Browsing by Author "Coleborn, K"

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    Caves: observatories of Australia’s diffuse groundwater recharge history
    (National Centre for Groundwater Research And Training, 2015-11-03) Baker, AA; Treble, PC; Andersen, MS; Markowska, M; Coleborn, K; Flemons, I; Kempsey Speleological Society
    Quantifying the timing and extent of diffuse groundwater recharge is crucial for our understanding of groundwater recharge processes. However, diffuse recharge is notably difficult to quantify. Our novel approach is to use caves as natural observatories of the diffuse recharge process, with the aim of improving our understanding of diffuse recharge in the context of climate change and climate variability. Since 2010, funded by the NCRIS Groundwater Infrastructure project, researchers from UNSW and ANSTO have established a long-term, national monitoring program of infiltration into caves using automated loggers. Five karst regions, in semi-arid, temperate, subtropical and montane climates from southwest WA to the mid- north coast of NSW, have been instrumented with automatic infiltration loggers. Over 200 loggers (between 10 and 40 per cave) have collected data on the timing and amount of diffuse recharge, from sites of contrasting limestone geology, starting in 2010. We present empirical data on the timing and relative amounts of diffuse recharge from 2010 to present. Caves with a range of depths from 0-40m show decreasing frequency of diffuse recharge events with depth below ground surface. Event-based rainfall intensity is confirmed to be the primary driver of diffuse groundwater recharge at all fractured rock sites, whereas annual rainfall amount is the primary driver at a site with high primary porosity. Inter-annual variability in the frequency and relative amount of recharge is compared to climate forcing variables such as the ENSO and surface temperature. Groundwater recharge is via both direct (river recharge) and diffuse processes. With anthropogenic global warming, increased temperatures will increase evaporation, and will likely change ENSO patterns, both of which will affect diffuse groundwater recharge. Our cave observatory system helps improve our understanding of the diffuse recharge process and provides a baseline monitoring network during a period of climate change.
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    Climate and groundwater recharge: the story from Australian caves
    (National Centre for Groundwater Research And Training, 2017-07-11) Baker, AA; Treble, PC; Markowska, M; Andersen, MS; Wang, ZYO; Mahmud, K; Cuthbert, MO; Coleborn, K; Rau, GC
    Quantifying the timing and extent of diffuse groundwater recharge is crucial for our understanding of groundwater recharge processes. However, diffuse recharge is notably difficult to measure directly. Caves can be used as natural observatories of ongoing diffuse recharge processes, and speleothems (cave carbonate deposits such as stalagmites) as archives of past recharge. Cave records can improve our understanding of diffuse recharge in the context of climate change and past climate variability. A long-term, national monitoring program of infiltration into caves has been undertaken since 2010 using a network of over 200 automated loggers. This has been supplemented by artificial irrigation experiments at one semi-arid site. The timing of past recharge can be determined from the periods of past stalagmite growth. Recharge characteristics can be elucidated from oxygen isotope composition, with increased 18O likely caused by evaporative fractionation and increased 16O from high intensity/magnitude rainfall events. Automated logger data identify the diffuse recharge thresholds that vary with climate and geology. Both the logged data of natural events and the artificial irrigation experiments identify significant spatial heterogeneity in recharge in these karstified systems. Water infiltrating into the karst is often depleted in the lighter oxygen isotope due to soil and shallow subsurface evaporative fractionation. Speleothem deposition is more frequent during glacial periods, presumably because recharge thresholds are lower, and their isotopic composition provides evidence of the characteristics of the recharge process. Caves provide direct access into the unsaturated zone. Direct observation of groundwater recharge can be used to complement data from the saturated zone (boreholes) and models. The heterogeneity of recharge in karst aquifers can be directly observed and quantified. Speleothems preserve a record of groundwater recharge that can extend back for hundreds of thousands of years, providing a long-term view on the timing and variability of groundwater recharge in Australia.
