Journal of Geophysical Research: Earth Surface最新文献

Escarpments and cliffs (hereafter termed escarpments) form topographic barriers that influence the spatial patterns of climate and biodiversity. The inherent extreme slope change across the escarpment edge promotes escarpment retreat over time. Typically, escarpments are divided into arch- and shoulder-types. In arch-type, the drainage divide is located inland, and knickpoints, located where channels flow across the escarpment, can retreat and embay the escarpment. In shoulder-type, the divide aligns with the escarpment edge, a setting expected to cause a slow and uniform escarpment retreat, preserving their integrity as barriers through time. However, observations from around the globe reveal shoulder-type escarpments are associated with deep embayments (i.e., alcoves) that destroy the linear appearance of the escarpment front. Yet, the processes that produce and sustain these embayments remain largely unexplored. Embayments of shoulder-type escarpments typically occur along reversed channels which were part of the antecedent drainage that used to flow away from the escarpment but now flow toward it, often resulting in a valley confined drainage divide called a windgap. Here, we hypothesize that feedback between knickpoint retreat and windgap migration away from the escarpment along reversed channels can sustain escarpment embayments, and use topographic analyses and numerical simulations to explore this hypothesis. Our analyses, focused on field sites in the Negev Desert, show that embayments of shoulder-type escarpments can be sustained through the hypothesized feedback, and quantify the sensitivity of this feedback to geomorphologic and lithologic parameters. Results suggest that this feedback may explain some of the global variability of escarpment morphologies.

Weathering of ultramafic rocks emplaced at low latitude during arc-arc and arc-continent collisions may provide an important sink for atmospheric CO₂ over geologic timescales. Accurately modeling the effects of ultramafic rock weathering on Earth's carbon cycle and climate requires understanding mass fluxes from ultramafic landscapes. In this study, physical erosion and chemical weathering fluxes and weathering intensity are quantified in 15 watersheds across the Monte del Estado, a serpentinite massif in Puerto Rico, using measurements of in situ ³⁶Cl in magnetite, stream solute fluxes, and sediment geochemistry. Despite high relief in the study watersheds, erosion fluxes are moderate (22–109 tons km⁻² yr⁻¹), chemical weathering fluxes are large (55–143 tons km⁻² yr⁻¹), and weathering intensities are among the highest yet reported for silicate-rock weathering (up to 0.88). We use these data to parameterize power-law relationships between weathering, erosion, and runoff. We interpret the relative importance of climate versus erosion in setting weathering fluxes and CO₂ consumption from the best-fit power-law slopes. Weathering fluxes from tropical, montane serpentinite landscapes are found to be strongly controlled by runoff and weakly controlled by the supply of fresh rock to the weathering zone through physical erosion. The strong runoff dependence of weathering fluxes implies that, to the extent that precipitation rates are coupled to global temperature, ultramafic landscapes may be important participants in the negative silicate weathering feedback, increasing (decreasing) CO₂ consumption in response to a warming (cooling) climate. Thus, serpentinite landscapes may help stabilize Earth's climate state through time.

Glacigenic bedforms such as glacial lineations and moraines on the Chukchi and East Siberian margins of the Arctic Ocean reveal recurrent waxing and waning of voluminous ice masses. Despite their paleoclimatic significance, the timing, geographic distribution, and mechanisms of these glaciations remain inadequately understood. To enhance our understanding of the Quaternary Arctic glacial history, we study high-resolution swath bathymetry and subbottom profiler data with lithostratigraphy and provenance of four sediment cores. These data characterize deposits of the last two glaciations at the Chukchi margin and adjacent basins. In all cores, multiple peaks of plagioclase are prominent in both glacial intervals, probably reflecting predominant glacigenic input from the East Siberian Ice Sheet (ESIS). Peaks of dolomite and quartz trace the Laurentide Ice Sheet sources around the last glacial/deglacial interval and in sediment preceding the older glaciation. By integrating seismostratigraphy with sediment cores, we constrain the formation of mid-slope moraines on the western side of the Chukchi Rise to the older glaciation (estimated age range MIS 4 or 6). Considering the coeval glacial erosion off the East Siberian margin, our results confirm that the ESIS at that time extended to water depths of ∼650/950 m on the Chukchi Rise/East Siberian margin. In comparison, the last ESIS (MIS 2 or possibly 4) was smaller, with the identified seafloor imprint limited to water depths of ∼450 m on the Chukchi Borderland, while its extent on the East Siberian margin remains to be determined.

