Re-evaluation of mid-Holocene deposits at Quebrada Puripica, northern Chile

During the middle Holocene (8–3 ka), wetland deposits accumulated in areas with emergent water tables in the central Atacama Desert (22–24°S), producing a stratigraphic unit (Unit C) that can be mapped and correlated across different basins and geomorphic settings. Wherever mapped, Unit C is located between 6 and 30 m above modern wetlands, and includes thick sequences of diatomite and organic mats. The origin, depositional environment, and paleoclimatic significance of Unit C is controversial and currently under debate. Grosjean [Science 292 (2001) 2391a] suggests the unit developed under a regime of falling lake and ground-water levels, whereas evidence presented here suggests that Unit C formed during a period of rising ground-water levels and increased vegetation cover. The debate is embedded in broader discussions about geomorphic processes in ground-water-fed streams, as well as the history and forcing of climate variability in the central Andes. ntral Atacama and Andes are remote, with few opportunities for different researchers to examine the same site. One exception is a sequence of mid-Holocene deposits at the confluence of Quebrada Seca and Rı́o Puripica in northern Chile (23°S). Grosjean et al. [Quat. Res. 48 (1997) 239–246] initially suggested the deposits accumulated in temporary lake basins, which formed when recurring debris flows from a side canyon (Quebrada Seca) dammed the main channel (Rı́o Puripica) during a period of drought and reduced stream flow. Upon our visit to the site, we found evidence that clearly demonstrates the deposits were not formed in temporary lakes, but rather were deposited in a wetland environment. Disagreement remains about the climatic interpretation of wetland deposits. Grosjean [Science 292 (2001) 2391a] now suggests that local ground-water levels rise and wetland deposits aggrade in deep canyon systems, such as Rı́o Puripica, when stream power and channel erosion is reduced during prolonged dry spells. However, sedimentological evidence and the presence of Unit C in many depositional environments, not just deep canyons, indicate that it formed during a period of higher regional ground-water levels that were sustained by enhanced precipitation and recharge in the High Andes.