HIGH-RESOLUTION δ18O RECORDS OF HYDROLOGIC VARIABILITY FROM THE SOUTH-CENTRAL PERUVIAN ANDES

Carbonate lakes in the Peruvian Andes can provide high-resolution records of the δ¹⁸O of lake water and, thus, of regional climate. Here we focus on δ¹⁸O records from authigenic calcite from two lakes in the central Peruvian Andes. Laguna Yuraicocha in the western cordillera of the central Peruvian Andes (12.53°S; 75.50°W; 4460 masl) is underlain and surrounded by Jurassic and Cretaceous limestone interbedded with siliciclastic rocks. A 6.9 meter-long sediment core is dominated by authigenic calcite (marl) with a mean concentration of 82 weight percent that has accumulated at a rate of ~ 1 mm yr^-1 for the past 6200 yr. Laguna Piuray is a carbonate lake in south-central Peru (13.42°S; 72.03°W; 3693 masl) that is surrounded by exposures of marl that span the interval from 14-8 ka. Collectively, these cores provide a nearly continuous composite δ¹⁸O record that spans the past ~14 ka. The age model for the cores is based on a combination of ²¹⁰Pb and AMS ¹⁴C from charcoal. Marl samples were taken with an average sampling interval of 8 years; samples were treated to remove organic matter, sieved to concentrate the <75 µm fraction, and the clay fraction was removed by pipette withdrawal. The <75 µm fraction contains abundant euhedral grains of calcite that are not abraded or corroded, thus reflecting their authigenic origin. The Piuray record reveals a pronounced two-step δ¹⁸O transition from the late glacial into the Holocene with a 4‰ enrichment from 14-12.5 ka, a 2‰ depletion from 12.5-11.5 ka, and, finally, a 6‰ enrichment from 11.5 -8 ka. The Yuraicocha record reveals a pronounced linear trend of δ¹⁸O depletion (-10.5 to -14.5 ‰) spanning the past ~6 ka, which likely reflects a progressive increase in hydrologic balance (i.e., the ratio of precipitation/evaporation) through the middle and late Holocene. This interpretation is consistent with basal core sediment that records lake low stands, and possible periodic dessication in the early-middle Holocene. The last 1200 yr of record reveals a 2‰ depletion culminating with the most depleted isotopes on record ~ AD 1800 followed by an abrupt 1.5 ‰ enrichment that began ~AD 1900 and continues to the present. These trends match those documented from ice cores, speleothems, and other carbonate lakes in Peru, thus implying a regional moisture balance control.