GM2.12

Denudation rates are routinely derived from concentrations of terrestrial in situ produced cosmogenic nuclides (TCN), particularly from ¹⁰Be concentrations in river sand. Denudation rates are calculated assuming that they remain steady throughout the integration time scale of the TCN. However, such an assumption is possibly unverified in settings with negligible tectonics, where rates typically range from 10 to 100 mm/ky. In those settings, the TCN conveys a signal that integrates denudation over a time span longer than a few thousand years. The signal may include periods when anthropogenic and climatic forcing on denudation was distinct from modern times. For instance, agricultural practices were limited before 6,000 years B.P. and climatic conditions were colder and drier before 10,000 years B.P. A variable forcing may produce variable and transient denudation rates. In that case, the assumption of steady denudation rates is invalid, and their derivation may introduce a bias.

To detect transient landscapes and resolve such a bias, we can take advantage of the different sensitivity of the ¹⁴C and ¹⁰Be TCNs to recent and short-term changes in surface denudation. In situ ¹⁴C is more sensitive than ¹⁰Be to such changes, because of a shorter half-life (5,700 y compared to 1.4 My). This potential application of coupled ¹⁴C - ¹⁰Be measurements has recently been discussed in several theoretical studies (Hippe, 2017; Mudd, 2017; Skov et al., 2019). Despite the improvement of ¹⁴C extraction lines and measurement facilities (Hippe et al., 2009; Lupker et al., 2019), sensitivity tests remain limited on natural cases (Hippe et al., 2012).

Here, we propose assessing this new application by in situ ¹⁴C - ¹⁰Be measurements on river sand from the Cevennes and the Monts Margeride within the Variscan Massif Central in France. With an average elevation of ~700 m, this mountain range presents an asymmetrical topography, composed of a low-relief surface reaching 1,700 m, and bordered by a gently sloping flank to the west and a steep escarpment to the southeast, along the Cevennes fault. This escarpment receives frequent and seasonal extreme precipitation events (300-700 mm in 48h) on its southeast flank.

The range is subject to very limited seismic activity and appears relevant for an application of the ¹⁴C-¹⁰Be couple. Basins are rich in quartz and have homogeneous lithology. The recent paleoclimatic context is well constrained, with substantial climatic variations but with limited Pleistocene glaciations (e.g. Fauquette et al., 1999; Magny et al., 2003; Mayewski et al., 2004). The Massif Central is subject to active erosion processes, without major contribution from stochastic events such as landslides. Denudation rates are in the range of the theoretical study of Skov et al. 2019 (Schaller et al. 2001; Molliex et al. 2016; Olivetti et al. 2016; Desormeaux et al., 2021) and several studies have suggested transient denudation patterns (Schaller et al. 2001; Olivetti et al. 2016). With our new measurements, we will verify whether the ¹⁴C-¹⁰Be couple has sufficient resolution to detect such transience in natural cases.

How to cite: Lenard, S. J. P., Lupker, M., Schimmelpfennig, I., Godard, V., Desormeaux, C., Haghipour, N., Wacker, L., Aumaître, G., Keddadouche, K., and Bourles, D. L.: Can 14C-10Be detect transient patterns of denudation ? Application to the French Massif Central., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5346, https://doi.org/10.5194/egusphere-egu21-5346, 2021.

Transient evolution and adjustment to changing tectonic and climatic boundary conditions is an essential attribute of landscapes. We present a new approach to detect and quantify transience in slow erosion landscapes over 100 ka timescales. We compare curvature and cosmogenic nuclides measurements (¹⁰Be and ²⁶Al) at hilltop sites with predictions of hillslope diffusion theory, in the slowly evolving quartzitic Serra do Cipó range in SE Brazil, and we observe a distinctive signature of an acceleration of denudation. The timing of this increase cannot be unequivocally associated with a single climatic event but is consistent with climatically-modulated important fluctuations in precipitation and erosion in this area during Middle and Late Pleistocene.

How to cite: Godard, V., Salgado, A., Siame, L., Fleury, J., and Team, A.: Detecting transience in slow evolution landscapes using cosmogenic nuclides and high resolution morphometry, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10051, https://doi.org/10.5194/egusphere-egu21-10051, 2021.

Deckenschotter (Cover Gravels in German) are Quaternary glacio-fluvial gravels, which unconformably overlie Tertiary Molasse or Mesozoic bedrock in the Northern Alpine Foreland. They comprise also the evidence of the Early Pleistocene glaciations. A significant phase of incision separated them into Höhere Deckenschotter (HDS: Higher Cover Gravels) and Tiefere Deckenschotter (TDS: Lower Cover Gravels) based on their topography. How the landscape evolved during Deckenschotter times is still not fully understood. The new cosmogenic nuclide chronology suggests that HDS deposited around 2 Ma and TDS around 1 Ma. In addition, 2 Ma old Deckenschotter are located at the same topographic elevation as the 1 Ma ones at Irchel (Canton of Zurich). This, indeed, points to cut-and-fill sequences and challenges the chronology based on the morphostratigraphy.

