Abstract
The 13C/12C 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 d13C 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 km2), 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 km2), 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 (cm2 to m2), 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 (lm2 to cm2), 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 d13C 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.