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

Within the framework of core-drilling through the permafrost of the active rock glacier Murtel-Corvatsch in the Swiss Alps, subfossil stem remains of seven different bryophyte species were found at a depth of 6 m below surfac e 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 recover ed mosses in the nearest surroundings of the drill site. A total of 127 pol len and spores captured by the mosses and representing 23 taxa were determi ned. The local vegetation during deposition time must be characterized as a moss-rich alpine grassland meadow rich in Cyperaceae, Poaceae, Chenopodiac eae and Asteraceae, comparable to today's flora present around the study si te. For C-14 analysis, accelerator mass spectrometry had to be used due to the small sample mass (about 0.5 mg Carbon content). The mean conventional C-14 age of 2250 +/- 100 years (1 sigma variability) corresponds to ranges in the calibrated calendar age of 470-170 BC and 800 BC to AD 0 at statisti cal probabilities of 68% and 95%, respectively. This result is compared wit h the present-day flow field as determined by high-precision photogrammetry and with information about the thickness, vertical structure and flow of t he permafrost from borehole measurements. Total age of the rock glacier as a landform is on the order of 10(4) years; the development of the rock glac ier most probably started around the onset of the Holocene, when the area i t now occupies became definitely deglaciated. The bulk of the ice/rock mixt ure within the creeping permafrost must be several thousand years old. Char acteristic 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 an d 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.