Climate change and aeolian activity in the Atacama Desert, northern Chile

Aims and objectives

The aim of this research was to identify, for the first time, evidence of environmental conditions during the Late Quaternary period in the near-coastal Atacama Desert, through the analysis and dating of aeolianite deposits (chemically cemented wind-blown dune sediments) in northern Chile. This was achieved by meeting the following objectives:

i. Aeolianite outcrops were identified, described and logged in order to determine the original cemented dune type and ascertain variations in past wind direction. Outcrops were also systematically sampled for dating and laboratory analyses.

ii. Samples were dated using the optically-stimulated luminescence (OSL) technique to determine the timing of dune sedimentation in relation to published climatic and sea level records.

iii. Samples were analysed using petrological, mineralogical and isotopic techniques in order to comprehensively characterise their environment of cementation and subsequent diagenetic history.

Background and rationale

The Atacama Desert of northern Chile is one of the Earth’s oldest deserts and is widely regarded as the driest and least hospitable place on Earth. Despite this, there is evidence for relatively wet conditions on a number of occasions during the Late Quaternary period. Paleoecological studies, for example, indicate greater winter precipitation at >52, 40-33 and 24-17 thousand years before present (ka BP), enhanced summer and winter precipitation from 17-14 ka BP and increased summer precipitation from 14-9.5 ka BP. 
The last of these periods is of particular interest as it coincides with evidence for a late glacial-early Holocene pluvial event in the central Atacama, when lake and groundwater levels were higher and humans occupied paleolake sites across the Altiplano. These lakes may have provided an inland migration route from Mesoamerica to the Monte Verde archaeological site in south-central Chile during the earliest human occupation of South America. In contrast, we know little about paleoclimates along the Pacific coast of the Atacama during the late Quaternary, the alternative routeway south.

This project sought to redress this imbalance and, for the first time, provide evidence of late Quaternary environmental conditions in the near-coastal Atacama through the analysis and OSL dating of aeolianite deposits. Aeolianite is of considerable geographical significance as it can provide insights into past changes in wind regime, moisture availability and, where found in near-coastal settings, marine-onshore sediment fluxes. Its formation is a two-phase process. First, conditions must favour the deflation of sediments to form dunes. In the case of coastal aeolianites, these sediments are moved onshore during sea-level fluctuations to form transverse, barrier, oblique, parabolic or, more rarely, climbing dunes. Second, prevailing weather conditions must be conducive to allow the alteration and cementation of these sediments by various geochemical processes. Such cementation increases the resistance to erosion and hence the preservation potential of the dune sediments, thereby enhancing their usefulness as a palaeoenvironmental indicator. There is, however, uncertainty about the timing of coastal aeolianite formation in relation to sea-level change. Work by the applicants in South Africa and by other academics in Australia, for example, indicates that the initial deposition of carbonate-cemented eolianites occurred during interglacial and interstadial sea-level highstands. In contrast, some studies report aeolianites formed during times of lower sea level but these are comparatively rare.

Methodology

Our research provided the first detailed analysis of aeolianite deposits in the Atacama and, indeed, the South American continent. Fieldwork took place over three weeks in July 2008, focussed upon two areas in northern Chile: (i) west of Copiapó, Atacama region, where extensive carbonate-cemented aeolian sediments have been mapped by the Chilean Servicio Nacional de Geología y Minería (Sernageomin); (ii) south of Iquique, Tarapacá region, where reconnaissance fieldwork by the applicants in January 2007 identified previously unmapped aeolianite outcrops. In both areas, aeolianite exposures occur in association with mid- to late-Pleistocene marine terraces and represent key transfer sites between coastal sediment sources and inland dunefields.

Areas of aeolianite outcrop were identified from geological maps, aerial photography and field survey, and subsequently described and logged to assess the original dune type and paleowind direction using standard sedimentological techniques. Outcrops were also sampled for dating and further laboratory analysis. OSL dating was used to determine the timing of dune sand deposition. Preliminary investigations near Iquique suggested that aeolianites formed through the stabilisation of mobile transverse dunes that migrated inland through breaks in the coastal mountain range. This being the case, the OSL-derived ages of sediment deposition corresponded closely with the onset of aeolianite cementation. Given the geomorphological context of the aeolianite sites, the majority of samples were anticipated to be late Quaternary in age. Results were compared against published climatic and sea level records to allow existing hypotheses linking desert dunefield development to the exposure of offshore sediments during glacial low sea-stands to be tested.

The diagenetic history of aeolianite development was determined through a variety of petrological, mineralogical and isotopic techniques. Analyses of resin-impregnated thin-sections was used to characterise the aeolianite mineralogy, and ascertain whether host sediment dissolution occurred during the early stages of formation to provide a local source of cementing solutions. Detailed micromorphological analyses were also used to characterise the range of cement types present, their inter-relationships and sequence of formation. This information was used to establish the environment of cementation, specifically whether formation occurred within the freshwater vadose zone as a result of percolating meteoric water or in the phreatic zone beneath a former higher water table. Preliminary analyses of samples from near Iquique suggested that, whilst the majority of aeolianites formed by carbonate-cementation within the vadose zone, some were at least partially cemented by sulfate minerals. These minerals were presumably supplied by coastal fog, the first time this had been identified, and offered the potential to assess former fluxes in fog moisture supply.

Finally, environmental and climatic conditions during the course of cementation were assessed by determining the stable carbon and oxygen isotopic signatures of aeolianite cements. Variations in δ13C values, for example, reflected changes in CO₂ sources within precipitating waters due to processes such as evaporation, plant transpiration and pedogenesis, whilst δ18O values revealed information about the temperature and composition of pore waters at the time of cementation.

Key findings were published in Nash et al. (2018) 'Late Quaternary coastal evolution and aeolian sedimentation in the tectonically-active southern Atacama Desert, Chile.'