GM2.9

EDI
Dates, Rates and Bytes: Quantifying Geomorphological Processes and Landscape Dynamics

Numerical frameworks are essential for understanding and interpreting landscape evolution. Over recent decades, geochronological techniques such as luminescence dating, cosmogenic nuclides and thermochronology have improved in accuracy, precision and temporal range. Developments in geochronological methods, data treatment and landscape evolution models have provided new insights into the timing, duration and intensity of landscape evolution processes. This combination of temporal constraints and numerical models has enormous potential to improve our understanding of landscape evolution. The session aims to bring together new research in using geochronology and data science to investigate Quaternary landscape change.

This session includes studies of erosional rates and processes, sediment provenance, burial and transport times, bedrock exposure or cooling histories, landscape dynamics, and the examination of potential biases and discordances in geochronological data and model-data comparisons. We welcome contributions that apply novel geochronological techniques, combine different geochronological techniques, combine numerical age constraints with landscape evolution models to determine rates and timing of landscape change as well as stochastic events, or that highlight the latest developments and open questions in the application of geochronometers to landscape evolution problems.

Convener: Ann Rowan | Co-conveners: Georgina King, Sebastian Kreutzer, Duna Roda-BoludaECSECS, Christoph Schmidt
vPICO presentations
| Wed, 28 Apr, 15:30–16:15 (CEST)

vPICO presentations: Wed, 28 Apr

Chairpersons: Ann Rowan, Christoph Schmidt
15:30–15:35
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EGU21-3513
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solicited
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Highlight
Greg Balco

This abstract describes a project to make large data sets of cosmogenic-nuclide measurements useable for synoptic global analysis of paleoclimate, glacier change, and landscape change. It is based on the 'ICE-D' (Informal Cosmogenic-nuclide Exposure-age Database), a transparent-middle-layer infrastructure for compiling and storing cosmogenic-nuclide measurements and generating internally consistent exposure-age data. The prototype implementation of this project focuses on a global data set of exposure ages from glacial deposits that are, potentially, useful for synoptic analysis of glacier change and paleoclimate. The aim is to address a number of messy data-management and analysis problems associated with cosmogenic-nuclide data, thus making it possible to apply unbiased, automated quantitative analysis to the entire globally-distributed data set. The presentation will highlight (i) examples of error-tolerant hypothesis testing using this approach; (ii) means of quantifying the importance of the details of cosmogenic-nuclide production-rate calculations to global paleoclimate inferences, and (iii) likewise, approaches to understanding the importance of geomorphic processes and landform evolution to global paleoclimate inferences drawn from exposure-dated landforms.

How to cite: Balco, G.: Synoptic analysis of globally-distributed data sets of cosmogenic-nuclide exposure ages, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3513, https://doi.org/10.5194/egusphere-egu21-3513, 2021.

15:35–15:37
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EGU21-12424
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ECS
Romano Clementucci, Paolo Ballato, Lionel Siame, Ahmed Yaaqoub, Abderrahim Essaifi, Laëtitia Leanni, Valery Guillou, and Claudio Faccenna

Rock erodibility plays a central role in setting topographic limits on relief development and is a key parameter in landscape evolution models. However, channel bed erodibility (K) is usually either fixed arbitrary or let varying over a wide range of values (10-12 – 10-3) because it is difficult to estimate. The topography of ancient orogens offers favourable conditions to quantify bedrock erodibility through the stream profile analysis, because the channel steepness is directly related to rock erodibility rather than rock uplift or climate variability.

The Anti-Atlas is a Variscan (Paleozoic) orogen of NW Africa that has not been drifted for long distances over the late Cenozoic and hence has not experienced an extended shift across climatic zones. Furthermore, it is characterized by a well preserved uplifted relict landscape with rather uniform erosion rates since at least the last 120 - 100 Ma. This specific configuration allows studying in detail landscape erosional dynamics and erodibility.

Here, we combine geomorphic analysis of stream profiles with in situ-produced cosmogenic concentrations (10Be) in river sediments, to decipher the surface evolution of the AntiAtlas and the adjacent Siroua Massif. In the Anti-Atlas, the basin-wide denudation rates determined for the relictal part of the landscape range between 5 and 20 m Ma-1, consistent with rates estimated from the volume of volcanics eroded from the Siroua Massif during the last 12 - 10 Ma (10 to 20 m Ma-1). The close agreement of short- and long-term erosion rates suggests a steady state landscape.

