The rise and fall of the CoAxial hydrothermal site, 1993-1996

In June 1993, a seafloor dike intrusion along the CoAxial segment of the Juan de Fuca Ridge was acoustically detected. A near-immediate field response and repeated plume mapping and sampling surveys during eight cruises over the next 3 years have provided a unique opportunity to estimate heat and mass fluxes from both event and chronic discharge during the life cycle of a newly created hydrothermal system. The intrusion triggered the release of at least three event plumes followed by chronic discharge focused at two sites: Flow, the site of a lava eruption 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 at both sites to estimate the chronic hydrothermal heat flux H-u. Initial 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 were 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 and measurements of the ratios Mn/heat, Fe/heat, and particulate S/heat in chronic plumes. While Mn and Fe fluxes mirrored the power curve decline of heat, the combined regional particulate S (PS) flux experienced a second pronounced maximum some months after the eruption owing to a sharp increase in the S/heat ratio at Floc. Integrated inventories from chronic discharge were similar to 4 x 10(17) J for heat, similar to 3 x 10(8) mol for Mn, similar to 2 x 10(8) mol for Fe, and similar to 1 x 10(8) mol for PS. Realistic uncertainties for all species are roughly a factor of 2. The three event plumes accounted for <5% of the chronic plume inventories.