A COOLING EVENT DURING THE YOUNGER DRYAS CHRON IN COSTA-RICA

A fossil pollen record from the Costa Rican Cordillera de Talamanca pr esents the first AMS radiocarbon-dated evidence of a temperature decre ase during the Younger Dryas Chron. This cooling event is named La Cho nta stadial after the bog at 2310 m altitude, located at the actual lo wer to upper montane forest boundary. High-resolution pollen analysis revealed that between 11,070 +/- 130 (AMS C-14 age) and 10,400 yr B.P. (interpolated age) vegetation comparable to present-day subalpine for est occurred about 300-400 m lower than in the previous warmer interva l, and 600 to 700 m lower than at present time. The radiocarbon dates ale close to the earlier estimated ages of 11,080-10,500 yr B.P. for t his interval, based on interpolation and pollen concentration rates (H ooghiemstra et al., 1992). The downslope shift of the upper forest lin e indicates an estimated temperature drop of 2-3 degrees C during the La Chonta stadial. The local vegetation development is indicative of a drier climate. From 10,400 to 9800 +/- 120 yr B.P. (AMS C-14 age) sub alpine rain forest was replaced by upper montane forest, a transitiona l period to Holocene environmental conditions.