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

Abstract

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 Floe, 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 Hu. Initial values of Hu at both sites were of order 104 MW, declining over time t as Hu = at-k, with k ≈ 1. Significant plumes were no longer detectable at Flow by June 1995, or at Floe by June 1996. Elemental fluxes from the CoAxial system have been derived from Hu 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 Floe. Integrated inventories from chronic discharge were ∼4 × 1017J for heat, ∼3 × 108 mol for Mn, ∼2 × 108 mol for Fe, and ∼1 × 108 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.