THE RISE AND FALL OF THE COAXIAL HYDROTHERMAL SITE, 1993-1996

In June 1993, a seafloor dike intrusion along the CoAxial segment of t he Juan de Fuca Ridge was acoustically detected. A near-immediate fiel d response and repeated plume mapping and sampling surveys during eigh t cruises over the next 3 years have provided a unique opportunity to estimate heat and mass fluxes from both event and chronic discharge du ring the life cycle of a newly created hydrothermal system. The intrus ion triggered the release of at least three event plumes followed by c hronic discharge focused at two sites: Flow, the site of a lava erupti on at the distal end of the intrusion; and Floc, 30 km back along the trend of the dike. We have combined measurements of plume temperature anomalies, plume areal extent, and year-long averages of current how a t both sites to estimate the chronic hydrothermal heat flux H-u. Initi al values of H-u at both sites were of order 10(4) MW, declining over time t as H-u = at(-k), with k approximate to 1. Significant plumes we re no longer detectable at Flow by June 1995, or at Floc by June 1996. Elemental fluxes from the CoAxid system have been derived from H-u an d measurements of the ratios Mn/heat, Fe/heat, and particulate S/heat in chronic plumes. While Mn and Fe fluxes mirrored the power curve dec line of heat, the combined regional particulate S (PS) flux experience d a second pronounced maximum some months after the eruption owing to a sharp increase in the S/heat ratio at Floc. Integrated inventories f rom chronic discharge were similar to 4 x 10(17) J for heat, similar t o 3 x 10(8) mol for Mn, similar to 2 x 10(8) mol for Fe, and similar t o 1 x 10(8) mol for PS. Realistic uncertainties for all species are ro ughly a factor of 2. The three event plumes accounted for <5% of the c hronic plume inventories.