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Rock fracturing sets the pace for a range of geomorphic processes. While experimental studies and modeling have provided invaluable insights into the mechanisms and rates of rock fracturing as a function of stress, time, and environmental conditions, fieldbased observations of subaerial fracturing evolution over geologic time are scarce. To address this knowledge gap, we conducted a systematic study of fractures that developed subaerially and in situ within clasts perched on abandoned late Quaternary alluvial surfaces (ca. 0, ca. 14, and ca. 62 ka in age) in the hyperarid Dead Sea Rift Valley, Israel. Using quantitative field observations, petrographic, and scanning electron microscopy, and micron-scale laser scans of fracture surfaces we found that fractures exhibit a consistent pattern of three distinctive weathering zones: (1) an "Outer Zone," where fracture surface morphology resembles the clast exterior; (2) an "Accumulation Zone," where fractures are infilled by "loose" accumulated particles; and (3) an "Inner Zone" where fractures extend inward to the crack-tip and preferentially follow grain boundaries. Crack-tips are characterized as a distinct micro domain that consists of fracture-parallel microcracks, chemical alteration, and dissolution morphologies. Altogether, the laboratory results indicate chemically enhanced fracturing and infiltration of water ahead of traction-free, open crack-tips. Field measurements also revealed an increase in fracture number density over geologic time. Our results highlight new details regarding the progressive nature of mechanical weathering through geologic time and the role of moisture as a potential rate-setting factor in the fracturing that allows mechanical weathering. [ABSTRACT FROM AUTHOR]

Documenting hillslope response to hydroclimatic forcing is crucial to our understanding of landscape evolution. The evolution of talus‐pediment sequences (talus flatirons) in arid areas was often linked to climatic cycles, although the physical processes that may account for such a link remain obscure. Our approach is to integrate field measurements, remote sensing of rainfall and modeling to link between storm frequency, runoff, erosion and sediment transport. We present a quantitative hydrometeorological analysis of rainstorms, their geomorphic impact and their potential role in the evolution of hyperarid talus‐pediment slopes in the Negev desert, Israel. Rainstorm properties were defined based on intensity–duration–frequency curves and using a rainfall simulator, artificial rainstorms were executed in the field. Then, the obtained measured experimental results were up‐scaled to the entire slope length using a fully distributed hydrological model. In addition, natural storms and their hydro‐geomorphic impacts were monitored using X‐band radar and time‐lapse cameras. These integrated analyses constrain the rainfall threshold for local runoff generation at rain intensity of 14 to 22 mm h‐1 for a duration of five minutes and provide a high‐resolution characterization of small‐scale runoff‐generating rain cells. The current frequency of such runoff‐producing rainstorms is ~1–3 per year. However, extending this local value into the full extent of hillslope runoff indicates that it occurs only under rainstorms with ≥ 100‐years return interval, or 1% annual exceedance probability. Sheetwash efficiency rises with downslope distance; beyond a threshold distance of ~100 m, runoff during rainstorms with such annual exceedance probability are capable of transporting surface clasts. The erosion efficiency of these discrete rare events highlights their potential importance in shaping the landscape of arid regions. Our results support the hypothesis that a shift in the properties and frequency of extreme events can trigger significant geomorphic transitions in areas that remained hyperarid during the entire Quaternary. © 2020 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Our understanding of sea‐cliff erosion processes and their response to recent and/or projected environmental changes such as sea‐level rise, climate change and anthropogenic development hinges on our ability to quantify sea‐cliff retreat rates and their variability through time. Here, we focus on Israel's Mediterranean 'Sharon' sea‐cliff as a case study for examining the significance of recent short‐term (i.e. annual to decadal) cliff‐top retreat rates that appear to exceed longer‐term rates of 'background' (i.e. centennial to millennial) retreat by 1–2 orders of magnitude. We demonstrate that an inherent sampling bias in rate estimates inferred from observation intervals shorter than process episodicity can also explain such a pattern. This potential ambiguity leads to a striking paradox where despite highly accurate and robust documentation of recent cliff‐top retreat, such as that obtained from aerial photographs and/or instrumental surveys, the short‐term retreat rates of episodically retreating sea cliffs remain poorly constrained. To address this key data gap along the Sharon sea cliff we employed a sediment budget approach that focuses on quantifying the continuous wave scouring of cliff‐collapsed material from the shore platform as a rate‐limiting process for episodic retreat of the cliff above. We used four high‐resolution (0.5 m/pixel) airborne LiDAR data sets acquired between 2006 and 2015 to determine short‐term maximum retreat rates of up to ~0.08 m/yr during this nine‐year period. These modern retreat rates compare to the cliff's background retreat rate of 0.03 to 0.09 m/yr since the mid‐Holocene, as determined herein from multiple geologic and archeological observations. Our results demonstrate that previously reported twentieth century cliff‐top retreat rates for this sea cliff, which range up to values of several meters per year, are biased and that sea‐cliff erosion rates have not yet been significantly impacted by recent environmental changes in the eastern Mediterranean basin, such as the restriction of sediment supply following emplacement of the Nile's Aswan dam system. © 2018 John Wiley & Sons, Ltd. Sea‐cliff retreat rates inferred from annual‐decadal observations of cliff‐top retreat are artefactually high because retreat episodicity has a time‐scale of several hundred years. Here, we use repeat airborne LiDAR to measure the continuous wave scouring of talus material from the cliff‐base as a rate‐limiting process for episodic retreat of the cliff itself above. Our results demonstrate that modern sea cliff erosion continues at background rates while rate estimates inferred from conventional measurements of recent cliff‐top retreat erroneously suggest otherwise. [ABSTRACT FROM AUTHOR]

Abstract: Erosion and inland retreat of coastal cliffs present one of the most dynamic earth‐surface processes presently challenging coastlines. The rate of cliff retreat is central to coastal planning and protection of shoreline infrastructure. Until now, the majority of retreat rate estimates have been based on aerial photos from the past century, and therefore do not provide multicentury estimates of retreat rates. Here, we studied Bronze Age to Crusader archeological sites (∼3700 years) located on Israel's coastal cliff and used their spatial relation to the cliff to estimate the long‐term centurial‐millennial retreat rates of the cliff. To accomplish this, original layouts of partially eroded archaeological structures were reconstructed and compared to the modern coastal cliff line. The eroded parts of the studied structures and their original age constrained the maximum timing of the retreat. The resulting retreat rates are significantly lower than those previously calculated using observations of around 100 years. The archeological data also record the episodicity of the cliff failure events. The research highlights both the issue of the loss of valuable archaeological cultural resources, and simultaneously the usefulness of eroding coastal archaeological features to resolve questions of modern significance. [ABSTRACT FROM AUTHOR]