IODP3–NSF Expedition 501 “New England Shelf Hydrogeology” Scientific Prospectus

Abstract. In many coastal settings worldwide, the distribution of freshwater within continental shelf sediments is far out of equilibrium with modern sea level. One of the most remarkable examples is found on the Atlantic continental shelf off New England where groundwater within shallow Pliocene-Pleistocene sand aquifers over 100 km offshore has low salinity (3000 mg L−1 or less). On Nantucket Island, a 514 m deep borehole penetrating the entire Cretaceous–Tertiary sedimentary package shows considerable variations in salinity with fresh (<1000 mg L−1) water in sand aquifers, higher salinity (30 %–70 % of seawater) in thick clay and/or silt layers, and intermediate-to-low salinity in thin confining units. Integrated Ocean Drilling Program Expeditions 313 (New Jersey Shallow Shelf) and 317 (Canterbury Basin New Zealand) also showed abrupt offshore freshwater–saltwater boundaries linked to lithology. This demonstrates the disequilibrium nature of such systems; diffusion tends to eliminate such patterns. Pore fluid within Pleistocene to upper Cretaceous sands beneath Nantucket Island are also found to be modestly overpressured at ∼ 4 m relative to the local water table. We hypothesise that the rapid incursion of freshened water on the continental shelf in New England could have been caused by one or more of the following mechanisms: (1) meteoric recharge during Pleistocene sea-level lowstands, including vertical infiltration of freshwater associated with local flow cells on the shelf; (2) sub-ice-sheet recharge during the Last Glacial Maximum; and (3) recharge from pro-glacial lakes. We further hypothesise that the overpressures could be due to (1) Pleistocene sediment loading or (2) fluid density differences associated with emplacement of a thick freshwater lens over saltwater (analogous to excess pressures in the gas legs of petroleum reservoirs). We argue these different recharge mechanisms and overpressure models can be distinguished through drilling, coring, logging, and fluid sampling. Noble gas and environmental isotope data will be necessary to completely evaluate recharge models. This work will extend our understanding of the current and past states of fluid composition, pressure, and temperature in continental shelf environments. It will help better constrain rates, directions, and mechanisms of groundwater flow and chemical fluxes in continental shelf systems. It will contribute to the development of new tools for measuring freshened water in marine environments. The apparent transient nature of continental shelf salinity patterns could have important implications for microbial processes and long-term fluxes of carbon, nitrogen, and other nutrients to the ocean. Successful drilling will test process-based models for shelf freshwater off New England. These models can then be applied to other shelf freshwater systems around the world.