Journal of Geophysical Research: Earth Surface最新文献

Wave-supported gravity flows are a key process driving near-bed sediment transport in deltaic systems. However, previous observations have focused primarily on active river deltas with abundant sediment supply. The sediment sources and dynamics of wave-supported gravity flows in abandoned deltas with low sediment availability remain poorly understood. In this study, in situ observations were conducted on the abandoned Yellow River Delta of northern Jiangsu Province, China, and four wave-supported gravity flow events were observed within 48 hr during a storm event. The sediment source mechanisms in the study area include local resuspension, wave-induced liquefaction, and tidal advection, and these different sources influence the dynamic characteristics of the flows. Wave-supported gravity flows developed solely by local resuspension from bed shear stress exhibited lower suspended-sediment concentrations and shorter durations. In contrast, when the sediment source mechanisms of wave-supported gravity flows involved wave-induced liquefaction or tidal advection, the wave-supported gravity flows achieved suspended sediment concentrations 7 to 14 times higher. Despite large variations in suspended sediment concentration, flow velocities among the events were not significantly different, ranging from 3.6 to 5.8 cm/s, a result attributed to increased bottom drag and viscosity in the high-concentration fluid mud layers. Furthermore, all observed events were short-lived (1–2 hr) and displayed a distinct tidal periodicity. These findings suggest that in fine-grained sediment limited abandoned deltas, tidal advection is the dominant mechanism triggering frequent, short-lived wave-supported gravity flows, effectively compensating for the lack of local sediment supply.

Most of Earth's present-day terrestrial surface is covered by regolith—the layers of soil, saprolite, and weathered bedrock that together comprise the critical zone. Recent research has focused on understanding fluxes of minerals, water, and energy through the critical zone under steady state assumptions. However, in eroding landscapes, regolith and soil are produced from the bedrock as it is exhumed. Therefore, at some point in time, every location on the Earth's surface currently mantled by regolith experienced an onset of weathering processes. This initial creation of a critical zone from rock is poorly understood. Here we study initial critical zone formation from exposed bedrock by combining surface and subsurface geophysical observations at a site where regolith appears to be forming from bedrock on a granodiorite outcrop in Panola Mountain State Park, Georgia, USA. Vegetation gains an initial foothold on the outcrop by colonizing microtopographic depressions created by differential weathering of contrasting bedrock compositions. We observe a range of colonization stages, from moss to grasses to small bushes and eventually to large trees. Subsurface signatures of the vegetation include enhanced radar reflectance and reduced seismic velocities, with larger vegetation associated with stronger subsurface signals. Using a space-for-time substitution approach, we propose an evolutionary sequence for critical zone development. While disentangling the chicken-and-egg questions that pervade this topic remains challenging, our results suggest that geological heterogeneity can provide the initial catalyst for colonization, but ultimately vegetation itself plays a strong role in producing subsurface structures associated with the critical zone.

The cycle of channel belt widening and narrowing, driven by changes in sediment supply relative to sediment transport capacity, leads to the formation and preservation of strath terraces, where channel lateral erosion typically plays a key role. Because channel widths were mostly ignored, few models can explain how strath terraces in bedrock valley form under external forcing. Here, we present a modified fluvial erosion-deposition landscape evolution model that incorporates channel width, governed by water discharge and slope. Focusing on climate-driven strath terraces, our model explicitly simulates channel width variations induced by climate-driven changes in discharge, assuming uniform uplift and lithology. Results show that increased channel width constants amplify the influence of climate signals on sediments because the same water discharge is distributed across more channel nodes, reducing the transport capacity. Moreover, our model shows that strath terraces form as the climate shifts from wet to dry, considering channel width and sediment deposition. When the climate shifts from dry to wet conditions, channel widening erodes the former valley walls and destroys pre-existing strath terraces, generating planation surfaces. This lateral erosion in our model results from channel widening, rather than channel mobility. Bedrock straths are subsequently exposed during channel narrowing. Our model has the potential to elucidate the evolution of bedrock valley and channel width, under the influence of tectonics, climate, rock properties, and sediment thickness on valley geometry.

Modification of river corridors, particularly deforestation and the removal of large wood, has greatly altered the abundance and influence of large wood in most rivers in the temperate latitudes. The conceptual framework of large wood process domains can assist in both directing research and facilitating large wood-related management and restoration in rivers. Large wood process domains are spatially or temporally distinct portions of a river network or region with distinct processes of wood recruitment, transport, and storage. Previous research has shown wood to be unevenly distributed across space and time. We use a data set of logjam distribution density (# of channel-spanning logjams/100 m length of channel) in 304 spatially distinct reaches of mountain streams in the Colorado Front Range, and up to 11 years of repeat measurements at some reaches, to (a) statistically evaluate whether a priori designated process domains for logjam distribution density are distinctly different and (b) evaluate the sensitivity of process domain delineations to spatial and temporal sample size. Our results indicate that the major spatial controls on logjam process domains for logjam distribution density in the Southern Rockies are drainage area, reach morphology, and wildfire disturbance history. Greater logjam distribution densities were present in wide reaches and undisturbed catchments. Using subsets of the data set composed of under 100 reaches created similar results. The relationship between geomorphic and hydrologic characteristics and their ability to describe logjam distribution density was minimally affected when using fewer than 10 years of data.

