A mid-late Holocene sapropelic sediment unit from the southern Marmara sea shelf and its palaeoceanographic significance

Abstract

A sapropelic layer, having an age of ca 4750 and 3500 (14)C y BP, was discovered at 0.90-2.35 m below the sea floor (mbsf) in gravity cores from the southern shelf of the Marmara Sea. It is a 15-50 cm thick, phosphorescent green to grey, plastic, clayey hemipelagic mud horizon, containing 1.5-2.9% organic carbon (C(org)). The increase in C(org) and biogenic carbonate, together with a rich planktonic foraminiferal fauna, indicate increased organic productivity and warm surface waters during the deposition of the sapropelic layer. The down-core profiles of Mn, Fe, Cu, Zn, Pb, Cr, Ni and Co suggest that the sapropelic layer was deposited through an essentially oxic water column. The benthonic foraminiferal fauna indicates reduced oxygen levels in bottomwaters. The sapropelic unit was deposited during a high stand of global sea level. Its deposition was initiated by a large input of terrestrial organic matter and nutrient-rich fresh waters under a relatively warm and wet climate. The fresh water supply caused a strong water stratification, which, in turn, together with high organic matter input, resulted in reduced oxygen levels in the bottomwaters. (C) 1999 Elsevier Science Ltd. All rights reserved.