Improved chronological constraints for Holocene rock glacier activity in the Ben Ohau Range, Southern Alps/New Zealand

Abstract

Schmidt-hammer exposure-age dating (SHD) was applied to selected rock glaciers in the south central Ben Ohau Range, Southern Alps, New Zealand, to expand the available regional data set following a previous pilot study. Additional SHD-sampling was conducted on high-altitudinal moraines at two locations with published cosmogenic 10Be surface-exposure ages and utilised to establish a new regional SHD-age calibration equation. SHD-age estimates available for a total of eight rock glaciers and based on >16,000 sampled boulders were analysed in a chronological, morphodynamic, and palaeoclimatic context. SHD-age estimates for initiation of rock glacier formation indicated by the age of their respective frontal ridges spread between 12.1 ± 1.6 and 7.2 ± 0.8 ka with a cluster of four individual features between 9.3 and 8.6 ka. Activity at some rock glaciers commenced immediately after local deglaciation and all are considerably older than previously anticipated. Successive transverse ridges dated on the rock glaciers reveal a clear general trend of increasing age with increasing distance from the rooting zone. This reflects conveyor-like transport of boulders in predominantly stable position on the surface. The SHD-age estimates of the ridges themselves are interpreted as morphological evidence for phases of strong morphodynamic activity connected to cold periods and increased debris supply by climate-driven frost weathering. All investigated rock glaciers show a climax of such activity during the Early Holocene. After c. 7.0 ka such phases became less frequent and subsequently rare during the Late-Holocene. These newly obtained chronological constraints for rock glacier activity are discussed in the context of related palaeoclimatic conditions and their potential to complement the hitherto limited regional palaeoclimatic records for the Early Holocene based on glacial activity.