Department of Geological Sciences Indiana University 1001 East 10th Street Bloomington IN47405 USA

During the 1960s and 1970s,scientific drilling campaigns along the Atlantic continental shelf offshore New England, USA (Hathaway et al., 1979) revealed that the freshwater-saltwater interface is far out of equilibrium with modern sea-level conditions (Fig. 13.1). Aquifer salinity levels are less than 5 parts per thousand (5ppt) over 100km offshore of Long Island (wells 6009, 6011 and 6020, Fig. 13.2). In addition, salinity levels within confining units beneath Nantucket Island are 30-70% of seawater levels and exhibit a parabolic profile consistent with ongoing vertical diffusion (well 6001, Fig. 13.2). Analytical models of vertical solute diffusion for the Nantucket confining units suggest that flushing of aquifers beneath Nantucket began in the late Pleistocene between about 195 and 21 ka assuming a diffusion coefficient of 3.0 x 10-11 m2s-1 (Person et al., 2003).

Kohout et al. (1977) proposed that the presence of unusually fresh water within the permeable units of the Atlantic continental shelf could be attributed to meteoric recharge during Pleistocene sea-level low stands. Marine isotopic records and precise dating of shoreline facies and corals suggest that Pleistocene sea-level varied by about 120 m with a period between 40 ka and 100 ka. Average sea-level for the Pleistocene was about 40 m below modern. During sea-level low stands, large portions of the continental shelf

Glacial Lake Tonawanda

Figure 13.1 Bathym├ętrie map (solid contour lines in metres) of the Atlantic continental shelf, New England. The dashed contours are maximum reconstructed Laurentide Ice Sheet thickness (in metres) for 21 ka (after Colgan et al., 1981; Hathaway et al., 1979). The circles indicate the location of exploration boreholes used to construct the hydrostratigraphic cross-sections presented in Fig. 13.3.

Figure 13.1 Bathym├ętrie map (solid contour lines in metres) of the Atlantic continental shelf, New England. The dashed contours are maximum reconstructed Laurentide Ice Sheet thickness (in metres) for 21 ka (after Colgan et al., 1981; Hathaway et al., 1979). The circles indicate the location of exploration boreholes used to construct the hydrostratigraphic cross-sections presented in Fig. 13.3.

1 100

J3 200

Salinity (ppt) 0 10 20 30

Salinity (ppt) 0 10 20 30

6020

0 100 200 300

Salinity (ppt) 0 10 20 30

6009

6009

0 00 200 300 400 500

Salinity (ppt) 0 10 20 30

6001

Pleistocene Tertiary

Triassic

Figure 13.2 Salinity profiles and stratigraphy of Pleistocene and Cretaceous continental shelf sediments below Nantucket Island, MA (well 6001) and offshore Long Island (wells 6009, 6011, 6020; no stratigraphy plotted). The parabolic salinity profiles within clay-rich confining units in well 6001 suggests ongoing vertical diffusion (after Folger et al., 1978; Hathaway et al., 1979). The locations of the wells are listed in Fig. 13.1. The well numbers are in italics at the bottom of each scattergram.

P leistocene

Modern Shoreline Mean Sea Level

Pleistocene Sea Level LowStand

P leistocene

Pleistocene Sea Level LowStand

Observed
Sea Level Simulation
Ice Sheet Simulation

Sand Silt/Clay Carbonate 10 - Salinity (ppt)

Laterally Continuous Upper Aquifer

Distance (km)

Sand Silt/Clay Carbonate 10 - Salinity (ppt)

Laterally Continuous Upper Aquifer

Distance (km)

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