HR: 15:30h
AN: PP53D-08    [Abstracts]
TI: A 10 Million Year, High-Resolution Record of C4 and C3 Plant Evolution from Arabian Sea ODP Site 722
AU: * Huang, Y
EM: Yongsong_Huang@brown.edu
AF: Brown University, Department of Geological Sciences, Providence, RI 02912 United States
AU: Clemens, S
EM: steven_Clemens@brown.edu
AF: Brown University, Department of Geological Sciences, Providence, RI 02912 United States
AU: Liu, W
EM: Weiguo_Liu@brown.edu
AF: Brown University, Department of Geological Sciences, Providence, RI 02912 United States
AU: Wang, Y
EM: Yi_Wang@brown.edu
AF: Brown University, Department of Geological Sciences, Providence, RI 02912 United States
AU: Prell, W
EM: warren_prell@Brown.edu
AF: Brown University, Department of Geological Sciences, Providence, RI 02912 United States
AB: The Siwalik paleosol sequence in Pakistan and India and the Bengal Fan sediments indicate a major expansion of C4 plants during the late Miocene, approximately 8 to 5 Ma. However, the depositional environments of paleosol sequences and deep-sea fans result in uncertainties in chronology and sediment discontinuities. The paleosol isotopic data are also local in nature. Here, we report new high-resolution carbon and hydrogen isotopic measurements of higher plant biomarkers from sediments in the Arabian Sea (ODP 722B). This site is situated at 2000 m water depth on the Owen Ridge, isolated from turbidite deposition on the adjacent Indus Fan. Continuous deposition allows more accurate age control, based on oxygen isotope and nannofossil stratigraphy. The principal source of terrestrial input to the site is from aeolian sources associated with monsoon circulation, which results in the transport of plant leaf waxes from adjacent continental regions, especially the Arabian Peninsula, the Middle East, and the Indian subcontinent. Therefore, our data represent the first continuous integrated large scale records of C3 and C4 plants for these continental regions. We analyzed ca 200 samples over the last 10 My. Our results show that C4 plants were already present by 10 Ma. Assuming end member 13C values for C3 (-34 %) and C4 (-20 %) plant leaf waxes, C4 plant percentage input increased from 25 to 45 percent from 10 to 7.5 Ma. Surprisingly, however, the C4 input decreased to 15 percent from 7.5 to 6.8 Ma. The major rise of C4 plant inputs occurred between 6.8 to 5 Ma, when the C4 percentage input increased from 15 to 65 percent. C4 percentage continued to rise slowly from 5 Ma, reaching 75 percent at 0.7 Ma. A C4 decrease from 57 percent took place from 0.7 Ma to present. Our hydrogen isotopic ratios of leaf waxes suggest a major increase in continental aridity between 8 to 6 Ma, which is followed by a slower rate of aridity increases from 6 to 0.7 Ma. Both carbon and hydrogen isotopic ratios become more variable on shorter time scales after 6 Ma than before 8 Ma. Notably, a decrease in C4 plants by ca. 15 percent, just prior to the major expansion of C4 plants, was also observed in published Siwalik paleosol carbonate 13C record. This pattern of C4/C3 plant evolution has not yet been observed in other continents, suggesting important regional climatic control, rather than a global change in atmospheric pCO2 may have played a key role on the rise of C4 plant during the late Miocene.
DE: 4870 Stable isotopes
DE: 3344 Paleoclimatology
DE: 3022 Marine sediments--processes and transport
DE: 1055 Organic geochemistry
DE: 0315 Biosphere/atmosphere interactions
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting