A 210Pbex accumulation-decay model for dating moraine soils to trace glacier retreat time

<p><strong>Abstract</strong><strong>: </strong>This report exploring the potential for using the natural fallout radionuclide <sup>210</sup>Pb<sub>ex</sub> to date moraine soils for tracing glacier retreat. Based on the physical processes of <sup>210</sup>Pb<sub>ex</sub> deposition, decay and losses due to runoff, a <sup>210</sup>Pb<sub>ex</sub> accumulation-decay model was developed, where = <sup>210</sup>Pb<sub>ex</sub> inventory (Bq·m<sup>-2</sup>); = annual inventory of <sup>210</sup>Pb<sub>ex</sub> deposition (Bq·m<sup>-2</sup>); = <sup>210</sup>Pb decay coefficient (0.969); = time span (years); <em>b</em> and <em>c</em> = <sup>210</sup>Pb<sub>ex</sub> loss coefficients for the runoff pathway. 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 1910–1990 along a retreat length of 1750 m in the valley. <sup>210</sup>Pb<sub>ex</sub> inventories increased from 3,669.6 ± 218.5 Bq·m<sup>-2</sup> at the site where the glacier retreated in 1990 to 10,718.9 ± 167.4 Bq·m<sup>-2</sup> in 1910. The coefficients of <em>b </em>= 0.6006 and <em>c </em>= 0.9764 were derived from the <sup>210</sup>Pb<sub>ex</sub> 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 <sup>210</sup>Pb<sub>ex </sub>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.</p><p><strong>Keywords: </strong><sup>210</sup>Pb<sub>ex</sub> accumulation-decay model, <sup>137</sup>Cs, dating moraine soils, glacier retreat time.  </p>