DEEP-SEA CORALS · Deep-Sea Coral Geochemistry and Climate: a Focus on the History of the Southern Ocean

Deep-Sea Coral Geochemistry and Climate: a Focus on the History of the Southern Ocean

The Earth has experienced large, abrupt and complex climate shifts through geologic time. For example, ice core records show that glacials had lower atmospheric pCO2 and cooler temperatures than today, and that the last deglaciation was punctuated by large, abrupt millennial-scale climate events. The ocean is a key component of this climate system, from storage and transport of heat across the globe and its key role in the carbon cycle. This project was directed towards combining existing and novel geochemical approaches to optimize the use of deep-sea coral skeletons as archives of past ocean conditions. Cold-water corals form an exciting new paleoclimate archive. Unlike reef-forming corals found in shallow tropical areas, cold-water corals do not have algal symbionts so they are not restricted to the photic zone or to warm waters. As a result, they are found in all of the ocean basins, including the Southern Ocean, and at depths from a few metres to greater than 5,000m. The robust calcium carbonate skeletons of many corals are well preserved and can be dated using the decay of uranium. The chemistry of the growth layers within the coral skeletons reflects external environmental conditions and can preserve a record of rapid climate events in the ocean similar in temporal resolution to the atmospheric records preserved in ice cores. Like other organisms, however, corals manipulate seawater chemistry as they precipitate (a phenomenon known as “vital effects”) causing substantial challenges in accurately interpreting geochemical proxy records. Few studies have taken advantage of cold-water corals as paleoclimate archives, in part due to challenges in collecting relevant samples, and in part due to these complicating “vital effects”. The benefits, however, make addressing these challenges well worth the effort. The project had five major goals: • To develop physical and chemical models for mechanisms controlling deep-sea coral geochemistry allowing a better understanding of vital effects • To develop new geochemical proxies for climate change and environment using deep-sea corals • To extend instrumental records of climate change in the Southern Ocean • To gain a unique, high-resolution perspective on the role of the Southern Ocean during the deglaciation • To determine temporal links between deep-sea coral population growth and climate change Overall the project has been successful with the outcomes closely aligned with the project goals. The science also included a more synoptic view of the Atlantic Ocean rather than the tight focus on the Southern Ocean. However, the linkages are scientifically well meshed through the formation of deep waters in the Southern Ocean that govern Atlantic Ocean dynamics, and through use of a wider sample set to assist in development of geochemical proxies. A review of the state of the art in deep-sea coral geochemistry was published in 2014 (Robinson et al 2014) documenting progress made across the deep-sea coral community. During the four years of this reintegration period, the Bristol-based research team used a raft of methods designed to take advantage of these deep-sea coral archives - including field programs and laboratory studies. Research papers have been published that review the current knowledge of deep-sea coral research, establish new geochemical proxies and dating techniques for using deep-sea corals as climate archives and that explore the past history of coral population dynamics in the Southern Ocean. Indeed the research carried out during the reintegration period has been published and recognised in the most influential journals in the scientific community – with two articles on Southern ocean climate from deep-sea corals published in the journal ‘Science’. Burke and Robinson (2012) showed the depth structure of the Southern Ocean during the last major deglaciation at a millennial scale (20,000 to 10,000 years ago). The most recent publication (Chen et al 2015) compared oceanic radiocarbon in the Southern Ocean to the Equatorial Atlantic and was able to show, for the first, time, that the sub-surface ocean carbon cycle changes at the same time as atmospheric carbon dioxide levels even on centennial timescales. This result would be difficult to establish using traditional sediment core records given the challenged of producing accurate and precise high resolution age models. The research team and collaborators have also been able to develop and refine a number of geochemical techniques that have markedly increased the possibilities for working with deep-sea coral archives – research that is likely to influence other researchers seeking to understand coral biomineralisation and past climate. These methods include using nitrogen isotopes to look at nutrient cycling (Wang et al 2014), clumped isotopes to examine temperature (Spooner et al in revision) and new methods for rapid dating of corals (Longworth et al 2013, Spooner et al 2015). At the conclusion of the project, the integration of the Fellow is complete, with career development shown through promotion from Lecturer to Reader in 2012, and the establishment of a coherent and productive research group at the University of Bristol. Further details of the research can be found at the following websites, and by watching the TEDx talk given by the Fellow in Brussels in 2014: Ocean Science at Bristol: https://bristoloceans.wordpress.com/ Robinson personal webpage: http://www.bristol.ac.uk/earthsciences/people/laura-f-robinson/index.html TEDx talk: https://www.youtube.com/watch?v=2R1lZ-N4-So

Data: CORDIS, © European Union