GM10.1

The topographic history of an orogen results from the interactions of climate and tectonics, and it can be reconstructed from the sedimentary record of its peripheral basins. Previous tectonostratigraphic studies, including flexural models, and sparse stable oxygen and carbon isotope data from the South-Pyrenean foreland basin suggest that a major period of topographic growth occurred in the late Paleocene-early Eocene. To further test this hypothesis, we present a stack of 658 stable isotope measurements on whole-rock marine carbonate mudstone from a 4800-m-thick composite sedimentary succession which provides a 12 Ma continuous record of environmental conditions during the early to middle Eocene (54 to 42 Ma). From the base of this record (at 54 Ma), oxygen isotopes  (δ¹⁸O values) show a faster decrease rate than the coeval global negative excursion associated with the Early Eocene Climatic Optimum (EECO). This local alteration of the global δ¹⁸O signal indicates that topographic growth during this period, associated with significant tectonic activity, perturbed the oxygen isotopic composition of foreland waters. Thus, our data suggest that significant topographic uplift of the Pyrenean orogen started from at least 54 Ma and continued until ca. 49 Ma, reaching the maximum elevations of 2000±500m in this phase from previous isotope and flexural studies. In addition, our record shows that the long-term carbon stable isotope composition during this period remained relatively stable with no similarity to the global bell-shaped long-term trend of the EECO. This is consistent with the restricted physiography of the South-Pyrenean foreland basin, mainly influenced by local sedimentary and water inputs. Overall, the Pyrenean topographic growth from the Late Cretaceous to the Miocene displays several growth stages that seem to be primarily determined by episodes of an increased rate of tectonic plate convergence. The duration of these growth stages (several millions of years) is possible documentation of the response time of mountain ranges to tectonic perturbations. The results of this work, therefore, demonstrate that stable isotope measurements on whole-rock sediments in foreland basins can provide key information for tectono-climatic and topographic reconstructions of mountain ranges.

How to cite: Castelltort, S., Honegger, L., Adatte, T., Spangenberg, J. E., Poyatos-More, M., Ortiz, A., Curry, M. E., Huyghe, D., Puigdefebregas, C., Garces, M., Vinyoles, A., Valero, L., Laeuchli, C., Nowak, A., Fildani, A., and Clark, J. D.: Oxygen stable isotopes signals of the early Eocene growth of the Pyrenees: implications for steady-state and response time of mountain ranges, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1771, https://doi.org/10.5194/egusphere-egu22-1771, 2022.

Foreland basins record the evolution of orogens through sedimentary accumulation and recycling and, as such, represent unique archives of the evolution of orogenic systems. Foreland basins are, however, complex source-to-sink systems responding to the uplift of the mountain range, thrusting, eustasy, climate, and the type of lithologies eroded. The respective contributions of these parameters has been numerically evaluated, the influence of the inherited geometries of the foreland on the sediment routing systems has not yet been investigated.

We use a Landscape Evolution Model (FastScape) to explore the effect of varying foreland paleo-topography on its stratigraphic architecture. Models consist of a half mountain range steadily uplifting (0.5 mm/yr) over 25 Myrs. Eroded material is transported and deposited in a foreland domain and a distal open marine domain. We present 4 setups with varying paleo-topographies in the foreland domain: an initial flat foreland at sea level (M1), an elevated flat continental foreland (+300 m, M2), a pre-existing 1 km-deep and 100 km-wide depression at the foot of the growing orogen with either a flat forebulge at sea level (M3) or an elevated forebulge (+300m; M4).

Our result show that an elevated foreland domain produces, after 25 Myrs, a thinner foreland basin because the faster and efficient sediment export of sediment out of the foreland to the open marine domain reduces the sedimentary load and, in doing so, the flexure. In contrast, a pre-existing depression at the foot of the range, produces a thicker foreland basin because the additional load filling the initial space increases the flexural response. We show that a pre-existing bathymetry is required to preserve marine deposits in the foreland basin. In our experiments, the landscape after 25 Myrs cannot be used to infer the initial foreland geometry as the initial foreland influences is smoothed out after ~10-15 Myrs. The stratigraphic architectures of the foreland basin are nonetheless different. In addition, we show that emplacement of an alluvial fan at the foot of the range results in a transient drop in erosion rate in the range by locally increasing the base-level (autogenetic feedback).

How to cite: Gérard, B., Rouby, D., Huismans, R., and Robin, C.: Impact of inherited geometries on syn-orogenic foreland basin, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2781, https://doi.org/10.5194/egusphere-egu22-2781, 2022.

Anorogenic reliefs (plateaus and plains) made up about 70% of the total emerged reliefs on Earth. They are characterized by nearly flat erosional surfaces upstream bounded by escarpments, called pediments/pediplains. Africa’s landforms are constituted by very long (several thousands of kilometers) wavelength relief of broad “basins” (depressions) and swells (Holmes, 1944) possibly controlled by mantle dynamics. This "basin" and swell pattern caused Africa to be consisted of numerous endorheic and exorheic systems. Consequently, the Nile River, the longest river on Earth and main object of this study, crosses today a set of two former endorheic systems (Ugandese and Sudanese “Basins”) and one exorheic system (Egyptian Margin) along its courses to the Mediterranean Sea. Our objective is here to unravel the paleorouting systems of the Nile River through relief growth, tectonic, and climate since the uppermost Cretaceous.

