The terrestrial cratering record: I. Current status of observations

The location, size, and principal characteristics of the currently known proven and probable terrestrial impact structures are tabulated. Of the 78 known probable structures, only 3 are Precambrian and the majority are <300 my in age. A survey of the variation in preservation with size and age indicates that, unless protected by sedimentary cover, a structure <20 km in diameter has a recognizable life of <600 my. The depth-diameter relationships of terrestrial structures are similar to lunar craters; however, it is believed that terrestrial craters were always shallower than their lunar counterparts. Complex structures formed in sedimentary targets are shallower than those in crystalline targets, and the transition from simple to complex crater morphology occurs in sedimentary strata at approximately one-half the diameter of the morphology transition in crystalline rocks. This is a reflection of target strength. Although observations indicate that crater size, target strength, and surface gravity are variables in the formation of complex craters, they do not permit an unequivocal choice between collapse and rebound processes for the formation of complex structures. It may be that both processes act together in the modification of crater morphology during the later stages of excavation. The major emphasis of recent shock metamorphic studies has been toward the development of models of cratering processes. An important contribution has been the identification, through meteoritic contamination in the melt rocks, of the type of bolide at a number of probable impact structures. This has served to strengthen the link between the occurrence of shock metamorphic effects and their origin by hypervelocity meteorite impact.     

Shapes of chondrules determined from the petrofabric of the CR2 chondrite NWA 801

The approximately spherical shapes of chondrules has long been attributed to surface tension acting on ~1 mm melt droplets that formed and cooled in the microgravity field of the solar nebula. However, chondrule shapes commonly depart significantly from spherical. In this study, 109 chondrules in a sample of CR2 chondrite NWA 801 were imaged by X-ray computed tomography and best-fitted to ellipsoids. The analysis confirms that many chondrules are indeed not spherical, and also that the chondrules’ collective shape fabric records a definite 13% compaction in the host meteorite. Dehydration of phyllosilicates within chondrules may account for that strain. However, retro-deforming all chondrules shows that a large majority were already far from spherical prior to accretion. Possible models for these initial shapes include prior deformation of individual chondrules in earlier hosts, and, as suggested by previous authors, rotation of chondrules as they were solidifying, and/or “streaming” of molten chondrules by their differential velocities with their gaseous hosts after melting. More in situ 3-D work such as this study on a variety of unequilibrated chondrites, combined with detailed structural petrography, should help further constrain these models and refine our understanding of chondrite formation.     

Electron microbeam analysis of urban road-deposited sediment,Manchester, UK: Improved source discrimination and metal speciation assessment

Road-deposited sediment (RDS) is an accumulation of particulates upon street surfaces in urban centres. It is commonly highly contaminated and has major potential impacts upon surface water quality and human health, as well as becoming a waste material upon street sweeping. Although significant research has been undertaken upon the fine fraction within these systems, there is a lack of detailed, high-resolution grain-specific mineralogical and chemical data for the coarser fractions, which contain the bulk of the contamination by mass. The study reported here utilizes backscatter electron microscopic, electron microprobe and Raman spectroscopic analysis to provide novel data on RDS material from Manchester, UK, with significant implications for the improved understanding of source discrimination and contaminant metal speciation. The RDS samples studied are highly heterogeneous and the abundant nature of anthropogenic grains is particularly apparent. The RDS material has been found to be composed of a number of grain types: (i) silicate and alumino-silicate grains derived from a wide range of sources; (ii) iron oxide grains derived from the corrosion of galvanized steel; (iii) iron-rich glass grains derived as slag material from metal and waste processing activities and (iv) spherical Fe oxide and Fe-rich glass grains derived from high temperature combustion processes. Elemental metallic grains (Fe, Cu, Pb) are also present in minor amounts.     

Present-day trends of vertical ground motion along the coast lines

Vertical ground motion (VGM) rates stand as crucial information, either for predicting the impact of the actual sea level rise along low-lying coasts or refining geodynamic problems. Because present day VGM rates have a magnitude smaller than 10 mm/yr, they remain challenging to quantify and often elusive. We focus on the quantification of global-scale VGM rates in order to identify global or regional trends. We computed VGM rates by combining tide gauges records and local satellite altimetry, which yield a new dataset of 634 VGM rates. We further compare this database to previous studies that use geodetic techniques and tide gauges records in order to evaluate the consistency of both our results and previous ones. The magnitudes differ by less than 5 mm/yr, and similar subsidence and uplift general tendencies appear. Even if the asset of our database stands in the greater number of sites, the combination of all studies, each with different pros and cons, yields a hybrid dataset that makes our attempt to extract VGM trends more robust than any other, independent study. Fennoscandia, the West coast of North America, and the eastern coast of Australia are uplifting, while the eastern coast of North America, the British Isles and Western Europe, the eastern Mediterranean Sea, Japan, and the western coast of Australia are subsiding. Glacial Isostatic Adjustment (GIA) is expected to provide a major contribution to the present-day signal. Aside from Fennoscandia, observed VGM often depart from the GIA model predictions of Peltier (2004). This either results from an underestimate of the model predictions or from the influence of other processes: indeed, the influence of the geodynamic setting appears in particular along the coasts of western North America or Japan, where the alternation of transform faults and subduction zones makes it possible to assign contrasted behaviours to the local geodynamic context. Local mechanisms like anthropogenic processes or sediment compaction, also contribute to VGM. This remains true for the critical cases of Venice, the Gulf of Mexico, the Ganges delta, and the Maldives, which are particularly exposed to the current sea level rise.     

