GM2.4
Presentations: Thu, 26 May | Room -2.31
During Late Quaternary time, the paleoclimate of the eastern Sahara was punctuated by multiple pluvial periods, then dramatically and cyclically transformed to hyperarid conditions, receiving less than 2 mm/yr of precipitation at present. Geologists, climate modelers, and archaeologists, therefore, have used various proxies to reconstruct past climates during that time, a crucial period for human habitation and migration. These reconstructions, however, lack the precipitation pattern during those pluvial periods, which represents a significant control on weighing the hypotheses of human migrations and occupations. Here we reconstruct the chronology and paleohydrology of a set of fossil rivers expressed by ridges in the modern landscape due to differential erosion. Our 14C and Optically Stimulated Luminescence (OSL) ages of sediments preserved in these ancient rivers cluster within the last African Humid Period (AHP; ca. 14.8 – 5.5 ka BP) and hence support more significant fluvial activity during this distinct humid epoch. Based on median grain size (D50), paleochannel geometry, and drainage area, paleohydraulic reconstructions indicate that typical precipitation intensities of 55–80 mm/h occurred during sediment transport events. When combined with previous annual rainfall estimates, we find that such rainfall intensities were likely 3–4 times more frequent during the AHP. These climatic perturbations may have rendered some parts of the Nile River Valley inhospitable for occupation, driving humans to migrate away in the northwest and west of the Nile Valley between 10.2 and 7.2 ka BP. Ultimately, our results, along with the archeological data, tell a tale from the past of the dramatic climatic changes that our planet undergoes, demonstrating the critical role of climate in sustaining human populations.
How to cite: Zaki, A., King, G., Haghipour, N., Giegengack, R., Watkins, S., Gupta, S., Schuster, M., Khairy, H., Ahmed, S., El-Wakil, M., Eltayeb, S., Herman, F., and Castelltort, S.: Intense precipitation during the African Humid Period inferred from east Saharan fossil rivers: Implications for human dispersal , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10420, https://doi.org/10.5194/egusphere-egu22-10420, 2022.
The impact of Quaternary climate cycles on denudation as well as fluvial aggradation and incision is debated, especially in regions that did not experience glaciation. Here we present a record of paleo-denudation rates, and geochronologic data constraining aggradation and incision from the Sfakia and Elafonisi alluvial sequences on the island of Crete, Greece. We report seven new optically stimulated luminescence (OSL) and ten new radiocarbon ages, as well as eight 10Be and eight 36Cl denudation rates from modern and terrace sediments. At the Elafonisi fan system, we identify four periods of aggradation, where marine isotope stages (MIS) 2, 4, and likely 6 correspond to aggradation periods, and MIS 1, 3, and likely 5e are characterized by incision. The dating of paleoshorelines indicates constant uplift over the past 71 ka, at rates of 1.2 mm/a. Aggradation occurred throughout the entire glacial cycle at the Sfakia fan, followed by up to 50 m of incision in the past 10 ka. Chronological constraints indicate that aggradation rates were particularly high during MIS 2 and 4, analogous to the Elafonisi fan system. However, our paleo-denudation rates indicate mostly constant denudation throughout the past 80 ka; with only two samples indicating an up to 50% increase in paleo-denudation rates compared to modern rates. Nearby climate and vegetation records show that MIS 2, 4, and 6 were characterized by cold and dry climate with sparse vegetation, whereas forest cover and wet conditions prevailed during MIS 1, 3, and 5. Our data suggest that variations in climate and vegetation cover were not sufficient to markedly alter landscape-wide denudation rates, but that changes in hydroclimate and vegetation exerted a strong control on the aggradation-incision behavior of the drainages. During relatively cold stages, low vegetation cover and river sediment transport capacity led to aggradation, whereas the increased river transport capacity during relatively warm stages caused subsequent incision. We therefore hypothesize that the studied catchments show a decoupling between transport-limited streams responding to climate forcing and near-steady hillslope denudation.