Several generations of stepped pediments, proxies of relief growth, were characterized and mapped on DEM and satellite images and dated using their geometrical relationships with dated magmatic rocks. To better constrain periods of relief uplift and the deformation wavelength through time, the stratigraphic record of the sedimentary basins located in between two swells were studied using biostratigraphy and sequence stratigraphy. The originality of the approaches is to integrate data of all the source-to-sink (S2S) systems to produce a coherent scenario of the evolution of the Nile.

We proposed the following model for the evolution of the Nile River. First, a main large pediplain is formed during the uppermost Cretaceous (?75-66 Ma), acting as a base of any kind of landforms that would be formed afterwards, bounded westward by the Darfur-Ennedi crest. Second, a major marine flooding during late Paleocene time (58-57 Ma) reached a subtle high bounding northward the endorheic Sudanese “Basin”. Third, Pre-Eonile started to form during the uppermost Eocene (~37 Ma) with a divide limited to the Egyptian Margin. Fourth, the Eonile was incised during late Miocene (~10 Ma) at time of a major uplift at the scale of north-east Africa. Finally, the Nile captured first the Sudanese endorheic system in the Early Pliocene (~4 Ma) and altered the Ugandese one in the Middle-Late Pleistocene (less than 1 Ma).

How to cite: Setiawan, I., Guillocheau, F., Robin, C., and Braun, J.: Evolution of the Nile River since 70 Ma: insights from surface processes and anorogenic reliefs controlled by mantle dynamics, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2680, https://doi.org/10.5194/egusphere-egu22-2680, 2022.

During the upper Cretaceous the South African Plateau underwent an uneven uplift period that coincided with the onset of a long-term climate cooling. This uplift recorded two pulses, an early pulse during the Turonian that affected the eastern margin and a later pulse occurred during the Campanian affecting the western margin (van der Beek et al., 2002; Braun et al., 2014; Baby et al., 2020). We aim here to determine the response of physical erosion and chemical weathering to this tectonic event, that may have played a role in the long-term climate cooling by promoting silicate weathering and associated atmospheric CO₂ drawdown. In this study we targeted the IODP 361 site located in the Cape Basin. We applied a new proxy (De_Hf) of chemical weathering intensity based on the coupled Lu-Hf and Sm-Nd isotopic systems in clay fractions of the sediments. This approach is combined with X-ray diffraction analyses to determine the concomitant evolution of clay mineralogical assemblages, which in active tectonic settings can provide insights on the evolution of mechanical erosion.

Twenty-two samples were analyzed for Hf-Nd isotopic compositions and 99 samples were analyzed for X-ray diffraction analyses. Clay e_Nd values remain relatively stable throughout the studied interval, mainly within -8 to -9 e-units, but down to -10 e-units in the Maastrichtian, suggesting a relatively stable source of the sediments. These values are coherent with a sediment source from the Karoo basin and/or the Cape Belt (Dia, 1990; Garzanti et al., 2014). Clay mineral assemblages are dominated by smectite (about 65 to 98%) but show an increase in illite proportions during the Campanian-Maastrichtian, up to about 25%. This increase in primary clay minerals may point to an increase in mechanical erosion, that could be linked to the uplift pulse affecting the western south African margin at that time. De_Hf values represent the departure of e_Hf from the clay array (Bayon et al., 2016), and display a marked increase during the Campanian-Maastrichtian as well, suggesting concomitant enhanced weathering of the nearby margin at that time.

Altogether, our data suggest a coupled response of mechanical erosion and chemical weathering of the southwestern African margin during the upper Cretaceous tectonic uplift of the margin, which also may have played as an active CO₂ consumption mechanism. 

How to cite: Gaitan Valencia, C. E., Pucéat, E., Pellenard, P., Blondet, J., Guillocheau, F., Robin, C., Bayon, G., and Adatte, T.: Deep basin record evolution of chemical weathering and physical erosion as response to the tectonic uplift of the South African Plateau during the upper Cretaceous, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6086, https://doi.org/10.5194/egusphere-egu22-6086, 2022.

The onset of the Siberian Traps Large Igneous Province at the Permian–Triassic transition significantly affected climate and depositional environments across the world. Known long term consequences of this event are (I) global warming, (II) increased continental weathering, (III) oceanic stagnation and acidification and (IV) mass extinction. These effects have the potential to strongly alter signals from source-to-sink systems in terms of petrography, sediment volumes and geochemistry. On the Finnmark Platform, a shift in provenance from a southern source to an eastern source during the middle Triassic is known. However, the impact of the environmental changes at the Permian-Triassic transition have so far not been investigated. The Barents Sea Basin contains a continuous record of sediments deposited before, during and after the Permian-Triassic event. The interval is present in several exploration wells, which show the transition in individual depositional environments. Therefore, it serves as an excellent area to investigate the response of source-to-sink systems to such extreme climatic changes.

The goal of this project is to investigate how the Triassic climatic changes were expressed in source-to-sink systems, mainly using techniques such as provenance, facies analysis, petrography, and sediment volumes. Herein we present preliminary provenance and petrography results mainly from Induan-aged sandstones and clasts from the Havert Formation. On the Finnmark Platform, upper Permian spiculate mudstones, limestones, and sparse sandstones are overlain by Lower Triassic mudstones and interbedded sandstones, which deposited as turbidites and prograding deltas. In order to determine how the signal from the catchment changed in relation to the great climatic changes, it is of high importance to examine changes within provenance and sediment volumes across the Permian-Triassic transition.

