Deep-water hydrographic variability in the Northeast Atlantic during MIS 4

Increased abyssal ocean density stratification across the Middle Pleistocene Transition

Middle to Late Miocene paleoceanographic evolution of the south-eastern Indian Ocean 1 (

Role of oceanic iron biogeochemistry in atmospheric CO2 variability between the last glacial maximum and the holocene

During the last glacial cycle, atmospheric CO2 varied by about 80–100 ppmv between glacial and interglacial periods. A substantial fraction of this change is thought to reflect variations in the strength of the marine biological pump, driven in part by changes in iron (Fe) supply from mineral dust to highnutrient, low-chlorophyll (HNLC) regions. In this PhD thesis, I use the Earth system model of intermediate complexity cGENIE to investigate how aeolian dust deposition and the solubility of dust-borne Fe have influenced the ocean carbon cycle and atmospheric CO2 between the Last Glacial Maximum (LGM) and the Holocene. This thesis is organized around two main modelling studies. In the first part, I prescribe time-varying dust deposition both globally and in key HNLC regions (Southern Ocean sectors, North Pacific and equatorial Pacific) along the last deglaciation, while keeping the remaining regions at Holocene conditions. Over the full glacial–interglacial contrast, these experiments show that changes in dust–borne iron supply can explain a global atmospheric CO2 drawdown of up to ∼30 ppmv between typical LGM and Holocene states. This represents roughly one third of the observed 80–100 ppmv glacial–interglacial pCO2 difference. For each HNLC region, I derive empirical relationships linking iron input, export production and atmospheric CO2. When the deglacial dust changes in all HNLC regions are combined, they lead to a pCO2 increase of about 18–19 ppmv, with the Southern Ocean contributing roughly half of this signal and the North Pacific almost one third. The timing of the response is not uniform: Southern Hemisphere dust changes influence CO2 throughout the termination, whereas the impact of Northern Hemisphere dust is concentrated in the later stages. These experiments confirm the dominant role of the Southern Ocean, but also show that high-latitude Northern Hemisphere dust sources make a non-negligible contribution to the deglacial pCO2 rise. In the second part, I investigate how uncertainties in Fe solubility modify the dust–iron–pCO2 connection. Using the same family of global dust fields for the pre-industrial period and the LGM, I perform a large ensemble of equilibrium simulations in which dust-borne Fe solubility is varied over a wide range, together with an updated ligand scheme for the internal Fe cycle. The results indicate that the pCO2 response to increasing Fe solubility is strongly non-linear: for spatially heterogeneous solubility, the additional drawdown of LGM pCO2 saturates at about 30 ppmv relative to Holocene conditions, again close to one third of the full glacial–interglacial pCO2 difference. The strongest sensitivity occurs in the Southern Ocean band between ∼35◦S and the Subantarctic–Polar fronts, although other HNLC regions also contribute. Across the different dust reconstructions, some robust features emerge, in particular the saturation of the pCO2 response at high solubility and the central role of polar oceans. Taken together, these two studies provide a consistent picture of how dust fluxes and Fe solubility control the efficiency of the soft-tissue pump at the global scale. They demonstrate that dust–iron fertilization can explain a significant but limited share of glacial–interglacial pCO2 changes, and they quantify how model assumptions about Fe solubility and ligand complexation modify this contribution. The thesis also identifies important missing processes such as temporally variable solubility, interactive ligands, sediment and hydrothermal Fe sources, and circulation changes—that should be explored in future work. In this sense, the results presented here offer both a quantitative estimate of the dust–Fe effect and a framework for more comprehensive simulations that combine dust, solubility and ocean circulation in a unified modeling approach

Evolution of deep-water circulation in the North-East Atlantic during the latest Miocene warming

Biogenic Magnetite Indicating Millennial‐Scale Evolution of Bottom Water Oxygenation in the Northern South China Sea Since About 30 ka

Previous studies indicate that bottom‐water oxygenation (BWO) during glacial–interglacial transitions varies complexly, particularly on millennial timescales. Here we reconstruct millennial‐scale BWO variations since ∼30 ka using biogenic magnetite preserved in sediments from the northern South China Sea. Magnetofossil proxies reveal a four‐stage evolution: low oxygenation (∼28–18 ka), a rapid deglacial increase (18–11 ka), an early–mid Holocene maximum (11–5.8 ka), and a gradual decline thereafter. This pattern parallels oxygenation records from the Pacific and Indian Oceans and broadly tracks atmospheric CO 2 , suggesting coupling between deep‐ocean oxygenation and the carbon cycle. Our results suggest that Western Pacific circulation exerted a primary control on long‐term BWO evolution. Low BWO during the Last Glacial Maximum likely reflects an enhanced influence of oxygen‐depleted deep waters through the Luzon Strait, followed by increased ventilation and oxygen‐rich BWO since the deglacial. On millennial timescales, however, biogeochemical oxygen consumption influenced dissolved oxygen variability. This coherent coupling between BWO and atmospheric CO 2 provides insights into future climate change under ongoing global warming.

Seasonal and Interannual Variability of Antarctic Bottom Water Downstream of the Four Key Formation Regions

Antarctic Bottom Water (AABW) is a cold, dense water mass formed around Antarctica that ventilates the global abyssal ocean. Recent studies have reported widespread warming, freshening, and thinning of AABW, yet its temporal variability remains poorly constrained because of sparse observations. This limits our ability to distinguish trends induced by climate change from natural variability. We address this limitation by examining seasonal and interannual AABW variability using passive dye‐like tracers in the high‐resolution ocean–sea ice model ACCESS‐OM2‐01. The tracers are released in four key AABW formation regions: the Ross and Weddell Seas, Prydz Bay and the shelf adjacent to Adélie Land. We find pronounced differences in the magnitude and time scales of AABW variability across formation regions. AABW from the narrow‐shelf regions (Prydz Bay and Adélie Land coast) is characterized by high seasonal variability, reaching about 0.16 tracer units (equivalent to 100% of its local time‐mean tracer concentration) along the continental slope. By contrast, seasonal variability of AABW formed on the wide shelves (Ross and Weddell Seas) remains below 50% of its local time‐mean concentration. Interannual variability is strongest in the Weddell Sea, where it reaches 0.12–0.15 tracer units (40%–50% of its local time‐mean concentration) along the continental slope. Advective timescales show that AABW from narrow‐shelf sources ventilates the abyss on much shorter time scales (1 year) than AABW from wide shelves (2 years). These findings provide new context for interpreting observed deep‐ocean changes, helping to distinguish anthropogenic trends from natural variability and sampling limitations.