Alluvial river networks are key components of sedimentary systems. They transport sediment, supplied from eroding source regions, to downstream depositional sinks. Their slopes and the rates at which they transport sediment are controlled by their water and sediment supplies, which are sensitive to environmental conditions: temperature and precipitation rates influence erosion rates, and hence sediment supply; precipitation rates are also a first order control on water supply. Changes in sediment and water supply therefore drive changes in channel slope, which may be preserved in fluvial terrace sequences, and sediment-transport rates, which may be preserved in downstream stratigraphic records. Consequently, if we can understand this behaviour, we may be able to use geomorphic and stratigraphic archives to reconstruct past environmental change and its influence on landscapes. Here, we apply a physically based model describing the coupled evolution of sediment transport and slope along alluvial rivers. We show that aggradation, incision, and variation in sediment-transport rates can be damped and lag significantly behind environmental change, depending on the forcing timescale, and on network geometry and hydrology. When water supply is varied, variation in sediment output can in some cases be amplified and appear to lead the imposed forcing. These effects should be taken into account when interpreting fluvial terrace and sedimentary records. Contrasting patterns of aggradation and incision compared with sediment-transport rates implies that integrating information from geomorphic and stratigraphic archives will provide deeper paleo-climatic insights and a powerful test of our understanding of sedimentary systems.

Along the southeastern margin of the Tibetan Plateau, the onset of rapid fluvial incision during the Miocene is commonly attributed to growth of high topography. Recent recognition of lacustrine strata preserved atop interfluves, however, suggest that headward expansion of river networks drove migration of the topographic divide. Here, we explore the impact of this process on fluvial incision and sediment transport along the Yangtze River. Landscape evolution simulations using TopoToolbox (TTLEM and TVD-FVM) demonstrate that expansion of the Yangtze watershed into previously internally-drained basins since the Late Miocene could be responsible for 1-2 km of fluvial incision. The distribution of modern knickpoints and river profiles are consistent with this hypothesis. We suggest that increased erosive power associated with capture and basin integration drove accelerated incision and sediment transport during the Late Miocene. Progressive captures and integration of sub-basins could have driven punctuated episodes of rapid incision as drainage divides were progressively breached. Our results support the notion that the low-relief landscape atop the eastern Tibetan Plateau was elevated prior to Miocene time, and that basin integration and headward incision into this high-standing plateau drive rapid incision downstream of this former topographic divide.

The measurement of cosmogenic nuclide (CN) concentrations in riverine sediment has provided breakthroughs in our understanding of landscape evolution. Yet, the link between this detrital CN signal and landscape evolution is based on hypotheses that are not easy to verify in the field. New applications could arise from a better understanding of the statistics of CN concentrations in sediment grains. In this work, we present the coupling between the landscape evolution model Cidre and a model of the CN concentration in distinct grains. These grains are exhumed and detached from the bedrock and then transported in the sediment to the catchment outlet with temporary burials and travels according to the erosion-deposition rates calculated spatially in Cidre. The concentration in the various CN can be monitored in these grains. Because the CN concentrations are calculated in a limited number of grains, they provide an approximation of the whole CN flux. Thus, this approach is limited by the number of grains that can be handled in a reasonable computing time. On the other hand, part of the variability in the erosion-deposition processes can be recorded in the grain-by-grain distribution of the CN concentrations by monitoring the CN concentrations in distinct grains using a Lagrangian approach. We illustrate the robustness, the perspective and the limits of this approach by deriving the catchment-mean erosion from the 10Be mean concentration of the grains leaving a synthetic catchment uplifting at different rates and by comparing this derived erosion rate to the actual one calculated by Cidre.

