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

海水在海洋地壳的上部循环，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，多次被埋藏的岩浆体加热，运输和释放了地球热量的25%。据估计，流体流经海洋地壳的速率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，大部分来自对东太平洋海洋地壳的研究，这些地壳是由中等和快速扩张的洋中脊形成的。这些研究表明，海水及其通过海底的环流显著影响海洋中的地壳演化和生物地球化学循环，并以刚刚开始被量化和理解的方式影响生物圈。为了扩大这一认识，本研究将重点放在缓慢扩张脊形成的海底流体流动上，如大西洋、印度洋和北冰洋，因为它在结构、矿物学和形态上与快速和中等扩张脊形成的海底流体流动有很大不同。本次研究回归北塘，长期；海底;流体流动监测点，由海洋钻探计划在大西洋钻探并安装仪器。这个研究地点被钻孔所穿透，在这些钻孔中已经部署了流体流动、地球化学和生物采样器，以收集数据和样本。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发展，这增加了科学基础设施并具有商业应用。它还在三个美国学术机构（其中一个是EPSCoR州（密西西比州））为学生提供培训和教育与研究的整合，并支持在科学和工程领域性别代表性不足的PI。公众宣传将与暗能量生物圈研究中心一起开展。本项目完成了对中大西洋洋脊西翼800万年缓慢扩张地壳脊侧热液过程的长期生物地球化学和水文研究。这个地点，北池，是一个孤立的东北走向的沉积物池，被海底山脉所包围，这些山脉自20世纪70年代以来一直在研究。在2011年的“综合海洋钻探计划”（Integrated Ocean Drilling Program）第336次考察和5个月后的一次考察（2012年）中，在钻入火山地壳的海底的4个钻孔观测站中部署了传感器、采样器和实验设备，目的是监测水文特性、地壳流体成分和矿物蚀变的变化，以及其他目标。2012年和2014年的井口采样已经揭示了地壳流体成分的变化；相关的压力数据证实，井眼处于封闭和超压状态，这反映了井眼从钻井干扰中恢复后地层的变化。这项研究包括为期13天的海洋考察，并使用现场机器人操作车辆在North Pond回收井下仪器包。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。船舶和岸基分析将用于解决与慢扩展地壳上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.