Unraveling the Influence of the North Atlantic Ocean on Atmospheric CO2 through Carbonate Chemistry: Insights from ODP Site 984

Abstract

In the modern climate, the North Atlantic - the portion of the Atlantic Ocean which lies north of the Equator - serves as a significant carbon dioxide (CO2) sink; however, its role in past atmospheric CO2 fluctuations remains enigmatic. To address this, the thesis employs a comprehensive examination of a long, continuous, and high-resolution marine sediment core from Ocean Drilling Program site 984 (ODP 984), situated at the heart of the Northern Component Water (NCW) endmember. Various analyses, including faunal and detrital counting, and benthic foraminifera stable isotopes and trace element compositions, are employed to interpret the chemical and physical attributes of past seawaters and porewaters, thereby illuminating the North Atlantic's role in atmospheric CO2 dynamics across different timescales.

The thesis probes the potential significance of the North Atlantic in glacial-interglacial atmospheric CO2 variability. Despite a more efficient glacial carbon pump, the biological production inferred from foraminiferal U/Mn might have been diminished by enhanced glacial stratification at various levels, as indicated by ice-rafted debris, and benthic foraminifera d13C and [CO32-] gradients between intermediate and deep Atlantic. These records imply the North Atlantic has sequestered a large deep carbon reservoir prior to the LGM therefore leaving a nutrient-depleted upper cell that support a more efficient but likely more static carbon pump.

A comparative analysis of the Last Glacial Maximum (LGM) and the Preindustrial ocean, based on the compilation of global ocean carbon cycle parameters, reveals that the LGM global ocean had a much higher alkalinity, facilitating greater carbon storage. An elevated regional alkalinity was also observed in the North Atlantic at a depth of 2-3 km, suggesting a unique water mass possibly acting as a buffer between the shallow and deep Atlantic.

The thesis further scrutinizes millennial scale climatic oscillations during periods of stable external forcing, exemplified by the Marine Isotope Stage 3 (MIS3, 29,000 - 57,000 years ago). The synthesis of carbon cycle parameters and ocean circulation state data indicates that Heinrich stadials were marked by low Dissolved Inorganic Carbon (DIC) and comparatively weak Atlantic Meridional Overturning Circulation (AMOC) in the intermediate depth North Atlantic, which may have contributed to atmospheric CO2 increases during these periods. In summary, this research elucidates the past role of the North Atlantic in atmospheric CO2 variability. Low glacial atmospheric CO2 concentrations are facilitated by high alkalinity and stratification in the ocean, both of which enable larger ocean carbon storage. On a shorter timescale, abrupt CO2 increases during Heinrich stadials are closely linked to marine biology in the North Atlantic. These findings improve the understanding of the ocean-atmospheric carbon cycle, offering invaluable insights for understanding our climate system.