Active microbial sulfur cycling across a 13,500-year-old lake sediment record

Abstract. The sulfur cycle is very important in lake sediments, despite the much lower sulfate concentrations in freshwater than seawater. To date, little is known about the formation and preservation of organic and inorganic sulfur compounds in such sediments, especially in the sulfate-depleted subsurface. Here we investigated the fate of buried S-compounds down to 10-m sediment depth, which represents the entire ~13.5 kya sedimentary history, of the sulfate-rich alpine Lake Cadagno. Chemical profiles of sulfate and reduced sulfur reveal that sulfate from lake water is 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 with ongoing sulfate consumption. Active sulfur cycling within this deep layer produces highly ³⁴S-depleted chromium-reducible sulfur (CRS) (δ³⁴S between -45 and -26 ‰ VCDT) and humic-bound sulfur compared to sulfate in lake (+24 ‰) or aquifer water (+12 to +15 ‰) or CRS in surface sediments (-12 to + 13 ‰). Overall, very similar 𝜀_{sulfate-pyrite} isotope differences in both surface and deep sediments suggest rather comparable closed-system sulfur cycling despite the large differences in sulfate concentrations, organic matter content, and microbial community composition. Although sulfate is depleted in the central part of the sediment column, dsrB gene libraries suggest potential for microbial sulfur reduction throughout the sediment column, with sequences in sulfate-depleted layers being dominated by Chloroflexota. Collectively, our data suggest an active sulfur cycle that is driven by uncultivated microorganisms in deep sulfate-depleted sediments of Lake Cadagno.