The aim of this study is to reconstruct the drainage patterns, base level changes, and thus the landscape evolution in the northern Alpine Foreland during the Early Pleistocene. Therefore, we focused on three Deckenschotter sites at Irchel and one in the area around Lake Constance. Sediments at these sites were analysed in detail to reveal their provenance, transport mechanism, depositional environment, and paleoflow regimes. Their chronology was established by isochron-burial dating. Our results indicate that the analysed sediments were transported from the Central and eastern Central Alps as well as from the Molasse to the foreland first by glaciers and then by rivers. They are deposited in a glacio-fluvial environment in the vicinity of a glacier. Based on the reconstructed chronology in this study and published cosmogenic nuclide ages, we propose that Deckenschotter are cut-and-fill sequences accumulated in three pulses between 2.5 Ma and 1 Ma. This cut-and-fill system implies that the regional base level was relatively constant during the Early Pleistocene. In addition, the depositional environment of Deckenschotter shows the presence of glaciers in the foreland. The 2.5 Ma old gravels, therefore, document the first advance of glaciers onto the Alpine Foreland. This seems to be synchronous with a first onset of glaciations on the northern hemisphere, which is assumed to occur at around 2.7 Ma.

How to cite: Dieleman, C., Christl, M., Vockenhuber, C., Gautschi, P., and Akçar, N.: Early Pleistocene complex cut-and-fill sequences in the Alps, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5643, https://doi.org/10.5194/egusphere-egu21-5643, 2021.

The oldest Quaternary deposits of the Swiss Northern Alpine Foreland are found on numerous hilltops, up to 300 m above the current valley bottoms. These Deckenschotter deposits consist mainly of glaciofluvial sediments intercalated with glacial sediments. Traditionally, the Deckenschotter are divided into two units: Höhere Deckenschotter (HDS – Higher Deckenschotter) and Tiefere Deckenschotter (TDS – Lower Deckenschotter). Elevation differences between the two suggest a phase of 100-150 m of incision (Graf, 2009).

Knowledge of their age of deposition is necessary for understanding the long-term landscape evolution as well as for assessing the long-term safety of the planned deep geological repository for nuclear waste in northern Switzerland (NTB 14-01, 2014). In this study, the method of isochron-burial dating was implemented to address the question of the age of the Deckenschotter. We aim to reconstruct the chronology of the alternating deposition and incision of the gravel units in the Northern Alpine Foreland. Our focus is placed on similar and complementary Deckenschotter sites located in the Northern Alpine Foreland in crucial locations in order to establish sound long-term landscape evolution scenarios. One of these is a former gravel pit, Feusi, situated in the southern slope of the hill chain called ‘Egg’ or ‘Schliniker Platten’, north of the village Oberweningen. The outcrop comprises several gravel units intercalated with glacigenic diamict layer in the upper part. Previous age estimates with the isochron-burial dating method indicate an age of 1.1 ± 0.2 Ma for the diamict layer (NAB 19-025, 2020). Knudsen et al. (2020) reported an age of 0.93 ± 0.13 Ma for the same layer based on a slightly different age calculation approach.

We sampled the lowermost accessible horizon, the Egg Schotter, of the Feusi outcrop at an altitude of ~580 m a.s.l. This horizon is located close to the base of the outcrop, just a few meters above the contact with the underlying Molasse and in a clear stratigraphic position, 20 m below the previously dated diamict. Study of the lowermost unit will allow us to temporally examine the earliest phases of Deckenschotter accumulation. Weathering horizons in the gravel layers overlying the Egg Schotter suggests periodic subaerial exposure. Therefore, the total time contained in the sediment package is difficult to estimate. Having two horizons dated at different depth in the same outcrop may provide insight into the timespan hidden between the deposition and weathering of different gravel layers. Indications of the timespan of HDS activity could be further gleaned by comparing to the age from the glacigenic sediment. In order to achieve this, eight clast samples of quartz-rich lithologies, of various shapes and sizes were collected in the Egg Schotter and processed for isochron-burial dating. The cosmogenic nuclides ¹⁰Be and ²⁶Al were extracted and measured with the new MILEA accelerator at the accelerator mass spectrometry facility, ETH Zurich. The first results of this study will be presented.

Graf, H.R. 2009: Quaternary Science Journal 58, 12–53

Nagra, NTB 14-01, 2014

Nagra, NAB 19-025, 2020

Knudsen, M.F. et al. 2020. Earth and Planetary Science Letters, 549, 116491

How to cite: Broś, E., Kober, F., Ivy-Ochs, S., Grischott, R., Christl, M., Vockenhuber, C., Maden, C., and Synal, H.-A.: Delving deeper into cosmogenic 26Al-10Be isochron-burial dating of Swiss Deckenschotter deposits, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12374, https://doi.org/10.5194/egusphere-egu21-12374, 2021.

