Glacial climate transitions in the Southern Hemisphere - New insights from cosmogenic 10Be in rocks and 14C in tree-rings

Full text: The study of global climate change recorded and archived in corals, tree-rings, glacial deposits, ocean sediments and ice cores reveals a complex scale of variations ranging from major glacial cycles (100,000 years), millennial (1,000 years) timescales and even decadal changes (i.e. ENSO). 'Global patterns' of climate cycles are inferred principally from Northern Hemisphere records and modelling approaches. For the major glacial-interglacial cycles this seems to be reliable - however data is now emerging that on the millennial scale of climate change particularly over the Last Glacial-Interglacial Transition (20,000 to 10,000 years ago) and through the Holocene, things are not so simple, let alone 'global'. Critical questions are now being asked as to the synchronicity, intensity and mode of abrupt climate patterns across Earth's hemispheres. However, with a paucity of Southern Hemisphere studies, answers are not readily available. Long-lived cosmogenic radioisotopes, such as 10Be, 14C, 26Al and 36Cl, are produced by cosmic ray bombardment of Earth's atmosphere and lithosphere. The measurement by Accelerator Mass Spectrometry of concentration profiles of these radioactive 'clocks' and 'tracers' in climate archives is emerging as key parameters to provide the essential chronological frameworks and rates of climate processes to address these vexing questions. This talk will present 2 examples of cosmogenic iisotopes in the study of Southern Hemisphere climate change records. The first relates to application of in-situ produced 10Be and 26Al in glacially transported surface rocks to determine the chronology of glacial cycles in Tasmania and New Zealand during the last deglaciation. The Younger Dryas (YD) is a major short term and intense climatic reversal towards colder temperatures occurring between 11,500-12,800 cal year BP superimposed on the last deglaciation. Its presence has now been identified in most, if not all, of the northern hemisphere archives of late Quaternary climate change. Recently, investigations have centered on searching for a cooling reversal coeval with the YD chronozone in Southern Hemisphere archives. The implications of such an appraisal are significant as a positive outcome directly supports the presence of a global triggering mechanism coupled with climate change synchronicity across the equator. The second deals with a detailed comparison in the variations of atmospheric radiocarbon recorded in tropical tree rings from Thailand to that in mid-latitude Huon Pine tree-rings from Tasmania during the Little Ice Age (LIA) from ∼1600-1800 AD. Such a study allows a better understanding of the mechanisms that control regional 14C atmospheric concentration offsets in terms of atmospheric mixing and the role of the Asian monsoon during periods of known Northern Hemisphere climate cooling such as occurred in the LIA. (author)