Carbon cycling in a warming world: a deglacial test case

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Projections of future climate change require us to understand the interactions between carbon and its main reservoirs, including the large natural exchanges between ocean, land and atmosphere. Studies have also shown that uncertainties in modelling the carbon cycle are as important as uncertainties in modelling the physical climate system. Thus it is vital that we develop a better quantitative understanding. The natural component of the carbon cycle has already been significantly perturbed today, so studying the natural processes before human interventions is an important additional test of our knowledge and understanding. This is particularly true when there were abrupt perturbations on the climate system that occurred on human-relevant timescales. A prime target is the transition out of the glacial period, some 20,000 years ago. The transition saw global temperatures increase by ~ 3-5C, sea level rise by 120m and atmospheric pCO2 increase by ~100ppmV. Importantly, a large portion of these changes occurred in rapid events which overprinted the long term deglacial transition. We still do not fully understand how CO2 levels in the atmosphere increased over this time although processes that alter the balance of carbon between the ocean, land and the atmosphere are certainly critical. Indeed, the leading hypothesis to explain the major rise in atmospheric CO2 calls upon release of carbon from the deep ocean that accumulated from the decay of organic matter as it rains down through to the ocean's depths. At the same time changes in the terrestrial biosphere are thought to have contributed through processes including permafrost melting. This link between biological productivity and the exchange of carbon between its reservoirs is a timely topic given concern over the fate of anthropogenic carbon emissions - including ocean acidification and suggestions of long-term excess storage of carbon in the deep sea. However recent studies have provided data that contradict this view of carbon storage and exchange. Evidence has pointed towards climatically-triggered release of geologically-held carbon. The solid Earth contains vast reservoirs of carbon, so small changes in release have the potential to be important to the carbon budget. If release of geologic carbon is indeed sensitive to changes in the climate system, then this process may also be triggered under future climate scenarios. Constraining the potential impact of geologic carbon released in the past has the potential to provide critical new information in projecting future change. The interdisciplinary project team proposes an ambitious programme combining field work, geochemical analyses and integrated modelling to provide specific tests of the competing hypotheses that have been used to explain the rising CO2 levels at the end of the glacial period. We will particularly focus on using radiocarbon to help diagnose the distinct sources of carbon. New data will be based on geochemical analyses on precisely dated deep-sea coral skeletons collected from locations which are key to these tests. We will mount the first Remotely Operated Vehicle research expedition to the Eastern Equatorial Pacific dedicated to systematic collections of corals from depths where release of geological carbon is hypothesised to have occurred. Our second focus will be on the North Atlantic (samples in hand) where changes in ocean circulation and linkages to isolated basins may have played a key role in releasing carbon from the deep ocean. We will integrate these data with existing knowledge and use them in conjunction with climate modelling (including intermediate complexity models and state of the art General Circulation Models) to explore which processes are most important to the Earth system in a warming world. We will also be well placed to formulate a broad synthesis of the processes controlling the carbon cycle during the most pronounced rapid warming period of recent Earth history.