Journal of Geophysical Research: Earth Surface最新文献

The determination of critical shear stresses is fundamental to bedload sediment transport prediction in gravel-bed rivers. Due to the heterogeneous shape and arrangement of the individual clasts in a riverbed, critical shear stresses typically show a large spatial variability, which is not adequately captured by the reach-averaged description followed in common studies. In this regard, there is a general paucity of field data on this spatial variability of the critical shear stress, largely due to the lack of a standardized measurement method. In an attempt to fill this gap, we propose a field-based workflow to estimate the frequency distribution of dimensionless critical shear stress (also named critical Shields number), which is based on the measurement of a series of variables related to the position, orientation and resistance to motion of individual clasts in a gravel-bed river, combined with a probabilistic approximation to drag and lift coefficients. Following this workflow, the patch-scale variability of particle incipient-motion conditions was determined in a gravel bar of the Upper Cinca River, Spain. The results are consistent with what is known about sediment entrainment in gravel-bed rivers. We consider this method to have great potential to advance our understanding of particle initiation of motion in gravel-bed rivers as it provides valuable systematic field information.

Segmented barrier islands can be found in regions with small tidal ranges. In contrast to tidally dominated barriers, where inlet dynamics are thought to control island length scales, the controls on barrier island length scales in wave-dominated environments have not been quantified. These microtidal barriers typically have a curved shoreline, suggesting the influence of wave-driven alongshore sediment transport. Microtidal barriers are also typically hydrodynamically isolated from one another, as weak tidal flows limit interactions between adjoining barriers. To better understand the controls on and scales of barrier segmentation in the relative absence of tides, here we develop a theoretical framework to estimate the alongshore length scales at which a barrier will either breach or heal following a disturbance in the barrier morphology. The non-dimensional framework compares the timescales of overwash (advective) and alongshore sediment transport (diffusive) processes along barrier island chains. We then apply this framework to modern barrier islands in the microtidal Gulf of Mexico using wave hindcast data and the lengths, widths, heights, and lagoon depths measured from remotely sensed geospatial data and topobathymetric data. We find that most of these barriers are currently longer than their critical length scale, often as a result of coastal restoration efforts. Our critical length scale analysis suggests that most of the Gulf of Mexico barriers are vulnerable to segmentation despite coastal restoration efforts intended to protect fisheries and the mainland coasts.

This paper studies hydrodynamic and morphodynamic field measurements of two storms with dune erosion in the swash-dune collision regime. It analyses (a) the behavior and change of the total dune profile over the course of both storms (b) the erosion rate at the dune base, (c) the slumping frequency, and (d) the volumes of individual slumps. The erosion rate at the dune base shows a strong positive correlation with the square of the total water levels that were exceeded for 2% of the time, recorded approximately 5–6 m in front of the dune face (r = 0.91). Individual slumping events occurred when nearly all sediments from previous slumps at the dune base were transported away from the dune. A strong positive correlation was found between the time between two consecutive slumps, and the volume of the first slump divided by the mean erosion rate between the two slumps (r = 0.90). As a consequence, smaller slumps were followed more rapidly by a new slump than larger slumps, under identical erosion rates. The majority of the slumping events occurred after the last wave impact before a slumping event, when the instantaneous water level in front of the dune was still retreating. No clear process based on the incident hydrodynamics could be identified that determined the size of individual slumps. Overall, the results of this study suggest that the morphodynamic behavior of the upper dune face and dune crest is primarily steered by the erosion at the dune base.

