Bimodal Plio-Quaternary glacial erosion of fjords and low-relief surfaces in Scandinavia

International audienceGlacial landscapes are characterized by dramatic local relief, but they also commonly exhibit high-elevation, lowreliefsurfaces. These surfaces have been attributed to glacial headward erosion and periglacial processes in Alpinesettings. However, the timing and processes responsible for their formation in northern high-latitude regions remainelusive. Here, we infer the topographic evolution of western Scandinavia during the Plio-Quaternary glaciations(0-2.8 Ma) by linking onshore erosion to offshore sedimentation. We estimate the rate of fjord erosion from geophysicalrelief and compare that with the erosion reflected by offshore sedimentation. We find that the sedimentsgenerated by fjord erosion (65-100 103 km3) over the entire western Scandinavia during the Plio-Quaternaryglaciations accounts for only 35–55% of the equivalent bedrock erosion deduced from total sediment volume depositedoff the coast of Norway. This large mismatch implies that during this period, significant erosion (300-400m) must have also taken place away from the fjords at high elevation and thus indicates a bimodal distribution ofglacial erosion. Furthermore, comparing the distribution of the high-elevation, low-relief surfaces with estimatesof the long-term glacier equilibrium line altitude supports the idea that effective erosion in extensively glaciatedareas limits topographic height, a process known as the glacial buzzsaw. We therefore conclude that glacial andperiglacial processes have a substantial impact on the formation of low-relief surfaces observed in glaciated mountainbelts and high-latitude continental margins

Worldwide acceleration of mountain erosion under a cooling climate

Climate influences the erosion processes acting at the Earth’s surface. However, the effect of cooling during the Late Cenozoic era, including the onset of Pliocene–Pleistocene Northern Hemisphere glaciation (about two to three million years ago), on global erosion rates remains unclear1, 2, 3, 4. The uncertainty arises mainly from a lack of consensus on the use of the sedimentary record as a proxy for erosion3, 4 and the difficulty of isolating the respective contributions of tectonics and climate to erosion5, 6, 7. Here we compile 18,000 bedrock thermochronometric ages from around the world and use a formal inversion procedure8 to estimate temporal and spatial variations in erosion rates. This allows for the quantification of erosion for the source areas that ultimately produce the sediment record on a timescale of millions of years. We find that mountain erosion rates have increased since about six million years ago and most rapidly since two million years ago. The increase of erosion rates is observed at all latitudes, but is most pronounced in glaciated mountain ranges, indicating that glacial processes played an important part. Because mountains represent a considerable fraction of the global production of sediments9, our results imply an increase in sediment flux at a global scale that coincides closely with enhanced cooling during the Pliocene and Pleistocene epochs10, 11