Walrus foraging marks on the seafloor in Bristol Bay, Alaska: a reconnaissance survey

A reconnaissance sidescan sonar survey in Bristol Bay, Alaska revealed extensive areas of seafloor with features related to walrus foraging. They are similar to those seen in areas such as the outer Bering Sea and Chukchi Sea. Two types of feature were observed: (a) small (≪1 m diameter) shallow pits, often in clusters ranging in density from 5 pits per hectare to 35 pits per hectare; and, (b) more abundant, narrow, sinuous furrows, typically 5 to 10 m long with some reaching 20 m or more. Most foraging marks were in less than 60 m water depth in areas of sandy seafloor that were smooth, hummocky or characterized by degraded bedforms; the absence of foraging marks in other areas may be related, in part, to their more dynamic nature. The distribution of foraging marks was consistent in a general way with walrus locations from satellite telemetry studies.     

Metallogeny and tectonic development of the tasman fold belt system in tasmania

In western Tasmania, Precambrian sedimentary sequences form the basement for narrow trough accumulations of Eocambrian and younger sequences. The main trough, the meridional Dundas Trough, is flanked to the west by the Rocky Cape region of Precambrian rocks within which major, apparently stratiform, exhalative magnetite-pyrite deposits are intercalated with metabasaltic volcanics and ultramafic bodies.The Eocambrian-Cambrian troughs apparently developed during extension of Precambrian continental crust. Early shallow-water deposition includes thick dolomite units in some troughs. Deepening of the troughs was accompanied by turbidite sedimentation, with minor limestone, and submarine basaltic volcanism with associated minor disseminated native copper. Ultramafic and related igneous rocks were tectonically emplaced in some troughs during a mild compressional phase. They contain only minor platinoids, copper-nickel sulphides and asbestos, but are source rocks for Tertiary secondary deposits of platinoids, chromite and lateritic nickel.In the Dundas Trough, Eocambrian-Early Cambrian rocks are separated by an inferred erosional surface from structurally conformable overlying Middle to Late Cambrian fossiliferous turbidite sequences. The structural conformity continues through overlying Ordovician to Early Devonian terrestrial and shallow-marine stable shelf deposits.A considerable pile of probable Middle Cambrian felsic volcanics accumulated between the sedimentary deposits of the Dundas Trough and the Tyennan region of Precambrian rocks to the east. A lava-dominated belt within the volcanics hosts major volcanogenic massive sulphide deposits, including those of the exhalative type, which in the south are enriched in copper, gold and silver, whereas in the north they are rich in zine, lead, copper, gold and silver. Cambrian movements along faults near the margin of the Tyennan region resulted in erosion of the mineralized volcanics, locally exposing sub-volcanic granitoids. Above the local unconformities occur unmineralized volcaniclastic sequences that pass conformably into Ordovician to Early Devonian shelf deposits. Ordovician limestone locally hosts stratabound disseminated and veined base metal sulphide deposits.Pre-Middle Devonian rocks of western Tasmania differ, for most part, from those in the northeast where deeper marine turbidite quartz-wacke sequences were deposited during the Ordovician and Early Devonian.The Eocambrian to Early Devonian rocks of Tasmania were extensively deformed in the mid-Devonian. The Precambrian regions of western Tasmania behaved as relatively competent blocks controlling early fold patterns. In northeastern Tasmania, folding is of similar age but resulted from movements inconsistent with those affecting rocks of equivalent age in western Tasmania.The final metallogenic event is associated with high-level granitoid masses emplaced throughout Tasmania during the Middle to Late Devonian. In northeastern Tasmania, extensive I-type granodiorite and S-type granite, with alkali-feldspar granites, are associated with mainly endogranitic stanniferous grelsens and wolframite ± cassiterite vein deposits. In contrast, scheelite-bearing skarns and cassiterite stannite pyrrhotite carbonate replacement deposits are dominant in western Tasmania, associated mainly with S-type granites. Several argentiferous lead-zinc vein deposits occur in haloes around tin-tungsten deposits. A number of gold deposits are apparently associated with I-type granodiorite, but some have uncertain genesis.The contrasting regions of western and northeastern Tasmania have probably been brought together by lateral movement along an inferred fracture. Flat-lying, Late Carboniferous and younger deposits rest on the older rocks, and the only known post-Devonian primary mineralization is gold associated with Creta ceous syenite.     

