https://nova.newcastle.edu.au/vital/access/ /manager/Index ${session.getAttribute("locale")} 5 https://nova.newcastle.edu.au/vital/access/ /manager/Repository/uon:33010 SO₄ =(mSO₄/mCO₃)_solid/(mSO₄/mCO₃)_solution. High crystal growth rates (driven by either pH or saturation state) encouraged higher values of D_SO₄ because of an increasing concentration of defect sites on crystal surfaces. At low growth rates, D_SO₄ was reduced due to an inferred competition between sulphate and bicarbonate at the calcite surface. These experimental results are applied to understand the incorporation of sulphate into speleothem calcite. The experimentally determined pH-dependence suggests that strong seasonal variations in cave air PCO₂ could account for annual cycles in sulphate concentration observed in stalagmites. Our new experimentally determined values of D_SO₄ were compared with D_SO₄ values calculated from speleothem-drip water monitoring from two caves within the Austrian and Italian Alps. At Obir cave, Austria, D_SO₄ (×10⁵) varies between 11.1 (winter) and 9.0 (summer) and the corresponding figures for Ernesto cave, Italy, are 15.4 (winter) and 14.9 (summer). These values approximate predicted D_SO₄ values based on our chamber experiments containing both low (2 ppm) and high (20 ppm) sulphate concentrations. Our experimental values of D_SO₄ obtained at crystal growth rates typical of stalagmites, closely match those observed in other cave sites from around the world. This validates the universality of the controls behind D_SO₄ and will enhance the use of speleothem CAS as a palaeoenvironmental proxy.]]> Tue 03 Sep 2019 18:19:27 AEST ]]> https://nova.newcastle.edu.au/vital/access/ /manager/Repository/uon:49220 Sun 07 May 2023 09:37:55 AEST ]]> https://nova.newcastle.edu.au/vital/access/ /manager/Repository/uon:12517 DIC values are already higher (by about 1‰) than those of soil water due to dissolution of the carbonate rock. A subsequent systematic shift to even higher δ¹³C values, from −11.5‰ in the cave drips to about −8‰ calculated for the solution film on top of stalagmites, is related to degassing on the stalagmite top and equilibration with the cave air. Mass-balance modelling of C fluxes reveals that a very small percentage of isotopically depleted cave air CO₂ evolves from the first phase of dripwater degassing, and shifts the winter cave air composition toward slightly more depleted values than those calculated for equilibrium. The systematic ¹³C-enrichment from the soil to the stalagmites at Grotta di Ernesto is independent of drip rate, and forced by the difference in pCO₂ between cave water and cave air. This implies that speleothem δ¹³C values may not be simply interpreted either in terms of hydrology or soil processes.]]> Sat 24 Mar 2018 08:16:02 AEDT ]]> https://nova.newcastle.edu.au/vital/access/ /manager/Repository/uon:10909 Sat 24 Mar 2018 08:07:41 AEDT ]]> https://nova.newcastle.edu.au/vital/access/ /manager/Repository/uon:18114 13C values are linked to low δ¹³C values of cave air and drip water during that time. This observation corresponds to times of reduced cave ventilation, high pCO₂ of cave air, low drip water pH, lower calcite supersaturation and typically high drip rates. In contrast, the translucent, dense laminae represent more or less complete lateral coalescence (inclusion-free) during the cold season (high calcite, drip water and cave air δ¹³C values), i.e. times of enhanced cave ventilation, low cave air pCO₂, increased drip water pH, relatively high calcite supersaturation and typically low drip rates. In essence, the relative development of the two lamina types reflects changes in the seasonality of external air temperature and precipitation, with a strong control of the winter air temperature on the intensity of cave-air exchange. Thick translucent, dense laminae are favoured by long, cold and wet winters and such conditions may be related closely to the North Atlantic Oscillation mode (weak westerlies) and enhanced Mediterranean cyclone activity during the cold season. Studies of speleothem lamination can thus help to better understand (and quantify) the role of seasonality changes, for example, during rapid climate events.]]> Sat 24 Mar 2018 08:04:36 AEDT ]]> https://nova.newcastle.edu.au/vital/access/ /manager/Repository/uon:6876 Sat 24 Mar 2018 07:49:46 AEDT ]]> https://nova.newcastle.edu.au/vital/access/ /manager/Repository/uon:26774 13C values range between ca -6⁰/₀₀ and +4⁰/₀₀, with the most 13C-depleted values in dolomites of the distal alluvial plain and flood basin, and the most ¹³C-enriched in dolomites of the tidal flat and lagoon. Uniform δ¹⁸O values ranging between 0⁰/₀₀ and +3⁰/₀₀ were found in all sedimentary facies. It is hypothesized that the primary dolomite with mosaic microstructure nucleated on extracellular polymeric substances secreted by sulphate reducing bacteria. A multi-step process involving sabkha and reflux dolomitization led to partial replacement and overgrowth of the primary dolomite, but replacement and overgrowth were facies-dependent. Dolomites of the landward, clay-rich portion of the sedimentary system were only moderately overgrown during late dolomitization steps, and partly retain an isotopic signature consistent with bacterial sulphate reduction with δ¹³C as low as -6⁰/₀₀. In contrast, dolomites of the marine, clay-free part of the system were probably transformed through sabkha and reflux diagenetic processes into calcian varieties, and exhibit δ¹³C values of ca +3⁰/₀₀. Major shifts of δ¹³C values strictly follow the lateral migration of facies and thus mark transgressions and regressions.]]> Sat 24 Mar 2018 07:24:46 AEDT ]]>