Journal of Geophysical Research: Earth Surface最新文献

Wave-driven erosion of marsh boundaries is a major cause of marsh loss, but little research has captured the effect of seasonal differences on marsh-edge retreat rates to illuminate temporal patterns of when the majority of this erosion is occurring. Using five surface models captured over a study year of a marsh with a steep escarped boundary in South San Francisco Bay, we find a pronounced seasonal signal, where rapid marsh retreat in the spring and summer is driven by a strong sea breeze but little change is found in the marsh-edge position in the fall and winter. We found accretion in the mudflat transition region close to the marsh boundary in the calmer seasons however, suggesting intertwined morphodynamics of mudflats and the eroding marsh-scarp. We observed large spatial heterogeneity in retreat rates within seasons, but less on longer (annual and decadal) timescales. The relationship between marsh-edge retreat rates and properties of the wave field nearby is explored and contextualized against extant relationships, but our results speak to the difficulty in addressing spatial erosion/accretion variability on short (seasonal) timescales with simple models.

Reconstructing historical climate change from deep ground temperature measurements in cold regions is often complicated by the presence of permafrost. Existing methods are typically unable to account for latent heat effects due to the freezing and thawing of the active layer. In this work, we propose a novel method for reconstructing historical ground surface temperature (GST) from borehole temperature measurements that accounts for seasonal thawing and refreezing of the active layer. Our method couples a recently developed fast numerical modeling scheme for two-phase heat transport in permafrost soils with an ensemble-based method for approximate Bayesian inference. We evaluate our method on two synthetic test cases covering both cold and warm permafrost conditions as well as using real data from a 100 m deep borehole on Sardakh Island in northeastern Siberia. Our analysis of the Sardakh Island borehole data confirms previous findings that GST in the region have likely risen by 5–9°C between the pre-industrial period of 1750–1855 and 2012. We also show that latent heat effects due to seasonal freeze-thaw have a substantial impact on the resulting reconstructed surface temperatures. We find that neglecting the thermal dynamics of the active layer can result in biases of roughly −1°C in cold conditions (i.e., mean annual ground temperature below −5°C) and as much as −2.6°C in warmer conditions where substantial active layer thickening (>200 cm) has occurred. Our results highlight the importance of considering seasonal freeze-thaw in GST reconstructions from permafrost boreholes.

Granular flows are ubiquitous in nature with single flows traversing a wide range of dynamic conditions from initiation to deposition. Many of these flows are responsible for significant hazards and can generate remotely detectable seismic signals. These signals provide a potential for real-time flow measurements from a safe distance. To fully realize the benefit of seismic measurements, basal-granular forces must be linked to macroscopic internal flow dynamics across a wide range of flow conditions. We utilize discrete element simulations to observe dry and submerged granular flows under plane-shear and inclined-flow configurations, relating bulk kinematics to basal-force distributions. We find that the power and frequency of force fluctuations scale with non-dimensional shear rate (I). This scaling tracks three pre-established regimes that are described by μ(I) rheology: (a) an intermittent particle rearrangement regime, where basal forces are dominated by low frequencies; (b) an intermediate regime where basal forces start to increase in frequency while showing correlations in space and (c) a fully collisional regime where the signal is nearly flat up to a cutoff frequency. We further identify a newly defined fourth regime that marks a “phase change” from the intermediate to collisional regime where increases in basal force fluctuations with increasing shear rates stalls as the granular bed dilates, partially destroying the contact network. This effort suggests that basal forces can be used to interpret complex granular processes in geophysical flows.

Estuaries worldwide are susceptible and adapting to climate change (CC) impacts from both the river and coastal boundaries. Furthermore, engineering efforts are undertaken to improve flood safety, to claim land for human use or for port operations, which change estuary morphology. This paper aims to gain an understanding of the combined effects of CC and human interventions on the estuarine-wide morphological response by analyzing the sediment infilling of highly engineered estuaries. A schematized process-based morphodynamic model is used (Delft3D-FM, in 2DH), resembling a highly engineered estuary in the Rhine-Meuse Delta, The Netherlands. Three types of changes were implemented, both in isolation and in combination: (a) local interventions (changing channel depth or wetland area), (b) upstream human interventions (changing fluvial sediment supply) and (c) extreme CC scenarios (with projections for the future forcings and bathymetry). Results show that a CC scenario can elicit both positive and negative changes in the estuary's sediment budget. The direction and magnitude of the change depend on the local intervention and can align with the effect of the local intervention, intensifying its impact. The combined effects can even reverse the sign of the sediment budget. This stresses the need of analyzing CC impacts in combination with human interventions. Additionally, a relationship was identified which quantifies how a change in peak flow velocity due to both local interventions and sea-level rise affects the annual sediment budget. These findings can help determine how local interventions affect morphodynamics of engineered estuaries in present and future climates.

