Terrestrially derived sediments in sedimentary archives are regularly used to reconstruct past climatic or tectonic conditions. Sediments are generally produced in mountainous areas and transported via sediment routing systems (SRS) to a zone of final deposition. Environmental reconstructions are based on the assumption that perturbations in climatic or tectonic conditions generate signals within the transported sediment. However, experimental and numerical studies have shown that not all signals are faithfully transmitted, but can be modified, buffered or even lost during transport along the SRS. Oftentimes, it is stated that signals can only be faithfully transmitted if the response time of the SRS is short relative to the period of the forcing. However, individual signals in response to a perturbation can already be generated early during the transient response phase. Hence, signals can be transported through and stored within the SRS as a measurable change of a sedimentary parameter (or ‘proxy’) even before the SRS has returned to steady state conditions, i.e. before the characteristic response time has passed. Therefore, it is important to gain quantitative insight into the time scales required for a portion of sediment, which carries a change in proxy information, to travel to the sedimentary archive.
We seek to bring together new concepts and results on sedimentary proxy generation during environmental changes, as well as proxy transport and archiving during the transient state of the SRS. We welcome studies addressing, but not limited to, the following themes:
• Numerical & analog modeling of proxy generation, transport and deposition along sediment routing systems
• Field studies of proxy generation, transport and deposition along sediment routing systems
• Comparison of proxy propagation measured on different grain-size ranges
• Tracing of environmental signals through a system
• Novel proxy methodologies
• The connectivity of sediment routing systems and proxy transport
This session will be supported by a special issue in Frontiers in Earth Sciences.
vPICO presentations: Thu, 29 Apr
Theory suggests that the response time of alluvial-channel systems to perturbations in climate can be related to the magnitude and direction of the forcing, and the length of the system; shorter systems may record a higher frequency of forcing compared to longer systems. The Toro Basin in the Eastern Cordillera of NW Argentina has preserved a suite of alluvial-fan deposits at the toe of the western flanks of the Pascha Range. Farther downstream, a series of cut-and-fill terraces have been linked to 100-kyr climate cycles since ca. 500 ka (Tofelde et al., 2017). The upper basin fan sequence therefore presents an excellent opportunity to explore (1) how climate-induced channel responses may or may not propagate downstream, and (2) the differences in landscape response to forcing frequency as a function of stream length when comparing the upper to the lower basin fan/terrace sequences.
The abandonment ages of eight fan surfaces based on our new 10Be-derived exposure ages of 30 boulders and a 10Be depth profile define two sets of fans: the first set records fan-surface activity and abandonment between ca. 800 and 500 ka, and the second set records activity within the last 100 kyr. By comparing the fan stratigraphy and surface ages with paleoenvironmental records, we can explore whether the complete fan sequence can be explained by the local expression of shifts in regional and global climate.
The older set of fans records an important phase of incision within the basin, punctuated by periods of surface stability and aggradation, between ca. 800 and 500 ka. We argue that this net incisional phase, which has been recognised within other intermontane basins throughout the Central Andes, was likely triggered by prolonged and enhanced global glacial cycles following the Mid-Pleistocene Transition (MPT). A period of relative fan surface stability followed in the upper basin, while 100-kyr cut-and-full cycles persisted downstream, suggesting a disconnect in the behaviour of the two regions.
The younger set of fans reflect higher frequency climate forcing, possibly the result of precessional forcing of climate (ca. 20-kyr timescales) without significant net incision. Within these climatic cycles, fan surface activity can be correlated to periods of enhanced warming and aridity within the Central Andes and the previous global interglacial. The incision and abandonment of these surfaces then coincide with periods of increased humidity from an intensified monsoon, regional glaciation and global cooling. The lack of this high-frequency signal farther downstream provides field support for theoretical predictions of a filtering of high-frequency climate forcing with increasing channel length.
Tofelde, S., Schildgen, T.F., Savi, S., Pingel, H., Wickert, A.D., Bookhagen, B., Wittmann, H., Alonso, R.N., Cottle, J. and Strecker, M.R., 2017. 100 kyr fluvial cut-and-fill terrace cycles since the Middle Pleistocene in the southern Central Andes, NW Argentina. Earth and Planetary Science Letters, 473, p.141-153.
How to cite: Orr, E., Schildgen, T., Tofelde, S., and Wittmann-Oelze, H.: Landscape response to the Mid-Pleistocene Transition (MPT) and higher frequency climate change in the Central Andes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8313, https://doi.org/10.5194/egusphere-egu21-8313, 2021.
