Pollen analysis and 14C age of moss remains in a permafrost core recovered from the active rock glacier Murtèl-Corvatsch, Swiss Alps: geomorphological and glaciological implications

Abstract

AbstractWithin the framework of core-drilling through the permafrost of the active rock glacier Murtèl–Corvatsch in the Swiss Alps, subfossil stem remains of seven different bryophyte species were found at a depth of 6 m below surface and about 3 m below the permafrost table in samples from massive ice. The composition of the moss species points to the former growth of the recovered mosses in the nearest surroundings of the drill site. A total of 127 pollen and spores captured by the mosses and representing 23 taxa were determined. The local vegetation during deposition time must be characterized as a moss-rich alpine grassland meadow rich in Cyperaceae, Poaceae, Chenopodiaceae and Asteraceae, comparable to today’s flora present around the study site. For l4C analysis, accelerator mass spectrometry had to be used due to the small sample mass (about 0.5 mg Carbon content). The mean conventional 14C age of 2250 ± 100 years (1σ variability) corresponds to ranges in the calibrated calendar age of 470–170 BC and 800 BC to AD 0 at statistical probabilities of 68% and 95%, respectively. This result is compared with the present-day flow field as determined by high-precision photogrammetry and with information about the thickness, vertical structure and flow of the permafrost from borehole measurements. Total age of the rock glacier as a landform is on the order of 104 years; the development of the rock glacier most probably started around the onset of the Holocene, when the area it now occupies became definitely deglaciated. The bulk of the ice/rock mixture within the creeping permafrost must be several thousand years old. Characteristic average values are estimated for (1) surface velocities through time (cm a-1), (2) long-term ice and sediment accretion rates (mm a-1) on the debris cone from which the rock glacier develops, (3) retreat rates (1–2 mm a-1) of the cliff which supplies the debris to the debris cone and rock glacier, and (4) ice content of the creeping ice/rock mixture (50–90% by volume). The pronounced supersaturation of the permafrost explains the steady-state creep mode of the rock glacier.