Climate variability over the last two millennia in the North American Monsoon region, recorded in laminated lake sediments from Laguna de Juanacatlan, Mexico

摘要

High-resolution titanium (Ti) data obtained using an ITRAX XRF core scanner from a laminated sediment core from the Laguna de Juanacatlan, western central Mexico yield a unique high-resolution record of runoff (precipitation) change for the last 2000 years. In the absence of reliable, long-term meteorological records, comparison of theTi data with information from the rich Spanish colonial period archives and the post-Independence period, confirms thatTi is a proxy for runoff. This interpretation is supported by comparison with other high-resolution archives from the surrounding region, primarily tree rings and other lake sediment sequences. The Juanacatlan Ti record is therefore a proxy for summer, monsoonal rainfall. The record provides new evidence from the Pacific margin of tropical North America of the occurrence of dry conditions through much of the Classic period (c. ad 300-900), and wetter conditions during the later Medieval period (cad 1200—1350). The period commonly known as the'Little Ice Age' (LIA) shows considerable variability, with dry conditions in the early part (cad 1400-1600) and wetter conditions,punctuated by multiyear droughts through the eighteenth century. A notable feature of the record is the apparent decoupling of lacustrine sedimentation from the climate since the mid-twentieth century, possibly resulting from anthropogenic disturbance. Preliminary interpretations of theTi record indicate that patterns are consistent with changes in monsoon strength associated with ENSO and solar forcing over the last two millennia.