Our results demonstrate the main role of rock-type on sustaining post-orogenic landscape. Specifically, we find a striking correlation between erosion rates and normalized channel steepness per different rock-types. This allows estimating the erodibility within a narrower range of values (10-7 - 10-4) as a function of the reference concavity values of the river network.

How to cite: Clementucci, R., Ballato, P., Siame, L., Yaaqoub, A., Essaifi, A., Leanni, L., Guillou, V., and Faccenna, C.: Does lithology control post-orogenic topography and rock erodibility? Insights from Anti-Atlas of Morocco, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12424, https://doi.org/10.5194/egusphere-egu21-12424, 2021.

15:37–15:39
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EGU21-6651
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ECS
Anna-Maartje de Boer, Wolfgang Schwanghart, Jürgen Mey, Jakob Wallinga, Basanta Raj Adhikari, and Tony Reimann

Mass movements play an important role in landscape evolution of high mountain areas such as the Himalayas. Yet, establishing numerical age control and reconstructing transport dynamics of past events is challenging. To fill this research gap, we investigated the potential of Optically Stimulated Luminescence (OSL) dating and tracing methods. OSL dating analyses of Himalayan sediments is extremely challenging due to two main reasons: i) the OSL sensitivity of quartz, typically the mineral of choice for dating sediments younger than 100 ka, is poor, and ii) highly turbid conditions during mass movement transport hamper sufficient OSL signal resetting prior to deposition which eventually results in age overestimation. In this study, we aim to bring OSL dating to the test in an extremely challenging environment. First, we assess the applicability of single-grain feldspar dating of mass movement deposits in the Pokhara valley, Nepal. Second, we exploit the poor bleaching mechanisms to get insight into the sediment dynamics of this paleo-mass movement through bleaching proxies. The Pokhara valley is a unique setting for our case-study, considering the availability of an extensive independent radiocarbon dataset (Schwanghart et al., 2016) as a geochronological benchmark.

Single-grain infrared stimulated luminescence signals were measured at 50°C (IRSL50) and post-infrared infrared stimulated luminescence signals at 150°C (pIRIR-150). As expected, results show that the IRSL50 signal is better bleached than the pIRIR150 signal. A bootstrapped Minimum Age Model (bMAM) is applied to retrieve the youngest subpopulation to estimate the palaeodose. However, burial ages calculated based on this palaeodose overestimate the radiocarbon ages by an average factor of ~8 (IRSL50) and ~35 (pIRIR150). This shows that dating of the Pokhara Formation with our single-grain approach was not successful. Large inheritances in combination with the scatter in the single-grain dose distributions show that the sediments have been transported prior to deposition under extreme limited light exposure which corresponds well with the highly turbid nature of the sediment laden flood and debris flows that emplaced the Pokhara Formation.

To investigate the sediment transport dynamics in more detail we studied three bleaching proxies: the percentage of grains in saturation (2D0 criteria), percentage of well-bleached grains (2σ range of bMAM-De) and the overdispersion (OD). Neither of the three bleaching proxies indicate a spatial relationship with run-out distances of the mass movement deposits. We interpret this as virtual absence of bleaching during transport, which reflects the catastrophic nature of the event. While single-grain feldspar dating did not provide reliable burial ages of the Pokhara mass movement deposits, our approach has great potential to provide insight in sediment transport dynamics of high-impact low-frequency mass movement events in mountainous region.

References

Schwanghart, W., Bernhardt, A., Stolle, A., Hoelzmann, P., Adhikari, B. R., Andermann, C., ... & Korup, O. (2016). Repeated catastrophic valley infill following medieval earthquakes in the Nepal Himalaya. Science, 351(6269), 147-150.

How to cite: de Boer, A.-M., Schwanghart, W., Mey, J., Wallinga, J., Raj Adhikari, B., and Reimann, T.: Insight into the sediment dynamics of a high-impact low-frequency mass movement event using single-grain feldspar luminescence in the Pokhara valley, Nepal, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6651, https://doi.org/10.5194/egusphere-egu21-6651, 2021.

15:39–15:41
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EGU21-289
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ECS
Marijn van der Meij, Arnaud Temme, Mark Verhoeven, and Tony Reimann

Agricultural systems are subject to severe land degradation, because anthropogenic erosion processes, such as tillage erosion, substantially increase erosion rates compared to natural settings. Optically Stimulated Luminescence (OSL) dating is often used to measure the age of depositional layers to quantify rates of landscape change. OSL dating in agricultural systems is however challenging, because the deposits are reworked in the tillage layer even long after their moment of deposition. This post-depositional mixing resets the built-up luminescence signal, which causes an offset between the apparent OSL ages and the actual deposition ages.

