Novel apatite provenance analysis reveals erosion of moraines as primary sediment source during glacial retreat

Abstract

Climate change-driven glacial retreat is transforming alpine landscapes at unprecedented rates. As ice retreats, newly exposed valley walls and moraine sediment become vulnerable to hillslope and fluvial erosion that in turn can significantly increase fluxes downstream. These increases have important implications for water quality and storage, presenting risks to biodiversity, ecosystem stability, and human inhabitants. However, the mechanisms driving these increases remain unclear, with both enhanced subglacial bedrock erosion and the remobilisation of glacial sediments hypothesised. Resolving this uncertainty requires the ability to trace the exact sediment source within a catchment. Recent advances in apatite analysis using LA-ICP-MS techniques enable the simultaneous collection of thermochronometric, geochronometric, and chemical data from individual detrital grains. This now allows us to trace sediment sources across a glaciated catchment based on lithology and source-rock elevation. In this study, samples were collected across the Bugaboo Glacier catchment, British Columbia, Canada, where ice retreat has exceeded 2 km in the last century. Detrital samples were collected from the outwash river and two moraine locations, coupled with bedrock samples from varying elevations. Bedrock samples encompass the catchment’s two principal lithologies, a Cretaceous granitic intrusion, and Neoproterozoic metasediments. Thermochronometric dates show a positive relationship with elevation, while geochronometric dates and trace chemistry data help highlight key differences between lithologies. Detrital mixture models and multi-dimensional scaling suggest moraine samples are composed of sediment derived from a wide range of elevations within the catchment, while the sediments of the modern outwash river appear to be derived entirely from erosion of the moraines, left exposed by retreating ice. These findings suggest that increased sediment flux during glacial retreat is primarily driven by the remobilization of unconsolidated moraine materials, rather than enhanced bedrock erosion. Furthermore, this study demonstrates the effectiveness of multi-method detrital apatite analysis as a powerful provenance tool in studying sediment dynamics.