Identifying spatial variations in glacial catchment erosion with detrital thermochronology

Abstract

Understanding the spatial distribution of glacial catchment erosion during glaciation has previously proven difficult due to limited access to the glacier bed. Recent advances in detrital thermochronology provide a new technique to quantify the source elevation of sediment. This approach utilizes the tendency of thermochronometer cooling ages to increase with elevation and provides a sediment tracer for the elevation of erosion. We apply this technique to the Tiedeman Glacier in the heavily glaciated Mount Waddington region, British Columbia. A total of 106 detrital apatite (U-Th)/He (AHe) and 100 apatite fission track (AFT) single-grain ages was presented from the modern outwash of the Tiedemann Glacier with catchment elevations between 530 and 3960 m. These data are combined with nine AHe and nine AFT bedrock ages collected from ã 2400 m vertical transect to test the hypotheses that erosion is uniformly or nonuniformly distributed in the catchment. A Monte Carlo sampling model and Kuiper statistical test are used to quantify the elevation range where outwash sediment is sourced. Model results from the AHe data suggest nearly uniform erosion in the catchment, with a preference for sediment being sourced from~2900 to 2700 m elevation. Ages indicated that the largest source of sediment is near the present-day ELA. These results demonstrate the utility of AHe detrital thermochronology (and to a lesser degree AFT data) to quantify the distribution of erosion by individual geomorphic processes, as well as some of the limitations of the technique.

Evaluation of detrital thermochronology for quantification of glacial catchment denudation and sediment mixing

“…The results are presented together with those of sample 9TETG15 (modern Tiedemann Glacier pro-glacial outwash) published by Ehlers et al (2015). For the comparison of detrital FT age distributions derived from different sample locations, and with the bedrock ages of the catchment, we present the single-grain apatite FT results as histograms and probability density functions for each sample (Figs.…”

“…The mismatch between the expected and the measured detrital age distribution reveals elevations within the catchment where erosion and weathering is less or more effective with respect to the entire catchment elevation (Brewer et al, 2003;Ruhl and Hodges, 2005;Stock et al, 2006;Reiners et al, 2007;Ehlers et al, 2015).…”

“…The entire region is composed of a granodiorite batholith (coarse to medium grained) containing good-quality apatite grains. There are no known faults that caused differential vertical exhumation within the catchment and bedrock apatite FT and (U-Th)/He cooling ages are available (Parrish, 1983;O'Sullivan and Parrish, 1995;Ehlers et al, 2006Ehlers et al, , 2015, and allow a cross check with our new detrital apatite FT age distributions.…”

“…Detrital material can also provide indirect access to unreachable locations such as under glaciers themselves. The analysis of apatite (U-Th)/He ages from detrital material was successful in quantifying the spatial distribution of erosion in formerly glaciated catchments of the Sierra Nevada, California (Stock et al, 2006), and in the Tiedemann Glacier catchment in British Columbia (Ehlers et al, 2015). In contrast to the (U-Th)/He dating technique, the FT dating approach has the advantage that large numbers of measurements are possible, which is crucial for a statistical robust analysis of detrital age distributions.…”

Resolving the internal and basal geometry of ice masses using imaging phase-sensitive radar

“…The glacier has a catchment of 35 000 km 2 and is 5 km wide at the terminus, where the glacier flows at ∼6300 m a −1 (Rignot and Mouginot, 2012;Morlighem and others, 2016). While many surrounding glaciers have recently experienced dynamic thinning due to acceleration and retreat of their termini, Store Glacier has exhibited little change in both mass balance and overall terminus position since 1968, exhibiting only a 200 m seasonal oscillation in the terminus position (Weidick, 1995;Howat and others, 2010;Box and Decker, 2011). This stability is due to topographic narrowing and grounding near the glacier's terminus (Todd and Christoffersen, 2014;Morlighem and others, 2016).…”