Cave monitoring to constrain the paleoclimate interpretation of δ18O proxy in speleothems from semi-arid areas
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Date
2015-07-09
Journal Title
Journal ISSN
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Publisher
University of New South Wales and Australian Nuclear Science and Technology Organisation
Abstract
The stable oxygen-isotope ratio (δ 80) is the most commonly used paleoclimate proxy in speleothems (i.e. cave deposits) to reconstruct records of past environmental change. Speleothems are excellent archives as they can be accurately and precisely dated (using the U-Th disequilibrium method) and analysed to a high temporal resolution (i.e. monthly). Over the past decades it has become apparent that the oxygen-isotope signal preserved within speleothems is complex, and reflects changes in the climate, hydrologic pathways, and the depositional environment within the cave. Disentangling the preserved δ180 signal is critical to the correct interpretation of high-resolution records and often requires site-specific hydrological and climatic understanding.
In semi-arid Wellington, NSW we conducted several artificial infiltration experiments to understand the climate-karst-cave interactions driving the δ180 cave drip water composition. Baseline conditions were established from 4-year drip monitoring and 3-year stable isotope analysis of both drip waters and rainfall. The responses inside the cave were monitored at three sites of varying depth (-5 m, -10 and -25 m below the surface). An applied deuterium (D20) tracer was used to mark the artificial infiltration events and the subsurface response was monitored inside the cave. The study revealed that variable evaporative processes in the unsaturated zone dominate the drip water isotopic composition, resulting in enriched drip water composition relative to annually weighted mean rainfall. The 'effectiveness' of infiltration events is highly controlled by the antecedent conditions in the soil zone, often requiring large soil moisture deficits to be surpassed. Physical processes including evapotranspiration, lateral flow and preferential flow are important for water movement in the unsaturated zone, with residence times exceeding 6 months. We suggest that records of past environmental change from speleothems in semi-arid regions are likely to contain information of paleoaridity, the variability of which is influenced by flow path and storage in the unsaturated zone. The wider findings of this study suggest a new approach in semi-arid areas to the traditional climatic interpretation of the δ180 proxy, which is more likely to be overprinted with a highly evaporated signal, strongly affected by the hydrogeology and cave climate and also the potential disequilibrium in the deposition of calcite. All of these factors require careful consideration for climate interpretations from such speleothem records.
Description
Keywords
Caves, Monitoring, Climates, Stable isotopes, Arid lands
Citation
Markowska, M., Baker, A., Andersen, M., Jex, C., Cuthbert, M., Rau, G., Graham, P., Rutlidge, H., Mariethoz, G., Marjo, C., Treble, P., & Edwards, N. (2015). Cave monitoring to constrain the paleoclimate interpretation of δ18O proxy in speleothems from semi-arid areas. Presentation to the 13th Australasian Environment Isotope Conference (AEIC), Sydney, 8-10th July 2015. (pp. 68).