GLACIAL LACUSTRINE RECORDS OF THE LAST DEGLACIAL HEMICYCLE IN THE TROPICAL ANDES

Cores from glacial lakes in the tropical Andes provide continuous, high-resolution proxy records of glacial flour input and paleoglacier extent, storm-induced sedimentation, and hydrologic balance. Here we review cores that span the last 12,000-25,000 cal years from glacial lakes located from 2°45’-13°54’S and from 2500-4920 m. Age-depth models for these cores are based on AMS ¹⁴C dating of macro-vegetal material, ²¹⁰Pb dating, and tephrochronology. Tephra layers are limited to cores from lakes in southern Ecuador where up to 8 late glacial-Holocene tephra are preserved. These tephra have a diagnostic major element geochemistry, and were deposited ~15,500, 15,100, 9,000, 8800, 7300, 5300, 2500, and 2200 cal yr BP.

The first-order change in sedimentation seen in all lakes is an abrupt transition from inorganic (<1% TOC) sediment to organic-rich (>10% TOC) sediment, which reflects deglaciation of upvalley cirques. Deglaciation from M.I.S. 2 ice limits commenced > 20,000 cal yr B.P.; lakes located in catchments with cirque headwalls <5000 m record complete deglaciation between 16,330 and 11,400 cal yr B.P. Lakes in catchments with headwall elevations >5500 m record the continuous presence ice throughout the late-Quaternary. Laguna Pacococha, adjacent to the Quelccaya Ice Cap (QIC; 4920 m) in southeastern Peru, reveals complete disappearance of the QIC 12,000 cal yr B.P. and reappearance 5000 cal yr B.P. The concentration of glacier flour in alpine lakes in catchments that have been continuously occupied by glaciers reveals rapid deglaciation midway through the Younger Dryas, a minimum in glacial activity during the early Holocene, and maximum Holocene glacial activity ~1600 cal yr B.P. Superimposed on these overall trends of glacial flour production are oscillations at millennial-centennial time scales.

Records of El Niño-induced sedimentation from southern Ecuador and a δ¹⁸O record of hydrologic balance from authigenic carbonates in Lake Junin, Peru reveal a progressive increase in moisture from both the tropical Pacific and tropical Atlantic beginning in the early-mid Holocene. Superimposed on this Holocene trend are oscillations at millennial-centennial scales. Regional changes in hydrologic balance are controlled by the strength of austral summer (DJF) insolation and by the state of the ENSO.