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