Displays

The Tor Exhumation Approach (TEA) traditionally uses large residual granitic landforms as indicators for surface lowering over time of their surrounding landscape. This study challenged the broad applicability of this approach by using large metamorphic residual landforms instead of the traditional plutonic rock formations. We applied in-situ cosmogenic nuclide techniques (¹⁰Be) along vertical landforms (schist-tors) on the Otago upland in southern New Zealand to investigate the capabilities of the TEA within another major rock type. The aim of the investigation was to decipher surface denudation models for the last ~100 ka. The experiment was coupled with fallout radionuclides (^239+240Pu) in nearby soils to compare the Pleistocene / Holocene surface denudation rates with the Anthropocene (last ~60 years).

The surface ages of the eight investigated schist-tors was between ~20 ka and ~300 ka. This allowed the numerical modeling of continuous surface denudation rates with highest values near 0.16-0.20 [mm year^-1] (about 140–180 [t km^-2year^-1]), and over a period that has not yet been achieved. The fallout radionuclide study resulted in two different mass redistribution rates. Average soil erosion along a ridge was ~400 to ~850 [t km^-2year^-1], whereas in an adjacent valley soil deposition rates reached ~130 to ~1,500 [t km^-2year^-1]. In conclusion, this study provides a new basis on how schist-tors emerge at two different landscape positions (ridges and valleys). In addition, differences between past surface denudation rates and modern ones could be revealed.

How to cite: Raab, G., Scarciglia, F., Norton, K., Martin, A., Christl, M., Ketterer, M., and Egli, M.: Surface denudation and soil erosion over 300 ka at the Otago upland (New Zealand) using 10Be and 239+240Pu, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2911, https://doi.org/10.5194/egusphere-egu2020-2911, 2020.

Landscape system components interact in ways which are not yet fully understood, and in tectonically active regions it is of particular interest whether endogenic or exogenic factors are the main drivers of landscape evolution. For example, fluvial terraces may form in response to exogenic disturbances like climatic changes or to endogenic forces like tectonic uplift. This study explores how temporal and spatial correlations between end moraines (denoting the advance of glaciers due to climate change) and fluvial terraces can yield insights about exogenic-endogenic processes determining landscape evolution during the Quaternary on the southern margin of the Shaluli Shan plateau, SE Tibet, a formerly glaciated and tectonically active region. A high-resolution TanDEM-X Digital Elevation Model (12 m) was used to produce detailed geomorphological maps of glacial valleys, marginal moraines, glacial lineations, and fluvial terraces. The geomorphological mapping was complemented with geomorphological and sedimentological field observations. Samples for Optically Stimulated Luminescence dating were taken from extensive and distinct terraces located in pull-apart basins bordering the plateau and samples for cosmogenic nuclide exposure dating were collected from selected boulders on end moraines formed by valley glaciers draining the Mt Genie massif on the Shaluli Shan plateau. Infrared stimulated luminescence (IRSL) signals from feldspar multi grains aliquots, and ¹⁰Be and ²⁶Al concentrations from quartz, were used to determine depositional ages of terraces and moraines, respectively. In combining both dating techniques, we compare the timing of glacial expansions with the depositonal ages of the terraces to tease out the effects of exogenic and endogenic drivers on terrace formation and to formulate a conceptual model of landscape evolution.

How to cite: Schneider, R. A. A., Stroeven, A. P., Blomdin, R., Gribenski, N., Caffee, M. W., Yi, C., Xu, X., Zeng, X., Hättestrand, M., Fu, P., and Owen, L. A.: Landscape evolution of the southeastern Tibetan Plateau – Temporal and spatial relationships between glacial and fluvial landforms, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9990, https://doi.org/10.5194/egusphere-egu2020-9990, 2020.

The determination of fault slip rate is often inferred from dating of Quaternary, deformed geomorphological surfaces affected by fault activity. For this reason, cosmogenic and luminescence methods now are widely applied to date the emplacement of geomorphic markers, but each method relates to different geomorphic processes. While the Terrestrial Cosmogenic Nuclides (TCN) method generally dates the exposure duration of the rock surface to cosmic rays, the Optically Stimulated Luminescence (OSL) method provides burial duration of the sediment after deposition. Age differences between these two methods may relate to the erosion-transport-deposition and aggradation processes experienced by the sediment prior its final deposition but combined may provide new insights into the processes affecting alluvial landforms.

Our case study is located in the Western Foothills, south of the Choushui River (Central Taiwan). There, slip on the Changhua blind thrust fault has caused the eastward tilt of a wide flight of fluvial terraces but slip rates on frontal faults are still debated due to large epistemic uncertainties in dating alluvial surfaces with OSL and TCN methods. To achieve a finer chronology of the deposits, a high-resolution sampling strategy has been deployed leading to a direct and unique comparison between OSL and TCN dating methods. Taking advantage of a natural exposure, we collected 10 samples for ¹⁰Be dating completed by 5 OSL samples along a 7 m depth profile. The depth distribution of ¹⁰Be concentrations show a complex depositional history with at least two depositional sequences, modelled to be older than ~38.7 ka.

As previous work has shown the difficulties of OSL dating in Taiwan, particular attention has been paid to luminescence characteristics of quartz and potential dosimetry issues. Our OSL analysis are in good agreement with ¹⁰Be and previous ¹⁴C dating and also reveal three depositional units, dated between ~9 ka and ~66 ka, that are evidenced by different OSL signal characteristics and variations in dosimetry.

This study shows that it is informative to have an exhaustive, detailed, and direct comparison between dating methods on a single depth profile to discuss the geomorphic processes and allow a more detailed understanding of the long-term rates of the Changhua Fault.

How to cite: Rizza, M., Lebrun, B., Siame, L., and Guillou, V.: How to reconcile OSL and TCN data: the potential of high-resolution sampling on the Choushui Tableland (West Central Taiwan), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4595, https://doi.org/10.5194/egusphere-egu2020-4595, 2020.

