Friday, 14 July 2006

Scale, Paleoclimate, and 13C in Pedogenic Carbonate: From the Rhizosphere to the Biome.

H. Curtis Monger, New Mexico State University, Dept of Agronomy and Horticulture, Box 30003 MSC 3Q, Las Cruces, NM 88003 and David R. Cole, Oak Ridge National Laboratory, Building 4500S, MS-6110, P.O. Box 2008, Oak Ridge, TN 37831-6110.

Soil and paleosol profiles record data about global processes that take place over different time and spatial scales. For example, the ratio of 13C/12C in pedogenic CaCO3 is controlled by vegetation that, in turn, is controlled by climate. Consequently, this isotopic ratio has become an important tool for understanding climates of the past. However, the connection between 13C/12C in pedogenic CaCO3 and paleoclimate involves crossing four scales where complexities arise that may lead to inaccurate interpretations. At the first scale, the rhizosphere scale, carbonates crystallize on a time scale of 100 to 102 years over distances of 10-6 to 10-2 meters. In this micro-environment, some plant species produce crystals on their roots that exibit 13C/12C fractionations greater than the 14 to 16 typically observed between soil CO2 and pedogenic carbonate. At the second scale, the soil profile scale (100 to 104 yr and 10-3 to 102 m), newly precipitated CaCO3 overprints previously precipitated CaCO3. At the third scale, the landscape scale (101 to 106 yr and 101 to 105 m), the lateral variability in soil type, topography, and hydrology produce heterogeneous C3 and C4 vegetation patterns within a small area. At the fourth scale, the biome scale (102 to 107 yr and 104 to 109 m), two boundaries exist between C3 and C4 vegetation: one at the C3 desert shrubland/C4 grassland boundary and one at the C4 grassland/C3 woodland boundary. Addressing these complexities at each of the four scales leads to more accurate paleoclimatic interpretations and an increased understanding of the climate-vegetation-soil system.

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