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    Dissolved organic matter in the unsaturated zone: the view from the cave
    (American Geophysical Union (AGU), 2017-12-14) Baker, AA; Duan, W; Rutlidge, H; McDonough, LK; Oudone, PP; Meredith, KT; Andersen, MS; O'Carroll, DM; Coleborn, K; Treble, PC
    Soil organic matter content is typically a few percent of the total soil composition. Diffuse recharge can mobilise some of this soil-derived organic matter. While soil pore water dissolved organic matter (DOM) concentrations are up to 100 ppm, the resulting groundwater dissolved organic matter concentration is typically less than 2ppm. Dissolved organic matter transported from the soil can be both biodegraded and sorbed to minerals, and the relative importance of these two processes in the unsaturated zone is poorly understood. Caves in karstified limestone uniquely provide direct access to water percolating from the soil to the groundwater. Cave percolation waters can be analysed for their DOM concentration and character. This provides insights into the extent and type of biological and chemical processing of DOM during transport from the soil to the groundwater. We determine the concentration and characteristics of DOM in cave percolation waters using liquid chromatography (LC-OCD) and optical spectrophotometry (fluorescence and absorbance). We sample DOM from multiple caves in SE Australia (Cathedral Cave, Wellington; South Glory and Harrie Wood Caves, Yarrangobilly), permitting comparison of unsaturated zone DOM properties at different depths (up to 30m below land surface) and different climate zones (montane and temperate). We use caves with long-term hydrological monitoring programs so that DOM in waters of contrasting residence times can be compared. Additionally, we compare these cave percolation water DOM characteristics to those from local and regional groundwater, sampled from nearby wells. Our results will help improve our understanding of how DOM is processed from soil to groundwater, and is also relevant to speleothem scientists interested in using organic matter preserved in speleothems as a paleoclimate or paleoenvironmental proxy. Plain Language Summary When plants die, they break down to organic matter, which forms part of the soil. When this organic matter is washed out of the soil and into the subsurface, we know very little about what happens next. Partly it is because we can't see and measure what is happening. There is a solution. We can use caves as observatories. We can collect the organic matter in the water which enters the caves, and analyse it back in the laboratory. There, we can determine not only the cocntration of organic matter, but also its chemical composition. Why is this important? There's lot of organic matter in soil. But only one or two organic molecues per million water molecules are present in groundwater. Where does it all go? One idea is that it is used as food by subterranean microbes. Another is that is sorbed to minerals. By measuring the chemical composition of organic matter in cave drip waters, we can work out which is more important, and help understand why there is so little organic matter in groundwater.
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    Hydrological and geochemical responses of fire in a shallow cave system
    (2019-04-20) Bian, F; Coleborn, K; Flemons, I; Baker, AA; Treble, PC; Hughes, CE; Baker, AC; Andersen, MS; Tozer, MG; Duan, WH; Fogwill, CJ; Fairchild, IJ
    The influence of wildfire on surface soil and hydrology has been widely investigated, while its impact on the karst vadose zone is still poorly understood. A moderate to severe experimental fire was conducted on a plot (10 m × 10 m) above the shallow Wildman's Cave at Wombeyan Caves, New South Wales, Australia in May 2016. Continuous sampling of water stable isotopes, inorganic geochemistry and drip rates were conducted from Dec 2014 to May 2017. After the fire, drip discharge patterns were significantly altered, which is interpreted as the result of increased preferential flows and decreased diffuse flows in the soil. Post-fire drip water δ18O decreased by 6.3‰ in the first month relative to the average pre-fire isotopic composition. Post-fire monitoring showed an increase in drip water δ18O in the following six months. Bedrock related solutes (calcium, magnesium, strontium) decreased rapidly after the fire due to reduced limestone dissolution time and potentially reduced soil CO2. Soil- and ash-derived solutes (boron, lead, potassium, sodium, silicon, iodine and iron) all decreased after the fire due to volatilisation at high temperatures, except for SO42−. This is the first study to understand the hydrological impact from severe fires conducted on a karst system. It provides new insights on the cave recharge process, with a potential explanation for the decreased d18O in speleothem-based fire study, and also utilise the decreased bedrock solutes to assess the wildfire impacts both in short and long time scales. Open access © 2021 Elsevier B.V
