| Popis: |
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. |
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