The Distribution of Dissolved Barium from US GEOTRACES cruises in the North Atlantic and Eastern Tropical South Pacific

Abstract:

Interest in the oceanic geochemistry of barium (Ba) stems from a variety of reasons including its use as a paleo-productivity indicator, its chemical similarity to Ra, and its utility as a water source tracer. To better constrain these uses of Ba, we have obtained trace element clean samples from both the North Atlantic and Eastern Pacific US GEOTRACES cruises. Analytical work on the Pacific samples is proceeding while work on the Atlantic samples is complete. For the Pacific, 36 stations were occupied from Peru to Tahiti. For the Atlantic, dissolved Ba was determined at 32 stations across the North Atlantic during US cruises GT10 and GT11 along the meridional transect from Lisbon to the Cape Verde Islands and the zonal transect from Cape Cod to the Mauritanian coast. In the Atlantic, the general distribution of dissolved Ba exhibits a vertical bifurcation at approximately 500 m into shallow versus deep water. The greatest variation is found on the eastern side of the basin with concentrations ranging from 35 nmol/kg at the near surface (100 m) to over 83 nmol/kg at depth. A reduction of Ba in excess of 20% compared to the average of mesopelagic depths less than 500 m is observed within the Canary Current upwelling zone east of the Cape Verde Islands and accompanied to some extent by a subsequent regeneration at depth. Below 500 m, dissolved Ba correlates well with dissolved Si, whereas the correlation with alkalinity is poor at depth and shows a decoupling above 500 m. There is evidence of hydrothermal Ba input at the TAG vent system of the Mid-Atlantic Ridge along transect GT11 as indicated by the rapid increase in the dissolved Ba below 2500 m in conjunction with increases in Fe and Mn. In addition to the hydrothermal source, a near surface (~40 m) maximum of 51 nmol/kg is found along the continental slope of North America in correspondence with a minimum surface salinity (34.75) and increased dissolved manganese indicating either fluvial or sediment input; however, a similar input is not evident along the North African continental margin. We will contrast the Atlantic distribution with the Pacific as data become available.