Constraining millennial-scale fault slip rates and understanding the related structural architecture in active orogenic wedges might provide important assessment of future seismic risk evaluations. An increasing number of Holocene and Late Pleistocene deformation rates have been reported throughout the Western Himalayan frontal fold-and-thrust belt (FTB), which have illustrated that the deformation is divided along several arc-parallel fault splays rooted to the Main Himalayan Thrust décollement. Studies also report complex pattern of out of sequence faulting, as well as spatiotemporal variations in fault growth and lateral heterogeneity of the FTB. However, what drives this lateral heterogeneity is debated until now.

Late Pleistocene – Holocene deformed fluvial strath terraces across active faults in western Himalayan provide a measure of time-averaged fault slip rates along the Himalayan Frontal Thrust (HFT) and the Medlicott Wadia Thrust (MWT). While the Quaternary slip rates along the HFT varies from 3 – 12 mm/y, the slip rates from the ~700 km- long MWT is steadily high (7 – 9 mm/y).In contrast, previously published balanced cross-sections proposed slip rates of ~1 – 2 mm/y along the MWT. So from million-year to millennial timescale, there exists a significant temporal variation in fault activity. Single fault system like the MWT accommodate up to 50 - 60% of the total measured geodetic rates on Holocene/Late Quaternary timescales. These results document significant strain partitioning within the Sub-Himalaya and steady - high slip on the MWT which, beside the HFT could host the next big seismic event and damage the human-infested north India.

The age of alluvial surfaces can play a key role in deciphering surface processes and landscape evolution. However, the most common dating methods (e.g. with cosmogenic nuclides like 10Be) are expensive and time-consuming. We propose an approach that utilizes a limited number of 10Be samples in combination with hyperspectral data to estimate surface ages.

Specifically, we make use of known alterations of the spectral reflectance of geochemical surfaces caused by weathering processes, e.g., clay mineral and iron oxide formation. Changes consist of an overall increase in reflection, but mainly the development of characteristic absorption features. Our aim is to detect these weathering features via hyperspectral satellite imagery to build a ground-truthed spectral-age model for estimating alluvial surface ages over large regions.

To test this approach along the Río Santa Cruz in southern Patagonia, we dated 7 out of 13 fluvial terrace levels that yielded exposure ages up to 1.5 My, and we conducted in situ spectral measurements using a field spectrometer. By comparing these observations to satellite data (Landsat 8 and EnMAP), we can estimate ages and make better correlations of undated surfaces along the 250-km length of the river. Surprisingly, the age range of the model indicates slower than expected weathering rates and a major methodological advance in the detection of weathering processes via hyperspectral satellite.

Chemical weathering of silicate minerals plays a vital role in maintaining the long-term habitability of Earth’s climate over geological timescales via a negative feedback mechanism. But much debate concerns the response strength of silicate weathering to each climatic factor and its evolution with land surface reorganisation. Such discrepancy arises from lacking weathering proxy validation and scarce quantitative paleo-constraints on individual forcing factors. Here we examine the catchment-scale link of silicate weathering intensity with various environmental parameters using a global compilation of modern sediment dataset for calculation of weathering intensity proxies, including chemical index of alteration (CIA) and weathering index of Parker (WIP).

We show the primary control of temperature on silicate weathering given the monotonic increase of feldspar dissolution (constrained by CIA) with it at 0-30 ^oC, while controls of precipitation or topographic-lithological factors are regional and subordinate¹. We interpret the non-linear forcing of temperature on feldspar dissolution as depletion of more reactive plagioclase (relative to orthoclase) at higher temperature. Accordingly, the surface temperature decrease could be accompanied with a higher proportion of more reactive plagioclase available for weathering, supporting the hypothesis that land surface reactivity has increased during the late Cenozoic cooling². We also propose a first-order quantitative relationship between surface temperature, feldspar dissolution and CO₂ consumption that will be of great potential for deep-time temperature reconstruction and carbon cycle modeling.

References

1. Deng, K., Yang, S. & Guo, Y. (2022) Nat Commun 13, 1781.

2. Caves Rugenstein, J.K., Ibarra, D.E. & von Blanckenburg, F. (2019) Nature 571, 99–102.