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UNraveling PAst Climate as a Key to understanding future CLIMATE

Final Report Summary - UNPACK CLIMATE (UNraveling PAst Climate as a Key to understanding future CLIMATE)

Executive Summary:

Earth has experienced tremendous climate change, from icehouse to greenhouse states and back again, over time spans of millions of years to a few decades. Past climate change has been recorded and preserved in the geological record, and understanding this record is the primary goal of palaeoclimate and palaeoceanography research. The study of past climates is often the only way, to learn about variability, mechanisms, and vulnerability in the climate system, as the time scale of natural fluctuations of many components in the climate system exceeds observational time spans. Lessons learned from the past have implications for present and future climates, and can thereby directly help to combat future climate change.

The research project UNPACK Climate brings forward novel ways to obtain accurate information on past climate, by means of isotope geochemistry. It is only recently that advances in mass spectrometry allow isotope geochemists to measure minute amounts of certain elements with high precision (new TIMS techniques), or produce high resolution records of climate change by improved sample throughput (MC-ICP-MS techniques). Times have never been more favourable to apply novel isotope geochemistry to palaeoclimate research.

With this proposal we tackle two fundamental questions in the integrated climate system:

1. What is the role of the ocean during past rapid climate change events?

The deep ocean stores and transports vast amounts of heat and carbon, and changes in its circulation are likely to influence global climate. Although there are numerous tracers of water mass position in the past, we know very little about the flux water masses. This hurdle can be overcome by a new proxy we developed to decipher past ocean ventilation rates, combined neodymium isotopes and radiocarbon measurements from absolutely dated deep-sea corals.

2. How stable was the East Antarctic ice sheet over the past 15 million years?

The East Antarctic Ice Sheet contains the largest amount of freshwater on earth. Understanding its past dynamics and potential mechanisms of destabilisation seem to be vital in the context of future global warming. For example, the Pliocene warm period (~4.5 – 3.0 Ma) was the last time Earth’s climate was significantly warmer than today for a sustained time period. We apply an innovative technique (provenance analyses of ice-rafted debris and fine continental detritus) to constrain extend and stability of the East Antarctic Ice Sheet during past warm periods.

Over the past four years, we made important steps towards addressing the two objectives set out above. For the first theme (‘Deglacial Atlantic Ocean ventilation rates’) we managed to attract NERC funding, and with it a postdoctoral research scientist. Ms Kirsty Crocket has been working on the project from May 2009 to April 2011 and devoted most of her time to developing a high precision, low blank methodology for low abundance neodymium (Nd) isotope measurements in deep-sea corals by thermal ionisation mass spectrometry (TIMS). This analytically demanding task has been completed, and has been a necessary precondition for the project. In brief, we are now using a revised chemical procedure for Nd separation and a refined mass spectrometry to measure samples down to the nanogram level with high precision (~20 ppm). The methodology has been presented at EGU 2011 in Vienna, has been published in brief in early 2012 (van de Flierdt et al., 2012), and has been applied to seawater measurements by a departmentally paid PhD student (Myriam Lambelet; Pahnke et al., 2012). In the following year, a large set of ~30 deep-sea corals from the North Atlantic Ocean has been analysed, covering the past ~19,000 years of water mass history. All of these corals were previously analysed for radiocarbon and have been subject to precise U-Th dating. Results are still in the process of being interpreted, and have been presented for the first time at the 2012 Goldschmidt conference in Montreal. Two papers, one on the detailed methodology development aspect of the work, and one on the first coral results, are in preparation and will be submitted for publication in 2013.The most exciting finding to date is that our Nd isotope measurements support changes in water mass chemistry during the life time (<100 yrs) of a single coral, and that our fine resolution of water mass structure during the last deglaciation provides unprecedented insights in water mass dynamics over the past 19,000 years. In 2011 we also started the expansion of the project to look at deep-sea corals from the Southern Ocean for the past 25,000 years (research cruise participation: May-June 2011; two papers submitted). A PhD student (Torben Struve) was supported by this grant for a year to start learning the TIMS methodology and to analyse deep-sea corals from the Southern Ocean. We started working on samples, which had already been dated and analysed for radiocarbon (collaboration with Laura Robinson, U Bristol) and first results will be presented at the Goldschmidt 2013 in Florence. The main scientific question we will try to tackle is the role of the Southern Ocean in sequestering carbon in the global ocean and subsequently releasing it during glacial terminations. Future plans, funded by the Grantham Institute for Climate Change and the Leverhulme Trust, include the establishment of some new tracers in deep-sea corals such as Pb isotopes and Cd isotopes and to carefully compare water mass histories in the South and North Atlantic with regards to deciphering causal relationships for rapid changes.

For the second theme (‘Cenozoic history of the East Antarctic Ice Sheet’), funding from the Grantham Institute for Climate Change could be secured to employ a graduate student starting October 2009. Ms Carys Cook PhD topic is the reconstruction of the East Antarctic ice sheet during the Pliocene warmth. She is co-supervised by our collaborators at Lamont-Doherty Earth Observatory, Prof Sidney Hemming and Dr Trevor Williams. Preliminary results of her work, following up from the pioneering study of Williams, van de Flierdt et al. (2012), have been presented at numerous conferences in 2011 and 2012 (Goldschmidt, ISAES, Fall AGU), and indicate a significant collapse of the East Antarctic ice sheet in the Aurora and Wilkes Subglacial basins during the Pliocene. This conclusion has been derived from a multi-proxy provenance approach on marine sediment cores, using argon-argon dating of ice-rafted mineral grains (e.g. hornblende), and analyses of the fine detrital fraction for Nd and strontium (Sr) isotopes. The two key sites we worked on for the past three years are ODP Site 1165 (Prydz Bay, Antarctica), and IODP Site U1361 (off Adélie Land, Antarctica). A paper on the latter project has just been submitted to a high profile journal. Furthermore, graduate student Elizabeth Pierce (Columbia University), has been working on completing the geochemical characterization of the East Antarctic margin, and the Miocene record from the newly recovered IODP cores (Expedition 318, Site U1356). Her work on ‘the weak underbelly of the East Antarctic ice sheet’ has been published in 2012 (Paleoceanography), and the work on the Miocene transition, which hints to significant outburst flooding events from an active ice margin, is currently in preparation for publication. Finally, having participated in January to March 2010 in one of the rare expeditions to drill the continental margin off Antarctica (IODP Expedition 318), much collaboration evolved that benefit from our novel provenance approach to track sediments around Antarctica. Undergraduate student Sang-Eun Kim worked on the new cores with the exiting goal to reconstruct initial ice growth on Antarctica. Her final year MSci thesis produced a nice pilot data set, which we can build on for future work. Graduate student Claire Huck joined the team in April 2010 and utilised similar provenance tools as described above in conjunction with reconstructions of predominant water masses and weathering patterns across the Eocene/Oligocene boundary. First results include a Paleoceanography paper on the age models of the cores (Tauxe et al.) and a Nature paper published on the temperatures off Antarctica in the warm Eocene based on pollen analyses (Pross et al.).

UNPACK Climate has resulted in 62 conference abstracts and eleven peer-reviewed journal articles, four additional non peer-reviewed articles, and many more in preparation. Additional funding could be secured from NERC, the Leverhulme Trust, the Royal Society, and the Grantham Institute for Climate Change.