The Southern Ocean around Antarctica is a major sink for atmospheric CO2, but the processes controlling present and future air-sea CO2 exchange in the region are poorly known. Atmospheric O2 measurements can provide valuable information on the factors controlling CO2 uptake, as physical and biological processes affect O2 and CO2 in different but interconnected ways.

The major goals of this project were to collect 18 months of high precision and well calibrated measurements of atmospheric O2 (reported as deviations in the O2/N2 ratio) and CO2 from the ARSV L.M. Gould operating in Drake Passage and along the Antarctic Peninsula, and to analyze them to gain knowledge regarding Southern Ocean biogeochemical cycling. These measurements were an extension of an existing 4-year record supported by previous grants. This project was a collaborative proposal between the University of Colorado and NCAR, and this report covers the NCAR component. Our instruments on the Gould ran well and collected continuous data on cruises through the end of November, 2017. The full record totals 50 research cruises in Drake Passage over a period of five and a half years.

We have completed an analysis of our Gould atmospheric O2 and CO2 time series with specific objectives of characterizing the annual means and seasonal cycles of CO2 and O2 at a range of latitudes between Punta Arenas, Chile and Palmer Station, Antarctica, and comparing them to a suite of coupled ocean-atmosphere model predictions and to measurements on flask samples collected at Palmer Station. The latitudinal O2 and CO2 gradients provide unique insights into air-sea CO2 exchange with the Southern Ocean and potential to resolve some growing discrepancies in estimates from other methods.

The summertime latitudinal CO2 gradient decreases to the south, consistent with significant uptake by the ocean around Antarctica, while the wintertime CO2 gradient is nearly flat, consistent with airborne observations from the HIPPO and ATom series of campaigns. The O2:CO2 ratio of the summertime latitudinal gradients suggests greater summertime CO2 uptake than previously estimated, consistent with both ORCAS and station gradients. The latitudinal gradient in the seasonal amplitude for O2 as measured on the Gould peaks between 62 and 64 S in Drake Passage. The seasonal amplitude provides estimates of zonal scale seasonal marine productivity and its latitudinal distribution indicates where air-sea fluxes are greatest.

Additionally, the atmospheric CO2 measurements made as part of this project have been used to evaluate the potential of underway pCO2 measuring vessels to contribute CO2 data to the global atmospheric CO2 network for improving estimates of global carbon fluxes. We have also used the Gould O2 record to provide spatial and temporal context for analysis of measurements from the O2/N2 Ratio and CO2 Airborne Southern Ocean (ORCAS) Study and are preparing a joint publication on ORCAS, Gould, and Palmer data.

There are very few atmospheric O2 observations in the Southern Hemisphere and this project has characterized the full latitudinal gradient from 55-65 S in all seasons, greatly expanding our data coverage and enabling novel constraints on Southern Ocean biogeochemistry.

Last Modified: 12/31/2019
Modified by: Britton Stephens