The sandy beach subterranean estuary as a potential organic carbon sink

Subterranean estuaries (STEs) beneath sandy beaches are biogeochemical reactors that can modify the chemical composition of fresh groundwater and recirculating seawater. Compared to surface estuaries, the mechanisms underlying the transformation of dissolved organic matter (DOM) in the STEs remain challenging to disentangle, particularly in high-energy beaches where DOM supplied from marine and terrestrial sources is exposed to alternating redox conditions, salinity gradients, and dynamic flows. Our study is aimed at elucidating the spatio-temporal patterns of DOM sources and sinks in high-energy beach STEs from a case study site on a barrier island in the German North Sea. We present a geochemical analysis of freshwater lens groundwater (FWL), seawater (SW), and beach STE groundwater samples (STEGW) collected over different seasons. STEGW samples were collected from multilevel wells with sampling depths of 6m, 12m, 18m, and 24m close to the dune base (ML1), near the high-water line (ML2), and the low-water line (ML3), and the FWLGW was collected from wells located at the northwestern part of the island. All samples were analyzed for their dissolved organic carbon (DOC) concentrations, and humic-like/terrestrial components of DOM (FDOM) were determined via fluorescence spectroscopy. DOM samples were desalted through solid-phase extraction and molecularly characterized via ultra-high-resolution electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. We found a decrease of FDOM along the salinity gradient in the land-sea continuum (FWL > STEGW > SW). DOC concentrations were highest in the FWL and SW but relatively lower in STEGW samples, suggesting a role of this STE as net organic carbon sink. The DOM composition of the groundwater samples from all sampling stations was highly diverse, with a total of up to 10,000 detected molecular formulas. The numbers of detected molecular formulas in FWL samples were twofold higher than those in SW samples, with intermediate values observed in the STEGW. There was a general decline in terrestrial DOM signature in the land/sea continuum, suggesting the loss of terrestrial DOM from land to the sea. Our results indicate that STEs under high-energy beaches are powerful sinks for organic carbon from both marine and terrestrial sources.