INVITED: PLEISTOCENE HYDROGEOLOGY OF THE ATLANTIC CONTINENTAL SHELF

Salinity profiles from deep scientific bore holes drilled 64 km offshore on Nantucket Island, Massachusetts and 150 km offshore of Long Island, New York indicate that the freshwater-saltwater interface is far out of equilibrium with modern sea level conditions within continental shelf sediments. Salinity concentrations within clastic Miocene through Pleistocene aquifers penetrated by these wells are relatively low (less than 3000 mg/l). Salinity levels within confining units of one 514 m deep Nantucket Island well are 30 – 80 % of seawater and exhibited parabolic profiles consistent with ongoing vertical diffusion. In this study we quantitatively test two competing hypothesis to explain the apparent flushing of the coastal-plain sediments in New England: (1) meteoric recharge during Pleistocene sea-level lowstands; and (2) subglacial recharge during the last glacial maximum. We constructed a one-dimensional diffusion model as well as two- and three-dimensional numerical models of variable-density groundwater flow, heat and solute transport for the Atlantic continental-shelf sediments to quantitatively test these competing hypotheses. Salinity profiles in the confining units of the Nantucket Island borehole were consistent with a mean diffusion time of about 21 ka; the approximate time of the last glacial maximum in New England. We could not reproduce the relatively low-salinity groundwaters observed within the shallow Pliocene-Miocene clastic aquifers 100 km offshore of Long Island with numerical models that represent the effects of sea-level fluctuations alone. Rather, we obtained the best agreement with observed salinity data by representing the effects of 2 ka of subglacial recharge following 2 Ma of freshwater incursion of meteoric recharge induced by sea-level cycles with amplitude and period of 60 m and 100 ka, respectively. We estimate that recharge rates induced by subglacial meltwater below Nantucket Island and Long Island were about 20 to 30 times greater than modern subaerial recharge estimates reported in the literature. Our study suggests that freshwater abundance in continental-shelf sediments is larger than would be estimated from modern hydrologic conditions and, consequently, should positively impact water-resource planning in the highly populated eastern seaboard of the United States.