Cohen
Denis
Cohen
Denis
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ArticleInfluence of late Pleistocene glaciations on the hydrogeology of the continental shelf offshore Massachusetts, USA(John Wiley & Sons, 2014-12-05) Siegel, Jacob ; Person, Mark ; Dugan, Brandon ; Cohen, Denis ; Lizarralde, Daniel ; Gable, CarlMultiple late Pleistocene glaciations that extended onto the continental shelf offshore Massachusetts, USA, may have emplaced as much as 100 km3 of freshwater (salinity <5 ppt) in continental shelf sediments. To estimate the volume and extent of offshore freshwater, we developed a three-dimensional, variable-density model that couples fluid flow and heat and solute transport for the continental shelf offshore Massachusetts. The stratigraphy for our model is based on high-resolution, multichannel seismic data. The model incorporates the last 3 Ma of climate history by prescribing boundary conditions of sea level change and ice sheet extent and thickness. We incorporate new estimates of the maximum extent of a late Pleistocene ice sheet to near the shelf-slope break. Model results indicate that this late Pleistocene ice sheet was responsible for much of the emplaced freshwater. We predict that the current freshwater distribution may reach depths up to 500 meters below sea level and up to 30 km beyond Martha's Vineyard. The freshwater distribution is strongly dependent on the three-dimensional stratigraphy and ice sheet history. Our predictions improve our understanding of the distribution of offshore freshwater, a potential nonrenewable resource for coastal communities along recently glaciated margins.
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ArticleIce sheet–derived submarine groundwater discharge on Greenland's continental shelf(American Geophysical Union, 2011-07-28) DeFoor, Whitney ; Person, Mark ; Larsen, Hans Christian ; Lizarralde, Daniel ; Cohen, Denis ; Dugan, BrandonIsotopically light (−1‰ to −8‰ δ18O) and fresh pore fluids (300–520 mM Cl−) were found in continental shelf sediments up to 100 km offshore of southeastern (SE) Greenland, suggesting infiltration and mixing of ice sheet meltwater with seawater to depths of 260 m. These geochemical anomalies may be associated with ice sheet–derived submarine groundwater discharge (SMGD). We present a continental-scale reconstruction of the late Pleistocene hydrogeology of SE Greenland using finite element analysis that incorporates ice sheet loading and solute and isotope transport. Results indicate that subglacial infiltration and SMGD are of the same order of magnitude and are highly dependent on the permeability of the subaerial basalt. Simulated infiltration and SMGD almost doubled during the Last Glacial Maximum, compared to ice-free conditions. Much of the present-day glacially induced groundwater discharge occurs on land. Subice infiltration on the continental shelf likely represents a mixture of seawater and meltwater during past glacial maximums. Simulated SMGD during the most recent interval of glacial retreat is about 4% of the total ice sheet melting. At present, the simulated rate of SMGD is about 11% of the estimated current melting rate.
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ArticleOffshore freshened groundwater in continental margins(American Geophysical Union, 2020-11-20) Micallef, Aaron ; Person, Mark ; Berndt, Christian ; Bertoni, Claudia ; Cohen, Denis ; Dugan, Brandon ; Evans, Rob L. ; Haroon, Amir ; Hensen, Christian ; Jegen, Marion ; Key, Kerry ; Kooi, Henk ; Liebetrau, Volker ; Lofi, Johanna ; Mailloux, Brian J. ; Martin-Nagle, Renée ; Michael, Holly A. ; Müller, Thomas ; Schmidt, Mark ; Schwalenberg, Katrin ; Trembath-Reichert, Elizabeth ; Weymer, Bradley ; Zhang, Yipeng ; Thomas, Ariel T.First reported in the 1960s, offshore freshened groundwater (OFG) has now been documented in most continental margins around the world. In this review we compile a database documenting OFG occurrences and analyze it to establish the general characteristics and controlling factors. We also assess methods used to map and characterize OFG, identify major knowledge gaps, and propose strategies to address them. OFG has a global volume of 1 × 106 km3; it predominantly occurs within 55 km of the coast and down to a water depth of 100 m. OFG is mainly hosted within siliciclastic aquifers on passive margins and recharged by meteoric water during Pleistocene sea level lowstands. Key factors influencing OFG distribution are topography-driven flow, salinization via haline convection, permeability contrasts, and the continuity/connectivity of permeable and confining strata. Geochemical and stable isotope measurements of pore waters from boreholes have provided insights into OFG emplacement mechanisms, while recent advances in seismic reflection profiling, electromagnetic surveying, and numerical models have improved our understanding of OFG geometry and controls. Key knowledge gaps, such as the extent and function of OFG, and the timing of their emplacement, can be addressed by the application of isotopic age tracers, joint inversion of electromagnetic and seismic reflection data, and development of three-dimensional hydrological models. We show that such advances, combined with site-specific modeling, are necessary to assess the potential use of OFG as an unconventional source of water and its role in sub-seafloor geomicrobiology.