Spatial and Temporal Patterns of Late Holocene Rockfall Activity on a Norwegian Talus Slope: A Lichenometric and Simulation-Modeling Approach

Abstract

Largest lichens (genus Rhizocarpon) were measured on 2800 boulders sampled systematically at 28 sites on a thin triangular talus. Sites with similar lichen-size frequency distributions were grouped and used to interpret the temporal and spatial patterns of rockfall supply. Most of the slope yields size-distributions of largest lichens that reflect the history of rockfall activity over at least the last 400 yr. A general increase in surface age diagonally downslope suggests boulders are supplied by rockfall rather than avalanching, with no evidence of postdepositional redistribution. At one corner of the talus foot the lichen-size distributions have reached equilibrium, suggesting negligible supply of boulders during the late Holocene. Sites at the apex of the talus are dominated by small lichens. Simulation modeling is used to reconstruct possible temporal patterns of debris supply to different parts of the talus, and competing models are tested by estimating the thickness of talus predicted to have accumulated during the Holocene. The normal rate of rockfall during the late Holocene is estimated to result in burial of about 4% of the talus surface each 25 yr. Rates of rockfall supply during the 18th century, the coldest phase of the Little Ice Age, are estimated to have been almost five times the normal late Holocene rate. Results are consistent with talus formation under periglacial conditions during the Holocene; a paraglacial origin is unnecessary.