Morphodynamic limits to environmental signal propagation across landscapes and into strata

The sedimentary record contains unique information about landscape response to environmental forcing at timescales that far exceed landscape observations over human timescales. However, stochastic processes can overprint and shred evidence of environmental signals, such as sediment flux signals, and so inhibit their transfer to strata. Our community currently lacks a quantitative framework to differentiate between environmental signals and autogenic signals in field-scale analysis of strata. Here we develop a framework and workflow to estimate autogenic thresholds for ancient sediment routing systems. Crucially these thresholds can be approximated using measurements that are readily attainable from field systems, circumventing the low temporal resolution offered by strata. This work demonstrates how short-term system dynamics can be accessed from ancient sediment routing systems to place morphodynamic limits on environmental signal propagation across ancient landscapes and into strata.

Time-Of-Flight monitoring reveals higher sediment redistribution rates related to burrowing animals than previously assumed

Burrowing animals influence surface microtopography and hillslope sediment redistribution, but changes often remain undetected due to a lack of autonomous high resolution field monitoring techniques. In this study we present a new approach to quantify microtopographic variations and surface changes caused by burrowing animals and rainfall-driven erosional processes applied to remote field plots in arid and mediterranean Chile. We compared the mass balance of redistributed sediment within plot areas affected and not affected by burrowing animals, quantified the cumulative sediment redistribution caused by animals and rainfall, and upscaled the results to the hillslope scale. The new instrument showed a very good detection accuracy. The cumulative sediment redistribution within areas affected by burrowing animals was higher (−10.44 cm3 cm−2 year−1) in the mediterranean than the arid climate zone ( −1.41 cm3 cm−2 year−1). Daily sediment redistribution during rainfall within areas affected by burrowing animals were up to 350 % / 40 % higher in the mediterranean / arid zone compared to the unaffected areas, and much higher than previously reported in studies not based on continuous microtopographic monitoring. Furthermore, 38 % of the sediment eroding from the burrows accumulated within the burrow entrance while 62 % was incorporated into overall hillslope sediment flux. The cumulative sediment excavation by the animals was 14.62 cm3 cm−2 year−1 in the mediterranean and 16.41 cm3 cm−2 year−1 in the arid climate zone. Our findings can be implemented into long-term soil erosion models that rely on soil processes but do not yet include animal-induced surface processes on microtopographical scales in their algorithms.

New Workflow of Sediment Mass Balancing, from Local Datasets, for Predicting Basin Scale Trends

Objectives/Scope Sediment volumetric budget estimates are very important input parameters for process-based depositional modelling (forward stratigraphic modelling). This paper presents a new integrated approach for analyze sediment volumetric budgets in sedimentary basins that is based on the reconstruction of regional grain size trends. In subsurface studies of sediment routing systems, noticeable uncertainties in estimated total sediment volumes occur when available datasets are limited to local areas that do not cover the entire sediment routing system. These uncertainties also affect models of catchment areas, structural uplift, and denudation rates as well as net:gross predictions. Methods, Procedures, Process The new integrated approach focuses on reconstructing sediment budgets for entire sediment fairways from limited local datasets. It uses a combination of sediment mass balancing and local grain size distributions to predict basin-wide grain size distributions. The comparison of local grain size to fairway-scale grain size trends is key in correcting sediment volumetrics for significantly reduced uncertainties in catchment reconstruction and net:gross ratios predictions at the scale of sediment fairways, sub-basins, prospects and exploration/production fields. Results, Observations, Conclusions The new approach has been applied successfully to two subsurface continental to marine delta systems. They cover periods of approximately 7 My in total and include four limited local areas of interest (AOI). These local AOIs measure 200×200 km, while the entire sub-basin measures 500×800 km. The new approach indicates that only up to 40% of the total sediment volume of each fairway could be captured by previous methodologies with limited local areas of interest. A maximum of 70% of the entire sink sediment volume could be incorporated in local areas of interest. The new approach presented in this paper significantly lowers the uncertainties in sediment volume estimates, depositional rates and lithology distribution input parameters in forward stratigraphic modelling. For the two case studies, previous sediment flux models indicated rates of 10,000 km/Myr. The new integrated approach indicates that sediment flux actually reached 30,000 km/Myr with major implications for sediment distribution, net:gross prediction and catchment size and denudation rates estimates. Novel/Additive Information The new integrated approach reduces uncertainties in catchment size and tectonic exhumation rate estimates for clastic depositional systems. It provides lower uncertainty parameters (sediment volume, source locations, sediment fractions, diffusion coefficients) for forward stratigraphic modelling, e.g., for reservoir quality prediction in hydrocarbon exploration. In fundamental research, provenance analyses can be better constrained by improved catchment size prediction and sediment grain size distribution models for sink areas

Landscape responses to dynamic topography and climate change on the South African source-to-sink system since the Oligocene

