Lichenometric and Radiocarbon Dating of Holocene Glaciation, Cordillera Blanca, Perú

Abstract

Measurements of Rhizocarpon subgenus Rhizocarpon thalli on moraines the ages of which are constrained by six radiocarbon dates, two pre-Colombian structures, and one historically-dated flood deposit indicate that lichenometry can be used to estimate the age of stabilization of glacial deposits that date to the last c. 7 ka on the west side of the Cordillera Blanca, Perú (8°40′-9°40'S; 77°00′-77°40'W). The preliminary growth curve for subgenus Rhizocarpon resembles those derived for alpine areas in Alaska and Colorado, and can be subdivided into an initial, rapid phase of growth (c. 0.4 mm yr-1) that lasts c. 100 years, a c. 500-to 1000-year-long phase of decelerating growth, and a period of slow linear growth (0.016 mm yr-1) that commences after c. 1000 years. Size clustering of subgenus Rhizocarpon thalli supports a four-fold subdivision of cirque moraines at altitudes between 4200 and 4650 m. The oldest moraines stabilized between c. 7000 and 6000 BP, and the three younger groups between c. 3350 and 1800 BP, between c. 1250 and 400 BP. and within the last 100 years. This subdivision is supported by limiting radiocarbon dates from the Cordillera Blanca and from several cordilleras in southern Perú, and indicates that the Holocene glacial chronology of the Cordillera Blanca is the most complete heretofore recognized in the tropical Andes.