How to cite: Kling, M., Sirevaag, H., and Eide, C. H.: Evolution of a clastic source-to-sink system through the Permian-Triassic transition: Provenance and petrography of the Havert Formation on Finnmark Platform, Barents Sea, Norway, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5009, https://doi.org/10.5194/egusphere-egu22-5009, 2022.

How to cite: Garcés, M., Bastida, J., López-Blanco, M., Beamud, E., and Cabrera, L.: The continental record of the Eocene-Oligocene Transition in the Eastern Ebro Basin. Decoding the paleoclimate signature from sediments and clay mineralogy., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10016, https://doi.org/10.5194/egusphere-egu22-10016, 2022.

We aim to determine the intensity of chemical weathering of detrital clays, as well as the lag time between the onset of the Paleocene-Eocene Thermal Maximum (PETM) and the chemical weathering response in a source-to-sink system. The PETM was a hyperthermal event characterized by an abrupt increase in global temperature (5–8 °C) over a short period (20 ka). A negative carbon isotope excursion marks the onset of the PETM, which reflects the fast injection of light carbon into the ocean-atmosphere system, triggering global climatic changes. Thus, physical and chemical erosion acted as feedback mechanisms to recover the global climate to pre-onset conditions. We focus on the continental section of the source-to-sink system, near the locality of Esplugafreda in the Southern Pyrenean foreland basin. We analyzed the evolution of the clay mineral assemblages in two clay-sized fractions (<0.5 µm and <2 µm), which can provide insights into the hydrolyzing conditions on the continents. We then measured oxygen and hydrogen stable isotopes as indicators of paleo-precipitation, temperature, and elevation of the catchment areas. The clay mineralogy results show an interplay of reworked clays during the extreme events, together with more hydrolyzing conditions marked by the production of authigenic clay during the onset and body of the PETM. The stable isotope geochemistry results point towards a climatically controlled response, where temperature fluctuations, as opposed to precipitation, played the main role in shaping the climatic regime. This is evidenced in a negative δ¹⁸O excursion at the onset and body of the PETM found in both size fractions. Further, we combine hafnium and neodymium isotope analyses of both clay fractions to track the silicate weathering intensity. This method will help us constrain the weathering regime and its response time relative to the onset and the body of the PETM. The results obtained in this project will serve to test numerical models of landscape evolution incorporating the chemical weathering response to climatic changes. This project has received funding from the European Union's Horizon 2020 research and innovation program under the Marie Sklodowska-Curie Grant Agreement No. 860383.

How to cite: Jaimes-Gutierrez, R., Adatte, T., Puceat, E., Tremblin, M., Pellenard, P., Braun, J., and Castelltort, S.: Chemical weathering linked to global warming during the PETM: Insights from the Spanish Pyrenees, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5824, https://doi.org/10.5194/egusphere-egu22-5824, 2022.

Luminescence has been developed as a dating tool for Quaternary deposits. One approach is the single-grain post-infrared luminescence (SG-pIRIR) protocol that provides high-resolution equivalent dose (D_e) distributions. This protocol is well-suited for fluvial deposits that often present large scatter in D_e distribution because of heterogeneous bleaching (zeroing) of the grains by sunlight exposure during transport.

Here we present a SG-pIRIR analysis of 14 samples of modern sediments from the Rakaia (RK) and the Waimakariri (WK) rivers in the South Island of New Zealand. Those rivers are output channels for tonnes of sediment eroded annually from the Southern Alps, they are braided in the Canterbury Plains on about 70 km, downstream of short sections of 10-20km where they are running into incised gorges.

The aim was to test and develop SG-pIRIR as a tool to document and quantify transport as proposed in some recent publications (McGuire and Rhodes, 2015; Gray et al.,2018; Sawakuchi et al., 2018). We focused on the fractions of saturated and well-bleached grains from D_e distributions, and on the mean D_e calculated with the central age model (CAM), as proxies for bleaching rates, transport and transient storage of particles in floodplains. In the Canterbury Plains, we found for both rivers that the percentage of saturated grains follow an exponential decay expressed as y= y₀.e^(-x/Lsat) with a characteristic length L_sat = 24 km, whereas on the opposite the quantity of well-bleached grains increase towards downstream at a rate of +4 to 7%/km. Similarly to the saturation, we observed an exponential decay of the CAM doses (characteristic length L_cam = 42 km). Those results reveal a strong alongstream bleaching of the grains.

We complement our natural-system analysis by building a numerical model that simulate the successive displacement and D_e evolution of a set of individual grains along a river of length l. The code includes three main processes that repeat until grains reach the river outlet: (1) displacement of a distance L_T set with an exponential probability density function (PDF); (2) temporary storage in the floodplain between two displacements, for a period R_t set with a PDF that follows a Pareto law (alpha=2). During R_t, D_e can increase by 3 Gy/kyr; (3) bleaching of grains during displacement (fluvial transport) or storage (if exposed at the surface of the floodplain during Rt) according to a probability P_Bl (tested from 0.01 to 0.3). We consider l=200 km and the input of 400 grains, 50%  with an initial D_e=50 Gy, which is the mean D_e measured upstream both rivers, and 50% at 1000 Gy (saturated). To first order, this simple model simulates well natural observations (L_{SAT and} L_CAM) along WK and RK for L_T on the order of a kilometer, Rt values of several decades and bleaching probability of ~0.05. This very simple transport model allows to better decipher SG-pIRIR data and to estimate transport length and resting times of sand-sized fluvial particles.

