Misleading old age on a young landform? The dilemma of cosmogenic inheritance in surface exposure dating: Moraines vs. rock glaciers

Abstract

Abstract Mount Geyikdag in the Taurus Mountain Range in southern Turkey was glaciated during the Late Pleistocene. Several glacial and periglacial landforms such as moraines and rock glaciers are present in Mount Geyikdag. Here we described two geomorphic units, a well-preserved moraine loop (so called Zor Moraine) and a rock glacier inside that loop, which have contrasting morpho-stratigraphic positions and terrestrial cosmogenic nuclide (TCN) 36Cl inventories. We measured 4 samples from each landform. The boulders of Zor Moraine have ages that range from 3.7 ± 0.3 ka to 7.8 ± 0.6 ka with a weighted average age of 5.5 ± 1.2 ka. This age agrees well with the Mid-Holocene glaciation observed within the same range. On the contrary, the boulders of the rock glacier have much older ages. They are in between 9.7 ± 0.9 ka and 14.2 ± 1.1 ka with a weighted average age of 12.5 ± 1.3 ka. We attribute this unlikely situation to excessive inheritance within the rock glacier boulders. On the contrary, Zor Moraine boulders contain no to negligible amount of inheritance, as glacial carving is a very efficient mechanism in removing pre-exposure component. Our study indicates that caution should be taken into account when interpreting the apparent age of a landform as boulders from different landforms might have considerably different prior exposure histories even within a closely related glacial/periglacial environment. It is possible that rock glacier boulder TCN results can yield seemingly maximum ages related to pre-exposure problems. Therefore an independent verification of rock glaciers’ age by other dating methods is advised.