Temporal changes in venting at Brothers volcano, as indicated by backscatter and oxidation-reduction potential
Few measurements have been conducted into the temporal variability of venting along volcanic arcs. To characterise any temporal changes, two survey track lines were conducted over Brothers Volcano, Kermadec arc, on March 6 and 7, 2009 on board the R/V Thompson. Conductivity (salinity)-temperature-depth-optical (CTD-O) and oxidation-reduction (ORP) measurements were taken to map the spatial distribution of hydrothermal fluids in the water column. Previous surveys conducted in 2002 and 2007 show that levels of ΔNTU had decreased from 2002 to 2007, followed by a small increase in 2009. Previous ORP data are only available from the 2007 cruise, and reveals an increasing anomaly with time. Changes in the salinity of the plume waters, combined with ΔNTU trends, suggest that from 2002 to 2007 hydrothermal venting shifted to the emission of vapours with low ion concentrations. This shift may indicate that the system has heated up, perhaps due to a pulse of magmatic activity. From 2007 to 2009 a large increase in the salinity of plume waters and ΔNTU was documented. ORP data does not match this hypothesis, but cannot be reliably quantified due to the constant variability of plume ORP signals, and its short spatial extent from the centre of the plume. Two different background waters were also observed in the 2009 track lines, representing an anomalous patch of warmer waters transported into the area. This anomaly was not present in the 2002 or 2007 tows, and may have implications for plume dynamics.