Quantification of Air-Sea CO2 exchange from the Southern Ocean during last deglaciation

Abstract

Glacial and interglacial cycles are natural, rhythmic patterns of global climate. These cycles are primarily driven by variations in solar forcing, commonly known as Milankovitch cycles. The solar forcing includes eccentricity (shape of the orbit), obliquity (earth's axial tilt) and axial precession (rotation axis orientation) with each having a characteristic frequency. However, atmospheric carbon dioxide (CO2) significantly influences the intensity and duration of these cycles. Over Milankovitch timescales (104 to 105 years), air-sea CO2 exchange predominantly regulates atmospheric CO2 concentrations. The influx of CO2 to the ocean is primarily driven by surface ocean primary productivity, or by the downwelling of CO2 rich deep-water masses in polar regions. Whereas ocean CO2 degassing depends on the release of respired organic carbon from depth through upwelling/overturning circulation. The high-latitude oceans, in modern times (interglacial), function as major sinks for atmospheric CO2. Whereas equatorial oceans act as significant source of CO2 (Eastern Equatorial Pacific and Western Equatorial Indian Ocean). As a result, high-latitude oceans significantly contribute to the regulation of the contemporary atmospheric carbon budget.

Among the high-latitude oceans, the Southern Ocean, because of its strategic location, expansive surface area, and dynamic circulation pattern (migration of polar front) plays a critical role in modulating Earth's overall climate. This is achieved through its impact on the ocean circulation pattern, facilitation of heat exchange, and contribution to carbon sequestration. It is estimated that a significant fraction (40%) of contemporary anthropogenic CO2 emission is absorbed by the Southern Ocean, thereby mitigating greenhouse effects. Despite its predominant absorbing nature, there is evidence of localized upwelling in the Southern Ocean. The substantial (~100 ppm) increase in atmospheric CO2 observed during glacial-interglacial transitions necessitates large-scale CO2 degassing from the global ocean. It is hypothesized that beyond equatorial oceans, the Southern Ocean significantly contributed to atmospheric CO2 rise through degassing during climate transitions. Therefore, a comprehensive understanding of the Southern Ocean's role in glacial-interglacial carbon dynamics, including the rate and timing of degassing coupled with the spatial extent of air-sea exchange, is of critical importance.

This Thesis focuses on reconstructing the direction and magnitude of air-sea CO2 flux in the Southern Ocean during the last termination. Faithful reconstruction of partial pressure of CO2 (pCO2) in the surface ocean requires utilization of a quantitative proxy. Boron isotopic composition of marine biogenic carbonates is an established proxy for seawater pH reconstruction. As seawater pH is primarily modulated by atmospheric pCO2, the combination of boron isotope ratio and trace element proxies (for temperature and carbonate system reconstruction) from foraminiferal calcite can be utilised to constrain paleo pCO2 of surface seawater.

During my doctoral studies, I developed a novel method for accurate and precise determination of boron isotope ratio using ICP-QQQ-MS (0.38‰, 2s). In addition, I also developed an improved method for accurately determining major, minor, and trace element-to-calcium ratios in mass-limited carbonate samples. Subsequently, foraminifera samples from a gravity core obtained from the Indian sector of Southern Ocean (45oS, 72oE) near the Kerguelen Island were analyzed for boron isotope ratio and trace element-to-calcium ratio to reconstruct the CO2 record and other physiochemical parameters. Additionally, radiocarbon dating was performed on planktic foraminifera samples to constrain the age (12.45 - 19.98 Kyr). Two species Globigerina bulloides and Globorotalia inflata were analyzed to reconstruct temperature using Mg/Ca and pH through boron isotope ratio. The selected species of planktonic foraminifera exhibit specific depth preferences, enabling us to delineate the water column and pCO2 structure. For instance, G. bulloides predominantly resides in the upper mixed layer of the ocean, typically near the surface, while G. inflata inhabits the deeper regions of the surface ocean, within the lower mixed layer and the thermocline. The pH values reconstructed from the analysis were then utilized to calculate surface seawater pCO2, enabling the determination of the flux of air-sea CO2 exchange in the Southern Ocean during climatic transitions. My research findings indicate that the Southern Ocean played a significant role as a source of CO2 to the atmosphere during the last deglaciation, providing valuable insights into Southern Ocean's contribution to shaping global climate patterns during the last deglaciation.