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Article Excerpt
Abstract
A number of archaeologists have suggested that significant climatic change with environmental and social consequences occurred between 1000 and 400 years ago in the Indo-Pacific region. We investigate this premise by examining the archaeological record of changes in hunter-gatherer economies in three geographically distinct coastal regions of tropical northern Australia. These case studies support the argument that Aboriginal mollusc exploitation reflects the altered local ecological habitats that accompanied broader coastal environmental change over the last few thousand years. Overlap between the phases and timing of climatic and behavioural changes within each region suggests that, given regional variation in the nature and of these changes, there was an associated human response to late Holocene climatic variability.
These case studies establish that archaeological and environmental evidence mutually support the argument for climate change influencing cultural change in northern Australia. We suggest that, while a direct physical link between environmental change and the interpretations of significant cultural change in the archaeological record have yet to be demonstrated unambiguously in this region, the analysis of mollusc exploitation has the potential to provide the direct link that is currently missing between changes in climate, environment and human responses over the last millennium.
Keywords: Arnhem Land, hunter-gatherer economies, mollusc exploitation, middens
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Archaeological sites have the potential to act as archives that record significant information on palaeoclimatic conditions to augment standard pollen and coral core palaeoclimate indicators (Sandweiss 2003). This becomes particularly useful in areas where few standard palaeoenvironmental studies exist, as is the case for the tropical north Australian coast, especially for the Late Holocene period. Some archaeologists argue that, in the Indo-Pacific region, significant climatic change between 1000 and 400 years ago had considerable environmental and social consequences. Most of these studies have dealt with horticulturalist societies, however, and the impact on hunter gatherer societies is less-well studied (cf. Habede and David 2004).
In tropical north Australia, radiometric dating indicates that substantial changes occurred in the shell-fishing practices of hunter-gatherer groups after approximately 1000 calibrated years ago (Bourke 2003, 2004; Brockwell et al. 2005; Faulkner 2006; Faulkner and Clarke 2004; Hiscock 1997, 1999). This paper presents an overview of the archaeological record of changes in hunter-gatherer economic and social practices in three geographically distinct regions of tropical northern Australia (Figure 1). This is done so within the context of what is known of significant phases of climate change in order to explore the relationship between human behaviour and climate change in the late Holocene.
Holocene patterns of climatic and environmental change
In the absence of detailed regional or location specific palaeoclimatic data, this discussion draws on information from the broader Indo-Pacific region to provide the context for this paper. It is widely acknowledged that there has been an increase in climatic variability in the Australasian region in the last few thousand years, in particular from approximately 2000 BP to the present (Gagan and Chappell 2000:44; Gagan et al. 1994; Kershaw 1983, 1995; Lees 1992a; Prebble et al. 2005:367-9; Shulmeister 1999:82; Wasson 1986). Many of the longer-term trends in climate change that have occurred during the period spanning the mid Holocene to the present day (Figure 2) are related to the El Nino/Southern Oscillation (ENSO) cycle, which has strongly influenced climatic patterns in Australia (Jones et al. 1999; McGlone et al. 1992; Shulmeister and Lees 1992), and at present represents the principal source of inter-annual climatic variability within the Indo-Pacific region (Allan et al. 1996; Cobb et al. 2003; Diaz and Markgraf 1992; Glantz 1991; Rowland 1999; Tudhope et al. 2001). Sea level changes too are inter-woven with long-term climatic shifts, particularly the intensity of the summer monsoon and cycling periods of aridity and increased precipitation linked to the ENSO cycle. This is due to the proximity of north Australia to the Western Pacific Warm Pool, which is responsible for the largest transfer of heat from the Pacific Ocean into the Indian Ocean and is implicated in the generation of El Nino/La Nina phases of the southern oscillation (Chivas et al. 2001:20; Gagan and Chappell 2000:35; Gagan et al. 2004; Thunell et al. 1994).
[FIGURE 1 OMITTED]
Within these longer-term environmental patterns, there appear to be several punctuated phases of relatively rapid climatic change throughout the Holocene, as well as higher levels of aridity in tropical areas (Figure 2). Most of these climate change events are characterised by polar cooling, tropical aridity and major atmospheric circulation changes (Haberle and David 2004:166-67; Mayewski et al. 2004: 243), events that are seen to occur during the mid to late Holocene in regional Australasian tropical palaeoclimate records (Haberle 1998:8-9; Haberle and David 2004:169).
While the long-term climate change patterns are important for contextualising the degree of variability in climate history throughout the Holocene, possibly the most significant period of climatic change for our purposes relates to the last 1200 years. In the northern hemisphere, there is evidence for a warm and dry period about between 1200 to 700 BP in low latitudes named the "Medieval Warm Period" or "Little Climatic Optimum" (LCO). Similarly, there is evidence for a cool dry period following the LCO between approximately 600 to 100 BP (Jones et al. 2001), referred to as the "Little Ice Age" (LIA). Based on glacial advances in both hemispheres and enhanced polar atmospheric circulation, it is strongly suggested that the LIA was a global scale event (Hendy et al. 2002:1511). However, the timing and nature of these significant events is not well established for the Southern Hemisphere, especially for the tropics (Allen 2006; Gagan et al. 2004; Hendy et al. 2002:1512).)
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The LIA and LCO and long-term El Nino events are interrelated, and have had strong ecological and economic consequences. The best-studied impacts of these processes on ecosystems are in marine environments, where El Nino is correlated with dramatic changes in the abundance and distribution of many organisms, and the collapse of fisheries (Holmgren et al. 2001:89). There are also documented impacts on terrestrial organisms as well, linked to effects on the structure of the vegetation. For example, El Nino events have been linked to the almost complete defoliation of mangrove forests (Haberle 2000:66; Holmgren et al. 2001:90-1). Therefore, ENSO-related climatic oscillations must have had significant impacts on human populations, primarily affecting the resource base in terms of the long and short-term availability, stability and structuring of these resources (e.g. Clarkson 2004:163).
In addition to the effects of climatic events, the resource base for past hunter-gatherers would have been influenced by processes of landscape alteration linked to climatic and sea level changes, which have created drastically different environments over time in Arnhem Land coastal areas. Following the cessation of sea-level rise c. 6000 BP, as extensive coastal plains developed through sedimentation and coastal progradation, the mangroves that had invaded drowned river valleys (the 'Big Swamp Phase') retreated seawards and had mainly disappeared from the floodplains by 4000 years BP (Woodroffe et al. 1985, 1986, 1988:98). There followed a so-called Sinuous Phase, during which meandering palaeochannels were created across the plains, resulting in a mosaic of estuarine, freshwater and mudflat areas (Woodroffe et al. 1986). Continued sedimentation and the slowing of coastal progradation during the Cuspate Phase, c. 2500 years ago, led to a cut-off of the tidal influence of the rivers. Freshwater ponded behind the cheniers and in palaeochannels creating the vast freshwater floodplains and wetlands that are a major feature of the northern coastal plains today (Chappell 1988; Woodroffe et al. 1988).
Coastal processes, of progradation by marine mud accretion and subsequent colonisation by mangroves may be indirectly related to climatic conditions. As explained by Woodroffe et al. (1993; Woodroffe and Mulrennan 1993: 63-5, 98-9), where macrotidal estuaries infill with deposition of tidal sediment from seaward, but where catchment areas are small, wet season flooding may not be large enough to reopen channels and move sediment downstream again. In other words, these estuarine plains develop...
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