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Chungará (Arica)

versión On-line ISSN 0717-7356

Chungará (Arica) v.40 n.especial Arica  2008

http://dx.doi.org/10.4067/S0717-73562008000300003 

Volumen 40 Número Especial, 2008. Páginas 245-259
Chungara, Revista de Antropología Chilena

ARQUEOLOGÍA

 

THE IMPACT OF ENSO IN THE ATACAMA DESERT AND AUSTRALIAN ARID ZONE: EXPLORATORY TIME-SERIES ANALYSIS OF ARCHAEOLOGICAL RECORDS1

EL IMPACTO DE ENSO EN EL DESIERTO DE ATACAMA Y LA ZONA ÁRIDA DE AUSTRALIA: ANÁLISIS EXPLORATORIOS DE SERIES TEMPORALES ARQUEOLÓGICAS

 

Alan Williams2, Calogero M. Santoro3, Michael A. Smith4, and Claudio Latorre5

2 HLA-Envirosciences Pty Limited, Level 5, 828 Pacific Highway, Gordon NSW 2072. Current address: Fenner School of Environment and Society, Australian National University, Canberra ACT 0200, Australia, alanw@arksolutions.com.au
3 Instituto de Alta Investigación, Departamento de Antropología, Universidad de Tarapacá, and Centro de Investigaciones del Hombre en el Desierto, Arica, Chile, calogero_santoro@yahoo.com
4 Address for Correspondence: National Museum of Australia, GPO Box 1901, Canberra ACT 2601. m.smith@nma.gov.au. Tel 61-2-62085335 Fax 61-2-62085130.
5 CASEB/Departamento de Ecología, Pontificia Universidad Católica de Chile, Alameda 340, Casilla 114-D. Institute of Ecology & Biodiversity, Casilla 653, Santiago, Chile, clatorre@bio.puc.cl


A comparison of archaeological data in the Atacama Desert and Australian arid zone shows the impact of the El Niño-Southern Oscillation (ENSO) over the last 5,000 years. Using a dataset of > 1400 radiocarbon dates from archaeological sites across the two regions as a proxy for population change, we develop radiocarbon density plots, which are then used to explore the responses of these prehistoric populations to ENSO climatic variability. Under an ENSO regime, precipitation is in anti-phase between Australia and coastal Chile. As ENSO also impacts marine resource productivity in Chile and advection of moisture from the Amazon Basin, the net effects of ENSO on subsistence economies on either side of the Tropical Pacific should be positively correlated. This is confirmed by cross-spectral analysis of the radiocarbon density plots, which shows that population responses on either side of the Tropical Pacific are synchronous (r = > 0.82). Both the Australian and Atacama desert records show a general increase in population from about 13 cal kyr BP, increasing through the mid-Holocene climatic optimum. Following the intensification of ENSO around 3.7 cal kyr BP, we can correlate 'boom and bust' cycles of occupation on both sides of the Pacific, including the collapse of the Atacama desert coastal economy and cultural system at 3 cal kyr BP and the decline of both the Atacama highlands and Central Australian dryland populations between 3-2 cal kyr BP. After 2 cal kyr BP adaptive responses to ENSO varied between these regions, though all dryland populations show resurgence in occupation.

Key words: Atacama Desert, Australian arid zone, population history, post-glacial time-series analysis, radiocarbon probability plots, ENSO variability, human-environmental interactions.


La comparación de datos arqueológicos del desierto de Atacama y de la zona árida de Australia muestra el impacto de El Niño-Oscilación del Sur en los últimos 5.000 años. Con base en un listado de > 1.400 dataciones radiocarbónicas de ambas regiones, utilizadas como fuente indirecta de fluctuaciones de la población, se realizaron gráficos de densidad de las dataciones para explorar las respuestas de las poblaciones prehistóricas a las variaciones climáticas provocadas por ENSO. Bajo un régimen ENSO las precipitaciones se encuentran en antefase entre Australia y la costa del norte de Chile. Dado que ENSO impacta en la productividad de los recursos marinos y suprime el ingreso de humedad de la cuenca del Amazonas, su efecto en las economías de subsistencia de ambos lados del Pacífico Tropical debería correlacionarse positivamente. Esto se confirma a través de análisis cruzados de espectros de gráficos de densidad de radiocarbonos, lo que muestra fluctuaciones sincrónicas de la población en ambos lados del Pacífico Tropical (r = > 0,82). Las bases de datos de Australia y del desierto de Atacama muestran un aumento gradual de la población desde los 13 mil cal años a.p., y un incremento importante durante el óptimo climático del Holoceno medio. La intensificación de ENSO a partir de los 3,7 cal años a.p. se correlaciona con ciclos de florecimiento y declinación de la ocupación humana en ambos lados del Pacífico, incluyendo el colapso del sistema económico y cultural en la costa del desierto de Atacama alrededor de 3 mil años cal a.p. y la declinación de la población en las zonas interiores áridas del Atacama y del centro de Australia, entre 3-2 mil años cal a.p. Posterior a 2 mil años cal a.p. las respuestas adaptativas varían entre ambas regiones, aunque se produce una recuperación general de las poblaciones en sus zonas áridas interiores.

