Linking the carbon and sulfur cycles in a historically brackish diked peatland: Stable Isotopes and FT-ICR-MS

Here we report on the porewater dissolved organic matter dynamics and underlying benthic biogeochemical processes in a historically brackish, diked, peatland located along the Baltic Sea in northeastern Germany.  The regeneration process of the “Heiligensee and Hütelmoor” includes a return of freshwater inputs as well as increased connection to the sea.  For porewater observations, two stationary multiport (about 0.5 m intervals) lances are located in the coastal sediments coastward of the sand-dune dyke, reaching down to ~5 meters through permeable sediments and peat layers.  Frequent sampling of these porewater lances indicates substantial influences by fresh submarine ground water discharge in the middle depths.  Therefore, we studied the impact by mixing of these groundwater (as, for example, a source of Fe, DOM, DIC, P, Ca) with saltwater (a source of SO₄ to fuel sulfate reduction) and the role of organic matter in the drowned peat layers.  We were particularly interested in the sulfurization of DOM, as biogenic sulfide can react both with Fe and DOM/POM.  Samples for a suite of analyses were taken in November 2020.  Characterizations included dissolved organic matter (21T FT-ICR-MS, National High Magnetic Field Laboratory), major and trace elements (ICP-OES), nutrient and sulfide concentration, as well as stable isotopes of sulfate, DIC, and water. Results are compared to nearby groundwater wells (a coastal sandy aquifer, a coastal peat layer, and an inland well), the brackish Baltic Sea and  the Hütelmoor surface waters, as well as the river Warnow.  Thus, we characterize the endmembers as well as the mixing zones in order to understand their influence on the chemical alterations of dissolved organic matter in this dynamic region.