of the partition coefficient K_D (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 CO₃^2- 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.