Rock fracturing regulates the topography, carbon cycle, geologic hazards, and infrastructure degradation of the Earth. Yet, there remains a paucity of constraints on long-term fracturing behavior. Here we use field measurements of 2221 clasts across a range of environments and rock types to show that the number and total length of fractures in natural surface rocks increase rapidly within the first ∼10 kyr of exposure, and then the rate of fracture growth slows exponentially over geologic time up to ∼150 ka. Similar rock breakdown deceleration trends were independently documented using a novel application of infrared photoluminescence (IRPL) dating for a visibly fractured boulder at one site. Previous work shows that increasing microcrack intensity correlates with enhanced compliance in the bulk rock over the same timescales that our bulk macroscale fracturing rates decrease. We hypothesize that enhanced compliance contributes significantly to the observed decrease in fracturing rates via increased material toughness. Our results contrast with current landscape-scale conceptual models that assume bulk fracturing rates and characteristics are invariant over time and are controlled by short term rock strength and external stress magnitudes alone. Instead, our findings indicate that, over geologic time, fracturing of rocks increases its effective toughness.

Whillans Ice Plain (WIP), a region of West Antarctica flowing into the southern Ross Sea, lurches forward 0.2–0.6 m over 30–60 min once or twice per diurnal ocean tidal cycle. Combining 11 years (2008–2019) of 30 s or better resolution Global Navigation Satellite System (GNSS) data from past field campaigns is necessary to provide insight into WIP dynamics by connecting individual slip events to decadal velocity trends. We processed GNSS data from 48 stations and used a linear least squares residual thresholding algorithm to automatically detect 5150 slip events during the 11-year observational period. Event frequency decreased over the 11-year record, due to an increased prevalence of one-slip diurnal ocean tidal cycles over two-slip cycles. Slip-event frequency varies at fortnightly, monthly, and semiannual ocean tidal periods, but does not explain the overall decrease in events. Combining the full complexity of the regional barotropic tides with secular changes over decadal timescales is necessary to interpret observed ice-stream dynamics.

The morphology of a coral reef terrace (CRT) is a key parameter in the interpretation and quantification of past sea-level changes, but it is directly influenced by local morphodynamic and hydrodynamic conditions. Spatial differences in terrace morphology may therefore result in over- or underestimation of paleorelative sea levels and their associated uncertainties. To investigate this, we integrate high-precision field surveys from the island of Aruba (Leeward Antilles, Caribbean Sea) with a stratigraphic forward model (DionisosFlow®) to quantify the intra-island variability of the Quaternary coral reef sequence. We establish that a possible slight North-South tectonic tilt of the island may drive differences in the elevation of CRTs and the number of emerged fossil coral reefs imprinted on the coastal landscape. However, terrace geometry is primarily defined by the basement slope and wave exposure. All together, our results show that even small-scale environmental and hydrodynamic variability can introduce meter-scale errors in sea-level reconstructions derived from CRTs.

Weathering is a fundamental driver of landslide evolution over geological timescales. Despite its ubiquity and importance, quantifying how weathering drives the progressive destabilization of rock slopes remains challenging. In this work, we develop a unified computational framework based on the particle finite element method to investigate the evolution of weathering-induced landslides, from long-term weathering to short-term slope failure and runout dynamics. The framework integrates key processes, including weathering front propagation, time-dependent strength degradation, rupture surface development, and post-failure runout dynamics. Through numerical simulation experiments, we elucidate how interactions among weathering characteristics (type, intensity, and rate law), bedrock strength, fracture distribution, and slope geometry govern the failure modes and kinematics of weathering-induced landslides. Simulations show that matrix-dominated weathering leads to shallow translational failures, whereas fracture-dominated weathering produces deep-seated rotational and compound landslides. Pre-existing fractures and slope morphology also strongly influence the movement of destabilized landmasses, affecting the failure pattern (e.g., kinematic mode and rupture surface geometry) and post-failure behavior (e.g., runout velocity). We further demonstrate that the failure time and volume of weathered slopes are governed by the competition between gravitational driving forces and cohesive resisting forces during progressive destabilization. These findings provide new insights into the fundamental mechanisms that drive the emergence of diverse failure patterns of weathering-induced landslides with important implications for landslide hazard assessment.

Long-term hillslope evolution involves complex continuous (i.e., soil creep) and discontinuous (i.e., landslide) processes. The nonlinearity introduced using discontinuous processes poses significant challenges to the numerical stability of long-term hillslope numerical simulations. Geomorphologists have proposed numerous sediment transport laws related to local slopes, each tailored to specific geomorphic processes. However, many complex hillslope sediment transport models, such as the depth–slope product model and the nonlinear depth- and slope-dependent model, lack available implicit solution schemes. We address this issue by proposing an implicit numerical scheme named “Modular Implicit Method.” Taking the nonlinear slope-dependent transport law as an example, our method splits the sediment transport law into gradient and coefficient functions and then directly computes the coefficient functions using a lagged strategy. Numerical experiments demonstrate that our method performs comparably to existing implicit methods in solving nonlinear transport models. Long-term evolution simulations on realistic hillslopes show that our method enables flexible switching and co-existence of multiple transport laws, demonstrating strong extensibility. Furthermore, the framework seamlessly couples with the stream-power incision model, exhibiting a more general compatibility.