How to cite: Ott, R., Scherler, D., Wegmann, K., D'Arcy, M., Ivy-Ochs, S., Christl, M., and Vockenhuber, C.: Decoupling between fluvial aggradation-incision dynamics and paleo-denudation rates during the last glacial cycle, Crete, Greece, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2141, https://doi.org/10.5194/egusphere-egu22-2141, 2022.
Drainage-divide migration, controlled by rock-uplift and rainfall patterns, may play a major role in the geomorphic evolution of mountain ranges. However, divide-migration rates over geologic timescales have only been estimated by theoretical studies and remain empirically poorly constrained. Geomorphological evidence suggests that the Sierra de Aconquija, on the eastern side of the southern Central Andes, northwest Argentina, is undergoing active westward drainage-divide migration. The mountain range has been subjected to steep rock trajectories and pronounced orographic rainfall for the last several million years, presenting an ideal setting for using low-temperature thermochronometric data and exhumation rates derived from in situ produced 10Be to explore its topographic evolution.
We perform three-dimensional thermal-kinematic modeling of previously published thermochronometric data spanning the windward and leeward sides of the range to explore the most likely structural and topographic evolution of the range. We find that the thermochronometric data can be explained by scenarios involving drainage-divide migration alone, or by scenarios that also involve changes in the structures that have accommodated deformation through time. By combining new 10Be-derived catchment-average denudation rates with geomorphic and stratigraphic constraints on fault activity, we conclude that the evolution of the range was likely dominated by west-vergent faulting on a high-angle reverse fault underlying the range, together with westward drainage-divide migration at a rate of several km per million years. Our findings place new constraints on the magnitudes and rates of drainage-divide migration in real landscapes, quantify the effects of orographic rainfall and erosion on the topographic evolution of a mountain range, and highlight the importance of considering drainage-divide migration when interpreting thermochronometer age patterns.
How to cite: Schildgen, T., van der Beek, P., D'Arcy, M., Roda Boluda, D., Elizabeth, O., and Hella, W.: Quantifying drainage-divide migration from orographic rainfall over geologic timescales: Sierra de Aconquija, southern Central Andes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3441, https://doi.org/10.5194/egusphere-egu22-3441, 2022.
*Georges Aumaître, Didier L. Bourlès, Karim Keddadouche
Located in northeastern France, the Vosges Mountains (VM) belongs to these Hercynian ranges strewn across the European alpine foreland. Peaking at ~1425 m of elevation, it presents four contrasting primary characteristics. Firstly, the geological basement allows a bipartite N-S subdivision: the Palaeozoic southern part (crystalline Vosges) composed of various igneous, metamorphic and sedimentary rocks contrasts with the much more homogeneous Triassic cover in the northern part (sandstone Vosges). Secondly, a clear E-W topographic gradient is reflected by steep hillslopes on the eastern side (Alsace) and gently-sloping hillslopes on the western side (Lorraine). Thirdly, a sharp W-E precipitation gradient (>1000mm/yr) is recorded between the windward and the leeward side. Finally, the imprint left by Quaternary climatic fluctuations yields a N-S gradient: whereas the crystalline Vosges hosted abundant valley glaciers, the sandstone Vosges were void of ice cover.
Owing to these advantageous characteristics, this contribution aims to present the first data of catchment-wide denudation at the massif scale and to explore the long-term interactions between denudation, lithological control, morphometry and climatic forcing. Modern stream sediments from 21 river catchments draining the whole massif were sampled for in situ 10Be and 26Al concentration measurements at the outlet of their mountainous reach. The mean Channel Steepness Index (ksn) was computed as a morphometric “predictor” of denudation rates. Groups of lithologically uniform catchments were statistically identified based on their lithological surficial proportions.
Catchment-wide denudation rates inferred from cosmogenic 10Be and 26Al concentrations range from 33 to 83 mm/ka and 38 to 337 mm/ka, respectively. The [26Al]/[10Be] ratio range from 1.43 to 7.96, highlighting a complex exposure history for the glaciated catchments. At the massif scale, results show (i) no relation between denudation and steepness, (ii) a strong positive relation between denudation and precipitations when lithological groups are considered and (iii) a negative relation between the surficial proportion of fluvio-glacial deposits in the catchment and the [26Al]/[10Be] ratio.
