Sarah G. Evans

Rock fracturing regulates the topography, carbon cycle, geologic hazards, and infrastructure degradation of the Earth. Yet, there remains a paucity of constraints on long‐term fracturing behavior. Here we use field measurements of 2221 clasts across a range of environments and rock types to show that the number and total length of fractures in natural surface rocks increase rapidly within the first ∼10 kyr of exposure, and then the rate of fracture growth slows exponentially over geologic time up to ∼150 ka. Similar rock breakdown deceleration trends were independently documented using a novel application of infrared photoluminescence (IRPL) dating for a visibly fractured boulder at one site. Previous work shows that increasing microcrack intensity correlates with enhanced compliance in the bulk rock over the same timescales that our bulk macroscale fracturing rates decrease. We hypothesize that enhanced compliance contributes significantly to the observed decrease in fracturing rates via increased material toughness. Our results contrast with current landscape‐scale conceptual models that assume bulk fracturing rates and characteristics are invariant over time and are controlled by short term rock strength and external stress magnitudes alone. Instead, our findings indicate that, over geologic time, fracturing of rocks increases its effective toughness.

The thawing of Arctic permafrost mobilizes previously sequestered carbon, amplifying positive feedback in the global climate system. Thaw patterns are heterogeneous and influenced by factors such as snow insulation and hillslope hydrology, including narrow (<10 m) features like water tracks (WTs) and thermoerosional gullies (TGs). These WT/TG features are distinguished by vegetation, soil moisture, and drainage patterns, yet their snow duration characteristics remain poorly understood. We applied the Blue Snow Threshold (BST) algorithm to high-resolution PlanetScope imagery (3 m) from 2017 to 2023 to quantify snow-on, snow-off, and snow cover duration for 34 field-delineated WTs and TGs on the North Slope of Alaska, USA. BST-derived snow duration metrics were compared to those derived from in situ time-lapse photographs. BST-derived results indicate that snow-on dates are similar between hillslopes and WT/TG features, while snow-off dates are delayed on average by 19 days in WTs and 32 days in TGs. Results from in situ photographs suggest a smaller offset in snow-off dates, 9 days on average, highlighting limitations of the remote sensing data’s temporal resolution. Despite these limitations, these findings suggest that WTs and TGs hold snow longer than the surrounding hillslope, which could facilitate their remote identification and clarify their contributions to permafrost thaw.

Increasing air temperatures in the Arctic cause permafrost to thaw, releasing carbon dioxide and methane into the atmosphere. Carbon in thawing permafrost is released approximately three times more readily when soils are unsaturated versus saturated. Therefore, understanding if the Arctic is wetting or drying as permafrost thaws is crucial to predicting soil carbon emissions. In upland permafrost regions, near‐surface soil moisture is influenced by unchannelized curvilinear zones of enhanced saturation known as water tracks. The ground underneath water tracks can collapse into thermoerosional gullies, altering their thaw depth and seasonal saturation. Water tracks and thermoerosional gullies frequently occur together on upland hillslopes but exhibit heterogeneous saturation dynamics. Thus, understanding saturation states in water tracks and gullies is crucial to predicting soil carbon emissions. In this study, we quantify saturation across water tracks and a gully and examine changes in near‐surface saturation metrics over time by leveraging ~30 years of meteorological data and remotely sensed wetness indices from Landsat (1994–2023) and PlanetScope (2017–2023) imagery for a permafrost hillslope on the North Slope of Alaska, USA. Results suggest that the studied water tracks are drying following the ground collapse event, decreasing the overall saturated area proximal to the collapse, but that the water tracks still have relatively high mean Normalised Difference Water Index (NDWI) values for all rainfall magnitudes. Given the importance of soil saturation for predicting carbon emissions, the results of this work may provide tools for improving estimates of carbon release from thawing continuous permafrost hillslopes.

