Understanding a submarine eruption through time series hydrothermal plume sampling of dissolved and particulate constituents: West Mata, 2008-2012

Four cruises between 2008 and 2012 monitored the continuing eruption of West Mata volcano in the NE Lau Basin as it produced plumes of chemically altered water above its summit. Although large enrichments in He-3, CO2, Fe, and Mn were observed in the plumes, the most notable enrichment was that of H-2, which reached concentrations as high as 14,843 nM. Strongly enriched H-2 concentrations in the water column result from reactions between seawater or magmatic water and extremely hot rocks. In 2008, the observation of elevated H-2 concentrations in the water column above West Mata pointed to vigorous ongoing eruptions at the volcano's summit. The eruption was confirmed by visual observations made by the ROV Jason 2 in 2009 and demonstrated that H-2 measurements are a vital instrument to detect ongoing volcanic eruptions at the seafloor. Elevated H-2 in 2010 showed that the eruption was ongoing, although at a reduced level given a maximum H-2 concentration of 4410 nM. In 2012, H-2 levels in the water column declined significantly, to a maximum of only 7 nM, consistent with visual observations from the Quest-4000 ROV that found no evidence of an ongoing volcanic eruption. Methane behaved independently of other measured gases and its concentrations in the hydrothermal plume were very low. We attribute its minimal enrichments to a mixture of mantle carbon reduced to CH4 and biological CH4 from diffuse flow sites. This study demonstrates that ongoing submarine volcanic eruptions are characterized by high dissolved H-2 concentrations present in the overlying water column.