Determining the timing and pattern of human colonisation in Australia: Proposals for radiocarbon dating 'early' sequences Chris S.M. ~ u r n e ~and l ? Michael I. ~ i r d l Abstract At present, radiocarbon dating estimates of the age of human occupation of archaeological sites in Australia are virtually all based on single, or comparatively few, radiocarbon dates at the depth associated with the earliest cultural remains. Several new radiocarbon 'calibration' curves for the early occupation period of Australia (>30,000 14CBP), some of which suggest significantly rapid and extreme shifts in apparent radiocarbon age, have become available in recent years. While there are currently major discrepancies between these curves, improvement and regular revision of the 'accepted' calibration curve is a recognised priority within the international radiocarbon community. Thus, the opportunity for more precise interrogation and confident interpretation of the conventional radiocarbon timescale is likely to emerge in the future. In the meantime, we suggest more intensive dating of the critical initial occupation levels of stratigraphic sequences to allow maximum exploitation of any future radiocarbon calibration curve structure (e.g. using 'wigglematching' andlor Bayesian statistics). Such an approach should assist in the future development of a more robust model of the patteming of human colonisation of Australia.
In addition to these results, dating of environmental change inferred to be as a consequence of human arrival support the older ages. These events include extinction of Genyornis newtoni and disruption of the vegetation cover in the Lake E re basin at approximately 50,000 cal. BP (based on AAR, l&, OSL and U-series; Miller et al. 1999; Johnson et al. 1999), continental-wide extinction of megafauna centred on 46,400 cal. BP (based on OSL and U-series; Roberts et al. 2001) and increases in burning recorded at Lynch's Crater and ODP-820 at around 45,000 years BP (based on I4cand oxygen isotope stratigraphy respectively; 120"E
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The conventional radiocarbon chronology for Australian human colonisation suggests an arrival time of approximately 40,000 I4C BP (all age estimates quoted in this paper are specified thus: calendarlcalibrated years = cal. BP; radiocarbon years (uncalibrated) = 14C BP), as suggested from archaeological sites such as Upper Swan (Pearce and Barbetti l98 l), Carpenter's Gap 1 (O'Connor 1995), Puritjarra (Smith 1987), Wareen Cave (Allen 1996:277), Ngarrabullgan (David et al. 1997) and Mimbi Caves (Balme 2001) (Fig. 1). In Figure 1 contrast, altemative dating techniques, such as luminescence and electron spin resonance (ESR) and recent developments in the pretreatment and graphitisation of samples for radiocarbon dating, at Malakunanja 11, Nauwalabilia I, Mungo 111 and Devil's Lair, have implied human arrival by at least 4555,000 cal. BP (Fig. 1) (Roberts et al. 1990, 1994a, 1998; Thorne et al. 1999; Turney et al. 2001a). P
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School of Archaeology and Palaeoecology, Queen's University, Belfast, BT7 INN, U.K. Email:
[email protected] Research School of Earth Sciences and Research School of Biological Sciences, Australian National University, Canberra, ACT 0200, Australia
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Key early occupation sites discussed in text. Sites given in solid circles ( 0 ) are those investigated using ABOX-SC 1% dating. Open circles (0) are those sites reporting original radiocarbon ages or dated using alternative methods, such as OSL or ESR. Moss and Kershaw 2000; Turney et al. 2001b; Figure l). The ages obtained by these new methods are significantly beyond the age of arrival of people suggested by traditional radiocarbon pretreatment methods and are considered unlikely to be a result of significant post-depositional disturbance of artefacts (Bird et al. 2002; Turney et al. 2001~).Several workers have suggested that the difference is due to an effective limit in the age at which conventional radiocarbon dating is reliable, the so-called 'radiocarbon barrier' (e.g. Roberts et al. 1994b). Although the issues discussed here are primarily aimed at the debate concerning human settlement of Australia, the issues are equally
Determining the timing and pattern of human colonisation in Australia
appropriate to other areas of the world, notably the expansion of Homo sapiens 'out of Africa' into Europe and south-east Asia and their interaction with Neanderthals and, potentially, with Homo erectus at this time (Duarte et al. 1999; Swisher et al. 1996). As a result of the apparent mismatch between colonisation dates proposed on the basis of ages produced by different techniques, work has concentrated on increasing the effective limit of the radiocarbon technique. Developments in sample pretreatment and line technology, using acid-base wet oxidation and stepped combustion (ABOX-SC) of natural charcoals, have provided reliable radiocarbon ages back to 55,000 W BP (Bird et al. 1999). This suggests that all but the earliest known human occupation sites can potentially be reliably dated using radiocarbon. Numerous sites have now been re-dated using ABOX-SC 14C across Australia that are considered to provide the earliest evidence of continental-wide occupation (Fig. l), but with mixed results (Fifield et al. 2001). In some instances, the ABOX-SC ages have made no significant difference to the timing of the earliest cultural horizons (e.g. Carpenter's Gap 1) (Fig. l), suggesting little or no modern carbon contamination (Bird et al. 1999). In others, the ages have significantly increased (e.g. Puritjarra, see Smith et al. 2001) indicating contamination by younger carbon is a significant problem -for some samples. Most recently, comprehensive dating of Devil's Lair in southwestern Australia, has demonstrated that not only can radiocarbon ages based upon charcoals from occupation levels significantly exceed 40,000 W years, but that excellent agreement can be reached between radiocarbon ages (using ABOX-SC), OSL and ESR ages (Fifield et al. 2001; Turney et al. 2001a) (Fig. 1).Overall however, the results in Figure 1 are enigmatic in that despite improving the accuracy of the radiocarbon ages associated with the earliest cultural horizons, we still cannot determine the fine details of settlement.