In this study we illustrate the effect of post-depositional mixing on geochronological OSL age-depth data from northeastern Germany and we developed tools to un-mix depositional and post-depositional ages. We analyzed 32 OSL samples from five locations in a kettle hole to reconstruct spatial and temporal deposition patterns. We were able to correct our chronologies for post-depositional mixing by tillage by accounting for (pre-)historical plough regimes. Next to these empirical data, we also modified a Soil-Landscape Evolution Model called Lorica to numerically simulate the effect of post-depositional mixing on depositional ages. This combination of measurements and simulations enabled us to constrain the spatial and temporal effects of post-depositional mixing on OSL age-depth data more accurately. This is an important step towards getting a better grip on the dynamics of agricultural landscapes including the associated dates and rates.

How to cite: van der Meij, M., Temme, A., Verhoeven, M., and Reimann, T.: Un-mixing the effect of post-depositional tillage turbation on OSL age-depth data through measurements and numerical simulations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-289, https://doi.org/10.5194/egusphere-egu21-289, 2021.

15:41–15:43
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EGU21-1721
Kathryn Fitzsimmons, Peter Fischer, Zoran Peric, Maike Nowatzki, Susanne Lindauer, and Andreas Vött

Loess – a homogeneous, predominantly silt-sized aeolian sediment – has long been recognised as a valuable terrestrial record of past environmental conditions. Loess deposits drape some 10% of the Earth’s land surface, accumulating almost continuously in some regions. Most aeolian dust is thought not to travel far, often deriving from fine-grained material transported by rivers from glaciated regions. The provenance of loess sediment is inferred from the trajectories of atmospheric circulation systems and how these may have changed in intensity and influence over a region through time. The most frequently used techniques for correlating aeolian dust deposits with likely source areas, including bulk geochemistry, age distributions of detrital zircons, and Sr-Nd isotope ratios in clays, remain limited in the information they may provide about loess provenance. Since loess is dominated by silicate minerals – namely, quartz and feldspars – it is advantageous to explore their potential as indicators of source changes within loess-paleosol sequences (LPS). Increasingly, researchers have been exploring variations in the luminescence characteristics of sedimentary quartz and feldspar as possible provenance tools. Of a range of approaches so far applied, luminescence sensitivity is the quickest to measure and provides a means to rapidly assess potential changes in sediment source down LPS.

Luminescence sensitivity – the signal intensity per absorbed radiation dose – arises from the efficiency of charge traffic between traps and luminescence centres within a crystalline framework. In a sedimentary context, sensitivity is the product of interplay between source lithology and the history of the mineral in question. Consequently, shifts in sediment provenance may be observed through variations in luminescence sensitivity down LPS. Despite the presence of thick loess deposits across Europe, however, this approach has yet to be tested on this continent.

Here we undertake an empirical investigation of the luminescence sensitivity characteristics of quartz and feldspar from different grain-size fractions at the Schwalbenberg LPS in the German Rhine valley. The Schwalbenberg LPS has recently been shown to respond to variability in Atlantic-driven climate oscillations in fine detail; it follows, therefore, that changes in source will likely be recorded in its sediments. We test the potential of luminescence sensitivity as an indicator of changes in sediment source through time, comparing samples from a 30 m core (REM3) spanning the last full glacial cycle, with samples of oxygen isotope stage (OIS) 3-2 age exposed within a c. 6 m profile on the southern margins of the deposit. The temporal overlap of the two localities during OIS3 enables comparison of luminescence characteristics with respect to possible provenance during that timeframe; we find an inverse relationship between quartz and feldspar sensitivity, as well as variability in sensitivity between different quartz grain sizes. There is some indication that feldspar sensitivity increases during periods of soil formation down the core. These observations may suggest source variability over millennial timescales.

How to cite: Fitzsimmons, K., Fischer, P., Peric, Z., Nowatzki, M., Lindauer, S., and Vött, A.: Luminescence sensitivity of German loess: indicators of source variability, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1721, https://doi.org/10.5194/egusphere-egu21-1721, 2021.