Longshore sediment transport (LST) is essential for shaping sandy shorelines. Many shorelines are complex and indented, containing headlands, offshore islands and tombolos. Tombolos often form between islands and the mainland; however, the conditions for LST across tombolos are unclear. This question is important because tombolos are often reinforced with anthropogenic infrastructure, potentially causing sediment starvation of downdrift beaches. Along many shorelines, the return to a tombolo's natural condition has been proposed to promote sediment connectivity and counteract erosion. Nevertheless, the implications of such restorations remain uncertain. In this study, we employ the Delft3D wave-current model to investigate hydrodynamics and sediment dynamics across a tombolo, examining its role as a connector between adjacent beaches. Contrary to expectations, our simulations show only diminutive longshore currents from the updrift beach across the tombolo unless offshore wave heights exceed 8 m. Instead, predominant currents crossing the tombolo originate from offshore of the island, driven by storm-induced water level differences and circulation cells on both sides of the tombolo. The offshore island shelters the downdrift domain, resulting in higher wave energy and dissipation updrift of the tombolo. Further, increasing wave height or wave approach angle not only intensifies water level differences but also relocates circulation cells, enhancing total sediment transport from the updrift beach across the tombolo. However, in general, the deposition of sediment from the updrift side of the domain does not compensate for the sediment loss on the downdrift beach. We conclude that LST across tombolos is limited and occurs only under extreme wave conditions.

The formation age of the middle Yellow River and the existence of a northward-flowing river have been fiercely debated. The age distribution of detrital zircon varied spatiotemporally and produced contradictory provenance interpretations. The Jinshaan Gorge, the main part of the middle Yellow River and key to studying fluvial evolution and clarifying disputes, developed its topography during the late Cenozoic. In this study, we systematically review the Cenozoic tectonic evolution of the North China Craton, perform detrital zircon U–Pb dating in the Neogene−Quaternary sediments and investigate the topography along the Jinshaan Gorge, and the sedimentology and chronological framework of these sediments. We propose that the Gorge of the middle Yellow River could have developed since the Neogene, controlled by the tectono-geomorphologic evolution of the North China Craton in a dominantly extensional environment. No evidence supports a northward-flowing river during the Early Pleistocene or even earlier in the Jinshaan Gorge. We attribute the provenance variations of the Cenozoic sediments to detrital mixing of diverse geological units, local and distant, and especially highlight the systematic provenance shift between the Neogene and Quaternary sediments caused by bedrock downcutting and recycling aeolian sediments. The increased 1.5−0.33 Ga component of the lower Yellow River during the Early Pleistocene was likely caused by enhanced loess accumulation and should not be individually used as a proxy for the Yellow River formation. We emphasize the significance of a comprehensive study of river evolution.

Gobi’s (gravel deserts) are one of the largest dust sources in northern China. Previous studies indicated that sand transport processes above the surface differed between Gobi and sand surfaces. However, the sand transport rate and related dust emission processes above Gobi (gravel) surfaces are still poorly understood. In this field study, we quantified this transport to provide important support for parameterizing Aeolian sediment transport models and clarifying the relationship between dust emission and transport. Threshold wind velocity can reach 0.38 ± 0.04 (mean ± SD) m s⁻¹ above Gobi surfaces. Compared to the most commonly used sand-transport models, we found that the Lettau and Lettau sediment transport model can be used to calculate horizontal sediment transport above a Gobi surface. The relationship between the vertical sediment transport (F_s) and shear velocity could be expressed using a power function. Although the horizontal sand transport and vertical flux (Q and F_s, respectively) above Gobi surfaces can be expressed similarly to previous results (i.e., using similar equation forms), the equation coefficients were much larger for the Gobi surface than for a shifting sand surface; that is, sediment transport was higher above the Gobi surface. This difference resulted from the larger sand transport rate and saltation height above the Gobi surface, and the larger transport and higher saltation height were related to the larger sand transport height and higher content of coarse sand transported above the Gobi surface.

