First hydrothermal discoveries on the Australian-Antarctic Ridge: Discharge sites, plume chemistry, and vent organisms

Abstract

The Australian-Antarctic Ridge (AAR) is one of the largest unexplored regions of the global mid-ocean ridge system. Here, we report a multiyear effort to locate and characterize hydrothermal activity on two first-order segments of the AAR: KR1 and KR2. To locate vent sites on each segment, we used pro- files collected by Miniature Autonomous Plume Recorders on rock corers during R/V Araon cruises in March and December of 2011. Optical and oxidation-reduction-potential anomalies indicate multiple active sites on both segments. Seven profiles on KR2 found 3 sites, each separated by 25 km. Forty profiles on KR1 identified 17 sites, some within a few kilometer of each other. The spatial density of hydrothermal activity along KR1 and KR2 (plume incidence of 0.34) is consistent with the global trend for a spreading rate of 70 mm/yr. The densest area of hydrothermal activity, named ''Mujin,'' occurred along the 20 km-long inflated section near the segment center of KR1. Continuous plume surveys conducted in January-February of 2013 on R/V Araon found CH4/3He (1-15 x 106) and CH4/Mn (0.01-0.5) ratios in the plume samples, consistent with a basaltic-hosted system and typical of ridges with intermediate spreading rates. Addition- ally, some of the plume samples exhibited slightly higher ratios of H2/3He and Fe/Mn than others, suggesting that those plumes are supported by a younger hydrothermal system that may have experienced a recent eruption. The Mujin-field was populated by <em>Kiwa </em>crabs and seven-armed <em>Paulasterias</em> starfish previ- ously recorded on the East Scotia Ridge, raising the possibility of circum-Antarctic biogeographic connections of vent fauna.