Palabras claves: desierto de Atacama, zona árida de Australia, historias de población, análisis de series temporales postglaciales, gráficos de probabilidad de radiocarbonos, variabilidad ENSO, interacción humana y medio ambiente.


 

The El Nifio-Southern Oscillation (ENSO) is a major driver of climatic variability on both sides of the Paciñc, linking the environmental histories of the Atacama Desert and Australian drylands. In this paper, we ask whether the impact of ENSO can be detected in archaeological records from these diverse regions. We use time-series analysis ofradiocarbon data from archaeological sites in the Atacama Desert and in the Australian arid zone to reconstruct and compare the long-term population history of these regions. Changing frequencies of 14C ages from archaeological sites serve as a rough proxy for the timing and direction of population change, as well as a broad indication of differences in relative intensity of human activity between regions.

It is important to move beyond regional analy-ses when looking at global climatic phenomena such as ENSO. This study is the ñrst attempt to compare archaeological records of ENSO on both sides of the Walker Circulation (in which ENSO is embedded), and as such is explicitly exploratory and comparative in approach. In the Atacama Desert, the intensiñcation of the current ENSO cycle is reflected in a major decline in the use of the desert coast after 3 cal kyr BP and greater use of the interior. In Australian drylands, there is a widespread population collapse around 3 cal kyr BP that we attribute to the initial impact of the current pattern of ENSO. The correlation in the timing of abrupt archaeological shifts on both sides of the Paciñc reinforces the interpretation that these changes reflect the zonal impact of ENSO-driven climate variability.

The Walker Circulation and ENSO

The Walker Circulation is a pattern of zonal airflow across the tropical south Pacific driven by differences in sea surface temperature (SST) between the west and east of this región (Sturman and Tapper 2006). High SST and rising air over Indonesia genérate a high-level flow of air that subsides off the west coast of South América and returns as easterly surface winds. This zonal atmospheric thermal circulation cell is coupled to westward circulation of surface waters, promoting upwelling of nutrient-rich cold water off the coasts of Perú and Chile. A strong Walker Circulation promotes convective rainfall over northern and eastern Australia and suppresses it over the coastal regions of Perú and Chile, but the system is inherently unstable and contributes to climatic variability and extremes of flood and drought on both sides of the Paciñc. The contrast between SSTs across the región (measured by the Southern Oscillation índex or SOI) determines the strength of the Walker Circulation. Fluctuations are known as the El Nifio-Southern Oscillation (ENSO). El Niño was ñrst described and recognized as a climatic anomaly in 1890, by Federico Pezet a geographer in Perú, and it was already well known amongst Peruvian ñshermen and sailors (Glantz 1996:3, 13; Frézier 1713). A strong +SOI (La Niña) is generally associated with heavy rainfall over Australia, dry conditions on the coast of Chile, and wet conditions in the altiplano. When the SOI is negative (an El Niño), the circulation weakens or reverses, suppressing the upwelling of cold waters off the coast of South América, and creating drought in Australia, warmer wetter conditions in coastal Perú and Chile, and dry conditions in the altiplano. Recent evidence suggests that the Interdecadal Paciñc Oscillation (IPO) and Indian Ocean Dipole (IOD) modify the extent to which ENSO affects Australian rainfall (Verdón et al. 2006; Samuel et al. 2006) but there is currently little data on their interplay with palaeo-ENSO (D' Arrigo et al. 2005).

Although palaeo-climatic data suggest that ENSO was a feature of global climate during past warm periods, most studies indicate that the current cycle dates from about 5 cal kyr BP and significantly intensiñed after 3.7-3.0 cal kyr BP (Andrus et al. 2002; Carré et al. 2005; Donders et al. 2007; Fontugne et al. 1999; Gagan et al. 2004; Keefer et al. 1998; Koutavas et al. 2002; Koutavas et al. 2006; Rein et al. 2005; Riedinger et al. 2002; Sandweiss et al. 1996; 2001; Shulmeister 1999). This is in line with predictions from climatic models (Clement et al. 1999). However, a few studies suggest an earlier onset at 7-6.5 cal kyr BP, using data from Laguna Pallcacocha in Ecuador (Moy et al. 2002; Rodbell et al. 1999) or from the Western Equatorial Paciñc (Tudhope et al. 2001). While such an early date for the onset of ENSO is yet to be corroborated, these studies also show a switch to a more vigorous pattern of climatic variability after 5-3.0 cal kyr BP.