The effects of climate change on eroding landscapes and the sedimentary record remains poorly understood. While sediment routing systems at the Earth’s surface should, in principle, record changes in past environmental boundary conditions, the extent to which landscapes are buffered to high-frequency, high-magnitude climate change is contentious. Mountain catchments and alluvial fans offer one way to address this question, as they are accessible sediment routing systems in which source and sink are closely coupled, sedimentation rates are high, and sediment budgets can be closed. Here we consider the extent to which the granulometry of sediment in stream-flow-dominated alluvial fans records changing environmental conditions. We focus on well-constrained field examples in Death Valley, California, such as the Hanaupah Canyon Fan, which have experienced hydroclimate forcing associated with recent glacial-interglacial cycles. Using field-derived measures of grain size, we compare three complementary methods that have recently been used to reconstruct sediment dynamics on alluvial fans. First, we use a self-similarity analysis of sediment calibre to reconstruct sediment mobility on the fan over time. Second, we use a downstream-fining model to evaluate the extent to which different depositional units on the fans record changing sediment fluxes from source catchments. Finally, we adopt a palaeohydrological approach to reconstruct unit discharges, bed shear stresses and instantaneous sediment transport capacities for fans in the study area, based on field measures of hydraulic geometry and grain size. We evaluate the extent to which these three methods provide consistent results, and we quantify the extent to which grain mobility, water and sediment discharge scale with documented variations in the regional climate. Our work shows that alluvial fans are highly sensitive to palaeo-environmental change, but our findings also illustrate the importance of considering which measures of past climate (particularly averages versus variance) are most relevant for landscape responses and sensitivity to climate change.
How to cite: Whittaker, A., Brooke, S., and D'Arcy, M.: Quantifying competing measures of sediment dynamics on alluvial fans: implications for reconstructing past environmental change, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12885, https://doi.org/10.5194/egusphere-egu21-12885, 2021.
The understanding of the sediment routing system and source-to-sink dynamics in a catchment is vital as it helps to assess areas undergoing erosion and deposition. This is significant in catchments which undergo active mining activities especially natural sand materials. The role of climate and natural erosional processes is vital in this as mining of sand is also affected by natural replenishment. In present study, we take a case study of a small catchment of 30km length ~ Chharri, situated in arid landscape of Kachchh of western India. Using geomorphic assemblage mapped using remote sensing and field investigation, we identified natural sub-sinks (depocenters) in the Chharri river valley. The investigation was validated by studying sediment profiles of the depocentral landforms in seasonal time series (pre-monsoon and post monsoon sessions). The changes in morphology, sediments accumulations were integrated to assess the natural sand replenishment in areas which had been undergoing mining activity. Based on time series data it was deduced that the small catchments in dry-land environments, the sand production and dynamics is modulated by type of vegetation, pattern in precipitation and human intervention. The results of such source-to-sink study have long-term implications on sand replenishment, mining activity and landscape evolution of such river basins.
How to cite: Singh, A. K. and Bhatt, N.: Geomorphic variability and sediment dynamics in small catchments of dryland environments: Application into sand mining, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2426, https://doi.org/10.5194/egusphere-egu21-2426, 2021.
Periodic variation in Earth's orbit leads to variation in temperature and precipitation at its surface that are expected to exert a profound influence on landscape evolution. Indeed, cyclical fluctuations in sediment yield and grain size are a ubiquitous feature of the geological record, and recurrence times of geomorphological features such as fluvial terraces and alluvial fans often appear to reflect orbital periodicities. However, making quantitative interpretations of these records requires a detailed understanding of the ways in which sediment is transported from mountainous source regions along alluvial channels to depositional sinks. Sediment transport processes may dampen (i.e. buffer, 'shred') or amplify climate signals, such as changes in channel elevation or sediment flux, and may introduce a lag between them and the responsible external forcing. Recent modelling studies, mostly focused on the potential transmission of climatic signals to sedimentary archives, have predicted a wide range of behaviour and have proven challenging to test in the field. Here, we aim to clarify this discussion and also consider the potential preservation of climatic signals by fluvial terraces along alluvial channels. Our starting point is a recently developed model describing the long-profile evolution of gravel-bed rivers. This model is the first of its kind to be derived from first principles using physical relationships that have been extensively tested in laboratory settings, and takes a non-linear diffusive form. We employ perturbation theory to obtain approximate analytical solutions to the relevant equations that describe how channel elevation and sediment flux vary in response to periodic fluctuations in discharge and sediment supply. Our solutions contain expressions for response amplitudes and lag times as functions of downstream distance, system 'diffusivity' and forcing frequency. Lag times can be a significant fraction of the forcing period, implying that care is required when interpreting the timings of terrace formation in terms of changes in discharge or sediment supply. We also show that at the onset of periodic forcing, or a change in the dominant forcing period, alluvial channels undergo a transient response as they adjust to a new quasi-steady state. Importantly, this result implies that suites of fluvial terraces can be preserved without the need for significant local base-level fall. Since the expressions presented here are defined in terms of fundamental properties of alluvial channels, they should be readily applicable to real settings.
How to cite: McNab, F., Schildgen, T., Turowski, J., and Wickert, A.: Responses of gravel-bed rivers to periodic climate change, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7904, https://doi.org/10.5194/egusphere-egu21-7904, 2021.
