Browsing by Author "Bradley, C"
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- ItemEight-years of cave monitoring at Golgotha Cave, SW Australia: implications for speleothem paleoclimate records(Australasian Quaternary Association Inc, 2014-06-29) Treble, PC; Fairchild, IJ; Baker, AA; Bradley, C; Wood, A; McGuire, ESpeleothems are an important archive of paleoenvironmental information but a thorough understanding of processes are necessary for their interpretation. In order to better understand speleothem records from the climatically-sensitive southwest region of WA, we have conducted a detailed eight-year monitoring study at Golgotha Cave, southwest WA. Oxygen isotopic data demonstrated that the majority of water moved through the porous Quaternary calcarenite as matrix-flow with an inferred transit time of <1 year. A zone of high-flow dripwater is fed by high-magnitude rainfall events (Treble et al., 2013). Prior calcite precipitation (PCP) signals of increased Mg/Ca and Sr/Ca in dripwater are attributed to stalactite deposition. This signal is enhanced at low-flow sites and minimised at the high-flow site as degassing and subsequent stalactite deposition are a function of drip interval. Long-term rising trends found in most solutes are attributed via a mass-balance approach to increasing forest bioproductivity, consistent with an increase in forest understorey following a low-intensity burn in 2006. A fundamental message from this study is that individual speleothem records from within Golgotha Cave will differ, e.g. speleothem δ18O at our high-flow site is biased to recording high-magnitude rainfall events, whilst PCP will be the main driver of speleothem Mg/Ca and Sr/Ca at low-flow sites. Forest biomass appears to be modulating transpiration-sensitive ions and these may serve as an indicator of fire history.
- ItemInterpreting past climate using southwest Australian speleothems(Australian Meterological & Oceanographic Society, 2012-01-31) Treble, PC; Azcurra, CS; Baker, AA; Bradley, C; Wood, A; Fischer, MJ; Fairchild, IJ; Hellstrom, JC; Gagan, MKThere is an identified need to extend our baseline climate information beyond the relatively short duration of instrumental records in Australia. An improved knowledge of natural rainfall variability would assist in our understanding of climate change. SW Australia (SWWA) is one region that has been identified as having a changing climate since the 1970s. Speleothems (cave stalagmites) are an effective archive of past climate variability and caves from the coastal region of SWWA are being studied for paleoclimate records. The modern speleothem record from this region has been assessed and shown to record the post-1970s rainfall decrease (Treble et al., 2003; 2005; Fischer and Treble, 2008). The extension of the speleothem record is currently underway, however, a long-term cave monitoring program was also deemed necessary to separate the climatic from non-climatic signals i.e. to reduce uncertainty when interpreting these records. This presentation outlines what we have learnt about the possible hydrological modification of the climate signal in speleothems. In particular, we present results from a five-year long monitoring study of rainfall and cave drip water O isotopes (!18O) from Golgotha Cave, SWWA. From this study, we have been able to characterize the probable flow paths feeding stalagmites in our monitored cave. These flow paths range from slow diffuse flow of isotopically-averaged rainfall to preferential routing of high-magnitude, 18O-depleted, events along fast flow routes into the cave. Hence, we offer a possible explanation for why paleoclimate records from coeval speleothems in our cave may differ. Our study suggests that this disagreement may simply be due to different flow paths resulting in a bias towards the preservation of high or low magnitude rainfall events.