Recent studies have revealed the presence of a complex freshwater system underlying the Aurora Subglacial Basin (ASB), a region of East Antarctica that contains ∼7 m of global sea level potential in ice mainly grounded below sea level. However, the impact that subglacial freshwater has on driving the evolution of the dynamic outlet glaciers that drain this basin has yet to be tested in a coupled ice sheet-subglacial hydrology numerical modeling framework. Here, we project the evolution of the primary outlet glaciers draining the ASB (Moscow University Ice Shelf, Totten, Vanderford, and Adams Glaciers) in response to an evolving subglacial hydrology system and to ocean forcing through 2100, following low and high CMIP6 emission scenarios. By 2100, ice-hydrology feedbacks enhance the ASB's 2100 sea level contribution by ∼30% (7.50–9.80 mm) in high emission scenarios and accelerate the retreat of Totten Glacier's main ice stream by 25 years. Ice-hydrology feedbacks are particularly influential in the retreat of the Vanderford and Adams Glaciers, driving an additional 10 km of retreat in fully coupled simulations relative to uncoupled simulations. Hydrology-driven ice shelf melt enhancements are the primary cause of domain-wide mass loss in low emission scenarios, but are secondary to ice sheet frictional feedbacks under high emission scenarios. The results presented here demonstrate that ice-subglacial hydrology interactions can significantly accelerate retreat of dynamic Antarctic glaciers and that future Antarctic sea level assessments that do not take these interactions into account might be severely underestimating Antarctic Ice Sheet mass loss.

Bedrock river incision drives the evolution of the Earth's landscape and is influenced by river hydraulic power. However, the relationship between plunging flows and the curvature of bedrock canyons is poorly understood, which encouraged us to explore how the bend's curvature and undulating beds affect plunging flows. A generalized physical model was built to investigate the hydrodynamic characteristics of plunging flows in a constant curvature flume with plain and undulating beds. Our experimental findings demonstrated that plunging flows were related to secondary circulation, topography, and width-to-depth ratios. Plunging flows occurred when secondary circulation reached its peak, and as secondary circulation vanished, the intensity of plunging flows decreased. The undulating bed topography in the bedrock bends suppressed secondary circulation and the development of plunging flows. Bed topography may be a dominant factor in plunging flows in bedrock bends. The potential erosion area in bedrock bends was related to the intensity of plunging flows, which caused velocity inversion to increase the shear stress near the bed. With higher discharges and undulating beds, the intensity of the transverse shear stress of the riverbed near the center was greater. The potential erosion area in bedrock bends was concentrated at the center of the cross-section around the bend apex upstream. Our experimental results can improve incision models in terms of the distribution of shear stress and flow structure in bedrock bends.

Forests overlap with large parts of the northern hemisphere permafrost area, and representing canopy processes is therefore crucial for simulating thermal and hydrological conditions in these regions. Forests impact permafrost through the modulation of radiative fluxes and exchange of turbulent fluxes, precipitation interception and regulation of transpiration. Forests also feature distinct soil layers of litter and organic matter, which play central roles for the infiltration and evaporation of water, while also providing thermal insulation for deeper ground layers. In this study, we present a new module within the CryoGrid community model to simulate forest ecosystems and their impact on the surface water and energy balance. The module includes a big-leaf vegetation scheme with adaptations for canopy heat storage and transpiration. Furthermore, we account for the effect of surface litter layers on water and energy transfer. We show that the model is capable of simulating radiation, snow cover and ground temperatures below a deciduous needleleaf forest on a north-facing slope in the Khentii Mountains in Central Mongolia. A sensitivity analysis of topographic aspect and ecosystem configuration confirms the important role of the litter layers for the energy and water balance of the ground. Furthermore, it suggests that the presence of permafrost is primarily linked to topographic aspect rather than the presence of forest at this site. The presented model scheme can be used to study the development of the ground thermal regime in forests, including the state of permafrost, under different climate, ecosystem, and land use scenarios.

