Quaternary geology and paleoclimate of Costa Rica: Evidence from glaciation, stable isotopes of surface waters, and a speleothem.

摘要

Quaternary climates of the Caribbean Region are poorly known. This study investigates the terrestrial evidence of Costa Rican Quaternary paleoclimates via analysis of the Quaternary glaciation of the Costa Rican highlands to estimate temperature reductions associated with the last local glacial maximum, an analysis of the spatial and temporal variability in stable isotope values of Costa Rican surface waters and precipitation, and analysis of the stable isotope values of speleothems to estimate past variations in precipitation amount. The highest peaks of Costa Rica were glaciated during the late Quaternary, attesting to a significantly different climate in the Central American isthmus. New evidence of glacial extent comes from striated, grooved, and channeled bedrock in previously undocumented sites. During the last local glacial maximum, estimated as >12,000 14C yr BP, an ice cap ∼35 km2 in extent covered the highest peaks of the Cordillera de Talamanca around Cerro Chirripo, ∼2 km 2 of ice existed around Cerro Kamuk, and ∼5 km2 existed on Cerro de la Muerte. In Chirripo Park, the paleo equilibrium line altitude (ELA) was ∼3500 m. Cirque floor elevations around Cerro Kamuk of 3260 m suggest a lower paleo ELA there. The modern 0°C isotherm of 5000 m suggest a late Pleistocene ELA depression of ∼1500 m, associated with a temperature depression of 8 to 9°C.; Analysis of a calcite speleothem from the Caribbean slope of Costa Rica provides direct evidence of past variations in precipitation amount on decadal time scales. delta18Ocalcite values prior to 10,140 yr BP are ∼1‰ higher than early Holocene values. These lower values may be explained by a temperature reduction of ∼5°C, decreased rainfall and relative humidity, or some combination of the two. To interpret the delta 18O record preserved in this speleothem, a calibration study was undertaken to determine the spatial and temporal variation in stable isotopes in surface waters and precipitation. delta18Owater values are most strongly correlated with precipitation amount and follow distinct regional trends. Deuterium excess values suggests that moisture recycling contributes to the moisture flux along the Nicaragua Trough.