Overview of Hydrographic Data From the GEOTRACES EPZT Cruise

Abstract:

During the US GEOTRACES East Pacific Zonal Transect (EPZT) cruise extensive use was made of the hydrographic data collected by the SIO Shipboard Technical Support team aboard ship to select suitable depths for water column sampling at all stations. Across the eastern half of the section, starting from the Peru Margin and spanning the Oxygen Minimum Zone (OMZ), attention was paid to not only the standard hydrographic sensors but also the fluorometer and dissolved oxygen sensors interfaced to the CTD, for example to select depths associated with the chlorophyll maximum in the uppermost water column and also the underlying low oxygen waters which formed a primary focus for sampling to investigate redox-cycling within the OMZ. In the western half of the section, attention turned toward the extensive hydrothermal plume which was first intercepted directly above the southern East Pacific Rise ridge axis near 15°S. There, the depth and intensity of the hydrothermal plume was revealed by a combination of the standard sensor suite interfaced to the CTD/rosette, augmented with an Oxygen Reduction Potential (ORP) probe provided by NOAA-PMEL and a SeaPoint optical back scatter sensor provided by WHOI. Thorough on-board salinity, dissolved oxygen, and nutrient analyses provided key data for CTD calibration, quality control and hydrographic interpretation including clear evidence for dissolved phosphate scavenging in the near-field hydrothermal plume. We will compare the EPZT CTD/hydrographic data with WOCE-era (1991-1994) sections including zonal section P21, which parallels the new transect, and crossovers with WOCE meridional sections P16, P17, P18, and P19.