Reconstructing the erosion history of glaciated passive margins: applications of in situ produced cosmogenic nuclide techniques

Abstract

Abstract Offshore sediment accumulations provide an intriguing record of the net sediment output resulting from geomorphological evolution of the circum-Atlantic continental margin since the commencement of Neogene glaciation. However, the onshore record of the timing, pattern and amount of bedrock erosion that produced these sediments is comparatively poorly constrained and understood, although there are good general models of glaciation history. The geomorphology of circum-Atlantic continental margin mountains, as assessed from remote sensing data and field observations, includes palimpsest landforms and landscapes that reflect a complex pattern of spatial and temporal variations in the impact of glacial, fluvial and periglacial processes. Perhaps most surprising is that, despite having been repeatedly overridden by large ice sheets, parts of the landscape appear to be relict, with nonglacial morphology. This has important implications both for glaciological conditions under ice sheets, and for sediment source areas and erosion rates. Conventional dating and analysis have provided an excellent way to begin unravelling the timing and pattern of erosion, landform development, and possible landform preservation under ice. However, testing hypotheses developed from current models, and addressing critical unresolved questions, requires additional approaches. The use of in situ cosmogenic nuclide production in bedrock is a new approach for investigating landscape evolution in mountainous areas. With careful interpretation of geomorphological settings, cosmogenic nuclides can be used to determine apparent surface exposure age and landscape preservation, and constrain erosion depths and duration of burial by ice. Here we provide a framework for the interpretation of cosmogenic nuclide concentrations in bedrock surfaces of landscapes affected by glacial, fluvial and periglacial processes, illustrated with examples from the northern Swedish mountains. This demonstrates potential uses of cosmogenic nuclide techniques, and provides a foundation for attempts to improve geomorphologically based reconstructions of relict landscapes, to reconstruct and analyse the dynamics of landscape change in glacial times, and to define the consequences of different process regimes in terms of erosion patterns, sediment transport, and the supply of sediments that are deposited offshore.