Trace element signatures in sedimentary pyrite as indicators of methane seepage activities

Abstract

Marine methane seeps are increasingly important to understand the impact of methane release into the water column and potentially the atmosphere. Among the various proxies used to reconstruct methane seepage, authigenic pyrite is a potentially important recorder. Pyrite is a significant carrier of trace elements, whose abundance is closely linked to the depositional environment where they form. Methane seepage environments are characterized by extensive pyrite enrichment. However, it remains to be determined whether the trace elements in pyrite can reliably provide information on the processes associated with methane seepage. We analyzed trace elements abundance in pyrite within sediment collected at the Haima cold seep area, South China Sea, using in-situ laser ablation-inductively coupled plasma mass spectrometry. Our results indicate that in a methane seepage environment, the concentrations of Ca, Mg, and Sr in pyrite are lower than in non-methane seepage environments due to the precipitation of authigenic carbonates that occurs in methane seepage settings. Additionally, the significant sulfate-driven anaerobic methane oxidation hinders the organoclastic sulfate reduction process, leading to a deficiency of Cr in pyrite. The reductive dissolution of Fe-Mn (oxy)hydroxides releases other elements, such as Mn and As, resulting in their enrichment in pyrite. Our study demonstrates that trace elements in pyrite can record information about methane seepage processes, highlighting their significance for understanding biogeochemical processes occurring in methane seepage environments.