HR: 08:00h
AN: PP41D-01    [Abstracts]
TI: Comparison of Vegetation Change Inferred From Palynology and Compound-Specific Carbon Isotopes of Lipid Biomarkers in the Maya Lowlands of Peten, Guatemala
AU: * Newell, S D
EM: sdnewell@stanford.edu
AF: University of Florida, Department of Geological Sciences 241 Williamson Hall P.O. Box 112120, Gainesville, FL 32611 United States
AU: Hodell, D A
EM: dhodell@geology.ufl.edu
AF: University of Florida, Department of Geological Sciences 241 Williamson Hall P.O. Box 112120, Gainesville, FL 32611 United States
AU: Curtis, J H
EM: curtisj@ufl.edu
AF: University of Florida, Department of Geological Sciences 241 Williamson Hall P.O. Box 112120, Gainesville, FL 32611 United States
AU: Brenner, M
EM: brenner@ufl.edu
AF: University of Florida, Department of Geological Sciences 241 Williamson Hall P.O. Box 112120, Gainesville, FL 32611 United States
AU: Venz-Curtis, K
EM:
AF: University of Florida, Department of Geological Sciences 241 Williamson Hall P.O. Box 112120, Gainesville, FL 32611 United States
AB: The Petén region of northern Guatemala has been occupied by humans for more than 3000 years. Expansion of the Maya civilization during the Preclassic (~1000 BC to AD 250) and Classic (AD 250 to AD 900) Periods was accompanied by increasing deforestation of Petén watersheds and accelerated rates of soil erosion. Palynological data from Petén lake cores illustrate the near elimination of high forest taxa and prevalence of disturbance taxa (grasses, weeds) during the height of Classic Maya occupation (~AD 500 to AD 800). After flourishing during the Classic Period between AD 250 and 800, Maya population densities declined significantly in the Petén, thereby curtailing human pressures on the landscape. This cycle of population expansion and decline in the Petén provides a natural historical experiment that has been used to study the response of tropical vegetation to long-term changes in land-use by humans. We measured the carbon isotopic composition of long-chain n-alkanes of leaf waxes in two cores from Lakes Sacnab and Salpetén in the Petén Lake District of the southern Maya Lowlands. The carbon isotopic composition of leaf waxes has been shown to be a reliable indicator of the relative proportion of C₃ to C₄ biomass in a watershed. Biomarker results were compared directly to a pollen profile from Lake Salpetén. Although the general pattern of increased C₄ abundance inferred from δ¹³C of long-chain n-alkanes and increased disturbance taxa from pollen studies agree during the period of Maya occupation, the two proxies differ in detail suggesting they are recording different characteristics of watershed vegetation. For example, the highest long-chain δ¹³C values (representing greatest C₄ biomass) occurred during early settlement of the basins in the early to middle Preclassic Period (1300 to 500 BC) when Maya population densities were relatively low. This period also corresponded to the time of greatest erosion rates in the Salpetén basin (Anselmetti et al., in prep.). In contrast, pollen profiles indicate the highest percentages of disturbance taxa and lowest high forest taxa from 250 BC to AD 700 in the late Preclassic and Classic Periods, when both the δ¹³C of long-chain n-alkanes and erosion rates declined. The high compound-specific δ¹³C values in the early to middle Preclassic Period may be greatly influenced by the early cultivation of maize (a C₄ plant) along the shoreline. In contrast, maize is generally under-represented in pollen profiles because of its large size and difficulty of transport. Pollen may be more representative of vegetation over a larger area than the lipid biomarkers, but is biased towards tropical vegetation that is pollinated by wind and does not reflect those plants that depend on pollination by insects or self-fertilization. The δ¹³C of long-chain n-alkanes, on the other hand, may provide a reliable proxy for estimating the relative changes in C₃ to C₄ biomass, but it cannot be used to interpret changes in forest structure and composition. We conclude that the δ¹³C of long-chain n-alkanes and pollen are recording different aspects of watershed vegetation in Petén, and should be used in tandem to infer the response of tropical vegetation to past changes in Maya land-use.
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1055 Organic and biogenic geochemistry
DE: 1632 Land cover change
DE: 1637 Regional climate change
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005