Search for: All records

Abstract Slack-water deposits are archives of paleoflood frequency, magnitude, and provenance. In the eastern Himalaya, deposits along the Siang River document Quaternary outburst megafloods originating from southeastern Tibet. Here we present new observations of slack-water deposits within aggradational terraces of the Siyom River, the Siang’s largest tributary. Terrace stratigraphy reveals distinct, regionally extensive sedimentary facies including laminated sands, clays, and detrital organic-rich deposits consistent with slack-water deposition from temporarily impounded waters. Radiocarbon dates from clay and organic horizons range from 34,020 to 10,630 cal yr BP, overlapping with age constraints for Tibetan paleolake deposits. Detrital zircon (U-Th)/Pb geochronology confirms a local source for the underlying fluvial facies, whereas event-deposit silts contain young zircons derived from Tibet, supporting their interpretation as megaflood deposits. This evidence, combined with the deposits’ temporal overlap with Tibetan paleolakes and distinctive slack-water sedimentology, demonstrates that event facies formed through megaflood backflooding sourced from southeastern Tibet. The results point to the likelihood of similar deposits in other tributaries, providing a framework for regional investigation. Our findings further show that megafloods in steep terrain can produce substantial deposition and terrace formation tens of kilometers upstream in tributaries—far beyond the main stem floodway—revealing an overlooked geomorphic imprint of extreme floods. 

ABSTRACT Anthropogenic reshaping of landscapes can increase landslide activity. One example of heavily modified, landslide-prone landscapes is the Appalachian region, which hosts weak geologic strata, steep slopes, a humid climate, geologically recent river incision, and large-scale landscape modifications resulting from surface mining. Surface mining conducted before the Surface Mine Control and Reclamation Act (SMCRA) of 1977 did not require reclamation to approximate original contour, so remnant highwalls and mine benches are common throughout Appalachia. This study investigated the occurrence of slope instability and failure beneath pre-law mine benches in the northern coalfields of West Virginia. Mine bench features were manually digitized based on interpretation of high-spatial-resolution digital terrain data and aerial orthophotography, and ancillary geospatial data (i.e., mine permit boundary and disturbance extent data sets). A subset of mine bench features, stratified by coal seam, was randomly selected for slope stability analysis. The lower margins of the randomly selected mine benches were differentiated into segments with and without evidence of slope failure or instability. Results suggest that local topographic slope and aspect were not strong determinants of slope instability beneath mine benches. However, the coal seam or unit that was mined was an important determining factor. The Pittsburgh seam, and to a lesser extent the Redstone seam, had a larger proportion of their lower margins showing evidence of slope failure than did other seams. This study highlights that local-scale geology controls the geomorphic response to human landscape modifications and suggests that proactive reclamation of surface coal mines might reduce slope-failure geohazards. 

Abstract We propose and unify classes of different models for information propagation over graphs. In a first class, propagation is modelled as a wave, which emanates from a set ofknownnodes at an initial time, to all otherunknownnodes at later times with an ordering determined by the arrival time of the information wave front. A second class of models is based on the notion of a travel time along paths between nodes. The time of information propagation from an initialknownset of nodes to a node is defined as the minimum of a generalised travel time over subsets of all admissible paths. A final class is given by imposing a local equation of an eikonal form at eachunknownnode, with boundary conditions at theknownnodes. The solution value of the local equation at a node is coupled to those of neighbouring nodes with lower values. We provide precise formulations of the model classes and prove equivalences between them. Finally, we apply the front propagation models on graphs to semi-supervised learning via label propagation and information propagation on trust networks. 

Abstract BackgroundThe increasing size, severity, and frequency of wildfires is one of the most rapid ways climate warming could alter the structure and function of high-latitude ecosystems. Historically, boreal forests in western North America had fire return intervals (FRI) of 70–130 years, but shortened FRIs are becoming increasingly common under extreme weather conditions. Here, we quantified pre-fire and post-fire C pools and C losses and assessed post-fire seedling regeneration in long (> 70 years), intermediate (30–70 years), and short (< 30 years) FRIs, and triple (three fires in < 70 years) burns. As boreal forests store a significant portion of the global terrestrial carbon (C) pool, understanding the impacts of shortened FRIs on these ecosystems is critical for predicting the global C balance and feedbacks to climate. ResultsUsing a spatially extensive dataset of 555 plots from 31 separate fires in Interior Alaska, our study demonstrates that shortened FRIs decrease the C storage capacity of boreal forests through loss of legacy C and regeneration failure. Total wildfire C emissions were similar among FRI classes, ranging from 2.5 to 3.5 kg C m⁻². However, shortened FRIs lost proportionally more of their pre-fire C pools, resulting in substantially lower post-fire C pools than long FRIs. Shortened FRIs also resulted in the combustion of legacy C, defined as C that escaped combustion in one or more previous fires. We found that post-fire successional trajectories were impacted by FRI, with ~ 65% of short FRIs and triple burns experiencing regeneration failure. ConclusionsOur study highlights the structural and functional vulnerability of boreal forests to increasing fire frequency. Shortened FRIs and the combustion of legacy C can shift boreal ecosystems from a net C sink or neutral to a net C source to the atmosphere and increase the risk of transitions to non-forested states. These changes could have profound implications for the boreal C-climate feedback and underscore the need for adaptive management strategies that prioritize the structural and functional resilience of boreal forest ecosystems to expected increases in fire frequency. 

The spatial sensitivity pattern of millimeter-wavelength receivers is an important diagnostic of performance and is affected by the alignment of coupling optics. Characterization can be challenging in the field, particularly in the decentered and tightly packed optical configurations that are employed for many astronomical millimeter-wave cameras. In this paper, we present the design and performance of a lightweight and reconfigurable beam mapper, consisting of a bank of thermal sources positioned by a planar cable-driven robot. We describe how the measurement requirements and mechanical constraints of the Tomographic Ionized-carbon Mapping Experiment optical relay drive the design of the mapper. To quantify the positioning performance, we predict the beam patterns at each surface to derive requirements and use a non-contact computer-vision-based method built on OpenCV to track the payload position with an accuracy better than 1.0 mm. We achieve an in-plane absolute payload position error of 2.7 mm (RMSE) over a ∼400×400 mm workspace and an in-plane repeatability of 0.81 mm, offering substantial improvements in accuracy and speed over traditional handheld techniques.