Wave-tide interaction modulates nearshore wave height

Ocean Dynamics - The combined hazard of large waves occurring at an extreme high water could increase the risk of coastal flooding. Wave-tide interaction processes are known to modulate the wave...     

Quantifying long-term post-fire sediment delivery and erosion mitigation effectiveness

Large wildfires can have profound and lasting impacts not only from direct consumption of vegetation but also longer-term effects such as persistent soil erosion. The 2002 Hayman Fire burned in one of the watersheds supplying water to the Denver metropolitan area; thus there was concern regarding hillslope erosion and sedimentation in the reservoirs. The efficacy of various treatments for reducing erosion was tested, including hand scarification on contour, agricultural straw mulch, wood mulch, burned controls and unburned reference plots. Simulated rill erosion experiments were used both immediately after the fire and again 10 years post fire. To better understand untreated recovery, the same experiments were applied to control plots in post-fire years 1, 2, 3 and 4, and in unburned reference plots in years 4 and 10. Results indicate that control and scarified plots produced significantly greater sediment flux rates – 1.9 and 2.8 g s⁻¹ respectively – than the straw and wood mulch treatments – 0.9 and 1.1 g s⁻¹ – immediately after the fire. Mulch treatments reduced runoff rate, runoff velocity, and sediment concentration and flux rate. The straw mulch cover was no longer present, whereas the wood mulch was still there in year 10. Vegetation regrowth was slow and mulch treatments provided effective cover to reduce sediment right after the fire. In post-fire year 10, there were no significant differences in sediment flux rates across treatments; it is notable, however, that the wood mulch treatment (0.09 g s⁻¹) most closely approached the unburned condition (0.07 g s⁻¹). The burned control plots had high sediment flux rates until post-fire year 3, when flux rates significantly decreased and were statistically no longer higher than the unburned levels from year 4 and 10. These results will inform managers of the longer-term post-fire sediment delivery rates and of the ability of post-fire emergency hillslope treatments to mitigate erosion rates. Published 2019. This article is a U.S. Government work and is in the public domain in the USA.     

Tracking denudation and sediment production and transport with cosmogenic 10Be in arid,high-altitude Himalayan half-grabens,Zanskar, northern India

Understanding the extent to which local factors, including bedrock and structure, govern catchment denudation in mountainous environments as opposed to broader climate or tectonic patterns provides insight into how landscapes evolve as sediment is generated and transported through them, and whether they have approached steady-state equilibrium. We measured beryllium-10 (¹⁰Be) concentrations in 21 sediment samples from glaciated footwall and hanging wall catchments, including a set of nested catchments, and 12 bedrock samples in the Puga and Tso Morari half-grabens located in the high-elevation, arid Zanskar region of northern India. In the Puga half-graben where catchments are underlain by quartzo-feldspathic gneissic bedrock, bedrock along catchment divides is eroding very slowly, about 5 m/Ma, due to extreme aridity and ¹⁰Be concentrations in catchment sediments are the highest (~60–90 × 10⁵ atoms/g SiO₂) as colluvium accumulates on hillslopes, decoupled from their ephemeral streams. At Puga, ¹⁰Be concentrations and the average erosion rates of a set of six nested catchments demonstrate that catchment denudation is transport-limited as sediment stagnates on lower slopes before reaching the catchment outlet. In the Tso Morari half-graben, gneissic bedrock is also eroding very slowly but ¹⁰Be concentrations in sediments in catchments underlain by low grade meta-sedimentary rocks, are significantly lower (~10–35 × 10⁵ atoms/g SiO₂). In these arid, high-elevation environments, ¹⁰Be concentrations in catchment sediments have more to do with bedrock weathering and transport times than steady-state denudation rates. © 2020 John Wiley & Sons, Ltd.     

The Upper Pleistocene deposits at Cassington,near Oxford,England

For much of the Middle and all of the Upper Pleistocene the Upper Thames valley has remained outside the limit of ice advance. The main agents of landform evolution have been the River Thames and its tributaries, which have cut down episodically and in so doing have abandoned a series of river terraces. This study reports the findings of an investigation into exposures in the deposits underlying the Floodplain Terrace at Cassington, near Oxford, England. The sequence exposed reveals a stratigraphy of basal, predominantly fine-grained, lithofacies overlain by coarser gravel lithofacies. The fluvial architecture of these deposits indicates a major change in fluvial style from a low-energy (meandering) to a high energy (braided) channel system. The flora and fauna from the lower fine-grained lithofacies display a marked change from temperate at the base, to colder conditions towards the top, indicating a close association between deteriorating climate and changing fluvial depositional style. Amino acid and luminescence geochronology from the basal fine-grained lithofacies suggest correlation with Oxygen Isotope Stage 5 and hence it is argued that the major environmental change recorded at the site relates to the Oxygen Isotope Stage 5–4 transition. Deposition of much of the overlying gravel sequence probably occurred during Oxygen Isotope Stage 4, suggesting that the latter half of the Devensian may be less significant, in terms of fluvial landscape evolution in the Upper Thames valley, than was believed previously. © 1998 John Wiley & Sons, Ltd.     

Relic permafrost structures in the Gobi of Mongolia: age and significance

Relict permafrost structures (ice-wedge casts and cryoturbation structures) are present in the Gobi of southern Mongolia. Luminescence dates of sediments are presented to constrain the age of formation of permafrost structures. These data show that there was a phase of permafrost development during the latter part of the Last Glacial (after about 22 to 15 ka) that resulted in cryoturbated sediments and ice-wedge casts. Furthermore, permafrost degradation occurred during late Pleistocene times (13–10 ka) and was absent during the early Holocene. These permafrost structures mark the southernmost evidence of permafrost in northern Asia during late Quaternary times and indicate that the mean annual air temperature was below approximately −6°C during their formation. © 1998 John Wiley & Sons, Ltd.     

3‐D elemental and isotopic composition of presolar silicon carbides

Abstract— Thirteen presolar silicon carbide grains—three of supernova (SN) origin and ten of asymptotic giant branch (AGB) star origin—were examined with time‐of‐flight‐secondary ion mass spectrometry (TOF‐SIMS). The grains had been extracted from two different meteorites—Murchison and Tieschitz—using different acid residue methods. At high lateral resolution of ～300 nm, isotopic and elemental heterogeneities within the micrometer‐sized grains were detected. The trace elemental abundances, when displayed in two‐element correlation plots, of Li, Mg, K, and Ca show a clear distinction between the two different meteoritic sources. The different concentrations might be attributed to differences of the host meteorites and/or of extraction methods whereas the stellar source seems to be less decisive. In one SN grain with ²⁶Mg‐enrichment from extinct ²⁶Al, the acid treatment, as part of the grain separation procedure, affected the Mg/Al ratio in the outer rim and therefore the inferred initial ²⁶Al/²⁷Al ratio. A second SN grain exhibits a lateral heterogeneity in ²⁶Al/²⁷Al, which either is due to residual Al‐rich contamination on the grain surface or to the condensation chemistry in the SN ejecta.