Temporal and Spatial Variability of Biogeochemical Processes in the Southeast Pacific Oxygen Minimum Zone and Surrounding Regions

Tracer ages are used to compare temporal and spatial variability of biogeochemical processes in the southeastern Pacific oxygen minimum zone (OMZ) and surrounding regions. Effects of variability on time scales of seasonal to interannual and decadal on the southeastern Pacific are investigated by putting the 2013 GEOTRACES data in the context of 1990s WOCE, and recent CLIVAR, GO-SHIP and ARGO data. Seasonally, the northern boundary of the OMZ shifts north in the southern hemisphere summer. In the southern hemisphere winter the OMZ shifts southward; this change increases the influx of ventilated water into the OMZ. Changes in ENSO cycle appear to have little effect on the location of the boundary between the South Pacific subtropical gyre and the OMZ. Spatial differences in oxygen, AOUR, nitrite and nitrate are analyzed over the density range of 25.5 – 27.2 sigma theta, which contains the OMZ. Oxygen concentrations are proportional to the concentrations of SF₆ through all the regions (coastal, OMZ and subtropical gyre). The oxygen-pSF­­₆ age correlation has a much steeper slope in the subtropical gyre than in the OMZ and coastal regions. This pattern is also observed in comparisons of oxygen to CFC-12 concentrations from WOCE, CLIVAR, GO-SHIP data over the same region. AOUR, calculated with pSF­­₆ ages, is larger in the coastal and OMZ regions as compared with the subtropical gyre. In the coastal region, the pSF₆ age decreases monotonically with depth through the nitrite double maximum. However, there is a linear relationship between nitrate and pSF₆ age zonally across the GEOTRACES transect, which also appears in the WOCE data sets where nitrate is compared to pCFC-12 ages. The combination of tracer, hydrographic and nutrient data allows estimates of the spatial and temporal variability of biogeochemical processes in the southeastern Pacific.