Climate change and tectonics can generate signals in a source-to-sink system in the form of changing sediment supply. The study of the propagation of this signal through the system might elucidate how different source-to-sink systems respond to a given perturbation, for instance, the Early to Middle-Pleistocene climate transition. Knowledge on the temporal and spatial responses to such perturbations in a catchment is still limited. Previous studies, with the support of landscape evolution models, demonstrate that several thousands of years might be needed for an extreme-climate-transition-induced signal to propagate through a river catchment (an example of source-to-sink system). The present work aims to contribute to the understanding of how such systems might react when submitted to rapid climate change events by studying the Meuse river catchment. The primary goals are to characterize and quantify the main controls on sediment flux of this fluvial system as a response to the Early to Middle Pleistocene climate transition as well as to assess how climate signals propagated through this source-to-sink system during the last four glacial-interglacial cycles, starting around 450.000 years ago.
To achieve our goals, three main tasks are proposed. In the first stage of this project, with the support of high-resolution DEM and high-resolution sedimentary cores, the different Meuse fluvial terrace maps are updated. For that, a new cross-border fluvial terrace map between the Netherlands, Belgium and Germany is produced. Characterization and mapping of sediment grain-size and provenance is also carried out. The new Meuse terrace map will guide the sampling campaign of Meuse terrace sediments. The samples will be used for cosmogenic-nuclide age-dating of the sampled terraces. Two dating methods will be used depending on how deeply buried and well-preserved the terraces are: burial isochrone (26Al/10Be) where sediment cover thickness is greater than 4,5 – 5 m, and depth profile (10Be) when the terrace surface is well preserved. These methods will be applied to specific terrace steps, in order to date those around the Mid-Pleistocene transition. Beryllium-10 age-dating will possibly also be applied to specific sedimentary levels (cores, outcrops), in order to infer averaged denudation rates and, consequently, the sediment fluxes, during the investigated climatic cycles. During the latter part of the project, all the data will be set in a temporal framework using the cosmogenic dating results and existing age controls.
How to cite: da Silva Guimarães, E., van Balen, R. T., Kasse, C., Busschers, F. S., and Bouroullec, R.: Impacts of climatic changes on fluvial sediment fluxes in north western Europe: The Middle and Late Pleistocene Meuse river system, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10234, https://doi.org/10.5194/egusphere-egu21-10234, 2021.
River discharge variability is a fundamental control on fluvial morphodynamics and, in principle, stratigraphic architecture. The ability to quantitatively constrain discharge variability from fluvial stratigraphy would newly enable us to reconstruct instantaneous or interannual responses of rivers to climatic perturbation in the geologic past. However, the extent to which we can extract quantitative information about discharge variability from fluvial stratigraphy is currently unknown. Recent experimental work indicates that preserved cross-set geometries can potentially be used to inform formative flow conditions and durations. However, to date, this has not been tested on field examples of ancient fluvial systems. Here we use detailed measurements of cross-sets to assess bedform kinematics and formative flow conditions in fluvial strata of three Late Cretaceous geologic formations: the Blackhawk Formation, Castlegate Sandstone, and Ferron Sandstone, which crop out in central Utah, USA.
Unanimously low coefficients of variation (CV) in preserved cross-set heights of 0.25–0.5 are consistent with the hypothesis that CV<<0.88 arises from preservation of bedforms in disequilibrium conditions, which typically occurs during rapid flood recession in a “flashy” flood hydrograph. Bedform preservation in disequilibrium conditions requires that formative flow durations are shorter than bedform turnover timescales. We reconstruct median turnover timescales of 2–3 days, with a 10–90 percentile range of ~1–10 days, which implies that formative flow durations were of order hours to a few days. These durations are consistent with storm-related flood durations in perennial discharge regimes, as opposed to the more sustained flood durations that are typical of subtropical/monsoonal climate regimes. However, it is also possible that this same CV signature (CV<<0.88) can be achieved simply by the presence of morphodynamic hierarchies, e.g. concurrently migrating bedforms and bars. We explore whether it is possible to disentangle the relative role of formative flow conditions and morphodynamic hierarchies on bedform preservation using our field data, models of flood stratigraphy, and estimates of bedform preservation ratios. Moreover, we identify future steps that will further our ability to quantitatively extract formative flow variability and, ultimately, discharge variability from the rock record.
How to cite: Lyster, S., Whittaker, A., Hajek, E., Ganti, V., and Allison, P.: River discharge variability in the rock record? Disentangling the relative role of flow variability and morphodynamic hierarchies on bedform preservation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13384, https://doi.org/10.5194/egusphere-egu21-13384, 2021.
There are two main statements of a long-accepted paradigm of fluvial morphodynamics formalized inter alia by S.A. Schumm: (i) changes in fluvial systems strongly depend on exceeding external and / or internal thresholds and (ii) they are always characterized by a nonlinear and asynchronous character. While the first aspect of this paradigm is part of numerous studies on fluvial morphology and river dynamics, the second aspect has so far tended to be sidelined in practical geomorphology.
