Coordination Funds

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Subterranean estuaries are hidden connective zones between inland aquifers and the open sea where meteoric freshwater and circulating seawater mix and undergo major biogeochemical changes. Hence, they are considered powerful biogeochemical reactors affecting elemental net fluxes to the sea. DynaDeep is motivated by the fact that we need to understand the extent and functionality of marine-terrestrial interactions in subterranean estuaries, in order to comprehend the current dynamics and future trajectory of ecosystem functioning within the sedimentary land-sea interface. In particular, the effect of hydro- and morphodynamics on subsurface flow and transport is unclear and related consequences on biogeochemical reactions and microbial habitat characteristics have not been investigated. We propose the existence of a so far unexplored biogeochemical reactor within the deep subsurface of high-energy beaches, where dynamic boundary conditions propagate tens of meters into the subsurface leading to strong spatio-temporal variability of geochemical conditions. Subterranean estuaries below high-energy beaches may thus form a unique habitat with an adapted microbial community unlike other typically more stable subsurface environments. DynaDeep will study groundwater flow patterns as a function of hydro- and morphodynamics. It will deliver rates of biotic and abiotic transformation of organic matter and investigate related redox processes. Furthermore, it will assess transformation and fractionation of trace metals and metal isotopes. Microbial interactions with organic matter will be investigated, as well as the diversity and metabolic functioning of microbial communities. Six subprojects will cooperate in joint field campaigns, shared sampling approaches as well as experimental work and use mathematical models in an integrative approach. In the first phase, the subprojects will jointly establish a Subterranean Estuary Online Observatory on the island of Spiekeroog to investigate one core site in detail, before verifying observations at other high-energy sites in a second phase. Ultimately, DynaDeep will elucidate the global relevance of the dynamic deep subsurface of high-energy beaches for coastal ecosystems as well as for carbon, nutrient and trace element cycling.