15:43–15:45
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EGU21-4514
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ECS
Anne Guyez, Stephane Bonnet, Tony Reimann, Sébastien Carretier, and Jakob Wallinga

Enlightenment of sediments pathways and storage patterns within river systems is critical to apprehend sediment transfer at the Earth’s surface and landscape response to tectonics and climate. Because direct tracing methods (painted, fluorescent or magnetic sediments) are of limited use in terms of their analytical resolution in time and space, alternative physico-chemical methods suitable for larger spatial-temporal scales have been developed (e.g. cosmogenic isotope, detrital thermochronology, isotopic geochemistry, etc). The study of the natural luminescence of sediment particles is emerging for this purpose and seems promising for providing new information complementary to existing methods. This method is based on the quartz/feldspar grains ability to store energy while buried below the Earth’s surface and to emit lumen with light exposure. Some recent studies have used this property to solve geomorphological questions regarding particle fluxes in soil or fluvial systems (Reimann et al., 2017; Sawakuchi et al., 2018) and to quantify rock exhumation (e.g. Herman et al., 2010). Here, we present an experimental testing of an innovative single-grain luminescence-based approach on feldspars. Focusing alongstream the Rangitikei River (RR), New Zealand, we carried out analysis on both modern sediment and Holocene terraces deposits.

We based our analysis on two complementarians proxies, the paleodose estimated using the bootstrapped minimum age model (Cunningham and Wallinga, 2012) and the percentage of grains eroded from bedrock and re-deposited in the river without signal resetting, i.e. saturated grains. We document changes in the luminescence signature of fluvial sediments while the RR evolves in response to uplift and climate change; from a late Pleistocene-early Holocene braided system to a Holocene incising canyon that subsequently widen.  This allows us to appraise temporal changes in the alongstream contribution of canyon flanks landsides to sediment supply to the river. Overall, we show that distinct landscape dynamics gives distinct luminescence signatures.

How to cite: Guyez, A., Bonnet, S., Reimann, T., Carretier, S., and Wallinga, J.: Imprint of landscape dynamics in the luminescence signal of fluvial sediments (Rangitikei River, NZ), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4514, https://doi.org/10.5194/egusphere-egu21-4514, 2021.

15:45–15:47
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EGU21-6721
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ECS
Aditi Krishna Dave, Alida Timar-Gabor, Johan De Grave, Dimitri Vandenberghe, Saida Nigmatova, and Kathryn E. Fitzsimmons

The entrainment, transport and deposition of wind-blown (aeolian) sediments is vital to our understanding of the links between the resulting depositional landforms and climate dynamics in a region. The nature of aeolian processes has varied over Quaternary timescales, and is dependent on changing sediment supply, existing topography and climatic parameters such as wind strength and moisture availability, at local to supra-regional spatial scales [1]. Hence, in order to understand long-term interactions between aeolian-dominated landscapes and climate, there is a need to investigate the mechanisms driving wind-blown processes. In this context, the Ili basin of southeast Kazakhstan, with its extensive piedmont loess deposits and desert dunefields, provides an excellent case study for investigating aeolian earth-surface processes and their links to climate.   

Quartz is one of the most abundant and robust minerals on the earth’s surface. The lattice defects and impurities in quartz provides a potential provenancing tool, which can be probed by luminescence and electron spin resonance (ESR) techniques [2,3]. Here we investigate the luminescence sensitivity and ESR signature of various paramagnetic centres (E’, peroxy and [AlO4]0) in quartz of different grain sizes from Pleistocene loess, as well as from surface sediments and rocks collected from various depositional and potential source contexts across the Ili basin. We observe significant difference between fine and coarse grain quartz samples from various depositional settings which highlights the difference in entrainment and transport processes for different grain sizes. Our observations allow us to assess the validity of hypothesised aeolian depositional models as well as the more recent back-trajectory models for dust transport [1] in the region, thus, establishing an empirical understanding of modern aeolian earth-surface processes. Based on these results, we extend our understanding of modern processes to investigate past aeolian dynamics. Our data provides first empirical insights into the origin of fine-grained sediments along the piedmont and what these can tell us about climate dynamics in Central Asia.  

 

References:
[1] Fitzsimmons, K.E., Nowatski, M., Dave, A.K., Harder, H., 2019. Intersections between wind regimes, topography and sediment supply: perspectives from aeolian landforms in Central Asia. Palaeogeography, Palaeoclimatology, Palaeoecology 540, 109-531.
[2] Gray., H.J., Jain, M., Sawakuchi, A.O, Mahan, S.A., Tucker, G.E. 2019. Luminescence as a Sediment Tracer and Provenance Tool. Reviews of Geophysics 57 (3), 987-1017
[3] Toyoda, S., Nagashima, K., Yamamoto, Y., 2016. ESR signals in quartz: Applications to provenance research – A review. Quaternary International 397, 258-266.

