05 Oct 2023
 | 05 Oct 2023
Status: this preprint is open for discussion.

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

Jasmine S. Berg, Paula C. Rodriguez, Cara Magnabosco, Longhui Deng, Stefano M. Bernasconi, Hendrik Vogel, Marina Morlock, and Mark A. Lever

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.

Jasmine S. Berg et al.

Status: open (extended)

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  • RC1: 'Comment on egusphere-2023-2102', Anonymous Referee #1, 06 Nov 2023 reply

Jasmine S. Berg et al.

Jasmine S. Berg et al.


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Short summary
The addition of sulfur to organic matter is generally thought to protect it from microbial degradation. We analyzed buried sulfur compounds in a 10-m sediment core representing the entire ~13,500 year history of an alpine lake. Surprisingly, organic sulfur and pyrite formed very rapidly and were characterized by very light isotope signatures that suggest active microbial sulfur cycling in the deep subsurface.