Abstract

Of the major subtropical deserts found in the Southern Hemisphere, the Atacama Desert is the driest. Throughout the Quaternary, the most pervasive climatic infl uence on the desert has been millennial-scale changes in the frequency and seasonality of the scant rainfall, and associated shifts in plant and animal distributions with elevation along the eastern margin of the desert. Over the past six years, we have mapped modern vegetation gradients and developed a number of palaeoenvironmental records, including vegetation histories from fossil rodent middens, groundwater levels from wetland (spring) deposits, and lake levels from shoreline evidence, along a 1200-kilometre transect (16–26°S) in the Atacama Desert. A strength of this palaeoclimate transect has been the ability to apply the same methodologies across broad elevational, latitudinal, climatic, vegetation and hydrological gradients. We are using this transect to reconstruct the histories of key components of the South American tropical (summer) and extratropical (winter) rainfall belts, precisely at those elevations where average annual rainfall wanes to zero. The focus has been on the transition from sparse, shrubby vegetation (known as the prepuna) into absolute desert, an expansive hyperarid terrain that extends from just above the coastal fog zone (approximately 800 metres) to more than 3500 metres in the most arid sectors in the southern Atacama. Our study focuses on rodent middens (cf. Betancourt et al. 1990). These are amalgamations of plant remains (including seeds, fl owers, leaves and wood), bones, insects, feathers and rodent faeces, glued together within a crystallised matrix of rodent urine. In arid climates, they can survive for thousands of years underneath rock slabs and within crevices and caves. Midden plant remains are from within the rodent’s foraging range (usually 200 metres or less) and include plants collected for consumption, nest building and protection. In the Atacama, middens are produced by at least four different rodent families: Abrocomidae (Abrocoma cinerea, ‘chinchilla rats’); Chinchillidae (Lagidium viscacia and Lagidium peruanum, ‘viscachas’); Muridae (Phyllotis spp. ‘leaf-eared mice’) and Octodontidae (Octodontomys gliroides, ‘brush-tailed rat’). Modern studies of Phyllotis, Lagidium and Abrocoma indicate that they are dietary generalists and probably not selective enough to introduce large biases into the midden record (Pearson 1948; Pizzimenti and DeSalle 1980; Cortés et al. 2003).