B095-0003
Determining the transit time of carbon in shale-rich soils

Tuesday, 15 December 2020
Poster
Elizabeth K Williams1, Corey R Lawrence1, Hsiao-Tieh Hsu2, Rania Eldam Pommer3, Jennifer L Druhan4, Alexis Navarre-Sitchler5 and Katharine Maher6, (1)US Geological Survey, Geosciences and Environmental Change Science Center, Denver, CO, United States, (2)Stanford University, Stanford, CA, United States, (3)Colorado School of Mines, Golden, CO, United States, (4)Stanford University, Geological and Environmental Sciences, Stanford, CA, United States, (5)Colorado School of Mines, Geology & Geological Engineering, Hydrologic Science & Engineering, Golden, CO, United States, (6)Stanford-Geology & Env Science, Stanford, CA, United States
Abstract:
Soils store a large amount of organic carbon (C) making them an important sink or potential source of biospheric CO2 in response to environmental or land-use changes. In order to model and predict the response of soil carbon to environmental perturbations, it is necessary to understand the sizes of the C pools in soils as well as the timescales of soil C storage. Shales represent a large reservoir of the Earth’s reduced C stocks and in soils derived from shales, there is often the additional contribution of large amounts of mobilized shale-derived organic matter which impede the accurate estimation of biospheric C amounts, reactivities, and transit times.

The objective of this work is to evaluate the effect of rock-derived C (shale) on calculated soil C transit times. This study was conducted in the East River Valley of the Rocky Mountains (near Gothic, CO, USA). The predominate parent material of the East River watershed is Cretaceous Mancos Shale and the study area consists of sites in sub-alpine and montane regions, which differ in mean annual temperature, snow cover, and soil moisture. Soil samples (up to 1 meter) were collected at select depth intervals. Soil samples were incubated as well as physically fractionated into free, occluded, and mineral-associated soil C pools. The C, 13C, and 14C contents were subsequently determined for the bulk soils, respired CO2, and the three soil fractions to determine the transit times of C in the soils. Preliminary results indicate the presence of shale-derived C in all soil pools, except the free light fraction, affecting model outputs of C transit times. In order to estimate the shale contribution to C pools and to better constrain biospheric soil C transit times in shale-rich environments, we explored the use of residual oxidizable organic C as a proxy for rock-derived shale C. Improvements in the modeling and determination of soil C transit times with the quantification of shale C as well as variation in soil C transit times with other environmental factors (depth, elevation, temperature, and soil moisture) will be presented.

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