Fluid and chemical fluxes across the seafloor of a passive margin

海底地下水排放（SGD）是将营养物质、碳和金属引入沿海海洋沃茨的重要途径。然而，SGD的全部影响是不确定的，因为我们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，但来自南大西洋湾的越来越多的证据表明，大多数SGD发生在离岸较远的地方（1-70公里）。远离海岸的排放机制尚未完全了解。从北卡罗来纳州近海20公里处的一组海底威尔斯井进行的观测表明，这种排放反映了盐水流体从深处缓慢向上迁移。这些孔隙流体还受到风暴或冷洋流迁移期间海水对桑迪海底沉积物的快速冲刷的影响。该项目将检验这样一种假设，即这些流动过程是常见的，并在大陆架的大面积区域上推动重大的流体交换。该项目还将估计整个海底的镭、营养物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，到达南卡罗来纳州查尔斯顿附近近海20公里处。地球物理勘探将绘制海底测深图，并确定可能引导地下水向上流向海底的埋藏沉积结构。将在10个地点安装热阵列，深度为海底以下2-5米。来自这些阵列的数据将表明海洋快速冲刷的深度和频率，并量化可能的长期向上流动。安装在海岸垂直断面上的三对威尔斯井将提供压力数据，这些数据将提供流体通量的独立估计。还将对威尔斯井进行营养物、碳和镭示踪剂取样，这将表明贫镭海水和富镭孔隙沃茨水之间的混合。将根据计算的流体通量和观测到的地球化学成分确定海底的地球化学通量。认识到地下水直接排入海洋，为沿海沃茨提供了大量的营养物质、碳和金属，这代表了我们对沿海海洋系统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（与更深海洋的交换）的输入。现在，越来越多的证据表明，通过广阔大陆架排放的含盐地下水的量至少与河流排放量一样大，但对近海地区地下水排放的流动机制和化学成分知之甚少。为了了解这些过程，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对海洋水文循环和地球化学预算的教科书观点进行重大修订。特别是，含盐地下水可能提供比以前认识到的更多的营养物质（氮和磷）。这些营养物质影响沿海生态系统的肥力，其中包括具有重要经济意义的渔业，并可能导致有害藻类大量繁殖和海洋“死区”的爆发。这项工作的目的是包括重要的学生参与，包括代表性不足的群体的参与。

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.