Sulphur enrichment in organic matter of eastern Mediterranean sapropels: Astudy of sulphur isotope partitioning

Sulphur isotope compositions and S/C ratios of organic matter were analysed in detail by combustion-isotope ratio monitoring mass spectrometry (C-irmM S) in eastern Mediterranean sediments containing three sapropels of differe nt ages and with different organic carbon contents (sapropel S1 in core UM2 6, formed from 5-9 ka ago with a maximum organic carbon content of 2.3 wt%; sapropel 967 from ODP Site 160-967C, with an age of 1.8 Ma and a maximum o rganic carbon content of 7.4 wt%; and sapropel 969 from ODP Site 160-969E, with an age of 2.9 Ma and a maximum organic carbon content of 23.5 wt%). Su lphur isotopic compositions (delta(34)S) of the organic matter ranged from -29.5 to +15.8 parts per thousand and the atomic S/C ratio was 0.005 to 0.0 38. The organic sulphur in the sediments is a mixture of sulphur derived fr om (1) incorporation of S-34-depleted inorganic reduced sulphur produced by dissimilatory microbial sulphate reduction; and (2) biosynthetic sulphur w ith an isotopic signature close to seawater sulphate. The calculated biosyn thetic fraction of organic sulphur in non-sapropelic sediments ranges from 68-87%. The biosynthetic fraction of the organic sulphur of the sapropels ( 60-22%) decreases with increasing organic carbon content of the sapropels. We propose that uptake of reduced sulphur into organic matter predominantly took place within sapropels where pyrite formation was iron-limited and th us an excess of dissolved sulphide was present for certain periods of time. Simultaneously, sulphide escaped into the bottom water and into sediments below the sapropels where pyrite formation occurred.