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

合作研究：新英格兰大西洋大陆架更新世水文地质学

• 批准号: 0337634
• 负责人: Mark Person
• 金额: $ 26.16万
• 依托单位: Indiana University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2004
• 资助国家: 美国
• 起止时间: 2004-02-01 至 2007-01-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0337634&HistoricalAwards=false
• 关键词: Collaborative; Research; Pleistocene; Hydrogeology; Atlantic

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.

0337634人在世界各地的许多沿海地区，大陆架沉积物中的淡水分布与现代海平面条件并不平衡。在新英格兰可以找到一个最极端的例子，在那里，纽约长岛离岸约150 km处的浅中新世砂层中的地下水非常新鲜（盐度约为3000 mg/l）。在靠近大陆的地方，马萨诸塞州南图克特岛上的一个512米的钻孔在所有可渗透单元中都含有淡水（2000毫克/升的盐度）。该井内的厚封闭单元具有与正在进行的垂直扩散一致的抛物线盐度剖面。分析计算表明，最近在21 ka时，限制单元内就开始了扩散。南图克特岛下方白垩纪剖面内的水头超压，超过当地地下水位约4 m。 我们建议利用数学模拟和环境同位素分析来确定新英格兰近海平原沉积物的冲刷是否是由以下因素引起的：（1）更新世海平面低水位期间的大气降水补给;（2）末次盛冰期冰盖下的补给;（3）冰前湖泊的补给;（4）粘土脱水反应;（5）与海平面低水位期间可能在大陆架上形成的局部流动单元有关的淡水垂直渗透。本研究的第二个目标是确定所观察到的过剩头南图克特岛是否可以解释仅由更新世沉积物加载或是否更奇特的机制，如粘土脱水反应必须调用。我们将建立一个三维的，变密度地下水流，热量和溶质质量输运模型之间的大西洋大陆架缅因州和新泽西。代码将在印第安纳州大学SP超级计算机上运行。同位素迁移和地下水停留时间的直接模拟将与从大陆架封闭含水层的威尔斯井收集的环境同位素数据（C-13、C-14、O-18、H-2）进行比较。为了限制前冰川湖补给的同位素组成，我们将钻一口井，从南图克特岛陆地表面以下230英尺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用于限制水文，化学和同位素边界和初始条件的时间变化。 在威斯康星州冰川期间，陆地表面地形的弯曲调整将被用来解释冰盖负荷对相对海平面的影响。