Cosmogenic nuclides and the dating of Lateglacial and Early Holocene glacier variations: The Alpine perspective
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.
References (55)
- et al.
Mid-Pleistocene cosmogenic minimum-age limits for pre-Wisconsian glacial surfaces in southwestern Minnesota and southern Baffin Island: a multiple nuclide approach
Geomorphology
(1999) - et al.
Chemical weathering and landscape development in mid-latitude alpine environments
Geomorphology
(2005) - et al.
Terrestrial in situ cosmogenic nuclides: theory and application
Quaternary Science Reviews
(2001) Quaternary moraines vs. catastrophic rock avalanches in the Karakoram Himalaya, northern Pakistan
Quaternary Research
(1999)- et al.
Late Pleistocene glaciation in the northwestern Sierra Nevada, California
Quaternary Research
(2002) - et al.
Cosmogenic nuclide chronology of pre-last glacial maximum moraines at Lago Buenos Aires, 46 degrees S, Argentina
Quaternary Research
(2005) - et al.
10Be and 26Al production rates deduced from an instantaneous event within the dendro-calibration curve, the landslide of Köfels, Ötz Valley Austria
Earth and Planetary Science Letters
(1998) Cosmic ray labeling of erosion surfaces: in situ nuclide production rates and erosion models
Earth and Planetary Science Letters
(1991)- et al.
Production rates of cosmogenic nuclides in boulders
Earth and Planetary Science Letters
(2003) - et al.
Influence of a glacial buzzsaw on the height and morphology of the Cascade Range in central Washington State, USA
Quaternary Research
(2006)
Degradation of unconsolidated Quaternary landforms in the western North America
Geomorphology
Accuracy of cosmogenic ages for moraines
Quaternary Research
Insights into alpine moraine development from cosmogenic 36Cl buildup dating
Geomorphology
Cosmogenic exposure dating of late Pleistocene moraine stabilization in Alaska
Geological Society of America Bulletin
Weathering features
Geomorphology and in-situ cosmogenic isotopes
Annual Review of Earth and Planetary Sciences
Cosmogenic analysis of glacial terrains in the eastern Canadian Arctic: a test for inherited nuclides and the effectiveness of glacial erosion
Annals of Glaciology
Spatial patterns of glacial erosion at a valley scale derived from terrestrial cosmogenic 10Be and 26Al concentrations in rock
Annals of the American Association of Geographers
Reconstructing the last glacial maximum (LGM) ice surface geometry and flowlines in the Central Swiss Alps
Ecologae Geologicae Helvetiae
Using relict rockglaciers in GIS-based modelling to reconstruct Younger Dryas permafrost distribution patterns in the Err-Julier area, Swiss Alps
Norsk Geografisk Tidsskrift
The contributions of cosmogenic nuclides to unraveling alpine paleo-glacier histories
Be-10 dating of the duration and retreat of the Last Pinedale glacial sequence
Science
Methodische Untersuchungen über die Schneegrenze in alpinen Gletschergebieten
Zeitschrift für Gletscherkunde und Glazialgeologie
The initiation of the Little Ice Age in regions around the North Atlantic
Climatic Change
Surface dating of dynamic landforms: young boulders on aging moraines
Science
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