Eitan Shelef

Permafrost degradation is expected to accelerate over the next century, with potentially important consequences for Arctic hillslope stability, channel erosion, sediment mobilization, water quality, and carbon release from previously frozen soils. However, the extent to which changes in permafrost cover alter the intensity and dominant mechanisms of catchment‐scale erosion remains poorly constrained. This study uses sediments from 12 Alaskan lakes across two regions to compare the intensity of hillslope and channel physical erosion in small catchments between areas with continuous and discontinuous permafrost cover. Sediment‐based erosion rates computed from dated lake sediment cores spanning the middle Holocene to present, combined with a Landscape Evolution Model suggest that (a) a simple model accounts for most of the variance in sediment‐based erosion rates between lakes, (b) diffusive (primarily hillslope) processes are more significant than advective (primarily fluvial) processes, and (c) the average intensity of diffusive processes is ∼2.1 to ∼3.0 times higher in areas of discontinuous compared to continuous permafrost landscapes. Additional processes, including shoreline wave action and thaw slumping, may also contribute to the variability in erosion rates among lakes. Overall, these results suggest that warming‐driven shifts from continuous to discontinuous permafrost could more than double the intensity of erosional processes; however, the limited data and associated uncertainties underscore the need for more data to better constrain the impacts of permafrost thaw on catchment‐scale erosion.

Shoulder‐type escarpments are defined by the coincidence of a drainage divide and a cliff line, which separates antecedent, low‐relief highland drainage systems flowing away from the escarpment from the steep channels draining the escarpment face. Although these escarpments often form as linear landforms, many shoulder‐type escarpments worldwide exhibit deep embayments whose width is similar to that of highland valleys that deviate from expectations of uniform retreat. Recent numerical models propose that these embayments form and extend through feedback between the migration of highland wind gaps (valley‐confined drainage divides) and the retreat of escarpment‐crossing waterfalls. The current study provides field‐based support for this mechanism and also develops general observational constraints that serve as diagnostic indicators of its operation. Towards these objectives, the deeply embayed Ovil Cliffs in the Negev Desert, Israel, are used as a natural laboratory to explore the parameters governing the feedback and the feedback's morphometric signatures. Detailed field mapping and morphometric analysis of high‐resolution topography support the wind gap‐waterfall feedback as the primary driver of the cliffs' evolution. The analysis reveals unstable wind gaps tending to migrate towards the highlands and shows correlations between the drainage area at the waterfalls and (i) wind gap degree of instability, (ii) embayment depth along the escarpment face, and (iii) the slope of the pre‐existing highland drainage network. These morphometric correlations support dynamic coupling between migrating wind gaps and retreating waterfalls that extend the embayments across the Ovil Cliffs and highlight the role of the antecedent drainage geometry in modulating the feedback. Our findings suggest that the wind gap‐waterfall feedback introduces spatial variability in escarpment retreat rates and increases escarpment sinuosity, contributing to the differential preservation of shoulder‐type cliffs and escarpments over geological timescales.

The timing and mechanisms of permafrost thaw and erosion prior to human influence provide essential context for permafrost carbon mobility under arctic warming. Because arctic lake sediments archive erosional products from permafrost that age significantly on the landscape before remobilizing into the lake, the range of radiocarbon ages (age-offset) present in the lake sediment can indicate the relative contribution of ancient permafrost-derived organic carbon (OC) through time. We use age-offsets from Lake E5, northeastern Alaska, to reveal changes in the contribution of aged permafrost-derived OC deposited in the lake sediment over the last 45 thousand years. To do this, we combine the ¹⁴ C dating of paired bulk sediment and plant macrofossils from the same stratigraphic layer of lake sediment and ramped pyrolysis-oxidation (RPO) ¹⁴ C analysis. Lake E5 revealed small age-offsets (1580 years) during the post-glacial period, large age-offsets (27 200 years) during the Last Glacial Maximum, and moderate age-offsets (15 100 years) during the MIS 3 interstadial. These temporal patterns broadly align with our recently published findings from Burial Lake, northwestern Alaska, highlighting the importance of vegetation, soil development, moisture availability, and erosional mechanisms in controlling carbon mobility in Arctic Alaska while key differences emphasize the individuality of lake basins.