Exposed bedrock is ubiquitous on terrestrial and planetary landscapes, yet little is known
about the rate of bedrock erosion at a granular scale on timescales longer than the
instrumental record. As recently suggested, using the bleaching depth of luminescence
signals as a measure of bedrock erosion may fit these scales. Yet this approach assumes
constant erosion through time, a condition likely violated by the stochastic nature of erosional
events. Here we simulate bleaching in response to power-law distributions of removal
lengths and hiatus durations. We compare simulation results with previously measured
luminescence profiles from boulder surfaces to illustrate that prolonged hiatuses are unlikely
and that typical erosion scales are sub-granular with occasional loss at mm scales,
consistent with ideas about microflaws governing bedrock detachment. For a wide range of
erosion rates, measurements are integrated over many removal events, producing
reasonably accurate estimates despite the stochastic nature of the simulated process. We
hypothesize that the greater or equal erosion rates atop large boulders compared to rates at
ground level suggest that subcritical cracking may be more influential than aeolian abrasion
for boulder degradation in the Eastern Pamirs, China.

How to cite: Brown, N. and Moon, S.: Interpreting erosion frequency and magnitude from luminescence profiles in boulders, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10720, https://doi.org/10.5194/egusphere-egu2020-10720, 2020.

Wave transported boulders represent important records of storm and tsunami impact over geological timescales. Their use for hazard assessment requires chronological information that in many cases cannot be achieved by established dating approaches such as radiocarbon and U-Th dating. To fill this gap, we investigated the potential of optically stimulated luminescence rock surface exposure dating for estimating transport ages of coastal boulders. The approach was applied to wave-emplaced calcarenite clasts at the Rabat coast, Morocco. Calibration of the OSL surface exposure model was based on samples with rock surfaces exposed for ~2 years, and OSL exposure ages were evaluated against age control deduced from satellite images. The dating precision is very limited for all boulders due to the local source rock lithology, which contains low amounts of quartz and feldspar and was formed after MIS 5 (OSL signals are not in field saturation). Nevertheless, we propose a robust relative chronology for boulders that are not affected by significant post-depositional erosion and that share surface angle of inclination with the calibration samples. The relative chronology indicates that most boulders were moved by storm waves; these storms lifted boulders with masses of up to ~40 t; indicating that the role of storms for the formation of boulder deposits along the Rabat coast is much more significant than assumed previously. While OSL rock surface exposure dating cannot provide reliable absolute exposure ages for the coastal boulders from Rabat, the approach has great potential for boulder deposits composed of rocks with larger amounts of quartz or feldspar, older formation histories and lower susceptibility for erosion.

How to cite: Brill, D., May, S. M., Mhammdi, N., King, G., Burow, C., Wolf, D., Zander, A., and Brückner, H.: OSL rock surface exposure dating as a novel approach for reconstructing transport histories of coastal boulders over decadal to centennial timescales, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18537, https://doi.org/10.5194/egusphere-egu2020-18537, 2020.

Lithic surface artefacts are anthropogenically formed stone materials (stone tools and spall generated during knapping) resting atop or being semi-embedded into the uppermost sediment layer of a stratigraphic sequence on a given landform. Such surface artefacts lack a secure stratigraphic context are encountered worldwide and often comprise a significant proportion of the archaeological record. Yet, direct absolute dating techniques for constraining the age of lithic surface scatters are currently not available.

Promising recent work has shown the potential of using the optically stimulated luminescence (OSL) signal from rocks to date the emplacement of gravel pavements and blocks in both, archaeological and geological contexts (e.g. Sohbati et al., 2015; Jenkins et al., 2018). We build on this work and introduce a novel way of directly dating lithic surface artefacts using OSL-rock surface burial dating. We use this approach to date a surface lithic artefact scatter site, in southern Tibet. By calculating spatially resolved OSL burial ages for slices at 1 mm increments into each artefact’s buried surface we (i) infer the timing of artefact discard by humans at the site, (ii) demonstrate that most artefacts are not in-situ but were transported downslope and/or flipped and (iii) for some samples constrain the timing and number of cycles of artefact burial and re-exhumation.

This is the first time that the OSL signal is used to date sunlight exposure of artefacts. The method is not limited to archaeological contexts but can be applied to other surface clasts that yield a reasonable OSL signal too. OSL rock surface burial dating of surface clasts and artefacts thus holds great potential to (i) constrain manufacture and artefact discard by humans and (ii) detect and reconstruct post-depositional disturbances and transport pathways. We discuss the application potential of this approach in archaeology and geomorphology. 

References:

Jenkins, G.T.H., Duller, G.A.T., Roberts, H.M., Chiverrell, R.C., Glasser, N.F., 2018. A new approach for luminescence dating glaciofluvial deposits – High precision optical dating of cobbles. Quaternary Science Reviews 192, 263-273.

Sohbati, R., Murray, A.S., Porat, N., Jain, M., Avner, U., 2015. Age of a prehistoric Rodedian cult site constrained by sediment and rock surface luminescence dating techniques. Quaternary Geochronology 30, 90–99.

How to cite: Meyer, M., Gliganic, L., and May, J.-H.: Direct dating of lithic surface artefacts using luminescence and application potential in geomorphology, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19195, https://doi.org/10.5194/egusphere-egu2020-19195, 2020.

Detrital tracer thermochronology exploits the relationship between bedrock cooling age and elevation, whereby detrital cooling age distributions can inform the pattern of erosion in the upstream area. Typically bedrock ages exhibit a positive relation with elevation, since the exhumation path from the closure isotherm to the surface is longer. Therefore, spatially uniform erosion should yield a detrital cooling age distribution that mirrors the catchment’s hypsometric curve, corrected for mineral fertility. Applying thermochronometers is time consuming and expensive, hence measured detrital age populations have a limited sample size (rarely exceeding 100 grains) and only provide an approximation of the natural continuous detrital age distribution. With such limited sample sizes, discerning between two detrital age distributions resulting from different erosional patterns may be statistically impossible at a high confidence level.

