FY 1998

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

Baker, E.T., G.J. Massoth, R.A. Feely, G.A. Cannon, and R.E. Thomson

J. Geophys. Res., 103(B5), 9791–9806, doi: 10.1029/97JB03112 (1998)

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 flow at both sites to estimate the chronic hydrothermal heat flux H. Initial values of H at both sites were of order 10 MW, declining over time t as H = at^-k, with k 1. Significant plumes were no longer detectable at Flow by June 1995, or at Floc by June 1996. Elemental fluxes from the CoAxial system have been derived from H 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 ~4 × 10 J for heat, ~3 × 10 mol for Mn, ~2 × 10 mol for Fe, and ~1 × 10 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.