Quantitative phytolith analysis: The key to understanding buried soils and to reconstructing paleoenvironments.

摘要

Scope and method of study. The purpose of this study is to quantitatively recover and analyze the phytoliths from modern and prehistoric prairie soils, and to use the resulting phytolith signatures to develop a better understanding of pedogenic processes and to determine past climatic conditions. Improvements in analytical laboratory protocols will be developed as needed to meet these objectives.;Findings and conclusions. Phytoliths were quantitatively recovered from A horizons of three modern prairies (Shortgrass, Mixedgrass, and Tallgrass Prairies) and from three sites with buried soils of known age. Phytoliths were separated from other soil particles based on differences in particle size and particle density. Using polarized light microscopy, the morphologic distribution of Poaceae short cell phytoliths present in the isolated soil sample fractions was ascertained. The phytolith distribution within buried A horizons reveals information about soil forming processes. The relative phytolith concentration mirrors the soil organic carbon content in well-developed melanized A horizons. In a normal melanized A horizon, the phytolith concentration decreases exponentially with depth, in a soil developed by cumulic growth the phytolith concentration is relatively constant, and in a new soil formed on an alluvial deposit the phytoliths are concentrated in the upper portion of the deposit. The phytolith signature of modem soils mirrors the environmental conditions at the time of soil formation. Comparison of modern prairie short cell phytolith signatures to the signature in buried soils permits determination of climatic conditions at the time of past stable environments. The various phytolith forms evaluated are indicative of C3 vs. C4 grasses thus revealing climatic information. A higher C3 phytolith content indicates a cooler moister climate whereas a stronger C4 signature indicates a warmer climate. Phytolith seasonality groupings proved to be more reproducible that the individual phytolith short cell morphotypes. It was discovered that saddle-shaped phytoliths appear to hold great potential for understanding changes in botanical signature due to climate. Significant improvements to available published laboratory protocols for phytolith isolation were developed and implemented. Phytolith analysis leads to a better understanding of soil genesis and provides a method to ascertain past climatic changes.