Here, we investigate the impact of sample size on the detrital cooling age distributions and the resulting uncertainty in addressing the erosional pattern of the upstream area. To do so, we forward model a continuous detrital age distribution as a function given parameters, such as catchment hypsometry, mineral fertility, exhumation rate, analytical uncertainty and erosion scenario. A random subsample of the entire detrital population is drawn for each possible sample size, from which an approximate cumulative distribution function (CDF) is calculated. Then we compute the divergence of the approximate CDF from the reference continuous CDF as well as from the continuous CDF obtained for a different erosion scenario. The confidence level at which an approximate CDF can be tied to a specific erosion scenario is iteratively estimated for each sample size. We carry out the outlined approach for a synthetic catchment with elevations ranging between 0.5-2.2 km, where bedrock cooling ages increase by 30 My/km and prescribing a 10% standard deviation for single grain ages.

We find that, if the location of the erosion maximum coincides with the peak of the hypsometric curve, 100 grains do not suffice to resolve a local tenfold increase in erosion at the 95% confidence level. In this worst case scenario 240 grains would be required. However, for the same case, 70 grains are enough at the 68% confidence level. This study provides a method to consistently quantify the uncertainty of detrital tracer thermochronology as a function of sample size, case-specific variables and the initial scientific question.

How to cite: Madella, A., Glotzbach, C., and Ehlers, T. A.: How many grains do we need for tracer thermochronology?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8960, https://doi.org/10.5194/egusphere-egu2020-8960, 2020.

A coupled accelerator mass spectrometer - gas interface system successfully has been operating at the Hertelendi Laboratory of Environmental Studies, Debrecen, Hungary since 2013. Over the last 6 years more than 500 gas targets were measured below 100 µg carbon content for carbon isotopic composition. The system was tested with blanks, OxII, IAEA-C1, IAEA-C2 and IAEA-C7 standards. The performance of our instrumentation shows good agreement with other published gas-interface system data and also shows a quite good agreement with the nominal value of international standard samples.  There is a measurable but quite small memory effect after modern samples, but this does not significantly affect the final results. Typical ion currents at the low energy side were between 10-15 µA with a 5% CO₂ in He mixing ratio. The relative errors average ±6% for samples greater than or equal to 10 µgC sample with mean count rates of 300 counts per microgram C for OxII. The blank is comparable with other systems, which is 0.0050 ± 0.0018 F¹⁴C or 34000-47000 yr BP, which allows for the routine measurement of both of small environmental and archeological samples.

The research was supported by the European Union and the State of Hungary, co-financed by the European Regional Development Fund in the project of GINOP-2.3.2-15-2016- 00009 “ICER”. This work was carried out in the frame of the János Bolyai Research Scholarship (Mihály Molnár) of the Hungarian Academy of Sciences

How to cite: Molnár, M., Janovics, R., Major, I., Hubay, K., Buró, B., Varga, T., Kertész, T., Gergely, V., Veres, M., and Jull, A. J. T.: EnvironMICADAS C-14 AMS Gas ion source performance and its applications at HEKAL Laboratory, Debrecen, Hungary, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9577, https://doi.org/10.5194/egusphere-egu2020-9577, 2020.

Interpretations of surface exposure ages derived from the accumulation of cosmogenic isotopes commonly are hampered by a lack of field documentation that is necessary to identify and constrain if any post-depositional surface modifications have occurred (e.g. surface erosion, burial) that will impact age interpretations. Previous authors have discussed these issues, but the community still has not fully adopted the practice of interpreting surface exposure ages in conjunction with detailed soil stratigraphic observations. We employ this “novel” approach by documenting a soil chronosequence from the Gould wash alluvial fan sequence near Cibola, AZ to demonstrate how soil stratigraphy can provide constraints for the relative stability of depositional surfaces and can influence interpretations of TCN ages.

The Cibola chronosequence represents a range of alluvial fan ages that extend well beyond those commonly observed in the desert southwestern US (typically <100 ka) and provides evidence for extended periods of surface stability. We identified seven different alluvial fan surfaces within the sequence, documented their soil morphological and chemical properties, and dated four of the fan surfaces with ³⁶Cl depth profiles. Fan deposits largely consist of volcaniclastic alluvium derived from the local Trigo Peaks and distal Castle Dome mountain blocks and show both a reduction of bar-and-swale surface topography and an increasing expression of desert pavement with relative surface age. The soil profiles consist of Av-Bk-BCky-Cky-Ck vertical horizon sequences (~125-cm thick) in the youngest fan units to Avk-Btky-Bkym-Bky-BCky-Cky-C (~400-cm thick) in the oldest fan unit that reflect systematic changes in soil thickness, structure, rubification of B horizons, and relative accumulations of eolian derived silt, clay, and salts as a function of relative surface age.

Chlorine-36 depth profile analysis yielded variable fan ages that are largely controlled by the magnitude of allowable erosion. Model results for which input data were parameterized to optimize unconstrained erosion rates indicate surface exposure ages of 46 (2A), 114 (2B), 268 (3A), and 386 (4A) ka. These are associated with best-fit erosion rates of 0-6 mm/kyr that indicate 0-136 cm net erosion. By comparison, results for which erosion rates were constrained to ~1 mm/kyr based on soil stratigraphic observations yielded exposure ages of 41 (2A), 114 (2B), 209 (3A), and 287 (4A) ka, resulting in differences of 10-25% of the unconstrained ages. The systematic morphological trends observed in the soil profiles do not support inferences of net erosion exceeding 30 cm and therefore cannot support the results from unconstrained parameter optimization. Although statistical optimization schemes provide better model fits to the data as indicated by chi-shared minimization routines, current models cannot account for field observations or for inferred constraints on surface modifications based on cosmogenic isotope concentrations alone. That task is better suited for and required by the sampling protocol to achieve more reliable surface exposure dates.

