Browsing by Author "Kosarac, N"
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- ItemReconstructing a fire and climate history using a stalagmite from Crystal Cave, south-west Western Australia(Australasian Quaternary Association (AQUA), 2021-07-08) Kosarac, N; Treble, PC; McDonough, LK; Paterson, DJ; Adler, L; Baker, AABushfires are a major natural hazard, with a growing impact on communities and ecosystems in Australia and worldwide. However, our understanding of long-term past fire intervals and behaviour is limited by low temporal resolution. Cave stalagmites offer a novel opportunity to reconstruct long, high-resolution datasets of past fire and climate with a precise chronology. CRY-S1 is a cave stalagmite from Crystal Cave, Margaret River, south-west Western Australia. Monitoring has shown that speleothems from shallow caves are more likely to record fire events as pulses of ash-derived elements and evaporative enrichment of δ18O. However, Crystal Cave is located approximately 30 m below the surface, indicating that past fire events can also be recorded in a deep cave system. This may be on account of the cave’s fracture flow contribution enabling a more direct connection with the surface, or the catchment morphology. This Honours project aims to compile and analyse fire and climate data over 300 years using new and existing datasets for stalagmite CRY-S1. We assess 18O and 13C isotope data, as well as a suite of trace elements. Past studies have determined that fire events can be identified through ash-derived elements in stalagmite laminae. Elements of interest include phosphorous, strontium and zinc. Results from 14C analyses will be used to test a hypothesis that large fires will eliminate young carbon in soils, altering the stalagmite 14C/12C ratio. An age depth model was obtained using annual lamina counts from Sr synchrotron mapping. Studying trace elements and isotope signals in this context increases our understanding of how past fire and climatic changes can be interpreted through stalagmite geochemistry. This Honours project will contribute to a world first project which uses stalagmites to construct annually-resolved records of fire frequency and intensity.
- ItemA review of speleothems as archives for paleofire proxies, with Australian case studies(American Geophysical Union, 2023-03-22) Campbell, M; McDonough, LK; Treble, PC; Baker, AA; Kosarac, N; Coleborn, K; Wynn, PM; Schmitt, AKWildfires affect 40% of the earth's terrestrial biome, but much of our knowledge of wildfire activity is limited to the satellite era. Improved understanding of past fires is necessary to better forecast how fire regimes might change with future climate change, to understand ecosystem resilience to fire, and to improve data-model comparisons. Environmental proxy archives can extend our knowledge of past fire activity. Speleothems, naturally occurring cave formations, are widely used in paleoenvironmental research as they are absolutely dateable, occur on every ice-free continent, and include multiple proxies. Recently, speleothems have been shown to record past fire events (Argiriadis et al., 2019, https://doi.org/10.1021/acs.analchem.9b00767; McDonough et al., 2022, https://doi.org/10.1016/j.gca.2022.03.020; Homann et al., 2022, https://doi.org/10.1038/s41467-022-34950-x). Here we present a review of this emerging application in speleothem paleoenvironmental science. We give a concise overview of fire regimes and common paleofire proxies, describe past attempts to use stalagmites to investigate paleofire, and describe the physical basis through which speleothems can record past fires. We then describe the ideal speleothem sample for paleofire research and offer a summary of applicable laboratory and statistical methods. Finally, we present four case studies from southwest Australia which: (a) explore the geochemistry of ash leachates, (b) detail how sulfate isotopes may be a proxy for post fire ecological recovery, (c) demonstrate how a catastrophic paleofire was linked to changes in climate and land management, and (d) investigate whether deep caves can record past fire events. We conclude the paper by outlining future research directions for paleofire applications. © 2023 The Authors. Open Access CC-By-NC Licence
- ItemSpeleothem 14C is unlikely to be impacted by wildfire – case studies from Western Australia and Tasmania(Australian Nuclear Science and Technology Organisation, 2021-11-17) Campbell, M; McDonough, LK; Kosarac, N; Treble, PC; Markowska, M; Baker, AA; Hua, QAccelerator Mass Spectrometry (AMS) can be used to measure ¹⁴ C in speleothems (cave formations). AMS derived ¹⁴ C measurements offer an alternative geochronological method to U-series dating for recently formed speleothems with low U concentrations and insufficient ²³⁰ Th ingrowth, and where speleothems are impacted by allogenic thorium. However, using ¹⁴ C for dating speleothems is complicated by variable ¹⁴ C sources and dilution by ¹⁴ C-depleted parent rock (dead carbon). Old carbon may be mobilised from soil and decaying organic matter, which can mix with new carbon from root respiration in the vadose zone. These processes and their incorporation in speleothems are non-stationary in time and can be difficult to correct for. While variable dead carbon is a complication for dating speleothems, hypothetically, a sudden decline in ¹⁴ C, indicating an influx of old carbon, could be expected to occur after a bushfire. This could be