Modelling the effects of ice transport and sediment sources on the form of detrital thermochronological age probability distributions from glacial settings

International audienceThe impact of glaciers on the Quaternary evolution of mountainous landscapes remains controversial. Although in situ or bedrock low-temperature thermochronology offers insights on past rock exhumation and landscape erosion, the method also suffers from potential biases due to the difficulty of sampling bedrock buried under glaciers. Detrital thermochronology attempts to overcome this issue by sampling sediments at e.g. the catchment outlet, a component of which may originate from beneath the ice. However, detrital age distributions not only reflect the catchment exhumation, but also spatially variable 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 formed by bedrock erosion, and their transport is restricted to ice or hillslope processes, neglecting subglacial hydrology, until they are deposited. We base our model on the Tiedemann Glacier (British Columbia, Canada), which has simple morphological characteristics, such as a linear form and no connectivity to 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. Results show that sediment sources, reflecting different processes such as glacier and hillslope erosion, can have distinct bedrock age distribution signatures, and estimating such distributions should help to identify predominant sources in the sampling site. However, discrepancies between the detrital and bedrock age distributions occur due 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 strong variability of glacier ice velocity, (iii) the heterogeneous pattern of erosion, and (iv) the advective nature of glacier sediment transport along ice streamlines. This last factor leads to a poor lateral mixing of particle detrital signatures inside the frontal moraine, and then local sampling of the frontal moraine is likely to reflect local sources upstream. Therefore, sampling randomly across the moraine is preferred for a more representative view of the catchment age distribution. Finally, systematic comparisons between synthetic (U−Th)/He and fission track detrital ages, with different bedrock age-elevation profiles and different relative age uncertainties, show that the nature of the age-elevation relationship and age uncertainties largely control the ability to track sediment sources in the detri-tal record. However, depending on the erosion pattern spatially, qualitative first-order information may still be extracted from a thermochronological system with high uncertainties (> 30 %). 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. However, when combined with bedrock age distributions, detrital thermochronology offers a novel means to constrain the transport pattern and time of sediment particles. Published by Copernicus Publications on behalf of the European Geosciences Union. 932 M. Bernard et al.: Modelling the effects of glacial processes on detrital age distribution

Lateglacial paleoglacier and paleoclimate reconstructions in the north-western Italian Alps

International audienc

Lateglacial paleoglacier and paleoclimate reconstructions in the north-western Italian Alps

International audienc

Lateglacial paleoglacier and paleoclimate reconstructions in the north-western Italian Alps

International audienc

A multi-nuclide approach to constrain landscape evolution and past erosion rates in previously glaciated terrains

Cosmogenic nuclides are typically used to either constrain an exposure age, a burial age, or an erosion rate. Constraining the landscape history and past erosion rates in previously glaciated terrains is, however, notoriously difficult because it involves a large number of unknowns. The potential use of cosmogenic nuclides in landscapes with a complex history of exposure and erosion is therefore often quite limited. Here, we present a novel multi-nuclide approach to study the landscape evolution and past erosion rates in terrains with a complex exposure history, particularly focusing on regions that were repeatedly covered by glaciers or ice sheets during the Quaternary. The approach, based on the Markov Chain Monte Carlo (MCMC) technique, focuses on mapping the range of landscape histories that are consistent with a given set of measured cosmogenic nuclide concentrations. A fundamental assumption of the model approach is that the exposure history at the site/location can be divided into two distinct regimes: i) interglacial periods characterized by zero shielding due to overlying ice and a uniform interglacial erosion rate, and ii) glacial periods characterized by 100% shielding and a uniform glacial erosion rate. We incorporate the exposure history in the model framework by applying a threshold value to the global marine benthic δ18O record and include the threshold value as a free model parameter, hereby taking into account global changes in climate. However, any available information on the glacial-interglacial history at the sampling location, in particular the timing of the last deglaciation event, is readily incorporated in the model to constrain the inverse problem. Based on the MCMC technique, the model delineates the most likely exposure history, including the glacial and interglacial erosion rates, which, in turn, makes it possible to reconstruct an exhumation history at the site. We apply the model to two landscape scenarios based on synthetic data and two landscape scenarios based on paired 10Be/26Al data from West Greenland, which makes it possible to quantify the denudation rate at these locations. The model framework, which currently incorporates any combination of the following nuclides 10Be, 26Al, 14C, and 21Ne, is highly flexible and can be adapted to many different landscape settings. The model framework may also be used in combination with physics-based landscape evolution models to predict nuclide concentrations at different locations in the landscape. This may help validate the landscape models via comparison to measured nuclide concentrations or to devise new effective sampling strategies