Award: OCE-2148468

This project collected inorganic carbon system and pH data on two US GEOTRACES research cruises in the Pacific Ocean. The first cruise, named GP17-OCE, sailed from Tahiti south to the Southern Ocean then eastward towards the southern tip of Chile. The cruise track was a nominal repeat of prior cruises carried out by the United States over that last 30 years. We compared data from this cruise with these prior occupations and documented a clear increase in dissolved inorganic carbon and lowering of seawater pH. We also found significant decadal scale variability with 2014-2023 having smaller uptake of carbon dioxide than 2005-2014 and 1991-2005 also being a period of lower uptake.

 

The second cruise was in the Amundsen Sea in the Pacific sector of the Southern Ocean. The cruise, named GP17-ANT, provided the first high resolution, high quality, full water column measurements of pH, dissolved inorganic carbon, and total alkalinity of the Amundsen Sea. This data provides extremely important baseline conditions for the region. The analysis of the data also provided important insights into how water flows into the sea and the carbon system is modified due to physical (sea ice melt, glacial melt, and mixing) and biological (photosynthesis and respiration) processes as it circulates through the sea, setting the properties of water that later sinks as bottom water is formed. We found that as relatively warm dense circumpolar deep water flows underneath the ice shelf, it melts the bottom of the ice sheet diluting the carbon system variables and lowering pH. The resulting water that flows out from the ice sheet has a lower partial pressure of carbon dioxide which reduces carbon dioxide outgassing or in some areas turns the water into a sink for atmospheric carbon dioxide. The impacts of this dilution and reduced out gassing warrants further study. 

 

The project also provided opportunities for method testing and development. We tested the use of HDPE bottles for collecting total alkalinity samples. Plastic bottles are lighter and easier to handle than glass bottles which would reduce shipping costs and storage space. However, we found total alkalinity to be unstable in these HDPE bottles, and therefore not suitable for long term storage of samples. We also compared two different methods for measuring total alkalinity and confirmed that they produced identical values within the uncertainty of the measurements. This is important because even a small systematic offset between the methods could complicate data interpretation or lead to incorrect conclusions. 

 

The project provided several opportunities for education with one post-doctoral scholar being trained, and participation in the research by three undergraduate students. 

 

Last Modified: 04/03/2026
Modified by: Ryan J Woosley