Collaborative Research: Identifying Controls on Weathering of Seafloor Massive Sulfides

高温（高达400°C）热液流体排入深海海水导致形成富含金属的硫化物矿床（海底块状硫化物或SMS）。 这些硫化物类似于在陆地上开采的某些类型的富铜和富铁矿床。 目前的估计是，大约有10亿公吨的SMS存在于沿着洋中脊和弧后盆地的海底扩张区附近。 这些矿床是金属和能源的重要储藏库，可以被生活在海底黑暗中的微生物利用。 这项建议的重点是确定这些矿床的命运，即，SMS沉积物是否完全风化（生锈），只留下氧化物矿物，或者早期的蚀变是否产生了密封表面的不可渗透层，保护富含金属硫化物的内部不与海水反应？了解SMS的命运很重要，因为沉积物1）可以作为深海微生物生命的能量来源（通过氧化还原反应），2）最终会再循环到地球？的内部和/或火山弧和弧后系统时，洋壳俯冲。该项目包括一个主要由本科生组成的机构的参与以及对本科生进行实验室和海上培训，将在华盛顿海岸外的胡安德富卡海岭进行六次潜水采集海底块状硫化物样品。 将使用岩相学、扫描电子显微镜、微X射线衍射和基于同步加速器的技术来鉴定样品（从新鲜内部到风化外部）的物理、化学和矿物学性质。 将通过DNA和RNA测序以及荧光标记微生物种群图谱确定主要微生物组的存在。这些数据将被纳入一个反应性运输模型，以实现以下目标：（1）再现观察到的组合，（2）估计孔隙流体组成，（3）计算代谢反应的自由能产额，（4）检验孔隙度和微破裂程度控制长期风化敏感性的假设，（5）估计风化的时间尺度，（6）微生物活动在风化过程中的作用。对SMS风化控制的深入了解将有助于更好地预测矿床富金属部分的寿命和这些反应支持的潜在生物量，并增加对地下生物圈及其在深海碳循环中的作用的了解。

Venting of high temperature (up to 400°C) hydrothermal fluids into seawater in the deep ocean results in formation of metal-rich sulfide mineral deposits (seafloor massive sulfides or SMS). These sulfides are analogous to some types of copper- and iron-rich ore deposits mined on land. Current estimates are that there are approximately a billion metric tons of SMS present in close proximity to areas of seafloor spreading along mid-ocean ridges and in back-arc basins. These deposits are a significant reservoir of both metals and energy that can be harnessed by microorganisms living in the dark at the seafloor. This proposal focuses on determining the fate of these deposits, i.e., do SMS deposits weather (rust) completely, leaving behind only oxide minerals, or does early alteration create an impermeable layer that seals the surfaces, protecting the metal sulfide rich interiors from reaction with seawater? Understanding the fate of SMS is important because the deposits 1) could act as sources of energy for microbial life in the deep sea (through oxidation-reduction reactions) and 2) are eventually re-cycled into Earth?s interior and/or volcanic arc and back-arc systems when ocean crust is subducted. The project includes participation of a predominantly undergraduate institution and training of undergraduate students in the laboratory and at sea.Seafloor massive sulfide (SMS) samples will be collected during six submersible dives on the Juan de Fuca Ridge off the coast of Washington. Physical, chemical, and mineralogical properties of the samples (from fresh interiors to weathered exteriors) will be identified using petrography, scanning electron microscopy, micro-X-ray diffraction and synchrotron-based techniques. The presence of major microbial groups will be identified through sequencing of DNA and RNA and mapping of fluorescently labeled microbial populations. These data will be incorporated into a reactive transport model to attain the following: (1) reproduce observed assemblages, (2) estimate pore fluid compositions, (3) calculate free energy yields of metabolic reactions, (4) test the hypothesis that porosity and extents of micro-fracturing control long-term susceptibility to weathering, (5) estimate time-scales of weathering, and (6) investigate the role of microbial activity in weathering. An enriched understanding of the controls on weathering of SMS will allow for improved predictions of the lifetime of metal-rich portions of deposits and the potential biomass supported by these reactions, as well as add to a growing understanding of the subsurface biosphere and its role in deep-sea carbon cycling.