Collaborative Research: Identifying Controls on Weathering of Seafloor Massive Sulfides

高温(高达400摄氏度)热液向深海海水中喷发，形成富含金属的硫化物矿藏(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藏。目前的估计是，在沿大洋中脊和弧后盆地扩散的海底区域附近，大约有10亿公吨的海底生物。这些矿藏是金属和能源的重要储存库，可以被生活在海底黑暗中的微生物利用。这项提议的重点是决定这些矿床的命运，即SMS矿床是否完全经受住了(锈蚀)，只留下了氧化物矿物，或者早期的蚀变是否创造了一层不渗透的层来封闭表面，保护富含金属硫化物的内部免受海水的反应？了解SMS的命运很重要，因为这些沉积物1)可以作为深海微生物生命的能源(通过氧化还原反应)，2)当洋壳俯冲时，最终重新循环到地球-S内部和/或火山弧和弧后系统。该项目包括一个主要由本科生组成的机构的参与，以及在实验室和海上对本科生的培训。在华盛顿海岸外的胡安德富卡海脊进行六次潜水时，将采集海床块状硫化物(SMS)样本。样品的物理、化学和矿物学特性(从新鲜的内部到风化的外部)将使用岩相学、扫描电子显微镜、显微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的测序以及荧光标记的微生物种群图来确定。这些数据将被纳入反应传输模型，以实现以下结果：(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，以及(6)调查微生物活动在风化中的作用。如果能够更好地了解海洋系统的风化控制，将有助于改进对富含金属的沉积物的寿命以及这些反应所支持的潜在生物量的预测，并有助于加深对次表层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.