Collaborative Research: Completing North Pond Borehole Experiments to Elucidate the Hydrology of Young, Slow-Spread Crust

海水在大洋地壳上部循环，就像地下水在大陆含水层中流动一样。然而，在海洋中，这种海水循环，被埋藏的岩浆体多次加热，输送和释放了地球25%的热量。据估计，流经洋壳的流体速度等于河流向海洋输送的水量。我们对这种海底流体流动的了解很大程度上来自对东太平洋洋壳的研究，这些洋壳是由中等和快速扩张的大洋中脊形成的。这些研究表明，海水及其在海底的循环对地壳演化和海洋中的生物地球化学循环产生重大影响，对生物圈的影响也只是刚刚开始量化和了解。为了扩大这一认识，这项研究侧重于由缓慢扩张海脊产生的海底流体流动，如大西洋、印度洋和北冰洋的海底扩张海脊，因为它在结构、矿物学和形态上与快速和中等扩张海脊形成的海底流体有显著不同。这项研究回到北池，这是一个长期的海底流体流动监测点，由大西洋大洋钻探计划钻探和指示。这一研究地点被钻孔穿透，其中流体流动以及地化和生物采样器已经部署了数年来收集数据和样本。它还为船上和岸上的地球化学和生物分析提供资源。这项工作的更广泛影响包括传感器和技术开发，这增加了科学基础设施并具有商业应用。它还为三个美国学术机构的学生提供培训，并将教育和研究结合起来，其中一个是EPSCoR州(密西西比州)，并支持在科学和工程领域性别比例不足的PI。将与暗能量生物圈调查中心一起开展公众宣传。该项目完成了对中大西洋海脊西侧缓慢扩展的800万年前地壳脊侧热液过程的长期生物地球化学和水文研究。北池是一个孤立的东北方向的沉淀池，周围是自20世纪70年代以来一直被研究的海底山脉。在2011年综合大洋钻探计划第336次考察和五个月后(2012年)的一次考察中，在钻入海底的四个钻孔观测站中部署了传感器、取样器和实验，目的是监测水文性质、地壳流体成分和矿物蚀变等方面的变化。2012年和2014年的井口采样已经显示出地壳流体成分的变化；相关的压力数据证实，钻孔是封闭和超压的，反映了随着钻孔从钻井扰动中恢复，地层发生了变化。这项研究包括一次为期13天的海洋探险，并使用现场机器人操作的车辆在北池打捞井下仪器包。它将允许对地壳流体进行采样，恢复压力数据，并测量流体流速。将利用船基和岸基分析来解决与缓慢扩张地壳上的热液过程的水文地质有关的基本问题。

Seawater circulates through the upper part of the oceanic crust much like groundwater flows through continental aquifers. However, in the ocean this seawater circulation, many times heated by buried magmatic bodies, transports and releases 25% of the Earth's heat. The rate of fluid flow through ocean crust is estimated to be equal to the amount of water delivered by rivers to the ocean. Much of what we know of this subseafloor fluid flow comes from studies in the eastern Pacific Ocean on ocean crust created by medium and fast spreading mid-ocean ridges. These studies indicate that seawater and its circulation through the seafloor significantly impact crustal evolution and biogeochemical cycles in the ocean and affect the biosphere in ways that are just now beginning to be quantified and understood. To expand this understanding, this research focuses on fluid flow of seafloor generated by slow spreading ridges, like those in the Atlantic, Indian and Arctic Oceans because it is significantly different in structure, mineralogy, and morphology than that formed at fast and intermediate spreading ridges. This research returns to North Pond, a long-term; seafloor; fluid flow monitoring site, drilled and instumented by the Ocean Drilling Program in the Atlantic Ocean. This research site was punctured by boreholes in which fluid flow and geochemical and biological samplers have been deployed for a number of years to collect data and samples. It also provides resources for shipboard and on-shore geochemical and biological analysis. Broader impacts of the work include sensor and technology development, which increases infrastructure for science and has commercial applications. It also provides training for students and the integration of education and research at three US academic institutions, one of which is an EPSCoR state (Mississippi), and supports a PI whose gender is under-represented in sciences and engineering. Public outreach will be carried out in conjunction with the Center for Dark Energy Biosphere Investigations. This project completes a long-term biogeochemical and hydrologic study of ridge flank hydrothermal processes on slow-spreading, 8 million year old crust on the western flank of the Mid-Atlantic Ridge. The site, North Pond, is an isolated northeast-trending sediment pond, bounded by undersea mountains that have been studied since the 1970s. During Integrated Ocean Drilling Program Expedition 336 in 2011 and an expedition five months later (2012), sensors, samplers, and experiments were deployed in four borehole observatories drilled into the seafloor that penetrated into volcanic crust, with the purpose of monitoring changes in hydrologic properties, crustal fluid composition and mineral alteration, among other objectives. Wellhead sampling in 2012 and 2014 already revealed changes in crustal fluid compositions; and associated pressure data confirm that the boreholes are sealed and overpressured, reflecting a change in the formation as the boreholes recover from drilling disturbances. This research includes a 13-day oceanographic expedition and use of on-site robotically operated vehicles to recover downhole instrument packages at North Pond. It will allow the sampling of crustal fluids, recovering pressure data, and measuring fluid flow rates. Ship- and shore-based analyses will be used to address fundamental questions related to the hydrogeology of hydrothermal processes on slow-spread crust.