Can a glacially-derived alluvial cover effect reconcile increased terrestrial erosion rates and steady marine sedimentation rates in the late Cenozoic?

Initiation of glaciation in the late Cenozoic increased erosion rates in mountainous regions, yet the impact on marine sedimentation rates and global production in that time is controversial. Reconciliation of these records is key for interpreting late Cenozoic impacts of climate, tectonics, and erosion. Here, we propose that sediment eroded from glaciers acted as a protective cover from erosion in downstream rivers. We use a numerical model that couples glacial and fluvial erosion to examine how river profiles evolve under different uplift and sediment size regimes. Following model initiation with fluvial processes, we impose glacial cycles and compare erosion results to model scenarios with no glaciation. The resulting river profiles can be split into two groups: those with uplift rates of 0.1 mm/yr and less, and those with rates greater than 0.1 mm/yr. The low uplift group is insensitive to uplift rate as the profiles are primarily controlled by grain size. The least amount of erosion is found at medium grain sizes, while higher rates of fluvial and glacial erosion occur at fine and coarse grains, respectively. For fine to medium grain sizes, total erosion in the glaciated scenarios is indistinguishable from total erosion in unglaciated scenarios, due to a small proportion of the initial channel profile above the glacial erosion limit. Channel profiles in the high uplift rate group are dependent on both uplift and grain size, with a minimum total erosion at a larger grain size than the low uplift scenarios. While total erosion is greater in the glaciated scenarios, the fluvial erosion contribution is less than in unglaciated model runs, reflecting a cover effect from glacially-derived sediment. In both high and low uplift scenarios with fine-medium grain sizes, total erosion increases by 9% on average (median 6%) from unglaciated scenarios. This modelled increase in erosion from glaciation is much less than the 4x increase in erosion found in terrestrial records of steep mountain terrain for the late Cenozoic, but is within range of the near-steady sediment accumulation rates. Thus, dampened fluvial erosion from a glacially-derived sediment cover may help reconcile rapid glacial erosion rates with constant sediment accumulation rates in basins during the late Cenozoic.