Water mass analysis of the 2013 US GEOTRACES eastern Pacific zonal transect (GP16)

The 2013 US GEOTRACES Eastern Pacific Zonal Transect (GP16) extended from the Peruvian coast to Tahiti, along a line that fell between 10 and 15°S. This transect sampled the Peruvian oxygen deficient zone (ODZ) and the hydrothermal plume extending from the East Pacific Rise (EPR) for a variety of trace elements and isotopes (TEIs). Here we report nutrient and hydrographic measurements collected on this cruise, as well as results from an Optimum Multiparameter Analysis (OMPA) to quantify the fractional contributions of endmember water masses in each sample. The primary goals of this study were to better understand the distribution of water masses in the eastern tropical Pacific, and to help interpret TEI measurements collected on this cruise, as well as related studies carried out in the region. In the thermocline, Equatorial Subsurface Water (ESSW) dominated the low oxygen waters of the eastern tropical South Pacific, blending into Eastern South Pacific Intermediate Water (ESPIW) and South Pacific Central Water (SPCW) further west. Below the thermocline, distributions of Antarctic Intermediate Water (AAIW) and Equatorial Pacific Intermediate Water (EqPIW) were relatively homogenous along the section between 800 and 1200 m depth. Deeper in the water column, distinct water mass signatures were found on opposite sides of the EPR: southward flowing Pacific Deep Water (PDW) dominated the basin east of the EPR, while the northward flowing Antarctic Bottom Water (AABW) and Lower Circumpolar Deep Water (LCDW) had the strongest contributions on the western side of the EPR. These findings support previous studies that indicate the Peruvian ODZ is largely contained within ESSW and that the EPR plays an important role in steering water mass distributions in the deep waters of the tropical Pacific. Overall, these results agree well with previous water mass analyses in this region and are consistent with the general circulation patterns in the eastern tropical Pacific.