SSP2.7

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.

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.

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/Al_d ratio (Al_d: 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.

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.

D’Arcy et al., 2017. Sedimentology, 64, 388-424.

Kempf, O. et al., 1999. Int. J. Earth Sci., 88(2), 253-275.

Kuhlemann et al., 2001. Tectonophysics, 330, 1-23.

Lane, 1955. ASCE Proceedings, 81(745), 1-15.

Schlunegger et al., 1997. GSA Bulletin, 109(2), 225-241.

Schlunegger and Castelltort, 2016. Sci. Rep., 6, 1-11.

Whittaker et al., 2010. Basin Res., 22, 809-828.

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.

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.