- ItemAn isotopic and modelling study of flow paths and storage in quaternary calcarenite, SW Australia: implications for speleothem paleoclimate records(Elsevier, 2013-03-15) Treble, PC; Bradley, C; Wood, A; Baker, AA; Jex, CN; Fairchild, IJ; Gagan, MK; Cowley, J; Azcurra, CSWe investigated the distinctive shallow sub-surface hydrology of the southwest Western Australia (SWWA) dune calcarenite using observed rainfall and rainfall δ18O; soil moisture, cave drip rate and dripwater δ18O over a six-year period: August 2005–March 2012. A lumped parameter hydrological model is developed to describe water fluxes and drip δ18O. Comparison of observed data and model output allow us to assess the critical non-climatic karst hydrological processes that modify the precipitation δ18O signal and discuss the implications for speleothem paleoclimate records from this cave and those with a similar karst setting. Our findings include evidence of multiple reservoirs, characterised by distinct δ18O values and recharge responses (‘low’ and ‘high’ flow sites). Dripwaters exhibit δ18O variations in wet versus dry years at low-flow sites receiving diffuse seepage from the epikarst with an attenuated isotopic composition that approximates mean rainfall. Recharge from high-magnitude rain events is stored in a secondary reservoir which is associated with high-flow dripwater that is 1‰ lower than our monitored low-flow sites (δ18O). One drip site is characterised by mixed-flow behaviour and exhibits a non-linear threshold response after the cessation of drainage from a secondary reservoir following a record dry year (2006). Additionally, our results yield a better understanding of the vadose zone hydrology and dripwater characteristics in Quaternary age dune limestones. We show that flow to our monitored sites is dominated by diffuse flow with inferred transit times of less than one year. Diffuse flow appears to follow vertical preferential paths through the limestone reflecting differences in permeability and deep recharge into the host rock. © 2013, Elsevier Ltd.
- ItemAn isotopic and modelling study of recharge to the Tamala Limestone, SW Australia: implications for speleothem paleoclimate records(International Association of Hydrogeologists, 2013-09-20) Treble, PC; Bradley, C; Fairchild, IJ; Baker, AA; Jex, CN; Azcurra, CS; Wood, AWe present a modern calibration of recharge pathways in the vadose zone of Quaternary dune calcarenite (Tamala Limestone in the Margaret River region, SW Western Australia. This study aimed to i. better understand groundwater recharge in these calcarenites, which has wider relevance to SW WA calcarenites and their use as a groundwater source; and to ii. interpret the O isotopic composition of these waters in order to assess the suitability of Golgotha Cave for reconstructing records of paleoclimate using speleothems. We present six years of data from our monitored site at Golgotha Cave (Treble et al. 2013). A lumped parameter hydrological model is developed to describe water fluxes and dripwater O isotopic composition. Comparison of observed data and model output allow us to assess the critical non-climatic karst hydrological processes that modify the precipitation isotopic signal and discuss the implications for speleothem records from this cave and those with a similar karst setting. Our findings include evidence of multiple reservoirs, characterised by distinct isotopic values and recharge responses ('low’ and ‘high’ flow sites). Dripwaters exhibit isotopic variations in wet versus dry years at low-flow sites receiving diffuse seepage from the epikarst with an attenuated isotopic composition that approximates mean rainfall. Recharge from high-magnitude rain events is stored <1%o lower in a secondary reservoir which is associated with high-flow dripwater that is 1%o lower than our monitored low-ow sites. One drip site is characterised by mixed-flow behaviour and exhibits a non-linear threshold response after the cessation of drainage from a secondary reservoir following a record dry year (2006). We show that ow to our monitored sites is dominated by diffuse ow with inferred transit times of less than one year. Diffuse flow appears to follow vertical preferential paths through the limestone reflecting differences in permeability and deep recharge into the host rock. Reference Treble et al. (Z013). An isotopic and modelling study of flow paths and storage in Quaternary calcarenite, SW Australia: implications for speleothem paleoclimate records, Quaternary Science Reviews 64; 90-103.