Several attempts have been made to determine the pattern of Australian settlement (Birdsell 1977; Bowdler 1977) but these have generally been constrained by a paucity of sites and the relatively low accuracy and precision of the occupation ages reported (Webb 1998). Even with the increased accuracy associated with improved dating techniques (of which the ages in Figure 1 represent only a small proportion of late Pleistocene sites; Smith and Sharp 1993), the dating of sites provides ages that are nearly all statistically indistinguishable at the 20 confidence limits, indicating that the precision is currently insufficient to resolve the finer detail of settlement patterns. However, recent studies into possibly large changes in atmospheric 14C content in the period during which colonisation of Australia is thought to have been occurring suggests that such changes may provide a means by which we can further refine our understanding the pattern of settlement.
Calibration curves In an attempt to extend the dendrochronological radiocarbon calibration beyond the Holocene and Lateglacial (e.g. Stuiver et al. 1998), several 'calibration' curves have now been produced that span part of or all of the period back to 50,000 years BP (Fig. 2). These curves include Lake Suigetsu (Kitagawa and van der Plicht 2000), the Nordic Sea (Voelker et al. 2000), Papua New Guinea (Yokoyama et al. 2000) and the Bahamas (Beck et al. 2001), and all display critical features in the radiocarbon Calibration curve relevant to the period of Australian human colonisation, although relatively coarse in resolution, compared to Holocene and Late-glacial calibration curves. The annually laminated sequence from Lake Suigetsu (Kitagawa and van der Plicht 1998, 2000) provides the most detailed radiocarbon curve, through the Late-glacial and back to approximately 37,000 cal. BP. Although there are uncertainties regarding the calendar timescale and the possibility of hiatuses (van der Plicht 1999), there is evidence for periods of constant radiocarbon age ('plateaux') throughout this time, relating to changes in the global environment and cosmogenic isotope production rates (Kitagawa and van der Plicht 1998). Alternatively, Voelker et al. (2000) report a calibration curve between 50-10,000 cal. BP, generated by correlqting Nordic Sea 6180 records to the GISP2 isotope chronology and measuring .the radiocarbon content of surface-dwelling foraminifera. Although uncertainties exist with regard 10 15 20 25 30 35 40 45 50 55 60 to the marine reservoir age and Calendar age (ka cal. B.P.) the GISP2 chronology, beyond Figure 2 Radiocarbon 'calibration' curves produced from Lake Suigetsu (solid line; 30,000 14C BP the offset Kitagawa and van der Plicht 2000),the Nordic Sea (grey line; Voelker et al. 2000), between the radiocarbon and and the Bahamas (vertical line infill; Beck et al. 2001). Confidence limits given at sidereal timescales appear to be negligible. In 95%. For the Nordic Sea, a marine reservoir age of 400 years is assumed and no statistically uncertainty is given for the 'calendar' age. addition, a significant -
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radiocarbon plateau is recorded near 34-33.000 BP (Fig. 2 ) , producing 14C ages that are statistically indistinguishable for 6000 calendar years from 41,000 to 35.000 cal. BP. Such a plateau is broadly coincident with the Leschamp geomagnetic low recorded in the same sequence and in 1°Be peaks found in marine and ice data-sets. indicating that the observed plateau resulted from increased 13C production in the upper atmosphere. The existence of such plateaux is significant as it may be interpreted in an archaeological sequence as an apparent period of high sedimentation and regionally may suggest that an artificially high number of sites were being occupied at the same time. In addition to radiocarbon plateaux, there is now strong evidence of extreme shifts in atmospheric radiocarbon content that are superimposed on the calibration curve, postulated to be a result (at least partially) of changes in the global thermohaline circulation (Beck et al. 2001; Yokoyama et al. 2000). The most intense excursion of these is recorded in a speleothem record from the Bahamas (Beck et al. 2001) that occurs between 44,300 and 43,300 cal. BP and appears to cause a 10,000 year offset with radiocarbon ages. The calendar ages in this record are provided by paired TIMS U and Th analyses and although there are uncertainties regarding the degree of dead carbon contribution to the speleothem, similar excursions (albeit smaller) are also recorded in paired U-series and 14C dating of corals from Papua New Guinea (Yokoyama et al. 2000), suggesting such events are real. The implications of this work are considerable. If corroborated by other studies, not only will 'age inversions' be expected in both marine and terrestrial sequences, but these might appear close to the 'limits' of radiocarbon dating and therefore be interpreted as indicating that inverted ages are not reliable. Although the precise timing of the above events, and even their existence is still debatable, the overall structure is generally observed in all of the calibration curves indicating that differences in the sidereal timescale need be resolved before such features can be chronologically fixed. Such radiocarbon features have important implications for many