GEOTRACES Eastern South Pacific: Characterizing Water Mass Properties and Timescales

Abstract:

In the fall 2013 an eastern South Pacific GEOTRACES transect was occupied along about 12°S from the coast to 150°W. The objective is to characterize the water mass properties as compared with historical data and considering climate modes of variability. Tracer ages (CFCs and SF₆) are used to provide constraints on time scales of physical and biogeochemical processes. The GEOTRACES transect contains eutrophic and oligotrophic stations. In between, there is a large oxygen minimum zone (OMZ) created by a shadow zone in the subtropical gyre circulation. The major water masses in the upper 1500 m are the Antarctic Intermediate Water (AAIW) found below the 27 σ_θ, Subantarctic Mode Water located above the AAIW and below the 26.5 σ_θ, South Pacific Eastern Subtropical Mode Water found along 25.5 σ_θ near 100°W, South Pacific Subtropical Under Water follows 25 σ_θ, and South Pacific Subtropical Water is located at the surface west of 110°W. Water with SF₆ ages of less than 30 years are found above 26.5 σ_θ. The highest apparent oxygen utilization rates (AOUR) found in the coastal region are likely due to the upwelling in this region. The central gyre region shows the lowest AOUR, corresponding with the oligotrophic conditions. In between, the OMZ, ranging from 80⁰W to about 120⁰W, has a median AOUR. An analysis of our data compared to WOCE data at 4 co-locations shows changes in the water properties and biogeochemical processes over a 20 year time period. This analysis suggests an expansion and a strengthening of the OMZ in the past 20 years. While there is a decrease in oxygen content within the OMZ, there is a slight increase in AOU in the western part of the OMZ.