Historical Impact of ENSO

In northern Chile, historical El Niño events are recorded as having a major impact on coastal waters, causing the collapse of local fisheries, mass mortality of marine organisms and birds, red tide, torrential rainfall, erosión of coastal lowlands and widespread flooding (Quinn and Neal 1992). The major effects of ENSO are on the marine biomass along the coast. El Niño events disrupt the trophic structure of marine ecosystems (Glynn 1988); créate a pulse of severe drought in the highlands; and extreme rain in coastal regions of Perú and northern Chile. Between 1819 and 1991 historical records show 32 episodes of rain in the normally dry Atacama Desert. Of these, 21 were linked to El Niño (Ortlieb 1995:535), though there is no direct relationship between the magnitude of rain events on the coast and the intensity of ENSO (Aceituno 1992; Caviedes and Waylen 1987; Hocquenghem and Ortlieb 1992; cf. Quinn et al. 1987).

In contrast, on the altiplano -SOI events are associated with dry years. An intensiñed subtropical jetstream blocks the advection of moisture from the Amazon Basin and Gran Chaco during El Niño events (Garreaud et al. 2003; Vuille et al. 2000). The intensity of coastal fog may also be related to ENSO variability (i.e. Cereceda et al. 2002; Fontugne et al. 1999; Larraín et al. 2002) but this seems to have been of marginal importance for prehistoric occupation on the coast of northern Chile (though see Fontugne et al. 1999 for a contrary view).

In Australian deserts and drylands, ethnographic hunter-gatherer populations were extremely susceptible to changes in the annual and inter-decadal variability in rainfall and the flow-on effects these have on plant and animal resources and on foraging range. Responses to historically-recorded droughts show a pattern of contraction to the better-watered ranges and river systems, and fragmentation of the population remaining in affected áreas (Kimber 1990), while episodic extremes of high rainfall, lake full conditions or high river discharge allowed major aggregations of people for ceremonies, social exchange - and in some cases communal exploitation of ephemeral but abundant resources. Not all of these periods of 'boom and bust' are at-tributable to ENSO events, but sufficient evidence exists to indicate that historical ENSO had major consequences for Aboriginal populations in the interior of the continent. Table 1 summarises the net impact of ENSO on subsistence economies in the Atacama and Australian deserts.

History of archaeological research and ENSO

There have been several attempts in the Andes to correlate climatic data and cultural change, but it is only recently that the effects of ENSO have been explored (Kolata 1993; Kolata and Ortloff 1996; Lanning 1963,1967; Sandweiss etal. 2001; Seltzer and Hastorf 1990; Shimada et al. 1991; Usselmann et al. 1999; van Burén 2001). On the central coast of Perú, in an early study, Parson (1970) linked the decline of vegetation dependent on coastal fog to El Niño. In southern Perú, Fontugne et al. (1999) showed that precipitation from winter fogs was enhanced when El Niño was weak and suggested a correlation with coastal occupation in the Quebrada de los Burros (Tacna). Sandweiss et al. (2001) suggested that the main development of monumental temples on the north coast of Perú corresponds to a period when El Niño events wereless frequent, between 5.8 cal kyr BP and 3.2-2.8 cal kyr BP. However, the present study is the first to identify the coincidence in timing between the intensiñcation of ENSO c. 3 cal kyr BP and abrupt changes in the archaeological record along the coast and interior of the Atacama Desert (Northern Chile) (Grosjean et al. 2005; Núfiez et al. 2002; Standen 2003; Standen et al. 2004).

In Australia, the influence of ENSO on sea-sonal rainfall was not widely recognized until publication of an influential paper by McBride and Nicholls (1983), and it was some time before archaeologists incorporated ENSO into their think-ing. Hiscock (1994) suggested that late Holocene changes towards microlithic tool-kits around 4-3 cal kyr BP reflected greater mobility, in part a response to the greater environmental variability at this time. Clarkson and Wallis (2003) attempted to test this idea in a regional study in the northern savannah, but results were inconclusive. In eastern Australia, Turney and Hobbs (2006) found that the onset of the current pattern of ENSO (4-3 cal kyr BP) correlated with major changes in population in inland Queensland. Similarly, Cosgrove et al. (2007) argued that ENSO and its effects on ñre regimes and vegetation were important catalysts in allowing permanent Aboriginal occupation of tropical rainforests in northeastern Australia. The present study develops ideas and approaches in Smith et al. (2008). In that study, we (AW and MAS) used time-series analyses of radiocarbon data to identify the impact of ENSO in Australian drylands. Here, we set that work in broader context by comparing Australian and South American records.

