Clastic sediment flux to tropical Andean lakes: records of glaciation and soil erosion

We developed records of clastic sediment flux to 13 alpine lakes in Peru, Ecuador, and Bolivia, and compared these with independently dated records of regional glaciation. Our objectives are to determine whether a strong relationship exists between the extent of ice cover in the region and the rate of clastic sediment delivery to alpine lakes, and thus whether clastic sediment records serve as reliable proxies for glaciation during the late Pleistocene. We isolated the clastic component in lake sediment cores by removing the majority of the biogenic and authigenic components from the bulk sediment record, and we dated cores by a combination of radiocarbon and tephrochronology. In order to partially account for intra-basin differences in sediment focusing, bedrock erosivity, and sediment availability, we normalized each record to the weighted mean value of clastic sediment flux for each respective core. This enabled the stacking of all 13 lake records to produce a composite record that is generally representative of the tropical Andes. There is a striking similarity between the composite record of clastic sediment flux and the distribution of 100 cosmogenic radionuclide (CRN) exposure ages for erratics on moraine crests in the central Peruvian and northern Bolivian Andes. The extent of ice cover thus appears to be the primary variable controlling the delivery of clastic sediment to alpine lakes in the region, which bolsters the increasing use of clastic sediment flux as a proxy for the extent of ice cover in the region. The CRN moraine record and the stacked lake core composite record together indicate that the expansion of ice cover and concomitant increase in clastic sediment flux began at least 40 ka, and the local last glacial maximum (LLGM) culminated between 30 and 20 ka. A decline in clastic sediment flux that began 20 ka appears to mark the onset of deglaciation from the LLGM, at least one millennium prior to significant warming in high latitude regions. The interval between 20 and 18ka was marked by near-Holocene levels of clastic sediment flux, and appears to have been an interval of much reduced ice extent. An abrupt increase in clastic sediment flux 18ka heralded the onset of an interval of expanded ice cover that lasted until 14ka. Clastic sediment flux declined thereafter to reach the lowest levels of the entire length of record during the early–middle Holocene. A middle Holocene climatic transition is apparent in nearly all records and likely reflects the onset of Neoglaciation and/or enhanced soil erosion in the tropical Andes. & 2008 Elsevier Ltd. All rights reserved.