Relations between rainfall–runoff‐induced erosion and aeolian deposition at archaeological sites in a semi‐arid dam‐controlled river corridor

Process dynamics in fluvial‐based dryland environments are highly complex with fluvial, aeolian, and alluvial processes all contributing to landscape change. When anthropogenic activities such as dam‐building affect fluvial processes, the complexity in local response can be further increased by flood‐ and sediment‐limiting flows. Understanding these complexities is key to predicting landscape behavior in drylands and has important scientific and management implications, including for studies related to paleoclimatology, landscape ecology evolution, and archaeological site context and preservation. Here we use multi‐temporal LiDAR surveys, local weather data, and geomorphological observations to identify trends in site change throughout the 446‐km‐long semi‐arid Colorado River corridor in Grand Canyon, Arizona, USA, where archaeological site degradation related to the effects of upstream dam operation is a concern. Using several site case studies, we show the range of landscape responses that might be expected from concomitant occurrence of dam‐controlled fluvial sand bar deposition, aeolian sand transport, and rainfall‐induced erosion. Empirical rainfall‐erosion threshold analyses coupled with a numerical rainfall–runoff–soil erosion model indicate that infiltration‐excess overland flow and gullying govern large‐scale (centimeter‐ to decimeter‐scale) landscape changes, but that aeolian deposition can in some cases mitigate gully erosion. Whereas threshold analyses identify the normalized rainfall intensity (defined as the ratio of rainfall intensity to hydraulic conductivity) as the primary factor governing hydrologic‐driven erosion, assessment of false positives and false negatives in the dataset highlight topographic slope as the next most important parameter governing site response. Analysis of 4+ years of high resolution (four‐minute) weather data and 75+ years of low resolution (daily) climate records indicates that dryland erosion is dependent on short‐term, storm‐driven rainfall intensity rather than cumulative rainfall, and that erosion can occur outside of wet seasons and even wet years. These results can apply to other similar semi‐arid landscapes where process complexity may not be fully understood. Published 2015. This article is a U.S. Government work and is in the public domain in the USA     

The Winchcombe fireball—That lucky survivor

On February 28, 2021, a fireball dropped ∼0.6 kg of recovered CM2 carbonaceous chondrite meteorites in South-West England near the town of Winchcombe. We reconstruct the fireball's atmospheric trajectory, light curve, fragmentation behavior, and pre-atmospheric orbit from optical records contributed by five networks. The progenitor meteoroid was three orders of magnitude less massive (∼13 kg) than any previously observed carbonaceous fall. The Winchcombe meteorite survived entry because it was exposed to a very low peak atmospheric dynamic pressure (∼0.6 MPa) due to a fortuitous combination of entry parameters, notably low velocity (13.9 km s⁻¹). A near-catastrophic fragmentation at ∼0.07 MPa points to the body's fragility. Low entry speeds which cause low peak dynamic pressures are likely necessary conditions for a small carbonaceous meteoroid to survive atmospheric entry, strongly constraining the radiant direction to the general antapex direction. Orbital integrations show that the meteoroid was injected into the near-Earth region ∼0.08 Myr ago and it never had a perihelion distance smaller than ∼0.7 AU, while other CM2 meteorites with known orbits approached the Sun closer (∼0.5 AU) and were heated to at least 100 K higher temperatures.     

Marine radar ocean wave retrieval’s dependency on range and azimuth

The strength of the surface wave signal in marine X-band radar (MR) images strongly depends on range and azimuth (i.e., the angle between antenna look and peak wave direction). Traditionally, MR wave analysis is carried out in a set of rectangular windows covering the radar field of view (FOV). The FOV is typically partially obstructed, e.g., due to the coastline or ship superstructures. Especially for ships that are subject to regular course changes, this results in an increased variability or error associated with wave parameters. Using MR measurements from R/P FLIP, acquired off California during the 2010 US Office of Naval Research (ONR) high resolution air–sea interaction (Hi-Res) experiment, this study quantifies the dependency of the radar-based 2D wave spectrum and parameters on range and azimuth. With the help of reference data from a nearby Datawell Waverider buoy, we propose empirical methods to remove the dependency and we illustrate their efficacy.     

The Cooma Metamorphic Complex, a low-P, high-T (LPHT) regional aureole beneath the Murrumbidgee Batholith