How to cite: Dave, A. K., Timar-Gabor, A., De Grave, J., Vandenberghe, D., Nigmatova, S., and Fitzsimmons, K. E.: Reconstructing aeolian transport processes using luminescence and electron spin resonance characteristics of quartz grains: A case study from the Ili basin, SE Kazakhstan, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6721, https://doi.org/10.5194/egusphere-egu21-6721, 2021.

15:47–15:49
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EGU21-1714
Alida Timar-Gabor, Aditi Dave, Zuzanna Kabacińska, and Kathryn Fitzsimmons

The sediment-routing concept [1] aims to integrate tectonic fluxes and climatically driven erosion, an approach that is at the core of modern studies into Earth-surface processes. The concept relies on the potential to track individual mineral grains from source to sink. Provenance studies are instrumental in this respect; until recently, almost all of these have focussed on accessory minerals. By contrast, the durability and abundance of quartz ensures that parent rocks containing quartz are represented by detrital quartz in their daughter sediment. Even the purest quartz crystal contains a vast number of point defects, which may be either intrinsic or due to impurities. Some of these defects remain unchanged under ionising radiation bombardment by natural environmental radioactivity, while others are transformed, generally in the form of charge trapping. Based on the dynamics of some of these radiation-sensitive defects under irradiation, quartz is frequently used for dating by luminescence or electron spin resonance (ESR). Another, less explored, application of these defects is the fingerprinting of sediment sources. For provenance applications to be successful, sedimentary quartz signals used should match the corresponding signals of quartz from the host rocks: they should remain unchanged during transport and/or weathering processes.

Here we conduct an exploratory study on fine (4-11 μm) quartz from loess from Central Asia (Kazakhstan and Tajikistan), a region dominated by westerly air transport. These study sites were chosen since recent studies based on geochemical fingerprinting, grain-size modelling and meteorological reanalysis suggest the contribution from different source areas [2,3]. We investigate the signature of E’ (≡Si·, an unpaired electron at an oxygen vacancy site) and peroxy intrinsic defect centers (≡Si-O-O·and ≡Si–O· non-bridging oxygen) using ESR, by measuring both quartz grains extracted from both untreated samples, and from samples irradiated to 2000 Gy and subsequently heated to 10 min at 350 °C (as suggested by Toyoda and others [4]). By investigating the dose response of these signals, with and without the application of thermal treatments, we conclude that natural E` signals hold great promise for provenance studies, thus considerably simplifying the currently used measurement protocols. We observe a significant difference between the E’ and peroxy signals between the Kazakh and Tajik samples, which is in tune with the hypothesis that the two loess sites derive from different dust sources.

References

1. Allen, P.A., 2008. From landscapes into geological history. Nature 451, 274-276.

2. Li, Y., Song, Y., Fitzsimmons, K., Chen, X., Wang, Q., Sun, H., Zhang, Z., 2018. New evidence for the provenance and formation of loess deposits in the Ili river basin, Arid Central Asia. Aeolian Research 35, 1-8.

3. Li, Y., Song, Y., Kaskaoutis, D.G., Chen, X., Mamadjanov, Y., Tan, L., 2019. Atmospheric dust dynamics in southern Central Asia: Implications for buildup of Tajikistan loess sediments. Atmospheric Research 229, 74-85.

4. Toyoda, S., 2011. The E1` centre in natural quartz: its formation and applications to dating and provenance reserarches. Geochronometria, 38 (3), 242-248.

How to cite: Timar-Gabor, A., Dave, A., Kabacińska, Z., and Fitzsimmons, K.: Investigations on different measurement protocols of E` paramagnetic defect centres in quartz for provenance studies, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1714, https://doi.org/10.5194/egusphere-egu21-1714, 2021.

15:49–15:51
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EGU21-6630
Sebastian Kreutzer, Sascha Meszner, Christoph Schmidt, Tobias Lauer, Melanie Bartz, Mathieu Duval, Moncef Bouaziz, Christopher-Bastian Roettig, Ulrich Hambach, and Dominik Faust

The terrestrial dust archives around Matmata (Tunisia) are unique in their morphological setting and grain-size composition (cf. Faust et al., 2020a, b). Located in front of the Grand Erg’s parlour in a critical zone at the northern edge of the Saharan desert, up to 35 m thick plateau-like loess accumulations cover pre-existing landscapes. In conjunction with intercalated palaeosols, the sandy loess, or rather loess like sediment records, tapped fluctuations in aeolian dynamics related to rapid and large-impact climate boundary shifts. Some of them may have severely threatened local ancient cultures, and future changes may put modern settlements and agriculture projects in this region at risk. Palaeolandscape reconstruction, supported by reliable chronologies, helps us to chart the past landscape, assess today's dynamics,  and maybe predict possible future scenarios.