The thermal field within the firn layer on the Greenland Ice Sheet (GrIS) governs meltwater retention processes, firn densification with surface elevation change, and heat transfer from the surface boundary to deep ice. However, there are few observational data to constrain these processes with only sparse in situ temperature time series that does not extend through the full firn depth. Here, we quantify the thermal structure of Western Greenland’s firn column using instrumentation installed in an elevation transect of boreholes extending to 30 and 96 m depth. During the high-melt summer of 2019, heat gain in the firn layer showed strong elevation dependency, with greater uptake and deeper penetration of heat at lower elevations. The bulk thermal conductivity increased by 15% per 100 m elevation loss due to higher density related to ice layers. Nevertheless, the conductive heat gain remained relatively constant along the transect due to stronger temperature gradients in the near surface firn at higher elevations. The primary driver of heat gain during this high melt summer was latent heat transfer, which increased up to ten-fold over the transect, growing by 34 MJ m⁻² per 100 m elevation loss. The deep-firn temperature gradient beneath the seasonally active layer doubled over a 270-m elevation drop across the study transect, increasing heat flux from the firn layer into deep ice at lower elevations. Our in situ firn temperature time series offers observational constraints for modeling studies and insights into the future evolution of the percolation zone in a warmer climate.

Land-ice flow in Antarctica has experienced multi-annual acceleration in response to increased rates of ice thinning, ice-shelf collapse and grounding-line retreat. Superimposed upon this trend, recent observations have revealed that land-ice flow in the Antarctic Peninsula exhibits seasonal velocity variability with distinct summertime speed-ups. The mechanism, or mechanisms, responsible for driving this seasonality are unconstrained at present, yet detailed, process-based understanding of such forcing will be important for accurately estimating Antarctica's future contributions to sea level. Here, we perform time-series analysis on an array of remotely sensed, modeled and reanalysis data sets to examine the influence of potential drivers of ice-flow seasonality in the Antarctic Peninsula. We show that both meltwater presence and ocean temperature act as statistically significant precursors to summertime ice-flow acceleration, although each elicits an ice-velocity response after a distinct lag, with the former prompting a more immediate response. Furthermore, we find that the timing and magnitude of these local drivers are influenced by large-scale climate phenomena, namely the Amundsen Sea Low and the El Niño Southern Oscillation, with the latter initiating an anomalous wintertime ice-flow acceleration event in 2016. This hitherto unidentified link between seasonal ice flow and large-scale climatic forcing may have important implications for ice discharge at and beyond the Antarctic Peninsula in the future, depending upon how the magnitude, frequency and duration of such climate phenomena evolve in a warming world.

Sources and fluxes of methane to the atmosphere from permafrost are significant but poorly constrained in global climate models. We present data collected from the variable permafrost setting of the outer Mackenzie River Delta, including observations of aquatic methane seepage, core determinations of in situ methane occurrence and seep gas isotope geochemistry. The sources and locations of in situ geologic methane occurrence and aquatic and atmospheric gas release appear to be controlled by the regional geology and permafrost conditions. Where permafrost is >250 m thick, thermogenic gas deposits at depth are isolated by laterally continuous, low permeability ice-bearing sediments with few through-going thawed taliks. Thus, the observed in situ methane and aquatic gas seepage appears to be dominated by microbial methane. In contrast, where permafrost is <80 m thick, taliks are more likely to be through-going, providing permeable conduits from depth and migration pathways for both thermogenic and biogenic gas. Continuous annual fluid sampling of two lakes and a river channel documents aquatic methane flux from microbial sources, more deeply buried thermogenic sources, and mixtures of both. Using estimates of in situ methane concentration from deep core samples and observations of in situ free gas occurrences, we conclude that the reservoir of in situ geologic methane within ice bonded permafrost is substantial and that this methane is presently migrating with ongoing atmospheric release. It is our assessment that the permafrost setting, and processes described are sensitive to future climate change as the permafrost warms.