The Cooma Complex of the Lachlan Fold Belt, south‐eastern Australia, is characterised by a large (c. 10 km wide) low‐P, high‐T metamorphic aureole surrounding a small (3 × 6 km) granite pluton. The aureole extends northward to envelop the eastern lobe of the Murrumbidgee Batholith and progressively narrows to a kilometre wide hornfelsic aureole some 50 km north of Cooma. At its northern extremity, the batholith has intruded its own volcanic cover. These regional relations suggest that the Murrumbidgee Batholith is gently tilted to the north, with the Cooma Complex representing the aureole beneath the batholith. Two main deformation events, D3 and D5, affected the aureole. The inner, high‐grade migmatitic domain contains upright F5 folds defined by a composite, transposed S3/S0 fabric and S3/S0 concordant leucosomes. The folded stromatic migmatites define the western limb of a F5 synform, with its axis located in the batholith. Lenses and sheets of the Murrumbidgee Batholith intruded along S3 but also preserve S3 as a strong, solid‐state foliation. S3 and the granite sheets but are also folded by F5, outlining a fanning positive flower structure. These relations indicate that most of the batholith was emplaced before and during D3, but intrusion persisted until early syn‐D5. Formation of the Cooma Granodiorite occurred post‐D3 to early syn‐D5, after formation of the wide metamorphic aureole during early syn‐D3 to early syn‐D5. The Murrumbidgee Batholith was emplaced between pre‐D3 to early syn‐D5, synchronous with the formation of the Cooma Complex. The structural and metamorphic relations indicate that the Murrumbidgee Batholith was the ultimate heat source responsible for the Cooma Metamorphic Complex. D3 structures and metamorphic isograds are subparallel to the batholith margin for over 50 km. This concordance probably extends vertically, suggesting that the isograds also fan outward from the batholith margin. This implies an inverted metamorphic sequence focused on the Murrumbidgee Batholith, although the base has been almost completely removed by erosion in the Cooma Complex. The field evidence at Cooma, combined with previous thermal modelling results, suggest that extensive LPHT metamorphic terranes may represent regional metamorphic aureoles developed beneath high‐level granitic batholiths.     

Testing Nano-Powder and Fused-Glass Mineral Reference Materials for In Situ Rb-Sr Dating of Glauconite,Phlogopite, Biotite and Feldspar via LA-ICP-MS/MS

Reference materials (RMs) with well-characterised composition are necessary for reliable quantification and quality control of isotopic analyses of geological samples. For in situ Rb-Sr analysis of silicate minerals via laser ablation inductively coupled plasma tandem mass spectrometry (LA-ICP-MS/MS) with a collision/reaction cell, there is a general lack of mineral-specific and matrix-matched RMs, which limits wider application of this new laser-based dating technique to certain minerals. In this work, pressed nano-powder pellets (NP) of four RMs, GL-O (glauconite), Mica-Mg (phlogopite), Mica-Fe (biotite) and FK-N (K-feldspar), were analysed and tested for in situ Rb-Sr dating, complemented by isotope dilution (ID) MC-ICP-MS Rb-Sr analyses of GL-O and Mica-Mg. In addition, we attempted to develop alternative flux-free and fused ‘mineral glasses’ from the above RMs for in situ Rb-Sr dating applications. Overall, the results of this study showed that among the above RMs only two NP (Mica-Mg-NP and GL-O-NP) were suitable and robust for in situ dating applications. These two nano-powder reference materials, Mica-Mg-NP and GL-O-NP, were thus used as primary RMs to normalise and determine Rb-Sr ages for three natural minerals: MDC phlogopite and GL-O glauconite grains, and also Mica-Fe-NP (biotite). Our in situ analyses of the above RMs yielded Rb-Sr ages that are in good agreement (within 8%) of published ages, which suggests that both Mica-Mg-NP and GL-O-NP are suitable RMs for in situ Rb-Sr dating of phlogopite, glauconite and biotite. However, using secondary RMs is recommended to monitor the quality of the obtained ages.     

El Niño- or La Niña-like climate change?

The potential for the mean climate of the tropical Pacific to shift to more El Niño-like conditions as a result of human induced climate change is subject to a considerable degree of uncertainty. The complexity of the feedback processes, the wide range of responses of different atmosphere–ocean global circulation models (AOGCMs) and difficulties with model simulation of present day El Niño southern oscillation (ENSO), all complicate the picture. By examining the components of the climate-change response that projects onto the model pattern of ENSO variability in 20 AOGCMs submitted to the coupled model inter-comparison project (CMIP), it is shown that large-scale coupled atmosphere–ocean feedbacks associated with the present day ENSO also operate on longer climate-change time scales. By linking the realism of the simulation of present day ENSO variability in the models to their patterns of future mean El Niño-like or La Niña-like climate change, it is found that those models that have the largest ENSO-like climate change also have the poorest simulation of ENSO variability. The most likely scenario (p=0.59) in a model-skill-weighted histogram of CMIP models is for no trend towards either mean El Niño-like or La Niña-like conditions. However, there remains a small probability (p=0.16) for a change to El Niño-like conditions of the order of one standard El Niño per century in the 1% per year CO₂ increase scenario.