Regional Setting

The Atacama Desert

The lower Atacama Desert forms a narrow coastal desert of ~180,000 km2 (Figure 1), running from Arequipa in southern Perú (16° S), to Copiapó in northern Chile (27° S). From the Paciñc Ocean, the land rises abruptly to the foot of the Andes, with local relief exceeding 6,000 m. This strip of desert, in many places, less than 100 km wide, is dissected by numerous large canyons or smaller quebradas (ravines), which in turn contain a range of mesic habitats and oases fed by stream flow and groundwater recharge from precipitation on the western slopes of the Andes.

The cold, north-flowing Humboldt Current constrains inland (upslope) penetration of Paciñc air moisture (Houston and Hartley 2003). The littoral zone is barren with no vegetation and, apart from the mouth of the quebradas, only brackish water (Núfiez and Várela 1967). The coastal escarpment supports lomas vegetation (Tillandsia tragophoba, Griselinia carlomunozü, Astragalus neobarnebyanus) dependent on winter fog (Dillon and Hoffmann 1997). Above 2,900 masl, the slopes of the Andes support a sparse vegetation of shrubs and small herbs, including the Prepuna (2,900-3,100 m) and Puna (3,100-3,900 m) vegetation belts (for details see Latorre et al. 2005).

Rainfall on the hyper-arid coast of the Atacama Desert is typically sparse and patchy (0.6 mm/yr in Arica) and up to 3,000 masl is strongly controlled by the effects of ENSO on local SST. In contrast, the interior of the Atacama receives much of its moisture from summer precipitation in the high Andes above 3,000 m (ranging from 40-350 mm/yr), which in turn is dependent on summer Amazon humidity in the Amazon Basin. El Niño (-SOI) events promote rainfall along the coast but suppress the inflow of moisture-bearing easterlies across the Andes, leading to a moisture déficit in the interior. La Niña (+SOI) events correspond to comparatively wet periods in thehighlands (Ortlieb 1995; Usselmann et al. 1999). However, overall rainfall is very low irrespective of these small variations: With a UNEP aridity index (P/ PET) of < 0.05 the Atacama is uniformly hyper-arid (Middleton and Thomas 1997).

The Australian arid zone

The Australian arid zone is a continental mid latitude desert, covering 3-5 million km2 (approxi-mately 70 % of the continental land mass) depending on how its boundaries are determined (Figure 2). Within this large región, fhere are a wide variety of desert habitats (Mabbutt 1971), each with different potential for human occupation. These include arid littoral áreas along the west coast and southern margin of the Nullarbor Plain, range and upland systems in the Pilbara and Central Australia, extensive áreas of sandy desert or dune ñelds (1.68 million km2) (Figure 3), and riverine tracts consisting of ephemeral or intermittent rivers and lakes, such as the Channel Country and Willandra Lakes system.


Compared to the Atacama Desert, the Australian arid zone is well vegetated throughout, with Acacia and Eucalyptus species forming an open tree and shrub cover, hummock grasses (mainly Triodia and Plechtrachne) and chenopods (e.g. Atriplex) forming a perennial sp. understorey, with a flush of ephemeral or annual species (e.g. Asteraceae) after seasonal rain. Mean annual rainfall varíes from 350-400 mm in the north to 250 mm in the south, but rates of evaporation are typically very high and for much of the región the UNEP arid-ity index (P/PET) falls between 0.05 and < 0.20 (Middleton and Thomas 1997). The entire región has low relief (< 1500 m asi) and local uplands rarely exceed 700 m above the surrounding plains. There are no oases or quebrada systems. Compared to the Atacama, water resources in the Australian desert are more widely distributed across the landscape, but are smaller, less reliable and have much lower rates of discharge. Variability in rainfall is also very high, compared to other desert regions: Alice Springs, in the centre of the región, has a median annual rainfall of 259 mm per annum, but received 903 mm in one year and only 54 mm in another year. A further point of contrast with the Atacama is that the west coast of Australia has a warm current, the Leeuwin Current, so is only moderately arid and lacks the rich ñsheries that the cold Humboldt Current brings to the coast of Chile.