Landslides occur where the stresses below the surface exceed the shear strength of the material. Landslide inventories thus offer opportunities to investigate patterns in subsurface strength provided that the stress conditions at failure can be estimated. Clues to the failure stresses are encoded in the inclination of the slope that failed and the thickness of the sliding mass. We use this insight to develop a two-dimensional (2D) landslide back-analysis model that estimates bedrock strength over the broad scales relevant to earthquake-triggered landslide hazard and landscape evolution. A unique aspect of our model is the incorporation of independent landslide thickness measurements for each landslide, which are provided by differencing pre- and post-failure elevation data or estimated from a volume-area scaling relationship. This approach represents an innovation compared to previous regional-scale models that have assumed constant thickness or have used projections to estimate the depth to the failure plane, and it provides rock strength estimates as a function of depth below the surface. We evaluate our modeling approach in applications to two landslide inventories and compare the results against geotechnical field data. The back-calculated strength estimates are low for rock, which we hypothesize to reflect the contribution of weathering and fracturing, as well as the fact that landslides represent a small part of the entire study area and are likely associated with particularly weak material that is susceptible to failure. Finally, the two applications of our model indicate systematic variations in strength parameters below the surface and along an elevation profile, which we attribute to gradients in chemical and physical weathering.

The forces exerted by geophysical granular flows on Earth's surface, and the resulting seismic signals, can be used to monitor natural geohazards and understand their dynamic evolution and characteristics. Substantial research has focused on linking basal force fluctuations and seismic signals to granular flow dynamics. However, the mechanisms behind the generation and evolution of seismic signals remain incompletely understood. In this study, we conducted laboratory flume experiments to gain insights into the evolution and characteristics of basal force fluctuations and seismic signals and explored their relationship with the macroscopic properties of granular flows. Our results show that the shear and normal components of basal force fluctuations exhibit different behavior during flow evolution, which are related to variations in flow velocity fluctuations. As the granular flow moves downstream, shear basal force fluctuations decrease due to weakening velocity fluctuations, whereas normal force fluctuations increase. Similar to basal force fluctuations, seismic signals follow a generalized Pareto distribution. Basal force fluctuations and seismic signals are strongly nonlinearly related to the bulk flow properties, indicating that thicker, denser and faster flows generate stronger basal force fluctuations and more intense seismic signals. However, particle size significantly influences this relationship. We demonstrate that the inertial number, characterizing the macroscopic rheological properties of granular flows, can unify basal force fluctuations and seismic signals across different particle sizes, exhibiting a negative correlation on the temporal scale. This implies that the macroscopic rheological behavior of granular flows may provide critical insights into the mechanisms of generation and evolution of seismic signals.

Rapid, transient, landscape-scale changes associated with deglaciation can condition slopes for failure and trigger bedrock landslides. However, the mechanisms leading to paleo rock slope failures following the last glacial period are challenging to infer because observations of how both landsliding and potential driving factors were distributed in space and time are limited. Here, we map and analyze the spatiotemporal pattern of 676 post-glacial bedrock landslides around Eyjafjörður in north-central Iceland using 2-m resolution digital elevation data generated from optical stereo satellite imagery. Frequency-ratio analysis demonstrates that after controlling for slope, landslides are most overrepresented within 2.6 km horizontal distances from surface projections of major Tertiary bedrock structures and at land surface elevations within 300 m of a modeled lower limit to permafrost. Surface roughness analysis of landslide deposits indicates that peak landslide frequency of at least 0.2 landslides yr⁻¹ in the 5,579 km² study area lagged deglaciation by several thousand years. This timing aligns well with that of rapid permafrost degradation from the Younger Dryas (12.9–11.7 cal ky BP) through the Holocene Thermal Maximum (∼10–7 cal ky BP). Landslide frequency has averaged about 0.014 landslides yr⁻¹ since the Holocene Thermal Maximum when the climate has generally been cooler and permafrost has been more extensive. However, present day warming is likely to reduce permafrost extent and increase the potential for bedrock landslides in north-central Iceland, as has already been observed for several recent shallower landslides in regolith.

