A Look at Amazon Basin Seasonal Dynamics with the Biophysical ...

50,000 year record of vegetation and climate change in Noel Kempff MercadoNational Park, Bolivian Amazon.Francis E. Mayle 1 , Rachel Burbridge 1 , Timothy J. Killeen 2,31 Department of Geography, University of Leicester, University Road, Leicester LE17RH, UK. E-mail: fem1@leicester.ac.uk (address for correspondence)2 Center for Applied Biodiversity Science, Conservation International, 2501 M Street,NW, Suite 200, Washington, DC 20037, USA. E-mail: t.killeen@conservation.org3 Museo de Historia Natural ‘Noel Kempff Mercado’, Avenida Irala 565, Casilla 2489,Santa Cruz de la Sierra, Santa Cruz, Bolivia.Noel Kempff Mercado National Park is a UNESCO World Heritage Site, ca. 15,000km 2 , located in NE Bolivia on the Precambrian Shield, adjacent to the Brazilian statesof Mato Grosso and Rondônia. This is an ideal study area because it occupies theclimatically-sensitive ecotone between humid evergreen rainforests to the north, semideciduousdry forests to the south, and upland cerrado savannas to the east. Theclimate is highly seasonal, with mean annual precipitation of ca. 1500 mm.The study sites are Laguna Chaplin (14 0 28’S, 61 0 04’W) and Laguna BellaVista (13 0 37’S, 61 0 33’W), 200-250 m above sea level, which are surrounded byhumid evergreen forest. They are large (4-6 km diameter), flat-bottomed, shallowlakes (2.0-2.5 m water depth), formed by subsidence along fault-lines of theunderlying Shield.Analysis of fossil pollen and charcoal from 3 metre cores from these 2 sites,located 100 km apart, reveals a pattern of regional vegetation change over the last50,000 years. Chronological control was provided by a suite of AMS 14 C dates. Amix of deciduous dry forest and savannas covered the park between 50,000 and40,000 yr BP, while stable carbon isotope values show that more open savannaspredominated between 40,000 and 18,000 yr BP. We suggest that this expansion ofC4 grasses at the last glacial maximum (LGM) was driven by a combination of lowatmospheric CO 2 concentrations and a slight reduction in precipitation. Galleryforests contained Podocarpus trees, providing further evidence that LGMtemperatures were significantly lower than today.Savannas continued to dominate throughout most of the Holocene, withrainforests restricted to riparian zones. Humid evergreen rainforests only expanded todominate the area between 3,000 and 2,000 yr BP, due to increased precipitation inSW Amazonia, an inference supported by comparison with ice-core (Thompson et al.,1998) and lake-level records from the Bolivian Andes (Baker et al., 2001), as well assoil carbon isotope data from more central parts of the Basin (de Freitas et al., 2001).This Late Holocene increase in precipitation in southern Amazonia can be attributedto more intense insolation in the austral summer, in turn explicable by orbital forcingaccording to Milankovitch Theory.