Elsevier

Earth and Planetary Science Letters

Volume 411, 1 February 2015, Pages 87-99
Earth and Planetary Science Letters

Contribution of seasonal sub-Antarctic surface water variability to millennial-scale changes in atmospheric CO2 over the last deglaciation and Marine Isotope Stage 3

https://doi.org/10.1016/j.epsl.2014.11.051Get rights and content
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Highlights

  • Assessment of seasonality and habitat of two common planktonic foraminifera.

  • Impact of hydrographic changes on net Southern Ocean CO2 fluxes of the last 70 ka.

  • Enhanced Southern Ocean upwelling and export production during rises in CO2,atm.

  • Strong impact of the southern high-latitude seasonal cycle on global carbon cycling.

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.

Keywords

South Atlantic
planktonic foraminifera
stable oxygen and carbon isotopes
atmospheric CO2
last glacial period

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