After the flood: Sulfur authigenesis and isotope discrimination in a rewetting coastal fen

Land-ocean interactions in the coastal zone are of particular interest regarding the exchange of substances, like nutrients, carbon, sulfur, metals, and water. The rising sea level is and will enhance the pressure of salty solutions on previously fresh water ecosystems. We present here new results on the isotope biogeochemistry of a rewetted peatland, at the southern Baltic Sea, that is impacted by event-type flooding with brackish seawater. Sediment cores on transects through the wetland were investigated for their pore water and solid phase (mineral and organic matter) composition. Different fractions of the soils and solutions were analyzed for the elemental composition, mineral micro-textures, and the stable isotope composition (H, C, O, S) to understand the changes in water and biogeochemical carbon-sulfur-metal cycles due to flooding and the consequence for the development of sulfur isotope signatures in authigenic mineral phases and organic matter.

Flooding events with brackish water increased the availability of sulfate as an electron acceptor for microbial carbon transformations. This added sulfur impacted the remineralization capacity of organic substrates and created space for mineral authigenesis, with related iron sulfide textures. It yields isotope signals that are indicative for non-steady state biogeochemistry of coastal ecosystems and allow for a transfer of proxy information to other modern and past coastal organic-rich peatlands.

The soil cores from the peatland reflects the intense activity of sulfate-reducing bacteria and the associated formation of iron sulfides (essentially pyrite) and provided the isotope evidence for site-dependent sulfurization of organic matter. Sedimentary sulfur fractions and their stable isotope signatures are controlled by the availability of dissolved organic matter and/or methane, reactive iron, and in particular dissolved sulfate and, thereby, from the relative position with respect to the coast line, and depend on the surface topography and soil characteristics. Further mechanistic investigations consider the role of DOS upon changing sulfur substrate availability.

 

Acknowledgement for support by DFG-Baltic TRANSCOAST, ERASMUS, DAAD, Leibniz-IOW