The ‘desert-loess’ records around Matmata seem to engulf a wide temporal range back to Marine Isotope Stage (MIS) 9. Trapped charge dating techniques, such as luminescence and electron spin resonance (ESR) dating, are versatile tools to decipher the timing of past landscape changes. However, for archives such as the one in the neighbourhood of Matmata, conventional luminescence methods (e.g., optically stimulated luminescence, OSL) exceed reported temporal limits. Kreutzer et al. (2018) have convincingly shown that a multi-method approach, using infrared radiofluorescence (IR-RF) and OSL in conjunction with ESR dating, has good potential to tackle long-term landscape dynamics. Our contribution reports first trapped charge dating results from Matmata in Tunisia. We provide preliminary luminescence (IR-RF, OSL) and ESR dating results from seven different sites and discuss the challenges encountered during our methodological work. Finally, we attempt to link our findings to regional climate fluctuations and drainage alterations observed for the large endorheic salt lakes in the Matmata plateau's close purlieu.

References

Faust, D., Kreutzer, S., Trigui, Y., Pachtmann, M., Mettig, G., Bouaziz, M., Recio Espejo, J.M., Diaz del Olmo, F., Schmidt, C., Lauer, T., Rezek, Z., Fülling, A., Meszner, S., 2020a. New findings of Middle Stone Age lithic artifacts from the Matmata loess region in southern Tunisia. E&G Quaternary Sci. J. 69, 55–58. doi:10.5194/egqsj-69-55-2020

Faust, D., Pachtmann, M., Mettig, G., Seidel, P., Bouaziz, M., Recio Espejo, J.M., Diaz del Olmo, F., Roettig, C.-B., Kreutzer, S., Hambach, U., Meszner, S., 2020b. Sandy soils in silty loess: the loess system of Matmata (Tunisia). Quaternaire 31, 175–186. doi:10.4000/quaternaire.14217

Kreutzer, S., Duval, M., Bartz, M., Bertran, P., Bosq, M., Eynaud, F., Verdin, F., Mercier, N., 2018. Deciphering long-term coastal dynamics using IR-RF and ESR dating: A case study from Médoc, south-West France. Quaternary Geochronology 48, 108–120. doi:10.1016/j.quageo.2018.09.005

How to cite: Kreutzer, S., Meszner, S., Schmidt, C., Lauer, T., Bartz, M., Duval, M., Bouaziz, M., Roettig, C.-B., Hambach, U., and Faust, D.: Deciphering Past Desert-Margin Dynamics in Matmata, Tunisia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6630, https://doi.org/10.5194/egusphere-egu21-6630, 2021.

15:51–15:53
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EGU21-9067
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ECS
Andrea Madella, Christoph Glotzbach, and Todd A. Ehlers

Detrital tracer thermochronology exploits the relationship between bedrock thermochronometric ages and elevation to study spatial variations of upstream erosion from the distribution of detrital grain ages. If ages increase linearly with elevation and analytical uncertainties are negligible, spatially uniform erosion is expected to yield a detrital age distribution that mirrors the catchment’s hypsometric curve. Alternatively, a mismatch between detrital and hypsometric distributions may indicate non-uniform erosion within a catchment. For studies of this sort, measured age populations rarely exceed 100 grains, because applying thermochronology is time consuming and expensive. With such limited sample sizes, discerning between two detrital age distributions produced by different catchment erosion scenarios may be statistically impossible with high confidence. However, there is no established method to quantify the sample-size-dependent uncertainty inherent to detrital tracer thermochronology. Here, we investigate how sample size affects the uncertainty of detrital age distributions and how such uncertainty affects the ability to uniquely infer the erosional pattern of the upstream area. We developed a new tool to consistently report confidence levels as a function of sample size and case-specific variables. The proposed tool will be made available as open-source script along with test data. Testing the hypothesized erosion scenarios will help tracer thermochronologists define the minimum sample size (i.e. number of grain ages) to answer their specific scientific question with high level of statistical confidence. Alternatively, in cases of unavoidable small sample size, the related confidence level can be quantified.

