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Earth Surface Dynamics An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/esurf-2020-7
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/esurf-2020-7
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 11 Feb 2020

Submitted as: research article | 11 Feb 2020

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A revised version of this preprint is currently under review for the journal ESurf.

The effects of ice and hillslope erosion and detrital transport on the form of detrital thermochronological age probability distributions from glacial settings

Maxime Bernard1, Philippe Steer1, Kerry Gallagher1, and David Lundbek Egholm2 Maxime Bernard et al.
  • 1Univ. Rennes, CNRS, Géosciences Rennes, UMR 6118, 35000 Rennes, France
  • 2Department of Geoscience, Aarhus University, Aarhus, Denmark

Abstract. The impact of glaciers on the Quaternary evolution of mountainous landscapes remains controversial. Although in situ low-temperature thermochronology offers insights on past rock exhumation and landscape erosion, the methods also suffer from biases due to the difficulty of sampling bedrock buried under glaciers. Detrital thermochronology attempts to bypass this issue by sampling sediments, at e.g. the catchment outlet, that may originate from beneath the ice. However, the age distributions resulting from detrital thermochronology do not only reflect the catchment exhumation, but also the patterns and rates of surface erosion and sediment transport. In this study, we use a new version of a glacial landscape evolution model, iSOSIA, to address the effect of erosion and sediment transport by ice on the form of synthetic detrital age distributions. Sediments are tracked as Lagrangian particles which can be formed by bedrock erosion, transported by ice or hillslope processes and deposited. We apply our model to the Tiedemann glacier (British Columbia, Canada), which has simple morphological characteristics, such as a linear form and no connectivity with large tributary glaciers. Synthetic detrital age distributions are generated by specifying an erosion history, then sampling sediment particles at the frontal moraine of the modelled glacier.

An assessment of sediment transport shows that 1500 years are required to reach an equilibrium for detrital particle age distributions, due to the large range of particle transport times from their sources to the frontal moraine. Next, varying sampling locations and strategies at the glacier front leads to varying detrital SPDFs, even at equilibrium. These discrepancies are related to (i) the selective storage of a large proportion of sediments in small tributary glaciers and in lateral moraines, (ii) the large range of particle transport times, due to varying transport lengths and to a strong variability of glacier ice velocity, (iii) the heterogeneous pattern of erosion, (iv) the advective nature of glacier sediment transport, along ice streamlines, that leads to a poor lateral mixing of particle detrital signatures inside the frontal moraine. Finally, systematic comparisons between (U-Th)/He and fission track detrital ages, with different age-elevation profiles and relative age uncertainties, show that (i) the nature of the age-elevation relationship largely controls the ability to track sediment sources, and (ii) qualitative first-order information may still be extracted from thermochronological system with high uncertainties (> 30 %) depending on erosion pattern. Overall, our results demonstrate that detrital age distributions in glaciated catchments are strongly impacted not only by erosion and exhumation but also by sediment transport processes and their spatial variability. Combined with bedrock age distributions, detrital thermochronology offers a means to constrain the transport pattern and time of sediment particles. However, our results also suggest that detrital age distributions of glacial features like frontal moraines, are likely to reflect a transient case as the time required to reach detrital thermochronological equilibrium is of the order of the short-timescale glacier dynamics variability, as little ice ages or recent glaciers recessions.

Maxime Bernard et al.

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Maxime Bernard et al.

Maxime Bernard et al.

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Latest update: 09 Jul 2020
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Short summary
Detrital thermochronometric age distributions of frontal moraines have the potential to retrieve ice erosion patterns. However, modelling erosion and sediment transport by the Tiedemann glacier ice shows that ice velocity, source of sediment and ice flow patterns affect age distribution shape by delaying sediment transfer. Computed transfer times fit with the observed variability in glacier dynamics, suggesting bias in the inference of erosion patterns from age distributions of frontal moraines.
Detrital thermochronometric age distributions of frontal moraines have the potential to retrieve...
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