of the earliest radiocarbon-dated sequences in Australia, particularly if we are to understand more fdly the pattern of human colonisation during the late Pleistocene. Large, rapid excursions in atmospheric radiocarbon can be interpreted as time-parallel marker horizons, and should be detected globally due to the relatively rapid mixing of the atmosphere. Although there is no general consensus for a detailed calibration curve spanning the entire radiocarbon timescale, identifying 14C features within archaeological sequences can still be undertaken prior to an agreed calibration curve. A further implication of the Beck et al. (2001) data is that large inversions in apparent radiocarbon age are to be expected in sedimentary sequences. It is therefore possible that published radiocarbon ages that have been considered 'doubtful' due to apparent stratigraphic reversals, may in fact be recording vital chronological information. Only where we can be confident that the radiocarbon backgrounds are low (equivalent to greater than 50,000 I4C years) can we be confident that such shifts are real, rather than as a result of contamination.
Proposals for improving the precision of the timing of early human colonisation A range of options is available for developing geochronological models on a site-by-site basis using radiocarbon. In the long-term, an agreed radiocarbon calibration curve will allow the conversion of an individual .-iustralian .?rchaeology, Number 53, 2002
radiocarbon age estimate to calendar time. However, as noted above, the potentially large changes in atmospheric radiocarbon content and the typically large errors associated with estimating the radiocarbon activity of samples >30,000 14C BP inevitably means any individual calibrated age will have a significantly large error range, which even at 10 (68% confidence limits) will make any meaningful interpretation of the pattern of Australian settlement virtually impossible. An alternative approach to the above is to identify the inflections at the beginning and end of postulated radiocarbon plateaux. For instance, numerous 'early' archaeological sites in Australia have had basal occupation layers dated to between 35-30,000 14C years BP. Virtually all of the available calibration curves appear to record a significant radiocarbon plateau at around this time (Fig. 2) making the calibrated sidereal age potentially anywhere between 42,000 and 32,000 cal. BP (assuming no error term on an age). Thus, it is problematic to determine accurately whether sites of this age were all settled over a very short interval or over a period of -10,000 years. Although such an age suggests considerable antiquity it is not possible, on the basis of a single age estimate, to make a more precise interpretation. A single age, though it might be concordant with the occupation level and in stratigraphic order with other ages from the sequence cannot provide a precise age for occupation in this interval. However, a comprehensive suite of radiocarbon ages through the critical portions of sequence should allow the identification of plateaux and their inflections, against which stratigraphic information can be compared. Wherever possible, multiple 14C ages should be obtained in conjunction with other dating techniques to develop as robust an age model as possible. In addition to the above, the short-duration (ca. 1000 years) massive deviations in atmospheric radiocarbon content (ca. 9000 years difference between 1% and calendar at 44,000 cal. BP) as reported from the Bahamas (Beck et al. 2001) provide global markers. Virtually irrespective of the level of precision of a 14C age estimate such an event should be detected in a stratigraphic sequence, providing a timeparallel marker horizon against which archaeological and/or environmental change can be compared. Indeed, because of the relatively rapid mixing of the atmosphere, such deviations (if real) should be found globally, allowing comparisons between Australia and the rest of the world and may therefore prove to be one of the most exciting recent developments in radiocarbon dating. Ideally, sequences should be dated using a range of techniques to give confidence in the geochronological model developed. If such an approach is taken, Bayesian statistics are available to help further constrain the age model on any calibration curve (Buck et al. 1991). This method allows the incorporation of other ages and stratigraphic information to identify statistical outliers, improving the precision of individual radiocarbon ages for calibrating. Using Bayesian statistics, the analysis can be constrained by recognising that the radiocarbon (and other) ages were obtained in stratigraphic order. Thus, a set of prior (unconstrained) and posterior (constrained) probability distributions can be generated, with the difference expressed as an agreement index. Strong agreement would give a value of 100% or more, allowing a methodology for rejecting individual ages and series of ages below a value of 60% (Ramsey 1998). The major benefit of this approach is in the improvement it can 3
Determining the timing and pattern of human colonisation in Australia
make to the individual precision on a calibrated radiocarbon age. However, such an approach requires an accepted calibration curve which is not yet available at present and must therefore remain an ideal for the future.