Relatively little is known about the geomorphological characteristics of floodplain secondary channels and the potential for floodplain flows to mobilize bed material within these channels. This study examines the geomorphological characteristics (channel form, material properties, wood jams) and bed-material mobilization potential of secondary channels on the floodplain of a meandering river in Illinois, USA. It also compares these attributes to those of the main channel. Results show that secondary channels are at most about one-third the size of the main channel but also vary in size over distance. Channel dimensions tend to be greatest near the proximal connection of secondary channels to the main channel, suggesting that flow from the main channel is effective in producing scour where it enters secondary channels. The beds of secondary channels consist mainly of mud in contrast to sand and gravel on the bed of the main channel, implying that secondary channels do not convey bed material from the main channel onto the floodplain. Secondary channels connected to the main channel at both ends have more abundant active wood jams than those connected only at the proximal end. Flow from the main channel enters secondary channels at sub-bankfull stages, but maximum mobilization of cohesive bed material in secondary channels only occurs during flows that exceed the average bankfull stage in the main channel. Overall, secondary channels are active conduits of flow, sediment, and large wood on floodplains and can contribute to floodplain sediment fluxes through entrainment of bed material.

On January 1, 2024, a destructive earthquake struck the Noto Peninsula in Japan, triggering a tsunami in the Sea of Japan. A 3D lidar, which was installed on a gravel beach 150 km from the epicenter before the earthquake, successfully measured nearshore waves and topographic changes during the tsunami event. This study is focused on the analysis of a novel data set to elucidate the swash zone processes caused by the tsunami, with a particular focus on the combined effects of the tsunami and wind waves on high wave runup and morphological changes. The observation data show that the mean sea level was elevated by 1 m when the largest tsunami arrived at the beach. In addition to the tsunami, wind waves reaching a height of 2 m induced wave setup of approximately 1 m and swash exceeding a height of 2 m. The empirical formulas considering the observed change in foreshore slope reasonably reproduced the observed runup heights, suggesting that an increase in the tsunami water level indirectly amplified wind wave runup by increasing the foreshore slope on the concave profile beach. High wind wave runup largely eroded the gravel beach, with the elevation of the upper beach face decreasing by 0.5 m during the tsunami event. We found that the area on the beach face affected by wind waves expanded in response to changes in the tsunami water level.

A field of marine dunes has been studied in the Southern Bight of the North Sea. These large dunes, 1–5 m in height and several hundred meters in length, are highly mobile: migration rates of up to 30 m/year have been observed in places. The area is dominated by tides and is characterized by strong currents. Winds are predominantly from the southwest and, to a lesser extent, from the north. A large-scale 3D numerical model was used to simulate the migration of this dune field over time. It is based on the process-based opentelemac system. The model has been calibrated and validated against in situ bathymetric data and is therefore suited to our objective: to explore the contribution of weather (wind and atmospheric pressure) to the propagation of large marine dunes, in relation to that of tidal currents. To do this, a 4-month period was simulated, with and without meteorological effects being taken into account in the numerical model. The results highlight the fundamental role of wind conditions in an accurate representation of seabed changes over time. They also show how meteorological events that are different from the prevailing conditions influence the short-term evolution of the dune field.

The weathering of silicate rocks exerts a significant control on the weathering fluxes of metals and atmospheric CO₂ consumption. In this study, we present new magnesium (Mg) isotope data from a basalt weathering profile in Hainan Island, South China, to investigate Mg isotope fractionation and calculate weathering fluxes and CO₂ consumption. The Mg mobility (τ_Mg,Ti) in saprolites decreases from −34.1% to −95.7%. The δ²⁶Mg values in saprolites vary from −0.25 ± 0.07‰ to 0.43 ± 0.07‰, higher than those of the parent rock (−0.25 ± 0.07 ‰). The significant Mg loss during the formation and decomposition of clay minerals influences Mg isotope fractionation, particularly with changes in kaolinite structure under different pH conditions, which prefer heavy Mg isotopes. By applying a mass balance model, we have developed a novel method to calculate weathering fluxes based on the weathering profile, yielding Mg elemental fluxes (Mg_Flux) of 2.45–5.85 mol/cm²/Myr, Mg isotopic fluxes (δ²⁶Mg_Flux) of −0.44 to −0.04‰/mol/cm²/Myr, and CO₂ consumption of 2.3 × 10¹² mol/yr for the weathering outputs of basaltic rocks. This highlights the crucial role of basalt weathering in global carbon sequestration. Our findings improve the understanding of Mg cycling and isotope fractionation in epigenetic environments and facilitate the quantification of weathering fluxes and atmospheric CO₂ consumption during basalt weathering.