CHEMICAL WEATHERING AND MINERAL TRANSFORMATIONS IN A SMALL WATERSHED IN CENTRAL NEW YORK

As part of a larger study of the Critical Zone (Shale Hills Critical Zone Observatory, www.czen.org) we conducted an investigation of shale weathering and soil development in glacial sediments deposited atop Clinton Group (Silurian) rocks at Roger’s Glen, a small watershed in central New York State. Our primary objectives were to determine the provenance of the soil clays and to define and explain the geochemical and mineralogical trends in the soil profile. X-ray diffraction analysis (XRD) and modeling with NEWMOD© indicates that illite is the most abundant phyllosilicate throughout the soil profile. At the base of the soil profile illite constitutes approximately 87% of the <2μm fraction, and chlorite and vermiculite are present at concentrations of ~9% and ~5%, respectively. At the top of the soil profile these values change to ~21% illite, 64% illite-vermiculite, ~4% chlorite-vermiculite, and ~11% kaolinite. The XRD data, along with laboratory weathering experiments suggest that illite is resistant to chemical weathering, whereas the Fe-rich chlorite weathers to vermiculite, likely through a mixed-layer chlorite-vermiculite intermediate. Tau (t) plots (loss or gain of elements relative to parent material) show depletions of all elements in near surface soil. Exchangeable base cations Ca, Mg and K are enriched in the upper 10 cm of soil as decomposition of litter releases them back to the soil. The same cations are depleted in the rooting zone (15-25 cm) where they are taken up by plants and are leached by acids.

Glacial sediments at our study site are underlain by the Sauquoit and Willowvale members of the Clinton Group. XRD analysis of bedrock samples shows that the shales are composed mainly of illite with smaller amounts of Fe-rich chlorite, quartz, and traces of K-feldspar. Energy dispersive analysis (SEM-EDS) of the clay minerals within the shale provides the following average chemical compositions for the illite and chlorite, respectively:

Illite - K_0.68[Al_1.79Mg_0.13Fe²⁺_0.18Ti_0.02(Al_0.72Si_3.28)O₁₀(OH)₂],

Chlorite - (Mg_3.36Fe²⁺_4.01Al_3.46)(Al_1.40Ti_0.16Si_6.44)O₂₀(OH)₁₆.

Additional XRD and SEM-EDS analyses of shale fragments taken from within the soil profile indicate that the Clinton Group shales are the primary, parental source of the soil clays.