Subterranean Estuarine Chemistry: The Role of the Fe/Mn Redox Cycles on the Transport of U, Ra and Th Isotopes to the Coastal Ocean

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ABSTRACTOCE-0095384Two geochemists at the Woods Hole Oceanographic Institution will conduct a study to gain a better understanding of the chemical transformations that take place in a subterranean estuary. Groundwater, nearly 97% of the Earth's freshwater reservoir, may contribute significant fluxes of dissolved chemical species to the oceans via transport through "subterranean estuaries". Subterranean estuaries are mixing regimes of seawater and fresh ground water that underlie many coastal regions). Since little is known about chemical reactions and fluxes associated with submarine groundwater, quantifying its impact on the coastal oceans requires a focused and in-depth study of an enclosed estuary that receives a well documented flux of ground waters. The investigators have identified Waquoit Bay, Massachusetts -- part of the National Estuarine Research Reserve system -- as an ideal location for an initial study of these processes. The project will consist of two parts: o Establishment of the temporal variability in chemical compositions across the subterranean estuary. External forcing functions such as tidal-pumping as well as rainfall recharge of the aquifer will likely play important roles in the location and structure of the groundwater/seawater interface. o Study of the geochemical reactions and transformations in subterranean estuaries of three important classes of elements - (1) redox sensitive elements (Fe, Mn and U), (2) scavenged elements (P, Th isotopes) and (3) mobile elements (Ra isotopes). How significant are the addition and removal processes operating on these three classes of elements in subterranean estuaries? Is the formation of an "Fe/Mn Curtain" in the sediments of a subterranean estuary an important process with respect to the transport and transformation of scavenged and mobile elements? Particular focus will be on the redox chemistries of Fe and Mn and on the systematics of U-Th-Ra series isotopes. This project combines relevance to important societal problems with the study of fundamental oceanographic and geochemical problems associated with the transport and reactivity in a complex interface.