The Role of Mineral Weathering Dynamics in the Calcium Cycle of Northern Hardwood Forest Soils in a Changing World: Analytical Approaches to Quantifying Mineralogy.

In the northeastern USA, long term mass balance studies and soil resampling suggest depletion of exchangeable Ca pools during the acid deposition era. The mineral sources for replenishment are uncertain and we explored a new technique to determine soil Ca mineralogy. Three watersheds with soils formed in late Wisconsinan glacial till were studied: Sleepers River, VT with parent material composed of calc-granulite with high Ca content; Hubbard Brook, NH with till composed of granite and schist with a moderately low Ca content; and Winnisook in the Catskills of NY with till composed of low-Ca sandstone. Horizon samples were taken throughout the entire soil profile, sieved to < 2 mm, homogenized, epoxy impregnated and thin sectioned. We used a Cameca SX50 electron microprobe to map the distribution of five elements (Al, Si, Ca, Fe and P) in 30 x 30 μm pixels over an area of about 2 x 2 cm. Quantitative point analysis was done on selected phases in order to establish elemental concentration ranges to reference map intensities. Similar mineral phases were grouped using cluster analysis in MultiSpec, a software package normally used for remote sensing. Identification of Ca-bearing minerals was based on ideal and actual elemental composition, confirmed with additional multi-element point quantification. In the lower horizons at Sleepers River, we found actinolite, apatite, calcite, clinozoisite, diopside, epidote, grossular, hornblende and plagioclase. Fewer Ca minerals were found in the upper horizons and at the other watersheds. Additional confirmation of some of the Ca phases was obtained by synchrotron-based micro-xrd on similar thin sections. Limitations of this mapping approach include a large edge effect from the distribution of particle sizes, even though the samples are coarse textured, making it difficult to assign all pixels to a particular mineral phase. Nonetheless, this method reveals dramatic differences in Ca mineralogy that suggest a broad range of Ca replenishment sources for glacial till soils in northeastern North America.