1. Department of Geographical & Environmental Studies, Adelaide University, SA., 5005.
Robin Clark's Holocene record from Lashmars Lagoon (Singh et al., 1981) has challenged the widely used interpretation for peaks in charcoal abundance - the arrival, or increased activity, of indigenous people (also used in Singh et al., 1981). Specifically, the increase in charcoal coincident with the departure of people from Kangaroo Island at 2.5 ka BP was interpreted as a shift from frequent to infrequent burning which enhanced fuel loads to increase the volume of material burnt and so the volume accumulated in the Lagoon's sediments. This was despite negligible change in the pollen record across this period. Recent diatom analyses reveal considerable hydrological and water quality change associated with the change in charcoal accumulation (Illman, 1998). The shift to shallow saline conditions may have provided the REDOX conditions suitable for the production of blackened particles. Such conditions probably prevailed in certain Victorian crater lakes at the beginning of the Holocene and may have produced peaks in pyrite at West Basin (Gell et al., 1994) and 'charcoal' at Lake Cartcarrong' (Walkley, unpublished data). Clark (1984) revealed a four-fold increase in material digested by nitric acid - the treatment recommended by Singh et al. (1981) to remove dark organic remains other than charcoal - after 2.5 ka BP at Lashmars Lagoon. Hypothesis 1: climate change at 2.5 ka BP increased the salinity of Lashmars Lagoon producing the limnological conditions which produced high volumes of dark, angular fragments.
Recent developments in the use of 13C nuclear magnetic resonance (NMR) to recognise non-living soil organic matter (NSOM) and the isolation of charcoal particles in soil using photo-oxidation (Skjemstad et al., 1998) have enhanced understanding of the taphonomy of charcoal particles. Charcoal particles appear to be largely derived from burnt grasses and are quite rapidly broken down into the clay/fine silt fraction with which they are readily transported horizontally, and vertically in the soil profile (Janik, pers. comm.). Australian soils contain a large pool of inert carbon, almost all of which is charcoal. "Soils developed on alluvium, therefore, often contain high concentrations of charcoal not because of on-site burning, but because these soils are zones of accumulation " (Skjemstad et al., 1998: 675). Hypothesis 2: climate change at 2.5 ka BP mobilised old, regional charcoal which was transported in the clay fraction to sites of sediment accumulation including Lashmars Lagoon.
Hypothesis Testing: quantify charcoal using NMR and photo-oxidation from Holocene records at Lashmars Lagoon and Boat Harbour Creek, Fleurieu Peninsula.
Postscript: other sites reported in Singh et al. (1981) show increased charcoal concentrations in association with dry interglacials.
Clark, R.L. 1984. Effects on charcoal of pollen preparation procedures. Pollen et Spores, XXVI: 559-576.
Gell, P., Barker, P., De Deckker, P., Last, W. & Jelicic, L. 1994. The Holocene history of West Basin Lake, Victoria, Australia; chemical changes based on fossil biota and sediment mineralogy. J. Paleolimn. 12: 235-258.
Illman, M.A. 1998. Reconstruction of the Late Holocene palaeosalinity and palaeoecology of Lashmar's Lagoon, Kangaroo Island, South Australia. . In Newall, P. (Ed) Proceedings of ëDiatoms & Nutrients - a Taxonomic Workshopí. Deakin University, Warrnambool, 1-3 Feb. 1997: 41-51.
Singh, G., Kershaw, A.P. & Clark, R.L. 1981. Quaternary vegetation and fire history in Australia. (In) Gill, A.M., Groves, R.H. & Noble, I.R. (Eds). Fire and the Australian Biota. Australian Academy of Science, Canberra: 23-54.
Skjemstad, J.O., Janik, L.J. & Taylor, J.A. 1998. Non-living soil organic matter: what do we know about it? A. J. Exp. Ag., 38: 667-80.