Little Llangothlin Lagoon on the New England Tablelands of northeast New South Wales possesses the most detailed and best verified [sup.210]Pb chronology yet available in Australia. Recent criticisms of the length of the record are shown to be based on a faulty understanding of the principles of [sup.210]Pb dating. Attempts to revise the chronology of the lower part of the dated sequence by several decades must be rejected given that (a) fundamentally dissimilar chronological models yield ages that are statistically indistinguishable and (b) the most extreme manipulation of the modelling data fails to alter the basal dates in the profile by more than three years. The most telling criticism of the revisionist view, however, comes from the exact concordance between the dates from the basal part of the sequence, the historical date of official European contact and the massive changes in palynology, geochemistry and soil erosion resulting from that contact.
The thesis that environmental disturbance immediately prior to the time of official European contact in Australia was the result of human activity is supported by a wealth of documentary evidence revealing the illegal or unsanctioned presence of Europeans throughout much of southern and eastern Australia years before official records began. Likewise, it is clear that many elements of the pre-contact Australian environment, including certain of its soils, were fragile and susceptible to rapid and dramatic disturbance under the impact of European land use. Finally, there is convincing evidence of stable chemical and mineralogical conditions in several southeast Australian lakes throughout the last millennium or more, conditions that were altered catastrophically with the arrival of the first Europeans and their stock.
Gale and Haworth's (2002) paper on human environmental disturbance in Australia prior to official European contact used information derived from lake sediments to add an additional dimension to our understanding of the early colonial history of the continent. Well-dated sedimentary sequences from the New England Tablelands of northeast New South Wales (Figure 1) yielded evidence of enhanced rates of soil erosion and disturbance to lake sediment chemistry perhaps decades before the accepted date of European arrival in the 1830s. This disturbance is unlikely to have been a consequence of natural processes. Instead, it is likely to indicate the presence of either Europeans or the ripples of their culture in New England well before the official date of settlement. These findings provide us with information that is not easily obtained through conventional documentary sources. They have implications for our comprehension of the timing and processes of European colonisation of the continent and for our understanding of the long-term response of the biophysical environment to human impacts. Tibby offers a number of comments on this work and on our broader research on the late Holocene environmental history of Australia. In the first part of this reply, we address his specific criticisms. In the second part, we tackle the more general issues arising from his discussion.
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[sup.210]Pb as a chronological tool
Tibby points out that the [sup.210]Pb chronology at Little Llangothlin Lagoon in northeast New South Wales extends back to 1806 [+ or -] 12, that this lies beyond the generally accepted range of 120-150 years of the technique and that this alone renders inappropriate any attempt to differentiate official and pre-official contact at the site. Although introductory reviews of the [sup.210]Pb procedure not unreasonably provide a single value for the range over which the technique is applicable, [sup.210]Pb analysis cannot be compared with more conventional isotopic dating techniques (indeed, strictly, it is not a dating technique at all). Thus, the activity of any sample down a sequence is dependent not only on time but also on the initial activity of the material. …