PaleoVertisols in Russia and USA: the Use of Stable Isotopes, Morphology and Microfabrics for Reconstructions of Environmental Changes and Soil Processes.
Irina V. Kovda, Institute of Geography, Staromonetny 29, Moscow, Russia, Claudia Mora, Univ of Tennessee, Dept Of Earth And Planetary Science, Knoxville, TN 37996-1410, and Lawrence P. Wilding, Dept of Soil and Crop Sciences, Texas A&M Univ, College Station, TX 77843.
Paleosols are widely used as climatic proxies and may provide the most direct geologic record of past climates. PaleoVertisols provide especially valuable records because they are (i) readily identified by their morphology; (ii) extremely common in the geological record; (iii) resilient to burial diagenesis; (iv) modernVertisols form over a range of environmental conditions and provide a strong basis for reconstructing paleo processes and environmental conditions. We investigated five Pleistocene paleosols with various expressions of vertic properties in a natural exposure on a high terrace (elev.~60 m a.s.l.) along the Laba river, North Caucasus (45 o 06' N, 39o 53' E). Multiple paleosols are embedded in the loess-paleosol sequence. Bulk samples for thin sections, physical, and chemical analyses were taken from the middle of each horizon. Pedogenic carbonates (PC), including nodules, soft masses, and concretions were collected separately for petrographic and isotopic studies.Carbon and O isotopic compositions were determined for carbonate pedofeatures and soil organic matter (SOM) ofbulk soil matrix in horizons with Corg >0.1%. The Pleistocene paleosols were compared with modern Vertisols in Russia and the USA in order to evaluate the range of environmental conditions during soil development and with Paleozoic paleoVertisols of the Appalachian basin (USA) in order to compare the variety of pedogenic features that preserve well into the geologic record. Vertic features first appear at ~550 cm depth, within the 2nd paleosol, which is likely correlated with the Miculino upper Pleistocene interglacial (~50,000-120,000 yrs. B.P.). Vertic features are best expressed in the middle part of each buried soil, similar to modern Vertisol pedons. The more deeply buried paleosols have increased density, hardness and clay content, are coarser in structure, have increased reddish-brown soil matrix color (at depths > 900 cm), and have more strongly expressed vertic features, including parallelepipeds, wedge-shaped structures, stress cutans and slickensides. While modern Vertisols are grey-to black in color, paleoVertisols have a reddish color. Cathodoluminescence of calcite suggests matrix reddening is the result of early diagenetic oxidation of iron (Driese and Mora, 1993) in Paleozoic paleoVertisols. Red coloration of the Pleistocene paleosols may also be, in part, lithogenic. The d 13 C values of bulk SOM vary over 2.46 o/oo (d 13 C = -25.63 to -23.17 o/oo V-PDB) during the whole period of paleopedogenesis, suggesting ~75-90% C3 vegetation in the soil ecosystem. The SOM in the loess is 13 C-enriched compared to the paleosols, consistent with more humid conditions during paleosol formation. Pedogenic carbonates (PC) have d 13 C=-12.18 to - 10.30 o/oo and d 18O= -13.07 to -8.08 o/oo (typical values: d 13 C ≈ -10 to -11 o/oo, d 18 O ≈ -11 o/oo. The d 13 C values are similar to values in modern Russian Vertisols, but d 18 O values are more negative, consistent with overall lower Pleistocene temperatures. PaleoVertisol PC show a negative correlation between C and O isotope ratios (except the 4th paleosol), which is opposite the normal continental trend for terrestrial carbonate. This same pattern is observed in Chinese Pleistocene loess-paleosol sequences, where it is interpreted to reflect arrival of wetter conditions prior to warming and the beginning of a more typical "warm-wet", paleosol-forming period (Li et al., 2002). The pedogenesis of PC morphologies may be further examined in light of the isotopic compositions. Significant variations in isotope compositions across soft masses are consistent with our previous hypothesis that soft masses form and recrystallize under a variety of pedoenvironments. Many PC nodules and especially extremely coarse concretions (~10-15 cm in length) are multiaggregate, with septaric voids and inclusions of iron stains, illuviated clay, micrite or microsparite central cores and/or recrystallized microsparite/sparite along internal voids. The compound morphology of PCs suggests a polygenetic origin with several stages of carbonate accumulation, dissolution and reprecipitation. In general, clearly recrystallized carbonates have more variable C and 18O enriched isotopic compositions. We hypothesize this reflects recrystallization and exchange over a variety of younger, and generally warmer, climatic conditions. The Pleistocene paleosols have more complicated micromorphology than most PC nodules observed in Paleozoic paleosols, reflecting a more complex, superimposed history of recrystallization and exchange. Only early Permian carbonate nodules, similarly formed during stadial-interstadial climatic conditions, have such complexity. On the basis of the isotopic data and morphological features, we speculate that the Pleistocene paleosols formed under a climate generally similar to the modern environment of Vertisols in the North Caucasus region (MAT ~10-12 oC), however, the early Pleistocene period of paleosol formation appeared to have been more humid, resulting in more significant development of vertic features.