Browsing by Author "Borsato, A"
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- ItemEstablishing robust chronologies for two young stalagmites from the tropical south Pacific using radiocarbon(Australian Nuclear Science and Technology Organisation, 2021-11-17) Faraji, M; Hua, Q; Borsato, A; Frisia, SIn the last couple of decades, speleothems have provided terrestrial paleoclimate reconstructions covering almost all latitudes and a with temporal resolutions varying from seasonal to millennial scales. The most important property of speleothems is their capability to be dated via radiometric methods. The most commonly used dating method is based on U-series disequilibrium. When this method is not applicable, for example, because of multiple sources of 230Th, robust chronologies can be acquired via integrating the counting of annual physical and chemical laminae in two dimensional maps. This approach may also entail uncertainties related to the exact age of the youngest parts of the speleothem, commonly assumed to be actively growing at the time of removal. An alternative/complementary method for obtaining accurate age models and/or further constrain the age of speleothems that grew in the last circa 100 years is using the radiocarbon “bomb-pulse”. However, when comparing the radiocarbon age of speleothems with other contemporaneous terrestrial samples which absorb carbon from atmospheric CO2, speleothems show older than expected ages. That is due to the contribution of ‘radioactively dead’ carbon, known as Dead Carbon Fraction (DCF), derived from 14C-depleted material from bedrock and aged soil organic matter. Yet, a thorough understanding of DCF variations in speleothems potentially generates reliable chronologies. Relatively young Tropical Pacific stalagmites are commonly difficult to date with the U-series, and radiocarbon is, potentially, the best dating method. We studied stalagmites Pu17 and Nu16, retrieved from Pouatea and Nurau caves in Atiu, the second largest island from the southern group of the Cook Islands in the South Pacific. Both were studied with the aim to construct robust radiocarbon chronologies and attain information about the hydrogeology of the caves. These caves are expected to provide an excellent opportunity for radiocarbon dating because the rock burden above caves is thin (4 to 8 m), characterized by high porosity, and a limited and patchy soil cover is. This ensures rapid transmission of surface climate parameters into the cave. Rapid transmission of rain signal into the cave reduces the interaction between rainwater and bedrock, thus, minimizing the contribution of bedrock-derived dead carbon, which is crucial for constructing accurate and robust chronologies. Forty samples from the top portions of Pu17 and Nu16 were analysed for 14C using the VEGA AMS Facility at ANSTO. By modelling the soil carbon pools, we evaluated the age distribution of soil carbon above the cave. Results indicate a young epikarst carbon pool (younger than 22 years) for both Pu17 and Nu16, which translates into less than 4% DCF contribution. Radiocarbon chronologies of the two stalagmites show a sharp and relatively early bomb peak at ca. 1970 CE. The radiocarbon bomb-pulse allows to anchor chemical lamina counting and obtain accurate chronologies for modern speleothem and thus obtain robust paleorainfall records for the climate vulnerable south Pacific Island communities. © The Authors
- ItemExploring the dating of “dirty” speleothems and cave sinters using radiocarbon dating of preserved organic matter(Elsevier, 2017-04) Blyth, AJ; Hua, Q; Smith, AM; Frisia, S; Borsato, A; Hellstrom, JCSpeleothems and other carbonate deposits such as tufa containing high proportions of detrital material can be difficult to chemically date due to detrital thorium levels causing a high level of error in conventional U-Th disequilibrium dating. Here we investigate the use of an alternative technique centring on radiocarbon dating of organic matter preserved within the detrital fraction. Non-acid soluble humic, particulate and detritally absorbed organic matter was recovered from eight samples from a flowstone sinter formed within a roman aqueduct at Trento in Italy with a maximum age of 100 CE (1850 cal yr BP), and two repeat samples from a dripstone formed within the 20th Century on a wire fence at Lilly-Pilly Cave, Buchan Caves Reserve in Victoria, Australia. In the aqueduct samples the median calibrated 14C ages ranged from 2232 to 2889 cal yr BP, with 95.4% probability age range in the youngest and oldest samples of 2153–2337 and 2342–3449 cal yr BP respectively. The median age of the more modern dripstone was 336 cal yr BP, with a 95.4% probability age range of 148–486 cal yr BP. These results provide very approximate ball-park estimates of the age of the sample, but are consistently too old when compared to the known maximum ages of formation. It is hypothesised that this offset is due to a combination of the nature of the organic carbon transported from the source organic matter pools, and reworking of both modern and old organic carbon by in situ microbial communities. ©2017 Elsevier B.V