How to cite: Sion, B., McDonald, E., and Bustarde, J.: Using Soil stratigraphic observations to constrain TCN depth profile ages of relict alluvial fan surfaces in the Sonoran Desert, USA, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10614, https://doi.org/10.5194/egusphere-egu2020-10614, 2020.

Understanding the degree of sensitivity of the Earth Surface to past climate changes is key to assess the strength of postulated links between weathering, denudation, rock uplift and climate. Numerous studies have investigated the response of surface processes to the evolution through time of temperature and precipitation, in various settings and over different time periods. In particular, an important question still actively debated concerns whether or not Late Cenozoic climate change had an effective impact on denudation rates. It is noteworthy that this Late Cenozoic climatic evolution is often described as a long-term cooling over several Ma. However, although it has been postulated to be an important control on this response, the impact of its orbitally-controlled high-frequency component has been less investigated.

Studies focusing on climate-denudation links have often been carried out in regions of high tectonic activity. Therefore, they encountered associated significant limitations, such as: (1) high denudation rates that reach the analytical limits of many measurement methods; (2) stochastic events introducing a high degree of variability in the denudation signal; and, (3) high rates of tectonic uplift that can limit the sensitivity to the low-frequency component of the climatic boundary condition. Less active tectonic settings with lower denudation rates may thus provide conditions allowing to focus specifically on the coupling between climate variations and surface processes. Additionally, approaches combining different cosmogenic nuclides have proven to be very effective to unravel changes in surface processes over several time scales.

We present a new cosmogenic nuclides dataset from the Sera do Cipo range in Minas Gerais, Brazil. The core of the range is made of resistant quarzite bedrock with a relief of 500 m with respect to the surrounding low lands, and reported denudation rates are <10 m/Ma. Streams sediments from small catchments near the summit divide, as well as clasts derived from massive quartz veins at hilltop locations, were sampled. Both ¹⁰Be and ²⁶Al concentrations were measured in the collected samples, as in such slow denudation settings the ratio between the two nuclides is sensitive to changes in denudation rates through time. A high-resolution (1 m) Digital Elevation Models was also produced from tri-stereo Pléiades satellite images. This allows to compute high resolution metrics such as hilltop curvature at the sampling sites. Hilltop denudation rates display a strong positive correlation with curvature. ²⁶Al/¹⁰Be values significantly departing from the theoretical steady state denudation ratio are interpreted at hilltop sites as reflecting the fluctuation of denudation through time. Concerning the catchments samples, the determined ratio can also be impacted by the sediment transport history along hillslopes. Combining cosmogenic nuclides and high-resolution topographic datasets, the measured concentrations were inverted to constrain the variation of denudation over the last 2 Ma. We observe a significant change in the denudation regime at 1 Ma, with different kind of responses between ridges and small catchments across the landscape.

How to cite: Godard, V., Salgado, A., Siame, L., Fleury, J., and Aster, T.: Temporal evolution of surface processes inferred from cosmogenic nuclides in a slow erosion setting, insights from the Sera do Cipo range, Brazil, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9427, https://doi.org/10.5194/egusphere-egu2020-9427, 2020.

The downstream increase in valley width is an important feature of fluvial landscapes that may be evident to anyone: even if local exceptions exist, wide fluvial valleys in plains are distinctive of narrow upstream mountainous ones. Yet, the processes and rates governing along-stream valley widening over timescales characteristic of landscape development (>1-10 ka) are largely unknown. No suitable law exists in landscape evolution models, thus models imperfectly reproduce the landscape evolution at geological timescales, their rates of erosion and probably their response to tectonics and climate. Here, we study two 1 km-deep canyons in northern Chile with diachronous incision initiation, thus representing two time-stage evolutions of a similar geomorphic system characterized by valley widening following the upward migration of a major knickzone. We use 10Be cosmogenic isotope concentrations measured in colluvial deposits at the foot of hillslopes to quantify along-stream valley flank erosion rates. We observe that valley flank erosion rate increases quasi-linearly with valley-bed slope and decreases non-linearly with valley width. This relation suggests that lateral erosion increases with sediment flux due to higher channel mobility. In turn, valley width exerts a negative feedback on lateral valley flank erosion since channels in wide valleys have a lower probability of eroding the valley sides. This implies a major control of river divagation on valley flank erosion rate and valley widening. Our study provides the first data for understanding the long-term processes and rates governing valley widening in landscapes.

How to cite: Zavala, V., Carretier, S., Regard, V., Bonnet, S., Riquelme, R., and Choy, S.: Along-stream variations in valley flank erosion rates measured using 10Be concentrations in colluvial deposits from Atacama canyons: implications for valley widening, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2316, https://doi.org/10.5194/egusphere-egu2020-2316, 2020.

The eastern seaboard of Australia is characterized by a passive margin and a continental divide that separates the inland-draining rivers from those that drain to the Coral and Tasman seas. Seaward of this divide lies the Great Escarpment (GE) of Australia that separates a moderate relief coastal plain from a low relief, high elevation plateau. Quantifying the spatial variation of erosion rates from temperate New England (NE), NSW and tropical Bellenden Ker (BK), Queensland, two regions with distinctly different climates and escarpment embayment, could help constrain erosional controls that contribute to escarpment form. In this study, we compared forty detrital 10Be samples collected from sediments in the main trunk and tributaries of five major rivers: the Macleay, Bellinger, and Clarence in NE and the Russel-Mulgrave and North Johnstone in BK. We then traced the escarpment position in ARCGIS and calculated a sinuosity ratio to better compare the degree of embayment in each region. Across both datasets we found that for NE, which has deep gorges cutting into the plateau, the degree of embayment was twice that of BK, where the escarpment position is significantly less embayed and erosion rates significantly more variable (ratio of .18 vs .38). Erosion rates in low slope areas, such as on the plateau, were universally low with no other significant controlling factors. There was no correlation between erosion rates and catchment area, and that our data echo previous studies that find that once mean rainfall passes an approximate threshold (around 2000mm/yr) basin characteristics that are known to control erosion rates, such as slope and lithology, are subdued.

