Glacial transport affects thermochronological age distributions at the glacier fronts

Article in Earth Surface Dynamics
Mount Tiedeman

Maxime Bernard, Philippe Steer and Kerry Gallagher from the Rennes GeosciencesLaboratory, and in collaboration with David L. Egholm from the University of Aarhus  (Denmark), published in the journal Earth Surface Dynamics last November; a article on the effects of the ice transport on the shape of thermochronological age distributions collected at the glacier fronts.


 Glacier insertion is suspected to have enhanced the global climate cooling observed at the end of the Cenozoic through a positive feedback loop whereby efficient glacial erosion and sediment transport into storage basins promoted the extraction and sequestration of atmospheric carbon. However, clear evidence showing a superior
efficiency of glacial erosion compared to river erosion is still lacking, and therefore there is a need to understand further how glaciers erode their substrate.
 
 Thermochronological analysis of minerals deposited at the front of glaciers (products of erosion), is a technique generally used to map the average glacial erosion within a basin. During their exhumation towards the surface, some minerals record their date of passage at a temperature in the earth's crust, and obtain a so-called thermochronological age. At the surface, these ages generally evolve with elevation, where the youngest are found in the valley bottoms and the oldest towards the summits. Erosion, by taking minerals from rocks at the outcrop, samples these ages
which will then be transported by surface processes (glaciers, rivers, landslides...) and possibly deposited at the glacier front in a frontal moraine (glacial deposit composed of heterometric and non-stratified sediments), where they can be sampled. The  collected data are interpreted via thermochronological age proportion distributions (PDFs) which, combined with an age-elevation relationship, allow the identification of  contributing source elevations and thus map erosion.
 
 So far, most studies have interpreted these age distributions neglecting the effect of glacier transport. In this new study, the authors are therefore interested in the impact of this transport on the shape of the PDFs collected at the front of a glacier. To do so, they model the flow of a real glacier, the Tiedemann Glacier located in British Columbia (Canada), and simulate the formation of sediment particles by erosion, their transport, and their possible deposition at the glacier front (Figure 1). The objective is to simulate actual field sampling in the frontal moraine and to compare
the shape of the PDFs obtained to those expected when the effect of glacial transport of sediments is neglected, under different erosion scenarios.

Tiedeman

  By simulating particle transport over 8500 years, the authors show that glacial transport affects the shape of PDFs in the frontal moraine through the storage of a
significant portion of sediments in the basin (>50%) upstream of the sampling site favoured by (1) low ice flow velocities in tributary glaciers and, (2) significant
deposition in a lateral moraine upstream of the sampling site. This storage impacts the shape of the age distributions at the glacier front by favouring the 
contribution of young ages ( 3-4 Ma) at the expense of older ages ( 6 Ma), relative to the expected PDF (Figure 2b). Particles originating from hillslopes are 
impacted by this storage, particularly for intermediate elevations corresponding to ages around 6 Ma (Figure 2c). For particles originating from the subglacial floor
, their continuous production favours the effect of distance from the sources to the sampling site by shifting the observed PDF towards younger ages ( 3-4 Ma) found 
in the valley bottom and close to the sampling site (the frontal moraine).
 
 In addition, particle transfer times show significant variability controlled by transport distances, distance from sources to main ice streams, and significant
spatial variability in ice flow velocities. Finally, by comparing model results with existing field data, the authors show that glacier transport is characterized 
by a poor lateral mixing of sediments. This results in fragmentation of the detrital signal from sources at the glacier front. In practice, one can link a local 
sampling in the moraine to a local source upstream. However, in order to obtain a representative distribution of the thermochronological ages of the glacial basin, 
and thus map erosion, the numerical results suggest sampling the frontal moraine diffusely.

transport glaciaire

This original work thus provides key elements on the role of glacial transport on the thermochronological age distributions at the glacier fronts, and consequently 
on their interpretations. By validating the models with field data, the authors show that such a modelling approach is relevant to explore the mechanisms at work 
in glacial basins, and provides crucial information to guide field sampling strategies to reduce the effect of glacial transport in the collected data

 

Bernard, M., Steer, P., Gallagher, K., and Lundbek Egholm, D.: Modelling the effects of ice transport and sediment sources on the form of detrital thermochronological age probability distributions from glacial settings, Earth Surf. Dynam., 8, 931–953, doi.:10.5194/esurf-8-931-2020, 2020.