Scale and the isotopic record of C4 plants in pedogenic carbonate: from the biome to the rhizosphere

The C-13/C-12 ratio in pedogenic carbonate (i.e., CaCO3 formed in soil) is a significant tool for investigating C-4 biomes of the past. However, the paleoecological meaning of delta C-13 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(2)), an isotopic record interpreted as C-3 vegetation replacing C-4 grasslands may indicate invading C-3 woody shrubs instead of expanding C-3 forests (a common interpretation). ( 2) At the landscape scale (several tens of m(2) to hundreds of km(2)), 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(2) to m(2)), 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 (mu m(2) to cm(2)), carbonate formed on roots lack the 14-17 parts per thousand enrichment observed at broader scales, revealing different fractionation processes at different scales. A multi-scale approach in dealing with delta C-13 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.