Rainfall in Australian drylands is influenced by several climate controls, all of which attenuate towards the centre of the continent (Gentilli 1972; Sturman and Tapper 2006): the Australian summer monsoon in the north; tropical cyclones in the Pilbara and adjacent coastal áreas; ENSO variability in the Lake Eyre basin; and the winter Westerlies in the south. Spring and summer precipitation in the north-ern, central and eastern sectors of the arid zone is known to be positively correlated with the Southern Oscillation índex (Sturman and Tapper 2006: Fig. 11.28). The Walker circulation, in which ENSO is embedded, appears to have strengthened at about 5 cal kyr BP (Shulmeister 1999). The initial effect in northern Australia was an enhancement of the monsoon, followed by an abrupt shift to a drier, more variable climate after 3.7 cal kyr BP. The number of ENSO events (in this context - major periods of below-average precipitation) appears to have sharply increased from about 4,000-3,000 years ago, peaked 1,500-1,200 years ago, then declined towards present (Moy et al. 2002; see also Rein et al. 2004) though some evidence indicates an increase in the frequency of extreme ENSO events during the last millennium (Riedinger et al. 2002: Fig. 3).

Methods and Materials

Time-series analysis

Time-series analysis, using ages-as-data, was pioneered by Rick (1987) in Perú. The method we use here treats radiocarbon density as a proxy for population history, on the premise that the number of radiocarbon ages is a measure of occupation intensity. In practice, a number of other factors also determine radiocarbon density distributions: the size of the dataset; the reduced visibility of older sites; the geomorphic history of sedimentary units; dif-ferential preservation of organics; and the selection of samples for radiometric dating (see Smith et al. 2008 for a review of methodology). The effects of these can be estimated and offset to some extent, but uncertainty must remain about the robustness of radiocarbon data as a proxy for trends in prehistoric population. Despite these problems, assembling sets of radiometric dates in this way has a long history in archaeology and is widely used as a ñrst-order approximation of long-term prehistoric population trends (Barton et al. 2007; Gamble et al. 2004; 2005; Holdaway and Porch 1995; Lourandos 1993; Russell 2004; Shennan and Edinborough 2007; Turney and Hobbs 2006; Vermeersch 2005).

Time-series analysis also provides a formal statistical procedure for exploring quantitative records of variability in archaeological or envi-ronmental records (Weedon 2003). In this paper, we used the ARAND program (Howell 2001) to identify periodicities in the radiocarbon plots for each región. Spectral analysis allows detection of regular series cycles in a time-series, while cross-spectral analysis allows correlation of spectra from two or more time-series.

Archaeological records

For the Atacama Desert, the available radiocarbon dataset consists of 861 ages from 295 archaeological sites (Figure 1). For analysis we have subdivided these into two groups:

•     33114C dates from the coastal strip from 16° S to 25° S, ranging from Quebrada Jaguay in southern Perú to Taltal in northern Chile;

•     530 14C dates from the interior and highland sectors of the Atacama Desert, including 289 samples for the northern Atacama highlands (including oases along the 12 quebradas from Camaná to Camina) and 241 samples for the central Atacama (including the inland basins of Pampa del Tamarugal, Loa/Calama, and Puna de Atacama).

For the Australian arid zone, we use AustArchl, a datábase of radiocarbon determinations from archaeological sites in Australian drylands (Williams et al. 2007). This is currently the most comprehensive listing of radiocarbon ages for this región, listing 971 ages from 286 sites derived from published and unpublished research over the past 40 years. Figure 2 shows the distribution of dated sites. Our analysis focuses on three clusters of sites: those along the arid west coast of the continent, and those in the interior of the arid zone and in the Pilbara-Murchison región (N = 570 ages).

Calibration and plotting

Radiocarbon ages were calibrated with OxCal 4.03 (Bronk Ramsey 1995, 2001). For terrestrial samples, ages were calibrated with IntCal04 following a Southern Hemisphere correction of 40 ± 13 years (Hogg et al. 2002). For marine samples, ages were calibrated using Marine04 with a local marine reservoir correction (ΔR) of 72 ± 55 years for Australia and 190 ± 40 for Chile (Reimer and Reimer 2001; Southon et al. 1995; Ulm 2006). Following the calibration of the dataset, OxCal was used to sum the probabilities (2-sigma level) to produce time-series graphs and outputs for sub-sequent statistical analysis.

Results: The Archaeological Impact of ENSO

Can we detect the impact of ENSO-driven climatic variability in these archaeological records? The historical dynamics of ENSO suggest that any increase in the frequency or strength of El Niño (-SOI) events will be marked by disruption of coastal ecosystems along the Atacama coast and suppression of rainfall in the interior of both the Australian and Chilean deserts. On this basis, we would expect the impact of ENSO on population levéis to follow much the same trend on both sides of the Pacific, and that ENSO-driven archaeological events would be broadly synchronous across this región, at least with regards to the initial impact of the current ENSO cycle (Table 1).

