Displays

Landslides are a dominant mechanism of erosion in mountain landscapes. Widespread triggering of landslides by large storms or earthquakes can lead rapid changes in short-term erosion rates. If landslides occur repeatedly in particular parts of a mountain range, then they will dominate the evolution of that section of the landscape and could leave a fingerprint in the topography. Despite this recognition, it has proved difficult to examine shifts in the focus of landslide erosion through time, mainly because remote sensing approaches from single events to a few decades at most. Here we turn to the depositional record of past erosion, attempting to track landslide occurrence and the provenance of eroded material using a novel combination of the isotopic and molecular composition of organic matter (bulk C and N isotopes, molecular abundance and isotopic composition) deposited in Lake Paringa, fed by catchments proximal to the Alpine Fault, New Zealand. In the modern day forest, we find correlations between elevation, soil depth and the bulk δ¹³C values of the organic matter and the carbon preference index of n-alkanes. We find large shifts in these measurements in the lake core. Using an empirical model based on modern soil samples we suggest that the erosion provenance shifts dramatically after each of four large Alpine Fault earthquakes in the last one thousand years. These shifts in inferred erosion altitude match shifts in the hydrogen isotope composition of long-chain n-alkanes (plant wax biomarkers) and the inferred shifts in depth track changes in organic matter radiocarbon activity and nitrogen isotope composition, lending support to our model. The combination of bulk isotopic composition and biomarker ratios has the potential to track erosion provenance in other settings. In the Lake Paringa record, we find that post-seismic periods eroded organic matter from a mean elevation of 722 ⁺³²⁹/_-293 m at the headwaters of source catchments and supplied 43% of the sediment in the core, while inter-seismic periods sourced organic matter primarily from lower elevations (459 ⁺²⁵⁶/_-226 m). These results demonstrate that repeated large earthquake consistently focus erosion at high elevations, while inter-seismic periods appear less effective at modifying the highest parts of the topography. 

How to cite: Wang, J., Howarth, J., McClymont, E., Densmore, A., Fitzsimons, S., Croissant, T., Gröcke, D., West, M., Harvey, E., Frith, N., Garnett, M., and Hilton, R.: Temporal shifts in erosion provenance through multiple earthquake cycles, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9817, https://doi.org/10.5194/egusphere-egu2020-9817, 2020.

The sediment residence time represents the time elapsed since the formation of the sediment in soils until its deposition. In order to better constrain timescales of sedimentary processes (erosion, transport, and deposition), it is important to understand to what extent sediment residence time is controlled by geomorphological parameters (e.g. elevation, curvature, slope). Uranium isotopes have been used to infer the time elapsed since the formation of fine detrital grains (<63 µm) by physical and chemical weathering (i.e. comminution age).

In this study, uranium isotopes were measured in fluvial sediments (<63 µm) sampled at different locations in a catchment (Var, France) to determine the variation of uranium activity ratio (²³⁴U/²³⁸U) along the river profile. The absence of fluvial plain implies that the sediment residence time mainly represents the storage time on hillslopes, as sediment transport is expected to be very rapid in this mountainous sedimentary system. 

The catchment was divided into 27 sub-catchments to investigate the variability of the geomorphological parameters that have been extracted from spatial analysis. Additionally, sediment residence time was estimated based on soil thickness prediction data combined with denudation rate information to compare this predicted residence time to the one calculated with (²³⁴U/²³⁸U).

The correlation between (²³⁴U/²³⁸U) and the estimated sediment residence time confirms that (²³⁴U/²³⁸U) can be modelled to infer sediment residence time. Furthermore, the correlations between the slope, the elevation and (²³⁴U/²³⁸U) highlight the geomorphological controls on the sediment residence time. The use of (²³⁴U/²³⁸U) in sedimentary archives will help to determine past geomorphological variations and re-construct past links between catchment erosion and climate change.

How to cite: Thollon, M., Dosseto, A., Toucanne, S., and Bayon, G.: Sediment residence time variations in an Alpine river system inferred by uranium activity ratio., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6181, https://doi.org/10.5194/egusphere-egu2020-6181, 2020.

River strath terraces reflect changes in lateral and vertical erosion rates within mountain valleys related to changes in the sediment to water discharge ratio. In contrast to the formation of terraces in high latitude glaciated catchments, little is known about the timing and mechanisms of river valley aggradation and incision in response to climate in low latitude, non-glaciated arid regions. To investigate the timing of river strath terrace formation in North-West Africa, we developed and applied a new approach to OSL dose rate correction of gravels. We sampled terraces in the M’Goun catchment crossing the thrust front and a thrust-sheet-top basin of the south-central High Atlas in Morocco, totalling 23 dated samples. Strath surfaces are elevated 10 to 40 m above the modern river plain, depending on local valley and bedrock configuration, and are overlain by 2 to 10 m of fluvial conglomerates. Burial ages of conglomerates in the first strath terrace level span from 180 to 60 ka, with widespread abandonment and incision post 60 ka throughout the catchment. This timing coincides with an eccentricity-driven decrease in African summer insolation and a decrease in the fluvial signature of Saharan dust recorded in an offshore Atlantic sediment core. We propose enhanced precipitation from the African summer monsoon during high insolation periods led to increased sediment yield and aggradation in the southern High Atlas, whilst low insolation and dry periods led to sediment-starved incision. To our knowledge, the M’Goun river terrace record is the most detailed record of long-term landscape evolution in response to climate fluctuations in northwest Africa to date.

How to cite: Zondervan, J., Stokes, M., Telfer, M., Boulton, S., Buylaert, J.-P., Jain, M., Murray, A., Belfoul, A., Mather, A., Taleb, N., and Hann, M.: Eccentricity forcing of Saharan climate drives fluvial strath terrace formation in the High Atlas, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-718, https://doi.org/10.5194/egusphere-egu2020-718, 2020.

