Abstract

In arid regions, groundwater table response to interannual recharge variability is driven mainly by water table depth, magnitude/frequency of precipitation and evaporation rates. Here, we evaluate past groundwater and riparian ecosystem responses to multi-millennial scale climate variability in the endorheic basin of Pampa del Tamarugal (PdT), northern Atacama Desert (17°45'–21°20´S). Fluvial terraces found within several incised, unvegetated canyons are indicative of past fluctuations in stream and groundwater discharge. These terraces abound with extraordinarily well preserved fossil wood and leaf litter deposits. Radiocarbon dates from leaves, indeterminate logs and roots indicate that they grew between 17.4 to 14 kyr and at 11.8 kyr. Macro and microfossil analyses of buried leaf-litter reveal the prevalence of mostly hygrophytic and phreatophytic taxa. These fossil assemblages are analogous to typical riparian formations found in perennial river canyons found north (i.e. Camiña valley at ~19ºS) or south (i.e. Loa River basin, ~22ºS). As none of the plant species identified are indicative of increased local rainfall over the PdT (i.e. hillslope taxa) we infer that the expansion of riparian formations indicates increased surface discharge in streams associated with heightened local groundwater tables. Our radiocarbon chronology indicates that increased discharge in the PdT was practically synchronous with paleolake Tauca (18–14.1 kyr) and with past wet phases documented from rodent middens in the Atacama Desert. A slightly younger increase in fluvial discharge at 11.8 kyr is likely related to a wet phase documented between 13–11 kyr on the Bolivian Altiplano. Our results indicate that the response of hydrological (water tables and streams) and biological systems in the PdT were likely coupled to precipitation variability along the adjacent eastern highlands. The synchroneity between regional paleoclimate and the response of the PdT riparian ecosystems and local groundwater levels indicates rapid response times to variable precipitation an recharge on millennial timescales. In fact, this was probably the most important discharge event in the hyperarid core of the Atacama Desert over the last 18,000 years. This implies that a significant part of present day groundwater resources in the region are likely fossil that date to the last deglaciation.