With particular respect to the field of paleo-environmental research, this is evident from studies that aim at determining ages of Pleistocene river terraces in order to provide a time frame for the reconstruction of paleo-environmental conditions. Typically, numerical dating approaches are only applied to a single location that is supposed to be exemplary for the respective terrace level. Numerical ages determined for this specific location are then extrapolated and interpreted in a generalizing way to derive "THE age" of the river terrace as a whole.
With respect to the concept of asynchronism of fluvial reactions to environmental changes, such an approach seems problematic. In fact, asynchronism implies different sections of a river showing different and specific reactions to environmental changes at a given point in time. For fluvial terraces, this means that the processes controlling their formation may already have started in some sections of a valley, while in other sections they do not yet have any impact on landscape evolution.
In this contribution, we present luminescence ages of fluvial deposits originating from an Upper Pleistocene river terrace in a small valley located in the headwater of the Main River, Germany. Here, several samples from various locations throughout the river longitudinal course have been analysed. The luminescence ages determined for the lowermost part of the valley are significantly older than those from the middle section, which in turn are older than those from the valley’s upper reaches.
Our results suggest a diachronic alignment of sedimentation ages for fluvial deposits, starting with old ages close the mouth of a river and getting progressively younger for locations approaching the upper reaches. If these findings are confirmed in other fluvial systems and are not only the result of specific local conditions, they will be of great relevance for geomorphological research in fluvial landscapes. As a result, the widespread approach of deriving age estimates for fluvial terraces from numerical results merely determined for a single location appears to be inadequate and should be subjected to a critical review.
How to cite: Kolb, T. and Fuchs, M.: Diachronism as process-inherent part of Pleistocene river terrace formation: First results based on luminescence dating for testing a well-established theoretical concept and possible implications for practical field work, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10429, https://doi.org/10.5194/egusphere-egu21-10429, 2021.
The Schwalbenberg Loess-Palaeosol-Sequences (LPS) in the Middle Rhine Valley, Germany, comprise unprecedented complete records of Upper Pleistocene terrestrial ecosystem response to global climate changes. However, direct correlation of the Schwalbenberg geochemical signals with climate archives of supra-regional northern hemispheric relevance remains complicated. This is due to the complex interplay of pre-, syn-, or post-depositional processes that left their traces in the terrestrial record. In particular, the use of different element ratios to derive weathering indices may be complicated as dust sources change through time, and as ecosystems respond to changing conditions. In this study, we decode interfering geochemical signatures and re-evaluate proxies, commonly applied, regarding their suitability and meaning for understanding the evolution of the Schwalbenberg LPS. We undertake a systematic approach, firstly dividing the 30 m long Schwalbenberg REM3 LPS according to our core description. In a second step, we integrate LOG-ratios indicative of provenance shifts, sediment reworking dynamics and weathering into multivariate analysis. We apply Principle Component Analyses (PCA) and Linear Discriminant Analysis (LDA) to datasets comprising sediments deposited under similar environmental conditions. In doing so, we sensitively quantify subordinate processes and conditions, such as the impact of varying source- and weathering-signals in all proxies. Our results show that in particularly K/Rb and Mg/Ca ratios contain a strong provenance signal in loess deposits, whereas the Ca/Ald ratio (Ald: dithionite extractable) best indicates the maturity state of Gelic Gleysols. Integration of our filtered datasets with a high-resolution age model enables direct correlation of the variability of principal components on sub-millennial scales with Atlantic-driven climate oscillations. More specifically, PC2 appears to reflect changes in mineral dust accumulation and indicates increasing dust input in response to climate cooling towards the end of interstadials, highlighting the accretionary nature of the Schwalbenberg LPS during transitional periods from interstadial to stadial depositional modes.
How to cite: Vinnepand, M., Fischer, P., Zeeden, C., Schulte, P., Fiedler, S., Jöris, O., Hambach, U., Fitzsimmons, K., Prud'homme, C., Péric, Z., Schirmer, W., Lehmkuhl, F., and Vött, A.: Decoding geochemical signals of the Schwalbenberg Loess-Palaeosol-Sequences — A key to Upper Pleistocene terrestrial ecosystem responses in western Central Europe , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10532, https://doi.org/10.5194/egusphere-egu21-10532, 2021.
Landscapes have the ability to transmit environmental signals or inhibit them. The mechanisms by which landscapes do this are largely unquantified, but is probably due to the ability of landscapes to transiently store and release sediment which acts as a medium for energy to propagate. Previous experiments using 1D avalanching rice piles suggest that stochastic collapses can overprint, or shred, periodic sedimentary signals (Jerolmack and Paola (2010), as measured using mass efflux from the experimental rice pile. Jerolmack and Paola (2010) defined a threshold for successful surface signal propagation: Tx, where signals with a period less than Tx are shredded, unless the magnitude of the signal is sufficiently large. We aim to utilise the rice pile to further investigate signal propagation across a landscape, and the thresholds for this, by quantifying inter-particle interactions and the mechanics of how signals propagate using a quasi-2D rice pile model, built using MFiX-DEM code. This open source, physics model utilises individual particles which compose the solid phase whilst treating the fluid as a continuum. The rice grains in the model are represented by spherical particles, where each individual particle, or cluster of particles, can be tracked through each time step using a coordinate axis system, allowing internal dynamics, such as avalanche sizes and sediment residence times, to be quantified. To certify the model replicates the self-organised nature of an experimental rice pile, sensitivity tests were performed by systematically changing two key parameters controlling grain interactions: the friction coefficient and the coefficient of restitution, alongside the sediment flux. To calibrate the results of the sensitivity analysis, mass efflux through time and the corresponding power spectra are compared to real experimental rice pile results and statistical rice pile models. It is hoped this work will provide fundamental insights into how a signal propagates through a landscapes, and how they are shredded in the process.
