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THESIS ABSTRACT

Drew Point, an unlithified ice-rich permafrost coastline along the Alaskan Beaufort Sea, is among the most rapidly eroding Arctic coastlines, with an average erosion rate of 19 m/yr from 2007 to 2019. We use 16 high-resolution remote sensing datasets (satellite, airborne, and UAV imagery) to analyze erosion mechanisms (thermal abrasion and denudation) in relation to environmental forcings along a 1.5 km stretch of coastline during the 2018 and 2019 open water seasons. In a striking contrast, 2019 exhibited the highest mean erosion rate (34.5 m) within the 2007–2019 record, while 2018 had the second lowest (11.2 m). Block failure contributed to sub-seasonal erosion rates 6 to 21 times higher than thermal denudation, with staggered block fall timing, lag responses post-storm, and non-storm block collapse influencing overall erosion magnitude and timing. To quantify wind effects, we developed wind sums, a metric combining cumulative wind speed and directional data that can be used as a proxy for integrated storm intensity capable of incorporating lagged responses that correlated strongly with erosion at sub-seasonal and annual scales. Our findings emphasize the dominant role of wind during periods of open water and air temperature during the thaw season in driving permafrost coastline erosion dynamics, while highlighting the importance of spatiotemporally high-resolution datasets for understanding Arctic coastal change dynamics.

Despite their very low relief and erosion rates, non‐orogenic (i.e., cratonic) continental domains account for over 60% of the Earth's exposed lands. Therefore, they contribute significantly to the clastic sediments and solutes exported to the ocean and should be accounted for in global studies. Nonetheless, they have been much less studied than orogenic domains. In this study, we establish the source‐to‐sink sedimentary budget of the sub‐saharan West African cratonic domain and its Equatorial Atlantic rifted margin using published low‐temperature thermochronological data to estimate onshore denudation and regional geological cross‐sections to estimate offshore accumulation. We show that during and immediately following rifting (130‐94 Ma), the build‐up and subsequent erosion of rift‐related relief resulted in a transient, 100–200 km wide strip along the margin recording high denudation rates (>50 m/Myr), while the inland domain underwent steady and very low denudation (<10 m/Myr). Afterward, the whole onshore domain underwent very low and steady denudation. Thus, the changes in post‐rift accumulation rates documented along the rifted margin were caused by changes in the climate and/or drainage network. During the Late Cretaceous, we document a regional rise in accumulation rates caused by the enlargement of drainage areas feeding the basins by a hinterlandward migration of the continental divide. During the Paleogene, we document a general drop in accumulation rates in all the basins of the African Atlantic margins caused by the global greenhouse climate, which enhanced the development of lateritic weathering mantles, storing clastic sediments on the continent and favoring solute exports to the ocean. Plain Language Summary Cratons, shields and continental platforms have low relief and erode very slowly. As a result, they are often neglected while investigating sediment routing systems at the surface of the Earth over geological times. In this study, we determine the source‐to‐sink sedimentary budget of the sub‐saharan part of West Africa and its equatorial Atlantic margin. To do this, we estimate the histories of (a) erosion on the continent and (b) accumulation in the offshore basins. We show that the sub‐saharan region of West Africa has experienced very little erosion since at least 200 Ma. The only exception was the temporary removal of relief caused by the rift that split Africa and South America. Following that rifting, variations in sediment accumulation in the basins were driven by changes in climate that modified the way the continent was being eroded or by modifications of the river courses carrying sediments to the ocean. Key Points The sub‐saharan West Africa cratonic domain underwent only very low steady denudation rates (<10 km/Ma) since Equatorial Atlantic rifting Equatorial Atlantic rifting altered the cratonic denudation only transiently, in a narrow domain corresponding to rift‐related relief erosion Post‐rift accumulation rates in the rifted margin basins were thus controlled by global climate change and/or drainage reorganizations

Marine accumulations of terrigenous sediment are widely assumed to accurately record climatic- and tectonic-controlled mountain denudation and play an important role in understanding late Cenozoic mountain uplift and global cooling. Underpinning this is the assumption that the majority of sediment eroded from hinterland orogenic belts is transported to and ultimately stored in marine basins with little lag between erosion and deposition. Here we use a detailed and multi-technique sedimentary provenance dataset from the Yellow River to show that substantial amounts of sediment eroded from Northeast Tibet and carried by the river’s upper reach are stored in the Chinese Loess Plateau and the western Mu Us desert. This finding revises our understanding of the origin of the Chinese Loess Plateau and provides a potential solution for mismatches between late Cenozoic terrestrial sedimentation and marine geochemistry records, as well as between global CO 2 and erosion records. Theories of Cenozoic mountain uplift and associated global cooling assume that eroded mountain sediments are stored in marine basins. Here, based on detailed provenance data, Nie et al . show that Northeast Tibetan sediments are in fact stored inland, in the Chinese Loess Plateau and Mu Us desert.