Conclusions With the continued refinement of radiocarbon calibration curves to 50,000 cal. BP and beyond, it should be possible comprehensively to date sequences (assuming negligible post-depositional movement of sediments and samples in situ) above and below the occupation levels and then match the ages to observed variations in atmospheric 14C. Better still, multi-dating using a suite of techniques including radiocarbon dating can further exploit a calibration curve through the application of Bayesian statistical methods. Such approaches can allow a relatively precise determination of occupation despite the existence of plateaux in the radiocarbon calibration curve, particularly in the 50-30,000 cal. BP range, a critical period in Australian archaeology. Although the errors of individual ages appear to limit any potential for more tightly defining the timing of occupation of individual sites, the proposals outlined above provide an opportunity to use the variations in atmospheric '4C to best advantage. Even if no accepted calibration curve becomes available for a further decade or more, sampling and analysis along the lines described above can be conducted now. The acquisition of comprehensive suites of 14C analyses for critical occupation intervals will ultimately allow calibration thqough refined calibration curves that become available in the future.
Acknowledgements We wish to thank Jim Allen, John Chappell, Charles Dortch, Rainer Grlln, Rhys Jones, Peter Kershaw, Giff Miller, Bert Roberts and Mike Smith for many informative discussions. CT and MIB acknowledge the provision of fellowships from the Australian Research Council. Bruno David and an anonymous referee provided helpful comments.
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Australia's oldest human remains: Age of the Lake Mungo 3 skeleton. Journal of Human Evolution 36: 59 1-6 12. Turney, C.S.M., Bird, M.I., Fifield, L.K., Roberts, R.G., Smith, M.A., Dortch, C.E., Grun, R., Lawson, E., Ayliffe, L.K., Miller, G.H., Dortch, J. and Cresswell, R.G. 2001a Early human occupation at Devil's Lair, southwestern Australia 50,000 years ago. Quaternary Research 55: 3-13. Turney, C.S.M., Kershaw. A.P., Moss, P., Bird, M.I., Fifield, L.K., Cresswell, R.G., Santos, G.M., di Tada, M.L., Hausladen, P.A. and Zhou, Y. 2001 b Redating the onset of burning at Lynch's Crater (North Queensland): Implications for human settlement in Australia. Journal of Quaternary Science 16: 767-77 1. Turney, C.S.M., Bird, M.I. and Roberts, R.G. 2001c Elemental 6'3C at Allen's Cave, Nullarbor Plain, Australia: Assessing post-depositional disturbance and reconstructing past environments. Journal of Quaternary Science 16: 779-784. van der Plicht, J. 1999 Radiocarbon calibration for the Middle/Upper Palaeolithic: A comment. Antiquity 73: 119-123. Voelker, A.H.L., Grootes, P.M., Nadeau, M.-J., and Sarnthein, M. 2000 Radiocarbon levels in the Iceland Sea from 25-53 kyr and their link to the Earth's magnetic field intensity. Radiocarbon 42: 437-452. Webb, R.E. 1998 Problems with radiometric "time": Dating the initial human colonization of Sahul. Radiocarbon 40: 749758. Yokoyama, Y., Esat, T.M., Lambeck, K. and Fifield, L.K. 2000 Last ice age millennia1 scale climate changes recorded in Huon Peninsula corals. Radiocarbon 42 : 383-40 1.
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