Sinking particles in the Black Sea waters: Vertical fluxes of elements and pyrite to the bottom, isotopic composition of pyrite sulfur, and hydrogen sulfide production

Abstract

The Black Sea is the largest anoxic basin with hydrogen sulfide at depths below 100 m. Anoxic condition significantly affects fluxes of particulate matter due to consuming organic material for sulfate reduction and syngenetic pyrite formation. Composition of sinking particles was studied for 6 water depths of the Black Sea (100–1750 m) with deployment period of over one year. As a result, the data on chemical and mineral composition of sinking particles including the contents of reduced sulfur species (acid volatile sulfides and Cr reducible species) and isotopic composition of Cr reducible sulfur species were obtained.Mineral composition of trapped material changes with depth mainly due to dissolution of plankton carbonate shells, which decreases to the bottom to reach 90%, and also due to oxidation of organic carbon (by 59%). While sinking, particulate matter loses Fe, S, U, Co, Mo, Tl, and Ag to the least extent. Framboidal pyrite appears in sinking particles in the upper part of anoxic water column shallower than 180 m. Its quantity increases down to the deeper waters amounting to 0.4 wt% of dry weight. The δ34S values in syngenetic pyrite from depths of 980, 1680, and 1775 m range from −42.2 to −42.6‰ vs. VCDT and were found to be close to those of sulfur isotopic composition in dissolved hydrogen sulfide.Based on a decrease of particulate organic carbon flux, the annual production of hydrogen sulfide in 2015–2016 was calculated to be 5.9 Tg H2S /year, the main part of which (80%) generates above a depth of about 980 m. Our estimate of H2S consumption for pyrite formation in the water column is 0.030 Tg H2S /year (0.5% of annual H2S production) which is believed insignificant for hydrogen sulfide balance in the Black Sea.