Collaborative Research: Pleistocene Hydrogeology of the Atlantic Continental Shelf, New England

在世界各地的许多沿海环境中，大陆架沉积物中的淡水分布与现代海平面条件不平衡。其中一个最极端的例子可以在新英格兰找到，在纽约长岛离岸约150公里的中新世浅层砂中，地下水非常新鲜（盐度~ 3000毫克/升）。在靠近大陆的马萨诸塞州楠塔基特岛，一个512米的井眼在所有可渗透单元中都含有淡水（含盐量为2000毫克/升）。该井内的厚围封单元具有抛物线盐度剖面，与正在进行的垂直扩散相一致。分析计算表明，扩散开始于最近的21ka围封单元内。在南塔开特岛下的白垩纪部分的头部受到超过当地地下水位约4米的超压。我们建议利用数学模型和环境同位素分析来确定新英格兰海岸平原沉积物的冲刷是否是由：(1)更新世海平面低洼期的大气补给引起的；(2)末次盛冰期亚冰盖补给；(3)原冰期湖泊补给；(4)粘土脱水反应；(5)与海平面低潮期间可能在大陆架上形成的局部流动单元相关的淡水垂直入渗。这项研究的第二个目标是确定在楠塔基特岛上观察到的多余头部是否可以仅仅通过更新世沉积物负荷来解释，或者是否必须调用更多的外来机制，如粘土脱水反应。我们将为缅因州和新泽西州之间的大西洋大陆架建立一个三维、变密度的地下水流量、热量和溶质质量运输模型。该代码将在印第安纳大学的SP超级计算机上运行。同位素输运和地下水停留时间的直接模拟将与从大陆架承压含水层的井中收集的环境同位素数据（C-13、C-14、O-18、H-2）进行比较。为了限制前冰川湖泊补给的同位素组成，我们将在楠塔基特岛地表以下230英尺处钻一口井，从湖泊沉积物中提取孔隙流体。更新世冰盖、气候和海平面重建将用于限制水文、化学和同位素边界和初始条件的时间变化。威斯康星冰期陆地表面地形的弯曲调整将用于解释冰盖负荷对相对海平面的影响。

0337634PersonIn many coastal settings around the world the distribution of freshwater within continental shelf sediments is not in equilibrium with modern sea level conditions. One of the most extreme examples of this can be found in New England where groundwater within a shallow Miocene sand about 150 km offshore Long Island, New York is remarkably fresh ( ~ 3000 mg/l salinity). Closer to the mainland, a 512m borehole on Nantucket Island, Massachusetts, contains freshwater ( 2000 mg/l salinity) in all permeable units. Thick confining units within this well have a parabolic salinity profile consistent with ongoing vertical diffusion. Analytical calculations suggest that diffusion began within the confining units as recently as 21ka. Heads within the Cretaceous section beneath Nantucket Island are overpressured by about 4 m above the local water table. We propose to use mathematical modeling and environmental isotope analysis to determine whether flushing of coastal plain sediments offshore New England was caused by: (1) meteoric recharge during Pleistocene sea-level low-stands; (2) sub- ice-sheet recharge during the last glacial maximum; (3) recharge from proglacial lakes; (4) clay dehydration reactions; (5) vertical infiltration of freshwater associated with local flow cells that may have developed on the continental shelf during sea level low stands. A second goal of this research is to determine whether the observed excess heads on Nantucket Island can be explained by solely by Pleistocene sediment loading or whether more exotic mechanisms such as clay dewatering reactions must be invoked. We will construct a three-dimensional, variable-density groundwater flow, heat, and solute mass transport model for the Atlantic continental shelf between Maine and New Jersey. The code will be run on the Indiana University SP Supercomputer. Direct simulation of isotope transport and groundwater residence times will be compared to environmental isotopic data (C-13, C-14, O-18, H-2) collected from wells tapping confined aquifers on the continental shelf. In order to constrain the isotopic composition of pro-glacial lake recharge, we will drill one well to extract pore fluids from lacustrine sediments 230 feet below land surface on Nantucket Island. Pleistocene ice sheet, climatic, and sea-level reconstructions will be used to constrain temporal variations in hydrologic, chemical, and isotopic boundary and initial conditions. Flexural adjustments of land surface topography during Wisconsin glaciations will be made to account for the effects of ice sheet loading on relative sea level.