Hillslope hydrology in upland permafrost regions (e.g., Alaska, High Canadian Arctic, Russia, Antarctica) is often dominated by water tracks, zones of enhanced soil moisture in unchannelized depressions that concentrate water flow downslope. Continued warming of permafrost regions may alter hydrologic cycling, leading to increased frequency of extreme hydrologic events like drought and flooding and modification to biogeochemical cycles. It is therefore imperative to parametrize the role of water tracks in the hydrology of the permafrost environments. In this study, we synthesize uniting and distinguishing hydrologic characteristics of water tracks across permafrost regions and then examine water track seasonality, occurrence, and contribution to the permafrost hydrologic cycle using field observation, remote sensing, and numerical modeling for permafrost hillslopes on the North Slope of Alaska, USA. Results suggest that water tracks occur across climate and hydrologically disparate permafrost landscapes but have ubiquitous surface wetness, vegetation, and snow duration patterns that can be identified remotely using 3-m resolution PlanetScope imagery. Detailed field investigation from 2022-2024 of three study sites with ~20 water tracks and ~15 gullies suggests that water tracks are hydrologically distinct from larger, variably channelized hillslope features and require more precipitation and time to initiate discharge following rainfall events. Across these study sites, concentration-discharge relationships reveal that water tracks can exhibit drastically different dynamics of particulate and dissolved organic carbon export based on landscape attributes. Young water fraction analysis found that in 2023, 24–78% of runoff from the study sites was young water less than 35 days old during the observed summer thaw season, and model-estimated young water fraction increased by two-fold when factoring in the fall shoulder season. Geophysical investigations indicate the presence of buried ice wedges on the margins of studied water tracks, supporting the idea that water tracks may form from the coalesced drainage of patterned ground. Over time, these drainage patterns likely evolve and widen into present-day water tracks that act as flow conduits and discharge liquid water into the fall shoulder season after the adjacent hillslope has frozen. Our ongoing analysis explores how water track flow seasonality may influence observed ground collapse and mediate or enhance the permafrost-carbon feedback.

Climate change drives disturbance in hydrology and geomorphology in terrestrial polar landscapes underlain by permafrost, yet measurements of, and theories to understand, these changes are limited. Water flowing from permafrost hillslopes to channels is often modulated by water tracks, zones of enhanced soil moisture in unchannelized depressions that concentrate water flow downslope. Water tracks, which dominate hillslope hydrology in some permafrost landscapes, lack a consistent definition and identification method, and their global occurrence, morphology, climate relationships, and geomorphic roles remain understudied despite their role in the permafrost carbon cycle. Combining a literature review with a synthesis of prior work, we identify uniting and distinguishing characteristics between water tracks from disparate polar sites with a toolkit for future field and remotely sensed identification of water tracks. We place previous studies within a quantitative framework of “top‐down” climate and “bottom‐up” geology controls on track morphology and hydrogeomorphic function. We find the term “water track” is applied to a broad category of concentrated suprapermafrost flowpaths exhibiting varying morphology, degrees of self‐organization, hydraulic characteristics, subsurface composition, vegetation, relationships to thaw tables, and stream order/hillslope position. We propose that the widespread occurrence of water tracks on both poles across varying geologic, ecologic, and climatic factors implies that water tracks are in dynamic equilibrium with the permafrost environment but that they may experience change as the climate continues to warm. Current knowledge gaps include these features' trajectories in the face of ongoing climate change and their role as an analog landform for an active Martian hydrosphere.

