UNDERSTANDING MODERN LANDSCAPE BEHAVIOR USING METEORIC AND IN SITU 10BE AND 137CS IN LARGE RIVER BASINS, SW CHINA
No significant relationships were found between meteoric 10Be concentrations and mean local relief, mean annual rainfall, or basin area. Areas with high erosion rates (>0.4 mm/yr) had low meteoric 10Be values (R2= 0.34, p= 0.0002). While meteoric 10Be and basin size did not strongly correlate, basins <104 km2 had highly variable meteoric 10Be values, while basins >104 km2 had significantly less variability (p= 0.11). Analysis of meteoric 10Be, mean annual rainfall, and mean local relief grouped by stream order suggests high variability in small basins may mask relationships found in large basins.
Erosion indices (EI) were calculated for each sample using in situ 10Be erosion rates to calculate long-term sediment flux and ranged from 0.11 to 1.48 (median = 0.55). The EI represents the ratio of total meteoric 10Be leaving the basin on sediment grains over the total estimated atmospheric delivery of 10Be. The highest EI’s were in the northern portion of the Yangtze drainage, and corresponded to moderate erosion rates of 0.06 – 0.13 mm/yr. Variability in EI was significantly higher in basins <104 km2 (p= 0.14).
EI’s calculated from long-term sediment yield allow modern events, such as land-use change, to be isolated from long-term geomorphic trends. A median EI of 0.55, with 91% of measurements below 1, suggests that less meteoric 10Be is exported than is incident upon the landscape. The absence of measureable 137Cs in the majority of samples indicates that significant erosion has occurred since 1954. EI and 137Cs activity suggest that there is a “disconnect” between long-term sediment yield estimates and modern erosion rates, potentially caused by human impact changing the style and/or distribution of erosion.