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

The bulk chemical composition, mineral chemistry and clay mineralogy of soils developed on granodiorite parent materials were investigated in Yosemite National Park in the Sierra Nevada, CA. The soils studied were developed on glacial moraines estimated to be ~20 Ka (Tioga), ~130 Ka (Tahoe) and ~300 Ka (pre-Tahoe) in age, thus constituting a soil chronosequence. In addition to granodiorite bedrock minerals such as plagioclase, K-feldspar, quartz, biotite and hornblende, volcanic glass is also found disseminated throughout the soils in amounts up to 15% of the <2-mm size fraction by volume. Glass concentrations decrease with increasing depth for all soils. This suggests initial deposition of volcanic ash on preexisting soil surfaces with subsequent translocation downward into the soil profile. The volcanic ash is believed to have been derived from Pleistocene and Holocene volcanic centers located in the Mono/Inyo Craters region, approximately 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. In the soil <2-μm size fractions, the weight proportion of kaolinite ranges from 20 to 70%, gibbsite ranges from 10 to 45%, vermiculite ranges from 10 to 40%, and soil illite ranges from 1 to 15%. Ti and Zr concentrations vary sporadically throughout the soils due either to heterogeneous distribution 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 concentrations with the addition of low Ti and Zr volcanic glass. The dominance of gibbsite as a weathering product in these young soils (which are located in a cool, temperate setting) is counter to the common notion that gibbsite-dominated soils are restricted to old soils in humid tropical regions. The surprisingly large quantities of gibbsite may be due to the rapid weathering of volcanic ash, as has been observed in other predominantly volcanic soils. We suggest that volcanic glass, especially due to its high reactivity, must be considered as a significant factor when studying the soil, streamwater and isotopic geochemistry of Sierra Nevada watersheds.