Fundamentals of biohydrochemistry of anoxic basins

Abstract

The formation of hydrogen sulfide waters in three anoxic basins—the Black Sea, the Cariaco trench, and the Framvaren fjord—is considered. The accumulation of H2S caused by the oxidation of organic matter (OM) with seawater sulfates is characterized by the relationship between the increase in the total (denoted by the T subscript) carbonate carbon (ΔCT) and the total hydrogen sulfide (ΔST), which is close to the stoichiometric ratio ΔCT/ST = 2. The values of ΔCT, ST, and the pH are inversely related. About 99% of the increase in the total alkalinity (ΔAT) is provided by the growth in the sum of the carbonate and sulfide alkalinities (ΔAC and ΔAS, respectively). The calculated values of ΔAT/ΔCT and ΔAT/ΔST correlate well with the experimental data obtained for the three basins. The concentrations of dissolved ammonium, phosphate, and silica increase proportionally to the growth in CT and ST in anoxic waters. The isotope composition of H2S sulfur (δ34S from −40 to −11‰) and the factor of isotope fractionation (Δ34S from 60 to 40‰) show that the intensity of the sulfate reduction in the waters of the basins differ by 3–4 orders of magnitude: from the maximum value in Framvaren fjord to the minimum value in the Black Sea. The isotope composition of the carbon in dissolved carbonates formed by OM mineralization is distinct from the composition of plankton and organic detritus, being enriched in 13C by 4–7‰. The hydrochemical character of an anoxic basin is determined by two factors: by the intensity of sulfate reduction related directly to the flux of detrital organic matter to the bottom and by the intensity of the anoxic process dependent not only on the sulfate reduction intensity but also on the exchange time for deep and surface waters in the basin (the lifetime of the anoxic zone).