Landslide-Induced Weathering in Tectonically Active Mountains: Evidence From Dissolved Radiogenic Uranium Isotopes

Gaojun Li,Gen K Li,Le Li,Laifeng Li

doi:10.3389/feart.2022.825818

Abstract

Understanding weathering processes in landslide-dominated catchments is critical for evaluating the role of landslides in chemical weathering and the global carbon cycle. Previous studies have focused on solute concentrations in landslide-impacted landscapes, but have paid less attention to developing isotopic tracers of landslide-induced weathering fluxes. Recent work found that the dissolved radiogenic uranium isotopes in river water are closely related to the denudation rates in catchments draining steep mountains where landslides are thought to be a major erosion mechanism, suggesting the potential of uranium isotopes to trace landslide-induced weathering fluxes. Here we compile the dissolved radiogenic uranium isotopes (234U/238U ratios) in the river water samples from a group of catchments with variable landslide activities in the Minjiang River Basin at the eastern margin of the Tibetan Plateau. We derive three metrics of landslide activity from the analyses of digital topography and an inventory map of the co-seismic landslides triggered by the 2008 Mw7.9 Wenchuan earthquake: the normalized volume of landslides, the mean catchment slope angle, and the fraction of slopes steeper than a threshold angle beyond which slopes are mechanically unstable. The riverine dissolved 234U/238U ratios correlate negatively with the metrics of landslide activity in each catchment, which likely reflect the influence of landslides on the dissolved 234U/238U ratios. Mechanistically, enhanced bedrock landsliding would accelerate the exposure of fresh rock, promoting bedrock weathering and congruent dissolution of 234U and 238U contained in minerals; reduced landslide activities and enhanced regolith weathering would lead to preferential accumulation of 234U against 238U in solutes through alpha-recoil ejection, thus increasing dissolved 234U/238U. Our findings provide field evidence of using the riverine dissolved 234U/238U ratio to trace weathering fluxes driven by landslides, shedding new light on chemical weathering processes in uplifting mountains.

Highlights

In uplifting mountains, hillslopes steepen in response to channel incision and become prone to landslides once hillslopes are steeper than the angle of repose (Burbank et al, 1996; Ouimet et al, 2008; Larsen and Montgomery, 2012)
Two catchments in the upper stream part of the Minjiang River Basin do not have identifiable landslides from the Wenchuan earthquake, but the other four catchments whose corresponding sampling sites are along the main Minjiang River have an increase in the normalized volume of landslides (LSnorm) from the upstream (JGS11, 121 m3 km−2) to downstream sites (ZPP, 64100 m3 km−2) (Figure 1 and Table 1)
Our work demonstrates that the riverine dissolved 234U/238U directly links to landslide activities in a river basin where conventional water chemistry metric TDS cannot trace landslide-induced weathering fluxes, providing a promising tool to better understand landslide-driven weathering processes in catchments with complex lithologies

Summary

Introduction

Hillslopes steepen in response to channel incision and become prone to landslides once hillslopes are steeper than the angle of repose (Burbank et al, 1996; Ouimet et al, 2008; Larsen and Montgomery, 2012). Because landslides have been identified as a primary mechanism of erosion in steep mountains (Hovius et al, 1997; Larsen and Montgomery, 2012; Li G. et al, 2017; Marc et al, 2019), the linkage between landslide activity and chemical weathering holds key implications for the interplay between tectonic uplift, erosion, and the carbon cycle. Understanding this linkage requires effective tracers of landslide-induced weathering fluxes (e.g., Li et al, 2018; Li S. et al, 2019), which have received less attention. Using 10Be (meteoric)/9Be to trace weathering processes relies on robust constraints on the absorption coefficients of 10Be to different minerals (von Blanckenburg et al, 2012)

Results

Discussion

Conclusion