HR: 16:20h
AN: U14A-02 INVITED     [Abstracts]
TI: Biomarkers Reveal Late Neogene Changes in East African Vegetation
AU: * Ingram, S J
EM: singram@ldeo.columbia.edu
AF: LDEO, PO BOX 1000, Palisades, NY 10964 United States
AU: deMenocal, P B
EM: peter@ldeo.columbia.edu
AF: LDEO, PO BOX 1000, Palisades, NY 10964 United States
AU: Eglinton, T I
EM: teglinton@whoi.edu
AF: WHOI, Quissett Campus, Woods Hole, MA 02543 United States
AB: Multiple paleoenvironmental records incorporating diverse proxies and sites are needed to build a complete picture of African environments during the late Neogene period of hominid evolution. Augmenting the picture of progressive aridification of Neogene environments indicated by the progressive enrichment of soil carbonates (Cerling and Hay, 1988; Cerling, 1992; Wynn, 2000), Hominid environment-evolution hypotheses have drawn on evidence for high frequency environmental variability to emphasize the potential importance of climate variability as an evolutionary selection factor (Potts, 1996). Dramatic evidence for Neogene climatic variability is seen in high benthic foraminiferal oxygen isotope records of high latitude climate (Shackleton, 1990, Mix et al., 1995). This `global' signal has also been detected in low latitudes as shown by orbital frequency climate variability in dust records from marine cores around the African continent (deMenocal, 1995). Terrestrial records may provide a more direct record of local habitat change than distal dust deposits, but are unable to resolve with continuity the orbital frequency climatic variability seen in marine records. We therefore turn to molecule-specific carbon isotopic analyses of plant biomarkers in marine sediments to indicate regionally integrative C3-C4 vegetation changes. The validity of this approach has already been demonstrated in marine cores off West Africa (Schefuss et al, 2003). In order to bridge the gap between large scale climatic conditions and local rift valley conditions, we select a marine core in close proximity to hominid sites in the East African rift valley to generate a high resolution, regional integrative record of vegetation change during the Neogene. We take the approach of high resolution (3ka) sampling, capable of resolving precessional variability, in `windows' downcore to provide a measure of the mean and variability of vegetation for timesclices during the last 10Ma. Our results show that the long term trend towards carbon isotopic enrichment is consistent with paleosol data. Biomarker results further demonstrate that orbital frequency isotopic variability in the Pliocene and Pleistocene was of equal or greater magnitude than the long term trend. High frequency environmental variability is also seen at ~9Ma, where although carbon isotopic compositions indicate exclusively C3 vegetation, biomarker fluxes indicate significant environmental variability on precessional timescales. These initial biomarker results suggest the importance of high frequency environmental variability in hominid habitats in excess of long term change.
DE: 8404 Ash deposits
DE: 3344 Paleoclimatology
DE: 1040 Isotopic composition/chemistry
DE: 1055 Organic geochemistry
SC: Union [U]
MN: 2004 AGU Fall Meeting