Morphological, Microscopic and Isotopic Studies of Carbonate Pedofeatures: Current Problems of Interpretation and Application In Biogeosciences.
Irina V. Kovda, Institute of Geography, Staromonetny 29, Moscow, Russia, Lawrence P. Wilding, Dept of Soil and Crop Sciences, Texas A&M Univ, College Station, TX 77843, and Claudia I. Mora, Dept. Of Earth And Planetary Sciences, Univ of Tennessee, Knoxville, TN 37996-1410.
Carbonate accumulation and formation of carbonate pedofeatures are common processes in many soils under various pedoenvironments. Carbonate types and distribution patterns, chemistry, macro-, micro- and submicromorphology, and isotopic composition of carbonate pedofeatures provide critical information that is used widely in soil genesis, paleo-reconstructions, modeling, and prediction of CO2 fluxes. Such foundational knowledge is essential, not only in soil science, but in archaeology, geology, biology and other environmental/biogeosciences. Current literature and new databases have revealed a number of problems in the interpretation of carbonate physico-chemical features to infer origin and genesis. Specifically, what is the confidence level of a given methodology compared to information gained from coupling of multiple analytical approaches. Carbonate pedofeatures could be systemized based on their genesis, morphology, microstructure, mineralogy, landscape position, and age, but these typologies are often interdependent. Specific attributes of carbonate pedofeatures including composition and architecture can be obtained by a variety of methods. Morphological investigations include macro-, micro-, and sub-micromorphology that provide information on complex microfabrics and distribution patterns inferring polygenetic genesis, age, and origin. The compatibility between carbonate micro-morphology and pedoenvironments is well developed. Total chemical composition has been used previously, but this methodology is too general and insensitive to differentiate different environments. Mineralogical methods (thermogravimetric, XRD, IR-spectroscopy, and optical microscopy) provide data on mineralogical composition and crystal morphology which are useful signatures of in situ pedogenesis and origin (pedogenic versus geogenic). Radiocarbon assay and stable isotopic compositions are used for the estimation of age and rate of carbonate formation, to distinguish pedofeature origin, and for reconstruction of paleo-enviroments. The methods listed above provide important information but each are subject to limitations without close scrutiny. A comprehensive investigation utilizing all of the above field and analytical tools may provide the most reliable basis to determine the interdependency and sometimes confounding nature of their signatures. We have investigated soil carbonate pedofeatures of various morphology and pedo-environments by coupling macromorphology, micromorphology, SEM and microprobe analysis, radiocarbon and stable isotopic composition of C and O. This study revealed several issues worthy of further discussion and deliberation: • Evolution and transformation of carbonates with various morphology including genesis of hard nodules (i.e. can soft carbonate masses transform to hard nodules?); • Carbonate pedofeatures with diverse macromorphology versus observed microfabrics in thin sections; • Relationship between macro-, micro- and submicromorphic assemblages of carbonates; • Development of criteria to differentiate contemporaneous and relic carbonate pedofeatures; • Rate dynamics of carbonate formation and transformation specific to given pedoenvironments; • Rate of rejuvenation of radiocarbon age specific to various pedoenvironment; • Recrystallization of carbonates: original and secondary morphology, rates of recrystallization, and changes in stable isotopic composition; • Terminological systematics. The detailed and well-directed study of these topics may greatly improve the interpretation of carbonate pedofeatures and the reliability of carbonate morphology and isotopic record for the reconstruction of paleoenvironments and pedogenesis. A systematic study of soil carbonates collected from various pedoenvironments using a combination of morphological and isotopic methods is strongly required.