To our knowledge, this contribution is the first massif-scale attempt to quantify denudation in an European low- to medium-altitude mountain range. This is especially relevant as long-term landscape evolution in the Variscan belt, by contrast to the numerous works focusing on denudation in high-mountains ranges (e.g. the Alps), has been regularly disregarded in recent geomorphological studies. Importantly, whereas a vast majority of studies measuring denudation rates rely on 10Be concentrations only, this study highlights the need of using a pair of cosmogenic nuclides (i.e. 26Al/10Be) to check whether stream sediments in formerly glaciated catchments have experienced complex exposure history.
How to cite: Jautzy, T., Rixhon, G., Braucher, R., Schmitt, L., and Team*, A.: Measuring 10Be and 26Al concentrations in stream sediments from the Vosges Mountains (NE France) to explore the respective role of lithologic, topographic and climatic control on massif-wide denudation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5435, https://doi.org/10.5194/egusphere-egu22-5435, 2022.
In global geochemical cycles, the solid flux from the continent to the ocean is usually reduced to the input of sediments from rivers [1]. However, regional studies have shown that the input of sediments from rocky coast erosion may be a significant part of this flux [2]. So, it is important to consider this input into global cycles and to quantify it over different timescales.
On short-term timescales, from the year to the century, coastal erosion is currently quantified with direct measurement of the coastline recession, between successive time intervals [3]. Extrapolating on timescales longer than a thousand years is difficult. This leads to a lack of data and therefore a gap in knowledge in longer term coastal erosion [4].
A solution to quantify long-term erosion of rocky coast is to reconstruct the initial geometry of the coastline and to know the age of its formation. Volcanic islands are suitable objects for this method. Indeed, their initial shape is simple and can be easily reconstructed, and their maximum extension can be dated [5,6,7], although this age can be difficult to estimate. Thus, the topographic reconstruction of a volcanic island allows, by comparison with its current topography, the quantification of volumes lost by erosion. In turn, it becomes possible to obtain values of the rocky coast total recession on timescales from thousands to hundreds of thousands of years [8]. Moreover, the wide geographic distribution of volcanic islands provides a diversity of climatic and geodynamic settings allowing to analyze the effects of various factors on long-term coastal erosion.
Here we propose an improvement of this erosion quantification by accounting for the submarine morphology. Applying this approach for different volcanic islands, we carried out a statistical analysis of the impact of several factors that control long-term coastal erosion. This analysis allows us hierarchize these factors. This is the first step towards the formulation of long-term coastal erosion universal laws and towards the quantification of rocky coast sediment influx in global cycles.
[1] Milliman and Farnsworth (2013). Cambridge University Press.
[2] Regard et al. (in press).
[3] Bird (2011). John Wiley & Sons.
[4] Prémaillon et al. (2018). Earth Surface Dynamics 6, 651-668.
[5] Lahitte et al. (2012). Geomorphology 136, 148-164.
[6] Ramalho et al. (2013). Earth-Science Reviews 127, 140-170.
[7] Karátson et al. (2016). Geomorphology 253, 123-134.
[8] Bossis et al. (in press).
How to cite: Bossis, R., Regard, V., and Carretier, S.: The hierarchy of factors controlling long-term coastal erosion: a statistical approach from topographic reconstruction of volcanic islands., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8000, https://doi.org/10.5194/egusphere-egu22-8000, 2022.
The Southern Alps of New Zealand are among the world’s most active mountain ranges, with extremely high rates of exhumation and erosion. This place is therefore well suited to observe and comprehend sediment production at catchment scale and to study Quaternary landscape evolution.
Common methods to quantify erosion of a landscape include estimation of the suspended sediment yield (SSY), which is a proxy for short-term erosion rates, or measurement of cosmogenic 10Be concentrations in fluvial sediments, a demanding method that offers reliable erosion rates representative of larger time-span (millennial). Here, we propose single-grain post-infrared luminescence (SG-pIRIR) as a potential new proxy for erosion rates. We test this approach by comparing SG-pIRIR results with catchment-wide erosion rates obtained using conventional 10Be measurements for eight catchments of the New Zealand Southern Alps.