The drivers behind the marked 2- to 3-km relief canyons carved into the ca. 3.7-km-high Andean Plateau remain debated, with proposed triggers ranging from plateau surface uplift to precipitation-enhanced erosion. We integrate a surface processes model with published cross-section–derived estimates of structural geometry, kinematics, and shortening rates, as well as precipitation estimates from global atmospheric circulation models, to assess whether the geometry, sequential fault motion, and resulting surface uplift can reproduce plateau morphology. Endorheic basins form where uplift on faults exceeds incision. A decrease in shortening after ~10 Ma allows rivers to compete with uplift and incise into endorheic basins at the plateau edge. Two modeled basin capture events at ~7 to 5 Ma and ~3.5 to 0.5 Ma, coupled with climate-driven discharge fluctuations, markedly increase rivers’ discharge, erosion, and sediment transport, facilitating canyon incision and highlighting the importance of basin captures in canyon formation along the northeastern edge of the Andean Plateau.

Escarpments and cliffs (hereafter termed escarpments) form topographic barriers that influence the spatial patterns of climate and biodiversity. The inherent extreme slope change across the escarpment edge promotes escarpment retreat over time. Typically, escarpments are divided into arch‐ and shoulder‐types. In arch‐type, the drainage divide is located inland, and knickpoints, located where channels flow across the escarpment, can retreat and embay the escarpment. In shoulder‐type, the divide aligns with the escarpment edge, a setting expected to cause a slow and uniform escarpment retreat, preserving their integrity as barriers through time. However, observations from around the globe reveal shoulder‐type escarpments are associated with deep embayments (i.e., alcoves) that destroy the linear appearance of the escarpment front. Yet, the processes that produce and sustain these embayments remain largely unexplored. Embayments of shoulder‐type escarpments typically occur along reversed channels which were part of the antecedent drainage that used to flow away from the escarpment but now flow toward it, often resulting in a valley confined drainage divide called a windgap. Here, we hypothesize that feedback between knickpoint retreat and windgap migration away from the escarpment along reversed channels can sustain escarpment embayments, and use topographic analyses and numerical simulations to explore this hypothesis. Our analyses, focused on field sites in the Negev Desert, show that embayments of shoulder‐type escarpments can be sustained through the hypothesized feedback, and quantify the sensitivity of this feedback to geomorphologic and lithologic parameters. Results suggest that this feedback may explain some of the global variability of escarpment morphologies.

Whether permafrost systematically alters the rate of riverbank erosion is a fundamental geomorphic question with significant importance to infrastructure, water quality, and biogeochemistry of high‐latitude watersheds. For over four decades, this question has remained unanswered due to a lack of data. Using remotely sensed imagery, we addressed this knowledge gap by quantifying riverbank erosion rates across the Arctic and subarctic. To compare these rates to non‐permafrost rivers, we assembled a global data set of published riverbank erosion rates. We found that erosion rates in rivers influenced by permafrost are on average nine times lower than non‐permafrost systems; erosion rate differences increase up to 40 times for the largest rivers. To test alternative hypotheses for the observed erosion rate difference, we examined differences in total water yield and erosional efficiency between these rivers and non‐permafrost rivers. Neither of these factors nor differences in river sediment loads provided compelling alternative explanations, leading us to conclude that permafrost limits riverbank erosion rates. This conclusion was supported by field investigations of rates and patterns of erosion along three rivers flowing through discontinuous permafrost in Alaska. Our results show that permafrost limits maximum bank erosion rates on rivers with stream powers greater than 900 Wm⁻¹. On smaller rivers, however, hydrology rather than thaw rate may be the dominant control on bank erosion. Our findings suggest that Arctic warming and hydrological changes should increase bank erosion rates on large rivers but may reduce rates on rivers with drainage areas less than a few thousand km².

