A major challenge confronting geochemists is to relate the chemistry of vented hydrothermalfluids to the local or regional tectonic and volcanic state of mid-ocean ridges. After more than15 years of sampling submarine hydrothermal fluids, a complex picture of spatial and temporalvariability in temperature and composition is emerging. Recent time-series observations andsampling of ridge segments with confirmed recent volcanic eruptions (CoAxial and North Clefton the Juan de Fuca ridge and 9-10°N on the East Pacific Rise) have created a first-orderunderstanding of how hydrothermal systems respond to volcanic events on the seafloor. Phaseseparation and enhanced volatile fluxes are associated with volcanic eruptions, withvapour-dominated fluids predominating in the initial post-eruption period, followed in time bybrine-dominated fluids, consistent with temporary storage of brine below the seafloor.Chemical data for CoAxial vents presented here are consistent with this evolution. Rapidchanges in output and composition of hydrothermal fluids following volcanic events may havea profound effect on microbiological production, macrofaunal colonization, and hydrothermalheat and mass fluxes. Size and location of the heat source are critical in determining how fastheat is removed and whether subseafloor microbial production will flourish. CoAxial eventplumes may be a direct result of dyking and eruption of lavas on the seafloor.
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