Trace metal biogeochemistry in methane seeps on the Koryak slope of the Bering Sea

Methane seeps are widely distributed in the ocean being represented at all depths and latitudes. Along with hydrocarbons, seeping fluids can supply many chemical elements, including trace metals which benthic communities are needed. Under the condition of climate warming, the methane diffuse flow in the ocean can affect chemical balance. In this study for the first time, in the methane seep ecosystem recently discovered on the Koryak slope of the Bering Sea, we examined the distribution of some essential and potentially toxic chemical elements. In the benthic organisms, biotope water, and surface sediments, concentrations of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Zr, Mo, Cd, Ba, W, Pb, and U were quantified. Dissolved methane in water, total carbon (TC), and total inorganic carbon (TIC) in organisms and sediments, as well as the mineral composition of sediments, were also determined. Our data showed that bioaccumulation of many trace metals in benthic organisms is related to high concentrations of these elements in near-bottom water of methane cold seep habitats. Significantly elevated concentrations of many trace elements, especially chalcophile, were recorded in the gills and to a lesser extent, in the rest of the soft body of chemosymbiothrophic bivalve mollusk Calyptogena pacifica. Together with the essential elements Cu, Zn, Co, Ni, Cr, Mo, and Cd, potentially toxic As, Pb, U, and W also exhibited noticeably higher accumulation in the soft tissues of organisms. In all cases, tissues and whole organisms which are enriched in TOC exhibited higher trace element concentrations and BCF values. Bioconcentration Factor (BCF) exhibited a trend to decrease from the gills of C. pacifica (n106), to whole bodies of suspension feeders (n104). Reducing conditions led to strong depletion in most elements of surface sediments with the enrichment factor (EF) being around 1. Exceptions were Mo and Ba whose EF values (up to 2.2) suggest detectable authigenic enrichment.