The impact of glaciers on the Quaternary evolution of mountainous landscapes remains controversial. While in-situ low-temperature thermochronology offers insights on past rock exhumation and landscape erosion, it also suffers from biases due to the difficulty of sampling bedrocks buried under the ice of glaciers. Detrital thermochronology attempts to bypass this issue by sampling sediments at, e.g. the catchment outlet, that may originate from beneath the ice. However, the age distribution resulting from detrital thermochronology does not only inform on the catchment exhumation, but also on the patterns and rates of surface erosion and sediment transport. In this study, we use a new version of a glacial landscape evolution model, iSOSIA to address the role of erosion and sediment transport by the ice on the form of synthetic detrital age distributions and thus, for inferred catchment erosion from such data. Sediments are tracked as Lagrangian particles that can be formed by bedrock erosion, transported by ice or hillslope processes and deposited. We apply our model to the Tiedemann glacier (British Columbia, Canada), which has simple morphological characteristics, such as a straight form and no connectivity with large tributary glaciers. Synthetic detrital age distributions are generated by specifying an erosion history, then sampling sediment particles at the frontal moraine of the modelled glacier. The detrital ages are represented as synoptic probability density functions (SPDFs).

A characterization of sediment transport shows that 1500 years are required to reach an equilibrium for detrital particles age distributions, due to the large range of particle transport times from their sources to the frontal moraine. Second, varying sampling locations and strategies at the glacier front lead to varying detrital SPDFs, even at equilibrium. These discrepancies are related to (i) the selective storage of a large proportion of sediments in small tributary glaciers and in lateral moraines, (ii) the large range of particle transport times, due to varying transport lengths and to a strong variability of glacier ice velocity, (iii) the heterogeneous pattern of erosion, (iv) the advective nature of glacier sediment transport along ice streamlines that leads to a poor lateral mixing of particle detrital signatures inside the frontal moraine. Third, systematic comparisons between (U-Th)/He and fission track detrital ages, with different age-elevation profiles and relative age uncertainties, show that (i) the age increasing rate with elevation largely controls the ability to track sediment sources, and (ii) qualitative first-order information about distribution of erosion may still be extracted from thermochronological system with high variable uncertainties (> 30 %). Overall, our distributions in glaciated catchments are strongly impacted by erosion and transport processes and by their spatial variability. Combined with bedrock age distributions, detrital thermochronology can offer a means to constrain the transport pattern and time of sediment particles. However, results also suggest that detrital age distributions of glacial features like frontal moraines, are likely to reflect a transient case as the time required to reach detrital thermochronological equilibrium is of the order of the short-timescale glaciers dynamic variability, as little ice ages or recent glaciers recessions.

How to cite: Bernard, M., Steer, P., Gallagher, K., and Egholm, D. L.: The effects of ice and hillslope erosion and detrital transport on the form of detrital thermochronological age probability distributions from glacial settings, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15550, https://doi.org/10.5194/egusphere-egu2020-15550, 2020.

Over geological timescales, we often assume the export of sediment, from mountainous source regions to depositional basins, is relatively instantaneous. As such, stratigraphic units are thought to capture erosional trends in their upstream catchment. The export of sediment from mountain basins, however, is a process heavily modified by sediment transport.

Here, we exploit a well-constrained field site in the Argentine Andes to demonstrate how the connectivity between hillslopes and mountain rivers modulates long-term sediment export in post glacial landscapes. We map out erosion trends in upstream catchments by combining an analysis of river profiles with geomorphic mapping of sediment deposits. We then use a comprehensive catalogue of clast lithology data to test to what extent upstream erosion trends are recorded downstream.

Despite their proximity to each other, we find adjacent catchments supplying sediment to the Iglesia basin have distinctly different degrees of hillslope-river connectivity, evident from the morphology of terraced and fan deposits within the catchments. Catchments with good hillslope-river channel connectivity also have a higher abundance of clasts sourced from the upper cordillera downstream of their mountain front. We place these observations within the context of a strong precipitation gradient across the cordillera and demonstrate the importance of climate and climate-controlled base-level on the spatial distribution of erosion within mountain catchments and fundamentally, on sediment export.

This work has implications for those using gravels to reconstruct the history of mountain ranges. Furthermore, it highlights the need to better constrain the potential for a disproportionate increase in sediment export to populated areas under future climate scenarios

How to cite: Harries, R., Kirstein, L., Whittaker, A., Attal, M., Gailleton, B., and Mudd, S.: Linking source to stratigraphy through sediment transport: The importance of spatially variable climate on the evolution of the Argentine Andes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22349, https://doi.org/10.5194/egusphere-egu2020-22349, 2020.

Fine particles in rivers comprise a substantial fraction (>50%) of the mass leaving a landscape, while at shorter timescales they represent significant carriers of nutrients and contaminants with the potential to both degrade and enhance aquatic habitats. Predicting fine particle dynamics within rivers remains challenging due to a complex relationship between sediment and water availability from the landscape. This inherent complexity results in watershed-specific understandings of suspended sediment dynamics, typically parameterized as empirical functions of catchment land use, geology, and climate. However, observations of significant fine particle storage within river corridors may indicate that the flux of suspended sediment depends on reach-scale hydraulics. To better understand these dynamics, we synthesized over 40 years of suspended sediment concentration (SSC), hydraulic geometry, river flow, and grainsize data collected by the US Geological Survey from hundreds of rivers spanning a large variety of environments across the continental United States. This data synthesis reveals a strong nonlinear trend between reach-scale hydraulics and the suspended sediment flux, with a secondary dependence on particle properties. The multi-site synthesis reveals that by normalizing the suspended sediment flux by the bankfull shear stress and flux results in a collapse of the observed data onto a single function that describes a self-organizing structure for suspended sediment transport in watersheds. This general relationship indicates strong support for the role of autogenic processes in setting the flux of fine particles and erosion rates of watersheds.