Future works should consider testing these tools on other contexts, either in other tectono-climatc context or on different flow styles.

How to cite: Guyez, A., Bonnet, S., Reimann, T., Carretier, S., and Wallinga, J.: Feldspar single grain luminescence of modern fluvial sediments as a new tool to study fluvial transport, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4036, https://doi.org/10.5194/egusphere-egu22-4036, 2022.

Can river management practices impact the grain size patterns along mountain streams? If yes, such practices may have important consequences for the gravel industry, fluvial ecosystems and for the interpretation of grain size patterns in geosciences. Since the motion of particles in a riverbed is dependent on the applied shear stress, which in turn depends on river slope and depth, there are reasons to expect some impact on the riverbed grain sizes after a river channelization or construction of check dams. With the aim to answer the presented question we analyse and compare a large dataset of grain sizes and slopes of exposed gravel bars obtained in two mountain streams, i.e. Sense and Gürbe Rivers. While the Sense River is maintained in its natural character, the Gürbe River has been engineered with more than 60 check dams along c. 5 km on its sediment supply area. Both rivers are situated at the northern border of the Swiss Alps, they share the same source area and experience identical hydroclimatic conditions. In addition, for both streams, landslides and high-concentration flows from steep tributaries supply the clastic material to the trunk. Field work in the Sense River has been carried out c. 8 km downstream of the main sediment supply area, and the corresponding catchment area is c. 120 km² large. The selected stretch of interest on the Gürbe River lies above an alluvial fan situated right downstream of the supply area. The corresponding size of the drainage area in the Gürber River is c. 12 km². We collected more than 15’000 grain sizes by applying the Wolman’s pebble count method conducted on orthoimages of exposed gravel bars. The orthoimages were generated from digital photos that were taken by an unmanned aerial vehicle (UAV) and processed with standard photogrammetric techniques. These photos were also used to create digital elevation models, which allowed us to calculate the surface slope at various scales. Despite the differences in catchment area and distance from the supply area, the results from both rivers reveal similar bar-scale slopes (2.0 ± 0.1 cm/m) and grain size D50 (4.2 ± 0.1 cm) and D84 (12.7 ± 0.7 cm) percentiles. Additionally, by calculating the slope around each grain within a 2 m diameter circle, called here « local slope », we found a linear dependence between the local slopes (0.5 to 20 cm/m) and the grain size percentiles in both rivers. Since the check dams are built to reduce the mass fluxes, we consider that the grain size data of the Gürbe River reflects a greater attenuation of the landslide signals than the Sense River data. Furthermore, we suggest that the local slope dependency of the grain size percentiles rather reflects the effects of hydrodynamic processes than those of hillslope processes as controls on the motion of the riverbed particles. It thus appears that the check dams can have an impact on the sediment routing as they attenuate signals related to mass movements.

How to cite: do Prado, A., Mair, D., Garefalakis, P., Whittaker, A., Castelltort, S., and Schlunegger, F.: Impact of river management on grain size patterns: example of the Sense and Gürbe Rivers in the Swiss Alps, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1374, https://doi.org/10.5194/egusphere-egu22-1374, 2022.

Variations in fluvial grain size have long been used to decipher past climatic and tectonic events within stratigraphy. Thus, a thorough understanding of grain size fining response to external forcing and autogenic dynamics over long timescales (Myr) has implications within the interpretation of the sedimentary record. This work presents a new method (GSFast) that generalizes the Fedele and Paola (2007) self-similar gravel grain size method into multidimensions (downstream, across the basin, and overtime/depth) using the FastScape (Braun and Willett, 2013) landscape evolution model. The self-similar model results in a fining rate that is dominantly controlled by the rate of deposition relative to flux. Previously, the Fedele and Paola (2007) grain size fining model has been applied along a single river long profile (1D) to infer a subsidence pattern from observed fining rates such as within the Eocene Montsor Fans in the Southern Pyrenees (Duller et al. 2010; Whittaker et al., 2011). Here, using GSFast, we demonstrate the role of across basin dynamics on grain size fining through a sensitivity analysis comparison with the Duller et al. (2010) single river profile approach. For this, we performed a series of simple numerical experiments to predict grain size fining rate in sedimentary systems of varying spatial extents that are fed by an orogenic source area and undergoing subsidence at a prescribed rate.

When applied in 1D or within shorter confined basins, where all the upstream catchment flux is deposited as gravel within the fan (no alluvial plain), GSFAST can capture the Duller et al. (2010) results under comparable subsidence and flux conditions. This is because both 1D systems and short basins are characterized by no or limited lateral channel mobility resembling sheet flow or incised channel flow.

Conversely, in wide or long (unconfined) basins, the rate of grain size fining predicted by GSFast deviates from the Duller et al. (2010) single profile solution. This deviation occurs due to multiple mobile drainage channels that form when the gravel flux leaving the upstream catchment is unconfined and deposited in both the alluvial fan and adjacent plain. Deviations in grain size fining trend from the Duller et al. (2010) approach correlate with channel mobility dynamics that preferentially form in wide basins or long systems that connect a fan to a large alluvial plain.

