Fluid and chemical fluxes across the seafloor of a passive margin

海底地下水排放(SGD)现在被认为是将营养物质、碳和金属引入沿海海域的重要途径。然而，SGD的全部影响尚不确定，因为我们并不了解所有的驱动机制。虽然大多数关于SGD的研究都集中在海岸线上，但来自南大西洋海湾的越来越多的证据表明，SGD的大部分发生在远离海岸(1-70公里)的地方。对远离海岸的排放机制还没有很好的了解。对北卡罗来纳州近海20公里处的一组海底井的观察表明，这种排放反映了盐分流体从深处缓慢向上迁移。这些孔隙流体还受到风暴期间海水对沙质海底沉积物的快速冲刷或寒冷洋流迁移的影响。该项目将检验这样一种假设，即这些流动过程是常见的，并在大陆架的大片区域内驱动显着的流体交换。该项目还将估计横跨海底的镭、营养物质和碳的地球化学通量。研究区域将是一个150平方公里的区域，在南卡罗来纳州查尔斯顿附近近海20公里处。地球物理勘测将绘制海底水深图，并确定埋藏的沉积结构，这些结构可能会引导地下水向上流向海底。热力阵列将安装在海底以下2-5米深的10个地点。来自这些阵列的数据将表明海洋驱动的快速冲刷的深度和频率，并量化从深度可能的长期向上流动。安装在岸边垂直断面上的三对井将提供压力数据，这些数据将提供对流体流量的独立估计。这些油井还将进行营养物质、碳和镭示踪剂的采样，这些示踪剂将表明贫镭海水和富镭孔隙水之间的混合。将根据计算的流体通量和观测到的地球化学成分来确定穿过海底的地球化学通量。认识到地下水直接排放到海洋中向沿海水域提供了大量的营养物质、碳和金属，这代表着我们对沿海海洋系统的地球化学通量理解的范式转变。以前，只考虑来自河流、大气和上升流(与更深的海洋交换)的输入。现在，越来越多的证据表明，通过广阔大陆架排放的咸水地下水的数量至少与河流排放的量一样大，但对近海地区地下水排放的流动机制和化学成分了解很少。为了理解这些过程，对海底地下水排放的研究必须集中在更远的近海。这项工作有可能促使教科书对海洋的水文循环和地球化学预算进行重大修订。特别是，咸水地下水提供的营养物质(氮和磷)可能比以前认识到的要多得多。这些营养物质影响沿海生态系统的肥力，包括具有重要经济意义的渔业，并可能导致有害藻类暴发以及海洋“死区”的爆发。这项工作旨在包括大量的学生参与，包括代表不足的群体的参与。

Submarine groundwater discharge (SGD) is now recognized as an important pathway for the introduction of nutrients, carbon, and metals into coastal ocean waters. The full impact of SGD is uncertain, however, because we do not understand all the driving mechanisms. Whereas most studies of SGD have focused at the coastline, increasing evidence from the South Atlantic Bight indicates that the majority of SGD occurs far (1-70 km) offshore. Mechanisms for discharge far from shore are not yet well understood. Observations from a cluster of seafloor wells 20 km offshore of North Carolina suggest that this discharge reflects slow upward migration of saline fluids from depth. These porefluids are also affected by rapid flushing of sandy seafloor sediments by seawater during storms or migration of cold ocean currents. This project will test the hypothesis that these flow processes are common and drive significant fluid exchange over large areas of the continental shelf. The project will also estimate the geochemical fluxes of radium, nutrients and carbon across the seafloor. The study area will be a 150 square-km area that reaches 20 km offshore near Charleston, South Carolina. Geophysical surveys will map seafloor bathymetry and identify buried sedimentary structures that could channel groundwater traveling upward toward the seafloor. Thermal arrays will be installed to depths of 2-5 m below the seafloor at 10 locations. Data from these arrays will indicate the depth and frequency of rapid ocean-driven flushing and quantify possible long-term upward flow from depth. Three pairs of wells installed in a shore-perpendicular transect will supply pressure data that will provide independent estimates of fluid fluxes. The wells will also be sampled for nutrients, carbon, and radium tracers, which will indicate mixing between radium-poor seawater and radium-enriched pore waters. Geochemical fluxes across the seafloor will be determined based on calculated fluid fluxes and observed geochemical compositions.The realization that groundwater discharging directly to the ocean supplies significant quantities of nutrients, carbon, and metals to coastal waters represents a paradigm shift in our understanding of geochemical fluxes to the coastal ocean systems. Previously, only inputs from rivers, the atmosphere, and upwelling (exchange with the deeper ocean) were considered. Now, increasing evidence suggests that the volume of saline groundwater that discharges across broad continental shelves is at least as large as river discharge, but the flow mechanisms and chemical compositions of groundwater discharge in offshore regions are very poorly understood. It is essential that studies of submarine groundwater discharge focus farther offshore in order to understand these processes. This work has the potential to spur significant revisions of textbook views of the hydrologic cycle and geochemical budgets for the ocean. In particular, saline groundwater may be supplying significantly greater nutrients (nitrogen and phosphorus) than has previously been realized. These nutrients affect the fertility of coastal ecosystems, which include economically significant fisheries, and may contribute to outbreaks of harmful algae blooms as well as ocean 'dead zones'. The work is designed to include significant student participation, including participation by under-represented groups.