THE ROLE OF THE HYDROLOGIC BALANCE OF THE SOUTH AMERICAN TROPICS ON LATE QUATERNARY GLACIATION OF THE DRY, WESTERN CORDILLERA OF PERU

The equilibrium lines of modern glaciers on the arid (western) side of the Peruvian Andes lies several hundred meters above the 0°C isotherm making these glaciers especially sensitive to changes in accumulation, and thus in regional hydrologic balance. Continuous records of late Quaternary glacial activity have been developed from the proportion of glacial flour in downvalley glacial lakes in the Cordilleras Blanca and Huayhuash, and at the northeastern edge of the Quelccaya Ice Cap. These records correlate well with continuous records of hydrologic balance derived from the d¹⁸O of authigenic carbonates in Lake Junin, central Peru, and with El Niño activity as recorded in the frequency of storm-derived laminae in Laguna Pallcacocha, Ecuador. The Holocene trend in glacial flour sedimentation corresponds closely to the trend in regional hydrologic balance. This latter is controlled by the strength of austral summer (DJF) insolation, which controls the vigor of the South American summer monsoon and the advection of moisture from the tropical Atlantic Ocean. The glacial flour trends reveal rapid deglaciation midway through the Younger Dryas, a minimum in glacial activity during the early Holocene, and an increase in glacial activity beginning ~8000 cal yr B.P. that culminated in maximum Holocene glacial activity ~1600 cal yr B.P. Millennial-centennial cycles in glacial flour production correlate with intervals of enhanced El Niño activity and with intervals of reduced hydrologic balance in the central Andes. This suggests a climatic connection among ENSO, glacier mass balance, and the advection of moisture from the tropical Atlantic to the central Andes. Apparently, since the middle of the Younger Dryas hydrologic variability has played a far greater role in governing the activity of glaciers on the western side of the Andes than has temperature. Hydrologic variability in the tropics during marine isotope stage 2 may have played an important role in the apparent asynchronous behavior of tropical glaciers during this interval, and this needs to be considered when making global comparisons of ice marginal change.