Methane dynamics in hydrothermal plumes over a superfast spreading center: East Pacific Rise, 27.5°–32.3°S

Abstract

Samples were collected from hydrothermal plumes along the East Pacific Rise (EPR) from 28° to 32°S during the Ridge Axis Plume and Neotectonic Unified Investigation (RAPANUI) cruise (5 March to 12 April 1998). Forty‐five vertical casts and tow‐yos were conducted: 3 off axis and 42 over the axes of two overlapping propagating ridges, the West and East ridges. These ridges are composed of several nontransform offset ridge segments. Spreading rates range from 149 mm/yr in the segment interiors to 0 mm/yr at the propagating rifts. These maximum spreading rates are considered the fastest on the mid‐ocean ridge system and affect the structure of the ridge. Segment‐averaged methane plume maxima ranged from 1.7 to 7.2 nM. Mean methane concentrations on the West Ridge were nearly double those of the East Ridge. Westwardly advecting hydrothermal methane persisted to our most distal station, nearly 480 km west of the East Pacific Rise. Background concentrations were less than 1 nM. The highest methane concentration measured was 50 nM in a buoyant plume. Methane did not covary with manganese or any other hydrothermal tracer in plumes over portions of a segment that exhibited recent magmatic activity, possibly as a result of the hydrothermal system's recovery from a phase separation event. In contrast, methane/manganese ratios on the other segments ranged from 0.077 to 0.091. Methane δ13C values in plume maxima ranged from −27 to −33‰ versus Peedee belemnite; background values were around −40‰. These data are compared with hydrothermal plume methane data from slower spreading ridges and illustrate similarities in hydrothermal processes and sources between these systems.