Timing of the late glacial and Younger Dryas cold reversal in southern Chile varved sediments

The timing of the last deglaciation in southern Chile is re-evaluated from a calendar varve chronology (Lago Puyehue, 40 degrees S). The climate shifts are analysed by continuous annual varve-thickness measurements through the similar to 17,100 cal. year to 10,800 cal. year BP time window (similar to 3.5 m sediment core). The varve years are determined by the alternation of light (phytoplankton-rich) and dark (terrigenous and organic-rich) layers forming graded annual couplets (similar to 0.2 to 0.8 mm/year). The varve chronology is constructed by conventional varve-counting methods on thin sections after correction for instantaneous volcanic and/or seismic events detected in the thin sections. The calibrated varve-age model derived from the manual varve counting is constrained by high-resolution grey-scale (GS) semi-automatic counts of the annual light phytoplankton-rich layers (similar to 120 mu m to 300 mu m thick). Due to physical sediment properties the GS constitutes a proxy record for the phytoplankton/terrigenous varve-thickness variations through the sediment record. The varve couplets are thicker/thinner during humid/dry phases and darker/lighter (negative/positive annual grey-scale index) during cold/warm phases. Our results show that at 40 degrees S the last deglaciation took place in two phases between similar to 17,100 cal. year and similar to 15,500 cal. year BP. We note a climate instability between similar to 15,500 cal. year and 13,300 cal. year BP and a significant dry phase between similar to 15,000 and 14,500 cal. year BP. We evidence a cold event in two phases between similar to 13,300 and 12,200 cal. year BP interrupted by a dry event between similar to 12,800 and 12,600 cal. year BP. The onset of a significant warmer period is observed after similar to 11,500 cal. year BP. Our results provide new evidence of a Younger Dryas cool reversal in southern Chile, i.e., the Huelmo/Mascardi event Hajdas et al. (2003) associated with an abrupt dry pulse at similar to 12,800-12,600 cal. year BP. The high-resolution grey-scale measurements performed on the biogenic varves from Lago Puyehue provide a reliable calibrated chronology of the regional environmental and climate shifts during the last deglaciation.