Publication Date

2019-12-13

Availability

Open access

Embargo Period

2019-12-13

Degree Type

Thesis

Degree Name

Master of Science (MS)

Department

Marine and Atmospheric Chemistry (Marine)

Date of Defense

2019-10-29

First Committee Member

Rana A. Fine

Second Committee Member

Dennis A. Hansell

Third Committee Member

Victoria Coles

Abstract

Two decades of observations in the eastern South Pacific show changes in oxygen concentrations and apparent oxygen utilizations (AOUs) in the lower thermocline 26.5-27.0 sigma-theta. The 2013 GEOTRACES O2 and AOU data along ~12°S from the oxygen deficient zone (ODZ; O2 < 50 umol/kg) are compared with repeat hydrography data from the 1990s to the 2010s; the recent data add detail and process information to earlier work. In 2013, transient tracer ratio ages suggest renewal times of over two decades at the core of the ODZ as compared with being at least 7 years younger at the same densities in the subtropical gyre. Related to the age differences in the two regions, we observe oxygen parameters changing in different directions over the two decades, also different processes and forcing dominate. Similar to earlier studies, we found subtropical gyre oxygen concentrations increase and AOUs decrease due to increases in ventilation between 1990s and 2013, coupled to changes in southern hemisphere westerlies. In the core of the ODZ, oxygen concentrations decrease at an annual rate of 0.2 umol/kg/yr, AOUs increase between 1990s and 2013, and the 5 umol/kg contour reaches ~300 m deeper in the water column. ODZ changes are estimated to be due to increased biological consumption, consistent with increased upwelling corresponding to increased trade winds during that period. Thus, the difference in oxygen between the 1990s and 2013 is due to - proximity of the ODZ to coastal upwelling and longer ventilation time scales in the ODZ than gyre. In the gyre, water masses are closer to their sources and ages are younger. Whereas, the ODZ is more isolated from water mass source regions, and it takes more than two decades for the ventilated gyre waters to reach the ODZ. We have shown that in the South Pacific changes in oxygen parameters in the ODZ and subtropical gyre – to date – appear to be due to independent processes and forcing acting in each region. Continuing ocean observations are needed to monitor changes in gases such as oxygen that are critical to life.

Keywords

ocean; oxygen; oxygen deficient zone; oxygen variability; South Pacific

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