- ItemFast high-resolution synchrotron micro-XRF mapping of annually laminated stalagmites(Copernicus GmbH, 2019-04-11) Borsato, A; Frisia, S; Hellstrom, JC; Treble, PC; Johnson, K; Howard, DL; Greig, AAnnual lamination in trace elements content allows to improve speleothem chronology as well as to extract paleoclimate information about fluctuations of the seasonal signal through time. Given the relatively slow growth and textural heterogeneity intrinsic in most speleothems, only high-resolution mapping techniques provide a viable approach to resolve trace elements variability at annual to sub-annual scale. Synchrotron radiation X-ray fluorescence microscopy (SR-μXRF) is, to date, the ideal technique as it provides quantitative, non-destructive fast scanning of large samples at the necessary high spatial resolution (0.5 – 5 μm). Here we present SR-μXRF investigation of U/Th dated stalagmites from caves in different geographic and climate setting including semi-arid (Flinders Ranges, South Australia), temperate (Central Italy) and tropical humid (Cook Islands, Northern Laos) climates. SR-μXRF analyses were coupled with high-resolution petrographic observation and LA-ICP-MS elemental analyses in order to verify the XRF elemental quantification and to test the relationship between fabric and trace element incorporation. All the analysed stalagmites revealed faint to sharp annual lamination in Sr, and, occasionally, in other elements such as Br, Cu and Zn. In climate setting characterised by strong seasonal contrast stalagmites often exhibit fabric changes at annual to centennial scale that are reflected in the internal porosity as well as in the incorporation of trace elements. Fabric control and spatial heterogeneity is also evident in the intensity of the annual cycles and, in some cases, the amplitude of Sr cycles vary considerably from one line scan to the adjacent ones. This poses the question of the significance and reproducibility of trace element analyses in speleothems characterised by fabric heterogeneity. © Author(s) 2019. CC Attribution 4.0 license.
- ItemPast fires and post-fire impacts reconstructed from a southwest Australian stalagmite(Australasian Quaternary Association (AQUA), 2021-07-09) McDonough, LK; Treble, PC; Baker, AA; Borsato, A; Frisia, S; Nagra, G; Coleborn, K; Gagan, MK; Fakra, SC; Paterson, DJStalagmites provide records of past changes in climate, vegetation, and surface events, with cave dripwaters shown to respond to fires. It is, therefore, most likely that these cave mineral deposits capture the environmental effects of palaeo-wildfires in their chemical and physical properties, as well as the climate conditions antecedent to palaeo-fire events. We analysed multiple proxies in stalagmite (YD-S2) from a shallow cave in south-west Western Australia. Principal Component Analysis revealed that short term peaks in combinations of phosphorus, copper, aluminium, lead and zinc in the stalagmite correspond to the timing of documented fire events occurring in the modern portion of the record. One particularly significant fire event is identified at 1897 ± 5 CE and shows a clear peak in P interpreted to be derived from ash, and a peak in 𝛿18O interpreted to indicate evaporation of sub-surface water during the heat of the fire. A post-fire threshold rise in organic matter content and a shift in calcite fabric associated with higher and more variable drip rates are consistent with a post-fire changes in surface-cave hydrology resulting from heat-induced deformation of the shallow karst bedrock brought about by the intensity of this fire. The combination of climate and fire sensitive proxies in YD-S2 indicates that the 1897 ± 5 CE wildfire was preceded by a multi-decadal dry period. We also identify lower and less variable peak phosphorus concentrations in the pre-European period that are consistent with low-intensity cultural burning by Indigenous Australians. The YD-S2 record shows the potential of stalagmites in capturing the climate-fire relationship and the effects of land-management practices on wildfire frequency and intensity.
