Ocean circulation, nutrient cycling and atmospheric carbon dioxide

The link between the physical circulation of the ocean, the cycling of nutrients in the ocean, and the storage of carbon in the deep ocean, is fundamental to the understanding of past and future changes in the greenhouse gas atmospheric carbon dioxide (CO2) and the Earth's climate. However, it is currently uncertain how sensitive ocean nutrient cycling and atmospheric CO2 are to changes in global ocean circulation patterns. Nutrients are supplied to the sunlit regions of the surface ocean by ocean circulation, living organisms then use these nutrients to grow, storing carbon within their soft tissues. Some of these organisms then die and fall out of the surface ocean, or are eaten by other organisms, which excrete their remains that also sink out of the surface ocean. As this material sinks it is consumed by bacteria which 'dissolve' it, releasing the carbon and nutrients it contains back into the deep ocean. This transport of biological soft tissue, from the surface ocean to depth, helps to isolate carbon away from the atmosphere, keeping atmospheric carbon dioxide concentrations low. Of particular interest are areas of the surface ocean in which supplied nutrients remain unutilised by living organisms, meaning less carbon is stored in the deep ocean. The largest such area is the Southern Ocean, which forms a ring around the planet north of Antarctica. If global ocean circulation changes such that more nutrients are circulated through the surface waters of the Southern Ocean, then overall there will be less efficient biological utilisation of nutrients, less carbon will be stored in the deep ocean, and atmospheric CO2 will increase. Conversely, if ocean circulation changes such that fewer nutrients are circulated through the Southern Ocean surface waters, then more nutrients will be utilised, more carbon will be stored in the deep ocean and atmospheric CO2 will fall. We propose to quantify the sensitivity of global biological nutrient utilisation, deep ocean carbon storage and atmospheric CO2 to changes in ocean circulation (for example, to the rates of formation of different water masses). We shall perform and analyse multiple runs of a numerical 3D ocean model to investigate and formalise the link between ocean circulation, nutrient cycling and the amount of carbon stored in the deep ocean by falling biological material. These findings will then be used to examine: (i) How ocean circulation changes could have contributed to past changes in atmospheric CO2 during ice age cycles (ii) How ocean circulation changes in the future might amplify or dampen current anthropogenic climate forcing by accelerating or decelerating the increase in atmospheric CO2, (iii) How much carbon could be sequestered by 'geo-engineering' proposals to try and boost productivity in parts of the surface ocean.