A 300 kyr record of past hydroclimate change from alluvial fans in the southern Central Andes

Abstract

Alluvial fans are hypothesised to record signals of past hydroclimate changes in their depositional chronologies and slopes. However, direct tests of this hypothesis have been limited due to challenges in precisely dating the responses of alluvial fans to past climate forcing. Here, we present a new chronology of alluvial-fan deposition at the Sierra de Aconquija in the southern Central Andes (27°S) spanning ~300 kyr and based on 35 cosmogenic 10Be-derived exposure ages and eight infrared-stimulated (IRSL) and optically stimulated luminescence (OSL) ages. The ages reveal that fan deposition was phased with past climate changes. Orbitally-forced weakening and strengthening of the South American Summer Monsoon coincided with fan aggradation during dry episodes and incision during wetter episodes, consistent with predictions from alluvial-channel models. These results are in precise agreement with independent palaeoclimate constraints spanning timescales of 103 to 105 years. Fan aggradation-incision cycles record a predominant influence of precession-induced variations in precipitation, although climate shifts as rapid as 1 kyr also triggered incision, and fan architecture appears to be further modulated by orbital eccentricity cycles. Incision events on the fans coincide with the ages of moraines in the headwater catchments, including those formed during the Younger Dryas and Last Glacial Maximum. Furthermore, the fan chronology indicates dry conditions in the southern Central Andes during Heinrich Stadial 1, suggesting that enhanced precipitation during this episode was limited to more northerly latitudes. Our results demonstrate that climate change exerts a primary control on simple, source-to-sink sedimentary systems and show that alluvial fans can, in some cases, be utilised as terrestrial palaeoclimate archives with which the spatio-temporal variability of past climate changes can be constrained.