How to cite: Madella, A., Glotzbach, C., and Ehlers, T. A.: How many grains do we need for tracer thermochronology?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9067, https://doi.org/10.5194/egusphere-egu21-9067, 2021.

15:53–15:55
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EGU21-13071
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ECS
Sabrina Kainz, Lon Abbott, Rebecca Flowers, and James Metcalf

Past work has used the Southern Rocky Mountains (SRM) in the U.S. state of Colorado to illustrate the important role that rock strength plays in the histories recorded by the apatite fission track (AFT) and apatite (U-Th)/He (AHe) low-temperature thermochronometers (Flowers & Ehlers, 2018). The SRM were initially raised during the Laramide Orogeny, ca. 70-45 Ma, but consensus exists that the region also experienced a later, post-Laramide exhumation event. Flowers & Ehlers (2018) pointed to the low erosion potential of the Precambrian crystalline basement rocks that crop out in most SRM ranges as a primary reason for the abundance of 55-70 Ma “Laramide” AFT and AHe dates in the region, compared to a paucity of younger dates that would presumably be produced through erosion triggered by the post-Laramide exhumation event. South-central Colorado offers a test of this hypothesis, due to lateral variations in rock erodibility provided by the presence here of both sedimentary and crystalline Laramide ranges and adjacent sedimentary basins. The combination of our ongoing AHe study with previous south-central Colorado AFT and AHe work reveals kilometer-scale post-Laramide (Oligo-Miocene) exhumation has occurred in areas that possess thick sedimentary rock sequences whereas exhumation has been negligible where crystalline basement comprises the land surface. 

South-central Colorado’s Sangre de Cristo Mountains consist of an imbricate stack of thrust sheets composed of Permian sedimentary rock. About 30 km farther east stand the Wet Mountains, another Laramide range – but one composed of Precambrian basement rock. The Raton Basin, a SRM foreland basin filled with 2 km of synorogenic fill underlain by a thick sequence of marine shale, lies south and east of the two ranges. The Wet Mountains thus form a peninsula of strong crystalline rock surrounded by more erodible sedimentary rocks to the west, south, and east. 

Our study and that of Landman (2018) records at least 2 km of erosion in the Raton Basin east and south of the Wet Mountains since 25 Ma. Lindsey et al (1986) obtained 24-15 Ma AFT dates from the Paleozoic sedimentary rocks of the Sangre de Cristo Mountains, demonstrating that kilometer-scale Oligo-Miocene exhumation occurred just west of the Wet Mountains. By contrast, Kelley and Chapin (2004) obtained only pre-Laramide AFT ages between 228-110 Ma for 17 samples of Precambrian basement from the crest of the Wet Mountains. A 32 Ma ash flow tuff unconformably overlies Precambrian basement on Greenhorn Mountain, the Wet Mountains’ highest and southernmost peak. Its presence reinforces the conclusion, based on the AFT dates, that Oligo-Miocene erosion of the Wet Mountain massif has been minimal simultaneous with kilometer-scale exhumation to the west, south, and east. These results illustrate the important role that rock strength plays in determining the dates recorded in low-temperature thermochronologic studies.

How to cite: Kainz, S., Abbott, L., Flowers, R., and Metcalf, J.: Effect of rock strength on exhumation and the thermochronologic record: The south-central Colorado example, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13071, https://doi.org/10.5194/egusphere-egu21-13071, 2021.

15:55–15:57
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EGU21-2608
Marco G. Malusà, Owen A. Anfinson, and Daniel F. Stockli