Thus, under the same tectonic and climatic boundary conditions, our more dimensionally complex model that incorporates channel mobility leads to different predictions of subsidence patterns from grain size fining curves. This is because our multidimensional approach leads to a growing disconnect between subsidence and deposition rates as channel mobility increases. It also predicts markedly different fining patterns between short systems (i.e., limited to a steep fan-like structure), and long systems (i.e., systems that also incorporate a low gradient alluvial plain).

Duller et al. (2010). JGR: ES 115(F3).

Fedele & Paola. (2007). JGR: ES 112(F2).

Whittaker et al. (2011). Bulletin, 123(7-8).

How to cite: Wild, A., Braun, J., Whittaker, A., Fillon, C., and Castelltort, S.: The Importance of Autogenic Dynamics in Multidimensional Models of Grain Size Fining, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5582, https://doi.org/10.5194/egusphere-egu22-5582, 2022.

Sediment archives in the terrestrial and marine realm are regularly analyzed to infer past changes in climate and tectonic boundary conditions. However, contradictory observations have been made regarding whether short period events are faithfully preserved in stratigraphic record. For example, short period events were hypothesized to be non-detectable in terrestrially derived sediments offshore large river system due to buffering during sediment transport. Other studies, however, have detected signals of short period events in sediments that were transported along large river systems. We think that this apparent discrepancy is related to the lack of a differentiation between different types of signals and the lack of distinction between river response times and times related to signal propagation.

To overcome these issues, we propose to define environmental signals more generally as “a measurable change in any sedimentary parameter of interest through time that can be linked to an environmental change” and to further group signals in sub-categories related to hydraulic grain-size characteristics. Also, we review the different types of ‘times’ and suggest a precise and consistent terminology for future use to clearly distinguish times of landscape response from times of signal transfer. We compile and discuss factors influencing the times of signal transfer along sediment-routing systems, how those times vary with hydraulic grain-size characteristics, as well as consequences regarding signal preservation in stratigraphy.

Unravelling different types of signals and distinctive time periods related to signal propagation addresses the discrepancies mentioned above and allows a more comprehensive exploration of event preservation in stratigraphy – a prerequisite for reliable environmental reconstructions from terrestrially derived sedimentary records.

How to cite: Tofelde, S., Bernhardt, A., Guerit, L., and Romans, B.: Times associated with source-to-sink propagation of environmental signals during landscape transience, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1328, https://doi.org/10.5194/egusphere-egu22-1328, 2022.

The source-to-sink (S2S) river systems in the Indo-Gangetic Basin (IGB) construct alluvial landscapes which have hosted some of the earliest human civilizations. These landscapes provide the vital sustaining resources  acting as a lifeline for settlement and agriculture. The factors contributing to its richness are climate, political security, and the alluvial landscapes formed by Indus, Ganga, and Yamuna rivers. Given the significant changes that have happened due to human exploitation of landscape, an understanding of the impact of geomorphic landscapes on human settlement patterns is lacking, hence it is important to understand changes in these source to sink landscapes over a range of spatial and temporal scales. For instance, given the risk to people on river floodplains due to natural processes such as flooding, it is important to understand the impact of geomorphic landscapes on human populations.

Here, we aim to investigate the impact of  the source to sink system on the human settlement patterns in the Indo Gangetic Plains at decadal and centennial historic timescales. Historical maps along with LANDSAT images are used for detailed mapping of the S2S landscape elements and study the long term changes in the river morphology. Previous studies mainly exploited the optical data like LANDSAT and MODIS data to map the urban areas. To investigate changes at short timescales, we use DMSP-OLS stable nightlights data to study changes in urban settlement patterns in the Gangetic basin from 1992-2013. Earlier research have demonstrated usefulness of nightlights data for urban settings for the assessment of the economic, demographic and environmental purposes. We are integrating these data sets with mapping of geomorphic elements in the Gangetic basin as well as the river network extracted from the SRTM-DEM. We use these data sets to evaluate the links between the nightlights and geomorphic features thus highlighting possible connections between population growth and geomorphic elements in the IGB.

The overall aim of this study is to provide new quantitative insights between the dynamics of human habitation and river morphodynamics in the Indo-Gangetic basin over a range of scales.

How to cite: Aggarwal, E., Gupta, S., and Whittaker, A. C.: How do Source to Sink Systems influence patterns of human settlement in the Indo-Gangetic Basin?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8230, https://doi.org/10.5194/egusphere-egu22-8230, 2022.

Stratigraphic successions can record sediment transport dynamics and grain size trends within a temporal and spatial framework. Information on sediment- and water-fluxes are thus preserved by the arrangement and distribution of grains and by proxies of water depth conditions found in conglomerates and their associated stacking patterns.

Here we quantify long-term and instantaneous bedload sediment fluxes for three Oligo-Miocene depositional systems consisting of km-thick conglomerate sequences situated in the western, central, and eastern Swiss Molasse basin. We analyse these paleo-megafans for their stratal patterns, preserved channel depths and for temporal and spatial grain size trends. Our three target sections expose km-long spatial relationships from proximal (closer to the paleofan-apex) to distal positions as confirmed by their stacking patterns and clast morphometries, as well as supported by published palinspastic restorations. We apply a self-similarity approach to estimate threshold sediment fluxes based on grain size fining models for each section and we use paleo-hydrological techniques to estimate instantaneous bedload transport capacities associated with bankfull flow conditions.

