Journal of Geophysical Research: Earth Surface最新文献

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.

Recent theoretical models and field observations suggest that fluvial bedload flux can be estimated from seismic energy measured within appropriate frequency bands. We present an application of the Tsai et al. (2012, https://doi.org/10.1029/2011gl050255) bedload seismic model to an ephemeral channel located in the semi-arid southwestern US and incorporate modifications to better estimate bedload flux in this environment. To test the model, we collected streambank seismic signals and directly measured bedload flux during four flash-floods. Bedload predictions calculated by inversion from the Tsai model underestimated bedload flux observations by one-to-two orders of magnitude at low stages. However, model predictions were better for moderate flow depths (>50 cm), where saltation is expected to dominate bedload transport. We explored three differences between the model assumptions and our field conditions: (a) rolling and sliding particles have different impact frequencies than saltating particles; (b) the velocity and angle of impact of rolling particles onto the riverbed differ; and (c) the fine-grained alluvial character of this and similar riverbeds leads to inelastic impacts, as opposed to the originally conceptualized elastic impacts onto rigid bedrock. We modified the original model to assume inelastic bed impacts and to incorporate rolling and sliding by adjusting the statistical distributions of bedload impact frequency, velocity, and angle. Our modified “multiple-transport-mode bedload seismic model” decreased error relative to observations to less than one order of magnitude across all measured flow conditions. Further investigations in other environmental settings are required to demonstrate the robustness and general applicability of the model.

Global climate change has been intensifying the scale and frequency of rock-ice avalanches and similar catastrophic mass movements in high-mountain regions. The difference in the physical characteristics of rock and ice particles leads to mixing and segregation during flow. Although, both particle segregation and the presence of ice fundamentally alter flow behavior, the joint influence and feedback of these two aspects are overlooked in state-of-the-art rock-ice avalanche models. Using discrete element simulations, we show that by controlling the distribution of inter-particle frictional interactions within the mixture, segregation patterns resulting from the size, density, concentration, and surface friction differences of rock and ice phases can induce sharp velocity gradients along the flowing thickness. Flowing layers where low friction contacts with ice are abundant tend to flow faster and can induce slow creeping motion in an otherwise static basal layer dominated by more frictional rocks. Based on these observations, we find that the effective friction of rock-ice flows for various mixture concentrations and size ratios can be obtained as a sum of the single-phase rheologies of rocks and ice weighted according to their microscopic contact probabilities. This effective friction for rock-ice mixtures allows us to extend a recent non-local granular fluidity framework that captures the complex segregation-flow feedback mechanism in rock-ice flows. The findings provide a deeper micromechanical understanding of how particle interactions influence rock-ice avalanche mobility, which ultimately improves flow models needed for hazard assessment and mitigation.

Passive seismic monitoring (PSM) is emerging as a tool for detecting rockfall events and pre-failure seismicity. In this paper, the potential of PSM for rockfall monitoring is assessed through a case study carried out in Gros-Morne, Eastern Québec, in a region with prominent roadside cliffs, where more than 500 fallen rocks are found on the main regional road each year. The proposed method relies on using sensitive STA-LTA windows to detect a very large number of seismic events and build a comprehensive catalog. In total, more than 70,000 seismic events were detected over one year. Gaussian mixtures are used to partition the data set. Based on visual inspection of the data, a main working hypothesis is that the seismic events can be clustered into three groups. After analyzing the spatio-temporal distribution of the events in each group, we find that the events of one cluster can be associated with anthropogenic activity. The frequency of occurrence of the events of the different clusters and their link with meteorological data is also examined through a regression exercise, to assess the importance of the meteorological variables as explanatory variables. The results allow us to postulate on the physical origins of the signals in the different clusters, attributing them to rockfall activity and wind-induced seismic noise.

Volcanic soils are among the most productive soils in the world as they can accumulate large amounts of organic carbon and nitrogen and have good water storage capacity. They are extensively used worldwide for agriculture, which makes it difficult to study the soil-landscape dynamics under natural conditions. By working in the Galápagos Islands, a UNESCO World Heritage Site, we aimed to constrain soil development over millennial timescales using empirical data. Our monitoring sites on Santa Cruz Island cover a 10 km long NW-SE transect with an 8-fold increase in precipitation and associated vegetation changes. By controlling for age and chemical composition of the basaltic parent material, we investigated the influence of precipitation rates on soil weathering. At the landscape scale, soil weathering degree increased with increasing precipitation, as shown by the spatial patterns in soil depth, pH, mass loss coefficients, chemical index of alteration, chemical depletion fraction, and total reserve in bases. In addition to the climatic effect, rock porosity strongly enhanced basalt weathering. Porosity-enhanced weathering is particularly important in the humid and perhumid precipitation regimes: soils developed on porous scoriae developed weathering mantles that are ∼10-fold thicker and have 10-fold higher mass losses due to weathering compared to soils developed on basalt lava flows. Our results demonstrated that variations in rock pore dimensions and distribution can lead to large variations in basalt weathering rates, particularly in humid and perhumid climates where deep leaching can be facilitated by rock porosity.

