Active microbial sulfur cycling in 13,500-year-old lake sediments

Abstract. The addition of sulfur (S) to organic matter to form organosulfur compounds is generally thought to protect organic matter from microbial degradation and promote its preservation. While most microbial sulfur cycling occurs in sulfate-rich sediments above the sulfate-methane transition zone, recently discovered active sulfur cycling in deeper sulfate-poor environments may have a yet-unquantified impact on the mineralization of organic matter. Here we investigated the fate of buried S-compounds down to 10-m sediment depth representing the entire ~13.5 kya history of the sulfate-rich alpine Lake Cadagno. Chemical profiles of sulfate and sulfur reveal that these oxidized species are depleted at the sediment surface with the concomitant formation of iron sulfide minerals. An underlying aquifer provides a second source of sulfate and other oxidants to the deepest and oldest sediment layers generating an inverse redox gradient. At both sulfate depletion zones, isotopes of chromium-reducible sulfur (CRS) and humic-bound sulfur are highly negative (−30 to −65 per mil) compared to background sulfate suggesting ongoing microbial sulfur cycling. Interestingly, humic-bound S from intermittent sediment layers within the sulfate-depleted methanogenesis zone consistently exhibits a lower δ³⁴S than CRS in lacustrine deposits but a higher δ³⁴S than CRS in terrestrial deposits, which could possibly be due to different reactivities of organic matter types (lacustrine versus terrestrial origin) to sulfide or the ability of microorganisms to form/degrade organic S. Although sulfate concentrations are extremely low between the sulfate depletion zones, dsrB gene libraries reveal a huge potential for microbial sulfur reduction throughout the sediment column.