- ItemPast fires and post-fire impacts reconstructed from a southwest Australian stalagmite(American Geophysical Union (AGU), 2021-12-17) McDonough, LK; Treble, PC; Baker, AA; Borsato, A; Frisia, S; Campbell, M; Nagra, G; Coleborn, K; Gagan, MK; Paterson, DJStalagmites provide records of past changes in climate, vegetation, and surface events, with cave dripwaters shown to respond to fires. It is, therefore, most likely that these cave mineral deposits capture the environmental effects of palaeo-wildfires in their chemical and physical properties, as well as the climate conditions antecedent to palaeo-fire events. We analysed multiple proxies in stalagmite (YD-S2) from a shallow cave in south-west Western Australia. Principal Component Analysis revealed that short term peaks in combinations of phosphorus, copper, aluminium, lead and zinc in the stalagmite correspond to the timing of documented fire events occurring in the modern portion of the record. One particularly significant fire event is identified at 1897 ± 5 CE and shows a clear peak in P interpreted to be derived from ash, and a peak in 𝛿18O interpreted to indicate evaporation of sub-surface water during the heat of the fire. A post-fire threshold rise in organic matter content and a shift in calcite fabric associated with higher and more variable drip rates are consistent with a post-fire changes in surface-cave hydrology resulting from heat-induced deformation of the shallow karst bedrock brought about by the intensity of this fire. The combination of climate and fire sensitive proxies in YD-S2 indicates that the 1897 ± 5 CE wildfire was preceded by a multi-decadal dry period. We also identify lower and less variable peak phosphorus concentrations in the pre-European period that are consistent with low-intensity cultural burning by Indigenous Australians. The YD-S2 record shows the potential of stalagmites in capturing the climate-fire relationship and the effects of land-management practices on wildfire frequency and intensity. Plain-language Summary Fires have the potential to be recorded in stalagmites as pulses of ash-derived elements that leach intro dripwater above the cave. These ash-derived elements can then be incorporated into stalagmites as they grow. We analysed a stalagmite from a shallow cave in south-west Western Australia which revealed increases in phosphorus and metals including copper, lead, aluminium and zinc during years when fires are documented to have occurred over the cave. We use peaks in these elements to extend the fire record back to the 1760’s and identify a particularly large fire event in 1897 ± 5. This fire event was intense enough to cause not only an increase in ash-derived elements, but also an increase in the transmission of rainfall between the surface and the cave due to fracturing of the limestone as a result of intense heating and cooling. This event occurred at the end of a known drought period and was likely enhanced by the dry conditions. We also identify evidence for lower-intensity cultural burning by Indigenous Australians in the pre-European period compared to the post-European period, suggesting that changes in land management may also be recorded in stalagmites.
- ItemPast hydroclimatic variability from southwest Australian speleothems during the last millennium(Australiasian Quaternary Association Inc., 2018-12-10) Treble, PC; Baker, AA; Griffiths, AD; Hellstrom, JC; Bajo, P; Abram, NJ; Fairchild, IJ; Borsato, A; Markowska, M; Gagan, MKSpeleothems from Golgotha Cave in SW Western Australia have been investigated to extend our knowledge of past climate variability for this region during the last millennium. O isotopic datasets, the primary paleoclimate proxy used for speleothems, were constructed for four stalagmites. A challenge in their interpretation has been the disagreement between these records, despite representing coeval growth from within the same cave. Resolving this conundrum has necessitated the characterisation of the hydrology, hydrochemistry, rainfall isotopes7 and development of proxy system forward models1,8 for Golgotha Cave. The findings of these studies will be summarised as a conceptual model in order to present the main karst hydrological features that give rise to each stalagmite’s isotopic response to hydroclimatic forcing. The paleoclimate interpretation will focus on the two continuous stalagmite records that were fed predominately by diffuse flow. This will be supported by evidence from the two stalagmites predominantly fed by fracture flow, which has resulted in a non-linear response to hydroclimatic forcing.