10Be results demonstrate North-South and East-West gradients in erosion rates, ranging from 0.2 to 4.0 mm/yr, with the fastest towards South-West. The North-South gradient is consistent with existing data of Larsen et al., (2014), that present even higher rates to the south of our study area. We suggest that spatial gradient in erosion rate reflects a tectonic uplift gradient related to northward segmentation of the Alpine fault, coupled to an East-West climatic gradient, related to orographic effect.
Recently, luminescence signals have been proposed as a new tool to study exhumation, exposure histories and erosion, with various approaches including luminescence-depth profiles (Sohbati et al., 2018), luminescence thermochronometry (Herman and King, 2018) or direct relations between quartz luminescence sensitivity and erosion rates (Sawakuchi et al., 2018). Here, we tested the potential of equivalent dose (De) distributions obtained using SG-pIRIR as a proxy for catchment wide erosion rates.
We measured SG-pIRIR De distributions from modern fluvial sediments at the outlets of the eight catchments where we estimated 10Be erosion rates. For each of the samples, we calculated the fraction of grains whose luminescence signal is saturated (Bonnet et al., 2019; Guyez et al., 2022) and the fraction of well-bleached grains from De distributions. In addition, we characterized the De distribution using central age model (CAM; Galbraith et al., 1999) and bootstrapped minimum age model (MAM; Cunningham et al., 2012). We found a relationship between those four proxies and erosion rates obtained from conventional 10Be approaches, but also with SSY (Adams, 1980; Hicks et al., 2011) and channel steepness index. These results confirm the potential for this new tool to inform on catchment-wide erosion rates.
Further work should be undertaken to test this relation in other settings, and also to better comprehend the interplay of processes affecting luminescence signals of feldspar grains in fluvial deposits, with the perspective to use it as an independent reliable tool to reconstruct and possibly quantify erosion and transport processes in a wide range of fluvial settings.
How to cite: Guyez, A., Bonnet, S., Reimann, T., Wilkinson, C., Carretier, S., Norton, K., and Wallinga, J.: Does luminescence of modern fluvial sediments vary according to erosion rate? A comparison between single-grain feldspar p-IRIR dose distributions and 10Be cosmogenic catchment-wide erosion rate in the Southern Alps of New Zealand, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5390, https://doi.org/10.5194/egusphere-egu22-5390, 2022.
Where, and how much sediment is produced and transported in hillslope and fluvial systems depends upon the topographic slope, soil production rate, lithology, precipitation, and biota. In this study we investigate the patterns of sediment production in two catchments of the Coastal Cordillera (Chile) situated in semi-arid and mediterranean bioclimates. We do this by measuring 29 bedrock and 6 detrital apatite trace elements and apatite cooling ages with the U/Pb, fission track, U-Th(-Sm)/He thermochronometric systems. Detrital samples were collected from fluvial sediment and provide a catchment-scale view of the upstream areas. The compositional and geochronologic data measured in bedrock are analized with a Principal Component Analysis and a clustering algorithm to find the parameters that are best suited to trace sediment provenance at the sub-catchment scale. Next, we analyse the distribution of the same parameters within the detritus to infer the relative contribution of different areas within the catchments. Results indicate that spatial variations of bedrock cooling age and geochemical composition are significant even within small-scale (10-100 km2) granitoid catchments. Therefore, the combination of detrital apatite geochronology and geochemistry allows discrimination among source areas with acceptable confidence. Preliminary results show that the impact of vegetation distribution, hillslope angles and bedrock weatherability on sediment production differs in the two bioclimatic settings. In particular, hillslope angles and lithology exert a greater impact in the semi-arid catchment.
How to cite: Madella, A., Glotzbach, C., and Ehlers, T.: Biotic, lithologic and geomorphic control on sediment production from detrital apatite geochemistry and thermochronology, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2762, https://doi.org/10.5194/egusphere-egu22-2762, 2022.
