TESTING RELATIONSHIPS BETWEEN PARTICLE SIZE, CHEMICAL WEATHERING, AND EROSION IN THE KLAMATH MOUNTAINS, CA

In soil-mantled landscapes, steady state describes the balance between regolith production, often mediated by chemical weathering, and erosion, both of which are regulated by climate and tectonic forcing. As minerals move from bedrock to surface soil, weathering rates are in part controlled by particle size and consequent reactive surface area. Therefore, in actively eroding landscapes where climate conditions are constant, changes in particle size should reflect changes in the extent of weathering associated with mineral supply. We test this relationship by measuring particle size distributions in ridgetop regolith profiles at 13 sites in the Klamath Mountains, CA, experiencing different rates of uplift associated with the migration of the Mendocino Triple Junction.

Regolith profiles were collected in ten-centimeter intervals from the top soil to auger refusal at each location. Soil samples were dry sieved and mass fractions were calculated for 5 grain size fractions from >2mm to <63µm. Measured particle size distributions separate into two groups – those with >2mm as the highest mass fraction and those with 2mm-500µm as the highest mass fraction. Changes in particle size show no distinct pattern with changes in latitude or erosion rate; however, our data suggest that concentrations of major mineral forming elements, such as silica and magnesium, correlate with particle size.

Charcoal fragments were collected from two profiles, found at varying depths and were dated using ¹⁴C. At one site charcoal was found in two depth intervals near the soil surface, at 15 cm and 35 cm, and at the other site, charcoal was found much deeper at 95 cm. The age of the deep charcoal was found to be 4560 BP, and is consistent with a steady soil accumulation rate of .021 cm/yr, similar to previously measured erosion rates using ¹⁰Be. However, the age of the charcoal found at 15 and 35 cm was 70 BP and 620 BP, respectively, consistent with a steady soil accumulation rate of .036 cm/yr, which is greater than previously measured erosion rates by a factor of three.