Cosmogenic production rates (PRs) are the essential conversion factor between AMS cosmogenic concentrations and absolute exposure ages. The accuracy of cosmogenic glacial chronologies and reliability in their comparison to other paleoclimate systems  is largely contingent on the precision and accuracy of the adopted production rate. This is particularly critical in determining past glacial geochronologies at the scale of millennial temporal resolution. Most PR calibrations are carried out at deglaciation sites where radiocarbon provides the independent chronometric control usually based on 14C ages in basal sediments or varves from lake or bog cores which is assumed to represent the minimum age for glacial retreat. Under these conditions and hence provide PRs as maximum values. Given that today most AMS facilities can deliver 10-Be, 26-Al and 36-Cl data with total analytical errors less than 2% ( for 10 ka exposure), the precision of a PR remains largely dependent  on the error in the independent chronology and accuracy of AMS standards. The history over the past 20 years of the ever-decreasing value of  SLHL 10-Be cosmogenic spallation PRs   from initial estimates of about 7 atoms/g/a to the current  ‘accepted‘ (global average) values of ~4 atoms/g/a,   is an interesting story in itself and demonstrates the complexity in such determinations.  

Over the past few years new web-based calculators are now available to calculate uniformly new production rates from either new data or combinations of any set of published data (CRONUS-Earth, CRONUS-UW, CosmoCalc, ICE-D, CREp). This delivers a means by which new production rates can be seamlessly integrated and compared using identical constants, methods and statistics that were used to generate (currently accepted) global average or regional production rates.

 For the British Isles, there are a number of 10-Be reference sites that give PRs (Lm scheme) between 3.89±3%  atoms/g/a  (Putnam, QG, v50, 2019) to 4.20±1% atoms/g/a (Small, JQS, v30, 2015) which convert to 3.95 and 4.28, respectively, using datasets in the ICE-D calculator). This difference in 10-Be spallation PRs has recently raised some debate and challenges for the timing of the local-LGM and demise of the British Ice Sheet. This work provides a new  British Isles site specific 10Be PR from the  Arenig Mountains in North Wales where radiocarbon dating of basal sediments from a bog core associated with a series of nearby cirque moraines provides independent age control.  Similarly in the South Island of New Zealand, the current accepted 10Be PR is 3.76±2% (Putnam, QG 2009; converts to 3.94±1% using ICE-D) and is the only available PR that is used for these southern hemispheric glacial sites. This work provides a new Australasian site specific 10Be PR from Arthurs Pass retreat moraines where radiocarbon dating of basal sediments from three cores extracted from a bog impounded by the moraine provides independent age control. 

How to cite: Fink, D., Hughes, P., Fulop, R., Wilcken, K., Adams, P., Woodward, C., Shulmeister, J., Fujioka, T., and Ryan, P.: A tale of two bogs - new 10Be production rates from UK and NZ calibrated by basal 14C ages, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14132, https://doi.org/10.5194/egusphere-egu21-14132, 2021.

Stable cosmogenic Ne isotopes are widely used to determine the erosion rate of slowly-eroding land surfaces through the Cenozoic. Constraining erosion and surface exposure back in Earth history remains a challenge largely due to the presence of Ne isotopes generated by natural decay processes over the lifetime of rocks.  Prospects are best when cosmogenic nuclide production has been significant and nucleogenic Ne production is low and can be quantified.  We have explored the limits of palaeo-cosmogenic Ne in one of the Earth’s most extensive erosion surfaces, the late Precambrian Great Unconformity in Estonia. Here deep kaolinitic saprolites formed on Baltica prior to the deposition of Late Ediacaran quartz sandstones. On the basis of geochemical mass balance the duration of saprolite development is estimated to be of the order of a few Myr.

Borehole F163 samples a section through still-buried weathered unconformity that includes a saprolite surface consistent with negligible erosion during the marine transgression. Samples from the unconformity have ²¹Ne concentrations (>10⁸ atoms/g) that are significantly higher than shielded samples from >20 m below the unconformity. This difference is borne out by Ne isotope composition, and leads to the tanatalising prospect that Precambrian cosmogenic Ne is present in the saprolite. Using modern ²¹Ne production rates the palaeosols appear to record a few million years irradiation. This is broadly consistent with geochemical estimates of saprolite development.  Samples from the uppermost preserved part of the weathering profile in borehole F231 have low ²¹Ne concentrations that are indistinguishable from deeper in the rock profile. This would require profile truncation or the redeposition of weathered material.  The borehole is located on the western flank of an uplifted basement block rising ca 130 m above the typical Precambrian basement level in the area and likely that the thick regolith contains material eroded from the uplifted basement units. Clearly these are early days and quantifying surface exposure in deep time will require effort in field as well as the lab.

How to cite: Hall, A. M., Stuart, F., Kirsimae, K., and Somelar, P.: Understanding 21Ne inventories in Precambrian basement below the Great Unconformity in Estonia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15013, https://doi.org/10.5194/egusphere-egu21-15013, 2021.