Geochemical and magnetic evidence of change from winter to summer rainfall regimes at 9.2 cal ka BP in northwestern Mexico

We present a multiproxy record from magnetic susceptibility, loss on ignition and chemical analysis of a sediment core collected from “Ciénega Las Taunas”, a peatland in the northwest of the Sierra Madre Occidental in Mexico. The oldest sediments were dated to 12,850 cal yr BP. The relatively cool and wet Younger Dryas (YD) shows higher concentrations of TiO2 and Chemical Index of Alteration (CIA), suggesting higher runoff into the cienega. The transition from the YD to the early Holocene is marked by a decrease in susceptibility, pointing to weakened erosional processes during a period of closed forest vegetation, suggesting a predominantly winter rainfall regime. A change in seasonality likely occurred after ca. 9200 cal yr BP, as shown by: 1) an increase in most major elements, suggesting increased dust fallout from distant sources, and 2) more pollen assemblages from temperate vegetation (pine-oak forest). We related these changes to increased summer rains and monsoons in NW Mexico. A possible hiatus or a very slow sedimentation period occurred during the middle Holocene at this site. Finally, high-frequency changes in sediment properties were especially observed in the Late Holocene, suggesting contrasting sediment inputs controlled by an unstable water table and/or related to ENSO.