Scale and the Isotopic Record of C4 Plants In Pedogenic Carbonate—Biome to Rhizosphere

The ¹³C/¹²C ratio in pedogenic carbonate is a significant tool for investigating C4 biomes of the past. However, the paleoecological meaning of carbon isotopes in pedogenic carbonate can change with the scale at which one considers the data. We describe studies of modern soils, paleosols, and C4/C3 vegetation boundaries in the Chihuahuan Desert of North America and elsewhere which reveal four scales important for paleoecologic interpretations. (1) At the broadest scale, the biome scale (tens to thousands of km), an isotopic record interpreted as C3 forests replacing C4 grasslands may actually represent C3 desertscrub replacing C4 grasslands as the result of aridity. (2) At the landscape scale (hundreds m to km), an isotopic record of paleoecology can be influenced by landforms, which has bearing on the accuracy of scaling up paleoclimatic interpretations to larger regions. (3) At the soil profile scale (m), an isotopic record contained in soils of stable geomorphic surfaces will be more overprinted (and less detailed) than isotopic records in soils of aggrading environments. (4) At the rhizosphere scale (µm to mm), carbonates formed on roots lack the 14-17‰ enrichment observed at broader scales, revealing different fractionation processes at different scales. A multi-scale approach applied to carbon isotopes in pedogenic carbonate will improve accuracy of paleoecologic interpretations and increase understanding of landscape processes that impact C4/C3 competition.