Barium in planktonic foraminifera

Abstract

Reconstructions of Ba distributions in ancient oceanic surface waters could provide new insight into paleoceanographic change. Calcite shells of planktonic foraminifera potentially provide a means of reconstructing such paleo-Ba distributions if lattice-bound Ba can be determined on shells recovered from deep-sea cores. Planktonic foraminifera shells from a series of cores were purified of non-lattice-bound Ba associated with organic or sedimentary phases by a combination of physical agitation, oxidative-reductive steps, acid leaches, and a novel alkaline-DTPA step to dissolve barite. A sequential dissolution of a large sample of cleaned shells of the planktonic foraminifer Globigerinoides conglobatus indicates homogeneous distribution of Ba in the shell material. Comparison of shells from sediments, sediment traps, and plankton tows indicates no significant differences in the Ba content of the purified shells. Cleaned samples of the planktonic foraminifera Globigerinoides sacculifera, G. ruber, G. conglobatus, Orbulina spp., and Globoquadrina dutertrei from the equatorial Pacific, North Atlantic, and Mediterranean Sea have Ba Ca ratios between 0.6 and 1.0 μmol/mol (0.8 to 1.4 ppm). Variation in foraminiferal Ba contents between the three basins is consistent with the trend in surface seawater Ba. The calculated distribution coefficient for Ba incorporation in these five species based on these data is 0.19 ± 0.05. Several species of the non-spinose planktonic foraminifera Globorotalia have Ba Ca ratios ranging from 2 to 13 μmol/mol; these high Ba contents might be explained by differences in the way these foraminifera precipitate their shells. A temporal record of Ba Ca in samples of Globigerinoides and Orbulina from a core in the northwest Atlantic suggests that the Ba concentration of surface waters at this site has not changed by more than 20% over the last 14 kyr.