MICROTOPOGRAPHIC ISOTOPE VARIATIONS IN A MODERN VERTISOL: IMPLICATIONS FOR PALEOCLIMATE STUDIES

Much of the current interest in paleosols is driven by their potential to serve as climate proxies. These proxies must be developed by investigation of modern soil response to climate, time, and other soil-forming factors. Recent studies of a Modern Vertisol (i.e., clay-rich soils characterized by macro- and micromorphology formed in response to shrink-swell phenomena associated with seasonal precipitation cycles) climosequence in coastal Texas (MAP ranging from 1500 to 750mm) have shown systematic responses in soil chemistry, morphology, and isotopic compositions with increasing MAP. Significant differences in stable isotope behavior were noted between the Lake Charles Series (LAC; MAP=1160mm) and Victoria Series (VIC; MAP=685mm) soils. LAC soils have well developed gilgai (soil microtopography), pedogenic slickensides and isotopic compositions suggesting equilibration between coexisting soil organic matter (SOM) and pedogenic carbonate (PC) at all depths in the soil, preserving a climate-depth relationship. VIC soils have weakly developed gilgai and SOM and PC approach equilibrium only at depths>2m. The low MAP/very large seasonal moisture deficit associated with VIC may be insufficient to establish SOM-PC equilibrium over the 35-40 kyr of pedogenesis. We investigated soil profiles in the Laewest Series (LAW; MAP=940mm), intermediate between LAC and VIC. Similar to soils formed under higher MAP, LAW soils have well-developed macromorphology: gilgai, chimneys, pedogenic slickensides. SOM d¹³C values are similar to all other sites in the climosequence, with more negative values at depth, interpreted to record a shift from C3- to C4-dominated flora. LAW pedogenic carbonate d¹³C ranges from –3 to –5‰ V-PDB. Microlow SOM-PC fractionations approach equilibrium values, however, equilibrium fractionations are not approached in the microhigh samples, except deep in the profile. Thus, only microlow carbonates in lithified equivalents of LAW soils would be acceptable to employ for soil carbonate CO₂ paleobarometry. Results from studies of modern soils are useful to better understand the application and limitations of paleosol climate proxies.