Southern Ocean CO2 fluxes: new year-round observations from moorings in the West Antarctic Peninsula and Subantarctic Zone

Elizabeth H Shadwick, Virginia Institute of Marine Science, Gloucester Point, United States; CSIRO Oceans and Atmosphere, Hobart, TAS, Australia, Hugh W Ducklow, Columbia University, Lamont Doherty Earth Observatory, New York, New York, United States, Douglas G Martinson, Lamont -Doherty Earth Observatory, Division of Ocean and Climate Physics, Palisades, NY, United States, Adrienne J Sutton, NOAA Pacific Marine Environmental Laboratory, Seattle, WA, United States and Thomas W. Trull, Commonwealth Scientific and Industrial Research Organisation, Marine and Atmospheric Research, Antarctic Climate Ecosystems Cooperative Research Centre, Hobart, TAS, Australia
Our understanding of seasonally ice-covered Antarctic systems is heavily biased by summer observations when primary production dominates CO2-system variability. Changes in the coastal Antarctic are already underway: warming, freshening and onshore intrusion of carbon-rich circumpolar deep water are all processes with associated feedbacks to the CO2-system and contemporary rates of air-sea CO2 exchange. In the subantarctic zone, the uptake of CO2 is driven by a combination of biological and physical processes, which vary over seasonal and shorter timescales. Deep winter mixing drives the formation of subantarctic mode water, which spreads equatorward delivering nutrients to broad areas of the global ocean. We present the first year-round estimates of air-sea CO2 flux from moorings deployed between 2016 and 2018 in the West Antarctic Peninsula (WAP), and between 2011 and 2019 at the Southern Ocean Time Series Station in the subantarctic (SAZ). Air-sea fluxes in the WAP are computed from both pH (using a SeapHOx sensor) and CO2 (using a Pro-Oceanus sensor); fluxes in the SAZ are computed from CO2 (using a Battelle system). Interannual variability at both sites is considerable; biological processes dominate surface CO2-system seasonality and annual air-sea CO2exchange. Winter outgassing is observed beneath the ice in the WAP, and to a lesser extent in the SAZ. Biological drawdown in the WAP precedes sea ice retreat, and is up to three times larger than the signal in the SAZ. These observations are broadly consistent with float-based, zonally averaged, air-sea CO2 flux climatologies, with some differences in seasonality at our specific locations. The WAP moorings extend float-based observations onto the continental shelf where different mechanisms are at play. This work reinforces the need for wintertime data in the Southern Ocean, and highlights the value of moored platforms for sustained observations of air-sea CO2 exchange.