How to cite: Phillips, C., Rogéliz, C., Horton, D., Higgins, J., and Packman, A.: Landscape and river self-organization limit the flux of fine particles , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12885, https://doi.org/10.5194/egusphere-egu2020-12885, 2020.

The gravel-sand transition is observed along most rivers. It is characterized by an abrupt reduction in median bed grain size, from gravel- to sand-size sediment, and by a shift in sand transport mode from wash load-dominated to suspended bed material load. We document changes in channel stability, suspended sediment concentrations, flux and grain size across the gravel-sand transition of the Karnali River, Nepal. Upstream of the gravel-sand transition, gravel-bed channels are stable over hundred to thousand-year timescales. Downstream, floodplain sediment is reworked by lateral bank erosion, particularly during monsoon discharges. Suspended sediment concentration, grain size and flux reveal counterintuitive increases downstream of the gravel-sand transition. The results demonstrate a dramatic change in channel dynamics across the transition, from relatively fixed, steep gravel-bed rivers with infrequent avulsion to lower gradient, relatively mobile sand-bed channels. The increase in sediment concentrations and near-bed suspended grain size may be caused by enhanced channel mobility, which facilitates exchange between bed and bank materials.   These results bring new constraints on channel stability at mountain fronts, and indicate that temporally and spatially limited sediment flux measurements downstream of gravel-sand transitions are more indicative of flow stage and floodplain recycling than of continental-scale sediment flux and denudation rate estimates.

How to cite: Dingle, E., Sinclair, H., Venditti, J., Attal, M., Kinnaird, T., Creed, M., Quick, L., Nittrouer, J., and Gautam, D.: Sediment dynamics across gravel-sand transitions: Implications for river stability and floodplain recycling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-573, https://doi.org/10.5194/egusphere-egu2020-573, 2020.

In mountainous regions, mass-wasting processes dominate landscape evolution and pose serious risk to life and socioeconomic development. In the Nepal Himalayas, annual rates of mass-wasting are primarily driven by the Asia Summer Monsoon (ASM), the strength of which is highly sensitive to changing global climate.  However, whilst relationships between precipitation intensity and suspended fluvial sediment flux in the Himalaya are well described, a longer-term empirical relationship between ASM strength and total mass-wasting volume has remained elusive. Here, we use a new 30-year landslide inventory for central-eastern Nepal to quantify an empirical relationship between ASM strength and total mass-wasting volume. As well as providing insight into how Himalaya hillslope denudation rates might change under possible future ASM strength scenarios, these data allow a quantification of how background rates of ASM-triggered mass-wasting have been perturbed by extreme climatic and tectonic forcing (e.g. earthquakes, storms) and anthropogenic activity (e.g. road building).

We find a strong exponential relationship (R² = 0.66 – 0.88) between total ASM precipitation and total ASM-triggered mass-wasting volume, suggesting that relatively small changes in ASM strength can lead to significant increases in mass-wasting.  This relationship also allows the calculation of a climate normalised annual rate of mass-wasting for the study region between 1988 and 2018. This normalised rate reveals several years (1993, 2002, 2015 – 2018) with mass-wasting rates perturbed significantly above the rates expected given the ASM strength. We find that the perturbations in 1993 and 2002 correlate with the occurrence of extreme cloud outburst or flood events, resulting in above-expected mass-wasting equivalent to that caused by 3.5 average ASM seasons. By contrast, the 2015 – 2018 perturbation is more complex. We interpret the perturbation in 2015 as being caused by landscape preconditioning associated with the M_w 7.9 Gorkha earthquake, which caused above-expected ASM-triggered mass-wasting equivalent to that caused by 2.0 average ASM seasons. However, the increased mass wasting across the period 2016 – 2018 is actually found to be the result of a sudden increase in road-construction, with mass-wasting due to road-tipping in this period equivalent to that caused by 2.6 average ASM seasons. These results show that, in the Himalayas, extreme events and human activity can cause significant hillslope denudation above that expected from background ASM-driven mass-wasting.

How to cite: Jones, J., Boulton, S., Bennett, G., Whitworth, M., and Stokes, M.: Himalaya mass-wasting: impacts of the monsoon, extreme tectonic and climatic forcing, and road construction, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8702, https://doi.org/10.5194/egusphere-egu2020-8702, 2020.

Many rivers of the Indo-Gangetic Plain are prone to abrupt switching of channel courses causing devastating floods over some of the world’s poorest and most densely populated regions. Recent work has identified the gravel-sand transition as an avulsion node for the channels; notably the avulsion of the Kosi River in 2008 occurred in close proximity to its gravel-sand transition. The gravel-sand transition is a geomorphic feature observed within all major mountain-fed, and smaller foothill-fed Himalayan rivers ranging from 10 to 20 km downstream from the mountain front. It is characterised by an abrupt downstream reduction in grain size from gravel to sand and is often associated with a break in channel gradient, which suggests it is a relatively stable feature over the last few thousands of years.

