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

https://doi.org/10.1016/j.jenvrad.2019.106124Get rights and content

Highlights

  • For the first time a210Pbex accumulation-decay model is developed on moraines.

  • The 210Pbex physically-based model considers the effect of water erosion.

  • Runoff coefficients should be established for each study area.

  • The model is suitable for dating moraine soils over the past 100 years.

Abstract

This paper reports work exploring the potential for using the natural fallout radionuclide 210Pbex to date moraine soils for tracing glacier retreat. Based on the physical processes of 210Pbex deposition, decay and losses due to runoff, a210Pbex accumulation-decay model (An=I[1λn+11λb(cn+1λn+1)cλ] ) was developed, where An = 210Pbex inventory (Bq·m−2); I = annual inventory of 210Pbex deposition (Bq·m−2); λ = 210 Pb decay coefficient (0.969); n = time span (years); b and c = 210Pbex loss coefficients for the runoff pathway. Furthermore, 137Cs was used to identify the ages of the study sites and to support the 210Pbex 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. 210Pbex 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 210Pbex 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 210Pbex 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.

Introduction

Owing to global warming, most glaciers in remote polar and high mountain regions of the world have retreated rapidly, particularly over the past 100 years, but only some glaciers have observational retreat data (Barry, 2006). There is an urgent need for information on recent changes in glacier retreat speeds for assessing the impacts of climate change and its effects on soil and water resources in polar and mountainous regions (Lizaga et al., 2019a). Such information will improve understanding of the impact of climate change on fragile polar and high mountain ecosystems at local and global scales, thereby permitting better management and conservation of soil and water resources (López-Moreno et al., 2016).

Fallout radionuclides (FRN), such as 137Cs and 210Pbex, have been widely used for assessing soil loss (Walling et al., 2002; Navas et al., 2013; Chen et al., 2019), tracing sediment sources (Collins et al., 2017; Palazón et al., 2015; Gaspar et al., 2019; Lizaga et al., 2019b), establishing sediment budgets (Navas et al., 2014; Walling et al., 2014) and dating ice and sediment deposits (Morellón et al., 2011). 210Pb is an environmental isotope of the 238U decay series with a half-life of 22.6 years, which is derived from the decay of 222Rn (half-life of 3.8 days). 222Rn is a decay product of 226Ra (half-life of 1622 years) and is widely found in natural soils and rocks. Following 226Ra decay to gaseous 222Rn, part of the resultant 222Rn decays to form 210Pb in soils and rocks. Another small part of 222Rn enters the atmosphere, decays into 210Pb in the atmosphere, and is then contained in fallout back to the surface of the soil in association with precipitation. This fallout is eventually absorbed by the surface soil, and it is this component of the total 210Pb derived from atmospheric fallout that is termed unsupported or excess 210Pb (Robbins, 1978). The deposition of fallout 210Pb from the atmosphere has been relatively constant over time because of its natural origin. This is different from the deposition of 137Cs which is characterised by peak fallout rates in 1963, which subsequently declined to very low values by 1972 in the northern hemisphere as a result of the ban on atmospheric testing of thermonuclear weapons (Larsen, 1985). The Chernobyl accident increased soil radioactivity levels in the northern hemisphere, especially in eastern Europe, but negligible activities of Chernobyl 137Cs fallout were detected in China in 1986 (Zhang, 2005).

210Pbex is commonly applied to assess soil redistribution (Gaspar et al., 2013; Porto et al., 2018), but the radionuclide is also used for dating sediment records retrieved from water bodies to reconstruct dynamic changes in sediment accumulation (Morellón et al., 2011). Source models comprised of a number of requirements related to the fallout rate of the radionuclide or the constant accumulation of sediments have been developed (Appleby and Oldfield, 1978). For example, the constant initial concentration (CIC) model (Krishnaswamy et al., 1971) was applicable when the amount of 210Pbex carried and captured by sediments per unit mass was constant and the deposition rate was constant. The constant rate of supply (CRS) model assumes that 210Pbex flux is constant and accumulation rate varies with time, the flux of 210Pbex entering the interface between atmosphere and water is dynamically balanced with its flux entering the interface between water and sediment (Sanchez-Cabeza et al., 2000). Other models were specifically developed to date fluvial deposits with 210Pbex considering the continuous sedimentation occurring in floodplains (He and Walling, 1996).

