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

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 (δ¹³C). 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 δ¹³C as an indicator of C₃ and C₄ plant abundance. Changes were referenced to an SOC δ¹³C value of -20‰, which is the approximate mid-point between C₃ and C₄ carbon isotope composition, i.e., an equal mixture of C₃ and C₄ carbon. Prior to 10,000 cal yr BP, the region was dominated by C₃ plants with the exception of the southernmost Texas sites. From 10,000 to 2000 cal yr BP, C₄ plants expanded their range, initially as a peninsula of C₄-predominant grasses extending northeastward and ultimately dominating all but the northernmost border of the region. Finally, the C₃-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 C₃ and C₄ plant distribution inferred from SOC δ¹³C conforms well to results from other paleoclimate proxies, and differences may be helpful in targeting future research.