Cosmogenic nuclides and the dating of Lateglacial and Early Holocene glacier variations: The Alpine perspective

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Abstract

Based on cosmogenic 10Be data from four sites in the Alps we discuss geological uncertainties associated with the dating of former cirque and small valley glacier margins. At the Early Lateglacial Gschnitz site (Trins, Austria), a 3000 yr spread in 10Be exposure ages points to prolonged boulder instability. Three out of seven ages are not included in the mean age calculation, which yielded 15,400±1000 yr (indistinguishable from the oldest boulder age of 16,130±1040 yr). As a result of the distinctive morphology at Julier Pass (Switzerland) site we are able to exposure date the early (12,300±1300 yr) and the late (11,300±600 yr) Egesen stadial glacier advances (Younger Dryas equivalent), not just final retreat. At the Kromer site (Austria), 10Be exposure ages from five clast-supported boulders are indistinguishable within the analytical uncertainties (mean age: 8400±500 yr). In addition to moraine age, key factors that may lead to “too young” ages include degree of matrix- vs. clast-support of the boulders, post-depositional periglacial activity and tree coverage. At the Nägelisgrätli bedrock site near Grimsel Pass (Switzerland) exposure ages of 10,760–11,720 yr are consistent with Early Holocene cirque glacier retreat, and underline the marked lack of nuclide inheritance in bedrock exposures in the Alps.

Introduction

Glaciers are very sensitive indicators of climate change responding rapidly and markedly to changes in both temperature and precipitation (Kerschner, 2005; Oerlemans, 2005). A striking example is the nearly synchronous behavior of mountain glaciers worldwide during the Little Ice Age (LIA) (Grove, 2001). For example, at that time in the Alps, the Great Aletsch, Gorner and Lower Grindelwald Glaciers advanced nearly synchronously (Holzhauser et al., 2005). The timing of variations in glacier size that took place during the Mid- to Late Holocene can for the most part be constrained with radiocarbon dating (Hormes et al., 2001; Joerin et al., 2006). But at and before the Pleistocene/Holocene transition organic material at moraine locations is sparse. In this time range, the direct dating of moraines with cosmogenic nuclides has become an invaluable tool for reconstructing the timing of past changes in glacier volume.

Detailed mapping of moraines in the Alps began more than a hundred years ago. Based on morphostratigraphic relationships relative age sequences were established. Systems of moraines in the Alpine valleys record repeated glacier advances (“stadials”) during the Lateglacial (e.g. Penck and Brückner, 1901/1909; Heuberger, 1966; Gross et al., 1977; Maisch, 1981, Maisch, 1982, Maisch, 1987). In these studies the “Alpine Lateglacial” refers to the time period between downwasting of the Last Glacial Maximum (LGM) piedmont lobes and the beginning of the Holocene. The Lateglacial stadial sequence is based on several parameters: (i) the relative morphostratigraphic position of the moraines, (ii) the morphology of the moraines and related periglacial features and (iii) the depression of the equilibrium line altitude (ΔELA) of the glacier with respect to the LIA ELA. In this way a system of families of moraines was constructed based on the concept that glacier positions with similar ELA depressions and similar morphological characteristics located in comparable climate regions occurred at the same time (Gross et al., 1977; Maisch, 1981, Maisch, 1987). This detailed framework affords a unique opportunity for the application of surface exposure dating with cosmogenic nuclides in a well-constrained field situation. On the other hand, although the approximate ages of the stadials could be estimated fairly well (all except Egesen, Kartell and Kromer were thought to be pre-Bølling in age) (Table 1), direct dating of them was not possible until the advent of surface exposure dating.

The purpose of this paper is to use data from four sites in the Alps to illustrate issues related to the use of cosmogenic nuclides to elucidate the timing of mid-latitude cirque and valley glacier variations. Here, we summarize boulder selection strategies and point out the implications of our results with respect to future applications of surface exposure dating in Alpine settings. We focus on the Lateglacial and the Early Holocene in the Alps. During this transitional time period first large valley glaciers then later cirque glaciers dominated. Moraines of the large piedmont lobes that prevailed during the LGM present different problems for exposure dating, such as possible delayed stabilization due to ice-cored moraines (Reuther et al., 2005). Similarly, the problems associated with exposure dating of moraines that are hundreds of thousands of years old and lack large boulders are not addressed here (e.g. Kaplan et al., 2005). Brief descriptions are given for the four sites in the Alps (locations shown in Fig. 1). These include: (1) the Gschnitz stadial moraine at Trins (Austria); (2) Egesen stadial moraines at Julier Pass (Switzerland); (3) the Kromer stadial moraine at Kromertal (Austria) and (4) the Nägelisgrätli bedrock site at Grimsel Pass (Switzerland).

No new data are presented here. Measured atoms per gram, details of age calculations and implications for Alpine Lateglacial stratigraphy are given in Ivy-Ochs et al. (2006b). We focus on the 10Be data, which are supported consistently by the 26Al and 36Cl data. We have used a 10Be production rate of 5.1±0.3 atoms 10Be (g SiO2)−1 year−1 and scaling to the site location based on Stone (2000). As we are comparing exposure ages to each other we quote analytical uncertainties only. Landform ages are averages and not error-weighted means. The errors on the mean age are the 1σ confidence interval about the mean based on cumulative probability density distribution. In Table 1 we give errors that also include systematic errors, as these are required to compare exposure ages to other chronological frameworks (radiocarbon, luminescence). We stress that some of the generalizations may be specific to moraines younger than 20 kyr and to the climate and lithologies of the Alps.

Section snippets

Gschnitz stadial: Trins

The Gschnitz stadial is defined by moraines found in many valleys that record the first clear and widespread readvance of Alpine valley glaciers after decay of the LGM piedmont lobes. At the type locality at Trins (1200 m a.s.l.) an end moraine and lateral moraines that extend more than 3 km upvalley are found. The glacier that left these moraines was about 18 km long with a surface area of 51 km2. The end moraine (Fig. 2) is single-walled and about 30 m high with a steep distal slope. Numerous

Geological factors affecting exposure ages

Cosmogenic nuclides build up predictably with time within mineral lattices. Therefore, measuring their concentrations allows calculation of how long a rock surface has been exposed to cosmic rays (Lal, 1991; Gosse and Phillips, 2001). In order to use the exposure age of a boulder to date a landform, the rock surface must have undergone single-stage (no pre-exposure), continuous (not covered) exposure in the same position (not shifted) since deposition. Before we look in detail at the Alps data

Summary

Based on 10Be data from four sites in the Alps we can make a few generalizations about exposure dating of glacial landforms in mid-latitude Alpine settings. In our data set the prevalence of “too young” ages far overshadows “too old” ages. Indeed up to now we have seen no inheritance in the dated rock surfaces in the Alps. The presence of several “too young” ages even from boulders meters in diameter at the Gschnitz Trins site indicates prolonged boulder instability long after the landform was

Acknowledgment

We thank D. Fabel and M. Kaplan for critical review and numerous insightful suggestions. We also thank F. Preusser for careful editing and endless patience.

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