A stable isotope study of pyrite formation in the Late Pleistocene and Holocene sediments of the Black Sea

Abstract

The abundance and isotopic composition of total and pyrite sulphur have 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 horizons of Unit 3 (Lake Beds). An intermediate mud-flow horizon and the upper part of the lake beds have similar values to those in the modern sediments. The degree of pyritization (DOP) is lowest (<0.05) in the lower lake beds and highest (0.55) in Unit 1; the sapropel has values of ca. 0.48. δ 34S pyrite varies from a minimum of −37‰ in the sapropel, to −33‰ in Unit 1, up to +15‰ in the upper lake beds, and 0‰ in the lower lake beds. The Unit 1 and sapropel values are similar to the δ 34S values reported by Fry et al. (1991) of dissolved sulphide immediately below the water-column oxic-anoxic interface (−36 to −38‰), but significantly heavier than those in the deep waters of the basin (ca. −41‰. These results are interpreted as a reflection of the presence of four different types of 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 immediately below the oxic-anoxic interface where δ 34S of dissolved sulphide is −36‰; (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 dissolved sulphate than those in which the pyrite in Units 1 and 2 were formed; (3) pyrite in the upper part of the Lake Beds (Unit 3) that was formed under closed-system conditions by reaction between excess reducible 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 formed from the ambient sulphate in the freshwaters of the Late Pleistocene lake. The isotopic information supports earlier suggestions that pyrite in the modern sediments of the Black Sea forms mainly in the water column, with only minor amounts forming in the sediment, because of the 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.