Spatial variability in beach biogeomorphology in a tropical archipelago

Beaches of tropical island coasts exhibit high levels of diversity in composition and form in comparison with their continental counterparts. To investigate the nature and origin of this diversity, individual beach morphology and sedimentology was investigated in the British Virgin Islands (BVI), a Caribbean archipelago of > 60 high volcanic and low reef islands. The islands exhibit a diversity of orientations (some facing the Atlantic and some the Caribbean), elevation and gradient, rock type and wave energy. An examination of 100 beaches in the archipelago revealed a first-order division into sand (70 beaches) and coral rubble (30 beaches). These beaches occur in seven planform types (determined by the antecedent geological framework) and are further subdivided according to shoreface type (seagrass, sandy shoreface, or reef). Mainland-attached headland-embayment beaches are the most common form of sand beach while coral rubble beaches usually occur as barriers that enclose salt ponds and wetlands. Among sand beaches, carbonate content is greatest on Atlantic-facing beaches, and coral rubble beaches are more common on Caribbean-facing beaches. Grain size characteristics on sandy beaches are highly variable and range from fine to very coarse sands while coral rubble beaches range up to boulder-sized clasts. The local source material is a primary determinant of sediment composition. The local factors such as the underlying geology, source and availability of sediments are the primary determinants of beach form, composition and texture in the BVI. Oceanographic and climatic conditions such as the prevailing easterly trade winds and waves which seasonally range in direction from east-northeast to southeast as well as beach orientation to Atlantic- or Caribbean-facing waves also contribute to the variability, but in a secondary role. © 2019 John Wiley & Sons, Ltd.     

Characterizing beach intertidal bar systems using multi-annual LiDAR data

Intertidal bars are common in mesotidal/macrotidal low-to-moderate energy coastal environments and an understanding of their morphodynamics is important from the perspective of both coastal scientists and managers. However, previous studies have typically been limited by considering bar systems two-dimensionally, or with very limited alongshore resolution. This article presents the first multi-annual study of intertidal alongshore bars and troughs in a macrotidal environment using airborne LiDAR (light detection and ranging) data to extract three-dimensional (3D) bar morphology at high resolution. Bar and trough positions are mapped along a 17.5 km stretch of coastline in the northwest of England on the eastern Irish Sea, using eight complete, and one partial, LiDAR surveys spanning 17 years. Typically, 3–4 bars are present, with significant obliquity identified in their orientation. This orientation mirrors the alignment of waves from the dominant south-westerly direction of wave approach, undergoing refraction as they approach the shoreline. Bars also become narrower and steeper as they migrate onshore, in a pattern reminiscent of wave shoaling. This suggests that the configuration of the bars is being influenced by overlying wave activity. Net onshore migration is present for the entire coastline, though rates vary alongshore, and periods of offshore migration may occur locally, with greatest variability between northern and southern regions of the coastline. This work highlights the need to consider intertidal bar systems as 3D, particularly on coastlines with complex configurations and bathymetry, as localized studies of bar migration can overlook 3D behaviour. Furthermore, the wider potential of LiDAR data in enabling high-resolution morphodynamic studies is clear, both within the coastal domain and beyond. © 2019 John Wiley & Sons, Ltd.     

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...     

Resistant rock layers amplify cosmogenically-determined erosion rates

Prior numerical modeling work has suggested that incision into sub-horizontal layered stratigraphy with variable erodibility induces non-uniform erosion rates even if base-level fall is steady and sustained. Erosion rates of cliff bands formed in the stronger rocks in a stratigraphic sequence can greatly exceed the rate of base-level fall. Where quartz in downstream sediment is sourced primarily from the stronger, cliff-forming units, erosion rates estimated from concentrations of cosmogenic beryllium-10 (¹⁰Be) in detrital sediment will reflect the locally high erosion rates in retreating cliff bands. We derive theoretical relationships for threshold hillslopes and channels described by the stream-power incision model as a quantitative guide to the potential magnitude of this amplification of ¹⁰Be-derived erosion rates above the rate of base-level fall. Our analyses predict that the degree of erosion rate amplification is a function of bedding dip and either the ratio of rock erodibility in alternating strong and weak layers in the channel network, or the ratio of cliff to intervening-slope gradient on threshold hillslopes. We test our predictions in the cliff-and-bench landscape of the Grand Staircase in southern Utah, USA. We show that detrital cosmogenic erosion rates in this landscape are significantly higher (median 300 m/Ma) than the base-level fall rate (~75 m/Ma) determined from the incision rate of a trunk stream into a ~0.6 Ma basalt flow emplaced along a 16 km reach of the channel. We infer a 3–6-fold range in rock strength from near-surface P-wave velocity measurements. The approximately four-fold difference between the median ¹⁰Be-derived erosion rate and the long-term rate of base-level fall is consistent with our model and the observation that the stronger, cliff-forming lithologies in this landscape are the primary source of quartz in detrital sediments. © 2020 John Wiley & Sons, Ltd.     

