T53 No Dates: No Rates—Utilizing Geochronometers to Quantify Rates of Geomorphic Processes or Archaeological Development over a Wide Range of Temporal and Spatial Scales

Sunday, 22 October 2017
1:30 PM - 5:30 PM
Room 3B

TRACERS DERIVED IN SITU REPLICATE AND MIX BETTER THAN FALLOUT TRACERS INCORPORATED INTO GRAIN COATINGS

Paper number: T53-2
Program: Technical Sessions
Time: 1:45 PM – 2:05 PM

Authors

Abstract

Cosmogenic isotopes, short-lived radionuclides, and thermochronometric indicators are measured in river sand to quantify erosion rates, trace sediment sources, and/or infer erosional processes. Interpretations are often based on the rarely-tested assumptions that (1) tracer concentration is time-invariant and (2) sediment is well-mixed and representative of the upstream area. Better understanding of when and where these assumptions break down and what sampling strategies minimize variance will improve science done using detrital river sediment. We tested these assumptions in SW China by measuring mixing at 13 river junctions in 3 watersheds and by collecting 113 replicate pairs at two different time periods or from two different nearby sites. All samples were analyzed for in situ 10Be (10Bei), meteoric 10Be (10Bem), 210Pbex, and 137Cs. Due to low 137Cs and 210Pbex activity in Chinese samples, we also tested replicability of 137Cs and 210Pbex at 3 sites in northern Ohio. We find that 2 of the 3 China watersheds mix well for 10Bei and only 1 mixes well for 10Bem when considering the concentration of 10Be at the outlet compared to the area-weighted mean of headwater samples. No watersheds mix well for 210Pbex. Individual junctions mix poorly for all three systems. Faster eroding basins, larger basins, and junctions where one tributary is much larger than the other tend to mix better. The most important factor in setting mixing efficacy is the fraction of the basin area contributing to the downstream sample but not the upstream samples. Individual pairs of samples often replicate poorly (-150-100% different), but the population of all replicate samples replicates within 2σ for 10Bei but not 10Bei. 210Pbex and 137Cs replicate poorly. 10Bei replicability is worse in steeper basins, suggesting that stochastic processes affect reproducibility. 10Bem­­ replicability is worse in larger basins suggesting that it is a non-conservative tracer­­. Poor mixing and replicability of short-lived fallout radionuclides suggest that these isotopes may be best used as tracers over short distances and that sampling should happen within as short a period of time in a study area. For 10Be, our analyses suggest that samples should be amalgamated from multiple sites or multiple time periods to increase representativeness.

Geological Society of America Abstracts with Programs. Vol. 49, No. 6

doi: 10.1130/abs/2017AM-299479

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