Late Quaternary hydroclimatology of a hyper-arid Andean watershed: Climate change, floods, and hydrologic responses to the El Niٌo-Southern Oscillation in the Atacama Desert

Although certain characteristics of the El Niño-Southern Oscillation (ENSO) are well known on contemporary timescales, less is known about the magnitude–frequency relationships of this atmospheric phenomenon on longer timescales or its relationship to widespread flooding, especially in its core zone along the sub-tropical Andes where La Niña or El Niño episodes control regional hydroclimatology. Using a combination of stratigraphic evidence, geochronologic dating (14C and OSL), stable isotope analyses, and water geochemistry along the Rio Moquegua in the northern fringes of the Atacama Desert, we assemble a paleoflood chronology for mainstem and tributary sections for the past ca. 20 ka and ascertain the variation in ENSO frequency and magnitude. Because of the inherent watershed structure and regional hydroclimatology, mid-valley tributaries of the Rio Moquegua only flood during El Niño episodes and thus provide an important proxy of extreme El Niños while mainstem stratigraphy records both La Niña and El Niño episodes. El Niño floods appear to have been pronounced during the Late Pleistocene and up to at least the Younger Dryas (~ 12,000 cal yr BP) while stratigraphic evidence of large El Niño floods is lacking in tributary systems during the Mid-Holocene. stratigraphy in a ~ 2 ka 7 m high terrace along the mainstem indicates an increased frequency and magnitude of large floods between ca. 700 and 1610 AD as compared to the period from ca. 160 BCE to 700 AD with “mega-Niños” occurring ca. 1330 AD and ca. 1650 AD. Water geochemistry and radiocarbon dating indicate that at least two major aquifers exist, with wells and springs in the mid-valley dating to 710 and 3100 14C yr BP, respectively, while water from a spring in the headwaters dates to 10,320 14C yr BP. This range in dates suggests that groundwater flow in the mid-valley is neither fossil water nor exclusively recharged from local precipitation while the older date for headwater sections suggests a more fossil groundwater source and lack of contemporary recharge. The 18O similarity between groundwater in the mid-valley and the regional meteoric water line (MWL) suggests a Pacific moisture source and the role of El Niño precipitation in recharging local aquifers. Thus, El Niños and Pacific-sourced moisture are an important component of the regional hydroclimatology and the variation in ENSO frequency and intensity has significant social and hydrologic repercussions in these hyper-arid settings.