Production and Transport of Supraglacial Debris: Insights From Cosmogenic ¹⁰Be and Numerical Modeling, Chhota Shigri Glacier, Indian Himalaya

Abstract

Many mountain glaciers carry some amount of rocky debris on them, which modifies surface ablation rates. The debris is typically derived from erosion of the surrounding topography and its supraglacial extent is predominantly controlled by the relative accumulation rates of debris versus snow. Because Global Warming results in shrinking glaciers as well as thawing permafrost worldwide, changes in both rates will most likely affect the evolution of supraglacial debris cover and thus the response of glaciers to climate change. Here we report 10 Be concentrations measured in five amalgamated debris samples collected from the main medial moraine of the Chhota Shigri Glacier, India. Results suggest headwall erosion rates that are~0.5-1 mm year −1 , and apparently increasing (10 Be concentrations are decreasing) toward the present. We employed a numerical ice flow model that we combined with a new Lagrangian particle tracing routine to explore the impact of spatial and temporal variability in erosion rates and source areas on 10 Be concentrations in the medial moraine. Our modeling results show that neither changes in source areas, related to the transient response of the glacier to ongoing climate change, nor four different scenarios of spatial and temporal variability in erosion rates could explain the observed trend in 10 Be concentrations. Although not accounted for in our modeling explicitly, we suggest that the observed trend could be due to transiently enhanced erosion of recently deglaciated areas, or to greater spatial variability in erosion rates than explored in our models. Plain Language Summary High and steep mountain ranges are currently undergoing changes due to increasing temperatures. These changes include rapidly shrinking glaciers as well as thawing permafrost, which together destabilize rock walls that surround valley glaciers. In consequence, slope failures and thus erosion rates in these environments are expected to increase. However, quantifying rock wall erosion in alpine landscapes is difficult and estimates of background erosion rates that are unaffected by Global Warming are rare. Here we estimate rock wall erosion rates above the Chhota Shigri Glacier, Indian Himalaya, by studying rocky debris from the glacier surface. This debris is sourced from the surrounding topography and we use geochemical tools to measure its residence time at the Earth surface. We combine our geochemical observations with a computer model of the glacier that allows us to explore the effect of Global Warming on the evolution of the glacier and the debris on its surface. Our results suggest recent changes in rock wall erosion rates that may be related to glacier retreat and an increase in the erosion of rock walls that were previously ice covered.

Rockwall Slope Erosion in the Northwestern Himalaya

“…(b) Swath 2 (S2). Rockwall slope erosion: this study, Scherler and Egholm (2020); catchment‐wide erosion: Dortch et al. (2011a), Dietsch et al.…”

“…White circles: location of catchments of this study. Gray circles: location of published rockwall slope erosion rate studies: Baltoro glacier system (Seong et al, 2009), Chhota Shigri (Scherler & Egholm, 2020), Bhagirathi glacier system (Orr et al, 2019). Major faults from Hodges (2000) and Schlup et al (2003).…”

“…A number of studies have underlined the importance of the erosion of bedrock‐dominated slopes, referred to here as rockwall slopes (Figure 1), in catchment sediment flux, relief production, topographic configuration, and glacier dynamics of steep alpine environments (Benn et al., 2012; Heimsath & McGlynn, 2008; MacGregor et al., 2009; Orr et al., 2019; Scherler & Egholm, 2020; Seong et al., 2009; Ward & Anderson, 2011). The lateral erosion of slopes has been shown to exceed rates of vertical incision through glacial and fluvial processes, and therefore to a greater extent than previously thought, contribute to denudation budgets and landscape change on the catchment and mountain range scale (Brocklehurst & Whipple, 2006; Foster et al., 2008).…”

“…Our new erosion data set is combined with existing rockwall slope erosion records from Seong et al. (2009), Scherler and Egholm (2020), and Orr et al. (2019).…”

The Challenge of Non-Stationary Feedbacks in Modeling the Response of Debris-Covered Glaciers to Climate Forcing

“…Although it is understood that lithology and weathering rates will affect debris supply (Matsuoka and Sakai, 1999;Nagai et al, 2013;Scherler, 2014), debris source and deposition areas, volumes, frequency and rates remain poorly constrained (Benn and Evans, 2010). Estimates of denudation rates from headwall retreat (Heimsath and McGlynn, 2008;Seong et al, 2009), given in units per century or per millennium are difficult to integrate with a glacier system model that requires representation of episodic, discrete debris supply events (Scherler and Egholm, 2020). Measurements of rock volumes from individual events (e.g.…”