For regions where lakes are nonexistent, there is a need to find alternative sedimentary deposits that can fulfill suitable stability conditions so as to develop a deposition model fulfilling the conditions for reconstructing the chronology of the sediment deposition. Special conditions in the Hailuogou Valley located at the foot of the Tibetan Plateau have prevented the preservation of lakes and therefore sedimentary records deposited at their bottoms from which to reconstruct the recent history of sediment accumulation generated after glacier retreat are absent.

In light of the situation described above, this study explored the potential for using 210Pbex to date moraine soils and trace glacier retreat over the past 100 years. Based on the physical processes of 210Pbex deposition, losses with runoff that remove fine particles where 210Pbex is fixed and decays, a preliminary 210Pbex accumulation-decay model was developed. The model also considered the change of erosion rate, especially water erosion (Berhe et al., 2018; Liu et al., 2019), which decreased with vegetation growth in the valley following the glacier retreat. To this aim, a field campaign was undertaken in the Hailuogou Glacier Valley in 2016 at Mt. Gongga (Hengduan Range, China) a benchmark site of the IAEA INT5153 Project, “Assessing the Impact of Climate Change and its Effects on Soil and Water Resources in Polar and Mountainous Regions.” Following the direction of glacier retreat along the valley, nine sampling sites were selected based on the known chronology of moraine deposits. The precise moraine position was marked by boundary stones at each site over the period from 1910 to 1990 (Luo et al., 2015). Furthermore, fallout of 137Cs was used to identify the ages of the study sites, labeled by the 137Cs peak in 1963 and to assess if soil mobilization by water erosion occurred at the study sites. 137Cs was also used to support the 210Pbex model results. The model was validated using these recorded retreat times and combining the data on 210Pbex inventories with the time spans between the glacier retreat year and the sampling year for the moraine soil at the nine measurement points.

Section snippets

Study area

This study was carried out in the glacier retreat area of Hailuogou Valley in Mt. Gongga, which is located in the Ganzi Tibetan Autonomous Prefecture of Sichuan Province, China (Fig. 1). Mt. Gongga is situated in the middle of the Hengduan Mountain range, which is located on the southeastern edge of the Tibet Plateau. The peak of Mt. Gongga is 7553 m asl and its snow line elevation varies between 4800 and 5200 m. Glaciers on Mt. Gongga have a total area of 255.1 km2 with a total volume of

Particle size composition

The particle size depth distributions of the moraine soils are shown in Fig. 2. The average proportions of clay (<2 μm) ranged between 0.19 and 0.97% with a corresponding mean of 0.31%, silt (2–63 μm) between 26.64% and 61.33% with a mean of 44.49% and sand (63–2000 μm) between 38.46% and 73.29% with a mean of 55.2%. The mean median diameter (d50) of the nine moraine sites ranged between 44 and 113 μm with a mean value of 77 μm. The silt content in the recent moraine sites (R1 and R2) was lower

Temporal variations of 137Cs and 210Pbex inventories

Since the sources of 137Cs and 210Pbex are different, there is a difference in the total 137Cs and 210Pbex inventory curves at the nine moraine sites on a temporal scale. In general, both radionuclides gradually increased with time. Apart from sites R1–R4 where moraine soils formed after the 1963 137Cs peak, the inventories of 137Cs at R5 and R6, prior to the 137Cs peak (Owens et al., 1997), were significantly lower than those at R7 and R9 because older sites from R7–R9 experienced increased 137

Conclusions

Both 137Cs and 210Pbex fallout inventories in moraine soils increase as time increases due to glacier retreat in the Hailuogou Valley. The 210Pbex inventories increased from 3,669.6 ± 218.5 Bq·m−2 at the site where glacier retreated in 1990 to 10,718.9 ± 167.4 Bq·m−2 at the site in 1910; however, little 137Cs fallout was detected in the soils where the glacier retreated after the 1960s because of limited nuclear weapon testing in the world since that time. Based on the physical processes of

Acknowledgements

The study reported in this paper was supported by the National Natural Science Foundation of China (41873025) and by the International Atomic Energy Agency (IAEA; program INT5153). The assistance of the Alpine Ecosystem Observation and Experiment Station of Mt. Gongga and the Hailuogou Scenic Area Administrative Committee are also gratefully acknowledged.

References (40)

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