SLOW RATES OF SOIL PRODUCTION ON HILLSLOPES OF THE APPALACHIAN PLATEAU, OHIO, USA

Background rates of soil production in a range of tectonic and climatic settings are often at least an order of magnitude slower than rates of hillslope erosion by humans. However, little previous work has quantified background soil production rates in landscapes closely linked to global or regional arable soils. Our work employs ¹⁰Be to quantify natural rates of soil production on forested hillslopes of the Appalachian Plateau for comparison to rates of soil loss driven by agriculture. Our study site, Neotoma Valley, OH, USA, is a small watershed (0.5 km²) a few kilometers south of the Illinoian ice margin with a mid-20^th century history of soil creep measurements. Using clay cylinders embedded along a SW-NE trending, cross-valley transect, Kaye Everett observed decadal-scale downslope soil creep rates of 0.8 cm/yr or 0r 0.0006 cm³/cm/yr. We reoccupied Everett’s transect and excavated soil pits at 10 m elevation intervals between the valley floor and ridges on each valley side. For each soil pit (n=8), we collected several hundred grams of friable sandstone bedrock from the bottom of the soil mantle for in-situ ¹⁰Be analysis. We also collected a bedrock sample (n=1) from the SW divide of Neotoma Valley, in line with our NE-aspect transect.

¹⁰Be-derived soil production rates over millennial timescales in Neotoma Valley are slow: 0.005-0.018 mm/yr (n=8; mean=0.011 ± 0.05 mm/yr). Our one ridgetop bedrock sample yields a ¹⁰Be-derived lowering rate of 0.008 mm/yr. These rates fall at the low end of previously reported ¹⁰Be-derived soil production rates, but are not unreasonable for a low relief, tectonically quiescent setting. Indeed, Portenga et al.’s ¹⁰Be-derived mountain top lowering rates for the central Appalachian orogen average 0.009 ± 0.001 mm/yr. Soil production rates modeled using topography (hillslope curvature) and a hillslope transport coefficient derived from Everett’s mid-20^th century decadal-scale soil creep measurements are 0.001-0.040 mm/yr, in agreement with our long-term, background rates. For the hillslope soils of Neotoma Valley, the USDA recommends a soil erosion limit of 0.7-0.5 mm/yr, one to two orders of magnitude faster than our measured and modeled rates. Soil erosion at the USDA rates is clearly unsustainable, and our new data can help inform realistic management of soil resources in this region.