Ocean carbon storage over the last glacial cycle : constraints and mechanisms

The deep ocean’s ability to store and exchange large quantities of carbon with the atmosphere on millennial to orbital timescales means it plays a central role in glacial–interglacial climate change. Determining the causes of these past changes are central for testing our understanding of Earth’s carbon cycle and climate system. However, the mechanisms involved and their relative contributions at different stages over the last glacial cycle remain debated. In this thesis I use box and Earth system modelling, (summarised in Chapter 2), to test established and novel mechanisms for ocean carbon drawdown and storage and to predict how these should be recorded in proxy records, in particular in δ¹³C and CO₃²⁻. We compare these results to newly generated and compiled planktic (Chapter 3) and benthic (Chapter 4) foraminiferal trace element and isotope reconstructions, to investigate the processes involved in promoting carbon storage over the last glacial cycle. We focus on the Southern Ocean, as the nexus of ocean-atmosphere CO₂ exchange, and the Indo-Pacific, which contains the ocean’s ultimate carbon store. Chapter 3 demonstrates the utility of planktic δ¹³C records for understanding the processes responsible for altering ocean-atmosphere carbon exchange. This highlights a central role of sea-ice and the disequilibrium carbon pump. Chapter 4 reconstructs clear carbon storage at mid-depths, while intermediate and abyssal depths feel competing controls from carbon drawdown in the upper ocean, storage in the interior, and carbonate compensation. Physical processes played the dominant role in the early stages, with biology only becoming important as glacial conditions intensified. Finally, Chapter 5 highlights the importance of understanding the different controls on the biological pump – including a novel examination of CO₂ limitation on biological productivity – in controlling regional patterns of nutrient limitation and export, and their role in determining the ability of the biological pump to sequester atmospheric CO₂.