THE USE OF 239+240PU AND STABLE CARBON ISOTOPES AS EROSION TRACERS FOR QUANTIFYING AND EVALUATING SOIL EROSION AND SEDIMENTATION RATES

Quantifying soil erosion and sedimentation rates using environmental tracers has been essential for providing retrospective information on natural and accelerated erosion processes. Soil erosion is an Earth surface process that plays a role in regulating chemical weathering rates, soil carbon stock, sediment delivery to streams, and cycling of nutrients. Among well-established erosion tracers (e.g. ¹³⁷Cs and ²¹⁰Pb), the summed ^239+240Pu activities are promising for quantifying soil erosion and sedimentation rates on a decadal timescale (≈ 60 years). Compared to ¹³⁷Cs and ²¹⁰Pb, weapons-test fallout Pu isotopes have long half-lives, offer low-cost, high-throughput analysis via ICPMS, and enable provenance of fallout origin using ²⁴⁰Pu/²³⁹Pu atom ratios; Pu appears to be advantageous vs. ¹³⁷Cs in Chernobyl-affected areas, which are prevalent in parts of Eurasia. Fallout plutonium appears to bind to soil surfaces when deposited on terrestrial ecosystems, and the initial ^239+240Pu inventory is redistributed across the landscape by erosion and sedimentation of soil particles, allowing comparison of stable and unstable surfaces for soil redistribution assessment. In combination with ^239+240Pu, recent studies have also used stable carbon isotopes to provide additional information on erosion processes. The correlation between δ¹³C and soil organic carbon content qualitatively indicates soil disturbance such as erosion and sedimentation, bioturbation, and cryoturbation. In this talk, I will provide an overview of the distribution and behavior of ^239+240Pu and stable carbon isotopes in forest environments and will discuss analytical measurements, field experimental design, and soil erosion and sedimentation assessment. Furthermore, I will discuss case studies from Southeast Alaska and the Central Rocky Mountain region that used ^239+240Pu together with stable carbon isotopes to understand soil formation and landscape evolution on post-glacial landscapes of varying ages.