LATE GLACIAL AND HOLOCENE GLACIER FLUCTUATIONS IN THE CORDILLERA BLANCA, PERUVIAN ANDES

Discerning the timing and pattern of Late Quaternary glacier variability in the tropical Andes is important for our understanding of global climate change. Terrestrial cosmogenic nuclide (TCN) ages on moraines, and radiocarbon-dated clastic sediment records from moraine-dammed lakes in the Cordillera Blanca in Peru, document the waxing and waning of alpine glaciers in the Western Cordillera during the last ca. 16 ka. The integrated moraine and lake records indicate that ice was advanced during the Antarctic Cold Reversal (ACR), and began retreating by ca. 13.7 ± 0.6 ka. Proxy indicators of glacigenic sediment remained low after ca. 13.7 ka and throughout the Younger Dryas. Clastic sediments and TCN ages indicate that an early Holocene advance occurred ca. 10.7 ± 0.5 ka. TCN ages indicate that the glacier retreated nearly half the distance up-valley from its ACR position by ca. 9.4 ± 0.1 ka. Proxy indicators of glacigenic and paraglacial sediment values generally increase from ca. 8-4 ka, followed by lowering values during much of the late Holocene through the Medieval Climate Anomaly. A single TCN age on a boulder down valley of the Little Ice Age (LIA) position dates to ca. 3.0 ± 0.1 ka, tentatively suggesting that there was a late Holocene re-advance or recessional moraine that was more extensive than the LIA. Based on the lake sediment data, a final Holocene advance occurred during the LIA under colder and wetter conditions as documented in regional proxy archives. Late Glacial and Holocene TCN ages are largely consistent across the tropical Peruvian Andes, however, the lake sediment records contain evidence of periods of advancing and retreating glaciers that are not captured in the moraine chronologies. Moreover, the pattern of late Holocene glacial variability, as recorded in the lake sediments, varies at times when comparing sites across the tropical Andes.