Our analyses reveal that all three sections consist of amalgamated massive to cross-bedded, several m-thick conglomerate-beds, with a higher occurrence of sandstone- and mudstone-interbeds at distal positions. From this we infer that the clastic material has been deposited on alluvial fans consisting of a network of multiple braided streams. In contrast to the similar stacking pattern, grain size and channel depth measurements disclose a unique trend for each section. The Western section shows a ~60% decrease for both the D₈₄ and D₅₀ grain size percentiles down-system (along ~8.5 km). The Central succession reveals a decrease of ~30% (D₈₄) and ~40% (D₅₀), respectively (along ~29 km). The Eastern section reveals a decrease of ~30% (D₈₄) and ~7% (D₅₀) down-system (along ~9 km). Bankfull channel depths for all sections increase towards distal positions (threefold increase for the Western, increase of ~1/3 for the Central and twofold increase for the Eastern section, respectively).

All three sections show self-similar grain size distributions both down-system and between sites. From this, we conclude that the mechanisms that led to selective deposition down-system behaved similarly for all sections. However, calculations of threshold sediment fluxes reveal significant differences between these fans. The Western section discloses unit sediment fluxes of 18.8 ± 1.45 km³*Myr^-1*km^-1, while the Central suite reveals sediment fluxes twice as high of 39.8 ± 3.74 km³*Myr^-1*km^-1 and the Eastern section of 6.6 ± 1.6 km³*Myr^-1*km^-1. We argue that these differences are mostly controlled by variations in erosion rates within the source area in the Central Alps and tectonically-controlled variations in sediment deposition rates between the fans. Comparison of our threshold sediment fluxes with own estimates of instantaneous bedload transport capacities indicate that the fans were only active c. 1-5% of the time to balance the estimated long-term sediment budget. Our data capture an exceptional record of how these Oligo-Miocene megafans in the Swiss Molasse reflect long-term landscape processes within the foreland basin and the adjacent source areas in the Central Alps.

How to cite: Garefalakis, P., Whittaker, A. C., do Prado, A., Mair, D., and Schlunegger, F.: Sediment transport dynamics and grain size trends recorded by Oligo-Miocene megafans in the Swiss Molasse basin, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2358, https://doi.org/10.5194/egusphere-egu22-2358, 2022.

The large-scale and long-term transport and deposition of sediment is commonly represented by one of two reduced-complexity models, namely the transport limited (or T-L) model or the under-capacity (or ξ−q) model more recently developed by Davy and Lague (2009). Using both models, I investigated the behavior of a sedimentary continental system fed by a fixed sedimentary flux from a nearby active orogen though which sediments transit to a fixed base level representing a large river, a lake or an ocean.

Firstly, I will show that the two models share the same steady-state solution, for which a simple 1D analytical solution exists that reproduces the major features of many orogenic sedimentary systems: a steep fan adjacent to the mountain front that connects to a shallower alluvial plain. The resulting fan geometry obeys basic observational constraints on fan size and slope with respect to the upstream drainage area, A0. I will also show that the solution is strongly dependent on the size of the system, L, in comparison to a distance L0 that is determined by the size of A0. This gives rise to two fundamentally different types of sedimentary systems: constrained systems where L < L0 and open systems where L > L0. Constrained systems contain only a steep, conical fan that connects directly to the base level, whereas open systems are made of a steep fan connecting to a broad, low slope alluvial plain.

Secondly, I will present simple expressions that show the dependence of the system response time on the system characteristics, such as its length, the size of the upstream catchment area, the amplitude of the incoming sedimentary flux and the respective rate parameters (diffusivity or erodibility) for each of the two models. The ξ−q model predicts significantly longer response times, which I relate to its greater efficiency at propagating signals through its entire length. I will also demonstrate that, although the manner in which signals propagates through the sedimentary system differs greatly between the two models, they both predict that perturbations that last longer than the response time of the system can be recorded in the stratigraphy of the sedimentary system and in particular of the fan.  Interestingly, the ξ−q model predicts that all perturbations (regardless of their period) in incoming sedimentary flux will be transmitted through the system whereas the T-L model predicts that rapid perturbations cannot. I will finally discuss why and under which conditions these differences are important and propose observational ways to determine which of the two models is most appropriate to represent natural systems.

Reference: Davy P. and Lague D., JGR-Earth Surface, 2009.

How to cite: Braun, J.: Models of sedimentary fans and alluvial plains and how they propagate sedimentary, climatic and tectonic signals, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2606, https://doi.org/10.5194/egusphere-egu22-2606, 2022.

Whether sedimentary signals propagate from the terrestrial source to the ocean sink depends on the efficiency of sediment transport across the shelf. Continental shelves are low-relief areas that vary widely in spatial extent along different continental margins and with varying sea level and can store large amounts of sediment. During high sea level, the efficiency of cross-shelf transport is increased if 1) the shelf is dissected by submarine canyons that convey terrestrial sediments derived from river outlets or near-coast longshore currents or if 2) shelf-bottom currents relocate sediment into submarine canyons and over the shelf edge.

