Collaborative Research: Identifying Controls on Weathering of Seafloor Massive Sulfides

高温（高达400°C）热液喷射到深海的海水中，形成了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含铜和铁的矿床。目前的估计是，在靠近沿洋中脊和弧后盆地扩展的海底区域附近，大约有10亿公吨的SMS。这些沉积物是金属和能量的重要储存库，可以被生活在海底黑暗中的微生物利用。该方案的重点是确定这些沉积物的命运，也就是说，SMS沉积物是否完全风化（生锈），只留下氧化矿物，或者早期的蚀变是否形成了一个不透水的层，密封表面，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)最终再循环进入地球。当洋壳俯冲时，S内部和/或火山弧和弧后体系。该项目包括一个以本科生为主的机构的参与，以及在实验室和海上对本科生的培训。海底大量硫化物（SMS）样本将在华盛顿海岸附近的胡安德富卡山脊进行六次潜水收集。样品的物理、化学和矿物学性质（从新鲜的内部到风化的外部）将使用岩石学、扫描电子显微镜、微x射线衍射和基于同步加速器的技术进行鉴定。主要微生物群的存在将通过DNA和RNA测序和荧光标记微生物种群的定位来确定。这些数据将被整合到一个反应输运模型中，以达到以下目的：(1)重现观察到的组合，(2)估计孔隙流体成分，(3)计算代谢反应的自由能产量，(4)验证孔隙度和微压裂程度控制风化长期敏感性的假设，(5)估计风化的时间尺度，(6)研究微生物活动在风化中的作用。对SMS风化控制的深入了解将有助于改进对矿床中富含金属部分的寿命和这些反应所支持的潜在生物量的预测，并有助于对地下生物圈及其在深海碳循环中的作用的日益加深的了解。

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.