Geochemistry of Aegean Sea sediments: implications for surface- and bottom-water conditions during sapropel deposition since MIS 5

Abstract

Piston cores collected from the Aegean Sea provide a record of sapropel sequence S1, S3-S5. Primary productivity calculations using the equations of Muller and Suess suggest surface paleoproductivities ranged from 180 to 995 g C m-2 year-1 for sapropels and from 40 to 180 g C m-2 year-1 for nonsapropel sediments with corresponding total organic carbon values of 9%-12% and 1%-3%, respectively. The higher paleoproductivities exceed those in the most fertile modern upwelling zones, so are probably overestimated. Instead, enhanced preservation, particularly for S4 and S5, likely resulted from poor bottom-water ventilation beneath a salinity-stratified water column. If the preservation factor in the equations of Howell and Thunell is increased to account for such conditions, more realistic paleoproductivity estimates ensue. The interpreted presence of a deep chlorophyll maximum layer for S3-S5 within the lower part of the photic zone may account for high marine organic carbon and increased export production. A deep chlorophyll maximum layer is not advocated for S1 because of the presence of N. Pachyderma (d) immediately below S1. The organic geochemical data show that both marine and terrestrial organic matter contributed equally to sapropels S3, S4, and S5. Sapropels S3-S5 were deposited under normal marine conditions with very limited and temporary establishment of near-euxinic bottom-water conditions. Highly depleted and somewhat uniform Δ34S values together with the absence of fully euxinic conditions during sapropel intervals suggest that bacterially mediated sulfate reduction took place consistently below the sediment-water interface. It is believed that climbing levels of primary productivity triggered the onset of sapropel deposition, but that other contemporaneous factors extended and enhanced the conditions necessary for sapropel deposition, including increased nutrient supply from riverine inflow, water column stratification and reduced oxygenation of bottom waters, and buffering of low bottom-water oxygen levels by accumulating terrestrial organic carbon.