CANADIAN ARCTIC METEORITE SEARCH: 1981

The recovery of several thousand meteorite fragments from Antarctica has led to speculation that accumulations may occur on the Earth's other major ice caps. Meteorites falling over the past 80,000 years on the Devon Island ice cap in the Canadian Arctic may be exposed at the surface near the ice cap margin. From the terrestrial meteorite flux, ice movement rates, and fragmentation factors it is calculated that 12,560 samples of 150 g mass are potentially concentrated in a 65 km² zone along the northwest margin. A search of this region on foot and by helicopter in July, 1981, failed to recover any specimens. Although metre-sized gneissic boulders, plucked from the underlying Precambrian basement, were concentrated in this zone it is postulated that the unseasonal 30 cm snow cover on the ice prohibited the recognition of possible meteorite specimens, which may average only 5 cm in diameter.     

Organohalogen contaminants and trace metals in North-East Atlantic porbeagle shark (Lamna nasus)

The North-East Atlantic porbeagle (Lamna nasus) population has declined dramatically over the last few decades and is currently classified as ‘Critically Endangered’. As long-lived, apex predators, they may be vulnerable to bioaccumulation of contaminants. In this study organohalogen compounds and trace elements were analysed in 12 specimens caught as incidental bycatch in commercial gillnet fisheries in the Celtic Sea in 2011. Levels of organohalogen contaminants were low or undetectable (summed CB and BDE concentrations 0.04–0.85 mg kg⁻¹ wet weight). A notably high Cd concentration (7.2 mg kg⁻¹ wet weight) was observed in one mature male, whereas the range observed in the other samples was much lower (0.04–0.26 mg kg⁻¹ wet weight). Hg and Pb concentrations were detected only in single animals, at 0.34 and 0.08 mg kg⁻¹ wet weight, respectively. These contaminant levels were low in comparison to other published studies for shark species.     

The Multiple Sources and Patterns of Methane inNorth Sea Waters

The methane concentration in the atmosphere andsurface water was surveyed along 58° N acrossthe North Sea. In addition, the vertical methanedistribution in the water column was determined at sixstations along the transect. The methane contents ofthe surface water as well as in the water column wereextremely inhomogeneous. Input by freshwater fromriver discharge and injection of methane from thesediment were both observed. The survey continued fromthe western side of the North Sea to the Elbe Riverestuary. The Elbe River appears to have low methaneconcentrations compared to other European rivers, itsaverage input into the North Sea is estimated to be70 nmol s-1 of methane. Near 58° N,1°40' E, an abandoned drill site releases about 25 % ofthe North Sea's emission of methane to the atmosphere.The advective methane transport induced by watercirculation was assessed for May 16, 1994, using a 3-DNorth Sea circulation model. For the period of thissurvey, the North Sea's source strength foratmospheric methane is estimated using in situwind velocities. In comparison to the advectivetransport by the water circulation, the gas flux tothe atmosphere appears to be the dominant sink ofNorth Sea methane. This flux is estimated to bebetween 1500 · 106 mol a-1 and 3100 ·106mol a-1, depending on the relationbetween wind speed and gas transfer velocity.     

Aliphatic and aromatic hydrocarbons in benthic invertebrates from two sites in Antarctica

Samples of marine benthic invertebrates collected from two sites in the Antarctic have been analysed for both aliphatic and aromatic hydrocarbons in order to establish baseline concentrations for some classes of hydrocarbons.Samples from Signy Island, a pristine site, contained low concentrations of the hydrocarbons determined, whereas those from King Edward Cove, South Georgia, contained significantly higher concentrations. King Edward Cove has a known history of pollution from whaling operations.Platt & Mackie (1979) have suggested that the hydrocarbons in sediments from King Edward Cove are a result of the world-wide dissemination of the pyrolysis products of fossil fuels. Our work suggests, in contrast, that the hydrocarbons in the benthos are derived from local sources.