- ItemRoles of forest bioproductivity, transpiration and fire in a nine-year record of cave dripwater chemistry from southwest Australia(Elsevier, 2016-07-01) Treble, PC; Fairchild, IJ; Baker, AA; Meredith, KT; Andersen, MS; Salmon, SU; Bradley, C; Wynn, PM; Hankin, SI; Wood, A; McGuire, EForest biomass has the potential to significantly impact the chemistry and volume of diffuse recharge to cave dripwater via the processes of nutrient uptake, transpiration and forest fire. Yet to-date, this role has been under-appreciated in the interpretation of speleothem trace element records from forested catchments. In this study, the impact of vegetation is examined and quantified in a long-term monitoring program from Golgotha Cave, SW Australia. The contribution of salts from rain and dry-deposition of aerosols and dissolved elements from soil mineral and bedrock dissolution to dripwater chemistry are also examined. This study is an essential pre-requisite for the future interpretation of trace element data from SW Australian stalagmite records, whose record of past environmental change will include alterations in these biogeochemical fluxes. Solute concentrations in dripwater vary spatially, supporting the existence of distinct flow paths governed by varying amounts of transpiration as well as nutrient uptake by deeply-rooted biomass. Applying principal components analysis, we identify a common pattern of variation in dripwater Cl, Mg, K, Ca, Sr and Si, interpreted as reflecting increasing transpiration, due to forest growth. Mass-balance calculations show that increasing elemental sequestration into biomass has the largest impact on SO4, providing an explanation for the overall falling dripwater SO4 concentrations through time, in contrast to the transpiration-driven rising trend dominating other ions. The long-term rise in transpiration and nutrient uptake driven by increased forest bioproductivity and its impact on our dripwater chemistry is attributed to (i) the post-fire recovery of the forest understorey after fire impacted the site in 2006 CE; (ii) and/or increased water and nutrient demand as trees in the overlying forest mature. The impact of climate-driven changes on the water balance is also examined. Finally, the implications for interpreting SW Australian speleothem trace element records are discussed. © Crown Copyright Published by Elsevier B.V.
- ItemRoles of transpiration, forest bioproductivity and fire on a long-term dripwater hydrochemistry dataset from Golgotha Cave, SW Australia(American Geophysical Union (AGU), 2015-12-15) Treble, PC; Baker, AA; Fairchild, IJ; Bradley, C; Mahmud, K; Andersen, MS; Meredith, KT; Mariethoz, GGolgotha Cave is located in a forested catchment in SW Australia where evapotranspiration losses from the vadose-zone are high and forest biomass has been disturbed by fire. The cave has been continuously monitored since 2005 and this extensive dataset has been used to quantify key processes determining dripwater hydrology and chemistry (Mahmud et al., 2015; Treble et al., 2013; 2015). In this paper we present a synthesis of these findings and derive a conceptual model to illustrate the main hydrochemical processes that will impact cave dripwater in similar environments. We applied mass-balance techniques to quantify sources (water/rock interactions and aerosol) and sinks (prior calcite precipitation and biomass uptake). Mass-balance results suggest that transpiration and elemental sequestration into biomass modifies dripwater ion concentrations. The vegetation uptake impacts dripwater Mg, K and SO4, with the largest impact on SO4, estimated to be up to 60% at some drip sites. Overall, our findings suggest that varying amounts of transpiration by deeply-rooted trees contribute significantly to spatial and temporal variability in dripwater solute concentrations. This is in addition to the partitioning of infiltrating water between characteristic flow pathways. Applying principal components analysis, we identify a common long-term rising trend in dripwater Cl, Mg, K, Ca, Sr and Si. We assess whether the long-term trends in dripwater solutes are driven by post-fire biomass recovery and/or the impacts of a drying climate in SW Australia. References Mahmud et al. (2015), Terrestrial Lidar Survey and Morphological Analysis to Identify Infiltration Properties in the Tamala Limestone, Western Australia, doi:10.1109/JSTARS.2015.2451088. Treble et al. (2013), An isotopic and modelling study of flow paths and storage in Quaternary calcarenite, doi:10.1016/J.Quascirev.2012.12.015. Treble et al. (2015), Impacts of cave air ventilation and in-cave prior calcite precipitation on Golgotha Cave dripwater chemistry, doi:10.1016/J.Quascirev.2015.06.001.