The age of abandoned alluvial surfaces is a key component in quantifying landscape evolution processes, tectonic activity, and paleoclimate. However, limited resources, restricted field accessibility, lacking dating material and analytical constraints are often encountered when dating such landforms. To help mitigate these limitations, we propose a new and complementary surface dating approach that is based on calculating the duration of gully incision into a surface since its abandonment using a locally calibrated landscape evolution model. The approach consists of calibration of incision model parameters for abandoned alluvial surfaces with known age and then using the calibrated model to calculate the time required for gully profiles to form on nearby undated alluvial surfaces as a proxy for surface abandonment. The approach was tested on previously dated late Pleistocene (34 and 70 ka) alluvial terraces in the hyper‐arid Negev desert (Israel). Calibrated model parameters were within the range reported for incision models in arid regions worldwide and late Pleistocene surface abandonment ages were recovered to within 10% accuracy. In contrast, modeled durations for gully incision into an older mid‐Pleistocene surface previously dated to 230–549 ka were grossly underestimated at <120 ka. Field observations and approach tests indeed indicated that late Pleistocene model parameters should not be extrapolated as‐is to model gully incision farther back through geologic time. Because the validity of the proposed ages depends on extrapolating the locally calibrated parameters of the incision model, we recommend that such dating be conducted with care and where assumptions will probably be valid.

Whether the presence of permafrost systematically alters the rate of riverbank erosion is a fundamental geomorphic question with significant importance to infrastructure, water quality, and biogeochemistry of high latitude watersheds. For over four decades this question has remained unanswered due to a lack of data. Using remotely sensed imagery, we addressed this knowledge gap by quantifying riverbank erosion rates across the Arctic and subarctic. To compare these rates to non-permafrost rivers we assembled a global dataset of published riverbank erosion rates. We found that erosion rates in rivers influenced by permafrost are on average six times lower than non-permafrost systems; erosion rate differences increase up to 40 times for the largest rivers. To test alternative hypotheses for the observed erosion rate difference, we examined differences in total water yield and erosional efficiency between these rivers and non-permafrost rivers. Neither of these factors nor differences in river sediment loads provided compelling alternative explanations, leading us to conclude that permafrost limits riverbank erosion rates. This conclusion was supported by field investigations of rates and patterns of erosion along three rivers flowing through discontinuous permafrost in Alaska. Our results show that permafrost limits maximum bank erosion rates on rivers with stream powers greater than 900 W/m-1. On smaller rivers, however, hydrology rather thaw rate may be dominant control on bank erosion. Our findings suggest that Arctic warming and hydrological changes should increase bank erosion rates on large rivers but may reduce rates on rivers with drainage areas less than a few thousand km2.

Landslides pose a threat to life and infrastructure and are influenced by anthr...

Erosion of landscapes underlaid by permafrost can transform sediment and nutrient fluxes, surface and subsurface hydrology, soil properties, and rates of permafrost thaw, thus changing ecosystems and carbon emissions in high latitude regions with potential implications for global climate. However, future rates of erosion and sediment transport are difficult to predict as they depend on complex interactions between climatic and environmental parameters such as temperature, precipitation, permafrost, vegetation, wildfires, and hydrology. Thus, despite the potential influence of erosion on the future of the Arctic and global systems, the relations between erosion‐rate and these parameters, as well as their relative importance, remain largely unquantified. Here we quantify these relations based on a sedimentary record from Burial Lake, Alaska, one of the richest datasets of Arctic lake deposits. We apply a set of bi‐ and multi‐variate techniques to explore the association between the flux of terrigenous sediments into the lake (a proxy for erosion‐rate) and a variety of biogeochemical sedimentary proxies for paleoclimatic and environmental conditions over the past 25 cal ka BP. Our results show that erosion‐rate is most strongly associated with temperature and vegetation proxies, and that erosion‐rate decreases with increased temperature, pollen‐counts, and abundance of pollen from shrubs and trees. Other proxies, such as those associated with fire frequency, aeolian dust supply, mass wasting and hydrologic conditions, play a secondary role. The marginal effects of the sedimentary‐proxies on erosion‐rate are often threshold dependent, highlighting the potential for strong non‐linear changes in erosion in response to future changes in Arctic conditions.