HR: 08:45h
AN: PP41B-04    [Abstracts]
TI: Terrestrial Plant Biomarkers Preserved in Cariaco Basin Sediments: Records of Abrupt Tropical Vegetation Response to Rapid Climate Changes
AU: * Hughen, K A
EM: khughen@whoi.edu
AF: Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543 United States
AU: Eglinton, T I
EM: teglinton@whoi.edu
AF: Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543 United States
AU: Makou, M
EM: mmakowski@whoi.edu
AF: Marine Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543 United States
AU: Xu, L
EM: lxu@whoi.edu
AF: Marine Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543 United States
AU: Sylva, S
EM: ssylva@whoi.edu
AF: Marine Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543 United States
AB: Organic-rich sediments from the anoxic Cariaco Basin, Venezuela, preserve high concentrations of biomarkers for reconstruction of terrestrial environmental conditions. Molecular-level investigations of organic compounds provide a valuable tool for extracting terrestrial signals from these annually laminated marine sediments. Differences in hydrogen isotopic fractionation between C$_{16-18}$ and C$_{24-30}$ {\it n}-alkanoic acids suggest a marine source for the shorter chain lengths and a terrestrial source for the longer chains. Records of carbon and hydrogen isotopes, as well as average carbon chain length (ACL), from long-chain {\it n}-alkanoic acids parallel millennial-scale changes in vegetation and climate between the late Glacial and Preboreal periods, 15,000 to 10,000 years ago. Data from all terrestrial chain lengths were combined to produce single $\delta$D and $\delta ^{13}$C indices through deglaciation, exhibiting enrichment during the late Glacial and Younger Dryas and depletion during the B$\o$lling-Aller$\o$d and Preboreal periods. $\delta$D reflects the hydrogen isotopic composition of environmental water used for plant growth, combined with evaporative enrichment within leaf spaces, and as such may act as a proxy for local aridity. Leaf wax $\delta ^{13}$C, which is a proxy for C$_{3}$ versus C$_{4}$ metabolic pathways, indicates that C$_{3}$ plants predominated in the Cariaco watershed during warm/wet B$\o$lling-Aller$\o$d and Holocene periods, and C$_{4}$ plant biomass proliferated during cool/dry Glacial and Younger Dryas intervals. Coupled carbon and hydrogen isotopic measurements together clearly distinguish deglacial climatic periods as wetter with C$_{3}$ vegetation versus drier with C$_{4}$ vegetation. High resolution biomarker records reveal the rapidity of vegetation changes in northern South America during the last deglaciation. The leaf wax data reveal that local vegetation biomass, although not necessarily entire assemblages, shifted between arid grassland and wetter forest taxa on timescales of decades. Comparison of ACL versus $\delta ^{13}$C for Cariaco Basin and NW African leaf waxes indicate that biomarkers reflect real changes in local South American vegetation and not contamination from long-distance transport during cold windy climates. The precise temporal relationship between tropical vegetation shifts and climate changes is measured by direct comparison of terrestrial vegetation and climate proxies from the same core. Abrupt deglacial climate shifts in tropical and high-latitude North Atlantic regions were synchronous, whereas changes in tropical vegetation consistently lagged climate shifts by several decades.
DE: 4825 Geochemistry
DE: 3344 Paleoclimatology
DE: 4802 Anoxic environments
DE: 1045 Low-temperature geochemistry
DE: 1615 Biogeochemical processes (4805)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting