Earthquake-induced turbidite deposition as a previously unrecognized sink for hydrogen sulfide in the Black Sea sediments

Abstract

The depth profiles of excess 210Pb, 137Cs, elemental sulfur, reactive iron and porewater hydrogen sulfide of a western central basin sediment core in the Black Sea collectively point to the presence of a 20 cm thick reactive iron rich turbidite layer. This layer was most probably deposited there after the 1999 earthquakes in Northwestern Turkey, which caused oxidation of porewater hydrogen sulfide and anomalous accumulation of the product elemental sulfur in the solid phase. A time-variable mathematical model was constructed to explore the non-steady state diagenesis of Fe–S species in a turbidite mud emplaced on normal anoxic–sulfidic deep Black Sea sediments. The 20 cm thick turbidite layer initially contained 80 µmol (g dry wt.) − 1 of Fe(III) in the form of goethite, which led to the rapid (1 day) oxidation and depletion of porewater sulfide and formation of solid phase sulfur intermediates such as S(0) and FeS. S(0) accumulated faster than FeS and was present in the sediment at high concentrations after 5–8 years following the deposition of the turbidite. Reduced iron was in the solid phase long after the consumption of all Fe(III), enabling the coexistence of S(0) and solid phase reactive iron (which has not been sulfidized yet) for a long period of time. The porewaters had sub-micromolar concentrations of dissolved sulfide in the first 2 years after deposition with Fe(II) exceeding 1 mM. The turbidite layer should serve as a sink for the sulfide from the overlying waters for about 10 years after its deposition. Considering the ubiquity of the deep-basin turbidites found in the Black Sea and the dense tectonic activity on the Northern Anatolian Fault in Northern Turkey, an estimated 0.144 Tg sulfur/year could be buried as S 8 and FeS 2 due to the earthquake-induced turbidites. This value is about 14% of the annual sulfur burial (as pyrite) and 3–5% of the annual sulfide oxidation due to lateral injections of oxic Mediterranean waters to the anoxic–sulfidic Black Sea waters. It is highly likely that earthquake-induced turbidites and subsequent sulfide oxidation in the deep basin represent a previously unrecognized sink in the budget of hydrogen sulfide in the Black Sea.