Salinity PPT

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f

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(e)

300 0

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Figure 13.3 (a) Observed (solid lines, in parts per thousand) salinity contour map for the Atlantic continental shelf sediments offshore Long Island, NY. (b) Computed salinity contours applying sea-level boundary conditions (20 sea-level cycles about the Pleistocene mean sea-level of 40 m below modern using a period of 100,000yr with an amplitude of 120 m). (d) Computed salinity contour map (dashed lines, ppt) for the Atlantic continental shelf sediments off shore Long Island, NY applying ice-sheet recharge and (f) ice-sheet model the same as (d) but allowing groundwater in shallow aquifer to discharge along submarine canyons. Both ice-sheet simulations used the computed salinity from the Pleistocene sea-level simulation shown in Fig. 13.3b as an initial condition. Both ice-sheet simulations are presented following 2000yr of imposed ice-sheet boundary conditions. A comparison between simulated and observed concentrations profiles for well 6009 for b, d & f are shown in c, e & g, respectively. The location of the cross-section is listed in Fig. 13.1 (after Person et al. 2003).

were exposed to meteoric recharge. However, numerical models of Person et al. (2003) which represented variable-density groundwater flow and solute transport could not reproduce the relatively low-salinity groundwaters observed off Long Island by applying boundary conditions consistent with Pleistocene sea-level fluctuations (Fig. 13.3a-c). These researchers also considered the effects of subice sheet recharge between about 21 and 18ka when the Laurentide Ice Sheet advanced out onto the continental shelf forming the islands of Nantucket, Martha's Vineyard and Cape Cod. Including the effects of ice-sheet recharge (i.e. applying a specified head boundary condition for 2000 years equal to 9/10th the ice-sheet height across much of Long Island) helped to drive the freshwater-saltwater interface much farther out onto the continental shelf (Fig. 13.3d & e). Observed salinity conditions were most closely matched by also allowing groundwater to discharge from Miocene/Pliocene aquifers along submarine canyons near the continental slope (Fig. 13.3f & g). Simulated recharge induced by Laurentide Ice Sheet meltwater was short lived (<2000 yr) but, on average, about two to ten times greater than modern subaerial levels.

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