The Bohemian Massif is the relic of a major Paleozoic mountain range that is known to have exhumed and its surface levelled in the Permian. The southern part of the Bohemian Massif with high grade metamorphic rocks and magmatic intrusions dips towards the south under the weakly consolidated Neogene sediments of the Molasse Basin. However, Neogene landscape evolution is largely unconstrained, but the occurrence of marine sediments several hundred meters above sea level is a clear indication of significant surface uplift during the last few million years. The landscape is characterized by rolling hills and extended planation surfaces above an elevation of about 500 m. However, at lower elevations deeply incised gorges confined by steep hillslopes are abundant and contrast impressively with the low relief landscapes above. A continental drainage divide follows the central ridge of the Bohemian Massif with the Vlatava (Moldau) and the Danube (Donau) draining the regions north and south of the drainage divide. In this study we aim quantifying spatial and temporal variations of landscape change in the Bohemian Massif during the last few million years. To characterize the two contrasting landscape states, we computed landscape metrics based on digital elevation models (e.g. normalized steepness index, geophysical relief). To determine the rate landscape change we determined catchment-wide erosion rates from the concentration of cosmogenic ¹⁰Be in river sands.

Results show that the landscape is characterized by out-of-equilibrium river profiles with knickpoints abundantly occurring at elevations between 450 m and 550 m separating steep channel segments at lower elevations from less steep channels at higher elevations. Hypsometric maxima at or close above knickpoint elevations along with high and low values in geophysical relief downstream and upstream of major knickpoints support the idea of landscape bimodality. Furthermore, we found a strong drainage divide asymmetry, which evidences for the reorganization of the drainage network of the region. Across-divide gradients in channel steepness predict the northward migration of the Danube-Vltava drainage divide including growth and shrinkage of tributary catchments. Erosion rates of the 20 investigated catchments are very low (20 – 50 m per million year) compared to the Alps or other active mountain ranges. The lowest erosion rates occur in catchments with a large fraction of planation surfaces at mid-altitudes. Highest erosion rates occur in elongated catchments of Danube tributaries. Based on our results we suggest that the occurrence of contrasting bedrock properties between Molasse sediments and the crystalline basement represents a superior control on the topographic evolution of the entire region. The transition from soft sediments of the Molasse basin to much less erodible basement rocks during progressive river incision in a setting of low but long last uplift distinctly changes the channel steepness and relief, the course of the receiving streams, and their susceptibility to sudden changes in flow direction (river capture) of the million years’ time scale.

How to cite: Robl, J., Stüwe, K., Dremel, F., Liebl, M., von Hagke, C., and Fabel, D.: Old orogen - young topography: Landscape Rejuvenation in the Bohemian Massif, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2068, https://doi.org/10.5194/egusphere-egu23-2068, 2023.

Understanding topographic relief evolution and its changes over hundreds of thousands to million-year timescales remains challenging. Recent approaches usually combine numerical modelling of terrestrial cosmogenic nuclide (TCN) exposure ages on strath terraces, exhumation histories based on thermochronology, drainage basin evolution, and basin stratigraphy. However, even when combined, these methods are unable to measure the rate changes with precisions needed to differentiate climate from tectonic drivers over multiple glacial cycles and longer timescales.

Muon-paleotopometry is a new approach that may address the methodological gap in determining relief generation. Muon-paleotopometry utilizes the dependence of cosmic ray muon flux on crustal shielding depth. The spatial pattern of concentrations of multiple muon-induced TCN measured along a near-horizontal transect under valleys and peaks relates directly to the history of changes (positive or negative) in crustal thickness. It enables paleotopometry above the sample datum over an isotope-specific monitoring duration. By sampling at depths of hectametres, long-lived TCNs are not sensitive to minor short-term (<10⁵-yr) changes owing to cut and fill terraces or transgressions for instance, but short-lived isotopes may provide constraints on this. The method uses concentration differences among samples, so is not significantly impacted by limitations in knowledge of muon flux and interactions at those depths. Early proof-of-concept investigations at Dalhousie (M. Soukup, Hon. Thesis, 2017) provided encouraging results to allow for the current large-scale relief investigation of the European Alps.

How to cite: Raab, G., Gosse, J., and Hidy, A.: Muon Paleotopometry – Measuring crustal thickness variations with muons?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17271, https://doi.org/10.5194/egusphere-egu23-17271, 2023.