 In temperate NE, where rainfall ranges from approximately 800-1200mm/yr, there was a moderate linear correlation with mean catchment rainfall and erosion rates (R² .50), which is likely due to a strong orographic effect due to the escarpment. Erosion rates from tributaries below the plateau were highly variable and ranged from 5m/Ma up to 60m/Ma and correlated strongly with mean catchment slope (R² .86). In addition, there were moderate inverse linear correlations between erosion rate and the catchment total percent granite and sedimentary rock (R² .53 and .63 respectively) and a moderate correlation between erosion rate and catchment total percent metamorphic rock (R² .57). Similar to previous studies, these data suggest that in temperate climates with moderate amounts of annual rainfall, individual basin characteristics play a significant role in controlling basin wide erosion rates.

In contrast, data from tropical BK, where mean rainfall amounts are in excess of 2000mm/yr, erosion rates from tributaries below the plateau were significantly less variable than NE. Rates had a mean of 37m/Ma ± 9 (standard deviation 5m/Ma, N=10) and were not significantly correlated with mean catchment slope nor catchment lithology. The mean erosion rate of BK is similar to that of other studies in the region, though with slightly less variability, and possibly reinforces the hypothesis from other researchers that in tropical climates with significant mean rainfall, soil depth effectively armors hillslopes and prevents bedrock erosion from occurring.

How to cite: Glass, J., Codilean, A., Fülöp, R., Wilcken, K., Cohen, T., and Abbott, L.: Spatial variation of erosion rates and passive margin escarpment embayment from New England, NSW and Bellenden Ker, Queensland, Australia: an analysis using GIS and in-situ 10Be basin-wide cosmogenic nuclides, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12751, https://doi.org/10.5194/egusphere-egu2020-12751, 2020.

Quaternary erosion through glacial and post-glacial processes has left an imprint on Alpine topography. There are few methods capable of resolving these processes on Late glacial to Holocene timescales. The aim of this study is to contribute towards a more detailed understanding of post-glacial erosion across the Central and Western Alps by better constraining the post-glacial erosion history of the Gorner glacier in Zermatt, Switzerland. This is done using a new approach that combines Optically Stimulated Luminescence (OSL) and ¹⁰Be cosmogenic nuclide surface exposure dating to invert for post-glacial erosion rates (Lehmann et al., 2019). Both dating methods are influenced by surface erosion but operate on different spatial scales- OSL signals form within the first 1-5 mm of a rock surface (Sohbati et al., 2011) whereas the ¹⁰Be signal accumulates within approximately the first 3 m (Lal, 1991). Six bedrock samples, exposed progressively since the Last Glacial Maximum, were collected along a vertical transect spanning an elevation of 641 m. Preliminary results show inheritance in the bottom three samples suggesting multiple advances and retreat. Further results for the post-glacial erosion rates down the transect, and comparison to other glaciers in the Western Alps, will be presented.

References:

Lal, D., 1991. Cosmic ray labelling of erosion surfaces: in situ nuclide production rates and erosion models. Earth and Planetary Science Letters, 104, 424-439.

Lehmann, B et al., 2019. Evaluating post-glacial bedrock erosion and surface exposure duration by coupling in situ optically stimulated luminescence and ¹⁰Be dating. Earth Surface Dynamics, 7.

Sohbati, R. et al., 2011. Investigating the resetting of OSL signals in rock surfaces. Geochronometria, 38(3), 249-258.

How to cite: Elkadi, J., Lehmann, B., King, G., Steinemann, O., Ivy-Ochs, S., Christl, M., and Herman, F.: Quantifying post-glacial erosion at the Gorner glacier, Switzerland, using OSL and 10Be surface exposure dating. , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8432, https://doi.org/10.5194/egusphere-egu2020-8432, 2020.

Sediment burial dating using optically stimulated luminescence (OSL) is a well-established tool in geochronology. Yet, an important prerequisite for its successful application is that the OSL signal is sufficiently reset prior to deposition. However, subaqueous bleaching conditions are vastly understudied, for example the effect of turbidity and sediment mixing on luminescence bleaching rates is only poorly established. The possibility that slow bleaching rates may dominate in certain transport conditions led to the concept that OSL could be used to derive sediment transport histories. The feasibility of this concept is still to be demonstrated and experimental setups to be tested. Our contribution to this scientific challenge involves subaquatic bleaching experiments, in which we suspend saturated coastal sand of Miocene age in a circular flume and illuminate for discrete time intervals with natural light. We further record the in-situ energy flux density received by the suspended grains in the UV-NIR frequency range by using a broadband spectrometer with a submersible probe.

Our analysis includes pre-profiling of each sample following a polymineral multiple signal protocol (Reimann et al., 2015), in which we simultaneously measured the quartz dominated blue stimulated luminescence signal at 125°C (BSL-125) and the K-feldspar dominated post-infrared infrared stimulated luminescence signal at 155°C (pIRIR-155). Preliminary results from the flume experiments show that the bleaching rates are indeed slow, differ for both signals and that the pIRIR155 seem to bleach faster than the BSL125. Besides the good prospects of acquiring a new tool for quantifying sediment transport, these results might have potentially far-reaching implications regarding the preferred target mineral for OSL dating in fluvial settings.  

How to cite: Mey, J., Schwanghart, W., de Boer, A.-M., and Reimann, T.: Illuminating the speed of sand – quantifying sediment transport using optically stimulated luminescence, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3737, https://doi.org/10.5194/egusphere-egu2020-3737, 2020.

