Surface exposure dating with cosmogenic nuclides

Im letzten Jahrzehnt hat sich die Methode der Oberflächendatierung mittels kosmogener Nuklide zu einer leistungsfähigen Methode in der Quartärchronologie und quantitativen Landschaftsanalyse entwickelt. Kosmogene Nuklide werden durch kosmische Strahlung in Fest- und Lockergestein gebildet. Die Konzentrationen der kosmogenen Nuklide kann mittels Massenspektrometrie ermittelt werden. Dies ermöglicht - je nach Verwitterungssrate - die Datierung von Landschaftselementen und Landschaftsformen mit Altern zwischen einigen 100 Jahren bis über 10 Millionen Jahren. Neben einem Abriss der historischen Entwicklung und Theorie der Oberflächendatierung mittels kosmogener Nuklide enthält dieser Artikel eine ausführliche Übersicht der zahlreichen Anwendungsgebiete dieser Methode. Probenahmestrategien und die Eigenheiten der einzelnen Nuklide werden im Detail besprochen. Die Vielzahl der mit dieser Methode in den verschiedensten Mineralien bestimmbaren Nuklide (Radionuklide 10Be, 14C, 26Al und 36Cl und Edelgase 3He und 21Ne) erlaubt die Beprobung und Analyse verschiedenster Lithologien. Der erreichte hohe Entwicklungsstand der Methode erlaubt es den Fokus auf die eigentlichen geomorphologischen Fragestellungen zu legen. Die Sensitivität der kosmogenen Oberflächendatierungsmethode muss trotzdem sorgfältig im Rahmen ausführlicher Feldstudien erfolgen, wie zum Beispiel durch die Analyse von lokalen und regionalen Terrassen- oder Moränenstratigraphien oder durch den Vergleich mit anderen Datierungsmethoden.researc

Post-depositional impacts on ‘Findlinge' (erratic boulders) and their implications for surface-exposure dating

Understanding and interpretation of ‘numbers' produced about the depositional age of an erratic boulder by cosmogenic nuclide surface-exposure dating is important in the construction of glacial chronology. We have sampled three ‘Findlinge' (glacially transported boulders) located on the right-lateral margin of the Aare glacier at Möschberg, Grosshöchstetten, southeast of Bern, with the aim of shedding light on this topic. The boulders have the same depositional, but different post-depositional histories: simple exposure; exhumation; and human impact. This sampling is specially selected for this study, since the boulders showing exhumation and human impact would not have been sampled in a regular surface-exposure dating application. We measured cosmogenic 10Be concentrations and calculated apparent exposure ages that are 13.6±0.5, 18.1±0.8, and 7.5±0.4ka, respectively. The exposure age of the first boulder reflects exhumation. The apparent exposure age of 18.1±0.8ka (erosion-corrected exposure age 19.0±0.9ka) from the second boulder correlates well with the end of the Alpine and global last glacial maximum. The third boulder shows evidence of quarrying as it is surrounded by a rim of excavation material, which is also reflected by the 7.5±0.4ka apparent exposure age. We modeled the variation of 10Be concentrations with depth down into the sediment in which the first (exhumed) boulder was once buried in, and down into the third (quarried) boulder. According to our modeling, we determined that the exhumed ‘Findling' was buried in sediment at a depth of around 0.5m, and around 2m of rock was quarried from the third ‘Findling'. Our results reveal the importance of sampling for surface-exposure dating within a well defined field context, as post-depositional impacts can easily hinder exposure-dating of surface

Timing and patterns of debris flow deposition on Shepherd and Symmes Creek fans, Owens Valley, California, deduced from cosmogenic 10Be

Debris-flow fans on the western side of Owens Valley, California, show differences in their depths of fan head incision, and thus preserve significantly different surface records of sedimentation over glacial-interglacial cycles. We mapped fan lobes on two fans (Symmes and Shepherd Creek) based on the geometry of the deposits using field observations and high-resolution Airborne Laser Swath Mapping (ALSM) data, and established an absolute fan lobe chronology by using cosmogenic radionuclide exposure dating of large debris-flow boulders. While both fans and their associated catchments were subject to similar tectonic and base level conditions, the Shepherd Creek catchment was significantly glaciated while that of Symmes Creek experienced only minor glaciation. Differences in the depth of fan head incision have led to cosmogenic surface age chronologies that differ in the length of the preserved depositional records. Symmes Creek fan preserves evidence of exclusively Holocene deposition with cosmogenic 10Be ages ranging from 8 to 3 ka. In contrast, the Shepherd Creek fan surface was formed by late Pleistocene and Holocene debris-flow activity, with major deposition between 86-74, 33-15, and 11-3 ka. These age constraints on the depositional timing in Owens Valley show that debris-flow deposition in Owens Valley occurred during both glacial and interglacial periods, but may have been enhanced during marine isotope stages 4 and 2. The striking differences in the surface record of debris-flow deposition on adjacent fans have implications for the use of fan surfaces as paleoenvironmental recorders, and for the preservation of debris-flow deposits in the stratigraphic record

Reconsidering the current stratigraphy of the Alpine Lateglacial

The sedimentary and morphological evidence for Lateglacial glacier fluctuations in the Lienz area provides a strong case against the currently used pentapartite stratigraphic subdivision of the Alpine Lateglacial (ALG; c. 19–11.7 ka) i.e. the timespan between the Würmian Pleniglacial (= Alpine Last Glacial Maximum; AlpLGM) and the beginning of the Holocene. The results of comprehensive geological mapping (including the detection of mass movements) supported by geochronological data and pollen analysis revealed that the ALG- record of the Schobergruppe mountains and the Lienz Dolomites can be subdivided into four unconformity-bounded (allostratgraphic) units which are linked to three climatostratigraphically-defined phases of glacier activity. Delta deposits and till of local glaciers document the phase of ice-decay after the AlpLGM. Between this period and the Bølling/Allerød (B/A) interstadial only one glacier stabilisation with massive end moraines, correlated with the Gschnitz stadial, is evident. Multiple end moraines prove the presence of very active glacier tongues during the Younger Dryas aged Egesen stadial. The 10Be exposure dating of an end moraine, previously attributed to the Daun stadial (pre-B/A interstadial) based on ΔELA values, provided an age of 12.8 ± 0.6 ka indicating it is of Younger Dryas age. This case highlights the pitfalls of the commonly used ΔELA-based stratigraphic ALG subdivision and the subsequent derivation of palaeoclimatic implications. ΔELAs are still considered as a useful tool for correlation on the local scale e.g. in one mountain group with a quite comparable topography and lithology and taking into account the limitations, especially the impact of debris cover. However, our results show that a stratigraphic correlation across the whole Alpine chain via ΔELAs is not a successful approach potentially leading to bias and, eventually, to circular arguments.researc

Glare, a GIS tool to reconstruct the 3D surface of palaeoglaciers

Acknowledgements This research has been supported by the Leverhulme Trust International Network Grant IN-2012-140. Processing and collecting of ground penetrating data in Forgefonna was part of Elend Førre's master's project that was completed in 2009 at the Department of Geography, University of Bergen. We also acknowledge Dr Andreas Bauder for providing the subglacial topography data for Griessgletscher and Simone Tarquini for granting access to the high resolution TIN of Italy, a cut of which is provided to the reader to practice the tools (see Appendix). Referees Dr. Iestyn Barr, Dr. Jeremy Ely and Dr. Marc Oliva are thanked for their constructive comments and tool testing, which significantly improved the final output.Peer reviewedPostprin