Background and aims Windblown sand movements, i.e., wind denudation and sand burial, pose a strong selective pressure on dune vegetation. Dune plants commonly receive repeated wind denudation or sand burial. Therefore, simultaneously examining the role of sand movement intensity and frequency in shaping dune vegetation is critical for dune biodiversity conservation and ecological restoration. However, studies of this nature are rare. Methods We studied the integrated effects of sand movement intensity and frequency on the seedling performance of a dominant semi-shrub, Artemisia ordosica , in the Mu Us sandland. We subjected A. ordosica seedlings to a total intensity of 10 cm wind denudation or sand burial treatments conducted once, twice or four times. Key results We found, given that the total intensity of sand movement remains the same, increasing frequency and decreasing intensity per time largely improved seedling survival. Furthermore, increasing frequency and decreasing intensity per time significantly alleviated the negative effects of wind denudation, although such alleviation effect was not detected for sand burial. Seedlings of A. ordosica increased specific leaf area, root length, and biomass allocation to root to adapt to wind denudation, while they developed adventitious roots to adapt to sand burial. Conclusions Our results demonstrate that a single heavy sand movement is more detrimental than multiple light ones to the performance of A. ordosica seedlings. Our findings suggest that windproof measures to prevent severe sand movements is necessary to allow the establishment of A. ordosica during the dune restoration process.

Physical erosion and chemical weathering rates beneath glaciers are expected to increase in a warming climate with enhanced melting but are poorly constrained. We present a global dataset of cations in meltwaters of 77 glaciers, including new data from 19 Asian glaciers. Our study shows that contemporary cation denudation rates (CDRs) beneath glaciers (2174 ± 977 Σ*meq+ m−2 year−1) are ~3 times higher than two decades ago, up to 10 times higher than ice sheet catchments (~150-2000 Σ*meq+ m−2 year−1), up to 50 times higher than whole ice sheet means (~30-45 Σ*meq+ m−2 year−1) and ~4 times higher than major non-glacial riverine means (~500 Σ*meq+ m−2 year−1). Glacial CDRs are positively correlated with air temperature, suggesting glacial chemical weathering yields are likely to increase in future. Our findings highlight that chemical weathering beneath glaciers is more intense than many other terrestrial systems and may become increasingly important for regional biogeochemical cycles.

The terrestrial cosmogenic nuclide dataset consisting of 1233 10 Be measurements of fluvial sediment samples across the Asian mountain belts is used to constrain the influence of climate, tectonics and topography on Earth-surface denudation processes. 10 Be-derived basin-wide denudation rates were recalculated using a Monte Carlo simulation approach that relied on pixel-by-pixel production rates. The rates derived span four orders of magnitude, from 7.4±0.7 mm kyr -1 (1 σ ; Central Tibetan Plateau) to 9,646 744 +777 mm kyr -1 (1 σ ; Southern Tibetan Plateau), with a median of 186 mm kyr -1 . Comparing our results with the traditional basin-wide production rate scaling model using a centroid coordinate and mean elevation, suggests that 96.6% of production rates will be underestimated using the latter method, especially for basins with a large relief. The bias between both methods reaches as large as 40%, caused by basin hypsometry and the nonlinear scaling of production rate with elevation and latitude. Quantification of the correlations between uniformly recalculated denudation rates and tectonics, climate, topography, and rock lithology will facilitate the extrapolation of denudation rates in unknown basins using available data. From the perspective of local scales, precipitation coupled with neotectonics-driven landslides is likely the most influential factor in subtropical Taiwan Island, while topography primarily constrains surface denudation rates in the eastern Tibetan Plateau within a narrow range of precipitation. In highland regions with glacier coverage, such as the Pamir, central and southern Tibetan Plateau, high denudation rates are affected by glaciation, whereas lower precipitation and high erosion base level may limit river incision and evolving relief in the Central Tibetan Plateau. In contrast to local-scale findings highlighting a dominant factor constraining denudation rate, our large-scale quantitative analyses across the Asian mountain belts finds that (1) topographic metrics including slope, relief and normalized channel steepness index show similar, first-order, power-law relationships with denudation rates over millennial timescales, as opposed to the linear relationships suggested to control the surface denudation rate; (2) precipitation, temperature, and vegetation cover relative to elevation and a modern tectonic metric of neotectonic activity based on the kernel density distribution of earthquakes, subordinately constrain the surface denudation, approximating a first-order power-law correlation with denudation rate; and (3) the lithology, as represented by an erodibility index, appears not to correlate with denudation rates at the macro-scale of the Asian mountain belts, possibly due to the complex mixture of different types of rocks and the relative low resolution of erodibility index database.

