The Southern Ocean's role in CO2 change

The cause of glacial-interglacial CO2 change has been described as the âceholy grailâc of palaeoclimate. Several decades of research have narrowed focus onto the Southern Ocean as the most important driver of glacial interglacial CO2 change (Sigman et al., 2010), due its intimate link to the deep ocean (Rae & Broecker, 2018) and its unique biogeochemistry: as a high nutrient low-chlorophyll region, CO2 brought to the surface from deep upwelling cannot be efficiently captured by biological productivity and may outgas to the atmosphere.

Several mechanisms have been proposed to explain how the Southern Ocean may have trapped more CO2 during glacials, including iron fertilisation, ocean stratification, and a âcelidâc of increased sea ice. However, while various tracers of these processes exist it has, until recently, not been possible to quantitatively assess their impact on CO2 storage and release.

The aim of this project is to quantify the role of the Southern Ocean in CO2 change over the last ice age, including the glacial-interglacial CO2 changes that help drive the ice ages themselves, and the CO2 excursions associated with rapid climate change. To do this we will use the boron isotope composition of fossil carbonates (Foster & Rae, 2016, Rae 2018) to reconstruct how Southern Ocean pH and CO2 changed over glacial cycles.

The student will make the first high-resolution pH and CO2 reconstructions from the Antarctic Zone of the Southern Ocean, using novel analytical approaches to overcome small sample sizes. These will be coupled with new records of CO2 storage and release in the deep Southern Ocean over the last glacial cycle.

By comparing these data to multi-proxy records of biological pump efficiency, sea ice, and ocean circulation, alongside experiments with an Earth system model, we will work out the relative importance of these processes in CO2 change on glacial-interglacial to centennial timescales.