Holocene hydrologic balance of tropical South America from oxygen isotopes of lake sediment opal, Venezuelan Andes

Precipitation in the South American Andes is derived from Atlantic Ocean evaporation which is modified by passage over lowland South America. The isotopic composition of Andean precipitation reflects evaporation conditions over the Atlantic Ocean, moisture recycling during advection across the South American lowlands and uplift to the Andes. Records of the oxygen isotope composition of precipitation in the Venezuelan Andes, derived from lake sediment diatom δ18O measurements, show a 2.4‰ decrease during the past 10,000 yr. A simple model of the evaporation, advection and uplift processes is used to understand the cause of the isotope shift. The data and model suggest that the decreasing δ18O reflects a decrease in the fraction of moisture entering South America that reaches the Andes. Ice cores from Peru and Bolivia exhibit similar isotope trends indicating that the shift occurred in both hemispheres. An isotopic record of Amazon River discharge is consistent with the Andean records, indicating increasing continental runoff was associated with the decreasing export of water vapor. Orbital changes in solar insolation cannot explain the synchronous trends in both hemispheres. Changing climate in the tropical Pacific is an attractive explanation for the trends because modern interannual variability in this region has similar effects in both hemispheres.