Chemistry and mineralogy of a granitic, glacial soil chronosequence, Sierra Nevada Mountains, California

The bulk chemical composition, mineral chemistry and clay mineralogy of soi ls developed on granodiorite parent materials were investigated in Yosemite National Park in the Sierra Nevada, CA. The soils studied were developed o n glacial moraines estimated to be similar to 20 Ka (Tioga), similar to 130 Ka (Tahoe) and similar to 300 Ka (pre-Tahoe) in age, thus constituting a s oil chronosequence. In addition to granodiorite bedrock minerals such as pl agioclase, K-feldspar, quartz, biotite and hornblende, volcanic glass is al so found disseminated throughout the soils in amounts up to 15% of the < 2- mm size fraction by volume. Glass concentrations decrease with increasing d epth for all soils. This suggests initial deposition of volcanic ash on pre existing soil surfaces with subsequent translocation downward into the soil profile. The volcanic ash is believed to have been derived from Pleistocen e and Holocene volcanic centers located in the Mono/Inyo Craters region, ap proximately 50 km east of the study site. The clay mineralogy of the soils is dominated by kaolinite, gibbsite and vermiculite, with lesser amounts of soil illite. The total amount of clay in the soils increases with soil age . Ln the soil < 2-mu m size fractions, the weight proportion of kaolinite r anges from 20 to 70%, gibbsite ranges from 10 to 45%, vermiculite ranges fr om 10 to 40%, and soil illite ranges from 1 to 15%. Ti and Zr concentration s vary sporadically throughout the soils due either to heterogeneous distri bution of Ti and Zr within the soil parent material, extensive mobility of Ti and Zr within the soil profile, and/or the dilution of soil Ti and Zr co ncentrations with the addition of low Ti and Zr volcanic glass. The dominan ce of gibbsite as a weathering product in these young soils (which are loca ted in a cool, temperate setting) is counter to the common notion that gibb site-dominated soils are restricted to old soils in humid tropical regions. The surprisingly large quantities of gibbsite may be due to the rapid weat hering of volcanic ash, as has been observed in other predominantly volcani c soils. We suggest that volcanic glass, especially due to its high reactiv ity, must be considered as a significant factor when studying the soil, str eamwater and isotopic geochemistry of Sierra Nevada watersheds. (C) 1999 El sevier Science B.V. All rights reserved.