How mineralogy and slope aspect affect REE release and fractionation during shale weathering in the Susquehanna/Shale Hills Critical Zone Observatory

To understand the factors that control rare earth elements (REE) release and fractionation during shale weathering, we investigate the REE contents in solid (bedrock, regolith, stream sediments), and natural waters (stream, and pore waters) from a first-order catchment developed entirely on gray shales in central Pennsylvania, USA. Up to 65% of the REE (relative to parent bedrock) is depleted from the weathering profiles in the acidic and organic-rich soils due to chemical leaching. In addition, newly formed fine particles were also lost along with the down-slope movement of soil waters. Weathering profiles on the south-facing slope show less depletion of REE than those on the north-facing slope (33% vs. 45% on average). We hypothesize that the different degrees of REE depletion on the two transects reflect a history of different chemical weathering rates and possible different surface erosion rates, controlled by contrasting slope aspect-induced microclimate conditions. In addition, weathering profiles, natural waters and sequential extractions all show a preferential removal of Middle REE (up to 22% more) relative to Light REE and Heavy REE during shale weathering, due to preferential release of MREE from rhabdophane. Furthermore, the long-term phosphate mineral dissolution rates (e.g., rhabdophane) were estimated at 10(-15) to 10(-41) mol m(-2) s(-1) under field conditions, based on REE depletion profiles. Strong positive Ce anomalies (average [Ce/Ce*](N) value: 1.79) observed in the regolith, stream sediments, and regolith extractions point to the fractionation and preferential precipitation of Ce as compared to other REE, due to the generally oxidizing conditions during release, transport, and redistribution of REE in the surface and subsurface environments. Positive Eu anomalies (average [Eu/Eu*](N) value: 1.30) observed in the natural waters of the catchment are attributed to weathering of plagioclases in the shale bedrock. This study highlights the use of REE as natural tracers for low-temperature geochemical processes. (C) 2011 Elsevier B.V. All rights reserved.