We present cosmogenic nucleide data that help to understand the ill-constrained  uplift history of the Eastern Aps. Well accepted, but indirect evidence  for this uplift history includes: (a) the onset of flexural subsidence of the foreland basins, or (b) the formation of the Periadriatic line (recording the onset of continent-continent collision in the Alps). Both are often used to suggest about 30 Ma as the starting date for the surface uplift of the range. Since this time the fine interplay of many kilometres of upwards rock uplift and downwards erosion resulted in net surface uplift of some 2-3 kilometres but reference frames that allow to discern between rock uplift and surface uplift are often hard to identify. One way of measuring surface uplift rates is through the study of areas where erosion did not occur. That is, dating and identifying relicts of ancient base levels for example in caves, sediments or paleosurfaces.

In this contribution we present ¹⁰Be, ²¹Ne, ²⁶Al cosmogenic nucleide data of fluvial sediments sampled in some 50 caves across the Eastern Alps from elevations between 300 and 2500 m surface elevation. We collected samples that were interpreted to have been deposited during cave formation at the vadose-phreatic transition. As such, they form markers for base level and  the age of their burial into the cave may be interpreted as the time the cave was at base level some few hundreds of meters above sea level. Interpretation of our data indicates that the uplift rate of the Eastern Alps may be in the order of 200 m – 500 m per Million years for much of the Pliocene. As such, much of the observed surface uplift of the Eastern Alps may have occurred since the late Miocene and surface uplift is thus much faster than previously thought.

How to cite: Stüwe, K., Gradwohl, G., Robl, J., Plan, L., Fabel, D., and Stuart, F.: Surface uplift of the Eastern Alps. Much faster than we thought?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2346, https://doi.org/10.5194/egusphere-egu23-2346, 2023.

The impact of Quaternary climate change on landscape evolution, and more specifically the timing of incision of the overdeepened Alpine valleys, remains difficult to quantify with existing thermochronometric methods. Thermochronometers are used to determine rates of rock cooling, however most techniques are insensitive to temperature changes <60 °C that occur within the last kms of Earth’s crust. Recording cooling rates within this temperature range is essential if the impact of glacial-interglacial cycles on rock exhumation is to be resolved.

Electron spin resonance (ESR) thermochronometry applied to quartz minerals has the potential to span this thermal (and temporal) gap. We are developing this method by building upon previous studies (e.g. Scherrer, 1993) with the ultimate aim of constraining the timing of incision of the Rhône valley. Preliminary data from the Japanese Alps (King et al., 2020) indicate that ESR thermochronometry could resolve rates of <1 mm/yr over Quaternary timescales.

To determine a rock cooling history using ESR thermochronometry, signal accumulation and signal thermal loss must be robustly determined within the laboratory. We have collected a series of geological samples including rocks from boreholes that have known isothermal histories to investigate the potential of this technique. Our objective is to use the latter rocks to confirm the validity of our laboratory measurements and data-fitting/numerical models. Specifically, we have investigated known-thermal history samples from the MIZ1 borehole (Japan) and the KTB borehole (Germany) as well as samples from Sion in the Western European Alps.

Preliminary data reveal that the ESR dose response and thermal decay of different quartz samples is highly variable. Whereas the Al-centre of some samples exhibits linear dose response to laboratory irradiation up to 15 kGy, the Al-centre of other samples exhibits exponential, or double-exponential growth and saturates at doses of 3-4 kGy. The Ti-centre of most samples is well described by a single saturating exponential function, however samples from the MIZ1 borehole exhibit pronounced sub-linearity in the low-dose response region. Furthermore, whereas for some samples the Al-centre is less thermally stable than the Ti-centre, for other samples the inverse is observed. These observations suggest that a uniform measurement protocol and data-fitting approach may not be appropriate for quartz ESR data.

Inversion of two KTB samples yielded temperatures within uncertainty of borehole temperature, however results for the MIZ1 borehole are more variable and can only recover temperature at best within ~10%. Investigations into the cause of the poor results for the MIZ1 borehole are ongoing (i.e. measurement protocol, data-fitting/numerical model) and will be discussed. Preliminary data from Sion are promising and reveal consistent cooling rates.

Scherer, T., Agel, A., and Hafner S. S.: Determination of uplift rates using ESR investigations of quartz, KTB Rep. 93-2. Kontinentales Tiefbohrprogram der Bundesrepublic Deutschland Niedersächs. Landesamt Bodenforsch., Hannover, 121–124, 1993.

King, G.E., Tsukamoto, S., Herman, F., Biswas, R.H., Sueoka, S., Tagami, T. Electron spin resonance (ESR) thermochronometry of the Hida range of the Japanese Alps: validation and future potential. Geochronology 2, no. 1 (2020): 1-15.

How to cite: King, G., Wen, X., Bartz, M., Anderson, L., Bossin, L., Tsukamoto, S., Li, Y., Herman, F., Ogata, M., and Sueoka, S.: Will ESR thermochronometry reveal the timing of Rhône valley incision?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11839, https://doi.org/10.5194/egusphere-egu23-11839, 2023.

