HR: 11:35h
AN: PP42A-06    [Abstracts]
TI: $\delta$$^{13}$C and $\delta$$^{15}$N Values of Soil Organic Matter Over Drought and Non-drought Affected Elevation Gradients in Ethiopia: Calibrating for Environmental Reconstruction
AU: * Terwilliger, V J
EM: terwilli@ku.edu
AF: University of Kansas, Department of Geography 213 Lindley Hall, Lawrence, KS 66045 United States
AU: Eshetu, Z
EM: zeshetu@telecom.net.et
AF: Southern University, Wondo-Genet College of Forestry P.O. Box 128, Shashemene, 000000 Ethiopia
AU: Colman, A S
EM: a.colman@gl.ciw.edu
AF: Carnegie Institution of Washington, Geophysical Laboratory 5251 Broad Branch Rd., NW, Washington, D.C 20015 United States
AU: Fogel, M
EM: m.fogel@gl.ciw.edu
AF: Carnegie Institution of Washington, Geophysical Laboratory 5251 Broad Branch Rd., NW, Washington, D.C 20015 United States
AB: Portions of Ethiopia today are experiencing increasing temperatures and drought frequencies. The longest known hominid record is in Ethiopia's Awash Basin. Reconstructing past environments in Ethiopia may, therefore, contribute both to understanding present day and past consequences of climate change. Studies suggest that at least 7000 years of environmental reconstruction may be possible from isotopic analyses of organic matter in some Ethiopian paleosols. We have measured $\delta$$^{13}$C and $\delta$$^{15}$N of organic matter from modern soils in Ethiopia to explore the climatic dependence of these signals and thus to determine the maximum resolution of climatic reconstruction possible by bulk isotopic analyses of soil organic matter (SOM). Surface soil samples were taken at elevations from 350 - 3500 m in drought affected regions and from 1050 - 3100 m in regions with no history of drought. Collections were made at an altitude resolution of better than 150 m. Deeper soil samples (max. 27 m) were also obtained at 22 elevations in sites of the Awash Basin that had already been studied using other paleoenvironmental proxies. Soils were sampled in grassland, shrubland, forest, and grass/sedge wetland. The $\delta$$^{15}$N values of SOM decreased significantly with increase in elevation and were sensitive to both overlaying vegetation type and drought proclivity. Our results support hypotheses that $\delta$$^{15}$N values vary with total nitrogen pools in soils which, in turn vary with humidity and associated microbial influences. The $\delta$$^{13}$C values of SOM had a quadratic relationship to elevation that most likely reflected the relative compositions of C3 and C4 biomass in overlying vegetation. Exposure to drought could not be detected by $\delta$$^{13}$C values. At sites in the Hadar region where depth profile measurements were made to 27 m, $\delta$$^{13}$C values decreased with depth. This result conforms to inferences from other proxy that a cooler, wetter climate previously existed. Nitrogen contents of soils below 3 m were often too low to be reliably analyzed for $\delta$$^{15}$N by on-line elemental analysis mass spectrometry. Our measurements of Ethiopian SOM $\delta$$^{13}$C and $\delta$$^{15}$N values correlate well with modern climate and vegetation. The mechanisms by which elevation and aridity strongly influence SOM $\delta$$^{15}$N values merit specific investigation.
DE: 1040 Isotopic composition/chemistry
DE: 1620 Climate dynamics (3309)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting