Trace metals and organic carbon in sediments of the northeastern Chukchi Sea

Concentrations of Al, Fe and selected trace metals (As, Ba, Be, Cd, Cr, Cu, Hg, Mn, Ni, Pb, Sb, Se, Sn, V, Zn) in surface sediments from the northeastern Chukchi Sea (NECS) varied considerably as a function of sediment texture (i.e., grain size); however, essentially all metals, when normalized to Al, were present at natural, background values. Total organic carbon content ranged from 0.06–1.56% with a marine origin for at least two-thirds of the organic matter at each station based on δ13C values and C/N ratios. Recent sedimentation rates from 137Cs and 210Pb dating were 0.03–0.14 cm yr−1. Corresponding vertical distributions for most metals in age-dated sediment cores showed >100-yr records of uniform metal/Al ratios, including Pb and Hg. Concentrations of As, Mn and Cd were modified in surface sediments at a few stations by early chemical diagenesis. Surveys around two exploratory oil and gas drilling sites that were occupied in 1989 showed that Ba concentrations were as high as 10,000 µg g−1 within 200 m of one drilling site relative to background values of ~700 µg g−1. Barium enrichment was from barite, a drilling mud additive that was discharged to the seafloor. Above background concentrations of Cu, Hg, Pb and Zn also were found in sediments from 3–4 stations within 200 m of the two drilling sites. At present, sediments in the NECS are essentially pristine with respect to trace metals of anthropogenic origin, excluding small areas around drilling sites. These data should be of value for detecting and tracking future chemical changes in the sediments of the NECS, particularly in light of planned oil and gas exploration and production as well as environmental changes that may be associated with climate change.