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Holocene palaeofire records in a high-level, proximal valley-fill (Wilson Bog), Mount Lofty Ranges, South AustraliaSchool of Earth and Environmental Sciences, University of Wollongong, NSW 2522, Australia
Minerals and Energy Resources Division, Primary Industries and Resources, South Australia (PIRSA), Adelaide SA 5000, Australia
School of Earth and Environmental Sciences, University of Wollongong, NSW 2522, Australia
School of Earth and Environmental Sciences, University of Wollongong, NSW 2522, Australia
School of Earth and Environmental Sciences, University of Adelaide, Waite Campus, SA 5064, Australia, Bushfire Cooperative Research Centre, Level 5/340 Albert Street, East Melbourne, VIC 3002, Australia
School of Earth and Environmental Sciences, University of Wollongong, NSW 2522, Australia
School of Earth and Environmental Sciences, University of Wollongong, NSW 2522, Australia
School of Earth and Environmental Sciences, University of Wollongong, NSW 2522, Australia
School of Natural and Built Environments, University of South Australia, Mawson Lakes Campus, SA 5095, Australia An elevated valley-fill peat bog (Wilson Bog) near Mount Lofty, South Australia, failed in November 2005 following a flooding event, and exposed representative sections of the sediment infill. Two distinct units were revealed: 2 m of coarse-grained, siliciclastic sand/gravel, overlain by 2 m of peat. A simple charcoal extraction technique based on floatation and skimming was developed to extract coarse charcoal from coarse-grained gravels to determine the palaeofire record at a proximal site of sedimentation. Optically stimulated luminescence (OSL) dating of basal sediments revealed a minimum age of deposition of 7.02 +0.50—0.56 ka, while the oldest charcoal peak yielded a radiocarbon age of 6000—5740 cal. yr BP. The lower half of the siliciclastic unit contains three distinct charcoal peaks suggesting there were infrequent but intense fires associated with wetter conditions during the Holocene climatic optimum 8000—5000 years ago. The period from 4000 to 2000 cal. yr BP is characterised by more frequent charcoal peaks and higher background levels of charcoal, which is consistent with more regular but less intense fires during drier, cooler conditions. The sharp transition from siliciclastic sedimentation to peat formation began ~1200 cal. yr BP, which may relate to a return to wetter conditions. However, fire frequency appears to have increased in this time suggesting augmentation by anthropogenic or ENSO-related factors. Charcoal-rich layers in the siliciclastic unit are associated with poorly sorted, bimodal sediments with high proportions of clay, silt and gravel, which supports the hypothesis that there is an association between past fire events and rapid, coarse-grained, post-fire aggradation. By analogy with active colluvial aggradation following recent fires at nearby Mount Bold, it is evident that fire plays a significant role in hillslope destabilization and subsequent sediment movement, leading to rapid valley-fill aggradation — a chain of events to which we apply the term ‘pyrocolluviation’.
Key Words: Fire • charcoal • valley-fill • peat bog • OSL • pyrocolluviation.
The Holocene, Vol. 19, No. 7,
1017-1029 (2009) |
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