Mass-transport complexes (MTCs) document subsidence patterns in a northern Gulf of Mexico salt minibasin

Mass-transport complexes (MTCs) dominate the stratigraphic record of many salt-influenced sedimentary basins. Commonly in such settings, halokinesis is invoked as a primary trigger for MTC emplacement, although the link between specific phases of salt movement, and related minibasin dynamics, remains unclear. Here, we use high-quality 3D seismic reflection and well data to constrain the composition, geometry and distribution (in time and space) of six MTCs preserved in a salt-confined, supra-canopy minibasin in the northern Gulf of Mexico, and to assess how their emplacement relate to regional and local controls. We define three main tectono-sedimentary phases in the development of the minibasin: (a) initial minibasin subsidence and passive diapirism, during which time deposition was dominated by relatively large-volume MTCs (c. 25 km³) derived from the shelf-edge or upper slope; (b) minibasin margin uplift and steepening, during which time small-volume MTCs (c. 20 km³) derived from the shelf-edge or upper slope were emplaced; and (c) active diapirism, during which time very small volume MTCs (c. 1 km³) were emplaced, locally derived from the diapir flanks or roofs. We present a generic model that emphasizes the dynamic nature of minibasin evolution, and how MTC emplacement relates to halokinetic sequence development. Although based on a single data-rich case study, our model may be applicable to other MTC-rich, salt-influenced sedimentary basins.     

Effects of diel period and tow duration on estuarine fauna sampled with a beam trawl over bare sediment: Consequences for designing more reliable and efficient surveys

The effects of diel period and tow duration (5, 10 and 20 min) on samples of estuarine fauna in a beam trawl, were tested over bare sediment in Tuggerah Lake (New South Wales, Australia). Mean catch rates (numbers of fish caught 5 min⁻¹) were significantly larger at night for the total numbers of individuals and abundant, economically important species of fish and invertebrates (e.g. Gerres subfasciatus, Metapenaeus macleayi, Penaeus plebejus). Greater proportions of larger fish were also caught at night for some species (e.g. G. subfasciatus, Acanthopagrus australis, Rhabdosargus sarba), but not across all tow durations. Multivariate analyses detected dissimilarities in the composition and structure of assemblages between diel periods, which were driven by species caught predominately, or in larger proportions, at night. Short tows (5 min) were more efficient than longer tows (10 or 20 min) for sampling the diversity of species (i.e. most species were caught in the first 5 min of a tow). There were, however, no clear or consistent patterns relating to the effect of tow duration on the catch rates of other variables, the size ranges of abundant species, or the structure and composition of assemblages. Our data confirm that at night, bare sediment is an important habitat for a wide size- and species-range of estuarine fish and invertebrates. In future, more cost-effective and reliable information concerning these taxa would be achieved by sampling with the beam trawl at night, using tow durations of 5 min. We also highlight a problem inherent in the design of many studies of diel variation of fauna (i.e. the potential non-independence of data among day and night periods) and discuss its solution.     

Modeling seasonal variations of ocean and sea ice circulation in the Beaufort and Chukchi Seas： A model-data fusion study

A 3.8-kin Coupled Ice-Ocean Model （C1OM） was implemented to successfully reproduce many observed phenomena in the Beaufort and Chukchi seas, including the Bering-inflow-originated coastal current that splits into three branches： Alaska Coastal Water （ACW） , Central Channel, and Herald Valley branches. Other modeled phenomena include the Beaufort Slope Current （BSC） , the Beaufort Gyre, the East Siberian Current （ ESC）, mesoscale eddies, seasonal landfast ice, sea ice ridging, shear, and deformation. Many of these downscaling processes can only be captured by using a high-resolution CIOM, nested in a global climate model. The seasonal cycles for sea ice concentration, thickness, velocity, and other variables are well reproduced with Solid validation by satellite measurements. The seasonal cycles for upper ocean dynamics and thermodynamics are also well reproduced, which include the formation of the cold saline layer due to the injection of salt during sea ice formation, the BSC, and the subsurface upwelling in winter that brings up warm, even more saline Atlantic Water along the shelfbreak and shelf along the Beaufort coast.     

Relative radiative forcing consequences of global emissions of hydrocarbons, carbon monoxide and NOx from human activities estimated with a zonally-averaged two-dimensional model

A global two-dimensional (altitude-latitude) chemistry transport model is used to follow the changes in the tropospheric distribution of the two major radiatively active trace gases, methane and ozone, following step changes to the sustained emissions of the short-lived trace gases methane, carbon monoxide and non-methane hydrocarbons. The radiative impacts were dependent on the latitude chosen for the applied change in emissions. Step change global warming potentials (GWPs) were derived for a range of short-lived trace gases to describe their time-integrated radiative forcing impacts for unit emissions relative to that of carbon dioxide. The GWPs show that the tropospheric chemistry of the hydrocarbons can produce significant indirect radiative impacts through changing the tropospheric distributions of hydroxyl radicals, methane and ozone. For aircraft, the indirect radiative forcing impact of the NO _x emissions appears to be greater than that from their carbon dioxide emissions. Quantitative results from this two-dimensional model study must, however, be viewed against the known inadequacies of zonally-averaged models and their poor representation of many important tropospheric processes.     

Evidence for equilibrium iron isotope fractionation by nitrate-reducing iron(II)-oxidizing bacteria

Iron isotope fractionations produced during chemical and biological Fe(II) oxidation are sensitive to the proportions and nature of dissolved and solid-phase Fe species present, as well as the extent of isotopic exchange between precipitates and aqueous Fe. Iron isotopes therefore potentially constrain the mechanisms and pathways of Fe redox transformations in modern and ancient environments. In the present study, we followed in batch experiments Fe isotope fractionations between Fe(II)_aq and Fe(III) oxide/hydroxide precipitates produced by the Fe(III) mineral encrusting, nitrate-reducing, Fe(II)-oxidizing Acidovorax sp. strain BoFeN1. Isotopic fractionation in ⁵⁶Fe/⁵⁴Fe approached that expected for equilibrium conditions, assuming an equilibrium Δ⁵⁶Fe_{Fe(OH)3-Fe(II)aq} fractionation factor of +3.0‰. Previous studies have shown that Fe(II) oxidation by this Acidovorax strain occurs in the periplasm, and we propose that Fe isotope equilibrium is maintained through redox cycling via coupled electron and atom exchange between Fe(II)_aq and Fe(III) precipitates in the contained environment of the periplasm. In addition to the apparent equilibrium isotopic fractionation, these experiments also record the kinetic effects of initial rapid oxidation, and possible phase transformations of the Fe(III) precipitates. Attainment of Fe isotope equilibrium between Fe(III) oxide/hydroxide precipitates and Fe(II)_aq by neutrophilic, Fe(II)-oxidizing bacteria or through abiologic Fe(II)_aq oxidation is generally not expected or observed, because the poor solubility of their metabolic product, i.e. Fe(III), usually leads to rapid precipitation of Fe(III) minerals, and hence expression of a kinetic fractionation upon precipitation; in the absence of redox cycling between Fe(II)_aq and precipitate, kinetic isotope fractionations are likely to be retained. These results highlight the distinct Fe isotope fractionations that are produced by different pathways of biological and abiological Fe(II) oxidation.     

Selenium isotope fractionation during reduction by Fe(II)-Fe(III) hydroxide-sulfate (green rust)

We have determined the extent of Se isotope fractionation induced by reduction of selenate by sulfate interlayered green rust (GR_SO4), a Fe(II)-Fe(III) hydroxide-sulfate. This compound is known to reduce selenate to Se(0), and it is the only naturally relevant abiotic selenate reduction pathway documented to date. Se reduction reactions, when they occur in nature, greatly reduce Se mobility and bioavailability. Se stable isotope analysis shows promise as an indicator of Se reduction, and Se isotope fractionation by various Se reactions must be known in order to refine this tool. We measured the increase in the ⁸⁰Se/⁷⁶Se ratio of dissolved selenate as lighter isotopes were preferentially consumed during reduction by GR_SO4. Six different experiments that used GR_SO4 made by two methods, with varying solution compositions and pH, yielded identical isotopic fractionations. Regression of all the data yielded an instantaneous isotope fractionation of 7.36 ± 0.24‰. Selenate reduction by GR_SO4 induces much greater isotopic fractionation than does bacterial selenate reduction. If selenate reduction by GR_SO4 occurs in nature, it may be identifiable on the basis of its relatively large isotopic fractionation.     

Magma genesis in the New Britain island-arc: Constraints from Nd and Sr isotopes and trace-element patterns

A selected suite of fresh volcanic rocks from the New Britain island arc has been analyzed for ¹⁴³Nd/¹⁴⁴Nd, ⁸⁷Sr/⁸⁶Sr, major and trace elements to investigate relationships between isotopes, trace elements and petrology, and depth to the underlying Benioff zone. From these relationships inferences about magma generation are made utilizing Nd and Sr isotope systematics in possible source materials. Lavas ranging in composition from basalt to rhyolite show minimal variation of ¹⁴³Nd/¹⁴⁴Nd. Small variations in ⁸⁷Sr/⁸⁶Sr do not correlate with depth to the Benioff zone, but are related to magma type. Nd-Sr isotopes suggest that island arc lavas in general are derived from a mixture of suboceanic mantle and hydrothermally altered mid-ocean ridge-type basalt, but the New Britain magma source appears homogeneous with little indication of either the involvement of oceanic crust or mantle inhomogeneity. Trace element patterns in New Britain lavas are not consistent with Nd isotope data for currently accepted petrologic and trace element models of magma genesis. Mafic lavas from New Britain and other island arcs have anomalously high Sr/Nd, possibly due to components derived from subducted oceanic crust.     

Reappraising the P–T evolution of the Rogaland–Vest Agder Sector,southwestern Norway

The Rogaland-Vest Agder Sector of southwestern Norway comprises high-grade metamorphic rocks intruded by voluminous plutonic bodies that include the ~1000 km~2 Rogaland Igneous Complex(RIC).New petrographic observations and thermodynamic phase equilibria modelling of three metapelitic samples collected at various distances(30 km,10 km and ~ 10 m) from one of the main bodies of RIC anorthosite were undertaken to assess two alternative P-T-t models for the metamorphic evolution of the area.The results are consistent with a revised two-phase evolution.Regional metamorphism followed a clockwise P-T path reaching peak conditions of ~ 850-950 ℃ and ~7-8 kbar at ~1035 Ma followed by high-temperature decompression to ~5 kbar at ~950 Ma,and resulted in extensive anatexis and melt loss to produce highly residual rocks.Subsequent emplacement of the RIC at ~930 Ma caused regional-scale contact metamorphism that affected country rocks 10 km or more from their contact with the anorthosite.This thermal overprint is expressed in the sample proximal to the anorthosite by replacement of sillimanite by coarse intergrowths of cordierite plus spinel and growth of a second generation of garnet,and in the intermediate(10 km) sample by replacement of sapphirine by coarse intergrowths of cordierite,spinel and biotite.The formation of late biotite in the intermediate sample may suggest the rocks retained small quantities of melt produced by regional metamorphism and remained at temperatures above the solidus for up to 100 Ma.Our results are more consistent with an accretionary rather than a collisional model for the Sveconorwegian Orogen.