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 and 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% (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.
Continental-scale glaciations cause deformation, geopotential, rotation and stress changes of the Earth. Subsurface stress changes have implications to future activities such as carbon capture and storage, enhanced oil recovery and deep geological disposal of nuclear waste. We model glacially induced stresses, strain changes and deformation for North America, with emphasis on the two potential sites for long-term storage of used nuclear fuel in Canada (Saugeen Ojibway Nation (SON)-South Bruce area in southwestern Ontario and Wabigoon Lake Ojibway Nation (WLON)-Ignace area in northwestern Ontario). We apply a revised, high-resolution ice history of the past glacial cycle from the University of Toronto Glacial Systems Model, assumed to be representative for future glacial cycles, together with a set of seven different one- and three-dimensional earth structures. We find that glacially induced stresses and strains can vary strongly throughout a glacial cycle, whereas especially the horizontal components can change from tensional to compressive in nature. Such changes can happen within a few 1,000 years, caused by drastic and rapid ice thickness increase or decrease above the potential site. Despite SON-South Bruce being located further away from the ice sheet center than WLON-Ignace and temporarily in the forebulge of the developing ice sheet during glaciation, stresses and strains are very similar in magnitude and range at both sites. We also see the potential that the glacially induced stresses can alter the direction of the pre-existing maximum horizontal stress at SON-South Bruce. These results will be incorporated in the site safety and site selection process.
Antarctica has an active subglacial hydrological system, with interconnected subglacial lakes fed by subglacial meltwater. Subglacial hydrology can influence basal sliding, inject freshwater into the sub-ice-shelf cavity, and impact sediment transport and deposition which can affect the stability of grounding lines (GLs). We used satellite altimetry data from the ICESat, ICESat-2, and CryoSat-2 missions to document the second recorded drainage of Engelhardt Subglacial Lake (SLE), which began in July 2021 and discharged more than 2.3 km3 of subglacial water into the Ross Ice Shelf cavity. We used differential synthetic aperture radar interferometry from RADARSAT-2 and TerraSAR-X alongside ICESat-2 repeat-track laser altimetry (RTLA) and REMA digital elevation model strips to detect 2–13 km of GL retreat since the previous drainage event in 2003–06. Combining these satellite observations, we evaluated the mechanism triggering SLE drainage, the cause of the observed GL retreat, and the interplay between subglacial hydrology and GL dynamics. We find that: (a) SLE drainage was initiated by influx from a newly identified upstream lake; (b) the observed GL retreat is mainly driven by the continued retreat of Engelhardt Ice Ridge and long-term dynamic thinning that caused a grounded ice plain to reach flotation; and (c) SLE drainage and GL retreat were largely independent. We also discuss the possible origins and influence of a 27 km grounded promontory found to protrude seaward from the GL. Our observations demonstrate the importance of high-resolution satellite data for improving the process-based understanding of dynamic and complex regions around the Antarctic Ice Sheet margins.
When water and sediment supply to a river change, the short-term channel response will be a combination of adjustments in bed grain size (texture) and the accumulation or evacuation of sediment in the channel (topography). This response is well documented for gravel bed rivers, but little attention has been given to sand bed rivers, the focus of this paper. If the channel response is predominantly textural, subsequent changes in channel geometry may be minor. If the channel response is predominately aggradation or degradation, long-term changes in channel dimension, planform, and slope may occur. Here, we use a mixed-size morphodynamic model to explore the interaction between the textural and topographic responses to an increase in sediment supply in sand bed rivers. First, we consider how the steady-state transport condition varies as a function of sediment supply rate and grain size. We then evaluate the path to steady-state using numerical morphodynamic experiments. We find that bed aggradation in response to an increase in sediment supply can be reduced, eliminated, or even reversed depending on the sediment supply grain size. The path to the new steady-state condition involves two adjustment phases: the first is textural-dominated, and the second is topography-dominated. Under the common condition of an increased supply of finer sediment, rapid textural adjustments can produce a transport capacity that is a large fraction of the new supply rate long before the system has reached complete equilibrium. Under conditions of supply coarsening, the initial textural adjustment is less dominant, and aggradation is immediate.
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