This is in fact what we observe. ENSO appears to have intensiñed after 3.7 cal kyr BP (Moy et al. 2002; Shulmeister 1999) triggering increased variability in settlement on both sides of the Pacific (Figure 4). Its most direct ecological effect was the collapse of the fisheries along the Atacama coast by 3 cal kyr BP. This date appears to represent a critical tipping point for hunter-gatherer societies in the Australian arid zone and in the interior of the Atacama, as well as for specialized ñshing communities along the Atacama coast (Figures 5 and 6). Cross-spectral analysis of detrended time-series data over the last 13 cal kyr BP, comparing the Atacama (coast and interior) and the interior of the Australian arid zone (regions B & D), gives a correlation of 0.7780 for the two series. If we restrict this analysis to the last 3 cal kyr BP - to focus on the impact of ENSO - the correlation strengthens to 0.8386. Alternatively, if we simply focus on the interior of these two deserts, comparing the Atacama interior with the interior of the Australian arid zone (regions B & D) (over the last 13 cal kyr BP), cross-spectral analysis gives a correlation of 0.8293 on detrended data, compared to a correlation coefficient of r = 0.8859 on unmodified ('trended') data. Collectively, these statistics confirm what we can see graphically in the time-series plots: The onset of ENSO-driven environmental variability has a marked and broadly synchronous impact on human societies on both sides of the Soufh Pacific. Following the initial crash, adaptive and cultural responses to ENSO-driven variability necessarily varied across these regions (see below).

Figures 4-6 present time-series plots of archaeological radiocarbon data. These curves should be interpreted in broad terms, concentrating on major trends, peaks or troughs that reflect actual changes in population or level of human activity rather than the 'noise' generated by differences in sampling. As a proxy for changes in ENSO-driven climatic variability, we also plot the number of El Niño (-SOI) events /100yr from Moy et al. (2002). Although Moy et al. (2002) may exaggerate the extent of ENSO activity before 5 cal kyr BP (see above), their data represents the only long time-series record available for cross-comparison with archaeological time-series data.



The Atacama Desert

Figure 4 shows the overall pattern for the Atacama. Population levéis were relatively stable throughout the early Holocene, following initial colonisation of the región shortly before 13 cal kyr BP. There was a stepwise increase between 6-5 cal kyr BP followed by a major population crash between 4-3 cal kyr BP. Population seems to have then rapidly recovered, peaking between 3-2 cal kyr BP before sharply declining again. Figure 5 breaks these trends down to contrast the population history of coast and interior.

On the Atacama coast, there was a short pulse of activity associated with initial colonisation of the littoral shortly before 13 cal kyr BP (Figure 5), followed by consolidation of regional settlement at lower population levéis. The process of colonization (Beatón 1991; Borrero and Franco 1997) was favoured by an increase in precipitation in the highlands that activated lowland drainage systems, and a marine environment dominated by nutrient-rich cold up-welling water (Baker et al. 2001; Carré et al. 2005; Keefer et al. 1998; Koutavas et al. 2002; Latorre et al. 2005; Moreno et al. 2007; Moy et al. 2002; Rodbell et al. 1999; Usselmann et al. 1999).

Once established, coastal populations grew strongly from 11 cal kyr BP. Between 11 and 4 cal kyr BP, Figure 5 shows a saw-toothed pattern of expansión and collapse, with oscillations around an overall trend towards increasing population. This period is also marked by a geographic expansión of settlement and increasing use of specialised marine technology. Both of these developments coincide with a decline in the intensity or frequency of ENSO events, especially after 9 cal kyr BP, and the productivity associated with upwelling coastal waters (Figure 5) (Carré et al. 2005; Keefer et al. 1998) and enhanced moisture from winter fogs (Fontugne et al. 1999). It appears that the Chinchorro culture in the northern Atacama (Arriaza 1995; Grosjean et al. 2007; Standen 1997,2003; Standen and Arriaza 2000; Standen et al. 2004), enjoyed a particularly rich, stable and highly predictable marine environment.

