Pinpointing abrupt climate change in Patagonia using tephrochronology and improved age modelling

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Reconstructing past climate change is fundamental for understanding the natural variability of Earth's climate, in particular the magnitude, timing and rates of past change. In order to successfully achieve these aims it is important to assess past changes quantitatively, from a wide global network, and that all records are accurately dated. This will allow precise comparisons between regions in order to test models of spatial, as well as temporal variability. Datasets from the Northern Hemisphere exist that follow these criteria, although quantitative palaeoclimate data are still relatively scarce. These data and particularly quantitative data are even rarer from the Southern Hemisphere, although we are in the process of collecting such a dataset covering the last ~20,000 years of climate change from Patagonia around 47 deg S. The data include detailed chironomid (non-biting midge) and stable isotope records that will be calibrated against instrumental data, as well as sedimentological, geochemical, pollen and macrofossil data. Together these will comprise a detailed, quantitative reconstruction of past climate change for the region. To some extent, though, every palaeoclimatic record is only as good as its chronological controls, and if these are poor the errors of comparison between other records increase hugely. In order to produce a very high precision chronological framework in this region we aim to develop a tephra-based chronology for sediment-based palaeoclimate reconstructions. Tephra is produced by explosive volcanic eruptions and its virtually instantaneous deposition, in geological terms, leads to the provision of time-parallel marker horizons being left in lakes and other sedimentary deposits. If these sedimentary environments, such as lakes, are cored, dated, and the sequences analysed for the presence of tephra horizons, one can determine the number of regional volcanic events over a period of time. Once the tephra shards have been extracted from the core sequences they can be analysed physically and geochemically in order to characterise tephras from source volcanoes and more specifically from individual eruptions. Our approach is to develop the regional tephrochronology for part of the Southern Volcanic Zone of Patagonia (44 to 47 deg S), as there are opportunities in this region to develop many excellent lacustrine palaeoenvironmental archives in addition to the records we are creating. We will core 3 lakes close to the major stratovolcanoes in the region. Each site is downwind of the volcanoes and should provide an excellent archive of all (even the smallest) explosive eruptions since the end of the last ice age. We will identify all the tephra layers in each core using a novel core scanning techniques which identifies major geochemical trends (ItraX) as well as magnetic profiling, which should also pick out tephra layers. We will then perform detailed geochemical and physical analyses on each tephra layer using a range of techniques, allowing us to source the tephra to a volcano and provide individual geochemical and/or physical characteristics for each tephra. This approach will allow us to build a regional tephrochronology as well as a detailed picture of the frequency and nature of regional volcanic history. The tephrochronology can then be applied to not only our palaeoclimate sequences (in which there are at least seven clear tephra horizons) but also to sedimentary palaeoenvironmental archives that will be sampled in the future from the region. Coupled with other age determinations we will be able to model age controls on the sequences with increasing accuracy as well as being able to correlate between sequences precisely through the use of the tephras.