A STABLE-ISOTOPE STUDY OF PYRITE FORMATION IN THE LATE PLEISTOCENE AND HOLOCENE SEDIMENTS OF THE BLACK-SEA

The abundance and isotopic composition of total and pyrite sulphur hav e been determined in a core in the central Black Sea. Pyrite varies in concentration from roughly 2 wt% in the sapropel (Unit 2), to 0.8 wt% in the modern horizon (Unit 1) and ca. 0.1 wt% in the lowermost horiz ons of Unit 3 (Lake Beds). An intermediate mud-flow horizon and the up per part of the lake beds have similar values to those in the modem se diments. The degree of pyritization (DOP) is lowest (<0.05) in the low er lake beds and highest (0.55) in Unit 1; the sapropel has values of ca. 0.48. delta(34)S(pyrite) varies from a minimum of -37 parts per th ousand in the sapropel, to -33 parts per thousand in Unit 1, up to + 1 5 parts per thousand in the upper lake beds, and 0 parts per thousand in the lower lake beds. The Unit I and sapropel values are similar to the delta(34)S values reported by Fry et al. (1991) of dissolved sulph ide immediately below the water-column oxic-anoxic interface (-36 to - 38 parts per thousand), but significantly heavier than those in the de ep waters of the basin (ca. -41 parts per thousand). These results are interpreted as a reflection of the presence of four different types o f pyrite in the sediments of the Black Sea: (1) pyrite (in Units 1 and 2) that is formed within the upper part of the water column immediate ly below the oxic-anoxic interface where delta(34)S of dissolved sulph ide is -36 parts per thousand; (2) pyrite in the mud-flow layer lying between Unit 1 and the sapropel (Unit 2), with a significantly heavier isotopic composition, that was formed around the margins of the basin under conditions that were probably more closed with respect to disso lved sulphate than those in which the pyrite in Units 1 and 2 were for med; (3) pyrite in the upper part of the Lake Beds (Unit 3) that was f ormed under closed-system conditions by reaction between excess reduci ble Fe remaining in these freshwater sediments and downward diffusing sulphate and/or sulphide from the overlying organic-rich sapropel; and (4) pyrite in the lower part of the Unit 3 representing a phase forme d from the ambient sulphate in the freshwaters of the Late Pleistocene lake. The isotopic information supports earlier suggestions that pyri te in the modern sediments of the Black Sea forms mainly in the water column, with only minor amounts forming in the sediment, because of th e severe Fe-limitation of the bottom sediments that accumulate 2,000 m below the main Fe and HS- reaction zone at the oxic-anoxic interface at shallow depths in the water column.