Abstract
The Southern Ocean is thought to have played a key role in past atmospheric carbon dioxide (CO2,atm) changes. Three main factors are understood to control the Southern Ocean's influence on CO2,atm, via their impact on surface ocean pCO2 and therefore regional ocean–atmosphere CO2 fluxes: 1) the efficiency of air–sea gas exchange, which may be attenuated by seasonal- or annual sea-ice coverage or the development of a shallow pycnocline; 2) the supply of CO2-rich water masses from the sub-surface and the deep ocean, which is associated with turbulent mixing and surface buoyancy- and/or wind forcing; and 3) biological carbon fixation, which depends on nutrient availability and is therefore influenced by dust deposition and/or upwelling. In order to investigate the possible contributions of these processes to millennial-scale CO2,atm variations during the last glacial and deglacial periods, we make use of planktonic foraminifer census counts and stable oxygen- and carbon isotope measurements in the planktonic foraminifera Globigerina bulloides and Neogloboquadrina pachyderma (sinistral) from marine sediment core MD07-3076Q in the sub-Antarctic Atlantic. These data are interpreted on the basis of a comparison of core-top and modern seawater isotope data, which permits an assessment of the seasonal biases and geochemical controls on the stable isotopic compositions of G. bulloides and N. pachyderma (s.). Based on a comparison of our down-core results with similar data from the Southeast Atlantic (Cape Basin) we infer past basin-wide changes in the surface hydrography of the sub-Antarctic Atlantic. We find that millennial-scale rises in CO2,atm over the last 70 ka are consistently linked with evidence for increased spring upwelling, and enhanced summer air–sea exchange in the sub-Antarctic Atlantic. Parallel evidence for increased summer export production would suggest that seasonal changes in upwelling and air–sea exchange exerted a dominant influence on surface pCO2 in the sub-Antarctic Atlantic. These results underline the role of Southern Ocean dynamics, in particular their seasonal variations, in driving millennial-scale variations in CO2,atm.
Highlights
To minimize a potential bias of the planktonic 18O and 13C compilation by size fraction variations (Niebler et al, 1999; Oppo and Fairbanks, 1989), we have omitted data obtained from very small (315 μm) size fractions, where documentation for the data was available
Gas age scales of CO2 records from Antarctic ice cores Deglacial CO2,atm data of the BYRD (Neftel et al, 1988; Staffelbach et al, 1991) and Siple Dome (Ahn et al, 2004) ice cores are reported on the GICC05 gas age scale obtained by Pedro et al, (2012)
The EDC1 age scale of deglacial CO2,atm data obtained from the EDC ice core (Monnin et al, 2001) has been converted to the AICC2012 gas age scale using the depth-gas age relation obtained by Veres et al, (2013)
Summary
To minimize a potential bias of the planktonic 18O and 13C compilation by size fraction variations (Niebler et al, 1999; Oppo and Fairbanks, 1989), we have omitted data obtained from very small (315 μm) size fractions, where documentation for the data was available. Age uncertainties of the N. pachyderma (s.)-based chronology (Fig. S1) may result from the choice of the reference ice core temperature record (e.g. EPICA Dome C (EDC) D versus EPICA DronningMaud-Land (EDML) 18O) and different alignment approaches (tiepoints in maximum versus transitional phases of Antarctic temperature change).
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