Geochemical characteristics of the recent sediments from the Sea of Marmara

Abstract

Six box-cores, one boomerang core and one hundred sixty-six surface sediment samples recovered in the Sea of Marmara were analyzed in an attempt to establish the main controls on distribution and origin of trace metals within the oxic to suboxic basin of this sea. Surface and downcore data indicate that the abundance and distribution of the total metal concentrations for Fe, Ni, Zn, Cr, Co, Cu and Pb can largely be explained by the variable admixtures of terrigenous components in response to textural variations and biogenic calcareous components poor in metal contents. The distribution of Cr and Ni seems to be controlled, in part, by a contribution from the weathering of basic/ultrabasic sources on the coastal hinterland. A small but significant amount of Cu, Zn and Pb in sediments from the northeastern Sea of Marmara probably suggests a pollution effect from the northerly located Golden Horn Estuary that is known as one of the most polluted coastal regions of Turkey. Looking at the data, it seems that the Mn concentrations are unusually high in the deep-water sediments (4498–9127 ppm, carbonate free), along the east-west-trending Marmara Trough but low in sediments from the adjacent shallow waters (307–2059 ppm, carbonate free). The Mn enrichment in deep-water relative to shallow-water Marmara sediments is further indicated by the Fe/Mn ratios being, on average, 11–23 in deep waters and 34–69 in shallow waters. The contents of other metals (Fe, Ni, Zn, Cr, Co, Cu, Pb) in both shallow- and deep-water sediments fall nearly within the same range as those found in average crustal and sedimentary rocks, suggesting no significant enrichment of these metals in the deep Marmara basins. the downcore changes in the color, from reddish-brown at the surface (top 2–5 cm) to greenish-gray in the subsurface (5 cm), together with the occurrence of high Mn concentrations in the surface sediment layers in cores can be attributed, at least in part, to the diagenetic Mn enrichment within the sediment column. The usually low organic carbon contents and biogenic components present in sediments reflect that the well-known biogenic mechanism for the enrichment of Mn seems to be of minor importance in the deep Marmara waters. The generally low Mn concentrations contained in the surface (grab) and core sediments from the Black Sea and Aegean Sea approaching those of the Sea of Marmara do not provide any conclusion to support significant Mn contribution from these two adjacent seas, particularly from the so-called “manganese pump” zones in the shallow Black Sea waters. Comparison with other deep-sea sediments which have been deposited under the influences of hydrothermal activities, worldwide, suggests Mn enrichment in deep-water relative to shallow-water sediments of the Sea of Marmara, mainly as a result of hydrothermal contributions from this seismically active marine region. Based on the average Fe/Mn ratios obtained in cores, which tend to increase gradually from ∼11 in the deep Central Basin to ∼67–69 in the northeastern and southwestern shelves, the Central Basin of Marmara Trough can be considered as a possible, potential source for hydrothermal solutions leading to the enrichment of Mn in the deep Marmara waters. However, this remains to be further investigated.