It is now possible to envision global-scale measurements of accurate, seasonal variations of oxygen concentrations in the upper 2000 meters of the ocean by improving the quality of in situ oxygen measurements. These measurements can presently be made remotely by floats, moorings, and gliders. Analysis of data returned so far from oxygen sensors on Argo floats indicates that accuracy of these measurements is presently not sufficient to solve the pressing problems of air-sea oxygen exchange and slow decadal-scale O2 change in the thermocline. However, recent work with oxygen measurements on moorings and gliders suggest that it is possible to determine oxygen concentrations using available sensors to better than +/- 0.5 % accuracy if calibration protocols are established, which would be sufficient to address these problems.
In this project, researchers at the University of Washington hope to demonstrate this using float-based Aanderaa Optode sensors calibrated before and during float deployment in a location of strategic oceanographic interest -- the Northwest Pacific Kuroshio Extension. This is a region of intense pCO2 draw down in winter caused by a combination of physical and biological processes. The team will evaluate the relative importance of biological processes in creating the carbon sink by determining the net biological oxygen production from O2 data determined on floats and on a surface mooring in the area.
The project thus has two main goals: (1) to test the procedures necessary to improve accuracy of in situ oxygen measurements on profiling floats so that a protocol for future expansion of O2 sensor deployment can be developed; and (2) to determine the role of biological production in controlling the dramatic wintertime draw-down of surface ocean pCO2 in the Northwest Pacific Ocean. The team will deploy 10 Argo floats (constructed at Argo expense) with oxygen sensors in the Kuroshio Extension region. An oxygen sensor and gas tension device will also be deployed on an existing surface mooring in the region to derive a much more detailed data time series to compare with the float measurements. The oxygen sensors will be calibrated both in the laboratory before deployment and in situ in the ocean after deployment by comparing their output to measurements by Winkler titration. The floats will be programmed to make measurements in the atmosphere when they surface to transmit data to shore via satellite as a means of in situ calibration. With this calibration redundancy they will be able to determine the protocol necessary to achieve accurate in situ O2 data on a much broader scale with less extensive calibration.
Broader Impacts. Broader impacts to the marine research community include development of a protocol for calibration of O2 sensors so they can be used more broadly in situ on autonomous platforms. Broader impacts to education involve a "student cruise" on the University of Washington research vessel RV T.G. Thompson. This cruise will expose 10-15 undergraduate students to the experience of field oceanography and marine research, while fulfilling their required capstone research for the undergraduate oceanography degree. The University of Washington has committed twelve days of the ship time to accommodate the educational aspects of the study.