Radiocarbon and δ13C depth profiles of soil organic carbon in the U.S. Great Plains: A possible spatial record of paleoenvironment and paleovegetation

Abstract Soil profiles from undisturbed grassland sites around the Great Plains of the USA were sampled for analysis of soil organic carbon (SOC) content (%), radiocarbon age and stable-carbon isotope composition ( δ 13 C). With the exception of a few pronounced dating anomalies, SOC radiocarbon age generally increases steadily with depth back to 10–15,000 cal yr BP, the deepest soil intervals in 9 of the 12 sites. The radiocarbon ages were used to establish the chronology of changes in past plant distribution over time and space, based on SOC δ 13 C as an indicator of C 3 and C 4 plant abundance. Changes were referenced to an SOC δ 13 C value of −20‰, which is the approximate mid-point between C 3 and C 4 carbon isotope composition, i.e., an equal mixture of C 3 and C 4 carbon. Prior to 10,000 cal yr BP, the region was dominated by C 3 plants with the exception of the southernmost Texas sites. From 10,000 to 2000 cal yr BP, C 4 plants expanded their range, initially as a peninsula of C 4 -predominant grasses extending northeastward and ultimately dominating all but the northernmost border of the region. Finally, the C 3 -predominant region re-expanded after 1000 cal yr BP, perhaps as a response to the Little Ice Age cooling. Despite uncertainties associated with using radiocarbon-dated SOC-depth profiles as a proxy, the past C 3 and C 4 plant distribution inferred from SOC δ 13 C conforms well to results from other paleoclimate proxies, and differences may be helpful in targeting future research.