Browsing by Author "McGuire, E"

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    Eight-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, E
    Speleothems 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.
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    Impacts of cave air ventilation and in-cave prior calcite precipitation on Golgotha Cave dripwater chemistry, southwest Australia
    (Elsevier, 2015-11) Treble, PC; Fairchild, IJ; Griffiths, AD; Baker, AA; Meredith, KT; Wood, A; McGuire, E
    Speleothem trace element chemistry is an important component of multi-proxy records of environmental change but a thorough understanding of hydrochemical processes is essential for its interpretation. We present a dripwater chemistry dataset (PCO2, alkalinity, Ca, SIcc, Mg and Sr) from an eight-year monitoring study from Golgotha Cave, building on a previous study of hydrology and dripwater oxygen isotopes (Treble et al., 2013). Golgotha Cave is developed in Quaternary aeolianite and located in a forested catchment in the Mediterranean-type climate of southwest Western Australia. All dripwaters from each of the five monitored sites become supersaturated with respect to calcite during most of the year when cave ventilation lowers PCO2 in cave air. In this winter ventilation mode, prior calcite precipitation (PCP) signals of increased Mg/Ca and Sr/Ca in dripwater are attributed to stalactite deposition. A fast-dripping site displays less-evolved carbonate chemistry, implying minimal stalactite growth, phenomena which are attributed to minimal degassing because of the short drip interval (30 s). We employ hydrochemical mass-balance modelling techniques to quantitatively investigate the impact of PCP and CO2 degassing on our dripwater. Initially, we reverse-modelled dripwater solutions to demonstrate that PCP is dominating the dripwater chemistry at our low-flow site and predict that PCP becomes enhanced in underlying stalagmites. Secondly, we forward-modelled the ranges of solution Mg/Ca variation that potentially can be caused by degassing and calcite precipitation to serve as a guide to interpreting the resulting stalagmite chemistry. We predict that stalagmite trace element data from our high-flow sites will reflect trends in original dripwater solutes, preserving information on biogeochemical fluxes within our system. By contrast, stalagmites from our low-flow sites will be dominated by PCP effects driven by cave ventilation. Our poorly karstified system allows us to highlight and quantify these in-cave (PCP) processes, which are otherwise masked at sites where karstification is more developed and hydrogeology is more complex. Our modelling also shows enhanced CO2 source production in the unsaturated zone that is attributed to deeply-rooted vegetation and increasing bioproductivity which we link to forest recovery after fires impacted our site during 2006 CE. Crown Copyright © 2015 Published by Elsevier Ltd.
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    Roles 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, E
    Forest 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.