A study on a ²¹⁰Pb_ex accumulation-decay model for dating moraine soils to trace glacier retreat time

This paper reports work exploring the potential for using the natural fallout radionuclide ²¹⁰Pb_ex to date moraine soils for tracing glacier retreat. Based on the physical processes of ²¹⁰Pb_ex deposition, decay and losses due to runoff, a²¹⁰Pb_ex accumulation-decay model (A_n=I[1-λⁿ⁺¹1-λ-b(cⁿ⁺¹-λⁿ⁺¹)c-λ] ) was developed, where A_n = ²¹⁰Pb_ex inventory (Bq·m^-2); I = annual inventory of ²¹⁰Pb_ex deposition (Bq·m^-2); λ = ²¹⁰ Pb decay coefficient (0.969); n = time span (years); b and c = ²¹⁰Pb_ex loss coefficients for the runoff pathway. Furthermore, ¹³⁷Cs was used to identify the ages of the study sites and to support the ²¹⁰Pb_ex model results. The model was validated with data obtained from the Hailuogou Glacier Valley, Mt. Gongga, in 2016, where nine glacier retreat moraine points were recorded from 1910 to 1990 along a retreat length of 1750 m in the valley. ²¹⁰Pb_ex inventories increased from 3,669.6 ± 218.5 Bq·m^-2 at the site where the glacier retreated in 1990 to 10,718.9 ± 167.4 Bq·m^-2 in 1910. The coefficients of b = 0.6006 and c = 0.9764 were derived from the ²¹⁰Pb_ex inventories at the nine sites with recorded glacier retreat times that were marked with special stone and terrain features. The goodness-of-fit (GOF) for the model predictions of glacier retreat times is 65.5%. The results obtained confirm that the fallout radionuclide ²¹⁰Pb_ex has potential for dating moraine soils in other cryosphere regions throughout the world as well as for other types of records forming sedimentary landform sequences such as soils on debris flows and alluvial fans.