How to cite: Griffin, C., Higham, J., Duller, R., and Straub, K.: Tracking signal propagation through landscapes using a granular avalanching system, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12905, https://doi.org/10.5194/egusphere-egu21-12905, 2021.
The assertion that stratigraphy will store environmental signals, such as sediment flux signals related to paleoclimate and tectonics, is debatable because that same stratigraphy can also store signals of autogenic processes that overprint and replace allogenic signals (“shred”). To establish the likelihood that strata will contain allogenic signals, the focus should be on quantifying autogenic processes. Models show that stratigraphic storage of allogenic sediment flux signals will only take place if it exceeds a threshold condition set by autogenic processes. This is supported by experimental and numerical models but its validation is hindered by low spatio-temporal resolution of stratigraphic datasets. We address this by reformulating a theoretical framework that dispenses with the need for exquisite temporal resolution. To demonstrate the applicability of our approach we explore the potential for environmental signal propagation and preservation in two ancient field systems: a small Pleistocene delta in Greece and a larger Eocene sediment routing system in the Spanish Pyrenees. This work demonstrates how short-term system dynamics can be integrated with long-term basin dynamics to provide a framework that assesses the capacity of sedimentary systems to store environmental signals.
How to cite: Duller, R., Toby, S., De Angelis, S., and Straub, K.: Quantifying environmental signal propagation and preservation in ancient sediment routing systems using field data, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15896, https://doi.org/10.5194/egusphere-egu21-15896, 2021.
The early Eocene was a period of the intense collision during the formation of the Pyrenees. The flexural response to loading of the overriding European plate led to the formation of an elongated foredeep on the subducting Iberian plate which connected westward to the Atlantic Ocean. A thrust salient formed in the central Pyrenees, where Mesozoic Cover units travelled southwards on top of Triassic salt detachment. This process resulted in the sequencing of the foreland basin in different isolated sub-basins such as the Ripoll basin in the East, the Tremp-Graus and Ainsa-Jaca basins in central and western south Pyrenees and the Ager basin located south of the Tremp-Graus basin. The precise timing and surface processes associated to this reorganization of the sedimentary routing system remains not totally understood. Indeed, various sedimentary provenance studies show that the sediments of the Tremp-Graus basin were sourced from a different catchment zone than those of the Ager basin. Besides, the Ripoll basin sediments provenance analysis shows major similarities with the Ager basin, suggesting a common catchment area in the Eastern Pyrenees. However, it has been pointed out that the clastic systems feeding the rapidly subsiding sink of the Ripoll through could not find their way towards the shallower Ager basin. In this PhD project we aim at providing further constraints to the paleogeographic reconstruction and sediment routing systems of the South Eastern Pyrenees in the light of a revised chronostratigraphic scheme. A Source-to-Sink approach will be followed to study the sediment Routing Systems and to decode the climatic and tectonic signal from the sedimentary record. It will follow a volumetric quantification of the sediment budget over the entire foreland, and a comparison with eroded rock volumes of the whole Pyrenees. The resulting revised scenario will seek conciliation of all available data from the stratigraphic, structural, petrologic, geochronologic and sedimentologic datasets with new radiogenic isotopes sedimentary provenance analysis.
How to cite: Juvany, P. and Garcés, M.: Sediment flux across the south-Pyreneean foreland basin. A contribution to the S2S-Future network. , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12796, https://doi.org/10.5194/egusphere-egu21-12796, 2021.
The formation of large-scale coarsening- and thickening-upward sedimentary sequences is commonly related to mountain belt evolution, where tectonic, lithological or climatic controlled changes (e.g. Whittaker et al., 2010; D’Arcy et al., 2017) have a major influence on the large-scale architecture of the adjacent foreland basin. Furthermore, as proposed by Lane (1955), trends in stratal patterns are also controlled by variations in hydrological conditions such as sediment and water discharge. These changes affect the dynamics of alluvial systems, which are commonly preserved by grain size and stacking patterns. A relation between grain size trends and sediment flux has been in the focus of previous research (e.g. Schlunegger and Castelltort, 2016) yet with a limited dataset only. Here, we extend this approach and investigate whether there exists a direct correlation between grain size patterns and shifts in sediment flux through time at the scale of an entire basin.
