ALBERT

Description: Globally, freshwater systems are degrading due to excessive water withdrawals. We estimate that if rivers’ environmental flow requirements were protected, the associated decrease in irrigation water availability would reduce global yields by ~5%. As one option to increase food supply within limited water resources, we show that dietary changes towards less livestock products could compensate for this effect. If all currently grown edible feed was directly consumed by humans, we estimate that global food supply would even increase by 19%. We thus provide evidence that dietary changes are an important strategy to harmonize river flow protection with sustained food supply.

Description: We present a study to estimate the large-scale landscape history of a continental margin, by establishing a source-to-sink volume balance between the eroding onshore areas and the offshore basins. Assuming erosion as the primary process for sediment production, we strive to constrain a numerical model of landscape evolution that balances the volumes of eroded materials from the continent and that deposited in the corresponding basins, with a ratio imposed for loss of erosion products. We use this approach to investigate the landscape history of Madagascar since the Late Cretaceous. The uplift history prescribed in the model is inferred from elevations of planation surfaces formed at various ages. By fitting the volumes of terrigenous sediments in the Morondava Basin along the west coast and the current elevation of the island, the landscape evolution model is optimized by constraining the erosion law parameters and ratios of sediment loss. The results include a best-fit landscape evolution model, which features two major periods of uplift and erosion during the Late Cretaceous and the middle to late Cenozoic. The model supports suggestions from previous studies that most of the high topography of the island was constructed since the middle to late Miocene, and on the central plateau the erosion has not reached an equilibrium with the high uplift rates in the late Cenozoic. Our models also indicate that over the geological time scale, a significant portion of materials eroded from Madagascar was not archived in the offshore basin, possibly consumed by chemical weathering, the intensity of which might have varied with climate.

Description: We use a Landscape Evolution Model including flexural isostasy to investigate the influence of inherited foreland relief on the stratigraphic evolution of the retro-foreland domain during mountain building. We show models with four different types of initial relief in the foreland domain: at sea level, elevated (+300 m), a 1 km-deep and 100 km-wide foreland basin associated with either a forebulge at sea level or elevated at +300 m. During the first 10 Myr of simulation, the landscape evolution of the foreland is significantly altered by its inherited bathymetry/topography. The impact is then smoothed out once the foreland slope has stabilized and develops a transverse drainage network. Models record a long-term shallowing-up mega-sequence driven by the increase in sediment production rate in the uplifting range and the decrease in the rate of flexural accommodation space creation in the foreland basin. The initial relief of the foreland domain alters the timing of its transition from the under-filled to the over-filled phase. An initially deep foreland basin is twice as thick as an initially elevated foreland. It records deep marine deposits while a foreland initially at sea level records thin shallow marine and an elevated foreland records continental deposits. The forebulge is buried by continental deposits in an initially elevated foreland while it is buried by marine sediments in other models. Alluvial fans at the foot of the range are more elevated in initially elevated forelands. We discuss our results of modeled stratigraphic architecture in comparison with the Pyrenean, Alpine and Andean retro-foreland basins.

Description: Arctic river deltas define the interface between the terrestrial Arctic and the Arctic Ocean. They are the site of sediment, nutrient, and soil organic carbon discharge to the Arctic Ocean. Arctic deltas are unique globally because they are underlain by permafrost and acted on by river and sea ice, and many are surrounded by a broad shallow ramp. Such ramps may buffer the delta from waves, but as the climate warms and permafrost thaws, the evolution of Arctic deltas will likely take a different course, with implications for both the local scale and the wider Arctic Ocean. One important way to understand and predict the evolution of Arctic deltas is through numerical models. Here we present ArcDelRCM.jl, an improved reduced-complexity model (RCM) of arctic delta evolution based on the DeltaRCM-Arctic model (Lauzon et al., 2019), which we have reconstructed in Julia language using published information. Unlike previous models, ArcDelRCM.jl is able to replicate the ramp around the delta. We have found that the delayed breakup of the so-called “bottom-fast ice” (i.e. ice that is in direct contact with the bed of the channel or the sea, also known as “bed-fast ice”) on and around the deltas is ultimately responsible for the appearance of the ramp feature in our models. However, changes made to the modelling of permafrost erosion and the protective effects of bottom-fast ice are also important contributors. Graph analyses of the delta network performed on ensemble runs show that deltas produced by ArcDelRCM.jl have more interconnected channels and contain less abandoned subnetworks. This may suggest a more even feeding of sediments to all sections of the delta shoreline, supporting ramp growth. Moreover, we showed that the morphodynamic processes during the summer months remain active enough to contribute significant sediment input to the growth and evolution of Arctic deltas and thus should not be neglected in simulations gauging the multi-year evolution of delta features. Finally, we tested a strong climate-warming scenario on the simulated deltas of ArcDelRCM.jl, with temperature, discharge, and ice conditions consistent with RCP7–8.5. We found that the ramp features degrade on the timescale of centuries and effectively disappear in under 1 millennium. Ocean processes, which are not included in these models, may further shorten the timescale. With the degradation of the ramps, any dissipative effects on wave energy they offered would also decrease. This could expose the sub-aerial parts of the deltas to increased coastal erosion, thus impacting permafrost degradation, nutrients, and carbon releases.

