Cosmogenic¹⁰Be and³⁶Cl geochronology of cryoplanation terraces in the Alaskan Yukon-Tanana Upland

Abstract

Cryoplanation terraces are prominent but enigmatic landforms found in present and past periglacial environments. Geomorphologists have debated for more than a century over processes involved in the formation of these elevated, step-like, bedrock features. Presented here are the first numerical surface exposure ages and scarp retreat rates from cryoplanation terraces in the Yukon-Tanana Upland (YTU) in Alaska, part of unglaciated eastern Beringia, obtained from terrestrial cosmogenic nuclides (TCN) in surface boulders. Ages comprise six 10Be TCN ages from two terrace treads near Eagle Summit and six 36 Cl ages from two treads on Mt. Fairplay. Based on these exposure ages, scarps at both locations were last actively eroding from 49 to 22.4 ka. Both locations exhibit time-transgressive development, particularly near scarp-tread junctions. Boulder exposure ages and distances between sampled boulder locations were used to estimate scarp retreat rates of 0.11 to 0.56 cm/yr. These numerical exposure ages presented here demonstrate that the cryoplanation terraces in the YTU are diachronous surfaces actively eroding during multiple cold intervals. With these results, hypotheses for cryoplanation terrace formation are discussed and evaluated for the YTU, including those based on geologic structure, nivation, and the influence of permafrost.

PERICLIMv1.0: a model deriving palaeo-air temperatures from thaw depth in past permafrost regions

“…The palaeo-MAAT modelled for two sites in the Czech Republic (Sect. 4.3) was between −7.0 ± 1.9 and −3.2 ± 1.5 • C and its corresponding reduction between −16.0 and −11.3 • C in comparison with the 1981-2010 period, which is relatively consistent with earlier reconstructions utilizing various relict periglacial features in the central European lowlands that suggested MAAT depressions mostly between −17 and −12 • C (Poser, 1948;Büdel, 1953;Kaiser, 1960;Frenzel, 1967;Goździk, 1973;Huijzer and Vandenberghe, 1998;Marks et al, 2016). By contrast, it disagrees with slightly milder MAAT reductions of at least −7 and −13 to −6 • C derived from groundwater data (Corcho Alvarado et al, 2011) and borehole temperature logs (Šafanda and Rajver, 2001), but these may not necessarily correspond to the lowest temperatures because groundwater cycling has been slowed or interrupted by permafrost, while ground temperature history may have been partly masked by latent heat effects.…”

What and where are periglacial landscapes?

“…Ideally, thresholds of geomorphic activity need to be defined (as with ice‐wedge cracking 165 ), although this is not possible at present, given current knowledge. Some information, nonetheless, is becoming available on rates of cryoplanation development 166,167 …”

“…Some information, nonetheless, is becoming available on rates of cryoplanation development. 166,167 Sensitivity also varies spatially within periglacial landscapes. On the Dartmoor granite landscape, interfluves and plateaux are thought be to largely unaffected by hillslope changes initiated along river valleys.…”

Characteristic periglacial topography: Multi‐scale hypsometric analysis of cryoplanated uplands in eastern Beringia

“…Because CTs are thought to be dependent on close (100-200 m) proximity to the position of the climatic snowline but develop primarily where glaciation has been marginal or absent, 10,38 they have considerable potential utility as tools in paleo-geomorphic work. 1,15,[39][40][41] Stepped slope profiles and flattened summits are common features in cold, unglaciated regions (Figure 3). Although they comprise a range of size, plan-view shape, and topographic position, individual terrace units appear similar in profile, consisting of a nearly planar to gently convex-upward tread and a steep riser (scarp) immediately upslope (Figure 1).…”