The Swiss Molasse basin, our study area, includes three major depositional systems situated in eastern, central and western Switzerland. These systems have been active during Oligocene- to Miocene-times between 31 and 13 Ma (Schlunegger et al., 1997; Kempf et al., 1999). We collected grain size data from several km-thick conglomerate sequences along 16 individual sections using state-of-the-art techniques and compared grain size patterns with published data of sediment flux to the Molasse basin (Kuhlemann et al., 2001).
Preliminary results reveal that from 31 to 21 Ma, both, the median grain size (D50) and the D84 percentiles show an increase from 4.4±0.8cm to 4.8±0.1cm (D50, +10%), and from 8.0±1.4cm to 9.5±1.2cm (D84, +20%). Simultaneously, sediment flux to the entire Molasse basin increased from c. 6’000 km3/Myr at 31 Ma to c. 23’000 km3/Myr at 21 Ma recording almost a fourfold increase (Kuhlemann et al., 2001). A substantial flux reduction at 20 Ma to c. 17’000 km3/Myr was followed by strong fluctuations, reaching c. 15’000 km3/Myr at 13 Ma. However, compared to the peak at 21 Ma, this corresponds to a 30% decrease in sediment discharge. In contrast, during the same time interval, the grain size data record a 20% increase in D50 from 3.7±0.5cm to 4.5±0.4cm and a 40% increase in D84 from 6.2±1.0cm to 8.7±1.0cm, respectively.
The preliminary results thus show that grain size patterns are positively correlated to shifts in sediment flux during Oligocene-times (31 to 21 Ma), but weak to no correlations between sediment flux and grain size trends were observed during Miocene-times (20 to 13 Ma). We interpret this data as showing that surface erosion and topographic development occurred in pace with the geological driving forces such as ongoing continent-continent collision during Oligocene-times. In contrast, we tentatively suggest that the landscape entered a transient stage after 20 Ma when large-scale tectonic exhumation started to interfere with the fluvial processes in the Alpine hinterland, thereby shifting the Alpine topography in a transient state.
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How to cite: Garefalakis, P., Whittaker, A. C., Hermann, J., Mair, D., and Schlunegger, F.: Do grain size patterns from Oligo-Miocene Swiss Molasse sequences reflect shifts in sediment flux through time?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3269, https://doi.org/10.5194/egusphere-egu21-3269, 2021.
Sedimentary archives are the main source of information for climatic and tectonic changes in Earth’s history. We investigate how the tectonically triggered early (28±1 Ma) exhumation of the Tauern Window in the Eastern European Alps is recorded in the Oligocene/Miocene Northern Alpine Foreland Basin (NAFB) in Upper Austria. We combined Nd isotopic compositions on clay-sized bulk sediment and of sand-sized single-grain apatites with additional trace-element geochemistry, fission track (AFT) and U-Pb dating to investigate the timing of when this tectonic signal reaches the sediment archive within these different grain-size fractions.
This well-investigated basin offers an excellent opportunity to investigate environmental signal propagation. From ~27 to 19 Ma, a deep-marine basin-axial, gravity-flow dominated channel controlled the West to East directed sediment transport in the Upper Austrian NAFB. The sediments were sourced in the Eastern and Central Alps. At 19 Ma, channel sedimentation ceased and clinoforms prograded from the southern margin northward into the basin. This change in sediment-routing direction cut off the Central Alpine sediment source.
Drill cuttings of one well on the northern basin slope and drill cores from 12 wells were sampled for clay and the single-grain analysis. Clay eNd values remain stable around -9.7 (±0.5) from 27 to 19 Ma but increase afterwards to -8.7 (±0.2) at 18.3 Ma. In contrast, apatite single-grain results significantly change at 23.3±0.3 Ma from a domination of apatites from low-grade (<upper amphibolite-facies) metamorphic sources, Permo-Mesozoic and late Variscan U-Pb ages and AFT ages >40 Ma to a domination of high-grade metamorphic apatites with late Variscan U-Pb ages and an increasing number of AFT ages <30 Ma. The high-grade metamorphic apatites have slightly more positive eNd values (-2.2 ± 3.9) than the low-grade metamorphic apatites (-4.4 ± 4.2).
The changes in the single-grain data sets have been previously interpreted to mirror the exposure of a new Upper Austroalpine nappe as a consequence of the ongoing early Tauern Window exhumation. The total signal lag time between the beginning of the exhumation and the arrival of the signal within the apatite assemblage in the sedimentary archive is therefore 3.4 to 6 Myrs. The clay eNd values do not record this change in provenance at 23.3±0.3 Ma as they stayed stable until 19 Ma when they increased slightly. This might point towards a delayed recording of the provenance change revealed at 23.3±0.3 Ma by the apatites. The difference in signal recording is caused by the characteristics of the applied methods. Whereas single-grain distributions of orogen-wide sediment-routing systems can be dominated by geographically small areas with high erosion rates and high mineral fertility, bulk-rock methods integrate over the entire drainage area, a process that diminishes extreme values. The disconnection of the Central Alpine sediment source at 19 Ma, increased the relative proportion of the more positive eNd values of the Upper Austroalpine Nappe in the drainage area, leading to an increase in clay eNd values in our data set. Our results show that different information from the hinterland is recorded in the different grain-size fractions and methods.
