Acantharians: a missing link in the oceanic biogeochemistry of barium

Inductively coupled plasma mass spectrometry was used to analyze the Ba and Sr concentrations of the celestite (SrSO4) skeletons and cysts of individual acantharian specimens obtained from four diverse areas of the world’s oceans. Acantharian celestite Ba/Sr mole ratios (χBa/χSr) averaged 2.6×10-3 with minimum and maximum values of 6.1×10-4 and 2.5×10-2. Celestite Ba/Sr mole ratios were compared to dissolved Ba and Sr concentration ratios ([Ba2+]T/[Sr2+]T) derived from GEOSECS stations that most closely corresponded to acantharian collection sites. Resultant Ba/Sr distribution coefficients (DBa/Sr=(χBa/χSr)/([Ba2+]T/[Sr2+]T)) in diverse areas of the world’s oceans are on the order of three or larger. These data, in conjunction with observations of acantharian mediated Sr2+ depletions in the upper ocean, indicate that acantharians play a substantial role in the global oceanic Ba budget. Observation of Ba enrichments during celestite formation is consistent with expectations based on solidsolution–aqueous-solution precipitation dynamics and the much lower solubility of BaSO4 compared to SrSO4. Furthermore, the small solubility product of RaSO4 relative to SrSO4 and BaSO4 indicates that Ra should be enriched in both celestite and barite. Consequently, acantharians may have a substantial influence on the oceanic distributions of both Ba and Ra.