A geochemical study of the Erdek Bay sediments in the Sea of Marmara, Turkey, was carried out to investigate the heavy metal distributions and sources. The bay has an average depth of 34 m and a two-layer water column stratification, with a pycnocline at 20 m. The distribution of "total" metal (Fe, Mn, Pb, Cu, Ni, Cr, Zn and Hg) concentrations, together with sequential selective extraction analyses, shows that the relatively high metal concentrations are mainly due to land-based natural inputs from the erosion products of mineralized zones and mafic-ultramafic rocks delivered to the bay by two rivers from the south. However, some Pb, Zn and Cu occur in the Fe-Mn-oxyhydroxide and organic fractions, suggesting, in part, some anthropogenic inputs in addition to the predominantly natural inputs. The vertical distribution of metal/Al ratios in a gravity core in the bay shows no significant change with depth, except for an early diagenetic enrichment at 0.7-1.6 m below sea floor (mbsf) above a 4750-3500-year-old sapropelic layer. This distribution is a further indication of the insignificant antropogenic metal pollution in the bay. (C) 2001 Elsevier Science Ltd. All rights reserved.