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

Abstract

Rock glaciers as typical periglacial landforms in alpine environments constitute valuable palaeoclimatic indicators due to their connection to the climate-driven permafrost conditions responsible for both their initial formation and continuing activity. In particular for the Late Glacial/Early Holocene transition in cases local morphological evidence for glacial activity is sparse, targeting rock glaciers may successfully complement investigations on contemporaneous landform evolution or climatic variability in mountain regions. The Ben Ohau Range southeast of the Main Divide of the Southern Alps in New Zealand is an example where such conditions prevail. Only few studies focusing on chronological aspects, mostly older ones applying weathering-ring thickness as main dating technique, have been conducted on the rock glaciers occurring in considerable numbers in this selected mountain range to date. Consequently, the chronological data available remain limited.   Following successful application during a previous pilot study, Schmidt-hammer exposure-age dating (SHD) regarded as suitable calibrated-age dating technique has recently been extensively applied in the Ben Ohau Range. Overarching aim of the related research project is to spatially expand and simultaneously improve the chronological data set for both initial formation and periods of morphodynamic activity of rock glaciers. The obtained chronological data are, furthermore, intended to eventually support the analysis of regional Holocene glacier activity in the Southern Alps. Additional to SHD-sampling on various rock glaciers, published numerical TCND 10Be-ages from glacial landforms at two independent sites in the Ben Ohau Range have been utilised to establish a new regional SHD age-calibration equation. Including the abovementioned pilot study SHD age-estimates are now available for eight individual rock glaciers placed in three separate cirques/valley heads located in the middle and southern part of the range. These improved chronological constraints are based on no less than 16,500 sampled boulders on individual transversal rock glacier ridges and SHD age-calibration equation's control points.  With SHD age-estimates for their initial formation of 11.4 ± 0.4 ka (Duncan Stream), between 9.4 ± 0.9 and 8.6 ± 0.8 ka (Double Basin), and between 11.8 ± 1.6 and 7.3 ± 0.8 ka (Irishman Stream) the studied rock glaciers appear to be significantly older than anticipated and previously reported. Some rock glaciers must have commenced their morphodynamic activity directly around the onset of the Early Holocene, what concurrently indicates that deglaciation of these cirques must have been completed at this point. Because the age difference between the innermost Late Glacial/Early Holocene moraines and the outermost rock glacier ridges sometimes averages only several hundred years, transition from glacial to periglacial processes must have been relatively rapid. With the palaeoclimatic interpretation of this development the significant precipitation gradient east of the Main Divide causing comparatively dry conditions in the middle and southern Ben Ohau Range needs to be taken into account. Some of the studied rock glaciers are currently active, whereas others need to be classified as inactive. All seem, however, have experienced longer periods of activity during the Holocene.