Mega retrogressive thaw slumps (MRTS, >10⁶ m³) are a major threat to Arctic infrastructure, alter regional biogeochemistry, and impact Arctic carbon budgets. However, processes initiating and reactivating MRTS are insufficiently understood. We hypothesize that MRTS preferentially develop a polycyclic behavior because the material is thermally and mechanically prepared for subsequent generation failure. In contrast to remote sensing, geophysical reconnaissance reveals the inner structure and relative thermal state of MRTS decameters beneath slump surfaces, potentially controlling polycyclicity. Based on their life cycle development, five (M)RTS were studied on Herschel Island, an MRTS hotspot on the Canadian Beaufort coast. We combine >2 km of electrical resistivity tomography (ERT), 500 m of ground-penetrating radar (GPR) and annual monitoring of headwall retreat from 2004 to 2013 to reveal the thermal state, internal structure, and volume loss of slumps. ERT data were calibrated with unfrozen-frozen transitions from frost probing of active layer thickness and shallow boreholes. In initial stage MRTS, ERT displays surficial thermal perturbations a few meters deep, coincident with recent mud pool and mud flow development. In early stage polycyclic MRTS, ERT shows decameter deep-reaching thermal perturbations persisting even 300 years after the last activation. In peak-stage polycyclic MRTS, 3D-ERT highlights actively extending deep-reaching thermal perturbations caused by gully incisions, mud slides and mud flows. GPR and headwall monitoring reveal structural disturbance by historical mud flows, ice-rich permafrost, and a decadal quantification of headwall retreat and slump floor erosion. We show that geophysical signatures identify long-lasting thermal and mechanical disturbances in MRTS predefining their susceptibility to polycyclic reactivation.

Substantial seafloor morphological changes are rapidly occurring along the Canadian Arctic shelf edge. Five multibeam bathymetric mapping surveys, each partially covering a 15 km² study area between 120- and 200-m water depth, were conducted over a 12-year time period. These surveys reveal that 65 new craters have developed between 2010 and 2022, averaging 6.5 m and reaching up to 30 m deep. Remotely operated vehicle investigations revealed massive ice outcrops exposed on two newly formed crater flanks. This ice is not relict subaerially formed Pleistocene permafrost because it is hosted in sediments which were deposited in a submarine setting post-deglaciation. Low salinity porewater and sediment core ice samples with depleted oxygen isotopic compositions indicate waters with a meteoric signature are discharging and freezing in this area. These ascending brackish groundwaters are likely derived in part from thawed relict permafrost hundreds of meters under the continental shelf. They refreeze as they approach the −1.4°C seafloor, leading to the development of widespread, near seafloor, sub-bottom ice layers. Conditions appropriate for ice melting also exist nearby where ice is exposed to seawater or warmed by ascending groundwater. Small variations in temperature and salinity lead to shifts between freezing of ascending brackish groundwater or melting of near seafloor ice layers. These conditions have produced a dramatic submarine thermokarst morphology riddled with multi-aged depressions. Thermokarst geohazards may exist, unmapped, on other Arctic margins with groundwater channeled toward the shelf edge by a relict permafrost cap, and sufficiently cold shelf edge bottom water temperatures.

The flow of glaciers and ice sheets is largely influenced by friction at the ice-bed interface that can trigger rapid changes in glacier motion ranging from seasonal velocity variations to cyclic surge instabilities or even devastating glacier collapse. This wide range of transient glacier dynamics is currently not captured by models, and its implications for long-term glacier evolution are uncertain. This highlights the need of developing improved descriptions for processes that occur at the glacier bed. Here, we present a model that describes the evolution of basal friction inspired by a “rate and state” approach, coupled to models of subglacial drainage and glacier flow, and investigate how these couplings affect the dynamics of glaciers. We show that a wide range of sliding behavior results from a feedback loop between subglacial drainage efficiency and friction which depends on the evolution of a frictional state that can be interpreted as the degree of cavitation or till porosity for hard and soft beds, respectively. In our simulations, we find that glaciers are susceptible to surging if they exhibit a transition to velocity weakening friction associated with a poor sensitivity of the drainage capacity to the frictional state. This potential materializes if the local topography and mass balance create the conditions for high water pressure to build up in an area sufficiently large to exceed a critical length. We advocate accounting for feedback loops between friction and drainage as a promising avenue for better understanding dynamical instabilities of glaciers and ice sheets.