Timothée Jautzy^1,2, Dominik Brill³, Laurent Schmitt¹, Gilles Rixhon⁴

Obtaining robust chronological data on landforms and their related deposits together with constraining rates of earth surface processes have constantly represented a major challenge in Quaternary science. In the fluvial context, Optically Stimulated Luminescence (OSL) is particularly well-established but still faces several limitations. It notably requires expensive and time-consuming sample processing and measurement, frequently resulting in a poor spatial and stratigraphical distribution of sampling which may negatively impact the chronological information. To overcome this main limitation, a Portable OSL reader (POSL) has been recently developed (Sanderson & Murphy, 2010). It consists in directly capturing a luminescence signal (counts per seconds) on unprepared sediment samples. This technique is quick and affordable but, unlike conventional OSL, is not able to yield numerical age estimates.

This contribution explores POSL capacities to provide useful relative age information on alluvial sediments from the last centuries. We study and compare 42 samples collected from three alluvial profiles located in the floodplain of a gravelly-sandy mid-sized river: the Bruche (i.e. a sub-tributary of the Upper Rhine, France). POSL stimulations, including both blue and infra-red signals, are performed in combination with grain size analysis. We observe (i) an overall increase of signal intensity with increasing depth, (ii) a very good match between blue and IR signals and (iii) no systematic correlation between signal intensity and grain size. Whilst this last point must still be confirmed (i.e. signal intensity does not primarily depend on grain size), our preliminary results positively suggest that POSL is a promising tool to provide a relative chronology for very young alluvial sediments. Furthermore, it may also provide information on geomorphic processes. These results will be combined soon to numerical dating (OSL and ¹⁴C) and compared to outcomes of a planimetric analysis to thoroughly reconstruct the historical lateral mobility of the Bruche river.

How to cite: Schmitt, L., Jautzy, T., Brill, D., and Rixhon, G.: Using the portable luminescence reader to assess the historical lateral mobility of river channels: preliminary promising results, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19218, https://doi.org/10.5194/egusphere-egu2020-19218, 2020.

Reconstructing exposure histories of rock surfaces with luminescence dating is a recently developed tool which has proven valuable to chronologically constrain archaeological rock structures (e.g. Sohbati et al., 2012). Here, we explore the possibility to use infrared stimulated luminescence (IRSL) exposure dating to constrain the period of usage of two dry stone enclosure complexes (MZ001S and MZ005S) in Val di Sole, Trentino, Italy. Archaeological investigations confirm that the enclosures were used to keep livestock and radiocarbon ages and archaeological finds from MZ005S restrain the oldest time of use to the Late Middle Ages or Early Modern Period (Carrer and Angelucci, 2018). One 19th-century potshard has been recovered from MZ001S and interviews with residents indicate that MZ001S may have been in use until the mid-20th century. Mica and quartz-rich gneiss rocks of both structures were sampled, together with calibration rock surfaces which had been exposed for one year. Cores were extracted from the rocks with a water-cooled bench drill and cut with a cooled precision saw. Whole rock slices (approx. 0.7 mm thin) were heated to 180 °C for 100 seconds and were subsequently measured with infrared diodes at 50 °C for 300 seconds to create IRSL-depth profiles. Exposure ages were calculated with the exposure dating model developed by Sohbati et al. (2011) for which we used de-trapping rates calculated from the exposed calibration surfaces. IRSL-with-depth profiles are presented from both natural and calibration surfaces. Preliminary ages severely underestimate expected exposure ages (decades of exposure, compared to expected centuries of exposure) and precision of the ages is low.  More investigations are necessary but possible reasons for this age discrepancy are denudation of the rock surface, heterogenic mineralogy with patches of opaque minerals which locally increase light attenuation, or the calibration samples do not represent good analogues for the rocks from the stone structures. The low precision of the ages appears to originate from variations in the IRSL-depth profiles between different cores cut from the same sample.

References

Carrer, F., Angelucci, D.E., 2018. Continuity and discontinuity in the history of upland pastoral landscapes: the case study of Val Molinac and Val Poré (Val di Sole, Trentino, Eastern Italian Alps). Landscape Research 43, 862–877. doi:10.1080/01426397.2017.1390078.

Sohbati, R., Murray, A., Jain, M., Buylaert, J.P., Thomsen, K., 2011. Investigating the resetting of OSL signals in rock surfaces. Geochronometria 38. doi:https://doi.org/10.2478/s13386-011-0029-2.

Sohbati, R., Murray, A. S., Chapot, M. S., Jain, M., and Pederson, J. 2012. Optically stimulated luminescence (OSL) as a chronometer for surface exposure dating J. Geophys. Res., 117, doi:10.1029/2012JB009383.

How to cite: Ageby, L., Angelucci, D. E., Brill, D., Carrer, F., Brückner, H., and Klasen, N.: Exploring the application of IRSL rock surface exposure dating of archaeological stone structures in Val di Sole, Italy , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20236, https://doi.org/10.5194/egusphere-egu2020-20236, 2020.

For a better understanding of the recent exhumation history of the Alps and the distribution of palaeo- and recent earthquakes within the orogen, it is important to elucidate the Quaternary activity of major faults. In this study, we test the applicability of luminescence and electron spin resonance (ESR) dating, which have ultralow closure temperatures, to directly date fault gouge of the Simplon Fault. A dark grey to black, fine-grained fault gouge was sampled near Visp, Switzerland, from an outcrop that exposes rocks that formed at ductile/brittle conditions. Quartz and feldspar grains were extracted from the sample; quartz grains were used for ESR dating, whereas feldspar grains were used for infrared stimulated luminescence (IRSL) dating.

The IRSL measurements reveal that the natural post-IR IRSL signal, stimulated at 225°C (pIRIR₂₂₅) was in saturation. The pIRIR₂₂₅ signal had an extremely low saturation dose, with a characteristic saturation dose (D₀) of ~90 Gy. The natural IRSL signal at 50°C (IR₅₀) is about 80 % of the laboratory saturation, so that this signal is presumably in the field saturation. The IR₅₀ also showed a small D₀ of ~250 Gy. Although these D₀ values are unexpectedly small, the IRSL signals can be used to calculate the minimum age of the last seismic movement of the fault.

