An isotope view on hydrology and carbon cycling in a tidal basin under impact by submarine groundwater discharge

The role that of fresh surface and ground water sources play on the coastal water and element balances, and the associated biogeochemical processes is still a matter of debate. Fresh and saline water mixing in coastal areas is allowed by stable water isotopes and further hydrochemical tracers. Here, we report on investigations of surface, pore and ground waters from Königshafen Bay (Sylt) and potential mixing endmembers. Besides dissolved major and minor elements, the stable water isotope composition is used to characterize the temporal and spatial distribution of different water sources to the bay and the seasonal dynamics in the water column. Porewater gradients indicate different degrees of freshening, locally already in the top 50 cmbsf with spatial heterogeneity. Different fresh water endmembers are indicated both by the water isotope and hydrochemical signatures. It turns that at least two fresh water sources can be identified for sediments under SGD impact, that differ in composition from surface water sources draining into the southern North Sea. Further work is on the way to investigate the dynamics in the (sub)surface fresh water sources for the tidal basin and the link to other geochemical tracers, as well as the coupling to the dissolved carbon system on different temporal and spatial scales.

Acknowledgement: DFG (KiSNet), BMBF (COOLSTYLE/CARBOSTORE), DAAD, DFG Baltic Transcoast, and Leibniz IOW.