MINERALOGICAL TRANSFORMATIONS DURING SHALE WEATHERING FROM PUERTO RICO TO WALES

Soil is the essential material that sustains all terrestrial life on Earth, yet the processes by which soil forms from parent rock are not well understood. To investigate factors controlling soil formation, we established a transect of sites across a climate gradient as part of the Susquehanna-Shale Hills Critical Zone Observatory (SSHO). To minimize variables influencing soil production, sites were located on organic-poor, iron-rich Silurian-age shale and include cold and wet sites in Wales, New York and Pennsylvania and warm and wet sites in Virginia, Tennessee and Alabama. A site in western Puerto Rico provides a warm/wet end member for the transect that is underlain by a geochemically similar, but younger, shale than the other transect sites. Here, we present geochemical and quantitative mineralogical data that will be useful in modeling weathering processes and mineral transformations across this transect.

Parent shales across the transect have a mineral assemblage dominated by quartz, illite and chlorite. One exception is the site in Puerto Rico, where the parent shale contains up to 50% calcite in contrast to the < 1% calcite observed at the other sites. Sodium is largely present in plagioclase feldspar, which generally constitutes < 5% of the shale mineralogy. Plagioclase feldspar weathering increases from north to south, with 20% of plagioclase feldspar weathered at the soil surface in Wales and 100% of plagioclase feldspar weathered in Puerto Rico soil profiles. Soils in Alabama and Puerto Rico – the most intensely weathered sites in the transect – show considerable mineral transformations from the parent rock to the soil surface with kaolinite increasing 20 to 30% and iron oxides increasing up to 10%. In comparison, the northern sites exhibit increases of roughly 5% and 2% in kaolinite and iron oxides, respectively. The weathering of feldspar at depth may initiate profile development; however, the weathering of chlorite and illite, which constitute roughly 50% of the parent shale mineralogy, is more likely controlling the depth of augerable soil at all sites across the climosequence. Quantifying the mineral weathering reactions across the transect will help us understand both the impact of climate on weathering rates and the depth to which weathering influences soil mineralogy at the Earth’s surface.