HR: 1340h
AN: PP43A-0607    [Abstracts]
TI: Carbon Isotopic Ratio Of Individual Lignin Moieties In Stalagmite As A Novel Indicator Of Past C3/C4 Plant Variations
AU: * Hou, J
EM: Juzhi\_Hou@brown.edu
AF: Department of Geological Sciences, Brown University, 324 Brook Street, Providence, RI 02912 United States
AU: Huang, Y
EM: yongsong\_huang@brown.edu
AF: Department of Geological Sciences, Brown University, 324 Brook Street, Providence, RI 02912 United States
AU: Tan, M
EM: tanming@mail.iggcas.ac.cn
AF: Institute of Geology and Geophysics, Chinese Academy of Sciences, Qijia Huozi, Dewai, Beijing, 100029 China
AB: Oxygen isotopes,layer thickness,and trace elements of stalagmite provide excellent records of past climatic and environmental change.However,carbon isotopic ratios of speleothem are difficult to interpret,because multiple sources of carbon contribute to the formation of calcite.Organic humic substances inbeded in the stalagmite layers are potential paleoenvironmental archives,but their molecular compositions have not been characterized.Here we report the first molecular-level characterization of stalagmite organic matter using pyrolysis-gas chromatograph-mass spectrometry.Py-GC/MS analyses reveal that speleothems contain a large suite of lignin derived aromatic moeties,and the sample-to-sample variations in composition are relatively small.These compounds were fragments of microbial-oxidized lignin macromolecules,which were derived from vegetation growing in the overlaying soil.Py-GC/IRMS was used to measure isotopic ratio of the compounds found in stalagmites.Two sets of samples were studied.The first set of samples was collected along the longitudinal section of a speleothem,TS0302,from Shihua Cave,Beijing.The \delta$^{13}$C records of organic compounds show temporal variation, which is compared with the \delta$^{13}$C of bulk carbonate. The other set of samples was the top sediment of five speleothems in China (TS0301,Shihua Cave(39.83N,115.67E),Beijing;TW02,Water Cave(41.31N,124.1E),Liaoning;YMB01,Bailong Cave(24.38N, 102.71E),Yunnan;TJ9401,Jiguan Cave(33.75N,111.6E),Henan).The \delta$^{13}$C of organic compounds in these samples is roughly consistent with the C4 grass photosynthesis.Based on the results,we have developed a new approach to unravel the past changes in C3/C4 plant ratios based on carbon isotopic ratios of individual monomeric lignin molecules by using Py-GC/IRMS.Our approach overcomes the problems associated with interpreting carbon isotopic data of speleothem calcite.The new approach allows high resolution reconstruction of past regional variations in the abundance of C3 and C4 plants.
DE: 4808 Chemical tracers
DE: 3344 Paleoclimatology
DE: 1845 Limnology
DE: 1045 Low-temperature geochemistry
DE: 1615 Biogeochemical processes (4805)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting