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United States Department of Agriculture

Agricultural Research Service


item Leavitt, S
item Follett, Ronald - Ron
item Kimble, John
item Pruessner, Elizabeth

Submitted to: Elsevier
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/3/2005
Publication Date: 12/1/2006
Citation: Leavitt, S.W., Follett, R.F., Kimble, J., Pruessner, E.G. 2006. Paleoenvironment and paleovegetation from delta 13c of soil organic carbon in 14c-dated profiles in the U.S. great plains. Elsevier Quaternary International. doi:10.1016/j. quaint.2006.10.033.

Interpretive Summary: Soil carbon 14 dating and its 13C/12C isotope ratios (expressed as Delta 13C as per mil -- 0/00) produce a portrait of environmental changes in the Great Plains that consistent with other studies and proxy indicators in the region. Based on the Delta 13C isopleth at -20 0/00, considered to represent equal contribution of carbon from cool season (C3) and warm season (C4) plants, prior to 10,000 calendar years before present (Cal BP), C3 plants dominated the region. Beginning around 10,000 Cal BP the area of C4 dominance began to expand, probably driven by warming and perhaps increased summer precipitation. The significant C4 expansion was from ~10,000 and 7,000 Cal BP, with a particularly rapid expansion between 8,000 and 6,000 Cal BP, consistent with increasing aridity found in the mid-Holocene. Area of C4 dominance was greatest around 2,000 to 1,000 Cal BP, near the time of the so-called Medieval Warm Period. Over the last 1000 years, the C3-dominant region appears to have re-expanded, perhaps associated with the Little Ice Age that has dominated the climate during the last millennium. Current global warming does not seem to have yet imparted any impression on the SOC isotopic composition caused by plant changes, but there are also possible competing effects of increasing CO2 concentration that may confound such changes.

Technical Abstract: Soil profiles from undisturbed grassland sites around the U.S. Great Plains were sampled for analysis of soil organic carbon content (0/0), radiocarbon age and stable-carbon isotope composition (Delta 13C). With few exceptions, SOC radiocarbon age decreases monotonically with depth, dating back to 10-15,000 Cal BP in the deepest soil intervals in 9 of the 12 sites. The radiocarbon ages were used to characterize changes in past plant distribution in time and space based on SOC Deleta 13C as an indicator of C3 and C4 plant abundance. Changes were referenced to SOC Delta 13C of -20 0/00, which is the approximate mid-point between pure C3 and C4 carbon isotope composition, i.e., an equal mixture of C3 and C4 carbon. The region was dominated by C3 plan Prior to 10,000 Cal BP except in the southernmost Texas sites. From 10,000 to 2,000 Cal BP C4 plants expanded their range throughout the region. Finally, the C3-predominant region apparently re-expanded after 1,000 Cal BP.

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