The Susquehanna/Shale Hills Critical Zone Observatory is a V-shaped first-order catchment, developed almost entirely on the Silurian Rose Hill shale. We have focused on this site to investigate the controls, mechanisms and rates of shale weathering and soil formation. The parent rock is comprised primarily of illite, quartz, chlorite and ankerite. Three weathering fronts were observed at different depths identified in the northern ridgetop: ankerite dissolution begins tens of meters below land surface; plagioclase feldspar begins to weather at about 5-6 meters; chlorite and illite dissolve in the soil profiles to form more stable vermiculite and kaolinite. Six soil profiles were characterized: ridge top, mid-slope and valley floor sites on south-facing (9°) and north-facing (12°) planar hillslopes. Particle size analyses revealed that the soil textures were different: silt loam on the S-facing slope and sandy loam on the N-facing slope. All 6 sites show depletion profiles of major elements due to dissolution of clay minerals and feldspar. However, compared to the soils from the N-facing slope, those from S-facing slope were less depleted. Also local variations in element chemistry were observed with depth on S-facing slope to be superimposed on a general depletion trend. U disequilibrium isotopes of the bulk soils as well as parent rock were measured and modeled to estimate the soil production rates. In spite of similar slope, soil was produced, eroded and transported out of the S-facing slope at a faster rate. We argue that periglacial activities broke up bedrock and promoted soil erosion especially on the S-facing slopes, as evidenced by finer soil texture and fractures inferred by zig-zag variations in the depletion profiles. As such, the residence time of soil particles is generally shorter on the S-facing slope, preventing minerals in soils from becoming extensively weathered. Duration for the clay dissolution reactions (kinetically slow) plays a primary role on the extent of shale weathering. More solar radiation and subsequently faster freeze-thaw cycling presumably cause the higher soil erosion rates on the S-facing slope. The climate conditions and topography control the tree distribution to affect chemical weathering rates in complex ways. For example, solar radiation controls soil moisture contents by evapotranspiration and availability of water for weathering, although the higher soil temperature presumably accelerates the dissolution rate and promotes biological uptake of nutrients if moisture is not limiting. Soil pore water chemistries are consistent with more pronounced nitrification as well as higher clay dissolution rates on S-facing soils. To summarize, in a dynamic hillslope system like the Shale Hills, physical, chemical and biological processes interplay to control the soil thickness and chemistry. Controlled by biota but ultimately driven by topography and climate, weathering and erosion processes have been changing the shape of the catchment by eroding the S-facing slope faster than the N-facing slope.
Jin, L., Eissenstat, D., Lin, H., Chabaux, F.J., Ma, L., Brantley, S.L. (2010): Soil production is faster on south-facing slopes in the Susquehanna/Shale Hills Critical Zone Observatory due to periglacial, vegetative, and climate factors (Invited). AGU Annual Fall Conference Proceedings.
This Paper/Book acknowledges NSF CZO grant support.