Temporal and spatial investigations of the isotope biogeochemistry of a coastal peatland area under sporadic flooding

The interface between the terrestrial and marine environments regulates the interactions between the adjoining domains. Natural and anthropogenic alterations influence the processes and exchanges of materials. This study aims to determine the biogeochemical processes before and after anthropogenic changes in the coastal protection at the southern Baltic Sea coast proximal to a peatland (Hütelmoor, Rostock, Germany). Spatial and temporal investigations, with the use of stationary porewater lances, characterize the dynamics of biogeochemical transformation processes. Porewaters were measured for in-situ physico-chemical parameters and analyzed for dissolved organic and inorganic carbon (DOC and DIC), major ions, redox-sensitive elements and nutrients concentrations. Stable isotopes (ẟ¹³C-DIC and -DOC, ẟ²H- and ẟ¹⁸O-H₂O, and ẟ³⁴S- and ẟ¹⁸O-SO₄, and ẟ³⁴S-H₂S) and non-stable isotopes (²²³Ra, ²²⁴Ra) were also measured. Results indicate high concentrations of DOC which may have originated from peat degradation and high concentrations of DIC, which may have been derived from organic matter mineralization, terrestrial and marine carbonate dissolution, and Baltic Sea-derived DIC. Minor contributions from CH₄ oxidation cannot be ruled out. Diagenetic transformations are also reflected in the vertical profiles of redox-sensitive ions (such as Fe, Mn, SO₄ and H₂S). Sulfate, dominantly from Baltic Sea water and microbial reduction-oxidation, influences diagenesis. Water is a mixture of different brackish and freshwater sources.  Moreover, submarine groundwater discharge (SGD) was observed from 150 cmbsf and also evident in ²²⁴Ra activities. With the changes in the coastal protection status, internal transport processes of porewaters in the sediment are evidently influenced by the hydrogeodynamics along the coastline on a local to regional scale.