Coastal settlement became markedly less stable after 4 cal kyr BP, with the onset of the current ENSO cycle, leading to a major collapse of the system at 2.5 cal kyr BP. Cross-correlation of time-series data supports this: For the period from 7-2 cal kyr BP, population levéis on the coast are inversely correlated with the number of ENSO events. The sharp decline evident in the time-series data also corresponds to an abrupt cultural change in excavated sites. Many coastal sites are abandoned. Others reflect an abrupt broadening of the economic base away from marine resources to include horticultural producís -such as yam, manioc, chilli pepper, and corn- coupled with more extensive trade networks (Arriaza 1995; Grosjean et al. 2007; Núñez et al. 2008; Rivera 1991; Rothhammer et al. 2007; Standen et al. 2004). There is also a shift away from artificial mummiñcation and collective burial, towards a pattern of individual funerary rituals. Coastal populations gradually recovered after 2.5 cal kyr BP but were reliant on a mixed agricultural/marine economy and did not reach the levéis seen in the mid Holocene. The archaeological data indicate that the onset of the current ENSO cycle had two main effects on coastal populations: it caused the collapse of a distinctive specialised marine economy; and the increase in environmental variability was an important catalyst for broadening the economic base to include tropical agricultural crops and the strengthening of trade and political links with inland resource zones. The interior of the Atacama shows a complementary pattern of events, with a series of pulses of occupation reflecting discrete phases of cultural use of the interior.

Colonisation in the interior was more gradual than on the coast, beginning about 12.5 cal kyr BP, when local precipitation increased on the slopes of the Andes. Population levéis then oscillated around a comparatively low mean, with well-documented declines at 8.2 and 6.5 cal kyr BP. The last of these is known to coincide with the beginning of drier and more variable environmental conditions in the interior. It also reflects an archaeological discontinu-ity in settlement in this región, with a shift towards a more dispersed population, more mobile hunter-gatherer groups, and to logistic stone tool kits. A second pulse of highland settlement, centred on 4.9 cal kyr BP, is characterized by low-density use of caves and open camps, focussed on ecological refuge áreas.

The onset of ENSO is marked by a pulse of occupation between 3-2 cal kyr BP, offsetting the collapse of coastal settlement at this time. In the interior, this phase of occupation was marked by increasing reliance on a pastoral economy, with domestic camelids and a semi-sedentary pattern of settlement with fixed camps and basic stone architecture, and imported goods from the Paciñc and the Atlantic coasts. This was a short-lived phase - lasting no more than 1000 yrs - brought to an end by another sharp decline at 2 cal kyr BP. The steady population growth after 2 cal kyr BP reflects the expansión of small-scale local polities (simple chiefdoms) linked to larger societal structures, such as the Tiwanaku state and the Inka Empire in the Andes (Berenguer 2000; Bruhns 1994).

The Australian arid zone

Our data suggests that Aboriginal settlement in the interior of the Australian arid zone (in particular in Central Australia and the Lake Eyre basin) was sensitive to ENSO-driven climatic variability. In support of this, cross-spectral analysis of radiocarbon density plots for the arid interior and the number of ENSO events/100 years (Moy et. al. 2002), over the last 13,000 years, shows that the two are positively correlated (0.4498). De-trended spectral analysis on archaeological data for the last 10 cal kyr BP (combining all regions except the Murray-Darling Basin) also indicates periodicities of 1340 years and 175 years, suggesting that arid zone populations responded to millennial and sub-millennial climatic variability. The former is similar to the 1470-year periodicity reported for ENSO.

Figure 4 shows the summed probability plots for the entire AustArchl dataset. Figure 6 breaks this down by región, concentrating on the past 15,000 years. Overall, there is a stepped pattern of population growth, with major steps at 8 and 1.5 cal kyr BP, though these thresholds are not uniformly expressed across the arid zone (see Smith et al. 2008 for more details). Within this pattern, our time-series data show a distinct saw-toothed pattern of population expansión and collapse, with a sharp decline in population around 3 cal kyr BP evident across all regions. This is a short, widespread collapse, about 500 years in duration, followed by a period of major population growth in the late Holocene.

Arid coastal regions show a sustained increase in site visibility from 8 cal kyr BP, as the last marine transgression created extensive and productive mangrove swamps (the 'Big Swamp' phase) across northern Australia (Mulrennan and Woodroffe 1998; Woodroffe et al. 1985). In the Murray-Darling Basin, there is a marked peak in site visibility at 8 cal kyr BP representing sites associated with major rivers, which experienced increased discharge around this time. Away from the coasts and rivers, this period may also have seen some increase in regional populations, but data is patchy at present.

Australian arid zone hunter-gatherer populations appear to have expanded during the early part of the current ENSO cycle (5-3 cal kyr BP), with a sharp population decline at 3 cal kyr BP across the arid zone as a drier, more variable climate became established.