Both natural and laboratory-irradiated ESR spectra did not contain detectable Ti centre. Therefore, only the Al centre was used for ESR dating. The natural Al centre from the fault was not in saturation, with a preliminary equivalent dose value of ~1500 Gy. Since the last seismogenic movement most likely only partially reset the Al centre, the ESR age can be regarded as the maximum age of the last event.  We show that by combining luminescence and ESR dating, it is possible to narrow down the age range of the last seismic activity on the fault.

How to cite: Tsukamoto, S., Tanner, D., Brandes, C., and von Hagke, C.: Testing direct dating of Alpine faults by luminescence and ESR, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7982, https://doi.org/10.5194/egusphere-egu2020-7982, 2020.

Dating of loess deposits using optically stimulated luminescence (OSL) enable us to extract important information about the climate during the last ~150 ka. A good estimation of the dose rate during the past is essential for OSL and depends, among others, on the history of the moisture content in the proximity of the dated sample. While the current moisture content can be measured by heating/drying, the history of the moisture variations of a sample is generally unknown. Reference values reported on similar materials and climate conditions may provide a range for the expected moisture variations in the past, but these values are generally rough guesses and not depth- and time-specific.

Nuclear magnetic resonance (NMR) relaxometry targeting the hydrogens of the pore fluid can estimate the current moisture content of a sample without heating. Additionally, the NMR relaxation time distribution yields information of the expected moisture content for a given field potential (e.g. wilting point). This can help to estimate a sample-specific range of likely moisture variation in a quick (several min) and no-invasive way.

We discuss this new approach on a loess profile from Toshan (Iran) published previously by (Lauer et al., 2017) and (Vlaminck, 2018). The later pointed out inconsistencies in the obtained age model. The estimated sample specific moisture content for the wilting point (15 to 35 wt.%) provide the low boundary for the moisture content estimate, which is higher than previously assumed (5 wt.%). The new dose rate calculated for these sample specific moisture content values lead to clearly older and more consistent ages (less age inversions).

We suggest that NMR derived moisture content data is valuable for obtaining information on the moisture content of samples. Especially the minimum moisture can be derived reliably, giving more robust water content estimates for OSL dating.

References

Lauer, T., Vlaminck, S., Frechen, M., Rolf, C., Kehl, M., Sharifi, J., Lehndorff, E., Khormali, F., 2017. The Agh Band loess-palaeosol sequence – A terrestrial archive for climatic shifts during the last and penultimate glacial–interglacial cycles in a semiarid region in northern Iran. Quaternary International, Loess, soils and climate change in Iran and vicinity 429, 13–30. https://doi.org/10.1016/j.quaint.2016.01.062

Vlaminck, S., 2018. Northeastern Iranian loess and its palaeoclimatic implications (PhD Thesis). Universität zu Köln.

How to cite: Dlugosch, R., Zeeden, C., Lauer, T., and Tsukamoto, S.: Can moisture content estimates from nuclear magnetic resonance improve optically stimulated luminescence dating - first results on Loess samples from Toshan/Iran, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1493, https://doi.org/10.5194/egusphere-egu2020-1493, 2020.

The sediment rooting concept [1] relies on the potential to track individual mineral grains from their source to their ultimate sinks. Quartz is the second most abundant mineral in the Earth's crust and occurs in a broad variety of rocks and sediments. It is resistant to weathering and does not form solid solutions, thus being considered a pure mineral. However, even the purest quartz crystal contains a vast number of point defects, which may be either intrinsic (e.g., O-vacancies and related defects or Si vacancies) or due to impurities, most often as combination of monovalent (H⁺, Li⁺, and Na⁺) and trivalent (Al³⁺, Fe³⁺, and B³⁺) cations. Some of these defects remain unchanged under ionising radiation bombardment by the omnipresent natural radioactivity, while others are being transformed, generally by charge trapping. Based on the dynamics of some of these radiation sensitive defects under irradiation, quartz can be used for dating by luminescence or by electron spin resonance.

Another less explored application of these defects is fingerprinting the sources of the sediments. For provenance applications to be successful, the signals used when looking at quartz from the sediment should match the corresponding signals of quartz from host rocks, thus they should remain unchanged during transport and/or weathering.

Here we are conducting an exploratory study on quartz from loess from Central Asia (Kazakhstan and Tajikistan). This specific study site was chosen as very recent studies based on geochemical fingerprinting, grain size modelling and present-day meteorological data suggest contribution from different source areas in this Westerlies dominated region [2,3]. Consequently, this area is an ideal test site to look for spatial and temporal variability in source change. We are investigating the signature of E’ (an unpaired electron at an oxygen vacancy site) and peroxy intrinsic defect centers (nonbonding oxygen) as well as the Al-hole ([AlO₄]⁰, a hole trapped by substitutional trivalent aluminum at a silicon site) paramagnetic signals by electron spin resonance in loess samples, as well as in rock samples. We are also investigating the behaviour of these defects during laboratory experiments that aim at reproducing natural conditions during transport. While work is still in progress, we have observed a significant difference between the E’ and peroxy signals for Kazakh and Tajik samples, which is in tune with the current hypothesis regarding the dust sources in the area presented above.

References

How to cite: Timar-Gabor, A., Dave, A., and Fitzsimmons, K.: Investigations on paramagnetic centres in quartz for provenance studies, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5889, https://doi.org/10.5194/egusphere-egu2020-5889, 2020.