How to cite: Hülscher, J., Sobel, E. R., Kallnik, N., Hoffmann, J. E., Millar, I. L., and Bernhardt, A.: Selective recording of a tectonic forcing in different grain-size fractions in the Oligocene/Miocene Eastern Alpine Molasse Basin, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14581, https://doi.org/10.5194/egusphere-egu21-14581, 2021.
The Vienna Basin is a rhombohedral SSW-NNE oriented Neogene extensional basin that formed along sinistral fault systems during Miocene lateral extrusion of the Eastern Alps. The basin fill consists of shallow marine and terrestrial sediments of early to late Miocene age reaching a thickness of 5500 m in the central part of the basin. The early Pannonian was a crucial time in the development of the Vienna Basin, as It coincided with the formation of Lake Pannon. The lake formed at 11.6 Ma when a significant regressive event isolated Lake Pannon from the Paratethys Sea, creating lacustrine depositional environments. At that time the delta of the Paleo-Danube started shedding its sediments into the central Vienna Basin. Based on an existing age model delta deposition commenced around 11.5 Ma and continued until 11.1 Ma. These subsurface deltaic deposits of the Hollabrunn-Mistelbach Formation represent the coeval fluvial deposits of the Paleo-Danube in the eastern plains of the North Alpine Foreland Basin. Therefore, the Palaeo-Danube represents an extraordinary case in where coeval fluvial and deltaic deposits of a Miocene river are continuously captured.
This study provides an interpretation of depositional architecture and depositional environments of this delta in the Austrian part of the central Vienna Basin based on the integration of 3D seismic surveys and well data. The mapped delta has an area of about 580 km2, and solely based on the geometry we classify the delta as a mostly river – dominated delta with significant influence of wave – reworking processes. For seven time slices paleogeographic maps are created, showing the interplay between the lacustrine environments of Lake Pannon, delta evolution and fluvial systems incising in the abandoned deltaplain. Onlaps between single deltalobes indicate a northward-movement of the main distributary channel. Approximate water-depth estimates are carried out with in-seismic measurements of the true vertical depth between the topset deposits of the delta and the base of the bottomset deposits. These data suggest a decrease of lake water depth from about 170 m during the initial phase of delta formation at 11.5 Ma to about 100 m during its terminal phase at 11.1 Ma. A major lake level rise of Lake Pannon around 11.1 Ma caused a flooding of the margins of the Vienna Basin, resulting in a back stepping of riverine deposits and termination of delta deposition in the study area.
How to cite: Borzi, A., Piller, W. E., Harzhauser, M., Siedl, W., and Strauss, P.: The Evolution of the Paleo-Danube Deltas of the Lower Pannonian in the Vienna Basin , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4070, https://doi.org/10.5194/egusphere-egu21-4070, 2021.
The nature and contribution of flexural isostatic compensation to subsidence and uplift of passive margin deltas remains poorly understood. We performed a series of simulations to investigate flexural isostatic responses to high frequency fluctuations in water and sediment load associated with climatically-driven sea-level changes. We use a parallel basin and landscape dynamics model, BADLANDS, (an acronym for BAsin anD LANdscape DynamicS) that combines erosion, sedimentation, and diffusion with flexure, where the isostatic compensation of the load is computed by flexural compensation. We model a large drainage basin that discharges to a continental margin to generate a deltaic depocenter, then prescribe synthetic and climatic-driven sea-level curves of different frequencies to assess flexural response. Results show that flexural isostatic adjustments are bidirectional over 100-1000 kyr time-scales and mirror the magnitude, frequency, and direction of sea-level fluctuations, and that isostatic adjustments play an important role in driving along-strike and cross-shelf river-mouth migration and sediment accumulation. Our findings demonstrate that climate-forced sea-level changes set up a feedback mechanism that results in self-sustaining creation of accommodation into which sediment is deposited and plays a major role in delta morphology and stratigraphic architecture.
How to cite: Morón, S., Blum, M., Salles, T., Frederick, B., Farrington, R., Ding, X., Mallard, C., Mather, B., and Moresi, L.: Isostasy amplifies relative sea-level change on continental-scale deltas, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13678, https://doi.org/10.5194/egusphere-egu21-13678, 2021.
Leaf wax n-alkanes are well known higher plant biomarkers. These molecules are widely found in geological archives, where their concentration, average chain length (ACL) and isotopic composition (δ13Calkanes and δ2Halkanes) serve as proxies for changes in continental vegetation and hydroclimate. While mechanistic relationships of these proxies with climate and vegetation are relatively well understood, little is known about the transport of those biomarkers into geological archives. In marine sedimentary records, leaf wax biomarkers are often interpreted to represent the contiguous continental catchments, but few studies have examined the fidelity with which n-alkanes in marine sediments record the corresponding continental conditions.