Populations along the arid west coast of the con-tinent were also affected by the collapse at 3 cal kyr BP, though in this case the use of marine resources appears to have buffered coastal populations from other environmental oscillations. Unlike the Atacama coast, the west coast of Australia did not have rich fisheries, ñor specialised marine economies. Ñor is it a región where ENSO is strongly correlated with marine productivity. In this región, site abandonment coincides with the decline of the mid Holocene mangrove swamps (Przywolnik 2005), as a result of silting and shoreline progradation. A connection with ENSO has not been shown, but increasing climatic variability would affect sediment loads driving such coastal morphodynamics.

The widespread collapse of inland desert populations around 3 cal kyr BP appears to reflect the combined effects of weaker summer monsoon rainfall and the onset of the current cycle of ENSO activity. Some of the archaeological impacts have been explored by Hiscock (1994), but the effects on site use and patterns of subsistence and land use in the arid zone are yet to be explored in any detail. After 3 cal kyr BP most regions saw a rapid recovery, but in the arid interior - and in particular in Central Australia (see Smith and Ross 2008) -this was delayed until 1.5 cal kyr BP.

In most regions, there is a signiñcant increase in human activity from 1.5-1 cal kyr BP. The decline in ENSO activity from 1.2 cal kyr BP (or the shift to fewer but more extreme ENSO events) appears to underpin both an expansión of grassland in Central Australia and Aboriginal settlement (as Turney and Hobbs 2006, also found in inland Queensland). The disproportionate amplitude of the population response suggests that one of the effects of the peak in late Holocene ENSO activity was to breach a critical human-environment threshold, leading to an abrupt shift to a new pattern of land use. In Central Australia, much of the archaeological evidencepoints directly to regional population growth, beginning 1.5-1 cal kyr BP (see Smith and Ross 2008, for a detailed review). Hunter-gatherer groups appear to have increased their use of marginal or outly-ing áreas, and extended their use of existing sites. Grindstones for processing Acacia and grass seeds became a more prominent part of site assemblages (Smith 1986, 2004). An increase in territoriality is reflected in the greater differentiation of rock art complexes after 1.5 cal kyr BP.

Conclusión

This exploratory research emerged from the idea that ENSO is a global climatic phenomenon likely to affect desert societies on both sides of the Pacific. In fact, we argüe that the effects of ENSO are best seen at a zonal rather than regional scale. In this paper we have compared time-series archaeological records from the Atacama Desert and the Australian arid zone, using radiocarbon densities as a proxy for human occupation. The intensification of ENSO after 3.7 cal kyr BP ap-pears to have triggered increased (and broadly synchronous) instability in settlement on both sides of the Pacific. The most direct ecologi-cal effect of ENSO was the decline of fisheries along the Atacama coast. The 3 cal kyr BP event appears to represent a critical tipping point for hunter-gatherer societies in the Australian arid zone and in the interior of the Atacama, as well as for specialized ñshing communities along the Atacama coast. Following this crash, adaptive and cultural responses to ENSO varied across these regions, but we conclude that the onset of ENSO-driven environmental variability had a marked impact on human societies on both sides of the South Pacific.

We put this forward as a testable hypothesis requiring further corroboration against archaeological data. The population trends shown in our radiocarbon density plots need to be tested against multi-proxy records for relative population density in each región, and larger 14C dataseis (as these become available). Where ENSO is the cause of population changes, we expect broadly synchronous impacts on societies on either side of the Pacific (though the character of responses may differ) and we expect archaeological records to show shifts in those aspects of the palaeo-economy most likely to be affected by ENSO-driven climatic variability. The latter may include coastal resources (in the case of the eastern Pacific), productivity of wild plant foods or cultigens, water harvesting strategies and changes in group mobility.

Acknowledgements: This paper is a contribution to the IHOPE (Australia) program at the Fenner School of Environment and Society, Australian National University. We thank C. Turney, University of Exeter, for advice on methods. In Australia, C. Santoro was supported by a DEST Endeavour Research Fellowship (Australian Government) and by Visiting Fellowships at the National Museum of Australia and the Fenner School, ANU. We acknowl-edge other research support from the University of New England (Armidale NSW, Australia), the Center of Advanced Studies in Ecology and Biodiversity (CASEB) and Institute of Ecology and Biodiversity (P05-002 ICM), and the National Museum of Australia. CS and CL also acknowledge support from FONDECYT Grant 1070140 and a Grant of the Dirección de Investigaciones y Post Grado, Centro de Investigaciones del Hombre en el Desierto CIHDE, and the Instituto de Alta Investigación of Universidad de Tarapacá, Arica, Chile.

 

NOTAS

1 Artículo seleccionado de las presentaciones al Second Southern Hemisphere Desert Conference (Arica, octubre 2005).

 

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Recibido: octubre 2007. Aceptado: abril 2008.

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