However, new subsurface data from the Kosi mega-fan in eastern Nepal reveals 10-20 Ka gravels located ~50 km downstream from the current gravel-sand transition. The implication is that this key geomorphic boundary can periodically prograde considerably further into the Ganga Plains. A greater long-term (>10⁶ yrs) understanding of the controls on the gravel-sand transition is achieved by studying the stratigraphic record of the Miocene Siwalik Group, which is exhumed as a series of thrusted fault blocks at the Himalayan mountain front. The Siwalik succession is divided into three lithofacies units that coarsens upwards from siltstones and sandstones to coarse conglomerates. The units are termed the Lower, Middle and Upper Siwaliks respectively and reflect the current depositional environments found on the Ganga Plains.
The gravel-sand transition is recorded as the contact between the Middle and Upper Siwaliks. Significant gravel pulses have been identified directly below the Middle to Upper Siwalik contact and suggests that the gravel-sand transition is indeed mobile and can episodically prograde far into the plains. Sedimentological characteristics of the gravel pulses and sediment entrainment calculations suggest that extreme events (e.g. enhanced monsoon, earthquakes and GLOFS) can force gravel far into the Ganga Plains, impacting the position the gravel-sand transition. These episodes of distant gravel progradation must represent extreme floods from which the sedimentological system must take many years to recover. Such events are beyond the historic timescales of human narrative, and hence have not been recognised as a risk to the populations of the plains.

How to cite: Quick, L., Sinclair, H., Attal, M., Sinha, R., and Kumar, R.: Stability of the gravel-sand transition of the Ganga Plains recorded in Siwalik stratigraphy; implications for extreme floods, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-675, https://doi.org/10.5194/egusphere-egu2020-675, 2020.

Co-seismic landslides can mobilise up to 3 km³ of loose sediment within minutes. However, the export rate of this sediment is largely unconstrained. For example, it is estimated that a decade after the 2008 Wenchuan earthquake at least 90% of the co-seismic sediment remains stored on the hillslope. Post-earthquake debris flows are the main conduit by which such hillslope debris reaches the fluvial network but the mechanics that govern the triggering and runout of such flows remain unclear and as such they appear to behave largely unpredictably.  Material grain size is a key control on both triggering and runout, since it affects both hydrological (e.g. water loss during flow; saturation state before triggering) and frictional properties of the system. However, our understanding of the role of grain size in the genesis and evolution of debris flows remains poorly explored, largely due to limitations in real field data. Existing estimates for landslide and debris flow deposit grain size distributions (GSDs) are currently limited by 1. inconsistency of applied methods; 2. the very poor sorting of these sediments; 3. inaccessibility, and 4. inherent intra-deposit variability in GSD. 

Our research aims to better understand the role of grain size using an unprecedentedly detailed set of field-constrained GSDs across the post-seismic landslides and debris flows of the 2008 Wenchuan earthquake. Here we present data quantifying the grain size distribution across two debris flows using two different techniques. The two debris flows occurred in response to prolonged rainfall in August 2019 and mobilised co-seismic debris from the 2008 earthquake. In the field, we selected four to eight 1 m x 1 m x 0.5 m pits along the centre line of each debris flow at regular intervals and sieved the pit material into 8 cm, 4 cm, 2 cm and 1 cm fractions at 10 cm depth increments. Boulders >8 cm were measured and weighed individually. Smaller samples were then collected from the finer fraction (<1 cm) and sieved further in the laboratory. The coarse fraction was independently constrained from calibrated photogrammetry, and this was coupled to drone surveying to ensure the coarsest fraction (≥1 m) was correctly represented. This study presents a detailed estimate of post-earthquake debris flow GSDs with the overarching aim to better understand sediment transport and deposition from debris flows in the years following an earthquake.

How to cite: Harvey, E., Fan, X., Hales, T., Hobley, D., Liu, J., Xu, Q., and Huang, R.: The remobilisation of seismically-sourced sediment by debris flows in Wenchuan: A grain size perspective, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19439, https://doi.org/10.5194/egusphere-egu2020-19439, 2020.

The 232 CE Hatepe Eruption of Taupo Volcano, New Zealand (also referred to as Taupo Eruption), was one of the most violent and complex silicic eruptions worldwide in the last 5,000 years. The pyroclastic sequence was subdivided into 7 distinct stratigraphic units that reflect diverse eruption mechanisms with pumice fallout unit 5 (Taupo Plinian) and unit 6 (Taupo Ignimbrite) contributing the largest volumes, an estimated 5.8 km³ and 12.1 km³ DRE respectively. The non-welded Taupo Ignimbrite was emplaced by a highly energetic flow over a near-circular area of 20,000 km² around the vent, reaching distances of 80±10 km. It consists of an irregular basal layer and a thicker pumice-dominated main unit containing varying proportions of pumice clasts, vitric ash and dense components, overlain by a thin co-ignimbrite ash bed. The main ignimbrite unit shows two distinct facies, a landscape-mantling veneer deposit that gradually decreases from 10 m proximal thickness to 15-30 cm distally and a more voluminous, up to 70-m thick valley-ponded ignimbrite that filled depressions and smoothed out the landscape.

The sudden influx of vast volumes of loose pyroclastic material choked the drainage systems around the volcano, resulting in a large-scale geomorphic and sedimentary response. While previous work focused on major river catchments north to southeast of the volcano, we aim at characterising and quantifying landscape adjustment and remobilisation processes to the west, using stratigraphic, sedimentologic and geomorphic field studies of the volcaniclastic sequences along the Ongarue and Whanganui River valleys. Our working hypothesis involves a four-stage landscape response model based on previously described mass-wasting processes in the aftermath of large explosive eruptions: 1) large-scale remobilisation of ignimbrite veneer material from sloping surfaces by series of debris and hyperconcentrated flows, emplacing lahar deposits across the ignimbrite dispersal area and beyond, 2) cutting of steep channels into valley-ponded ignimbrite and resedimentation as lahar deposits downstream, 3) gradual widening of channels leading to establishment of an active channel with adjacent floodplains as sediment yields decrease and the landscape restabilises, represented by normal stream flow and flood deposits in the ignimbrite dispersal area and a shift from lahar to fluvial- dominated sequences downstream, and 4) return to pre-eruption sediment yields resulting in further downward incision to the original bedrock channel bed and prevailing fluvial sedimentation processes with remnants of primary and reworked deposits preserved as terraces along the valley walls.