This contribution investigates the use of the (U-Th-Sm)/He dating method to unravel the exhumation history of ultramafic ophiolite rocks. Magnetite-bearing rocks are widely distributed on the Earth's surface and are associated with a large range of geological and geodynamic settings. However, little is known of the crystallization and exhumation history of in case of oceanic accretion to orogenic zones, due to a lack of datable minerals. In the past few years, the (U-Th-Sm)/He method applied on magnetite or spinel appears to be very relevant and promising. However, the applicability of this method to access the thermal history has never been quantitatively investigated, limiting the age interpretation. To highlight the applicability and to access geological information using magnetite (U-Th-Sm)/He method (MgHe), we applied it on a well-known high-pressure low-temperature alpine ophiolite (Rocher Blanc ophiolite, Western Alps) where the P-T-t exhumation history is well constrained. A study of magnetite petrology, mineralogy and geochemistry has allowed us to characterize that magnetite crystallize at T>250°C. MgHe ages that range between apatite and zircon fission track (AFT and ZFT) ages of surrounding rocks in agreement with the known thermal sensitivity of those methods. MgHe data were co-inverted with AFT and ZFT data to determine the most robust thermal history associated with the ophiolite cooling. This first MgHe age inversion is consistent with experimental He diffusion data, opening the use of MgHe as a thermochronometer. This result allows us to refine the thermal history and to precise the geodynamical context associated to the final exhumation of this alpine ophiolite.

How to cite: Schwartz, S., Gautheron, C., Ketcham, R. A., Brunet, F., Agranier, A., and Corre, M.: Contribution of magnetite (U-Th-Sm)/He thermometer to quantify the final exhumation of high-pressure ultramafic rocks : example of the Rocher Blanc ophiolite (western Alps), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4505, https://doi.org/10.5194/egusphere-egu2020-4505, 2020.

The Cenozoic growth of the Ecuadorian Andes has been strongly influenced by the compressional reactivation of inherited crustal anisotropies, strike-slip faulting and uplift, and the erosional effects of a wet tropical climate superposed on the deforming orogen. Some authors have linked uplift in the Western Cordillera to the interaction between the South American Plate and the subduction of the oceanic Carnegie Ridge. However, recent studies have alternatively suggested that the tectonic evolution of a northward-escaping crustal sliver in western Ecuador along the Pallatanga strike-slip zone may equally well explain mountain building and topographic growth in this region. While the importance of the Pallatanga Fault has been recognized in the context of seismic hazards, its long-term impact on the development of topography and relief has not been explored in detail. To evaluate the possible roles of oceanic ridge subduction and/or strike-slip motion in prompting the growth of the Western Cordillera, we present new thermochronological data to constrain the deformational history of the Western Cordillera at different latitudes. We focus on two sites in the vicinity of the Pallatanga strike-slip fault (3°S and 1°30’S) and a location farther to the north (0°30’N). Our apatite and zircon (U-Th-Sm)/He dates range from 26.0 ± 0.4 Ma to 3.9 ± 0.1 Ma and from 23.7 ± 0.3 to 5.9 ± 0.1 Ma, respectively. The three sampled sites record a clear age-elevation relationship. The inverse modeling of apatite and zircon (U-Th-Sm)/He dates and upcoming apatite fission-track data is expected to provide new constraints on the recent uplift and exhumation history of the Western Ecuadorian Andes and thus furnish information on the paleo-geographical evolution of the northern Andes.

How to cite: Margirier, A., Reiners, P., Casado, I., Thomson, S., Alvarado, A., and Strecker, M.: Uplift history of the Western Ecuadorian Andes: new constraints from low-temperature thermochronology, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5228, https://doi.org/10.5194/egusphere-egu2020-5228, 2020.

Foreland basins represent a unique record of the evolution of mountain building processes in the adjacent hinterland. In the southern Colombian Andes and the adjacent foreland basin (i.e. Caguán-Putumayo Basin) no detrital U-Pb and heavy mineral studies have been conducted. This is due to the fact that the geochronological characterization of the basement rocks is poor, complicating the interpretation of source areas for provenance analysis.  Here we present a complete provenance study using U-Pb and Heavy mineral data. In order to gain a better understanding of the spatial distribution of the different potential basement sources we planned a characterization of the different basement provinces west of the Caguan-Putumayo basin. Here we present results from samples of active sediments (N=21), basements (N=16) and sedimentary rocks (N=4) older than Cretaceous. This characterization allowed the identification of eight (8) different domains with different age ranges. (1) The southern part of the Central Cordillera with populations of 150 - 250 m.y., (2) Southern part of the eastern flank of the Eastern Cordillera with ages around 150 - 180 m.y., (3) south of the Garzón Massif with age ranges between 1000 - 1150 m.y, (4) north of the Garzón Massif where rocks of 1500 m.y. dominate, (5) Paleozoic sedimentary rocks above the basement to the north of the Garzón Massif and the Serrania de la Macarena with a distinct population of 1300 m.y, (6). The basement of the Serrania de la Macarena with ages between 1650-1800 m, (7). The Serranía de Lindosa with ages around 500 m.y and (8). Amazonian Craton with ages between 1500 - 2000 m.y. Additionally, the relationship between Epidotes and Garnets displays a special behavior in each area. The provinces related to the Garzon Massif have a high amount of Garnets and low amount of Epidotes. On the other hand, the behavior of the areas away from the Garzon Massif is different. Based on the U-Pb detrital signal and the Epidote/Garnet relationship, we suggest that the stratigraphic intervals where we observe ages between 1000 and 1150 m.y. for the first time and high Garnet contents reflect uplift peaks of the Garzon Massif.

How to cite: Sandoval, J. R., Perez-Consuegra, N., Gomez, R. A., Mora, A., Parra, M., Valencia, V., Horton, B., Bueno, R., Reyes, A., Beltran, A., and Cardenas, A.: Spatial distribution of U-Pb ages across a basement uplift in the Northern Andes and its implications for the interpretation of the detrital record in adjacent basins., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12624, https://doi.org/10.5194/egusphere-egu2020-12624, 2020.