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    Impact of fire on hydrological and chemical signatures in karst vadose zone water, Wombeyan Caves, New South Wales, Australia
    (National Centre for Groundwater Research And Training, 2017-07-11) Bian, F; Coleborn, K; Flemons, I; Treble, PC; Baker, AA; Baker, AC
    Impact of wildfire on karst-vadose-zone hydrology and hydrogeochemistry is hard to evaluate owing to the complexity of subsurface environment. The aim of this study is to understand the variation of hydrogeochemical components and drip discharge in response to a moderate-intensity 10m x10m experimental fire above the shallow Wildman’s Cave at Wombeyan, Australia, in May, 2016. Water isotopes and cation analyses were conducted on drip waters collected pre- and post-fire. Ongoing drip water collection began in Dec, 2014. And drip rate has been monitored continuously using acoustic data loggers. Discharge into cave is discontinuous, indicative of limited soil and karst storage. The post-fire drip data demonstrate decreased duration of recharge, with approximately x3 increase in peak discharge, which we hypothesize is caused by the decrease of soil-storage capacity. Water isotope compositions have significantly changed after fire, with d2H isotope composition up to ~56 per mil lower and d18O ~6.3 per mil lower in the week after fire. With time, isotopic values return to pre-fire values. We hypothesize that this temporary depletion in water isotopic composition reflects a combination of post-fire rainfall isotope composition, loss of pre-fire evaporatively enriched soil and shallow karst stored water.Drip water concentrations of bedrock-related elements (Calcium, Strontium) and soil-related elements (Zinc, Nickle) decreased after the fire. We hypothesize that these reflect the loss of soil and soil biological activity above the cave, and agree with a decrease of soil storage capacity. This research demonstrates that even in complex hydrogeological settings, understanding the impact of local wildfire on subsurface system can be improved through the combination of drip water hydrograph analysis and geochemical analysis. This will provide opportunities to broaden the insights into improved fire management in karst environments and a better understanding of the relationship between surface environment conditions and vadose zone hydrology.
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    The impact of fire on the geochemistry of speleothem-forming drip water in a sub-alpine cave
    (Elsevier, 2018-11) Coleborn, K; Baker, AA; Treble, Pauline C; Andersen, MS; Baker, AC; Tadros, CV; Tozer, MG; Fairchild, IJ; Spate, A; Meehan, S
    Fire dramatically modifies the surface environment by combusting vegetation and changing soil properties. Despite this well-documented impact on the surface environment, there has been limited research into the impact of fire events on karst, caves and speleothems. Here we report the first experiment designed to investigate the short-term impacts of a prescribed fire on speleothem-forming cave drip water geochemistry. Before and after the fire, water was collected on a bi-monthly basis from 18 drip sites in South Glory Cave, New South Wales, Australia. Two months post-fire, there was an increase in B, Si, Na, Fe and Pb concentrations at all drip sites. We conclude that this response is most likely due to the transport of soluble ash-derived elements from the surface to the cave drip water below. A significant deviation in stable water isotopic composition from the local meteoric water line was also observed at six of the sites. We hypothesise that this was due to partial evaporation of soil water resulting in isotopic enrichment of drip waters. Our results demonstrate that even low-severity prescribed fires can have an impact on speleothem-forming cave drip water geochemistry. These findings are significant because firstly, fires need to be considered when interpreting past climate from speleothem δ18O isotope and trace element records, particularly in fire prone regions such as Australia, North America, south west Europe, Russia and China. Secondly, it supports research that demonstrates speleothems could be potential proxy records for past fires. © 2018 Elsevier B.V.