Here we present initial results on the stratigraphy of the volcaniclastic sequence and the sedimentary characteristics and dispersal of the identified lithofacies associations, which range from debris-flow and hyperconcentrated-flow to pumiceous fluvial deposits. Tempo-spatial variations in deposit characteristics are due to differences in source material, flow type, and nature of the source area and depositional environment.

How to cite: Zernack, A. V. and Procter, J. N.: Large-scale mass-wasting processes following the 232 CE Hatepe Eruption of Taupo Volcano, New Zealand - Sedimentary features and dispersal of reworked Taupo Ignimbrite in the Ongarue River valley, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11852, https://doi.org/10.5194/egusphere-egu2020-11852, 2020.

Over the past decades, luminescence has been widely used for dating sedimentary deposits. Several recent publications suggest luminescence signals can also be used to investigate fluvial transport. Here we explore what information luminescence signals yield in past and present sediment dynamics in the Rangitikei River (RR), New Zealand (Bonnet et al., 2019).

We present a dataset of 30 samples from fluvial terraces and modern river sediments of the RR. For each of the samples, we measured pIRIR luminescence signals of 300 individual sand-sized grains of feldspar (Reimann et al., 2012). We interpret results to evaluate differences between past and modern transport conditions, and to infer information on lateral input of bedrock particles in different river sections.

The information obtained from the single-grain analysis is incredibly rich, and requires new metrics for interpretation. To quantify the percentage of grains that were eroded from bedrock (or very old deposits) and re-deposited with minimal light-exposure, we identified grains for which the pIRIR signal is above 85% of full saturation (Wintle, 2006). For grains below this saturation threshold, we used the bootstrapped minimum age model (Galbraith et al.,1999; Cunningham and Wallinga, 2012) to determine the palaeodose, the best estimate of the natural radiation dose received by grains since their last deposition and burial event. For the modern deposits, we interpret the palaeodose to indicate the light-exposure of the best-bleached grains. Thereby, it provides a proxy of fluvial transport distance of the sediment grains.

For the modern river sediments we obtain palaeodoses between 2 and 6 Gy. A decreasing trend in palaeodose downstream suggests that part of the grains are transported through the entire system and are gradually bleached through light exposure during this process. The downstream trend in palaeodose of the RR is influenced by the connection of a major tributary, the Kawhatau River (KR), characterized by higher palaeodoses. Based on the observed trends, we estimate that the KR contributes three times more to modern sediment flux down the confluence than the upstream RR. Moreover, we observe that downstream of the confluence the percentage of saturated grains increase, which implies significant local input of bedrock particles from valley sides.

Data from recent (Holocene) autogenic fluvial terraces were acquired downstream the RR/KR confluence. They show a high to very high ratio of saturated grains (30-70%). We also document a downstream increasing trend of the percentage of saturated grains in these fluvial terraces, much stronger than for modern deposits. The maximum is observed for terraces at elevation of +28/+34 m, with an input of saturated grains that doubles over a distance of 100 km. As a consequence, saturated grains represent up to 70 % of the grain population in the most downstream sample. This implies a stronger lateral input of bedrock particles in the past, during recent incision of the river and a significant contribution of valley walls to the sediment flux of the RR, probably through landslides and/or lateral fluvial erosion.

How to cite: Guyez, A., Bonnet, S., Reimann, T., and Wallinga, J.: Fluvial transport dynamics in the Rangitikei River (New Zealand) unravelled through single-grain feldspar luminescence, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8160, https://doi.org/10.5194/egusphere-egu2020-8160, 2020.

Total 99 surface sediment samples were obtained from eastern continental margin of Korea from Uljin to Busan below water depth 500 m to investigate the spatial variability of surface sediments. Mean grain size (Mz) of surface sediments ranges from 1.74 to 9.70 Φ (mean of 6.19±2.28 Φ), fine-grained sediments were mainly deposited along the coastal line on the Korea Strait Shelf Mud (KSSM) and Hupo Basin, whereas, coarse-grained sediments were covered on the Hupo Bank and southern continental margin. TOC content of surface sediments ranges from 0.09 to 3.27% (mean of 1.36±0.83%) and spatial variation is similar with that of Mz. Spatial distribution patterns of Al (1.56~10.98 %), K (0.94~3.29%), Ti (0.04~0.37%), Ni (1.97~38.18 mg/kg), Co (1.28~14.31 mg/kg), Cs (0.78~10.47 mg/kg), and total REEs (39.11~173.80 mg/kg) were also similar with that of Mz (r > 0.70). Generally, contents of geochemical element were lower in coarse-grained sediments on the Hupo Bank and southern continental margin and relatively higher in fine-grained sediments on the KSSM and Hupo Basin. On the contrary, Ba (126.58~476.35 mg/kg) showed opposite pattern, high Ba contents were observed in coarse-grained sediments on the Hupo Bank and southern continental margin while, lower contents showed in fine-grained sediments. Surface sediments of the eastern continental margin of Korea could be divided into four types based on characteristics of geochemical element: Type I sediments were obtained from on the Hupo Bank and outer shelf/shelf break of the southern continental margin and might be composed with relic sediments formed during the Miocene and/or Last Glacial Maximum. Type II sediments were obtained from outer shelf of the southern continental margin especially beside of the Korea Trough and believed to be coarse-grained sediments deposited during the glacial age derived through the Korean Trough. Type III sediments which covered on the KSSM were mostly composed with fine-grained sediments. KSSM was deposited during the Last Glacial Maximum and consist of mixtures of sediments discharged from Chinese rivers and Nakdong River. Type IV sediments were mostly covered on the Hupo Basin. Sediments on the Hupo Basin were deposited during the Quaternary but sediment provenance should be differ from KSSM and it might be originated from small streams near the Hupo Basin.

