Biogenic barium fluxes to the deep sea: Implications for paleoproductivity reconstruction

Dymond et al. (1992) have recently proposed an algorithm to reconstruct paleoproductivity from biogenic Ba (bio‐Ba) accumulation rates in sediments. Their equation is based on sediment trap data which indicate that C org /bio‐Ba ratios in settling particles are higher in the western Atlantic compared to the Pacific. From this observation they have suggested that the flux of bio‐Ba to the seafloor may depend on dissolved Ba concentrations in intermediate and deep waters which are significantly higher in the Pacific compared to the Atlantic. Accordingly, they have introduced a factor related to dissolved Ba concentration in their equation as a variable which strongly influences paleoproductivity estimates. In an attempt to confirm the proposed dependency of bio‐Ba fluxes to the seafloor on dissolved Ba concentrations in seawater we have compiled additional data on organic carbon and bio‐Ba fluxes in the deep sea. These data confirm Dymond et al.'s findings that settling particles have significantly higher C org /bio‐Ba in the western Atlantic compared to the Pacific. However, we also found lower ratios in traps deployed in the North Atlantic, similar to those found in the Pacific, while in the Panama Basin we found ratios as high as those in the western Atlantic. From these observations we conclude that dissolved Ba concentration is not an important factor in regulating the flux of bio‐Ba to the seafloor. Instead, we propose that high C org /bio‐Ba ratios found in the western Atlantic, the Panama Basin, the Arabian Sea, and some stations in the Nordic Seas result from the addition of refractory organic carbon from nearby continents, shelves, or slopes. If that is confirmed, the algorithm proposed by Dymond et al. (1992) could be simplified and could provide a powerful means to estimate paleoproductivity. In addition, deviations from the C org /bio‐Ba ratios in settling particles could be used to estimate the input of continental or shelf‐derived refractory organic matter into the deep sea.