Here we assess the variability of n-alkane composition in terrestrial and marine sediments from 26°S to 41°S along the Chilean coast. The sample sites are located along a strong vegetation and precipitation gradient, with precipitation ranging from 25 to 2300 mm/yr. We evaluate riverbed sediments from twenty catchments, draining the western slopes of the Andes to the Pacific Ocean and compare the values to the ones of 19 samples from marine surface sediments recovered directly offshore each catchment.
The correspondence between terrestrial and marine n-alkanes signals changes along the precipitation gradient. Where precipitation rates range between 100 to 500 mm/yr, ACL and δ2Halkanes values agree well between continental and adjacent marine sediments. At precipitation rates below 100 mm/yr, ACL and δ2Halkanes values recorded in marine sediments are consistently lower than the ones found in continental sediments. At precipitation rates higher than 500 mm/yr, ACL and δ2Halkanes values registered in marine sediments are consistently higher than in the adjacent catchments.
Multiple factors, including mixing of sediment from different sources along the transport pathway from continent to ocean or variability in catchment storage capacity, likely cause marine n-alkane composition to be offset from their terrestrial source. Nevertheless, the consistent change in behavior along the gradient suggest that precipitation is the dominant factor on the transmission of n-alkane signals along the sedimentary routing systems of the study area. Moreover, since correspondence decreases at high and low precipitation, our data suggest that the sensitivity of the leaf wax biomarker proxy in marine archives towards hydrological change may be subdued due to sedimentary integration. This may have implications for quantitative interpretations derived from n-alkanes and their isotopes in marine paleorecords.
How to cite: Gaviria-Lugo, N., Läuchli, C., Wittmann, H., Bernhardt, A., Frings, P., and Sachse, D.: Assessing the fidelity of leaf wax signals in marine sediments: n-alkane sensitivity to change along a precipitation gradient, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10323, https://doi.org/10.5194/egusphere-egu21-10323, 2021.
Active continental margins play a major role in exporting terrestrial sediment to the oceans. Yet, the response of a sediment-routing system to a specific climate change is poorly quantified. The terrestrial-to-marine sediment-routing systems on West Sumatra can be used as a natural laboratory to quantify these changes. Rivers efficiently deliver sediment from the high-relief Barisan mountains to the enclosed marine forearc Mentawai Basin along this active subduction margin. The Holocene climatic history of the region is well known and shows an abrupt precipitation increase at the Pleistocene-Holocene transition (~12 ka BP) as recorded previously in δ18O time-series measured on planktonic foraminifera.
Here we analyze multiple late Quaternary sediment records spanning the last 14 ka from the Mentawai Basin, offshore Sumatra. We use volumetric sediment budgets and multivariate statistics on high-resolution elemental compositions to assess changes in sediment flux and composition based on ~150 km of high-resolution sub-bottom sediment profiles (covering 435 km²) and six sediment cores. Radiocarbon ages were taken at every ~35 cm within the sediment cores.
Our results, from sediment budget calculation and sedimentation rates within the cores, show that absolute rates of sediment supply to the marine forearc basin remained constant throughout the Holocene (542-566 g/m²/yr, 68% quantile, 0.1 and 0.2 cm/yr, respectively). However, the sediment composition varies drastically with comparable patterns all over the basin. The compositional changes are characterized by an 8000-year damped oscillation between terrestrial clastic (Al, Si, Ti, Zr, Fe, K, Rb) and marine authigenic (Ca, Sr) sediment supply. From ~12 to 8 ka BP, the relative amount of clastic rock-forming elements is increasing, then decreasing from ~8 to 4 ka BP, and finally increasing again from 4 ka BP to the present. Using a dynamical system modeling approach, we link the beginning of this oscillation to the abrupt precipitation increase at the interglacial transition at ~ 12 ka BP. We interpret the oscillation to reflect a persistent erosional feedback between physical and chemical weathering caused by ongoing landscape adaptation to the abrupt Pleistocene-Holocene transition. The total amount of sediment accumulation, however, does not follow this trend. Thus, the sedimentary composition shows ongoing landscape transience, whereas the uniform sediment accumulation rates imply a landscape in steady state.
These results have two major implications for future landscape evolution: a) the time scale of landscape transience far exceeds the time scale of abrupt climate transition, b) in this study, the sediment flux to the ocean floor does not mirror landscape transience because the excess terrestrial clastic sediment is compensated by decreased deposition of marine carbonates. These implications call for the implementation of compositional changes of exported sediments into landscape evolution models since so far most landscape models focus on the amount of exported sediment.
How to cite: Mosser, S., Bernhardt, A., Mohtadi, M., and Schwenk, T.: Transient landscape response to abrupt precipitation increase at the Pleistocene-Holocene transition, offshore Sumatra, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6237, https://doi.org/10.5194/egusphere-egu21-6237, 2021.
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