IODP3-NSF Expedition 501 - New England Shelf Hydrogeology - Scientific Prospectus

Q: What main problem does Expedition 501 investigate on the New England shelf?

It investigates why continental shelf groundwater remains out of equilibrium with modern sea level, focusing on freshwater occurrence and its sharp boundaries with seawater.

Q: How does the Nantucket Island borehole help characterize salinity patterns?

It penetrates the full Cretaceous–Tertiary sedimentary package and shows salinity variations that correspond to sand aquifers, clay/silt layers, and confining units.

Q: What methods will distinguish between proposed recharge mechanisms and overpressure models?

The expedition plans drilling, coring, downhole logging, and pore-fluid sampling, with noble gas and environmental isotope data needed for complete evaluation.

Aug 25, 2026

Uploaded by Paura

Freshwater distribution in continental shelf sediments often remains out of equilibrium with modern sea level, with a striking example on the Atlantic shelf off New England. Groundwater in shallow Pliocene–Pleistocene sand aquifers more than 100 km offshore shows low salinity, while Nantucket Island drilling reveals salinity contrasts by lithology and modest overpressures. Multiple recharge mechanisms and overpressure origins are proposed, testable by drilling, coring, logging, and fluid sampling. Noble gases and environmental isotopes will refine models and constrain flow, fluxes, and microbial and carbon–nitrogen impacts.

LanguageEnglish

CategoryResearch & Report

FormatPDF

Pages16

Reading Time40 min

AccessFree to download

Abstract

Expedition 501 overview

Scientific basis and hypotheses

Schedule and field operations

References and linked resources

Proc. Int. Ocean Drill. Prog., 501, 1, 2025 [https://doi.org/10.5194/piodp-501-1-2025](https://doi.org/10.5194/piodp-501-1-2025)[ ](https://doi.org/10.5194/piodp-501-1-2025)© Author(s) 2025 . This work is distributed under the Creative Commons Attribution 4 .0 License. IODP3–NSF Expedition 501 “New England Shelf Hydrogeology” Scientiﬁc Prospectus Brandon Dugan 1 , Karen Johannesson2 , and Jeremy Everest3 1Department of Geophysics, Hydrologic Science and Engineering Program, Colorado School of Mines, Golden, Colorado 80401, United States of America 2 School of the Environment, University of Massachusetts, Boston, Massachusetts 02125, United States of America 3British Geological Survey, the Lyell Centre, Edinburgh, EH14 4AP, United Kingdom Correspondence: Brandon Dugan ([dugan@mines.edu](dugan@mines.edu)) Received: 11 December 2024 – Revised: 19 December 2024 – Accepted: 20 December 2024 – Published: 3 November 2025 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. Published by Copernicus Publications on behalf of the IODP3 Science Ofﬁce, University of Plymouth. IO DP 3–NSF Expedition 501: New England Shelf Hydrogeology 2 B.

FAQs

What main problem does Expedition 501 investigate on the New England shelf?

It investigates why continental shelf groundwater remains out of equilibrium with modern sea level, focusing on freshwater occurrence and its sharp boundaries with seawater.

How does the Nantucket Island borehole help characterize salinity patterns?

It penetrates the full Cretaceous–Tertiary sedimentary package and shows salinity variations that correspond to sand aquifers, clay/silt layers, and confining units.

What methods will distinguish between proposed recharge mechanisms and overpressure models?

The expedition plans drilling, coring, downhole logging, and pore-fluid sampling, with noble gas and environmental isotope data needed for complete evaluation.

Aug 25, 2026

Uploaded by Paura