LATE QUATERNARY GLACIAL HISTORY IN THE ENCIERRO VALLEY, NORTHERN CHILE (29°S), DEDUCED FROM 10BE SURFACE EXPOSURE DATING

The arid Central Andes (18°S-27°S) are of particular paleoclimatological interest, because they mark the transition zone between tropical and extratropical circulation systems. Intensity changes or shifts of these systems should therefore have been sensitively recorded in suitable archives, e.g. moraines. Despite of altitudes above 6000 m, no glaciers exist in the Central Andes today due to the extreme aridity. However, huge moraines just north and south of the so-called ‘Arid Diagonal' document considerable Late Quaternary glaciation, indicating substantially increased precipitation. Here, we present first results form surface exposure dating using cosmogenic ¹⁰Be from the Encierro Valley, northern Chile (29°S), which were obtained in order to establish an absolute glacial chronology. Exposure ages from eleven boulders (calculated with a SLHL production rate of 5.25 at/a g SiO₂, and scaling according to Desilets and Zreda (2003, EPSL 206, 21-42) suggest that: (i) The most prominent moraine was deposited by a glacial advance at ~15 ka BP (14.9, 14.4, 13.8, 13.1, 11.6 and 9.7 ka BP). (ii) A recessional moraine was dated to ~12 ka BP (12.4, 12.1, 11.6 and 11.1 ka BP). (iii) Based on one sample, we tentatively suggest a preceding glacial advance ~25 ka BP. Sample-specific corrections for topographic shielding, sample geometry and rock surface erosion are small (<3%). The observed data scatter can not be explained by uncertainties due to these effects or by AMS measurement errors (<5%). Instead, we suggest that post-depositional geomorphological instability caused boulders to rotate and to be exhumed from the moraine matrix; boulder ages can easily underestimate the real deposition age of moraines (Zech et al., 2005, EPSL, in press). Further investigations are necessary to reduce the systematic uncertainties (~10%) and thus to clarify the correlation with northern hemispheric cooling events (Oldest and Younger Dryas, respectively). Besides, possible correlations with results north of the Arid Diagonal (Smith et al., 2005, Science 308, 678-681; Zreda et al., 2001, Fifth Iberian Quaternary Meeting, Lisboa, Portugal) deserve further attention.