Massive ground ice in Arctic regions underlain using continuous permafrost influences hydrologic processes, leading to ground subsidence and the release of carbon dioxide and methane into the atmosphere. The relation of massive ground ice such as ice wedges to water tracks and seasonally saturated hydrologic pathways remains uncertain. Here, we examine the location of ice wedges along a water track on the North Slope of Alaska using Ground‐Penetrating Radar (GPR) surveys, in situ measurements, soil cores, and forward modeling. Of nine unique GPR surveys collected in the summers of 2022 and 2023, seven exhibit distinctive “X”‐shaped reflections above columnar reflectors that are spatially correlated with water track margins. Forward modeling of plausible geometries suggests that ice wedges produce reflection patterns most similar to the reflections observed in our GPR profiles. Additionally, a large magnitude (∼71 mm) rain event on 8 July 2023 led to a ground collapse that exposed four ice wedges on the margin of the studied water track, ∼100 m downstream of our GPR surveys. Together, this suggests that GPR is a viable method for identifying the location of ice wedges as air temperatures in the Arctic continue to increase, we expect that ice wedges may thaw, destabilizing water tracks and causing ground collapse and expansion of thermo‐erosional gullies. This ground collapse will increase greenhouse gas emissions and threaten the Arctic infrastructure. Future geophysical analysis of upland Arctic hillslopes should include additional water tracks to better characterize potential heterogeneity in permafrost vulnerability across the warming Arctic.

Climate change is driving disturbance in hydrology and geomorphology in terrestrial polar landscapes underlain by permafrost, yet measurements of, and theories to understand, these changes are limited. Water flowing from permafrost hillslopes to channels is often modulated by water tracks, zones of enhanced soil moisture in unchannelized but curvilinear depressions that concentrate water flow downslope. Water tracks, which dominate hillslope hydrology in some permafrost landscapes, lack a consistent definition and identification method, and their global occurrence, morphology, climate relationships, and geomorphic roles remain understudied. We combine a literature review with a synthesis of prior work to identify uniting and distinguishing characteristics between water tracks from disparate polar sites with a toolkit for future field and remotely sensed identification of water tracks. Then, we place previous studies within a quantitative framework of “top-down” climate and “bottom-up” geology controls on track morphology and hydrogeomorphic function. We find that the term “water track” is applied to a broad category of concentrated suprapermafrost flowpaths that exhibit varying morphology, degrees of self-organization, hydraulic characteristics, subsurface composition, vegetation, relationships to thaw tables, and stream order/hillslope position. We propose that the widespread occurrence of water tracks on both poles across varying geologic, ecologic, and climatic factors implies that water tracks are in dynamic equilibrium with the permafrost environment but that they may transition as the climate continues to warm. Current knowledge gaps include these features’ trajectories in the face of ongoing climate change and their role as an analog landform for an active Martian hydrosphere.

Rock fractures are a key contributor to a broad array of Earth surface processes due to their direct control on rock strength as well as rock porosity and permeability. However, to date, there has been no standardization for the quantification of rock fractures in surface process research. In this work, the case is made for standardization within fracture-focused research, and prior work is reviewed to identify various key datasets and methodologies. Then, a suite of standardized methods is presented as a starting “baseline” for fracture-based research in surface process studies. These methods have been shown in pre-existing work from structural geology, geotechnical engineering, and surface process disciplines to comprise best practices for the characterization of fractures in clasts and outcrops. This practical, accessible, and detailed guide can be readily employed across all fracture-focused weathering and geomorphology applications. The wide adoption of a baseline of data collected using the same methods will enable comparison and compilation of datasets among studies globally and will ultimately lead to a better understanding of the links and feedbacks between rock fracture and landscape evolution.

Rock fracturing comprises a key component of a broad array of Earth surface processes due to its direct control on rock strength as well as rock porosity and permeability. However, to date, there has been no standardization for the quantification of rock fractures in surface processes research. In this work, we make the case for standardization within fracture-focused research and review prior work to identify various key datasets and methodologies. We then present a suite of standardized methods that we propose as ‘baseline’ for fracture-based research in surfaces processes studies. These methods have been shown in preexisting work from structural geology, fracture mechanics, and surface processes disciplines to comprise best practices for the characterization for cracks, clasts, and outcrops. These practical, accessible and detailed methods can readily be employed across all fracture-focused weathering and geomorphology applications. The wide adoption of a baseline of data, all collected using the same methods, will enable comparison and compilation of data among studies globally, and ultimately will lead to a better understanding of the links and feedbacks between rock fracture and landscape evolution.