Source fingerprinting loess deposits in Central Asia using elemental geochemistry with Bayesian and GLUE models

Abstract

The provenance of loess deposits in Central Asia is largely unexplored. Accordingly, the goals of this research were to test and compare the performance of two different models (generalized likelihood uncertainty estimation - GLUE and a Bayesian model) for quantifying the uncertainty in source apportionment estimated for 46 target loess samples collected in the Ili basin, in eastern Central Asia. Model performance was evaluated using goodness-of-fit (GOF), mean absolute fit (MAF) and virtual mixtures (VM) in combination with root mean square error (RMSE) and mean absolute error (MAE). Our dataset comprised 132 surficial samples collected from three potential sources comprising river alluvium (n = 29), sand dunes (n = 35) and topsoils (n = 68). All samples were analysed for elemental geochemistry. Six geochemical properties (Co, Er, Y, Ga, Dy and Pb) were selected in a composite fingerprint which classified 83% of the samples from the three source categories correctly. Based on both models, source contributions to the loess samples were in the following order: topsoils > river alluvium > sand dunes. Based on the GOF and MAF tests, both models were accurate in predicting measured tracer concentrations in the loess samples. The Bayesian model was slightly more accurate (mean RMSE 1.6%, mean MAE 1.8%) than the GLUE (mean RMSE 5.0%, mean MAE 4.7%) model in predicting known source contributions. Overall, our results provide confirmation that application of source fingerprinting with elemental geochemistry and uncertainty modelling techniques is useful for identifying the provenance of loess sediments in arid and desert environments.