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University of New South Wales (UNSW) 2018

Geochemical signatures of surface environmental changes in two Australian caves : fire signals and organic carbon archives

Bian, Fang

Titre : Geochemical signatures of surface environmental changes in two Australian caves : fire signals and organic carbon archives

Auteur : Bian, Fang

Université de soutenance : University of New South Wales (UNSW)

Grade : Master by Research in Applied Geology 2018

Résumé
Wildfire and climate changes have been broadly reported to generate accumulated effects on ecosystem flora and fauna. Although enormous of studies have been conducted to illustrate these surface events, geochemical evolutions into the subsurface system are difficult to quantify. This thesis aims to combine the isotope analysis, hydrograph analysis and geochemical analysis to provide insights on the karst hydrology and paleoenvironment reconstructions. In short term, wildfire has been reported to modify the burnt surface properties including soil hydrophilicity, and nutrient generation as ashes dissolve. A severe 10 m x 10 m experimental fire was conducted above the Wildman’s Cave in Wombeyan Caves, New South Wales, Australia in May 2016. After the fire, loss of water was observed associating with the decreased drip water d18O value by 6.3 per mille. A lag corresponding to the depth of each site was discovered. Both increased preferential flows and decreased capillary flows were reflected via hydrograph analysis in respect of individual discharge events. Fire-derived volatilization of the soil and ash elements occurred. Bedrock-derived elements decreased due to decreased dissolution. Post-fire monitoring shows a recovery trend based on the stable water isotopes to the initial values in the following half year. Over longer timescales, organic matter in cave stalagmites has the potential to assist in paleoenvironment reconstruction. In this thesis, an innovative d13C analysis method was proposed for entrapped organic matter in stalagmites. Acquired organic d13C from Easter Cave Flowstone 1 (ESF1) were compared with the calcite d13C and d18O. Our results show that this proposed method is reliable for the analysis of organic matter (OM) under constant conditions, with a low average standard deviation (0.2 per mille) and low sample consumption (25 mg). The fluctuations on the time series of calcite d13C for the ESF1 were recognised to be the results from fractionation, without climate and surface vegetation changes factors. As a consequence, in short term, it reveals the story occurring in the soil, with increased preferential flows and decreased capillary flows after fire. In longer timescales, an innovative insight onto the carbon isotope fractionation mechanism and paleoenvironment reconstruction is created. These results have improved current understandings to the importance of surface environments and linked it to the subsurface system.

Mots clés : Geochemistry ; Wildfire ; Paleoenvironment ; Organic matter

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Page publiée le 9 novembre 2019