FRACTIONATION OF RARE EARTH ELEMENTS DURING SOIL FORMATION ALONG FEATHER RIVER BASIN HILL SLOPES IN THE CALIFORNIA SIERRA NEVADA

Rare earth element (REE) mobility in surficial systems is poorly understood making their use as geochemical tracers a debated topic. Using soil data, this study examines how REEs fractionate as rock weathers into soil along the Middle Fork Feather River in Northern California. An erosional signal propagating through the basin was studied by comparing the traditional methods of REE normalization with the elemental loss method based on mass balance calculations known as tau (τ). Soil samples were collected along three differentially eroded hill slopes. The application of the traditional method of REE normalization showed chemical fractionation in the saprolite, however, the details of profile variation were unclear and difficult to interpret. The mass-balance method, which uses Zr as an immobile element, also portrayed REE fractionation within the saprolite. Results show that the bedrock REE signature is altered as the bedrock weathers to form saprolite and undergoes further alteration as the saprolite weathers to produce overlying soils. Most chemical weathering and chemical mass loss occurred in the saprolite, which corresponds to the fractionation observed in the saprolite. Considering the magnitude and variable changes of REEs in the first meter of rock-to-soil weathering, studies seeking to use REEs as tracers over longer spatial scales should also consider sampling and testing weathered material, such as saprolite, as sedimentary source instead of unweathered bedrock alone.