ADIRONDACK TILL TO SOIL: GEOCHEMISTRY AND MINERAL TRANSFORMATIONS

We are studying the mineralogy and geochemistry of Adirondack till and soil as part of a larger study to better understand chemical weathering and soil formation in the Critical Zone (the thin veneer of surface material on our planet which sustains all terrestrial life - www.czen.org). Four sites were chosen for study in the town of Webb, near Old Forge, in the west-central Adirondack Mountains of New York State. Using x-ray diffraction (XRD) analyses and RockJock, a quantitative modeling program, we determined that quartz, K-spar, plagioclase (average ~An₃₀), and hornblende were the most abundant minerals in these soil profiles, with clay minerals illite and vermiculite present at approximately 2-3%. The percentage of quartz drops substantially with depth, decreasing in concentration from 68.5 wt.% in the E horizon (of pit 10-01, for example) to 36.9 wt.% in the C-horizon. The B-horizons of all four pits contain vermiculite that takes the form of an hydroxy-interlayered clay, as determined by XRD patterns that show the gradual collapse of the clay interlayer structure with K-saturation and heat treatments.

Concentrations of exchangeable calcium, potassium, sodium, and magnesium were determined on soil leachates extracted with 1N NH₄Cl and analyzed by atomic absorption spectrophotometry. Extractable calcium and potassium values are highest in the O and A horizons, and decrease with depth. High calcium and potassium values in these upper horizons result from the decomposition of litter-layer material releasing these base cations back to the soil. Tau plots show depletion of base cations and the accumulation of Fe and Al in B-horizons as a result of chemical weathering and pedogenic processes.