Sulphur speciation in anoxic hypersaline sediments from the eastern Mediterranean Sea

Abstract

The anoxic hypersaline Tyro and Bannock Basins are among the most sulphidic bodies of water in the marine environment (H 2S > 2 mM). We report the distribution of elemental sulphur, Acid Volatile Sulphur (AVS), organic polysulphides, humic sulphur (0.5 M NaOH extractable), and pyritic sulphur in the sediments from these basins. Pyritic sulphur appears to be the main phase of inorganic reduced sulphur (50–80% of the total sulphur pool) and is at the same level (about 250 μmoles per gram dry weight) in cores recovered from the two basins. Remarkably, humic sulphur has been found to account for 17–28% of the total sulphur pool in the Tyro Basin, and for 10–43% in the Bannock Basin. Sulphur isotope data show negative δ 34S values for both pyritic sulphur (δ 34S: −19%c to −39%) and humic sulphur (δ 34S: −15% to −30%). This indicates that both pyritic and humic sulphur originate from microbially produced HZS. Neither basin shows a significant correlation between pyritic sulphur and organic carbon, suggesting syngenetic pyrite formation. Additionally, the degree of pyritization in these sediments (DOP is 0.62) indicates that pyrite formation is limited by the reactivity of Fe. Humic sulphur correlates with the pyritic sulphur distribution and seems to be related with gelatinous pellicles (bacterial mats at the interface between oxic seawater and brine). Both pyritic and humic sulphur can be formed in the water column at the interface of oxic seawater and brine by a syngenetic pathway; about 5% of the pyrite is formed in the sediments by diagenetic processes. The presence of a bacterial mat and high HZS content in the brines favour such conditions. However, most of the humic sulphur inventory can be supported by diagenetic processes. Although these basins differ in their major element chemistry, their reduced sulphur species chemistry appears to be similar.