The Yarlung Tsangpo River follows the Indus-Tsangpo suture through the southern Tibetan Plateau, and then becomes the Brahmaputra, following a bend into India through the Yarlung Tsangpo Gorge. In the middle reaches, narrow gorges alternate with broad valleys (Zhang, et al. 1998). In the section with steep and narrow gorges, the river is easily dammed by landslides, glaciers and/or moraines, rock avalanches and debris flows. Palaeo-lake sediments were discovered in the broad Xigazê valley and Dazhuka-Yueju gorge in the middle reach of the Yarlung Tsangpo River in Tibet. The river was likely dammed by a glacier and/or moraine at the eastern end of the Dazhuka-Yueju gorge. AMS 14C and OSL ages of lacustrine sediments indicate the palaeo-lake was formed during the period from ~30.2 to 32.3 cal. kyr BP, and failed at ~13.2 cal. kyr BP (Hu et al., 2018). The elevation of the dammed lake was 3811 m a.s.l., and its length, maximum water depth, and volume were 185 km, 211 m, and ~22.55 km3, respectively (Hu et al., 2022). The volume of the sediment was ~11.56 km3, which was calculated from the dam location, sediment surface elevation, and the ASTER GDEM2 data. Therefore, the backwater volume was 10.99 km3, and the peak flood possibly exceeded 3.4 × 105 m3/s during the dam failure. The dammed palaeo-lakes in the vallyes downstream of the middle Yarlung Tsangpo River were also discharged during ~13 ka, and they were likely interconnected by hydrological processes. Hence, the failure of the dam and related flooding from the Dazhuka-Yueju gorge probably triggered a chain reaction of dam failures downstream, forming a megaflood. However, the dammed event in the Dazhuka-Yueju valley probably had a limited effect on the landforms at downstream because of the presence of another dammed palaeo-lake in the broad Zetang valley. So the ages of the dammed palaeo-lakes at the middle Yarlung Tsangpo River need to constrained more precisely.
References
Hu, H.-P., Feng, J.-L., Chen, F., 2018. Sedimentary records of a palaeo-lake in the middle Yarlung Tsangpo: Implications for terrace genesis and outburst flooding. Quaternary Science Reviews, 192, 135-148.
Hu, H.-P., Liu, J.-H., Feng, J.-L., Ye, C.-S., Lv, F., Chen, F., Gong, Z.-J., Chen, L.-Q., Du, D.-D., 2022. Geomorphic processes of a dammed palaeo-lake in the middle Yarlung Tsangpo River, Tibet. Science of the Total Environment, 811C, 151949.
Zhang, D.D., 1998. Geomorphological problems of the middle reaches of the Tsangpo River, Tibet. Earth Surface Processes and Landforms, 23(10): 889-903.
How to cite: Hu, H.-P., Feng, J.-L., and King, G.: A dammed palaeo-lake at the middle Yarlung Tsangpo River, Tibet, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6716, https://doi.org/10.5194/egusphere-egu22-6716, 2022.
The Wutach capture is one of the most prominent river captures in central Europe. Subsidence of the Upper Rhine Graben and subsequent incision of tributaries to the Hochrhein led to the piracy of the Danube-Wutach draining the Feldberg region of the Black Forest at ~18 ka. The sudden lowering of the base level led to headward incision of the Wutach and the formation of numerous fluvial knickpoints along the trunk river and its tributaries. These knickpoints represent excellent markers that enable testing several hypotheses including the role of bedrock erodibility, the impact of deglaciation, and mechanisms that control the diversion of the Wutach. Here we present a geomorphometric approach predicated on the stream-power incision model to test these hypotheses. We show that the spatial distribution of knickpoints upstream of the capture is consistent with predictions by the stream-power model. Including proxy information about deglaciation change the parameters of the model, but only slightly increase the fit between modelled and observed knickpoint locations. By comparing estimates of the erodibility derived from knickpoints to those derived from catchment-wide denudation rates in nearby catchments, we observe differences of the order of two magnitudes. The difference in these estimates may can be explained by several processes including channel-bed armouring by sediment. We conclude that stream power parameters derived from the spatial distribution of knickpoints in the Wutach catchment are representative of exceptional (short-term) erosional conditions right after the capture event, rather than of landscape evolution on longer-term geological timescales.
