Concepts & Synthesis Emphasizing New Ideas to Stimulate Research in Ecology

The C/C ratio in pedogenic carbonate (i.e., CaCO3 formed in soil) is a significant tool for investigating C4 biomes of the past. However, the paleoecological meaning of dC values in pedogenic carbonate can change with the scale at which one considers the data. We describe studies of modern soils, fossil soils, and vegetation change in the Chihuahuan Desert of North America and elsewhere that reveal four scales important for paleoecologic interpretations. (1) At the broadest scale, the biome scale (hundreds to millions of km), an isotopic record interpreted as C3 vegetation replacing C4 grasslands may indicate invading C3 woody shrubs instead of expanding C3 forests (a common interpretation). (2) At the landscape scale (several tens of m2 to hundreds of km), the accuracy of scaling up paleoclimatic interpretations to a regional level is affected by the landform containing the isotopic record. (3) At the soil-profile scale (cm to m), soil profiles with multiple generations of carbonate mixed together have a lower-resolution paleoecologic record than soil profiles repeatedly buried. (4) At the rhizosphere scale (lm to cm), carbonate formed on roots lack the 14–17ø enrichment observed at broader scales, revealing different fractionation processes at different scales. A multi-scale approach in dealing with dC in pedogenic carbonate will increase the accuracy of paleoecologic interpretations and understanding of soil–geomorphic– climatic interactions that affect boundaries between C4 and C3 vegetation.