Surface exposure ages of marine and fluvial terraces based on in situ ¹⁰Be dating were determined to estimate formation ages and long-term rates of coastal uplift along the northern Pacific coast of Northeastern Japan. Surface rock samples were collected along the northern and southern coasts of Sanriku based on a reinterpretation of dispersed marine and fluvial terraces using DEM and aerial photographs. We collected three samples at the Samuraihama site from outcrops of pairs of marine and fluvial terraces distributed over the east-facing flank of the Kitakami Mountains. At the Yoshihama site, in contrast, where bedrock surfaces could be better exposed, we took vertical samples from weathered granite rocks on small trench walls dug on the middle marine terrace at Yoshihama Bay. Surface exposure ages from ¹⁰Be concentrations in quartz calculated from the measured ¹⁰Be/⁹Be ratios commonly suggest slow in both sites, whereas steep (~ 10°) dip domains on the marine terraces along the northern Sanriku coast may imply localized permanent strain accumulation.

How to cite: Wakasa, S., Ishiyama, T., Hirouchi, D., Matta, N., Fujita, N., and Echigo, T.: 10Be dating of middle-late Pleistocene uplifted marine terraces in northern Pacific coast of Northeastern Japan, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10655, https://doi.org/10.5194/egusphere-egu23-10655, 2023.

Two published cosmogenic radionuclide (CRN) ²⁶Al/¹⁰Be burial age calculation methods developed to correct for post-depositional production of nuclides in settings with low sediment overburden are compared. The advantages and limitations of simple (ISO; [1], [2]) and inverse modelling (INV, [3]) isochrons are investigated.

The studied dataset originates from the gravel of a Danube terrace in the Central Vienna Basin (Austria) [4]., where two horizons (5.5 m and at 11.8 m subsurface depth) were sampled. Each sample set contained 6 quartz or quartzite cobbles.

The advantage of ISO is that it is uninfluenced by changes in sample depth over time. However, the initial ²⁶Al/¹⁰Be ratio is fixed and no pre- and post-burial denudation rates can be calculated. In addition to age, INV models source and sink denudation rates, but assumes constant depth over burial time.

For correct application of ISO and INV outliers, must be excluded. The robustness of both methods is tested by systematically including or excluding data points (bootstrapping) to estimate the dependence of numerical ages on sample selection either in the field, or during outlier identification.

For outlier identification the traditional method of data exclusion of points above or below the isochron line is used. In addition, a new way is introduced here: the post-burial production is calculated using the modelled burial age and denudation rate and compared to the measured inventories of ¹⁰Be and ²⁶Al. If the fraction of post-burial production is equal or higher compared to the measured inventory and its ratio is considerably different for the two isotopes from the same sample, the datapoint is invalid.

In addition, the influence of each sample on the modelled burial age, tested by bootsrapping, is used to exclude samples with a large effect on the age.

The resulting ages at both levels using ISO and INV agree within errors with ISO being systematically slightly younger. The importance of outlier removal is stressed by the fact that inclusion of all samples results in a considerably older age of the stratigraphically higher level compared to the underlying one.   When outliers are excluded, burial ages of the two sampled horizons overlap within uncertainty, suggesting one single deposition event for the whole sediment package.

Interestingly, when the entire dataset is merged, both methods provide similar ages regardless of the outliers being excluded or kept in. This demonstrates that a larger sample number increases the robustness of a dataset considerably and decreases the sensitivity of either method to potential outliers.

In summary, both ISO and INV are robust ways of CRN burial age determination, provided that model presumptions are not violated and outliers are excluded or the sample number large enough to overprint the influence of outliers.

Funding: NKFIH FK124807; OMAA 90ou17; OMAA 98ou17.

References

[1] Balco, G., Rovey, C.W., 2008. American Journal of Science 308(10), 1083-1114.

[2] Erlanger, E.D., et al., 2012. Geology 40(11), 1019-1022.

[3] Pappu, S. et al., 2011. Science, 331(6024), 1596-1599.

[4] Ruszkiczay-Rüdiger, Zs. et al., 2021. Journal of Radioanalytical and Nuclear Chemistry, 329(3), 1523-1536.

How to cite: Ruszkiczay-Rüdiger, Z., Neuhuber, S., Hintersberger, E., Nørgaard, J., and Braucher, R.: Calculation of cosmogenic radionuclide burial ages: a comparison of two models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7899, https://doi.org/10.5194/egusphere-egu23-7899, 2023.

