Browsing by Author "Nagra, G"

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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.