Collaborative Research: Identifying Controls on Weathering of Seafloor Massive Sulfides
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.