The South African landscape displays important lithological and topographical heterogeneities between the eastern, western margins and the plateau. Yet the underlying mechanisms and timings responsible for this peculiar layout remain unclear. While studies have proposed a post-Gondwana uplift driver, others have related these heterogeneities to a more recent evolution induced by deep mantle flow dynamics during the last 30 million years. This theory seems supported by the rapid increase of sediment flux in the Orange basin since the Oligocene. However, the triggers and responses of the South African landscape to dynamic topography are still debated. Here we use a series of numerical simulations forced with Earth data to evaluate the contribution of dynamic topography and precipitation on the Orange river source-to-sink system since the Oligocene. We show that, if the tested uplift histories influence deposits distribution and thicknesses in the Orange sedimentary basin, they poorly affect the large-scale drainage system organisation and only strongly impact the erosion across the catchment for two of the four tested dynamic topography cases. Conversely, it appears that paleo-rainfall regimes are the major forcing mechanism that drives the recent increase of sediment flux in the Orange basin. From our simulations, we find that climate strongly smoothed the dynamic topography signal in the South African landscape and that none of the currently proposed dynamic topography scenarios produce an uplift high enough to drive the pulse of erosion and associated sedimentation observed during the Palaeocene. These findings support the hypothesis of a pre-Oligocene uplift. Our results are crucial to improve our understanding of the recent evolution of the South African landscape.

Sediment load determines the shape of rivers

Understanding how rivers adjust to the sediment load they carry is critical to predicting the evolution of landscapes. Presently, however, no physically based model reliably captures the dependence of basic river properties, such as its shape or slope, on the discharge of sediment, even in the simple case of laboratory rivers. Here, we show how the balance between fluid stress and gravity acting on the sediment grains, along with cross-stream diffusion of sediment, determines the shape and sediment flux profile of laminar laboratory rivers that carry sediment as bedload. Using this model, which reliably reproduces the experiments without any tuning, we confirm the hypothesis, originally proposed by Parker [G. Parker, J. Fluid Mech. 89, 127–146 (1978)], that rivers are restricted to exist close to the threshold of sediment motion (within about 20%). This limit is set by the fluid–sediment interaction and is independent of the water and sediment load carried by the river. Thus, as the total sediment discharge increases, the intensity of sediment flux (sediment discharge per unit width) in a river saturates, and the river can transport more sediment only by widening. In this large discharge regime, the cross-stream diffusion of momentum in the flow permits sediment transport. Conversely, in the weak transport regime, the transported sediment concentrates around the river center without significantly altering the river shape. If this theory holds for natural rivers, the aspect ratio of a river could become a proxy for sediment discharge—a quantity notoriously difficult to measure in the field.

Gas Emissions From the Western Aleutians Volcanic Arc

The Aleutian Arc is remote and highly active volcanically. Its 4,000 km extent from mainland Alaska to Russia’s Kamchatka peninsula hosts over 140 volcanic centers of which about 50 have erupted in historic times. We present data of volcanic gas samples and gas emission measurements obtained during an expedition to the western-most segment of the arc in September 2015 in order to extend the sparse knowledge on volatile emissions from this remote but volcanically active region. Some of the volcanoes investigated here have not been sampled for gases before this writing. Our data show that all volcanoes host high-temperature magmatic-hydrothermal systems and have gas discharges typical of volcanoes in oceanic arcs. Based on helium isotopes, the western Aleutian Arc segment has minimal volatile contributions from the overriding crust. Volcanic CO2 fluxes from this arc segment are small, compared to the emissions from volcanoes on the Alaska Peninsula and mainland Alaska. The comparatively low CO2 emissions may be related to the lower sediment flux delivered to the trench in this part of the arc.

Spatio-temporal variability and controlling factors for postglacial erosion dynamics in the Dora Baltea catchment (western Italian Alps)

Disentangling the influence of bedrock erodibility from the respective roles of climate, topography and tectonic forcing on catchment denudation is often challenging in mountainous landscapes due to the diversity of geomorphic processes in action and of spatial/temporal scales involved. The Dora Baltea catchment (western Italian Alps) appears the ideal setting for such investigation, since its large drainage system, extending from the Mont Blanc Massif to the Po Plain, cuts across different major litho-tectonic units of the western Alps, whereas this region has experienced homogeneous climatic conditions and glacial history throughout the Quaternary. We acquired new 10Be-derived catchment-wide denudation rates from 18 river-sand samples collected both along the main Dora Baltea river and at the outlet of its main tributaries. The inferred denudation rate results vary between 0.2 and 0.9 mm/yr, consistent with values obtained across the European Alps by previous studies. Spatial variability in denudation rates was statistically compared with topographic, environmental and geologic metrics. 10Be-derived denudation records do not correlate with the distribution of modern precipitation and rock geodetic uplift. We find, rather, that catchment topography, in turn conditioned by bedrock erodibility (litho-tectonic origin) and glacial overprint, has the main influence on denudation rates. We calculated the highest denudation rate for the Mont Blanc Massif, whose granitoid rocks and long-term tectonic uplift support steep slopes and high relief and thus favour intense glacial/periglacial processes and recurring rock fall events. Finally, our results, in agreement with modern sediment budgets, demonstrate that the high sediment input from the Mont Blanc catchment dominates the Dora Baltea sediment flux, explaining the constant low 10Be concentrations measured along the Dora Baltea course even downstream the multiple junctions with tributary catchments.