Seawater sulphate heritage governed early Late Miocene methane consumption in the long-lived Lake Pannon

Abstract

Sulphate deficiency makes lake environments a key source of atmospheric methane because sulphate-driven anaerobic oxidation of methane (SD-AOM) is hindered. Miocene to Pliocene Lake Pannon was the largest European lake, although the extent of its methane production remains unclear. When sulphate is available, SD-AOM occurs commonly in anoxic sediments and is recorded by authigenic iron sulphides. Here we identify abundant tubular pyrite and greigite aggregates from Lake Pannon sediments, which record brackish early lake conditions at ~11.3 Ma. The iron sulphides are morphologically similar to those reported elsewhere from methane seep and gas hydrate environments. Microscale sulphur isotope analysis and multiple sulphur isotope patterns of iron sulphides, and elevated sedimentary Ba/Al and Mo/Al ratios, are consistent with their derivation from SD-AOM under marine conditions. These results indicate substantial methane consumption in early Lake Pannon, and that sulphate was the dominant oxidant for AOM, which reduced the release of sedimentary methane to the atmosphere. SD-AOM signals archived by iron sulphides are valuable for evaluating methane consumption in lacustrine sediments.