A Combination of Relative-Numerical Dating Methods Indicates Two High Alpine Rock Glacier Activity Phases After the Glacier Advance of the Younger Dryas
Summary (3 min read)
INTRODUCTION
- Rock glaciers as distinct tongue-shaped landforms are perennially frozen and ice-rich debris on non-glacierised mountain slopes creeps steadily under the influence of gravity [1].
- With time, the surface of rock glaciers is increasingly subject to weathering processes.
- Relative and numeric age dating by measuring the weathering rind thickness or the Schmidt-hammer rebound value was successfully performed on moraines and rock glaciers developed on sandstones in New Zealand [4-8], basaltic and andesitic boulders in North America and Japan [9, 10] and on granites and gneiss in the Alps [2, 11].
- A ‘cross-check’ of these methods allows an extended interpretation and reciprocal control of the results.
STUDY AREA
- The study area is located in the eastern Swiss Alps (Fig. 1).
- The area of the Albula pass is characterised by many glacial and geomorphic features such as moraines, polished bedrock with roches moutonnées, scree slopes, rock glaciers, traces of glacial erosion (such as grooves, striations, etc.) and plucking processes.
- The mean annual precipitation is rather low with around 900 mm in the lower parts near Preda and up to 1100 mm towards the mountain ridges [21].
- While the active rock glaciers show a simple structure with a clearly identifiable rooting zone and a compact steep tongue, the conditions in case of the rock glacier Alvra are more complex.
- In close vicinity, previously-reported data of a peat bog was available [25].
Schmidt-Hammer Rebound Values
- The measured Schmidt-hammer rebound values for all rock glaciers are given in Fig. (8A-D).
- The standard error after [30] was in most cases relatively small.
- Close to the cirque, the values were usually near 50-55.
- This suggests that the activity of rock glacier Alvra began soon after the retreat of the Egesen-age glaciers.
- The measured Schmidt-hammer values at the distal front of these rock glaciers correspond approximately to the initial values of rock glacier Alvra (near 40-45).
Soil Chemistry and Physics
- The soil samples were air-dried, large aggregates were gently broken by hand and sieved to <2 mm.
- Total C and N contents of the soil were measured with a C/H/N analyser (Elementar Vario EL, Elementar Analysensysteme GmbH) using oven-dried and ball-milled fine earth.
- Oven-dried samples were dissolved using a mixture of HF, HCl, HNO3, and H3BO3 in a closed system [38] (microwave oven and under high pressure, 25 bar).
- Concentrations were determined by AAS (Atomic Absorption Spectrometry – AAnalyst 700, Perkin Elmer, USA).
- The dithionite- and oxalate-extractable fractions were measured for the elements Fe, Al and Si [39].
Fractionation of Organic Matter
- Acting on the assumption that chemical oxidation mimics natural oxidative processes, the authors treated the soils with 10% H2O2 to eliminate the more labile organic material from the more refractory organic matter [41-44].
- Profile photos and horizon designations of the investigated soils (see also Fig. 1).
- First organic matter formed in the sediment after glacier retreat [45] and may, under certain circumstances, provide a minimum age of deposition of the moraines and of deglaciation.
- The beakers were closed with two layers of parafilm to avoid evaporation of the reagent.
- At the end of the treatment the samples were washed three times with 40 ml deionised water, freeze-dried, weighed, analysed for total C and N and 14 C dated.
Cosmogenic Dating
- The cosmogenic nuclide concentrations (e.g. 10 Be) in the surface of moraine boulders and polished bedrock exposure reflect the time that has passed since the moraine stabilised [48] and the ice retreated and exposed the landform to cosmic ray flux.
- The rock samples were crushed, sieved and leached in order to obtain pure quartz following [50, 51].
- The Be was isolated using anion and cation exchange columns followed by selective pH Be(OH)2 precipitation [51].
- Production-rate scaling for latitude and altitude was based on [52] and corrected for sample thickness assuming an exponential depth profile, a rock density of 2.65 g cm -3 and an effective radiation attenuation length of 155 g/cm 2 [48].
- The theoretical snow height for the sample site was estimated using a mean snow height gradient of 0.08 m/100 m altitude difference [54].
Weathering rind Thicknesses
- The weathering rinds were generally very thin (Fig. 7).
- Nonetheless, the median and mean values showed a clear increase from the youngest part near the rockwall to the frontal talus of the rock glacier (Fig. 7A-E).
- Thicknesses of weathering rinds increased towards the distal front of the rock glaciers.
- The literature indicates that the modal values should be used to delineate trends along a rock glacier.
- The time dependent tendency looked, however, more disturbed.
Soils and Weathering Losses
- The physical and chemical characteristics of the soils are given in Tables 1 and 2.
- The parameter Alo + 0.5Feo (Table 2) clearly reflects the spodic characteristics of the soil at Igls Plans (see [55]).
- Long-term weathering rates of soils were derived using immobile element contents [56-58].
- This gives another indication of the minimum age of the present-day active rock glaciers and an indication of when the rock glacier Alvra became inactive.
DISCUSSION
- Frauenfelder et al. [2] and Laustela et al. [11] measured Schmidt-hammer rebound values that varied from approx.
- The median as well as the modal values of weathering rind measurements [11] are equally suitable to delineate the increasing weathering rind thicknesses with the duration of exposure.
- The soil “Igls Plans” is more weathered than soil “Peidra Mora” and shows relative element losses that are close to soils that developed over post-Egesen time [60, 61, 64, 65].
- This finding was only made possible using a multi-parameter approach that consequently clearly shows its advantages [66, 67].
- Two main phases of rock glacier activity could be derived: one starting immediately after the retreat of glaciers during the Younger Dryas (Egesen glacial states) and the other one during the Holocene.
CONCLUSIONS
- The authors obtained the following main findings: A distinct increase in weathering rind thickness and decrease of the Schmidt hammer rebound values along the rock glaciers (i.e. with increasing age) could be measured.
- Both methods reflect equally well the effect of weathering with increasing age.
- Soil analyses confirmed and added an additional time constraint.
- The older soil, in front of the relict rock glacier, was highly weathered and must have started its evolution in the Lateglacial (shortly after the Younger Dryas).
- This leads to additional datasets, provides the ability to cross-check each of the methods and to revise previous interpretations.
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References
258 citations
"A Combination of Relative-Numerical..." refers background in this paper
...Trimline and other erosional features indicate that during LGM the Albula pass formed a transfluence with ice flowing from the Engadine into the Rhine river system [24]....
[...]
183 citations
"A Combination of Relative-Numerical..." refers methods in this paper
...Acting on the assumption that chemical oxidation mimics natural oxidative processes, we treated the soils with 10% H2O2 to eliminate the more labile organic material from the more refractory organic matter [41-44]....
[...]
180 citations
"A Combination of Relative-Numerical..." refers methods in this paper
...Since the 1980s the method has also been successfully used for relative age dating of gemorphologic features such as moraines [14, 29, 30], rock glaciers ([2, 31] or rockfall deposits [32]....
[...]
169 citations
"A Combination of Relative-Numerical..." refers background in this paper
...soils that developed over post-Egesen time [60, 61, 64, 65]....
[...]
...The soil “Peidra Mora” shows typical weathering rates of a soil that must have an age of < 10 ky [60, 61]....
[...]
...The weathering status of the soil “Igls Plans” is typical for Alpine soils which have developed over the entire postEgesan period [60, 61]....
[...]