How to cite: Um, I.: Spatial variability of surface sediments on the eastern continental margin of Korea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2491, https://doi.org/10.5194/egusphere-egu2020-2491, 2020.

In tectonically active landscapes, fault movement drives both the creation of accommodation space (i.e., basins), and the production of topography on which geomorphic processes act (i.e., mountains). The action of fluvial processes on those mountains will route eroded sediment into the basins; in many extensional mountain belts, this leads to the deposition of coarse alluvial fans or Gilbert deltas in the hanging-walls of normal faults as they slip and create accommodation space. The stratigraphic architecture and sedimentary characteristics of such deposits clearly respond to and thus in principle can record the tectono-climatic environment controlling the system. This implies that key stratigraphic variables, such as grain size and unit thicknesses, can be quantitatively inverted to recover a tectono-climatic history. However, confounding variables also active in erosional-depositional systems (e.g., far-field base level control, stochastic processes, signal degradation during transport) may complicate attempts to decode this archive and may buffer or shred tectono-climatic signals before they are preserved.

The well-exposed early to middle Pleistocene deltaic stratigraphy of the Corinth Rift, central Greece, provides a rare opportunity to test these ideas quantitatively. Here, we present a preliminary data set attempting to decode the geomorphic and hence tectono-climatic history of a key section of the rift directly from the grain size and architecture of a very large (~500 m thick), fault controlled, and now uplifted Gilbert delta in the Kerinitis valley, located on the southern margin of the Gulf of Corinth. We used a series of high-resolution drone surveys to obtain 27 vertical transects through the incised delta, from which detailed grain size and sediment thickness data were obtained from photogrammetric analyses (~10,000 images). Our data enabled us to produce a highly detailed correlation of stratal horizons within the deltaic package, from which we were able to describe the evolution of grain size trends both downstream and through the ca. 800 ky lifespan of the delta. We are able to resolve a marked acceleration of the driving fault from the delta stratigraphy itself, which is recorded in a sudden increase in downstream fining rate, driven by more rapid extraction of sediment from the river supplying material to the delta. The timing of this increase correlates with independent constraints from stratigraphic form on the onset of “rift climax” in this delta. Post fault acceleration, we demonstrate that the fining rates begin to fall back, consistent with transient response to tectonic perturbation in the upstream catchment and with a wave of incision sweeping up through the terrestrial system. Our results demonstrate that sophisticated insights into fault evolution can be drawn from deltaic stratigraphy, and emphasise the importance of transient landscape response in creating rift zone sedimentary archives.

How to cite: Hobley, D. and Whittaker, A.: Quantitative reconstruction of landscape dynamics and tectonics from sediment calibre and architecture: an example from the Kerinitis megadelta, Gulf of Corinth, Greece, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10229, https://doi.org/10.5194/egusphere-egu2020-10229, 2020.

A large portion of freshwater and sediment is exported to the ocean by just several of the world's major rivers. Many of these mega-rivers are under significant anthropogenic pressures, such as damming and sand mining, which are having a significant impact on water and sediment delivery to deltaic ecosystems. However, accurately measuring the total sediment flux and its mean physicochemical composition is difficult in large rivers due to hydrodynamic sorting of sediments. To account for this, we developed an updated semi-empirical Rouse modeling framework, which synoptically predicts sediment concentration, grain size distribution, and mean chemical composition (organic carbon wt%, Al/Si ratio) with depth and across the river channel.

We applied this model to derive new sediment flux estimates for the Irrawaddy and the Salween, the last two free-flowing mega-rivers in Southeast Asia, using a newly collected set of suspended sediment depth samples, coupled to ADCP-measured flow velocity data. Constructing sediment-discharge rating curves, we calculated an annual sediment flux of 326 (68% confidence interval of 256-417) Mt/yr for the Irrawaddy and 159 (109-237) Mt/yr for the Salween, together accounting for 2-3% of total global riverine sediment discharge. The mean flux-weighted sediment exported by the Irrawaddy is significantly coarser (D₈₄ = 193 ± 13 µm) and OC-poorer (0.29 ± 0.08 wt%) compared to the Salween (112 ± 27 µm and 0.59 ± 0.16 wt%, respectively). Both rivers export similar amounts of particulate organic carbon, with a total of 1.9 (1.0-3.3) Mt C/yr, contributing ~1% of the total riverine POC export to the ocean. These results underline the global significance of the Irrawaddy and Salween rivers and warrant continued monitoring of their sediment fluxes, given the increasing anthropogenic pressures on these river basins.

How to cite: Baronas, J. J., Tipper, E. T., Bickle, M. J., Hilton, R. G., Stevenson, E. I., Hackney, C., Parsons, D., Darby, S., Larkin, C. S., and Khaing, A. M.: Revised sediment transport model for estimation of suspended sediment flux and chemical composition of the Irrawaddy and Salween rivers, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10918, https://doi.org/10.5194/egusphere-egu2020-10918, 2020.

The St. Lawrence Estuary is a large seasonally ice-covered estuarine system in eastern Canada. The suspended particulate matter (SPM) dynamic in this estuary is strongly influenced by winds, tides, river runoff, and coastal jets. The particle size distribution (PSD) is an important property of the SPM as it may affect sinking rates, particle re-suspension and distribution of pollutants. A deeper understanding of the PSD helps to determine the vertical and horizontal fluxes of the matter in the water column.

Although information exists concerning the composition and the SPM dynamic in the St. Lawrence Estuary in summer, there is a lack of recent spatial and vertical characterization while no winter data is available. Thus, the purpose of this study is to better characterize the SPM particle size and sedimentological properties in the St. Lawrence Estuary during summer and winter conditions.