The long-term cooling, decline in the partial pressure of carbon dioxide, and the establishment of permanent polar ice sheets during the Neogene period 1 , 2 have frequently been attributed to increased uplift and erosion of mountains and consequent increases in silicate weathering, which removes atmospheric carbon dioxide 3 , 4 . However, geological records of erosion rates are potentially subject to averaging biases 5 , 6 , and the magnitude of the increase in weathering fluxes—and even its existence—remain debated 7 – 9 . Moreover, an increase in weathering scaled to the proposed erosional increase would have removed nearly all carbon from the atmosphere 10 , which has led to suggestions of compensatory carbon fluxes 11 – 13 in order to preserve mass balance in the carbon cycle. Alternatively, an increase in land surface reactivity—resulting from greater fresh-mineral surface area or an increase in the supply of reactive minerals—rather than an increase in the weathering flux, has been proposed to reconcile these disparate views 8 , 9 . Here we use a parsimonious carbon cycle model that tracks two weathering-sensitive isotopic tracers (stable 7 Li/ 6 Li and cosmogenic 10 Be/ 9 Be) to show that an increase in land surface reactivity is necessary to simultaneously decrease atmospheric carbon dioxide, increase seawater 7 Li/ 6 Li and retain constant seawater 10 Be/ 9 Be over the past 16 million years. We find that the global silicate weathering flux remained constant, even as the global silicate weathering intensity—the fraction of the total denudation flux that is derived from silicate weathering—decreased, sustained by an increase in erosion. Long-term cooling during the Neogene thus reflects a change in the partitioning of denudation into weathering and erosion. Variable partitioning of denudation and consequent changes in silicate weathering intensity reconcile marine isotope and erosion records with the need to maintain mass balance in the carbon cycle and without requiring increases in the silicate weathering flux. A carbon cycle model constrained by weathering-sensitive isotopic tracers reveals that long-term cooling in the Neogene period reflects a change in how surface denudation is partitioned into weathering and erosion.

Atherosclerotic lesions develop preferentially at sites of disturbed blood flow. As shown by Christian Weber and his coworkers, this predilection stems from effects of disturbed blood flow on endothelial expression of the microRNA miR-126-5p, which maintains the proliferative reserve of endothelial cells through repression of the Notch pathway inhibitor Dlk1. Atherosclerosis, a hyperlipidemia-induced chronic inflammatory process of the arterial wall, develops preferentially at sites where disturbed laminar flow compromises endothelial cell (EC) function. Here we show that endothelial miR-126-5p maintains a proliferative reserve in ECs through suppression of the Notch1 inhibitor delta-like 1 homolog (Dlk1) and thereby prevents atherosclerotic lesion formation. Endothelial recovery after denudation was impaired in Mir126 −/− mice because lack of miR-126-5p, but not miR-126-3p, reduced EC proliferation by derepressing Dlk1. At nonpredilection sites, high miR-126-5p levels in endothelial cells confer a proliferative reserve that compensates for the antiproliferative effects of hyperlipidemia, such that atherosclerosis was exacerbated in Mir126 −/− mice. In contrast, downregulation of miR-126-5p by disturbed flow abrogated EC proliferation at predilection sites in response to hyperlipidemic stress through upregulation of Dlk1 expression. Administration of miR-126-5p rescued EC proliferation at predilection sites and limited atherosclerosis, introducing a potential therapeutic approach.

Following the 130 ± 5 × 106 m3 detachment of the Sedongpu Glacier, south-eastern Tibet, in October 2018, the Sedongpu Valley, which drains into the Yarlung Tsangpo (Brahmaputra) River, underwent rapid large-volume landscape changes. Between December 2018 and 2022 and in particular during summer 2021, an enormous volume of in total∼ 335 ± 5 × 106 m3 was eroded from the former glacier bed, forming a new canyon of up to 300 m depth, 1 km width, and almost 4 km length. The 2021 erosion peak happened through massive but still gradual retrogressive erosion into the former glacier bed. Several rock–ice avalanches of in total ∼ 150 ± 5 × 106 m3 added to the total rock, sediment, and ice volume of over 600 × 106 m3 (0.6 km3) that has been exported from the basin since around 2017. The recent erosion volumes at Sedongpu are by order of magnitude equivalent to the average annual denudation volume of the entire Brahmaputra basin upstream of the location where the river leaves the Himalayas. This high-magnitude low-frequency event illustrates the potential for rapid post-glacial landscape evolution and associated hazards that has rarely been observed and considered at such high intensity so far.