1) Since the Last Glacial Maximum, sea level has risen by ~120 m. As a consequence, most coastlines have migrated landward, inundating large shelf areas, some of which are now dissected by submarine canyons. However, with only 4% of the world’s canyons (n=183) reaching today's coastline, these canyons remain the exception. We identified the main controls on whether a submarine canyon head remains connected to terrestrial sediment input during sea-level rise. Shore-connected canyons preferentially occur along continental margins with narrow and steep shelves, such as the Mediterranean margin and the Pacific coast of Central and South America. Moreover, our analysis supports the occurrence of such canyons offshore river basins that are characterized by resistant bedrock and high water discharge. Such rivers deliver coarse-grained sediment to submarine canyons, which erode the canyon head and floor and such systems are most likely to efficiently propagate environmental signals to the deep sea.

2) Offshore the narrow shelf of north-central Chile (29-33°S) turbidite activity ceased with increasing Holocene aridity. In contrast, offshore the humid south-central Chile coast (36-40°S), shelf-bottom currents transport sediment into canyons but also across the across a wide shelf (40-60 km) and onto the continental slope. Here, sediment archives on the continental slope record frequent turbidite deposition during highstand conditions, although most of the depocenters are not connected via canyons to terrestrial sediment sources. High sediment supply, combined with a wide shelf on which shelf-bottom currents move sediment towards the shelf edge, controls Holocene turbidite sedimentation. Moreover, shelf currents can move sediment along large shore-parallel distances deviating sediment from their intuitive transport pathway. Sediment from the Nile river is transported along the Levant shelf. Offshore northern Israel, the shelf narrows and its slope is incised by submarine canyons. Consequently, sediment is re-routed down the continental slope and builds upslope-migrating sediment waves on the continental rise. These strata hence integrate climatic signals from the Nile outlet, sea-level modulations of shelf currents and authigenic sediment-wave development 500 km away but not simply down slope of the original sediment source.

Our analyses offer new insights into the formation and maintenance of submarine canyons and shelf currents that are required to efficiently transport sediments, pollutants, and organic carbon from rivers to the deep ocean floor. The position of canyon heads with regard to the sediment source and the extent and transport capacity of shelf currents have to be accounted for in source-to-sink signal propagation research.

How to cite: Bernhardt, A., Schattner, U., and Schwanghart, W.: Over the edge: The role of cross-shelf sediment transport by submarine canyons and bottom currents in signal propagation research, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2226, https://doi.org/10.5194/egusphere-egu22-2226, 2022.

Trough mouth fans are important depocenters for glacial sedimentation in high latitude margins, recording sedimentological processes and their relationship with paleoclimatic fluctuations on short timescales - from decades to millennia. The volume of sediments accumulated in these fans varies significantly depending on the phase of the glacial cycle, with higher values typically reached during early retreats. The input of large volumes during short time intervals can potentially trigger submarine landslides and overpressure build-up, making the understanding of processes and proportions related to sedimentation in glaciated margins crucial, especially during periods of global warming. In this study we use high-quality 3D seismic cubes (vertical resolution of 2 m and bin size of 6.25 x 18.75 m) to delimitate different types of deposits on an area over 14000 km² on the North Sea Trough Mouth Fan during a full glacial-interglacial cycle of the last glaciation (Weichselian). After mapping the corresponding top and base surfaces of each type of deposit their volumes were calculated using the mean thickness of the beds multiplied by their extent. The base of the studied package comprises a contourite body deposited on top of the Tampen Slide, whose failure is estimated to have happened around 130 ka ago. Next we have a thick (> 400 m) mixed package of debris flows and meltwater turbidites, with its rapid deposition happening during approximately 4 ka (~23 to 19 ka). The package is then completed with plume settling related to the full retreat of the ice stream. Although not completely interpreted due to limitations of the dataset extent and remobilization of a part of the fan by the Storegga Slide in the northern part, the last glacial cycle comprises a total of 7160 km³ of sediments, with more than half of it (4850 km³) originated from the downslope processes. The plumites and contourites comprise volumes of 1105 km³ and 1205 km³. This accounts to a significant variability of the magnitude of sediment volume coming into the sink per year, with the downslope deposits having over 100 times more sediment input and the plumites 5 times more when compared to the contourites. These results highlight the range of sediment volume that can be delivered in a glaciated margin depending on changes in processes and climatic fluctuations, which may also entail changes in the potential geohazards.

How to cite: Garcia, A., Bellwald, B., Midtkandal, I., Planke, S., Anell, I., Sternai, P., and Myklebust, R.: Mixed sedimentation of the North Sea Fan – insights on volumes of contourites, plumites and downslope deposits during a full glacial-interglacial cycle, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10658, https://doi.org/10.5194/egusphere-egu22-10658, 2022.

Oceanic Anoxic Events (OAEs) are short-lived global carbon cycle perturbations characterized by significant changes in the chemistry, temperature and circulation of the oceans of the world. The forcing mechanisms behind these events remain a subject of discussion, especially for the Jurassic and Cretaceous periods. Typically, these events are thought to be triggered by a contemporaneous emplacement of a large igneous province (LIP) associated with a significant release of CO₂ into the atmosphere and leading to rapid global warming. In turn, induced sharp climatic change promotes an acceleration of the hydrological cycle which intensifies continental weathering, increases input of nutrients into the oceans and drives elevated rates of primary productivity. Organic productivity increase causes high oxygen demand leading to anoxia accompanied by enhanced carbon burial.

