Identifying Solar Energetic Particle events of differing magnitudes through increases in 14C concentrations from high-resolution sampling of Southern Hemisphere tree rings
Dr Heather Haines1,2,3, Prof Chris Turney1,2,3, Dr Jonathan Palmer1,2,3, Dr Zoë Thomas1,2,3,4, Dr William Hiscock3,4, Mrs Juee Vohra3,4, Dr Kathy Allan2,5, Dr Brendan Buckley6, Asst Prof Ana Carolina Barbosa7, Dr Christopher Marjo3,4
1School of Biological, Earth and Environmental Sciences, The University of New South Wales (UNSW), Sydney, Australia, 2ARC Centre of Excellence for Australian Biodiversity and Heritage, UNSW Node, Sydney, Australia, 3Earth and Sustainability Science Research Centre (ESSRC), The University of New South Wales, Sydney, Australia, 4Chronos 14Carbon-Cycle Facility, Mark Wainwright Analytical Centre, University of New South Wales, Sydney, Australia, 5School of Geography, Planning, and Spatial Sciences, University of Tasmania, Hobart, Australia, 6Lamont-Doherty Earth Observatory, Columbia University, Palisades, USA, 7Department of Forest Sciences, Federal University of Lavras, Lavras, Brazil
Solar Energetic Particle (SEP) events cause increases in 14C concentrations in tree rings which are measurable at high resolution time scales. Known as Miyake events, two large 14C excursions in AD 774-775 and 993-994 have been identified in numerous tree-ring chronologies by laboratories around the world. These events generally are not observed in the standard 5-10 year resolution noted in most long-term tree-ring series. Annual ring analysis is required to pick up these short-term excursions. However, undertaking annual AMS 14C dating can be costly, time consuming, and, depending on the method, requires larger sample volumes than may be available. The Chronos 14Carbon-Cycle Facility at the University of New South Wales employs a Mini CArbon DAting System (MICADAS) which allows for high throughput of samples with only 10mg of wood needed to produce a full sized 1000μm graphite cathode using the Chronos cellulose preparation methods. This small sample requirement provides an ideal opportunity to search for SEP events using high-resolution dating.
In order to confirm our abilities to identify SEP events we have examined Manoao colensoi (New Zealand Silver Pine) samples from Oroko Swamp and observe both Miyake events in this species. Due to the ability for the MICADAS to process small samples sizes and the desire to better understand the seasonality of SEP events, we sampled the 6-year periods, AD 772-777 and 991-996, as sub-annual slices creating between 2 and 4 samples per annual ring. These slices were cut to 5-10mg in size and in the majority of cases 1000μm graphite cathodes could be prepared that were suitable for radiocarbon analysis. For seven cathodes smaller amounts of graphite were produced ranging from 559-848μm of graphite. However, detailed analysis at Chronos has determined that samples of this size produce reliable dates comparable to full sized cathodes. The results from this subsampled study helps to confine these events seasonally in the Southern Hemisphere.
Based on positive results identifying these large SEP events in Southern Hemisphere samples, we attempted to identify a smaller SEP event, the Carrington 1859 event, which had only been identified in Northern Hemisphere tree-ring samples. Here we again sampled the Oroko Swamp M. colensoi rings as well as Athrotaxis seleganoides rings from Tasmania, Australia, Callitris intratropica rings from the Northern Territory, Australia, Pinus merkusii rings from Thailand, Pseudotsuga menziesii rings from Vietnam, and Cedrela odorata rings from Brazil. A core from each site was sampled from 1857-1861 again using sub-annual ring sampling to represent possible seasonal varaibility. The Carrington SEP event occurred on September 1-2, 1859 which, with the exception of C. odorata, falls at the very start of the growing season for each of these tree species. Our results represent the ability of locating this small SEP event from across a range of locations and species in the Southern Hemisphere.
Dr Heather Haines is a dendrochronologist with a specialization in tropical regions, non-traditional species and methods, and a focus on radiocarbon dating. Her interests lie in developing high-resolution paleoclimate reconstructions, understanding hydroclimate variability, protecting native vulnerable species, and helping to extend the Australian tree-ring network. She has a preference for working with tree species that present a challenge for dendrochronological analysis due to anomalous ring boundaries, of which there are many within Australia.