HR: 09:15h
AN: PP41B-06    [Abstracts]
TI: Lignin Phenol and $\delta^{13}$C Evidence of Quaternary Vegetation Change in South America
AU: Ficken, K J
EM: k.j.ficken@swan.ac.uk
AF: Dept. of Geography, University of Wales Swansea, Singleton Park , Swansea, SA2 8PP United Kingdom
AU: * Filley, T R
EM: filley@purdue.edu
AF: Dept. of Earth and Atmospheric Sciences, Purdue University, 550 Stadium Mall Drive, West Lafayette, IN 47907 United States
AU: Street-Perrott, F
EM: f.a.street-perrott@swan.ac.uk
AF: Dept. of Geography, University of Wales Swansea, Singleton Park , Swansea, SA2 8PP United Kingdom
AB: The very low CO$_{2}$/O$_{2}$ ratio at the last glacial maximum would have been physiologically challenging for C$_{3}$ land plants by enhancing photorespiration and water stress (Cerling {\it et al}., 1997). Strong palaeoclimatic gradients, along a transect from Venezuela to NE Chile, make it possible to separate the effects of lower pCO$_{2}$ (global) and variations in precipitation (regional). All lake sites lie below the upper limit of C$_{4}$ graminoids in the Andes ($\sim$ 4200m a.s.l) and should have been sensitive to past increases in C$_{4}$ biomass resulting from lower atmospheric CO$_{2}$ and/or decreased summer-monsoon rainfall. Lakes situated at the southern end of the transect exhibit lower $\delta^{13}$C values during the last glacial, when periods of moister climate and higher lake levels occurred, and higher values during the Holocene. In contrast, lakes at the northern end exhibit an opposite temporal trend, with higher isotope values during the Late Pleistocene and lower values during the Holocene. During the last glacial, lignin phenol distributions suggest a dominance of woody plants at the southern end of the transect and a dominance of grasses at the northern end. The opposite occurred during the Holocene, with grasses dominant in the south and woody plants in the north. We conclude that glacial / interglacial variations in precipitation amount and seasonality, driven mainly by orbital forcing, had a greater impact on the distribution of C$_{3}$ and C$_{4}$ biomass in the tropical Americas than low glacial-age CO$_{2}$, which on its own would have favoured the spread of C$_{4}$ plants. Cerling T.E. {\it et al}. (1997) Nature 153-158.
DE: 1040 Isotopic composition/chemistry
DE: 1055 Organic geochemistry
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting