Widespread rocky reef occurrence in the central English Channel and the implications for predictive habitat mapping

Reefs are one of the marine habitats listed in Annex I of the European Union's Habitats Directive, which aims to establish a coherent European ecological network of Special Areas of Conservation. EU Member States are required to prepare and propose a national list of sites for evaluation under the scheme, but currently the occurrence of reefs in the United Kingdom's nearshore and offshore areas is not well documented. Here we report on our search for rocky reefs in the central English Channel, which unexpectedly revealed an extensive reef system covering an area of 1100 km². Prior to our work, it was generally perceived that the seabed in this area comprised mostly gravel, with a few isolated rock outcrops.Our approach to determining the location, extent and character of these reefs incorporated broad, medium and fine-scale analyses over a 3200 km² area of seabed, using single- and multi-beam acoustic data, ground-truthed by underwater video and stills imagery. A benthic terrain model was developed in ArcGIS to map topographic features at the broad and medium scales. Biotope assignments were made at the fine scale through detailed analysis of video footage obtained from 30 sampling stations. The study area has a complex geological history and lies at the centre of a major bedload-parting zone. Together, these strongly influence the seabed character and the distribution of biotopes. An integrated assessment of the physical and biological features was used to map the study area to level 4 of the EUNIS habitat classification system.Similar physical conditions exist in other areas of the UK continental shelf, raising the prospect of predicting where other rocky reef systems might occur. In the absence of a co-ordinated national seabed survey programme, such predictions, coupled with interpretation of existing single-beam bathymetry data, can help prioritise areas where limited survey resources could be most effectively deployed.     

Reconstructing pre-colonial oyster demographics in the Chesapeake Bay, USA

Recent estimates of growth and mortality rates in extant Chesapeake Bay, USA oyster (Crassostrea virginica) populations are used to quantify changes in both population abundance (dN/dT) and shell accretion (dS/dT) associated with modern population demographics. The demographics of oyster populations that would be required to maintain reef accretion rates commensurate with sea level rise over geological time frames are examined using estimates of oyster longevity in pre-colonial (pre -1600) times combined with parallel estimates of pre-disease endemic mortality. The analysis demonstrates that modern populations, with their disease related, age-truncated demographics, are generally not capable of maintaining and building biogenic reefs through accretion. Estimates of filtration rates associated with Chesapeake Bay oyster populations prior to 1600 considerably underestimate actual benthic-pelagic coupling during that period. Pristine oyster populations would have supported water column turnover rates on the order of minutes to hours. Thus, the spatial footprint of oyster reefs was limited by available productivity in the estuary. Accretion rate calculations for pristine (pre-1600) oyster reefs describe the intimate relationship between benthic-pelagic coupling and the presence or absence of oyster reefs and the associated communities.     

Rocks from the Gibbs fracture zone and the Minia seamount near 53°N in the Atlantic Ocean

Rocks dredged and drilled from both the rift mountains of the Mid-Atlantic Ridge (Minia seamount) and from the northern wall and the median ridge of the adjacent Gibbs fracture zone near 53°N include tholeiites, serpentinized and mylonitized peridoties, and gabbroic rock.The tholeiites include: (1) pyroxene-tholeiites, commonly without phenocrysts and containing less than 15% Al₂O₃; (2) plagioclase-tholeiites with small (1.1–2.2 mm length) plagioclase phenocrysts and an Al₂O₃ content varying between 14–17%; and (3) high-alumina plagioclase tholeiites with large (> 2.5 mm length) plagioclase phenocrysts and more than 17% Al₂O₃. The apparently transitional differences among the three groups support the possibility that differentiation by crystal fractionation of the high-alumina plagioclase-tholeiites gave rise to the plagioclase-tholeiites with less Al₂O₃ and smaller phenocrysts and to the pyroxene-tholeiites. A small portion of the basalts ampled show effects of low-grade metamorphism. The peridotites may represent evidence of intrusive emplacement of peridotitic material beneath the tholeiitic rocks.     

Nickeliferous pyrite tracks pervasive hydrothermal alteration in Martian regolith breccia: A study in NWA 7533

Martian regolith breccia NWA 7533 (and the seven paired samples) is unique among Martian meteorites in showing accessory pyrite (up to 1% by weight). Pyrite is a late mineral, crystallized after the final assembly of the breccia. It is present in all of the lithologies, i.e., the fine‐grained matrix (ICM), clast‐laden impact melt rocks (CLIMR), melt spherules, microbasalts, lithic clasts, and mineral clasts, all lacking magmatic sulfides due to degassing. Pyrite crystals show combinations of cubes, truncated cubes, and octahedra. Polycrystalline clusters can reach 200 μm in maximum dimensions. Regardless of their shape, pyrite crystals display evidence of very weak shock metamorphism such as planar features, fracture networks, and disruption into subgrains. The late fracture systems acted as preferential pathways for partial replacement of pyrite by iron oxyhydroxides interpreted as resulting from hot desert terrestrial alteration. The distribution and shape of pyrite crystals argue for growth at moderate to low growth rate from just‐saturated near neutral (6 < pH<10), H₂S‐HS‐rich fluids at minimum log fO₂ of >FMQ + 2 log units. It is inferred from the maximum Ni contents (4.5 wt%) that pyrite started crystallizing at 400–500 °C, during or shortly after a short‐duration, relatively low temperature, thermal event that lithified and sintered the regolith breccias, 1.4 Ga ago as deduced from disturbance in several isotope systematics.     

Effects of Recharge Wells and Flow Barriers on Seawater Intrusion

The installation of recharge wells and subsurface flow barriers are among several strategies proposed to control seawater intrusion on coastal groundwater systems. In this study, we performed laboratory‐scale experiments and numerical simulations to determine the effects of the location and application of recharge wells, and of the location and penetration depth of flow barriers, on controlling seawater intrusion in unconfined coastal aquifers. We also compared the experimental results with existing analytical solutions. Our results showed that more effective saltwater repulsion is achieved when the recharge water is injected at the toe of the saltwater wedge. Point injection yields about the same repulsion compared with line injection from a screened well for the same recharge rate. Results for flow barriers showed that more effective saltwater repulsion is achieved with deeper barrier penetration and with barriers located closer to the coast. When the flow barrier is installed inland from the original toe position however, saltwater intrusion increases with deeper barrier penetration. Saltwater repulsion due to flow barrier installation was found to be linearly related to horizontal barrier location and a polynomial function of the barrier penetration depth.     

Highly evolved hypabyssal kimberlite sills from Wemindji,Quebec, Canada: insights into the process of flow differentiation in kimberlite magmas

Kimberlite sills emplaced in granite located near the town of Wemindji (Quebec, Canada) range from 2 cm to 1.2 m in thickness. The sills exhibit a wide variation in macroscopic appearance from fine-grained aphanitic dolomitic hypabyssal kimberlite to ilmenite/garnet macrocrystal hypabyssal kimberlite. Diatreme or crater facies rocks are not present. Multiple intrusions are present within the sills, and graded bedding and erosional features such as cross-bedding are common. The sills exhibit a wide range in their modal mineralogy with respect to the abundances of spinel, apatite, phlogopite and dolomite. Olivine is the dominant macrocryst, with an average composition of Fo₉₀. Garnet macrocrysts are low chrome (2–3 wt. %) pyrope (G1/G9 garnet). Ilmenite occurs as rounded macrocrysts (7–13 wt. % MgO). Phlogopite microphenocrysts are Ti-poor and represent a solid solution between phlogopite and kinoshitalite end members. Spinel compositions mainly represent the Cr-poor members of the qandilite–ulvöspinel–magnetite series. The principle carbonate comprising the groundmass is dolomite, with lesser later-forming calcite. Accessory minerals include apatite, Sr-rich calcite, Nb-rich rutile, baddeleyite, monazite-(Ce) and barite. While some of these accessory minerals are atypical of kimberlites in general, it is expected that differentiation products of an evolved carbonate-rich kimberlite magma will crystallize these phases. The Wemindji kimberlites offer insight into the process of crystal fractionation and differentiation in evolved kimberlite magmas. The macroscopic textural features observed in the Wemindji sills are interpreted to represent flow differentiation of a mantle-derived, very fluid, low viscosity carbonate-rich kimberlite. The diverse modes and textural features result entirely from flow differentiation and multiple intrusions of different batches of genetically related kimberlite magma. The mineralogy of the Wemindji kimberlites has some similarities to that of the Wesselton and Benfontein calcite kimberlite sills but differs in detail with respect to dominant carbonate (i.e. dolomite versus calcite), and the character of the rare earth-bearing accessory minerals (i.e. monazite-(Ce) versus rare earth fluorocarbonates).     

In situ U–Pb,Sr and Nd isotopic analysis of loparite by LA-(MC)-ICP-MS

This work describes the in situ analysis of loparite [(Na,REE)Ti₂O₆], a perovskite group mineral with extremely low Rb/Sr ratios and high rare earth contents, by LA-(MC)-ICP-MS for the determination of U–Pb ages together with Sr and Nd isotopic composition. The reliability of these data were validated by analysis of a loparite standard by TIMS solution methods. Data are given for loparite from the Lovozero and Khibiny peralkaline complexes of the Kola Alkaline Province (Russia). For Lovozero loparite the Tera–Wasserburg intercept age for 15 loparites analysed is 373 ± 11 Ma, and the weighted ²⁰⁷Pb corrected ²⁰⁶Pb/²³⁸U age is 373 ± 2 Ma. For Khibiny loparite, the intercept age for 5 loparites analysed is 375 ± 10 Ma, and the weighted ²⁰⁷Pb corrected ²⁰⁶Pb/²³⁸U age is 374 ± 3 Ma. The common Pb compositions for Lovozero and Khibiny loparites are identical i.e. ²⁰⁷Pb/²⁰⁶Pb = 0.898 ± 0.009 and 0.898 ± 0.007, respectively. The ⁸⁷Sr/⁸⁶Sr initial ratios of Lovozero loparite range from 0.703552 to 0.703682 (av. 0.703611), and εNd (t₃₇₀) from + 3.8 to + 4.4 (av. + 4.0). The ⁸⁷Sr/⁸⁶Sr initial ratios of Khibiny loparite range from 0.703560 to 0.703871, and εNd (t₇₃₀) from + 4.0 to + 4.8. Our data indicate that in situ LA-(MC)-ICP-MS analysis of loparite provides accurate and precise estimates of the intrusion ages and isotopic composition of peralkaline rocks.     

Relation between the deposition and petrology of Shanxi coals and the composition of their extracts

The soluble organic material present in a set of 12 coal samples representing three major episodes of coal deposition in Shanxi Province, North China, has been examined. Shanxi coals, like those from Gondwanaland, contain little liptinite and accordingly the material extracted by pyridine yields only small quantities of n-hexane soluble n-alkanes, steranes and hopanes identifiable by gas-chromatography/mass spectrometry. The distribution of the n-alkanes is characteristic of the environment from which each of the Shanxi coals was formed but the distribution of the branched alkanes is generally determined by the dia- and catagenesis each coal experienced. Aromatic compounds identified in the extracts typify the small molecules present within the vitrinite lattice. Technologically significant benzene-methanol extracts deriving from vitrinite-inertinite have been characterised by proton NMR spectroscopy and partially separated by HPLC.