We recorded aerodynamic roughness and shear velocity along transects on and around mature crescent-shaped barchan dunes of $��4.5��m�$ and $��27��m�$ height above the horizontal rock-covered Qatar desert by fitting to the log-law time-averaged vertical velocity profiles acquired from triads of ultrasonic anemometers penetrating the inner turbulent boundary layer. Shear velocity first decreased, then recovered as air climbed on the dune, with a local maximum ahead of the crest as predicted by the Jackson and Hunt (1975, https://doi.org/10.1002/qj.49710143015) theory. Unlike flows over gentler bedforms without a slope discontinuity, an anomalous peak of shear velocity also arose on the dune centerline at the brink, which the theory attributed to skewness in the dune transect profile. The onset of aeolian transport produced a log-law passing through the Bagnold (1941, https://doi.org/10.1007/978-94-009-5682-7) focal point. It was bracketed by noticeable hysteretic peaks in the correlation between wind speed and entrained sand flux. The dunes' rocky surroundings and topography produced an aerodynamic roughness at odds with the Nikuradse (1933, https://ntrs.nasa.gov/citations/19930093938) data for fully developed turbulent boundary layers. Large-eddy numerical simulations illustrated the sensitivity of shear velocity to wide changes in aerodynamic roughness from desert floor to dune surface.

One of the most conspicuous signals of climate change in high-latitude tundra is the expansion of ice wedge thermokarst pools. These small but abundant water features form rapidly in depressions caused by the melting of ice wedges (i.e., meter-scale bodies of ice embedded within the top of the permafrost). Pool expansion impacts subsequent thaw rates through a series of complex positive and negative feedbacks which play out over timescales of decades and may accelerate carbon release from the underlying sediments. Although many local observations of ice wedge thermokarst pool expansion have been documented, analyses at continental to pan-Arctic scales have been rare, hindering efforts to project how strongly this process may impact the global carbon cycle. Here we present one of the most geographically extensive and temporally dense records yet compiled of recent pool expansion, in which changes to pool area from 2008 to 2020 were quantified through satellite-image analysis at 27 survey areas (measuring 10–35 km² each, or 400 km² in total) dispersed throughout the circumpolar tundra. The results revealed instances of rapid expansion at 44% ($��\pm �$15%) of survey areas. Considered alone, the extent of departures from historical mean air temperatures did not account for between site variation in rates of change to pool area. Pool growth was most clearly associated with upland (i.e., hilly) terrain and elevated silt content at soil depths greater than one meter. These findings suggest that, at short time scales, pedologic and geomorphologic conditions may exert greater control on pool dynamics in the warming Arctic than spatial variability in the rate of air temperature increases.

By defining the attributes of river networks, we can quantitatively extract records of climatic and tectonic changes from them. The stream power incision model (SPIM) provides a framework within which this can be achieved, as it facilitates the calculation of the relative rock uplift from river characteristics. One parameter that has been widely employed in tectonic and fluvial geomorphology is the channel steepness index, a metric that can represent the normalized rock uplift rate experienced by a river. However, to accurately infer the channel steepness index, we must accurately estimate m/n, the ratio between the two positive exponents of the SPIM. Present methodologies to constrain m/n rely on an assumption that rock uplift and erodibility are spatially invariant. These conditions are rarely present on Earth. In this study, we use a synthetic example and examples from the Siwalik Hills and Olympic Mountains to demonstrate how existing methodologies to constrain m/n produce systematic errors when there is spatial variation, and particularly spatial gradients, in the processes driving landscape evolution. To solve this problem, we present a methodology to estimate m/n based on a large river network inversion that accounts for spatial variation in landscapes. After demonstrating that the methodology can accurately recover m/n in our synthetic landscape, we show that our methodology can reconcile contrasting observations in the Siwaliks, and is critical to inferring accurate values of channel steepness index in the Olympic Mountains. This highlights the utility of large topographic inversions for investigating landscape dynamics.