The primary carbon source of the Aptian OAE1a, (~120 Ma, D. forbesi, D. deshayesi and D. furcata ammonite zones or G. blowi and L. cabri planktonic foraminiferal zones) is suggested to be the Greater Ontong Java Plateau (G-OJP) which peaks of activity are interpreted to be coeval to specific phases of the global crisis, such as a global biocalcification crisis. It is also known that the GOJP initial emplacement preceded the onset of OAE1a. What remains unclear is therefore what other processes could have triggered the OAE1a and what is the exact timing of its onset. OAEs commonly relate to periods of world scale major marine transgression (eustatic maximum flooding) as a consequence of global warming, potential deglaciation and/or addition of massive volcanogenic rock volume into the ocean. Nevertheless, the relationship of the onset of OAEs with the maximum flooding and the exact position of their time interval with respect to the global eustatic sea-level curve are still ambiguous.

This study focuses on the Aptian of the Tethyan margin in Tunisia, which sediment record covers the OAE1a and aims at understanding the exact mechanisms and their interplay in relation to the initiation and onset of this event, with a particular focus on the eustatic factor. The study methods include sedimentology, biostratigraphy, sequence stratigraphy and chemostratigraphy integrated with petroleum exploration well data coupled with published datasets and studies. Emphasis will be given to the reconstruction of the accommodation space with the aid of paleo-bathymetry estimations based on facies sedimentology and benthic foraminifera. At a later stage, the results will be correlated with data from SE Spain and Northern Europe (Greensand facies). One of the main goals of this project is to provide a global assessment and overview of the positioning of the OAE1a in relation to the early Aptian maximum floodings and understand in depth the critical factors, confluences, as well as thresholds that were necessary to be exceeded to trigger this event in a qualitatively and quantitatively manner.

The project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie Grant Agreement No 860383 (https://www.s2s-future.eu/).

How to cite: Giannenas, P. A., Robin, C., Guillocheau, F., Vennin, E., and Gréselle, B.: The role and contribution of eustasy as a triggering force of the onset of Oceanic Anoxic Events: A case study of the early Aptian OAE1a in the Tethyan margin of Tunisia, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6309, https://doi.org/10.5194/egusphere-egu22-6309, 2022.

In the East Shetland Platform (ESP) during the Paleocene, thick third order sequences (0.5 – 3 Myrs) were deposited during overall shelf progradation into the central North Sea, resulting in shelf - basin floor depositional profiles being preserved from Danian to Ypresian times. We interpreted the depositional record east of Shetland using over 40 000 km² of 3D seismic data and circa 100 wells with biostratigraphic and lithostratigraphic picks, drawing key comparisons between the geometries of individual sequences along strike. We constructed multiple chronostratigraphic charts and relative sea-level curves for the area, which will later be used to study uplift and the influence of emplacement of large igneous provinces on source-to-sink systems.Deposition during the Danian is marked by a switch from quiescent carbonate and chalk platforms to strongly progradational clastics at the onset of uplift in the hinterland. This results in the first third-order pulses of clastic input in the ESP and in the adjacent Viking Graben, which correspond to sediments of the Maureen (Upper Danian – Middle Selandian) and Lista Formations (Upper Selandian to Middle Thanetian). These systems are dominated by sediment gravity flows in channel-lobe complexes, and are separated by a top Selandian Unconformity. From Thanetian to early Ypresian, multiple fourth order cycles of relative sea-level change can be recognized in shelfal sequences dominated by normal and forced regression. These include a broad domain of forced regressive to normal regressive shelf-margin – scale clinoforms (Dornoch Formation “Highstand” – sequence D1) that are correlated to an Upper Thanetian Unconformity in the proximal platform and systems of channelized sediment transport in the basin. This is followed by a set of rapidly prograding, flat trajectory clinoforms with wave-dominated shoreline delta geometries and considerable deposition in inner shelf, prodelta lobes (D2 – D4). These systems are interdigitated with a larger shelf-margin – scale clinothem of seemingly coeval age in the southern ESP, closer to the Piper Shelf. In the central ESP, close to the Beryl Embayment, basement reactivation during the Paleocene created structural highs that controlled estuarine or lagoonal - like systems during the Dornoch - Beauly cycles, which ended after significant clinoform progradation beyond the ESP and into the Viking Graben, although the exact nature of these clinoforms (sequence D5) is still unknown. The final sequence B1 is marked by the progressive onlap advance towards the continent (including coastal plain aggradation and backstepping) and eventually complete transgression of the Dornoch-Beauly shelf, which helped preserve erosional landscapes developed during the Dornoch progradation and also the posterior tidal invasion of the shelf. In the south, transgressive deposits are almost 200 ms thick (150 – 200 m) in some seismic profiles, but in the northern ESP shelf often only a single reflector of this same age is identified (< 40 ms). This highlights the marked influence of both tectonic tilting/doming and differential sediment supply versus accommodation rates along strike in the ESP, which are interpreted as a direct result of the activity of the Icelandic Plume in the continent.

How to cite: Valore, L., Eide, C., and Sømme, T.: Stratigraphic framework and sedimentary environments of the East Shetland Platform in the Paleocene – Preliminary Results, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4577, https://doi.org/10.5194/egusphere-egu22-4577, 2022.