Orbital forcing signal in sediments of two Amazonian lakes

Paleolimnological data from two ancient lakes at 0degrees latitude in Amazonia indicate that past lake level changes reflect precessional (19,000 to 22,000 years) variations in insolation over the last 170,000 years. Water level in Lakes Pata and Verde, Brazil, is determined by the ratio of precipitation:leakage. Times of low lake level are indicated in sediments by oxidized clays, evidence of algal blooms and high K+ concentrations. Peak K+ values are attributed to biogenic concentration when the lake was reduced to a shallow, productive pool. Low lake phases correlate with maximum insolation during the dry season (June-July-August). Thus the last glacial maximum 18,000 to 22,000 yr BP was a wet time in northern Amazonia, and the driest period of the last 170,000 years was from 35,000 to 27,000 yr BP. These results from near the equator do not imply that precipitation changed synchronously across Amazonia, because geographic locations throughout the vast watershed were undoubtedly influenced by local moisture sources.