AMS Dating of Rock Art in the Laura Region, Cape York Peninsula, Australia-Protocols and Results of Recent Research

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Introduction

More than a decade ago, a programme of rock art dating was initiated by the authors in the Laura (Quinkan) region of Cape York Peninsula, in collaboration with other regional projects (see Morwood & Hobbs 1995a; Campbell et al. 1996). Since then a substantial body of data has been collected in several phases of field work and analysis (e.g. see Watchman 1990, 1993a, 1998; Cole & Watchman 1992; Watchman & Cole 1992; Cole et al. 1995; Cole 2000). Where possible, the programme has targeted well-studied rock art sites for which there is existing data from archaeological excavations and/or detailed rock art recording. This paper presents protocols and results of recent research, which has extended to selected cultural sites of the East Quinkan Reserve (Figure 1).

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The science of 'direct dating' represents a relatively new development in the study of rock art, involving the application of micro-techniques to investigate the complex processes of rock surface weathering. Some direct dating results have been unexpected, producing interesting debates (see Watchman 2001b for a discussion of this phenomenon; see also Pettitt & Bahn 2003; Valladas & Clottes 2003 for examples of recent debate). Although Watchman has published extensively elsewhere on 'direct' techniques of dating rock art, this paper reports recent protocols in some detail in view of ongoing interest in methodologies of this emerging field of study. In Cape York Peninsula and Australia more generally, the research involves important cultural and ethical considerations as well as sophisticated technical applications and careful contextual analysis.

Methods

As in previous stages of the project, field research was undertaken with the permission of Ang-Gnarra Aboriginal Corporation and the Queensland Environmental Protection Agency (formerly Queensland Department of Environment and Heritage). In 2001, field work on the East Quinkan Reserve was conducted in association with a broader archaeological project sponsored by Ang-Gnarra Aboriginal Corporation. The field team included Aboriginal Elders and advisors from Ang-Gnarra Aboriginal Corporation and Aygarra Timara Land Trust.

Having established access to key sites such as Early Man and Possum rock shelters, the next field task was to identify the presence of mineralised skins (rock surface accretions) in association with selected rock art motifs. It is these mineralised skins that provide the potential for dating by accelerator mass spectrometry radiocarbon (AMS 14C, see Watchman & Jones 1998; Watchman 2000). Typically, the skins are formed on stable rock surfaces (quartzites and sandstones) within the dripline of rock shelters across the tropical regions of Australia, as in the Laura Sandstone Province. Their complex mineral compositions often include carbon-bearing oxalate salts, such as the minerals whewellite and weddellite. These may be removed in layers and the powders treated chemically before submission to dating laboratories for AMS methods (Watchman 2001a). The removal and AMS 14C analysis of sub-samples of rock surface accretions such as oxalate and/or silica skins has been undertaken on previous occasions, as a method of providing age estimates for rock art in Australia and elsewhere (see Watchman 1987, 1993a, 1993b, 1995, 2001b; Cole et al. 1995; Dorn 1997; Watchman et al. 2000, 2001).

Oxalate is a possible product from several carbon pathways of micro-organisms and plants as described in detail by Watchman (2001 a, 2001 b). The formation of oxalate-mineralised skins in northern Australia rock shelters is a matter of ongoing research (e.g. see Watchman et al. 2001), and it is currently believed that the process involves microbiological agents such as certain bacteria and fungi using carbon for growth from contemporary animal body fats and organic acids in rain water. Lichen are not involved in the process and ancient carbon from the underlying rock is not a source for the carbon that exists in the oxalate salts associated with rock art. …