EGU22-3715, updated on 10 Jan 2024
https://doi.org/10.5194/egusphere-egu22-3715
EGU General Assembly 2022
© Author(s) 2024. This work is distributed under
the Creative Commons Attribution 4.0 License.

Isotope hydrogeochemistry investigations (223,224Ra, DI13C) on submarine groundwater discharge in a tidal bay (eastern North Sea)

Catia Milene Ehlert von Ahn1, Anna-Kathrina Jenner1, Jan Scholten2, Antonia Schell1,3, Iris Schmiedinger1, Jasper Hoffmann4, Patricia Roeser1, Carla Nantke1, and Michael Böttcher1,3,5
Catia Milene Ehlert von Ahn et al.
  • 1Geochemistry & Isotope Biogeochemistry Group, Leibniz Institute for Baltic Sea Research (IOW), Warnemünde, Germany.
  • 2Institute of Geosciences, Kiel University, Kiel, Germany.
  • 3Marine Geochemistry, University of Greifswald, Greifswald, Germany.
  • 4Wadden Sea Station, AWI Sylt, Germany.
  • 5Interdisciplinary Faculty, University of Rostock, Rostock, Germany

The impact of submarine groundwater discharge (SGD) on coastal biogeochemistry is currently under intense investigation. SGD can impact diagenesis and in general act as a potential source of elements, especially dissolved carbon, to coastal surface waters. However, qualitative and quantitative assessments of SGD are challenging since it requires the identification of suitable geochemical tracers for the complex hydrological and biogeochemical processes in the subterranean estuary. In this communication, we report on combined investigations carried out in Königshafen Bay (North Frisian island Sylt, Germany), a tidal area in the eastern North Sea. Sampling encompassed vertical porewater gradients, and surface waters collected through transects in the bay, and in tidal cycles at the outlet of the bay. Potential surface and subterrestrial freshwater endmembers are used to assess the results. Besides major and minor elements, this study focuses on the stable carbon isotope composition of dissolved inorganic carbon (DIC) and the activity of radium (Ra) isotopes. Our main aim is to characterize the interaction between diagenesis and the composition of SGD, as well as the resulting impact on the carbon system of the water column, and, via tidal exchange extended to the coastal North Sea. Porewaters showed usually an increase of isotopically light DIC with depth and a freshening already in the top 50 cmbsf at some sites. This indicates that both, carbon diagenesis and mixing of seawater with fresh groundwaters at depth impact the distribution of DIC. The activities of the short-living Ra isotope (224Raex) were higher in the bay compared to the open North Sea. Porewater activities were up to 30 times higher than in the bay’s surface waters with a maximum development at intermediate salinities. In the water column at the outlet of the bay, 224Raex and 223Ra showed maximum activities during low tide as a consequence of the highest contribution of waters in contact with the sediments of the bay. Moreover, due to the high hydraulic gradient developed during low tide more contribution from potential endmembers enriched in Ra can be expected. Further work is on the way to quantify the impact of SGD on the tidal basin and the indirect role for the North Sea carbon system on different temporal and spatial scales.

 The investigations are supported by the DFG-project KiSNet, the BMBF project COOLSTYLE (CARBOSTORE), the DAAD, the DFG RTG Baltic TRANSCOAST, and the Leibniz IOW.

How to cite: Ehlert von Ahn, C. M., Jenner, A.-K., Scholten, J., Schell, A., Schmiedinger, I., Hoffmann, J., Roeser, P., Nantke, C., and Böttcher, M.: Isotope hydrogeochemistry investigations (223,224Ra, DI13C) on submarine groundwater discharge in a tidal bay (eastern North Sea), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3715, https://doi.org/10.5194/egusphere-egu22-3715, 2022.