After the flood: Impact of salt water intrusions on the isotope biogeochemistry of a rewetted coastal peatland

<p><span>Land-ocean interactions in the coastal zone (LOICZ) are of particular interest regarding the exchange of water and elements, like nutrients, carbon, sulfur, and metals. </span><span>Processes impacting </span><span>groundwater</span><span> fluxes at these boundaries belong to the still unsolved problems in hydrology (Bl&#246;schl et al., 2019). </span><span>Stable isotope signatures (H, C, O, S), major and trace element contents in surface waters of a rewetted coastal peatland were investigated to understand the impact of storm-induced flooding by brackish seawater on hydrology and biogeochemical element cycling.</span></p><p><span>The study area is the H&#252;telmoor, a wetland located at the coastline of the southern Baltic Sea. The area is characterized by a continuous release of fresh water to the Baltic Sea via submarine groundwater discharge (Jurasinski et al., 2018). Surface water is partly drained to a nearby river, but the introduction of brackish waters into the peatland is typically precluded by a small dune and limited to storm-induced flooding events. In the present study, the spatially distributed composition of surface waters was investigated briefly after a flooding event. The results are compared with previous campaigns without actual salt water impact. </span></p><p><span>Conservative elements and water isotopes demonstrate the importance of seasonal variations due to varying evapotranspiration during pre-flood times and allow for a quantification of mixing processes in the post-flood waters. The impact of soil respired CO</span>_{<span>2</span>}<span>, and/or the mineralization of organic matter or methane on the surface waters is indicated by a shift of the C isotope composition of DIC towards lighter data. The S and O isotopic composition of dissolved sulfate indicates an impact by solutions modified by net microbial sulfate reduction on pre-flood surface waters and a potential oxidation of reduced sulfur species in post-flooding solutions. </span></p><p><span>Previous flooding events already impacted element cycling in the peatland&#8217;s past and are also reflected by a sulfidization of peat layers (Fern&#225;ndez-Fern&#225;ndez et al., 2017) and the observation of local areas with enhanced dissolved concentrations in the central part of the peatland.</span></p><p><span>The study is supported by DFG during GK Baltic TRANSCOAST, DAAD, and Leibniz IOW.</span></p><p align="justify">&#160;</p><p><span>References:</span></p><ul><li><span>Bl&#246;schl G. et al. (2019) Twenty-three unsolved problems in hydrology (UPH) &#8211; a community perspective. Hydrol. Sci. J. 64, 1141-1158.</span></li> <li><span>Jurasinski G. et al. (2018) Understanding the coastal ecocline: Assessing sea-land-interactions at non-tidal, low-lying coasts through interdisciplinary research. Front. Mar. Sci. 5, 1-22</span><span>.</span></li> <li>Fern&#225;ndez-Fern&#225;ndez L.E. et al. <span>(2017) Sulfur isotope biogeochemistry of soils from an episodically flooded coastal wetland, southern Baltic Sea. Geophys. Res. Abs. 19, EGU2017-14335</span><span>.</span></li> </ul>