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    The impact of wildfire on the geochemistry and hydrology the vadose zone
    (National Centre for Groundwater Research And Training, 2017-07-12) Coleborn, K; Baker, AA; Treble, PC; Baker, AC; Andersen, MS; Tozer, MG; Fairchild, IJ; Spate, A; Meehan, S
    Wildfire can dramatically modify the surface environment by removing surface vegetation, killing microbial communities and changing the soil geochemical and physical structure. Wildfires are a widespread phenomenon in Australia with 87,810 ha burnt in 2015-2016 in NSW alone (New South Wales Rural Fire Service, 2016). However, there has been little research on the impact of wildfire on vadose zone hydrology in Australia or elsewhere. Limestone caves can be used as natural laboratories to study the impact of the surface environment on vadose zone hydrology in real time. We conducted a two year monitoring program to examine the short term (less than one year) post-fire impacts on the geochemistry and hydrology of vadose zone water in Glory Hole Cave, Kosciuszko National Park, NSW. We ignited an experimental wildfire on the surface over the cave after 1 year of monitoring and compared the pre- and post-fire data. The findings indicate that there is a short term post-fire response in the organic and inorganic geochemistry vadose zone water. There was a post-fire spike in dissolved organic carbon, dominated by the hydrophilic fraction more than one month post-fire which was most likely due to high influx of ash from the fire. There was a multi-month increase in organic-associated metals such as Cu and Zn. The concentration of trace metals such as Fe and Si increased by an order of magnitude less than three months post-fire and was attributed to a flush of small colloidal organic matter. This study enabled us to establish that there is an organic and inorganic cave vadose zone water response to wildfires. The findings of this study can inform fire management strategies in order to protect sensitive karst environments in addition to water resource managers concerned with fluxes of mobilised metals nutrient fluxes to the vadose zone.
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    Past 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, DJ
    Stalagmites 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.
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    Past 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, DJ
    Stalagmites 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.
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    A post-wildfire response in cave dripwater chemistry
    (Copernicus Publications, 2016-07-21) Nagra, G; Treble, PC; Andersen, MS; Fairchild, IJ; Coleborn, K; Baker, AA
    Surface disturbances above a cave have the potential to impact cave dripwater discharge, isotopic composition and solute concentrations, which may subsequently be recorded in the stalagmites forming from these dripwaters. One such disturbance is wildfire; however, the effects of wildfire on cave chemistry and hydrology remains poorly understood. Using dripwater data monitored at two sites in a shallow cave, beneath a forest, in southwest Australia, we provide one of the first cave monitoring studies conducted in a post-fire regime, which seeks to identify the effects of wildfire and post-fire vegetation dynamics on dripwater δ18O composition and solute concentrations. We compare our post-wildfire δ18O data with predicted dripwater δ18O using a forward model based on measured hydro-climatic influences alone. This helps to delineate hydro-climatic and fire-related influences on δ18O. Further we also compare our data with both data from Golgotha Cave – which is in a similar environment but was not influenced by this particular fire – as well as regional groundwater chemistry, in an attempt to determine the extent to which wildfire affects dripwater chemistry. We find in our forested shallow cave that δ18O is higher after the fire relative to modelled δ18O. We attribute this to increased evaporation due to reduced albedo and canopy cover. The solute response post-fire varied between the two drip sites: at Site 1a, which had a large tree above it that was lost in the fire, we see a response reflecting both a reduction in tree water use and a removal of nutrients (Cl, Mg, Sr, and Ca) from the surface and subsurface. Solutes such as SO4 and K maintain high concentrations, due to the abundance of above-ground ash. At Site 2a, which was covered by lower–middle storey vegetation, we see a solute response reflecting evaporative concentration of all studied ions (Cl, Ca, Mg, Sr, SO4, and K) similar to the trend in δ18O for this drip site. We open a new avenue for speleothem science in fire-prone regions, focusing on the geochemical records of speleothems as potential palaeo-fire archives. © Author(s) 2016.
