HISTORY OF WATER-COLUMN ANOXIA IN THE BLACK-SEA INDICATED BY PYRITE FRAMBOID SIZE DISTRIBUTIONS

A detailed study of size distributions of framboidal pyrite in Holocen e Black Sea sediments establishes the timing of a change from depositi on under an oxic water column to deposition under an anoxic and sulfid ic water column. In the most recent carbonate-rich sediments (Unit I) and in the organic carbon-rich sapropel (Unit II), framboid size distr ibutions are remarkably uniform (mean diameter = 5 mu m); over 95% of the framboids in Unit I and Unit II are < 7 mu m in diameter. These pr operties of framboidal pyrite are consistent with framboid nucleation and growth within an anoxic and sulfidic water column, followed by tra nsport to the sediment-water interface, cessation of pyrite growth due to the exhaustion of reactive iron, and subsequent burial. In contras t, the organic carbon-poor sediments of lacustrine Unit III contain py rite framboids that are generally much larger in size (mean diameter = 10 mu m). In Unit III, over 95% of the framboids are < 25 mu m in dia meter, 40% of framboids are between 7 mu m and 25 mu m, and framboids up to 50 mu m in diameter are present. This distribution of sizes sugg ests framboid nucleation and growth within anoxic sediment porewaters. These new data on size distributions of framboidal pyrite confirm tha t the development of water-column anoxia in the Black Sea coincided wi th the initiation of deposition of laminated Unit II sapropels.