The PSD was measured using a laser diffractometer LISST-100X directly in the water column during the summer of 2010 and in the laboratory using water samples taken at discrete depths for winter 2019. X-ray diffraction and fluorescence analysis were used for the characterization of the particles’ mineralogical and chemical composition from which the detrital sources were evaluated.

Results show that SPM concentration is spatially more variable during summer than in winter. In contrast, the PSD’s is inverted in winter with relatively smaller size particles upstream and larger particles downstream. The depth distribution of the PSD shows slight differences between the seasons. In summer, larger particles are mostly present at the pycnocline whereas in winter, larger particles reach deeper depths and are mostly of inorganic origin. Throughout the estuary for both seasons, particulate inorganic matter contributed the most to total SPM. The winter mineralogical and chemical composition of the SPM was similar throughout the estuary confirming previous studies indicating an origin from the Canadian Shield. Taken as a whole, this study provided valuable new information on suspended matter dynamics in a large Subarctic estuarine environment.

How to cite: Fabris, A.-S., Larouche, P., and Montero-Serrano, J.-C.: Characterization of the St. Lawrence Estuary's suspended matter size and composition, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4280, https://doi.org/10.5194/egusphere-egu2020-4280, 2020.

Marine stratigraphy contains time-resolved information about the erosion of continents and its tectonic and climatic drivers. Quantitatively inverting marine stratigraphy for long-term terrestrial erosion histories requires numerical models that encompass the entire source-to-sink (S2S) system. Because inversion schemes require many model realizations to constrain free parameters against a misfit function, S2S models must be efficient (both in terms of allowing large time steps and scaling well for large problems) and have only a few parameters. Accordingly, most previous S2S models have treated seafloor evolution as a diffusion problem where sediment flux depends linearly on local topographic gradient. Such approaches have shown success in shallow marine settings like the continental shelf. However, they are less likely to apply to deeper marine environments where large deposits are observed and where nonlocal sediment transport processes (e.g., turbidity currents or marine debris flows) dominate sediment fluxes.

We present a unified modeling approach for coupling terrestrial and marine erosion, sediment transport, and deposition from the continent to the abyssal plain. Our model is based on the erosion-deposition family of models, where sediment flux is tracked across the landscape and seascape. Above sea level, erosion and deposition depend on river discharge, local slope, and sediment flux. Below sea level, local slope and sediment flux drive topographic change. The equations governing the terrestrial and marine domains take the same basic form such that a single semi-implicit numerical solution based on Gauss-Seidel iteration can be used across the whole S2S system. The solution scheme is near O(N) complexity in that the number of iterations required typically does not increase significantly with increasing grid resolution. The S2S model contains only five total parameters. We show preliminary analytical and numerical results and sensitivity analyses, and discuss the applicability of the model for the efficient inversion of deep marine stratigraphic data.

How to cite: Shobe, C. M., Braun, J., Yuan, X., Campforts, B., Guillocheau, F., and Robin, C.: Toward a unified model for sediment transport from terrestrial source to abyssal-plain sink, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11052, https://doi.org/10.5194/egusphere-egu2020-11052, 2020.

Since the 1960’s, large landscape modifications were carried out to improve agriculture productivity. One of these changes was the ploughing of humid plains together with the installation of subsurface drainage, which currently represents 10 % of arable lands in the world. Studies have shown the impact of subsurface drainage on the water regime, and especially decreases in flow peaks. Drainage increases water and sediment connectivity. Less effort was devoted to investigate the impact on the erosion dynamics and very few studies were designed at the catchment scale. However, the understanding of water and suspended solids dynamics from field to catchment outlet is a key to set efficient conservation measures to reduce erosion up. Here we focus on water and suspended solids dynamics from the soil profile scale to the field scale. We propose to trace both water and suspended solids to determine the relative contributions between surface and subsurface sources. Water tracing gives indication on  pathways while suspended solids trace sources (i.e. soil surface vs. deeper soil). The study site is composed of a 5ha field within a 2500 ha agricultural catchment representative of the French agricultural intensive openfield catchments. The studied field is representative of the catchment. It is a cereal crops openfield. Two drainage methods exist in the field: subsurface drainage with drains 120 cm-deep and surface drainage with artificial channels created after the winter seeding. The soil in this field is a loamy clay soil with clay floor at 45 cm of depth. Quantification of suspended solids and water fluxes (surface and subsurface) are monitored at high temporal resolution both at the field (since January 2019) and catchment (since September 2013) scale. Since November 2019, we trace water flows (rain, soil water subsurface flow and overland flow) using water ions and stable isotopes. Suspended solids are analysed through their mineralogy and primary particle size. At the field scale, the first results show a rapid response of surface drainage to rain inputs - confirmed by ions tracing - and suspended solids are mainly coming from surface drainage. Subsurface drainage reacts with a significant delay. Ions tracing shows that subsurface runoff seems to result from a replacement of older soil water by rain inputs.

How to cite: Gaillot, A., Delbart, C., Vanhooydonck, P., Cerdan, O., and Salvador-Blanes, S.: Impact of tile-drainage on the hydro-sedimentary responses of hydromorphic agricultural soils by tracing water and suspended solids from the field to the catchment scale., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21409, https://doi.org/10.5194/egusphere-egu2020-21409, 2020.

Soil erosion and its accompanying on- and off-site effects represent a serious threat to the environment. Over the last years many studies have been successfully carried out using compound-specific stable carbon isotopes of fatty acids (FA) and n-alkanes to characterize source soils and attribute suspended sediments or sedimentary archives to the characterized sources. One worthy next aim would be the extrapolation to large catchments. Important for this is a deepened knowledge about the variability of the signals over different temporal and spatial scales, which has so far been largely neglected, with the exception of a handful of studies. With this knowledge it should be possible to understand processes better in the catchment and deliver improved interpretation and representation of empirical data, ultimately supporting suitable mitigation actions to minimize sediment transport to aquatic environments.