- ItemStalagmite carbon isotopes and dead carbon proportion (DCP) in a near-closed-system situation: An interplay between sulphuric and carbonic acid dissolution(Elsevier, 2017-08-01) Bajo, P; Borsato, A; Drysdale, RN; Hua, Q; Frisia, S; Zanchetta, G; Hellstrom, JC; Woodhead, JDIn this study, the ‘dead carbon proportion’ (DCP) calculated from combined U-Th and radiocarbon analyses was used to explore the carbon isotope systematics in Corchia Cave (Italy) speleothems, using the example of stalagmite CC26 which grew during the last ∼12 ka. The DCP values in CC26 are among the highest ever recorded in a stalagmite, spanning the range 44.8–68.8%. A combination of almost closed-system conditions and sulphuric acid dissolution (SAD) are proposed as major drivers in producing such a high DCP with minor contribution from old organic matter from the deep vadose zone. The long-term decrease in both DCP and δ13C most likely reflects post-glacial soil recovery above the cave, with a progressive increase of soil CO2 contribution to the total dissolved inorganic carbon (DIC). Pronounced millennial-scale shifts in DCP and relatively small coeval but antipathetic changes in δ13C are modulated by the effects of hydrological variability on open and closed-system dissolution, SAD and prior calcite precipitation. Hence, the DCP in Corchia Cave speleothems represents an additional proxy for rainfall amount. © 2017 Elsevier Ltd.
- ItemUbiquitous karst hydrological control on speleothem oxygen isotope variability in a global study(Springer Nature, 2022-02-15) Treble, PC; Baker, AA; Abram, NJ; Hellstrom, JC; Crawford, J; Gagan, MK; Borsato, A; Griffiths, AD; Bajo, P; Markowska, M; Priestley, SC; Hankin, SI; Paterson, DJSpeleothem oxygen isotopic (δ18O) records are used to reconstruct past hydroclimate yet records from the same cave do not always replicate. We use a global database of speleothem δ18O to quantify the replicability of records to show that disagreement is common worldwide, occurs across timescales and is unrelated to climate, depth or lithology. Our global analysis demonstrates that within-cave differences in mean speleothem δ18O values are consistent with those of dripwater, supporting a ubiquitous influence of flowpaths. We present a case study of four new stalagmite records from Golgotha Cave, southwest Australia, where the isotopic differences between them are informed by cave monitoring. It is demonstrated that karst hydrology is a major driver of within-cave speleothem and dripwater δ18O variability, primarily due to the influence of fractures on flowpaths. Applying our understanding of water movement through fractures assists in quantitative reconstruction of past climate variability from speleothem δ18O records. © The Authors - Open Access under a Creative Commons Attribution 4.0 International License.
- ItemUbiquitous karst hydrological control on speleothem oxygen isotope variability in a global study(Springer Nature, 2022-02-15) Treble, PC; Baker, AA; Abram, NJ; Hellstrom, JC; Crawford, J; Gagan, MK; Borsato, A; Griffiths, AD; Bajo, P; Markowska, M; Priestley, SC; Hankin, SI; Paterson, DJSpeleothem oxygen isotopic (δ18O) records are used to reconstruct past hydroclimate yet records from the same cave do not always replicate. We use a global database of speleothem δ18O to quantify the replicability of records to show that disagreement is common worldwide, occurs across timescales and is unrelated to climate, depth or lithology. Our global analysis demonstrates that within-cave differences in mean speleothem δ18O values are consistent with those of dripwater, supporting a ubiquitous influence of flowpaths. We present a case study of four new stalagmite records from Golgotha Cave, southwest Australia, where the isotopic differences between them are informed by cave monitoring. It is demonstrated that karst hydrology is a major driver of within-cave speleothem and dripwater δ18O variability, primarily due to the influence of fractures on flowpaths. Applying our understanding of water movement through fractures assists in quantitative reconstruction of past climate variability from speleothem δ18O records. © Crown 2022 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.