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

Abstract

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.

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

“…As glacier dynamics are linked to climate, an active area of research aims to characterize the role of glacial erosion on the dynamics and relief development of mountain belts during the recent Quaternary glaciations (e.g. Zachos et al, 2001;Molnar and England, 1990;Beaumont et al, 1992;Montgomery, 2002;Brozović et al, 1997;Whipple et al, 2009;Steer et al, 2012;Champagnac et al, 2014). To address these questions, two timescales have typically been considered: a longer timescale (10 5 -10 6 years) to assess the potential glacial imprint on the landscape, and a shorter timescale (10 1 -10 4 years) to understand how ice actually erodes the landscape.…”

“…For example, some studies integrated glacial sediment records worldwide and in situ low-temperature thermochronology data, to estimate glacial erosion rates. They showed average erosion rates of 10 0 -10 3 mm yr -1 on short timescales (10 3 -10 5 years) and long-term (>10 6 years) average erosion rates of 10 -2 -10 0 mm yr -1 (Hallet et al, 1996;Koppes and Montgomery, 2009;Valla et al, 2011b;Koppes et al, 2015;Bernard et al, 2016). Herman et al (2013) used a global compilation of in situ thermochronological data and an inverse approach to infer an increase in erosion rates for all mountain ranges in the Quaternary period.…”

“…supraglacial and subglacial areas), and their contribution to erosion, define the relative proportion of sediments that enter the glacier transport system (Small, 1987). Sediments are transported by glaciers by (i) subglacial water flow through cavity or channel systems (Kirkbride, 2002;Alley et al, 1997;Spedding, 2000), and (ii) by ice internal deformation for sediments incorporated within or above the ice (Hambrey et al, 1999;Goodsell et al, 2005).…”

“…Therefore, sampling and analysing these sediments (detrital thermochronology) has the potential to provide a more spatially integrated view of the catchment erosion pattern on short timescales (10 1 -10 4 years) and thereby a more representative indication of how ice is eroding the landscape (e.g. Stock et al, 2006;Tranel et. 2011;Ehlers et al, 2015).…”

“…Thermochronological age distributions of detrital samples are generally interpreted in terms of synoptic probability density functions (SPDF) (Brewer et al, 2003;Ruhl and Hodges., 2005). Analysis of many glacial catchments has been based on the interpretation of these SPDFs (Stock et al, 2006;Tranel et al, 2011;Avdeev et al, 2011;Thomson et al, 2013). More recently, Enkelmann and Ehlers (2015) and Ehlers et al (2015) investigated the erosion patterns and the degree of mixing of ice-cored sediments at the terminus of the Tiedemann glacier (Coast Mountains, Canada) using both detrital and in situ low-temperature thermochronology.…”