Subglacial landforms as proxy for offshore freshened groundwater in formerly glaciated margins

Abstract Offshore freshened groundwater (OFG) is a significant yet underexplored component of the global water cycle, and its spatial distribution, particularly its links to past glacial processes, remains poorly constrained. The southeastern North Sea, characterized by extensive tunnel valley networks, provides an ideal setting to test how Pleistocene meltwater shaped offshore groundwater systems. Using newly acquired time-domain controlled-source electromagnetic data integrated with high-resolution seismic reflection surveys, we present a resistivity-based mapping of OFG in this region. The models reveal a laterally continuous OFG body with resistivities of 8–14 Ω·m, extending across multiple stratigraphic units and well beyond individual subglacial landforms. This configuration is best explained by subglacial meltwater flushing of sediment porewaters during Pleistocene glaciations. Subsequent preservation was aided by density contrasts that limit vertical mixing and by fine-grained glaciomarine deposits sealing the system from above. The connection between resistivity anomalies and mapped subglacial landforms shows that the latter can serve as predictive proxies for OFG occurrence. These findings shift the perspective from OFG as isolated anomalies to regionally extensive systems linked to glacial processes, establishing subglacial landforms as a testable screening proxy for locating OFG in formerly glaciated margins worldwide.