How to cite: Schwanghart, W., Tofelde, S., Scherler, D., Ott, R., Ludwig, A., and Landgraf, A.: Geomorphometric constraints on the development of the Wutach capture, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7356, https://doi.org/10.5194/egusphere-egu22-7356, 2022.
Quantifying the rate and timing of landscape evolution is required to better understand the interaction between tectonic and surface processes and the potential influence of climate change on million-year time scales. Pecube is a 3D thermo-kinematic model capable of predicting low-temperature thermochronometric ages based on variable rock exhumation within an evolving topography driven by tectonic and surface processes. Pecube has been widely used over the past two decades to constrain the timing and rate of relief development, model paleo topographies, establish robust sampling strategies, and track the evolution of glaciated landscapes. Since its initial development by Braun (2003), many new functionalities have been added to Pecube to incorporate lower-temperature thermochronometers (e.g., OSL), lateral advection along faults, and the ability to provide topography evolution scenarios resulting from a surface processes model (SPM). Although widely used, the current version of Pecube (1) still necessitates the use of a non-user-friendly terminal, and (2) lacks sample-specific thermochronometric predictions, including radiation-damage dependent helium diffusion models for (U-Th)/He-based thermochronometers. These two shortcomings may limit the use of Pecube by the community.
Here, we introduce a newly developed graphical user-friendly interface for Pecube, called PecubeGUI, which incorporates new tools intended to clearly guide the user through all model input parameters for all functionalities of Pecube. Among them, the user is now able to simply load topographic files from a digital elevation model (DEM) or a SPM, and interactively set (i) the topographic evolution scenario by direct visual inspection, and (ii) the corresponding steady-state geotherm. PecubeGUI also enables the ability to predict ages in specific locations on a DEM with the use of up-to-date models for helium production and diffusion in apatite. For a single synthetic grain, the user can choose between several diffusion models, and can define zonation. In addition, 4He/3He release spectra can be predicted and compared with observed data.
To illustrate the capabilities of PecubeGUI, we present a case-study that couples a glacial landscape evolution model (iSOSIA, Egholm, 2011) with updated Pecube functionalities. We use a forward modelling approach to assess the capability and sensitivity of apatite (U-Th)/He and 4He/3He thermochronometric data, to constrain the spatial and temporal distribution of glacial erosion at exceptionally high-resolutions in the Rhone valley (Swiss Alps) over multiple Quaternary glaciations. There, apatites show a potential for variable damage accumulation (eU = 12-280 ppm), implying variable single-grain closure temperatures. Previous modelling works suggest glacial incision at 1 Ma (Valla et al., 2011; Valla et al., 2012). With the observed data as constraints, we discuss how single-grain age predictions with detailed production-diffusion models (including the effect of radiation damage), can be used to (1) strategically establish the most effective sampling sites; and (2) constrain the spatial and temporal distribution of glacial erosion at the scale of a landscape, as well as at individual sampling sites.
How to cite: Bernard, M., van der Beek, P., Colleps, C., and Amalberti, J.: PecubeGUI: a new graphical user interface for Pecube, introduction and sample-specific predictions of apatite (U-Th)/He and 4He/3He data in the Rhone valley, Switzerland., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2277, https://doi.org/10.5194/egusphere-egu22-2277, 2022.
