Arctic-alpine blockfields in northern Sweden: Quaternary not Neogene

Abstract

Slowly-eroding, blockfield-mantled, non-glacial surface remnants may serve as markers against which to determine Quaternary glacial erosion volumes in high latitude mountain settings. To investigate this potential utility of these surfaces, chemical weathering , erosion rates, and origins of mountain blockfields are investigated in northern 5 Sweden. This is done, firstly, by assessing the intensity of regolith chemical weathering along altitudinal transects descending from three blockfield-mantled summits. Clay/silt ratios, secondary mineral assemblages determined through X-ray diffraction, and the presence of chemically weathered grains visible on scanning electron microscopy, in fine matrix samples collected from pits excavated along the transects are each used 10 for this purpose. Secondly, erosion rates and total surface histories of two of the summits are inferred from concentrations of in situ-produced cosmogenic 10 Be and 26 Al in quartz at the blockfield surface. An interpretative model is adopted that includes temporal variations in nuclide production rates through surface burial by glacial ice and glacial isostasy-induced elevation changes of the blockfield surfaces. Together, 15 our data indicate that these blockfields are not derived from remnants of intensely weathered Neogene weathering profiles, as is commonly considered. Evidence for this interpretation includes minor chemical weathering in each of the three examined block-fields, despite some differences according to slope position. In addition, average erosion rates of ∼ 16.2 mm ka −1 and ∼ 6.7 mm ka −1 , calculated for two blockfield-mantled 20 summits, are low but of sufficient magnitude to remove present blockfield mantles, of up to a few meters in thickness, within a late-Quaternary timeframe. Hence, block-field mantles appear to be replenished by regolith formation through, primarily physical , weathering processes that have operated during the Quaternary. Erosion rates remain low enough, however, for blockfield-mantled, non-glacial surface remnants to 25 provide reasonable landscape markers against which to contrast Quaternary erosion volumes in surrounding glacial landscape elements. The persistence of blockfield mantles over a number of glacial-interglacial cycles and an apparently low likelihood that 49 they can re-establish on glacially eroded bedrock, also discounts the operation of a "glacial buzz-saw" on surface remnants that are presently perceived as non-glacial. These interpretations are tempered though by outstanding questions concerning the composition of preceding Neogene regoliths and why they have apparently been comprehensively removed from these remnant non-glacial surfaces. It remains possible 5 that periglacial erosion of perhaps more intensely weathered Neogene regoliths was high during the Pliocene-Pleistocene transition to colder conditions and that periglacial processes reshaped non-glacial surface remnants largely before the formation of block-field armours.