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    Reconstructing past environmental change at Yarrangobilly Caves
    (Sydney Speleological Society, 2016-01-01) Treble, PC; Markowska, M; Tadros, CV; Jex, CN; Coleborn, K; Dredge, J; Baker, AA; Roach, R; Spate, A
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    A 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, AK
    Wildfires 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
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    Sulphur: a proxy for wildfires in stalagmites
    (Australasian Quaternary Association Inc., 2018-12-12) Coleborn, K; Baker, AA; Treble, PC; Wynn, P
    Bushfires are a global hazard that can have catastrophic impacts on communities and ecosystems. However, there is limited baseline data on how fire frequency and intensity have responded to climate change in the past. A speleothem-based bushfire record will allow us to determine long-term natural fire regimes and better understand the relationship between bushfires and climate. Recent research has demonstrated the potential of using sulphur as a speleothem paleofire proxy1 but a full characterisation of S in the karst environments of SW Western Australia is lacking. Here, we quantify the relative contributions of different sources of sulphur in a modern cave environment through the characterisation of rainfall, soil, bedrock, vegetation and cave drip water sulphate concentration and isotopic values (δ18O- SO4 and δ34S- SO4) to assess the role of sulphur cycling through the biomass in response to burn events at this site. This information will be used to interpret the SO4 isotope record, supported by other proxies in a 2-12 ka speleothem from SW Western Australia. This is the first speleothem SO4 isotope study in the southern hemisphere and the first in the world to use speleothem S isotopes in a paleofire context. © Author(s)
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    Using speleothem SO4 isotopes to elucidate S cycling in a fire prone region
    (Copernicus GmbH, 2019-04-11) Coleborn, K; Baker, AA; Treble, PC; Wynn, PM
    Wildfires are a global hazard that can have catastrophic impacts on communities and ecosystems. A speleothem based wildfire record will allow us to determine long-term natural fire regimes and better understand the relationship between wildfires and climate. Recent research has demonstrated the potential of using S as a speleothem paleofire proxy1 but a full characterisation of S in a fire-prone environment is lacking. Firstly, we used modern monitoring to quantify the relative contributions of S sources in a cave environment with overlying post-fire forest regrowth. Sulphate concentration and isotopic values (δ34S- SO4 and δ18O-SO4) for rainfall (+18.8hand +8.1h respectively), cave drip water (+20.3hand +3.2h), bedrock (+21.7hand +10.6h) vegetation (δ34S- SO4 +22.1h) and soil (δ34S- SO4 +19.5h) were characterised. Results showed the SO4 rainfall input was from a marine source. A 1-2h fractionation of drip water δ34S- SO4 compared to rainfall δ34S- SO4 revealed there was assimilation of SO4 into vegetation above the cave. δ18O- SO4 indicated biogeochemical cycling of S by mineralisation of organic S compounds to sulphate within the soil. These results demonstrate the necessity of dual isotope analysis of δ34S- SO4 and δ18O- SO4 to correctly identify S sources and biogeochemical cycling prior to incorporation of SO4 into a speleothem. Secondly, the S isotope proxy was applied to a 2-12 ka speleothem record from the same region and forest cover. More than fifty 200 mg samples from a flowstone were analysed for δ34S- SO4 and δ18O- SO4:Mean speleothem δ34S- SO4 was enriched in 34S by >2 h compared to modern rainfall, indicative of vegetation fractionation. LGM mean δ34S- SO4 was more negative than Holocene (+22.1hand +23.2h respectively). Fast growth periods at δ10 and 7ka provided multi-annual resolution and 1.5h variability in δ34S- SO4 was observed over decadal timescales, indicative of changes in vegetation cycling of S, which we interpret as a possible fire record.