High-resolution 4He/3He thermochronometry involves stepped-heat degassing of U and Th-bearing accessory minerals with simultaneous measurement of natural 4He (non-uniform bulk distribution) and synthetically produced 3He (uniform bulk distribution) at each step. The ratio evolution of 4He/3He measured across all heating steps reflects the spatial distribution of 4He within a single crystal, which can be coupled with its (U-Th)/He date to model high-resolution low-temperature thermal histories. Although an exceptionally powerful tool to elucidate disputed drivers of crustal exhumation in various geologic settings (e.g., climatic vs. tectonic mechanisms), the 4He/3He method is commonly hindered by the necessity to uniformly synthesize 3He within crystals at concentrations >1x109 atoms/mg for single grain analysis. This high concentration is required to ensure that the 3He released at initial heating steps—where the most important geological information is preserved—is sufficiently above blank-detection limits of modern, highly-sensitive noble gas mass spectrometers. Synthesis of high 3He concentrations is conventionally achieved via the spallation of targeted nuclei during high-energy proton irradiations to fluences >1x1015 protons/cm2; however, facilities capable of, or willing to, efficiently carry out such anomalously high-fluence irradiations using previously defined methods remain few and far between. Here, we summarize the current state-of-the-art of synthesizing uniform distributions of 3He in geologic materials, and present preliminary 4He/3He measurements on gem-quality Durango apatite using conventional and alternative approaches to induce 3He to sufficient concentrations. Alternative approaches include (1) in-vacuum proton-irradiation with a narrowly focused proton beam to maximize intensities for short-duration experiments, and (2) direct uniform 3He implantation via sample exposure to an energy-modulated 3He beam. We discuss the advantages and disadvantages of both conventional and alternative methods in regards to 3He uniformity, concentration limitations, crystal lattice damage, efficiency, post-experiment ‘cool-down’ times, and accessibility. Both alternative approaches are considerably less demanding on particle accelerator facilities, and can significantly reduce the post-experiment waiting time required to safely handle activated samples. Accordingly, these approaches, if proven successful, yield great promise to improve the accessibility and efficiency of routine 4He/3He analyses for geologic applications.
How to cite: Colleps, C., van der Beek, P., Amalberti, J., and Bernard, M.: Synthesizing uniform 3He concentrations in accessory minerals for 4He/3He thermochronology: Current status, complications, and prospects, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3544, https://doi.org/10.5194/egusphere-egu22-3544, 2022.
We present published bedrock apatite fission track (AFT) and apatite (U-Th)/He (AHe) ages from the Northern Apennines to provide new insights into the spatial and temporal pattern of erosion across the mountain range. The spatial pattern of time-averaged erosion rates derived from AFT ages illustrates similar erosion rates on the Ligurian and Adriatic sides of the range. However, erosion rates derived from AHe ages are higher on the Adriatic side of the range, relative to the Ligurian side. The temporal pattern of time-averaged erosion rates illustrates an overall decrease in erosion through time on the Ligurian side, but suggests an increase in erosion through time on the Adriatic side. These results are corroborated by an analysis of paired AFT and AHe thermochronometer samples, which illustrate that erosion rates have generally increased through time on the Adriatic side, but have decreased through time on the Ligurian side. We infer that such regional scale-differences must be controlled by first-order features of the Northern Apennines, so we present an updated kinematic model to understand what could control these differences. Using imposed erosion rates on the Ligurian side that are a factor of two slower relative to the Adriatic side, we demonstrate that cooling ages and maximum burial depths are able to replicate the pattern of measured cooling ages across the orogen and estimates of burial depth from vitrinite reflectance data. These results suggest that horizontal motion is an important component of the overall rock motion in the wedge, and that the asymmetry of the orogen has existed for at least several million years.
How to cite: Erlanger, E., Fellin, M. G., and Willett, S.: Reexamining the temporal and spatial patterns of exhumation and erosion in the Northern Apennines: new insights from low-temperature thermochronometers, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2342, https://doi.org/10.5194/egusphere-egu22-2342, 2022.
The impact of Quaternary glaciation on rates of erosion and the timing of Alpine valley incision remains disputed. This is mainly due to a lack of geochronological methods that cover the timespan of 103-106 years. While conventional thermochronometers like (U-Th-He) in apatite can constrain timescales of 106 years, and cosmogenic nuclide (CN) dating timescales of 101-103 years, it remains difficult to resolve changes in erosion rates at the timescale of glacial/interglacial cycles. To fill this temporal gap, we develop electron spin resonance (ESR) thermochronometry using both the Al and Ti centres in quartz. The combination of ESR and optically stimulated luminescence (OSL) thermochronometry, as well as numerical modelling approaches, will allow the development of a multi-thermochronometric system to understand rock cooling histories, enabling changes in erosion rates to be related to glacial advance and retreat.
In this study, we focus on the western European Alps, which were intensively glaciated during the Quaternary. Three vertical transects are targeted in the Rhône valley, which is thought to have substantially deepened around 1 million years ago[1]. The first transect consists of seven quartz samples, which were used for (i) optimizing the measurement protocols (i.e., preheat conditions, dose recovery), (ii) analyzing ESR signal growth and thermal stability of the Al and Ti signals to estimate kinetic trap parameters; and (iii) inverting the ESR data to constrain rock cooling histories.
