Lake records of Holocene climate change, Cordillera de Merida, Venezuela

Multi-proxy sediment records from four lakes in the Venezuelan Andes document changes in tropical climate over timescales of decades to millennia. The results are grouped into three topics: the Little Ice Age, the Holocene climate history, and atmospheric moisture balance of South America from oxygen isotopes.; A 1500-year reconstruction of climate history and glaciation indicates four glacial advances occurred between 1250 and 1810 A.D. These advances are coincident with solar activity minima. Temperature declines of 2.3 to 3.4° C and precipitation increases of 25 to 70% are required to produce the observed glacial responses. These results highlight the sensitivity of high-altitude tropical regions to relatively small changes in radiative forcing, implying even greater responses to future anthropogenic forcing.; On longer timescales, the Venezuelan Andes were generally wetter during the early Holocene. The middle Holocene was a time of low lake levels and reduced moisture balance whereas the late Holocene was wetter, with the wettest period occurring during the Little Ice Age. The pattern of millennial climate variability in Venezuela appears to be either a wet-dry-wet (Andes) or dry-wet-dry (lowlands) sequence. Comparison with climate records from North, Central and South America suggests this pattern is widespread near the northern and southern edges of the tropical monsoon climate regime and along the Andes near the equatorial Pacific.; The isotopic composition of Andean precipitation reflects evaporation conditions over the Atlantic Ocean, moisture recycling over the South American lowlands and uplift to the Andes. The isotopic composition of precipitation in the Venezuelan Andes, reconstructed from lake sediment diatom oxygen isotope records, show a 2.4 ‰ decrease during the Holocene. This decrease reflects a reduction in the moisture entering South America which reaches the Andes. Ice cores from Peru and Bolivia exhibit similar isotopic trends.; Direct orbital changes in solar insolation cannot explain the synchronous trends in both climatic and isotopic histories throughout the neotropics. However, sea surface temperature variation in the tropical Pacific may explain these trends because modern interannual variability in this region has similar effects in both hemispheres.