Palaeoenvironmental investigation of sapropelic sediments from the Marmara Sea: A biostratigraphic approach to palaeoceanographic history during the last glacial-Holocene

Abstract

High-resolution micropalaeontological examination of two cores recovered from the Central Basin of the Marmara Sea distinguishes seven biostratigraphical zones within pre-sapropelic (Pr1 & 2 zones), sapropelic (Sap1 & 2 zones) and post-sapropelic (Po1-2-3 zones) sediments. The pre-sapropelic sediments at the basal section of the cores reflect the lake stage of the Marmara Sea when it was isolated both from the Black and Mediterranean seas. First colonization of foraminifers at about 11-10.4 ky BP indicate the beginning of marine conditions following the entry of Mediterranean waters through the Canakkale Strait. Sapropelic sediments deposited at about 10.3-6.2 ky BP were associated with enhanced productivity of the surface waters, as inferred from the confined abundance of Globigerina bulloides in sapropelic zones. The first stage of the sapropelic deposition started under anoxic-close to anoxic bottom water conditions and continued in dysoxic-suboxic conditions, as deduced from benthic foraminiferal assemblages. Towards the end of the sapropelic deposition, suboxic conditions dominated. The most enriched 8180 values occur within the sapropelic sediments suggesting that a relative freshening of the surface water must have occurred after deposition of the sapropelic sediments; this observation casts doubt on the postulated strong Black Sea outflow during their formation.