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Tracing the oceanic mechanisms for the deglacial increase of atmospheric CO2: the role of Southern Ocean ventilation

Final Report Summary - DEGLACIALCO2 (Tracing the oceanic mechanisms for the deglacial increase of atmospheric CO2: the role of Southern Ocean ventilation)

It is now well documented that atmospheric carbon dioxide concentration (pCO2) changed in parallel to the growth and decay of polar ice sheets over the last 800,000 years. However, the mechanisms that cause these variations in pCO2, and exactly how they are related to Earths’ climate remains much debated. This is a crucial issue given the current anthropogenic increase in pCO2, and it was recently identified by the IPCC as a key uncertainty in our understanding of the climate system as a whole. Although the exact processes are contested based on the magnitude and rapidity of the rates of carbon exchange, it is generally accepted that changes in oceanic carbon storage play a vital role in causing glacial-interglacial CO2 change

The overall aim of the project “Tracing the oceanic mechanisms for the deglacial increase of atmospheric CO2: the role of Southern Ocean ventilation” has been to better understand the role of intermediate waters ventilated in the Southern Ocean in the rise of CO2 during the last deglaciation. To achieve this aim, we have used the boron isotope-pH proxy in planktic foraminifera from a suite of key locations to reconstruct oceanic pH changes, with the objective of documenting where and when CO2 was leaked to the atmosphere from the deep Southern Ocean during the transition between the Last Glacial Maximum and the Holocene.

In order to generate the high quality d11B records required to achieve our aims, a substantial effort has been directed to the improvement of d11B analysis. New available 10^12 Ω amplifiers and SC-FAST introduction system were installed and extensively tested in the MC-ICPMS during the project, in order to improve the accuracy and reproducibility of d11B analysis. In addition, ESI’s prepFAST fully-automated system has been adapted to the needs of the boron isotope chemical separation procedure, and allowed the automated chromatographic pre-concentration of boron prior to analysis, increasing sample throughput.

Records of d11B-pH, trace elements and stable isotopes (d13C and d18O) in planktic foraminifera have been generated from a suite of sediment cores from the Southern Ocean and the Equatorial/Tropical Pacific during the last deglaciation. By combining d11B-pH data with temperature records and assumptions regarding total alkalinity, we have reconstructed the upper ocean carbonate system in the past and used this information in a novel way. By applying this method to oceanic areas in disequilibrium with the atmosphere, we can directly document the changing magnitude of ocean CO2 outgassing through time. We are thus able to confirm that that surface waters in the Subantarctic Atlantic and the Eastern Equatorial Pacific became a significant source of CO2 to the atmosphere during the last deglaciation. This provides the first direct evidence for the role of the ocean as a CO2 source and supports the view that a deep ocean carbon reservoir was upwelled via the Southern Ocean during the last deglaciation. We are therefore able to highlight the vital role of the Southern Ocean and the low latitude upwelling regions in glacial-interglacial atmospheric CO2 variations. The data generated during the project (together with other data provided by project partners) have been used to evaluate the GENIE model output and to further quantify the role of the ocean in driving deglacial CO2 change.

Atmospheric CO2 fluctuations over glacial-interglacial cycles remain a major challenge to our understanding of the carbon cycle and the climate system. We have addressed this major challenge in our project and the resulting scientific articles will provide insights into the natural mechanisms that control Earth’s atmospheric CO2 concentrations, identifying key areas, processes and feedbacks. Besides improving our knowledge of the climatic history of the Earth, the outcomes of this project have scientific, social and economic implications, given the current increase in atmospheric CO2 levels due to anthropogenic emissions. A better understanding of the carbon cycle will allow a better representation of Earth’s climate in models, which will ultimately lead to more accurate projections of future climate under different emission scenarios. Ultimately, these projections influence the mitigation and adaptation measures to be taken by governments and the related international climate negotiations.