Description: Large swaths of the Earth's high-northern latitude land surface consist of permafrost landscapes. Permafrost is a key factor for hydrology, ecology, biogeochemistry, and for human infrastructure. It contains one of the largest soil carbon reservoirs on Earth, which will partially be mobilised upon thaw and will accelerate global climate warming. A quantitative understanding of permafrost thaw is therefore essential. Surface manifestations of permafrost thaw include thermokarst lake growth and drainage, gully formation, and retrogressive thaw slumping. These processes exert important influences on carbon, energy, water, and sediment balances, with implications also on habitats and surrounding ecosystems. The polar regions, where most of the permafrost landscapes are found, have been experiencing enhanced warming due to Arctic amplification. The warming leads to changes in the balance of and feedback between surface processes associated with permafrost thaw. Being able to predict the future of these processes on a pan-Arctic scale under various climate scenarios is therefore important. One widely used tool for modelling energy balance and the freeze-thaw process of permafrost is CryoGrid. We aim to develop a physics-informed machine-learning framework to upscale and accelerate CryoGrid to predict the future evolution of surface processes arising from permafrost thaw. We first calibrate the ability of CryoGrid to reproduce observed surface manifestations of permafrost thaw. Next, we move towards utilising physics-constrained deep-learning techniques to enable the usage of CryoGrid on a pan-Arctic scale to efficiently make various predictions. The resulting predicted observables can be used to constrain or indicate specific climate variables.

Description: It is widely recognized that collisional mountain belt topography is generated by crustal thickening and lowered by river bedrock erosion, linking climate and tectonics. However, whether surface processes or lithospheric strength control mountain belt height, shape and longevity remains uncertain. Additionally, how to reconcile high erosion rates in some active orogens with long-term survival of mountain belts for hundreds of millions of years remains enigmatic. Here we investigate mountain belt growth and decay using a new coupled surface process and mantle-scale tectonic model. End-member models and the new non-dimensional Beaumont number, Bm, quantify how surface processes and tectonics control the topographic evolution of mountain belts, and enable the definition of three end-member types of growing orogens: type 1, non-steady state, strength controlled (Bm 〉 0.5); type 2, flux steady state, strength controlled (Bm ≈ 0.4−0.5); and type 3, flux steady state, erosion controlled (Bm 〈 0.4). Our results indicate that tectonics dominate in Himalaya–Tibet and the Central Andes (both type 1), efficient surface processes balance high convergence rates in Taiwan (probably type 2) and surface processes dominate in the Southern Alps of New Zealand (type 3). Orogenic decay is determined by erosional efficiency and can be subdivided into two phases with variable isostatic rebound characteristics and associated timescales. The results presented here provide a unified framework explaining how surface processes and lithospheric strength control the height, shape, and longevity of mountain belts.

Description: We introduce the first solution for simulating the formation and evolution of glaciers, together with their attendant erosive effects, for periods covering the combination of glacial and inter-glacial cycles. Our efficient solution includes both a fast yet accurate deep learning-based estimation of highorder ice flows and a new, multi-scale advection scheme enabling us to account for the distinct time scales at which glaciers reach equilibrium compared to eroding the terrain. We combine the resulting glacial erosion model with finer-scale erosive phenomena to account for the transport of debris flowing from cliffs. This enables us to model the formation of terrain shapes not previously adequately modeled in Computer Graphics, ranging from U-shaped and hanging valleys to fjords and glacial lakes.

Description: We present an O(n) complexity and implicit algorithm for the two-dimensional solution of the Stream Power Incision Model (SPIM) enriched by a discharge threshold term and taking into account variability in rainfall and thus discharge. The algorithm is based on the formulation developed by Deal et al. (2018) and the generalization of the FastScape algorithm (Braun & Willett, 2013) where the slope is approximated by first-order accurate finite difference. We consider a variety of discharge thresholds that vary in their dependence on channel slope. The algorithm requires finding the root of a non-linear equation using a Newton-Raphson iterative scheme. We show that the convergence of this scheme is unconditional, except for a narrow range of model parameters where the threshold increases with the slope and for low discharge variability. We also show that the rate of convergence of the iterative scheme is directly proportional to the slope exponent n in the SPIM. We compare the algorithm to analytical solutions and to numerical solutions obtained using a higher-order finite difference scheme. We show that the accuracy of the FastScape algorithm and its generalization presented here is comparable to other schemes for values of n 〉 1. We also confirm that the FastScape algorithm and its generalization to variable discharge+threshold conditions does not need to satisfy the CFL condition and provides an accurate solution for both small and very long time steps. We finally use the new algorithm to quantify how the existence of an erosional threshold strongly affects the length of the post-orogenic decay of mountain belts.