From drought to flood: A water balance analysis of the Tuolumne River basin during extreme conditions (2015–2017)

The degree to which the hydrologic water balance in a snow-dominated headwater catchment is affected by annual climate variations is difficult to quantify, primarily due to uncertainties in measuring precipitation inputs and evapotranspiration (ET) losses. Over a recent three-year period, the snowpack in California's Sierra Nevada fluctuated from the lightest in recorded history (2015) to historically heaviest (2017), with a relatively average year in between (2016). This large dynamic range in climatic conditions presents a unique opportunity to investigate correlations between annual water availability and runoff in a snow-dominated catchment. Here, we estimate ET using a water balance approach where the water inputs to the system are spatially constrained using a combination of remote sensing, physically based modelling, and in-situ observations. For all 3 years of this study, the NASA Airborne Snow Observatory (ASO) combined periodic high-resolution snow depths from airborne Lidar with snow density estimates from an energy and mass balance model to produce spatial estimates of snow water equivalent over the Tuolumne headwater catchment at 50-m resolution. Using observed reservoir inflow at the basin outlet and the well-quantified snowmelt model results that benefit from periodic ASO snow depth updates, we estimate annual ET, runoff efficiency (RE), and the associated uncertainty across these three dissimilar water years. Throughout the study period, estimated annual ET magnitudes remained steady (222 mm in 2015, 151 mm in 2016, and 299 mm in 2017) relative to the large differences in basin input precipitation (547 mm in 2015, 1,060 mm in 2016, and 2,211 mm in 2017). These values compare well with independent satellite-derived ET estimates and previously published studies in this basin. Results reveal that ET in the Tuolumne does not scale linearly with the amount of available water to the basin, and that RE primarily depends on total annual snowfall proportion of precipitation.     

A model for the growth and development of wave-dominated deltas fed by small mountainous rivers: Insights from the Elwha River delta,Washington

Observations from ground-penetrating radar, sediment cores, elevation surveys and aerial imagery are used to understand the development of the Elwha River delta in north-western Washington, USA, which prograded as a result of two dam removals in late 2011. Swash-bar, foreshore and swale depositional elements are recognized within ground-penetrating radar profiles and sediment cores. A model for the growth and development of small mountainous river wave-dominated deltas is proposed based on observation of both the fluvial and deltaic settings. If enough sediment is available in the fluvial system, mouth-bars form after higher than average river discharge events, creating a large platform seaward of the subaqueous delta plain. Swash-bars form concurrently or within a month of mouth-bar deposition as a result of wave action. Fair-weather waves drive swash-bar migration landward and in the direction of littoral drift. The signature of swash-bar welding to the shoreline is landward-dipping reflections, as a result of overwash processes and slipface migration. However, most swash-bars are eroded by the river mouth, as only 10 of the 37 swash-bars that formed between August 2011 and July 2016 survived within the Elwha River delta. The swash-bars that do survive either amalgamate onto the shoreline or an earlier deposited swash-bar, forming a single larger barrier at the delta front. In asymmetrical deltas, the signature of swash-bar welding is more likely to be preserved on the downdrift side of the delta, where formation is more likely and accommodation behind newer swash-bars preserves older deposits. On small mountainous river deltas, welded swash-bars may be more indicative of a large sediment pulse to the system, rather than large hydrological events.     

Combining IR and X‐ray microtomography data sets: Application to Itokawa particles and to Paris meteorite

In the near future, a new generation of sample return missions (Hayabusa2, OSIRIS‐REx, MMX, etc.) will collect samples from small solar system bodies. To maximize the scientific outcome of laboratory studies and minimize the loss of precious extraterrestrial samples, an analytical sequence from less destructive to more destructive techniques needs to be established. In this work, we present a combined X‐ray and IR microtomography applied to five Itokawa particles and one fragment of the primitive carbonaceous chondrite Paris. We show that this analytical approach is able to provide a 3‐D physical and chemical characterization of individual extraterrestrial particles, using the measurement of their 3‐D structure and porosity, and the detection of mineral and organic phases, and their spatial co‐localization in 3‐D. We propose these techniques as an efficient first step in a multitechnique analytical sequence on microscopic samples collected by space missions.     

A review of the chondrite–achondrite transition,and a metamorphic facies series for equilibrated primitive stony meteorites

Here, the petrological features of numerous primitive achondrites and highly equilibrated chondrites are evaluated to review and expand upon our knowledge of the chondrite–achondrite transition, and primitive achondrites in general. A thermodynamic model for the initial silicate melting temperature and progressive melting for nearly the entire known range of oxidation states is provided, which can be expressed as T_m = 0.035Fa²?3.51Fa + 1109 (in °C, where Fa is the proportion of fayalite in olivine). This model is then used to frame a discussion of textural and mineralogical evolution of stony meteorites with increasing temperature. We suggest that the metamorphic petrology of these meteorites should be based on diffusive equilibration among the silicate minerals, and as such, the chondrite–achondrite transition should be defined by the initial point of silicate melting, not by metal–troilite melting. Evidence of silicate melting is preserved by a distinctive texture of interconnected interstitial plagioclase ± pyroxene networks among rounded olivine and/or pyroxene (depending on ?O₂), which pseudomorph the former silicate melt network. Indirectly, the presence of exsolution lamellae in augite in slowly cooled achondrites also implies that silicate melting occurred because of the high temperatures required, and because silicate melt enhances diffusion. A metamorphic facies series is defined: the Plagioclase Facies is equivalent to petrologic types 5 and 6, the Sub‐calcic Augite Facies is bounded at lower temperatures by the initiation of silicate melting and at higher temperatures by the appearance of pigeonite, which marks the transition to the Pigeonite Facies.