A series of laboratory experiments show the potential of the single aliquot regenerative dose protocol. The Al and Ti signals show similar thermal stability between different samples in the same transect and yield mid-Pleistocene ages. Preliminary inversion of the data shows that the low closure temperatures of the Al and Ti signals in quartz allow the Late Quaternary exhumation of the Alpine valleys to be resolved. Our new ESR thermochronometry results will be supported by OSL thermochronometry measurements, CN dating and also the high density of existing thermochronometric data [e.g. 1] providing new insights into the glacial incision history during the Quaternary and especially how erosion rates varied temporally under a changing climate.
Keywords glacial erosion; landscape evolution; ESR; European Alps
Reference:
[1] Valla, P.G., D.L. Shuster, and P.A. van der Beek. 2011. Significant increase in relief of the European Alps during mid-Pleistocene glaciations, Nature Geoscience. 4(10): p. 688-692.
How to cite: wen, X., Bartz, M., Anderson, L., and King, G.: Deciphering rock cooling histories in the European Alps using ESR and OSL thermochronometry, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8054, https://doi.org/10.5194/egusphere-egu22-8054, 2022.
The Variscan orogeny lasted from the Late Devonian to the Early Permian and resulted in a mountain range whose remnants can be found today in North America, Northern Africa, Europe and Asia. Although the mountain range was almost completely eroded to peneplains in the Permian, today the Variscan massifs (e.g., Appalachians, Massif Central, the Black Forest, Bohemian Massif) feature hilly to mountainous topography with peak elevations exceeding 1500 metres. This indicates surface uplift during the last million years. Clearly, the latest surface uplift is unrelated to the original mountain building phase, but cause, wavelength, timing and rates are still disputed.
Several Variscan massifs are characterised by low relief surfaces, rounded hilltops and graded river profiles with low channel gradients at higher elevations, but deeply incised rivers with migrating knickpoints and steep valley flanks prone to mass wasting at lower elevations near the base level of the receiving streams. This landscape bimodality may indicate temporal and/or spatial variations in uplift rates. Although these massifs have been studied extensively, the driving forces for relief rejuvenation are still unknown.
We investigate relief rejuvenation using two approaches, landscape metrics and low-temperature thermochronometry. This allows us to constrain landscape dynamics on different timescales, with both approaches covering the post-orogenic period. We use the Bohemian Massif as pilot study area, encompassing parts of Germany, Austria, Czech Republic and Poland. First results from the geomorphic analyses allow quantifying the observed landscape bimodality, with highest ksn values at lower reaches of tributaries of the Danube River. Distinct across divide gradients in χ with low χ values on the Danube side indicate that the Danube tributaries feature a higher channel steepness on average than those of the Vltava. Assuming spatially uniform uplift rates and bedrock properties, across-divide gradients in χ may provide evidence for a northward migration of the watershed. In this case, the Danube catchment would grow at the expense of the Vltava catchment.
In addition, we compiled existing cooling ages from the Bohemian Massif to see if similar patterns can be observed on longer timescales. First results show that in the Sudetes in the NE of the Massif, cooling ages found at high altitude areas are predominantly Late Cretaceous, while in lower areas Late to Middle Paleogene cooling ages prevail. South of the Sudetes, in the Austrian Mühlviertel region, this trend seems to be reversed. Local younger ages (late Mesozoic) are found in the higher reaches, while Jurassic cooling ages dominate in the lower sections. However, the relief rejuvenation identified in the geomorphological analysis does not appear to be reflected in this thermochronological data. To reconcile these findings and determine the spatial extent of the different cooling patterns, more low-T thermochronological data is currently processed.
How to cite: Dremel, F., Robl, J., von Hagke, C., and Stüwe, K.: Combining landscape evolution modelling and low T thermochronology to determine the driving forces of relief rejuvenation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11065, https://doi.org/10.5194/egusphere-egu22-11065, 2022.
