Environmental History at Laguna Yaguarú, Bolivia: Evidence from Pollen, Stable Carbon Isotopes, and an Embedded Mineral Facies

Lowland eastern Bolivia is an important area for paleoenvironmental research because of its location near the climatically controlled boundary between the Amazon basin rain forest and the seasonally dry savannas to the south. I present a multiple proxy study from Laguna Yaguarú (15˚36’S, 63˚13’W, 195 m) a large (250 ha) lake located in the forest-savanna ecotone. A 2.4 m long core recovered in May 2003 spans approximately in the last 5000 years. Chronological control is based on excess 210Pb sedimentation in the uppermost sediments and three AMS dates on plant macrofossils in deeper sediments. I undertook pollen and microscopic charcoal analysis of the Yaguarú core, supplemented with stable carbon isotope analysis, to provide a more complete history of vegetation and of possible climate shifts. Stable carbon isotope ratios in the lake sediments record the relative dominance of C3 and C4 plants in the watershed, and may reveal shifts in grassland communities not apparent in pollen spectra. The most notable feature of the sediment core is a 15 cm layer of gray clay which dates to ~5000 BP. Particle size analysis indicates that this layer represents a flood deposit. Pollen percentages changed significantly following the deposition of the clay layer. Pollen assemblages prior to the flood interval are dominated by Typha pollen, whereas subsequent assemblages contain mainly grass and forest pollen. Stable carbon isotope values within the flood deposit show high variability, perhaps indicating extra-basin carbon input. An increase in forest taxa beginning about 3000 BP correlates with nearby records and is attributed to southward migration of the ITCZ due to increased summer insolation in the southern hemisphere.