Transient topography represents an opportunity for extracting information on the combined effect of tectonics, mantle-driven processes, lithology and climate across different temporal and spatial scales. The geomorphic signature of transient conditions can be used to unravel the landscape evolution and to assess perturbations in uplift rates, especially in areas devoid of stratigraphic constraints. The Atlas-Meseta system experienced a large scale topographic rejuvenation during the Cenozoic through a combination of different processes. Despite the uplift, the Western Moroccan Meseta (WMM) represents a quiescent tectonic domain with deeply incised valleys and high-standing erosional surfaces (relict landscape). This topography is characterized by elevated non-lithological knickpoints, that delimit an uplifted relict landscape, implying a transient response to a change in uplift rates. Here, we determine denudation rates of selected watersheds and bedrock outcrops from cosmogenic nuclides and perform stream profile, regional and basin-scale geomorphic analysis. Denudation rates of the relict and the rejuvenated landscape range from 15 to 20 m/Myr and from 30 to 40 m/Myr, respectively. These results allow estimating the erodibility parameter for performing river-profile inversions and hence extracting rock uplift rates through time. Inverted rock uplift rates are 10-25 m/Myr from 45 to 22 Ma and 30-55 m/Myr from 22 to 10 Ma. Despite the different time scales, the inverted rates are consistent with ¹⁰Be averaged denudation rates (15-20 and 30-40 m/Myr) and river incision values from Pleistocene lava flows (<10 and ~50 m/Myr) for the rejuvenated and relict regions of the WMM. These results agree with geological data and indicate that the observed 400 m of surface uplift in the WMM started to develop at ~22-20 Ma. Given the wavelength of the topographic swell forming the topography of the WMM, uplift is here interpreted to reflect localized crustal thickening through magma addition or lithospheric thinning through mantle delamination. This event, however, represents only a first episode of uplift. The occurrence of ~7-Myr-old marine sediments at ~1200 m of elevation indicates that the adjacent Folded Middle Atlas experienced a more recent surface uplift at ~170 m/Myr. Considering the cumulative amount of surface uplift that varies eastward from 400 to 800 and 1200 m from the Meseta to the Tabular and the Folded Middle Atlas, as well as the spatio-temporal pattern of alkaline volcanism (middle Miocene and Pliocene to Present), we suggest that the most recent episode (second phase) of surface uplift was induced by a larger-scale process that most likely included upwelling of asthenospheric mantle and to a lesser extent crustal shortening and thickening in the Folded Middle Atlas.

How to cite: Clementucci, R., Ballato, P., Siame, L., Claudio, F., Simone, R., Giacomo, T., Riccardo, L., Laetitia, L., and Guillou, V.: Cenozoic uplift history and topographic rejuvenation of the northern Atlas-Meseta system (Morocco), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11243, https://doi.org/10.5194/egusphere-egu23-11243, 2023.

In recent decades, most mountain glaciers have been losing mass in response to climate change, and the area of the ablation zone covered by rock debris is expanding. Debris-covered glaciers are expected to have a longer life expectancy than climatically equivalent clean-ice glaciers because supraglacial debris insulates the underlying ice surface and reduces ablation. In order to accurately predict how debris-covered glaciers will evolve under a changing climate it is essential to quantify the processes controlling their behaviour. We used luminescence rock surface burial dating to constrain the englacial transport time of debris within an alpine debris-covered glacier. We collected 24 samples embedded in the ice in the ablation zone of the Miage Glacier, in the Mont-Blanc Massif (Italy). The natural luminescence signal of rock slices was measured from the surface to a depth of ~10 mm using a protocol comprising IRSL50, IRSL225 and OSL125 measurements. Nine of our samples showed a plateau within the first 2 to 3 discs suggesting that the luminescence signal has the potential to be used to date the burial duration of debris. Among them, 5 and 7 samples passed the dose recovery test for the IR50 signal within 10% and 20% of unity respectively. Only 3 samples passed the dose recovery test for the IR225 signal within 10% of unity. After 24h bleaching in the solar simulator, typical residual doses are as high as 20-40% of the natural equivalent dose measured. We obtained preliminary non fading corrected ages for 5 samples in the range from ~0.8 to ~11 ka. Glacier model estimated englacial rock debris transport times are an order of magnitude lower than the oldest ages obtained suggesting either that some clasts were stored on hillslopes or within moraines prior to englacial transport or that calibration issues may have contributed to age overestimation. Further luminescence signal processing quality checks are required to assess the quality of our ages. If ultimately successful, our results, and the application of luminescence rock surface burial dating to englacially transported debris, will enhance understanding of the dynamics of debris-covered glaciers and inform the use of glacier models for debris covered glaciers, which will improve projections of the contribution of mountain glaciers to the sustainability of water resources in vulnerable catchments such as those in High Mountain Asia and South America.

How to cite: Margirier, A., King, G., Schmidt, C., Brondex, J., and Rowan, A.: Englacial transport time of rock debris: new constraints from luminescence rock surface burial dating, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8283, https://doi.org/10.5194/egusphere-egu23-8283, 2023.