In our study we present compound-specific stable isotope data of long-chain FAs from two neighbouring yet distinct (in terms of soils and land use) catchments, Aller and Horner Water (17.6km² and 22km² respectively), Exmoor, South-west England. To capture the spatial heterogeneity, we analysed possible source soils from different land-uses, including moorland, heather, forest, permanent grassland, arable and ley grassland on different soil textures (clay, loam, and peat) for their FA stable isotope signature. A very interesting outcome is the apparent influence that soil texture has on the stable isotope signal of the FAs of the same land-use units. To consider temporal variability, we present isotope data for FAs of high flow events from the main outlet and 4 sub-catchments of Aller and Horner waters over the course of one year. Three of these events have been sampled at a high temporal resolution of up to 24 sediment samples per event.

Previous research by our group found a significant importance of the seasonal variability in the suspended sediment origin in the Baldegg Lake catchment, Switzerland.  In addition to such seasonal understanding, this study will allow us to understand the short-term variability in the origin of the transported sediments during storm events and to link it with high spatial resolution of the characterized source soils.

How to cite: Birkholz, A., Glendel, M., Brazier, R. E., and Alewell, C.: Variability of sediment source attribution with CSSI over temporal and spatial scales – from soil texture to land-use unit and from event to seasonality., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6933, https://doi.org/10.5194/egusphere-egu2020-6933, 2020.

Abstract: Based on the deep studies of existing mathematical models, a mathematical model that expresses the dynamic of transport and transformation of heavy metals in the rivers has been presented. In this model, the basic principles of chemistry in the environment, hydraulic and fluid transfer dynamics have been used as well as recent studies of researchers. The effects of sediment on the transfer and evolution of heavy metals pollution can be investigated by the proposed models. For example, the evolution and transport of heavy metal pollutants in a steady state flow containing sediment are studied using the present model. The results of theoretical analysis and calculations show that transport and transformation of heavy metal pollution in sediment laden flows, not only have common characteristics of general pollutant but also have features of transport and transformation induced by the movement of sediments.

Keywords: Numerical Simulation; Heavy Metal; Pollution; Sediment; Finite Difference Method.

How to cite: khalilzadeh poshtegal, M., Noury, M., and mirbagheri, S. A.: Numerical Modeling of Heavy Metal Pollution in the Rivers, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3691, https://doi.org/10.5194/egusphere-egu2020-3691, 2020.

Grain size distribution (GSD) data have been widely used in Earth sciences, especially Quaternary Geology, due to its convenience and reliability. However, the usages of GSD are still oversimplified. The geological information contained in GSD is very abundant, but only some simplified proxies (e.g. mean grain size) are widely used. The most important reason is that GSD data are hard to interpret and visualize directly.

To overcome this, some researchers have developed the methods to unmix the mixed multi-modal GSD to some components to make the interpretation and visualization easier. These methods can be divided into two routes. One is end-member analysis (EMA) which takes a batch of samples for the calculation of the end-members. Another is called single-specimen unmixing (SSU) (Sun et al., 2002) which treats each sample as an individual. The key difference between the two routes is that whether the end-members of a batch of samples are consistent. EMA believes that the end-members between different samples are consistent, the variations of GSD are only caused by the changing of fractions of the end-members. On the contrary, SSU has no assumption on the end-members, i.e. it admits that the end-members may vary between different samples. Some mature tools (Paterson and Heslop, 2015; Dietze and Dietze, 2019) taking the EMA route have appeared, but there is no available public and easy-to-use tool for SSU.

Here we introduce a free and open-source GUI tool which is called QGrain, it can help researchers to analyze the GSD data easily and bring new insights for the interpretation of GSD. QGrain is based on SSU but applied some algorithms (e.g. data preprocessing and global optimization) to improve its precision and robustness. It supports Lognormal or Weibull as the base distribution and it is easy to add more base distributions. QGrain can handle different types of sediments (e.g. aeolian, fluvial and lacustrine deposits). QGrain can export all detailed data and generate the charts automatically.

How to cite: Liu, Y., Liu, X., and Sun, Y.: A new easy-to-use tool for grain size distribution analysis, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2498, https://doi.org/10.5194/egusphere-egu2020-2498, 2020.

Laser diffraction grain size data have been widely used in paleoenvironmental reconstructions as physicochemical alteration-related proxies. Many studies are available on comparison of different laser diffraction devices, optical theories and optical settings. The ignorance of some uncertainty factors can lead to poorly comparable granulometric datasets. Other important factor leading to the aforementioned effect is the inadequate chemical pretreatment procedures which are often overlooked, but are capable to basically affect the results. In this study we examine a few past and recent sediment types from different geomorphological environments from the Carpathian Basin: lake and fluvial sediments, paleosols and loess. Our aim is to review and create a reliable methodology for laser diffraction particle size analysis and optical particle shape investigations. We compare widely used pretreatment methods -which can be found in the literature- with each other. We are also taking into account that different sediment types need different pretreatment methods. We can state that the duration of chemical pretreatment can affect the optical properties (color), the texture and the mineral composition of the sediments, as well as the size and shape of mineral particles in the samples. The changes in these significant parameters can mislead the researcher’s main objectives. The study is supported by the ÚNKP-19-3 New National Excellence Program of the Ministry for Innovation and Technology. Support of the National Research, Development and Innovation Office NKFIH K120620 is gratefully acknowledged.

How to cite: Gresina, F., Varga, G., Szabó, L., Király, C., and Szalai, Z.: The effect of chemical pretreatment on grain size results of past and recent clastic sediments , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21401, https://doi.org/10.5194/egusphere-egu2020-21401, 2020.