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

在世界各地的许多沿海环境中，淡水在大陆架沉积物中的分布与现代海平面条件并不平衡。最极端的例子之一可以在新英格兰发现，在纽约长岛外约150公里处中新世浅层砂岩中，地下水非常新鲜(约3000毫克/L盐度)。在距离大陆更近的地方，马萨诸塞州南塔基特岛上一个512米长的钻孔，在所有可渗透单元中都含有淡水(2000毫克/L盐度)。这口井内的厚封闭单元具有抛物线盐度剖面，与正在进行的垂直扩散一致。分析计算表明，早在21ka，封闭单元内的扩散就开始了。南塔基特岛白垩纪部分的海头比当地地下水位高出约4米。我们建议使用数学模型和环境同位素分析来确定新英格兰近海沿海平原沉积物的冲刷是否是由以下原因引起的：(1)更新世海平面低位期间的大气补给；(2)末次冰盛期的次冰盖补给；(3)来自前冰期湖泊的补给；(4)粘土脱水反应；(5)与海平面低位期间可能在大陆架上发育的局部流动单元有关的淡水垂直渗透。这项研究的第二个目标是确定南塔基特岛上观察到的过剩水头是否可以仅由更新世沉积物负载来解释，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、可变密度的地下水流动、热量和溶质质量传输模型。该代码将在印第安纳大学SP超级计算机上运行。对同位素迁移和地下水停留时间的直接模拟将与从开发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。为了限制前冰期湖泊补给的同位素组成，我们将钻一口井，从南塔基特岛230英尺以下的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被用来限制水文、化学和同位素边界和初始条件的时间变化。将对威斯康星州冰川期间的陆地表面地形进行弯曲调整，以考虑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.