Evidence for iron and sulfur enrichments in hydrothermal plumes at Axial Volcano following the January–February 1998 eruption

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Abstract
In response to 12 days of seismic activity at Axial Volcano, Juan de Fuca Ridge in January–February 1998, the NSF/NOAA‐sponsored Axial Response Team conducted three response cruises in February, August, and August/September to map and sample hydrothermal plumes over the region. Vertical profiles of particulate Fe and S over the eruption site show high concentrations from about 1400 m to the bottom. Chemical and scanning electron microscope analysis of the February plume samples revealed anhydrite, Fe‐ferrihydrites, elemental sulfur, and angular glassy basalt shards up to 190 µm in the longest dimension. Many of these shards had halite coatings, which is consistent with subseafloor basalt seawater reactions at temperatures >450°C at 1500 m depth. In August and September cruises the basalt shards were no longer present in the hydrothermal plumes. Instead, the plumes were predominantly composed of Fe‐ferrihydrites, elemental sulfur, and sulfur filaments. A unique feature of this data set is the high concentrations of elemental sulfur in the lower 60 m of the water column. The sulfur results are suggestive of a significant enrichment of the bacterial biomass in the water column over the eruption site with time. Within the Axial caldera, approximately 10–20% of the total sulfur is present as sulfur filaments. These post‐eruption particle compositional changes have strong similarities to the results from the 1993 CoAxial eruption on the Juan de Fuca Ridge.