HR: 09:00h
AN: PP51D-05    [Abstracts]
TI: Stable Carbon Isotope Ratios of Individual Pollen Grains as a Proxy for C3- Versus C4-Grass Abundance in Paleorecords: A Validation Study
AU: * Nelson, D M
EM: dmnelson@life.uiuc.edu
AF: Institute for Genomic Biology, University of Illinois, 1206 W. Gregory Dr., Urbana, IL 61801, United States
AU: * Nelson, D M
EM: dmnelson@life.uiuc.edu
AF: Department of Earth and Planetary Sciences, Harvard University, 20 Oxford St., Cambridge, MA 02138, United States
AU: Hu, F
EM: fshu@life.uiuc.edu
AF: Department of Plant Biology, University of Illinois, 265 Morrill Hall, 505 S. Goodwin Ave, Urbana, IL 61801, United States
AU: Hu, F
EM: fshu@life.uiuc.edu
AF: Department of Geology, University of Illinois, 245 Natural History Building, 1301 W. Green St., Urbana, IL 61801, United States
AU: Pearson, A
EM: pearson@eps.harvard.edu
AF: Department of Earth and Planetary Sciences, Harvard University, 20 Oxford St., Cambridge, MA 02138, United States
AB: C₃ and C₄ grasses have distinct influences on major biogeochemical processes and unique responses to important environmental controls. Difficulties in distinguishing between these two functional groups of grasses have hindered paleoecological studies of grass-dominated ecosystems. We recently developed a technique to analyze the stable carbon isotope composition of individual grass-pollen grains using a spooling- wire microcombustion device interfaced with an isotope-ratio mass spectrometer (Nelson et al. 2007). This technique holds promise for improving C₃ and C₄ grass reconstructions. It requires ~90% fewer grains than typical methods and avoids assumptions associated with mixing models. However, our previous work was based on known C₃ and C₄ grasses from herbarium specimens and field collections and the technique had not been test using geological samples. To test the ability of this technique to reproduce the abundance of C₃ and C₄ grasses on the landscape, we measured δ¹³C values of >1500 individual grains of grass pollen isolated from the surface sediments of 10 North American lakes that span a large gradient of C₃ and C₄ grass abundance. Results indicate a strong positive correlation (r=0.94) between % C₄-grass pollen (derived from classifying δ¹³C values from single grains as C₃ and C₄) and the literature-reported abundance of C₄ grasses on the landscape. However, the measured % C₄-grass pollen shows some deviation from the actual abundance at sites with high proportions of C₄ grasses. This is likely caused by uncertainty in the magnitude, composition, and variability of the analytical blank associated with these measurements. Correcting for this deviation using regression analysis improves the estimation of the abundance of C₄ grasses on the landscape. Comparison of the % C₄-grass pollen with C/N and δ¹³C measurements of total organic matter in the same lake-sediment samples illustrates the distinct advantage of grass-pollen δ¹³C as a proxy for distinguishing C₃ and C₄ shifts. At 9 of the 10 sites C/N values indicate that surface-sediment organic matter was derived primarily from aquatic production. At the one site where organic matter was produced primarily by vascular plants the δ¹³C value (-29.3‰) suggests organic matter derived exclusively from C₃ plants. However, ~80% of the grasses on the landscape at this site are C₄ grasses. The C₃- like bulk-sediment δ¹³C value likely represents woody species, which comprise >90% of the pollen spectra. Thus δ¹³C analysis of single grains of grass pollen offers a new tool to classify grass pollen into two major functional groups and promises to advance our understanding of grassland ecology and evolution. Reference Nelson, D.M., Hu, F.S., Mikucki, J., Tian, J., and Pearson, A., 2007, Carbon isotopic analysis of individual pollen grains from C₃ and C₄ grasses using a spooling wire microcombustion interface: Geochimica et Cosmochimica Acta, v. 71, p. 4005-4014.
DE: 0454 Isotopic composition and chemistry (1041, 4870)
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 4950 Paleoecology
DE: 4952 Palynology
DE: 4994 Instruments and techniques
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Fall Meeting