Browsing by Author "Hartmann, A"

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    Global analysis reveals climatic controls on the oxygen isotope composition of cave drip water
    (Springer Nature, 2019-07-05) Baker, AA; Hartmann, A; Duan, WH; Hankin, SI; Comas-Bru, L; Cuthbert, MO; Treble, PC; Banner, J; Genty, D; Baldini, LM; Bartolomé, M; Moreno, A; Pérez-Mejías, C; Werner, M
    The oxygen isotope composition of speleothems is a widely used proxy for past climate change. Robust use of this proxy depends on understanding the relationship between precipitation and cave drip water δ18O. Here, we present the first global analysis, based on data from 163 drip sites, from 39 caves on five continents, showing that drip water δ18O is most similar to the amount-weighted precipitation δ18O where mean annual temperature (MAT) is < 10 °C. By contrast, for seasonal climates with MAT > 10 °C and < 16 °C, drip water δ18O records the recharge-weighted δ18O. This implies that the δ18O of speleothems (formed in near isotopic equilibrium) are most likely to directly reflect meteoric precipitation in cool climates only. In warmer and drier environments, speleothems will have a seasonal bias toward the precipitation δ18O of recharge periods and, in some cases, the extent of evaporative fractionation of stored karst water. © 2019, The Author(s)
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    Global distribution and controls on cave drip water oxygen isotope composition
    (Australasian Quaternary Association Inc, 2018-12-10) Baker, AA; Hartmann, A; Duan, WH; Hankin, SI; Comas-Bru, L; Cuthbert, MO; Treble, PC; Banner, J; Genty, D; Baldini, LM; Bartolomé, M; Moreno, A; Pérez-Mejías, C
    The oxygen isotope composition of speleothems is a widely utilised paleoclimate proxy that is responsible for the current state-of-knowledge of past Asian monsoon dynamics, the timing of glacial-interglacial cycles, and the insolation control on inter-tropical convergence zone position, among other climate processes. Because speleothems are deposited by cave drip water, and this is derived from meteoric precipitation, it is critical to understand the empirical relationship between precipitation and cave drip water d18O. Here, we present the first global analysis, based on data from 148 drip sites, 38 caves, and five continents. Globally, drip water d18O is most similar to the amount-weighted precipitation d18O where mean annual temperature (MAT) is < 10 °C. For seasonal climates with MAT > 10 °C and < 16 °C, we demonstrate that drip water d18O records the recharge-weighted d18O. Our analysis implies that speleothems (formed in near isotopic equilibrium) are most likely to have d18O that directly reflect meteoric precipitation only in cool climates. In warmer and drier environments, speleothems will have a seasonal bias toward the precipitation d18O of recharge periods and, in some cases, the extent of evaporative fractionation of stored karst water. We highlight the implications of our analysis for the interpretation of oxygen isotope records in Australasian speleothems. © The Authors
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    Global distribution and controls on cave drip water oxygen isotope composition
    (International Union for Quaternary Research (INQUA), 2019-07-30) Baker, AA; Comas-Bru, L; Hartmann, A; Duan, WH; Hankin, SI; Cuthbert, MO; Treble, PC; Banner, J; Gentry, D; Baldini, LM; Bartolomé, M; Moreno, A; Pérez-Mejías, C
    The oxygen isotope composition of speleothems is a widely utilised paleoclimate proxy that is responsible for the current state-of-knowledge of past Asian monsoon dynamics, the timing of glacial-interglacial cycles, and the insolation control on inter-tropical convergence zone position, among other climate processes. Because speleothems are deposited by cave drip water, and this is derived from meteoric precipitation, it is critical to understand the empirical relationship between precipitation and cave drip water δ18O. Here, we present the first global analysis, based on data from 148 drip sites, 38 caves, and five continents. Globally, drip water δ18O is most similar to the amount-weighted precipitation δ18O where mean annual temperature (MAT) is < 10 °C. For seasonal climates with MAT > 10 °C and < 16 °C, we demonstrate that drip water δ18O records the recharge-weighted δ18O. Our analysis implies that speleothems (formed in near isotopic equilibrium) are most likely to have δ18O that directly reflect meteoric precipitation only in cool climates. In warmer and drier environments, speleothems will have a seasonal bias toward the precipitation δ18O of recharge periods and, in some cases, the extent of evaporative fractionation of stored karst water.
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    Insights into recharge processes and speleothem proxy archives from long-term monitoring networks of cave drip water hydrology
    (Copernicus GmbH, 2020-05-04) Baker, AA; Treble, PC; Hartmann, A; Cuthbert, MO; Markowska, M; Berthelin, R; Tadros, CV; Leopold, M; Hankin, SI
    Since 2010 we have established cave drip water hydrological monitoring networks in four contrasting climate zones (Mediterranean, montane, semi-arid and sub-tropical) across continental Australia. Deploying over one hundred automated drip loggers, we combine these long-term monitoring datasets with climate and water isotope data, lidar mapping, electrical resistivity imaging and karst hydrological modelling to provide insights into recharge processes and the impact of hydrological variability on speleothem proxy archives. We identify increases in drip discharge and compare the timing of those events to antecedent climate conditions (rainfall, evapotranspiration). We find rainfall recharge thresholds vary with climate. At our montane site, recharge occurs after 13 to 31 mm rainfall events, depending on antecedent conditions. At the semi-arid site, recharge occurs after 40 mm rainfall events, and at our sub-tropical sites, recharge occurs following all instances where > 93 mm / week of precipitation occurs, with lower precipitation thresholds (down to 33 mm / week) possible depending on antecedent conditions and at sites with limited vegetation cover. We use these recharge thresholds to constrain simple soil moisture balance models to better understand soil and karst storage volumes. Combined with electrical resistivity imaging, we can relate recharge to the caves to subsurface water flow paths and karst water stores. At our montane and Mediterranean climate sites, relatively consistent drip water isotopic composition confirms the presence of well-mixed water stores. This allows us to quantify the extent of speleothem oxygen isotope variability due to fractionation associated with changes in drip rate. We identify significant differences in long-term mean drip rates between different drip sites within a cave, and significant differences in event-based drip rate responses within a cave. Drip hydrological variability helps explain the within-cave variability of speleothem oxygen isotope composition observed at both sites, and helps identify the primary drip water oxygen isotope signal. At our semi-arid site, drip water isotopic composition is dominated by epikarst evaporation and our drip water monitoring demonstrates that recharge events are infrequent (~1.6 per year). Using both observational and modelling data, we quantify the relative importance of evaporative fractionation in the epikarst and fractionation during calcite precipitation. Using modern speleothem samples, we demonstrate that the oxygen isotope signal in this water limited environment reflects the balance between the oxygen isotope composition of recharge and its subsequent fractionation in the soil, epikarst and cave. © Author(s) 2020. This work is distributed under the Creative Commons Attribution 4.0 Licence.
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    A study of transition metal implanted single crystal YBa2 Cu3 O7-δ
    (Australian and New Zealand Institutes of Physics, 1994-02-10) Martin, JW; Russell, GJ; Cohen, DD; Evans, PJ; Hartmann, A
    The modification of materials by ion-implantation is a method by which the structure and properties of a material can be altered in a controlled fashion and the study of these modified structures can lead to a better understanding of the parent material. The application to the Held of high temperature superconductors has been present ever since the discovery of the new oxides. In this study, high quality single crystal YBa2 Cu3 O7-δ was implanted separately with several different transition metal ions. The implantation was carried out using a metal vacuum vapour arc (MEVVA) ion source operated at 30kV with an approximate dose of ixlO17 ions/cm2 being applied. The resulting crystals were subsequently annealed in two separate anneal cycles in an oxygen atmosphere at 55O°C for a total of 108 hours. The sample analysis techniques involved the use of Rutherford backscattering spectroscopy (RBS), at both 2 and 3.07MeV, a.c. susceptibility measurements, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) for compositional, superconductivity, crystal structure and bonding analysis respectively. This paper will report on the results of this study which have shown that the implantation of nickel increased the rate of oxygenation in the near surface region, obtaining a transition temperature of 92°K after only 108 hours anneal time, whereas the implantation of iron was deleterious with a transition temperature of only 78°K reached after the same anneal cycles.