because old carbon previously sequestered in soil and biomass may be mobilised, or there could be a decline in the previous oversupply of biological CO₂ while root respiration is in recovery following tree death. Using speleothems as proxy archives for past fire is an emerging field, and recent research has shown multiple proxies are needed to accurately characterise a fire event. To test the hypothesis that ¹⁴ C may serve as a proxy for palaeofire, we chose four sites which have experienced bushfires; Crystal, Golgotha, and Yonderup Caves in Western Australia, and Frankcombe Cave in central Tasmania. We used the AMS facilities at the Australian Nuclear Science and Technology Organisation to quantify the ¹⁴ C content along growth axes for the speleothems from Western Australia. Previously published AMS ¹⁴ C data for the Tasmanian speleothem were measured at the Australian National University. Crystal and Yonderup Cave speleothems had additional age-constraints of annual laminations. The Crystal Cave speleothem shows no decline in ¹⁴ C after an historical fire event. The Yonderup Cave speleothem does show a decline in ¹⁴ C after a reconstructed fire event, but the decrease is within analytical uncertainty. The Golgotha speleothem ¹⁴ C shows no change after a documented fire. The Tasmanian speleothem does show a significant decline in ¹⁴ C during a period where the region experienced successive large bushfires, as recorded in the historical record, but the temporal uncertainty of the measurements is so high (± 15 years), that the decline cannot reliably be attributed to the bushfires. These case studies provide only limited evidence to support our hypothesis that ¹⁴ C in speleothems may be a proxy for past fire events. This may be because the response is too short to be observed, sampling resolution is insufficient, or the carbon flux associated with a bushfire is too small relative to the total CO₂ flux in the vadose zone. While this is discouraging for palaeofire researchers, results offer relief to those who use ¹⁴ C dates to create speleothem chronologies, as it is one less source of uncertainty to consider when correcting ¹⁴ C ages. Our results may also serve to elucidate the importance of CO₂ source on ¹⁴ C, and by extension, on δ¹³C. © The Authors
- ItemSpeleothems as palaeofire archives – a synthesis and meta-analysis of data and methods(American Geophysical Union (AGU), 2021-12-18) Campbell, M; Treble, PC; McDonough, LK; Baker, AA; Kosarac, NSpeleothems, mineral deposits found in caves, have recently been developed as a suitable archive to reconstruct palaeofires. Speleothems are excellent candidates for palaeoenvironmental research as they can be absolutely dated using radiometric methods and annual lamina, they incorporate a large range of chemical and physical proxies, and many are resolvable at the seasonal to annual scale. Large databases such as the Speleothem Isotope Synthesis and Analysis V2 may be used to identify and access suitable samples (Figure 1). Here, we present a meta-analysis and synthesis of published research and unpublished data to provide a toolbox for the use of speleothems as proxies of past fire. Drawing on our data from monitored caves and modern speleothems that overlap with recorded fire history, we will outline the approaches that we have adopted and identify which proxies have been validated as fire sensitive in the speleothem record. Fire sensitive proxies include ash-derived elements, in particular P and some transition metals, and proxies which reflect changes in soil productivity, hydrology, and evapotranspiration (e.g. δ18O, growth rate, organic matter, fabric). However, the success of these proxies as fire sensitive signals can differ between events recorded in speleothems. We discuss the complexities and demonstrate that an understanding of local surface-cave hydrological connectivity is required for the correct interpretation of the palaeofire signal. Improved understanding of palaeofire is necessary to characterize pre-industrial fire regimes, especially in colonised countries, where the landscape management systems of Indigenous people was disrupted. This will be of interest to the palaeoclimate community, land managers and traditional owners. Plain-language Summary Stalagmites (natural formations found in caves) have been used to reconstruct past fire events. These formations are useful for reconstructing past environments because we can find out when they grew, how quickly they grew, and so any change in chemicals included in them can be used to describe the environment that they grew in. Here, we analysed published reconstructions of past fires and unpublished data, and demonstrate which methods are best for analysing stalagmites, and which chemicals (and combinations of chemicals) best indicate that a fire occurred. The chemicals included in stalagmites include some from the ash left after a fire (especially phosphorous), and some which can change due to changes in the soil and limestone over the cave (such as the ‘type’ of oxygen included in infiltrating rainwater). How these chemicals react to a fire changes between caves, and between fire events. We need to understand past fire patterns so that we can better manage the landscape. This is especially true in colonised countries where Indigenous land management was stopped by colonialism. This research will interest land managers, traditional owners, and other scientists who research past environments.