Weathering refers to in situ physical (mechanical), chemical, and biological breakdown of a mineral or rock and it's replacement with secondary minerals.
Until ~30 yrs ago when microorganisms were discovered living in and around deep sea hydrothermal vents more than 2.5 km beneath the sea surface where no light penetrates, all life was though to be dependent on light energy directly or indirectly. Weathering in these systems was believed to be solely the result of chemical reactions due to the differences between the cold oxic ocean water and the hot reduced rocks that had recently erupted.
Early studies focused on textural and chemical analysis of basalt collected from the seafloor (Thorseth et al, 1995; Fisk et al, 1998; Torsvik et al, 1998; Furnes et al, 1999). These studies suggested that microorganisms might play a significant role in the alteration of seafloor rocks, particularly volcanic glasses. More recent studies have called this into question. Templeton et al (2009) incubated cleaned slabs of basalt in hydrothermal fluids at the Loihi seamount for a year and then used scanning electron microscopy (SEM) coupled with a focused ion beam (FIB) to slice through metal encrusted microbial biofilms to observe the basalt surface underneath. They observed very little weathering of basaltic glass beneath the biofilm and suggested that the heavily encrusted biofilms were the result of microbial interactions with the metal rich hydrothermal fluids alone. As a result of these contradictory findings, the role of microorganisms in dissolution reactions, or the break down of promary mineral phases, in hydrothermal systems remains an open question.
Despite this, microorganisms have been shown to play an important role in precipitation of secondary minerals in hydrothermal systems. For example, Fouke et al (2000) showed that microorganisms increase the rate of precipitation of carbonates at Angel Terrace in Yellowstone by an order of magnitude. Lalonde et al (2005) showed that microorganisms mediate siliceous sinter formation and Konhauser et al (2002) similarly showed they mediated clay formation in volcanic systems.
While the full role of microbial influence in hydrothermal systems is not yet quantified, it is clear that they do influence some weathering reactions, particularly the rates of formation of some secondary phases.
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Fouke, B. W., Farmer, J. D. , Des Marais, D. J. , Pratt, L. , Sturchio, N. C., Burns, P. C., and Discipulo, M. K. (2000) Depositional facies and aqueous-solid geochemistry of travertine-depositing hot springs (Angel Terrace, Mammoth Hot Springs, Yellowstone National Park, U.S.A.). Journal of Sedimentary Research, 70, 565-585.
Furnes, H., Muehlenbachs, K., Tumyr, O., Torsvik, T. and Thorseth, I. (1999) Depth of the active bio-alteration in ocean crust: Costa Rica Rift (Hole 504B). Terra Nova, 11, 228-233.
Konhauser, K. O., Schiffman, P., & Fisher, Q. J. (2002). Microbial mediation of authigenic clays during hydrothermal alteration of basaltic tephra, Kilauea Volcano. Geochemistry Geophysics Geosystems, 3(12), 1-13. doi: 10.1029/2002GC000317.
Lalonde, S. V., Konhauser, K. O., Reysenbach, A.-L., & Ferris, F. G. (2005). The experimental silicification of Aquificales and their role in hot spring sinter formation. Geobiology, 3(1), 41-52. doi: 10.1111/j.1472-4669.2005.00042.x.
Templeton, A. S., Knowles, E. J., Eldridge, D. L., Arey, B. W., Dohnalkova, A. C., Webb, S. M., et al. (2009). A seafloor microbial biome hosted within incipient ferromanganese crusts. Nature Geoscience, 2(12), 872-876. Nature Publishing Group. doi: 10.1038/ngeo696.
Thorseth, I. H., Torsvik, T., Furnes H. and Mulenbachs, K. (1995). Microbes play an important role in the alteration of oceanic crust. Chemical Geology, 126, 137-146.
Torsvik, T., Furnes, H., Muehlenbachs, K., Thorseth, I. H. , and Tumyr, O. (1998). Evidence for microbial activity at the glass-alteration interface in oceanic basalts. Earth and Planetary Science Letters, 162, 165-176.