Aquifer systems extending far offshore on the U.S. Atlantic margin

Abstract

Low-salinity submarine groundwater contained within continental shelves is a global phenomenon. Mechanisms for emplacing offshore groundwater include glacial processes that drove water into exposed continental shelves during sea-level low stands and active connections to onshore hydrologic systems. While low-salinity groundwater is thought to be abundant, its distribution and volume worldwide is poorly understood due to the limited number of observations. Here we image laterally continuous aquifers extending 90 km offshore New Jersey and Martha’s Vineyard, Massachusetts, on the U.S. Atlantic margin using new shallow water electromagnetic geophysical methods. Our data provide more continuous constraints on offshore groundwater than previous models and present evidence for a connection between the modern onshore hydrologic system and offshore aquifers. We identify clinoforms as a previously unknown structural control on the lateral extent of low-salinity groundwater and potentially a control on where low-salinity water rises into the seafloor. Our data suggest a continuous submarine aquifer system spans at least 350 km of the U.S. Atlantic coast and contains about 2800 km 3 of low-salinity groundwater. Our findings can be used to improve models of past glacial, eustatic, tectonic, and geomorphic processes on continental shelves and provide insight into shelf geochemistry, biogeochemical cycles, and the deep biosphere.

Complex Eyed Pockmarks and Submarine Groundwater Discharge Revealed by Acoustic Data and Sediment Cores in Eckernförde Bay, SW Baltic Sea

“…Numerous methods have been used to detect and quantify SGD (Burnett et al, 2006) including optical systems (Karpen et al, 2004); geochemical water column investigations of tracers like the natural radionuclides radium and radon (Burnett & Dulaiova, 2003; Moore, 1996; Moore et al, 2008; Scholten et al, 2013), dissolved silicon (Oehler et al, 2019), methane, and chloride (Dulaiova et al, 2010; Schlüter et al, 2004); remote sensing (Shaban et al, 2005; Tamborski et al, 2015; Wilson & Rocha, 2012); and direct investigations using cores and different types of seepage meters (Bugna et al, 1996; Burnett & Dulaiova, 2003; Cable et al, 1997; Sauter et al, 2001). Only a few geophysical methods, like geoelectric (Stieglitz, 2005; Viso et al, 2010), controlled source electromagnetic (Gustafson et al, 2019; Müller et al, 2011), or autonomous underwater vehicle investigations (Sauter et al, 2003) have been used to explore SGD. Other hydroacoustic studies often postulate SGD solely based on geomorphological characteristics but without any geochemical verification (e.g., Goff, 2019; Hillman et al, 2015; Jakobsson et al, 2020).…”

3D characterisation and quantification of an offshore freshened groundwater system in the Canterbury Bight

“…Controlled source electromagnetic methods using a horizontal electric dipole source are sensitive to bulk electrical resistivity and can detect resistivity contrasts between the OFG and the surrounding seawater-saturated sediment, in a similar manner to hydrocarbon reservoirs 26,27 . In this study we integrate offshore time-domain controlled-source electromagnetic (CSEM) data with multichannel seismic reflection data, borehole data and hydrological modelling to quantitatively characterise an OFG system at high spatial resolution.…”

Deep Submarine Groundwater Discharge—Evidence From Achziv Submarine Canyon at the Exposure of the Judea Group Confined Aquifer, Eastern Mediterranean

“…This study suggests that DSGD of water with salinity lower than ambient seawater is possible also in any marginal sea, where an exposed, confined, aquifer serves as a conduit between terrestrial recharge of freshwater and deep submarine discharge as postulated by hydrogeological modeling . Additionally, the results presented here raise the possibility that underground submarine water bodies with salinities lower than seawater in the marginal sea (e.g., Gustafson et al, 2019;Hathaway et al, 1979;Johnston, 1983;Robb, 1984) may result from active, modern DSGD systems. Another possible explanation for such underground water bodies is that they are relicts of past, lower sea level that were placed in the ancient shoreline during the last glacial maximum.…”