GRAIN-SIZE BIAS IN DETRITAL THERMOCHROMETRY: IMPLICATIONS FOR INTERPRETING SEDIMENT PROVENANCE AND LANDSCAPE EVOLUTION
To evaluate the influence of grain-size bias on provenance studies, we used a forward model of catchment erosion to determine the probability of detecting an age population that occurs in a small fraction of the catchment. This simulates the challenge of fully characterizing age distributions of tectonically complex terrain. We found that stream sand underrepresents higher elevations when sediment size increases steeply with elevation. As a result, the chance of detecting an age population originating from the highest 20% of the catchment can be as low as 50%, depending on the number of grains analyzed.
To evaluate the influence of grain-size bias on quantifying spatial patterns in erosion rates, we considered catchments in which bedrock age increases with elevation. Our results show that higher-elevation sources are underrepresented in smaller grain sizes, such that the age distribution in sand is skewed toward younger ages from lower elevations. We explored this effect over a range in catchment relief and area, and found that age distributions from low-relief (<1.5 km) catchments are not sensitive to changes in sediment size. However, in large, high-relief catchments, age distributions in sand do not reflect spatial variations in erosion rates.
Both scenarios considered here show that changes in sediment size across landscapes can influence interpretation of detrital thermochronology in large, high-relief catchments. We show that limitations imposed by grain-size bias can be mitigated via analysis of a sufficient number of grains across a range of sediment sizes.