The Antarctic Circumpolar Current is the strongest wind-driven ocean current on the planet with transports estimated to between 95 - 184 x 10^6 m3s-1. [For comparison the Earth's entire river flow is estimated to be ~ 1^6 m3s-1] . Encircling the Antarctic continent, it has a natural 'chokepoint' in the form of the Drake Passage. This project is a renewal of a long term monitoring time series effort measuring the underway surface partial pressure of carbon dioxide (pCO2) in several annual transects across the Drake Passage on the R/V-IB LM Gould. In all some 100 plus transects over the past 8 years have now been accumulated of pCO2, along with discrete samples of other parameters of interest to studying the carbon system such as total CO2 (TCO2) values, isotopic (13C/12C and 14C/12C) ratios in surface TCO2. Additionally, measurements of oxygen, nutrients and such physical hydrography as may be determined from ship launched XBTs and XCTDs along the cruise tracks will be continued. The measurement set provides an opportunity to describe seasonal cycles on CO2 and biogeochemical nutrients in the Drake Passage, to compare temporal trends and inter-annual variability in the carbonate system for an important part of the Southern Ocean, and to further understand regional and hemispheric atmospheric-ocean exchanges associated with the biogeochemical controls on Southern Ocean air-sea CO2 fluxes.
The project will also consider collocated measurement of atmospheric oxygen along with a more rigorous determination of atmospheric CO2 to better anchor the surface pCO2 determinations with the atmospheric record.
The Drake Passage stretches from the southern tip of Chile to the northern tip of the Antarctic Peninsula (Fig. 1) and is the smallest (~780 km) chokepoint for almost 135 Sv (106 m3 s-1) of water that flows clockwise around the Antarctic continent within the Antarctic Circumpolar Current (ACC). The "cruises of opportunity" provided by the more than 18 crossings per year by the ARSV Laurence M. Gould provide an ideal opportunity to monitor changes in the surface water of the Southern Ocean. This study was successful in many ways: 1. Observations: By the end of this grant (fall 2014) the Drake Passage Time-series (DPT) had made more than a million measurements of surface water pCO2 (pCO2surf) during 278 crossings of the Drake Passage starting in March 2002. Over the last 12 years, 1237 discrete measurements of total CO2 (TCO2), the isotopic compostion of TCO2, nutrients and salinity have been made to better understand the processes controlling the pCO2surf and the air-sea flux of CO2. During this grant period a new atmospheric O2/CO2 measurement system has been added to enhance our understanding of large-scale processes controlling surface fluxes of CO2 in the Southern Ocean. At the end of this grant period two full seasonal cycles of atmospheric O2/CO2 had been captured (Fig. 6). 2. Representativeness: An analysis of the Drake Passage suggests that measurements made there do provide a representative picture of a large fraction of the Southern Ocean and are therefore critical for monitoring trends that may represent the broader Southern Ocean. Although the narrowness of the Drake Passage may cause anomalously high vertical mixing, the rapid current speeds of the ACC result in a physical and biological footprint that extends both west into the Pacific sector and east into the Atlantic sector of the Southern Ocean. Satellite pictures of sea surface temperature and ocean color suggest that the mean state and deviations from the mean state are highly correlated with areas that extend as far as 50 degrees west and 20 degrees east of the DPT (Fig. 2). The DPT sampling region also has the advantage that oceanographic front positions are tightly constrained allowing for better attribution of observed trends to specific frontal zones relative to sectors where fronts wander significant distances north and south. 3. Biogeochemical controls of the seasonal cycle in surface pCO2surf: A detailed analysis of the biological impact on the seasonal cycle in pCO2surf suggests that although temperature, winds and upwelling are not insignificant factors in the seasonal cycle (Fig. 3), the dominant control is the uptake of surface CO2 by phytoplankton (Fig. 4), which implies that it is important to continue to explore better ways to monitor the seasonal cycle of primary production in the Southern Ocean. 4. Evaluation of satellite-based estimates of biological productivity: A comparison of satellite-based and in situ estimates of biological carbon uptake, or net community production (NCP), indicates that different methodologies range over a factor of two within the Drake Passage. This study suggests that the in situ measured NCP falls in the middle of the satellite-based estimates in terms of actual magnitude and is slightly more variable from year to year. It is important to note that while satellite imagery suggests a slow progression of carbon uptake by phytoplankton over the course of the growing season, the in situ measurements indicate that most of the phytoplankton-mediated uptake of carbon happens early in the growing season (November and December) (Fig. 5). The timing of the biological CO2 uptake is a critical component in accurate modelling of the seasonal cycle of pCO2surf in the Southern Ocean because a slight phase shift in surface temperature, CO2 supply, and/or CO2 uptake/loss will dramatically change the amplitude of the seasonal cycle. 5. Trends: Global analyses of pCO2surf and hydrographic data indicate that the Southern Ocean has become a dominant region for absorbing atmospheric CO2 (CO2atm). This is confirmed by an analysis of 12 years of pCO2surf observations in the Drake Passage where an extensive number of measurements that span every season over this period provide a unique opportunity to understand more precisely what processes are driving the increase in CO2 uptake. The simplest explanation for this phenomena is that winter upwelling of waters south of the Antarctic Polar Front contain very low anthropogenic CO2 and are thus relatively unaffected by the ever-increasing CO2 mole fraction in the overlying atmosphere. As a result, each winter the air-sea difference in pCO2 increases proving a critical driver for uptake of atmospheric CO2 which is eventually transported north beneath the subtropical gyres. It should be noted that if this process continues the saturation points of aragonite (Ωarag) and calcite (Ωcalc) will not be realized as quickly as current models predict since winter trends in Ωarag and Ωcalc are less than expected given the increase in pCO2atm.