Detrital thermochronologic analyses are increasingly employed to develop quantitative models of landscape evolution and constrain rates of exhumation due to erosion. Crucial for this kind of application is a correct discrimination between thermochronologic ages that record cooling due to exhumation, i.e., the motion of parent rocks towards Earth’s surface, and thermochronologic ages that record cooling independent from exhumation, as expected for example in volcanic and shallow-level plutonic rocks. A suitable approach for the identification of magmatic crystallization ages is provided by double dating, which combines for example U–Pb and (U–Th)/He analyses of the same mineral grain. Magmatic zircon crystallized from volcanic or shallow-level plutonic rocks should display identical U–Pb and (U–Th)/He (ZHe) ages within error, because of rapid magma crystallization in the upper crust where country rocks are at temperatures cooler than the partial retention zone of the ZHe system. Conversely, zircon grains crystallized at greater depth and recording cooling during exhumation should display ZHe ages younger than the corresponding U–Pb ages. These latter ZHe ages may constrain the long-term exhumation history of the source rocks according to the lag-time approach, provided that a range of assumptions are properly evaluated (e.g., Malusà and Fitzgerald 2020). Here, we explore the possibility that detrital zircon grains yielding ZHe ages younger than the corresponding U–Pb ages may record country-rock cooling within a contact aureole rather than exhumation. To tackle this issue, we applied a double-dating approach including U-Pb and ZHe analyses to samples of the Gonfolite Group exposed south of the European Alps. The Gonfolite Group largely derives from erosion of the Bergell volcano-plutonic complex and adjacent country rocks, and its mineral-age stratigraphy is extremely well constrained (Malusà et al. 2011, 2016). Analyses were performed in the UTChron Geochronology Facility at University of Texas at Austin. For U-Pb LA-ICPMS depth-profile analysis, all detrital zircon grains were mounted without polishing, which allowed for subsequent ZHe analysis on the same grains. Zircon for ZHe analyses were selected among those not derived from the Bergell complex or other Periadriatic magmatic rocks, as constrained by their U-Pb age. We found that ca 40% of double-dated grains, despite yielding a ZHe age younger than their U-Pb age, likely record cooling within the Bergell contact aureole, not exhumation. These findings have major implications for a correct application of the lag-time approach to detrital thermochronology and underline the importance of a well-constrained mineral-age stratigraphy for a reliable geologic interpretation.

Malusà MG, Villa IM, Vezzoli G, Garzanti E (2011) Earth Planet Sci Lett 301(1-2), 324-336

Malusà MG, Anfinson OA, Dafov LN, Stockli DF (2016) Geology 44(2), 155-158

Malusà MG, Fitzgerald, PG (2020) Earth-Sci Rev 201, 103074

How to cite: Malusà, M. G., Anfinson, O. A., and Stockli, D. F.: Double dating (U–Pb and (U–Th)/He) of detrital zircon from the Gonfolite Group (European Alps) and implications for the lag-time approach to detrital thermochronology, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2608, https://doi.org/10.5194/egusphere-egu21-2608, 2021.

15:57–15:59
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EGU21-10288
Sergei Medvedev, Jan Inge Faleide, and Ebbe Hartz

Morphology evolution of the Barents Shelf is a key question in understanding how erosion and glaciation reshape the face of the Earth. The Cenozoic history is a subject of a longstanding debate in the Barents realm, in part, due to decades of petroleum exploration in the region. We address here the whole-region study of the influence of a set of mechanisms and factors on the erosion and sedimentation estimations. Several local studies along the edges of the Barents shelf enlighten the relation between sediments accumulated off-shelf and amount of erosion in the adjacent areas. There are only few studies of the entire Barents region but precision of these studies is limited due to uneven distribution of measurements and uncertainties in paleo-conditions. We compare the masses of sediments accumulated along the edges of the Barents shelf with erosion predicted in Henriksen et al. (2011) and estimate that erosion is significantly overestimated. Local corrections to this erosional model do not bring balance close. The major part of the erosional estimate is based on seismic methods and well logs reflecting sedimentary rock’s compaction changes caused by (now partly removed) load from above. This load, however, may be caused not by eroded material alone, but also by ice cap during the glacial cycles. Reduction of erosional estimates by accounting for ice load bring balance between existing erosional model and accumulated sediments close. We also model the glacial erosion using the numerical approach erosion backward in time (Medvedev et al., 2018). The method was modified for this study to account for difference in the lateral length scale of on- and off-shore erosion and flexural isostasy. We compare this erosional model with estimated masses of glacial-induced sediments off-shelf the Barents Sea. The results performed for a range of controlling parameters show that the Barents shelf was mainly submarine at the beginning of glaciation.

Henriksen, E., Bjørnseth, H., Hals, T., Heide, T., Kiryukhina, T., Kløvjan, O., Larssen, G., Ryseth, A., Rønning, K., and Sollid, K., 2011, Uplift and erosion of the greater Barents Sea: impact on prospectivity and petroleum systems. Geological Society, London, Memoirs 35, 271-281.

Medvedev, S., Hartz, E. H., and Faleide, J. I., 2018, Erosion-driven vertical motions of the circum Arctic: Comparative analysis of modern topography. Journal of Geodynamics 119, 62-81.

How to cite: Medvedev, S., Faleide, J. I., and Hartz, E.: Cenozoic reshaping of the Barents Shelf: Influence of erosion, sedimentation, and glaciation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10288, https://doi.org/10.5194/egusphere-egu21-10288, 2021.

15:59–16:15