Controls of sulfur authigenesis in lacustrine sediments of the (pre-)Anthropocene: Sulfur and oxygen isotopes can tell

Benthic sulfur cycling in fresh-water lakes is typically characterized by low concentrations of dissolved sulfate in the overlying water column. This electron acceptor is the major driver for the anaerobic mineralization of organic matter in brackish-marine systems. Post-glacial development in marginal seas like the Baltic or Black Sea are often characterized by a transition from fresh to brackish water conditions, and initial sulfur isotope signatures of lacustrine sediments are often found to be superimposed by later diagenesis. To better understand the link between sulfate sources and the developing sedimentary sulfur isotope signatures in lake systems, Lake Constance’s main inflows, vertical water column profiles and sediment samples were geochemically and isotopically (S-34, O-18) characterized.

We found that dissolved sulfate concentrations and stable isotope signatures for the two major riverine contributors, Alpenrhein and Bregenzer Aach, differed substantially in their isotopic composition. The Alpenrhein dominates sulfate contribution into the lake system and its contribution could be traced throughout the lake with some indication for potential minor sulfur cycling within the water column. Wells demonstrated that the Bregenzer Aach lost water to an underground passage towards Lake Constance. Water-rock interactions also provided minor amounts of sulfate to the migrating groundwater.

The top 10 cm of surface sediments, representing the Anthropocene, indicated fast gross and net dissimilatory sulfate reduction and the formation of iron sulfides that are isotopically close to water column sulfate, but they were found to be depleted in the heavier isotope at greater depth, indicating lower ratios of net sulfate reduction versus sulfate replenishing rates in the past.