On the origin of microrhythmic layering in arfvedsonite lujavrite from the Ilímaussaq alkaline complex, South Greenland

Microrhythmic layering is locally developed in agpaitic arfvedsonite lujavrite from the Ilímaussaq alkaline complex, South Greenland. Three–15-cm-thick laminated dark layers alternate with 1–10-cm-thick, light-coloured granular urtitic layers. Dark layers are uniform (isomodal) but the urtitic layers are enriched in early nepheline and eudialyte in their lower parts and in late analcime and REE phosphate minerals in the upper parts. The layers are separated by sharp contacts; they are draped around rafts from the overlying roof zone and lack structures indicative of current processes or post-cumulus deformation. Compared with the background arfvedsonite lujavrite of the complex, the dark layers are richer in sodalite, microcline and arfvedsonite and poorer in analcime and eudialyte. They have higher K₂O, Cl, FeO and S but lower Na₂O, H₂O⁺, Zr and P contents, the opposite of the light-coloured layers. The complementary chemistry of the two types of layers oscillates about the composition of the background arfvedsonite lujavrite. Layers probably formed in a stagnant bottom layer of the lujavrite magma chamber. Each layer started as a liquid layer which exchanged components with the underlying crystallization front. On cooling, it crystallized primocrysts and exchanged components with the overlying magma which became a new, complementary liquid layer and, during further cooling and burial within the sequence of layers, it underwent largely closed-system interstitial crystallization. Exhaustion of Cl and a sharp decrease in a_NaCl relative to a_H2O terminated the crystallization of a sodalite-rich dark layer and initiated abundant crystallization of nepheline in the overlying liquid layer (urtitic layer). The layered sequence represents a local K₂O-, Cl-rich but Na₂O-, H₂O-poor facies of arfvedsonite lujavrite and may have formed by exchanging components with sodalite-bearing rafts from the roof zone.     

Oberrheinisches Grundgebirge und Rheingraben

Zusammenfassung Anstelle des Rheingrabens befand sich während der variszischen Faltung eine von Blattverschiebungen durchsetzte Scherzone. Sie diente granitischen und gabbroiden Intrusiva sowie den Vulkaniten des Rotliegenden als bevorzugte Aufstiegsbahn. Vorzerklüftet und isostatisch unkompensiert behielt der Krustenspan bis zum Mitteldogger seinen kratonischen Zustand, gehalten von der Klammer variszischer Konsolidation. Vom Mitteleozän an hatten neuerliche Blattverschübe das basale Kluftgitter für die vertikale Grabentektonik vollends gangbar gemacht. Keilförmig trieb sich der Lineamentkörper in die Kruste vor, junge Füllmassen einsaugend und die Grabenschultern isostatisch aufstemmend. Die randlichen Widerlager wurden auseinandergetrieben, wodurch Raum für die sich steigernde Taphrogenese und den im Aquitan folgenden antithetischen Zerfall der Grabenscholle geschaffen wurde. Der dem aufwallenden basaltischen Substrat entspringende Grabenvulkanismus bildete den Höhepunkt krustaler Zerspaltung.

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The Rhine-Graben is preceded by Hercynian transcurrent faulting favouring intrusion and extrusion of granitic and gabbroide plutonites and lower Permian extrusives. Though fractured and isostatically out of balance, the Rhine-block remained cratonic until Middle-Dogger times on account of its "Hercynian" nature. From middle-Eocene times on, renewed transcurrent movements facilitated dip-slip movements along the graben-edge-faults. The Rhine block, sucked down the young fill and isostatically buckled the graben edges. Marginal buttresses were driven apart; they gave room for increasing thaphrogenesis and for anti-thetic collapse during Aquitanian times. Basaltic graben volcanism underlined the peak of tectonic activity.

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Résumé Lors du plissement varisque, il y eut à l'endroit du fossé rhénan, une zone d'écailles; celle-ci favorisa la montée de roches intrusives granitiques et gabbroïques ainsi que de volcanites du Rotliegend. Préalablement fracturée et dans un état isostatique non compensé, cette zone d'écaillés conserva son état cratonique jusqu'au Dogger moyen, enserrée entre des masses de consolidation varisque. A partir de l'Eocène moyen, de nouveaux déplacements des écailles préparèrent le réseau de fissures pour une tectonique verticale en fossé. Un segment linéamentaire s'enfonça en coin dans l'écorce, déterminant l'aspiration de nouvelles masses de remplissage et la montée isostatique des compartiments flanquant latéralement le fossé. Les contreforts ont été écartés les uns des autres, ce qui a fourni l'espace pour la taphrogenèse et pour la fragmentation antithétique du massif affaisé qui s'est produite à l'Aquitanien.La fissuration crustale a atteint son point culminant avec le volcanisme à caractère basique qui a fait éruption dans le fossé.

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Phase relationship and surface water mass change in the Northeast Atlantic during Marine Isotope Stage 11 (MIS 11)

Planktic foraminiferal census data, faunal sea surface temperatures (SSTs) and oxygen isotopic and lithic records from a site in the northeast Atlantic were analyzed to study the interglacial dynamics of Marine Isotope Stage (MIS) 11, a period thought to closely resemble the Holocene on the basis of orbital forcing. Interglacial conditions during MIS 11 persisted for approximately 26 ka. After the main deglacial meltwater processes ceased, a 10- to 12-ka-long transitional period marked by significant water mass circulation changes occurred before surface waters finally reached their thermal maximum. This SST peak occurred between 400 and 397 ka, inferred from the abundance of the most thermophilic foraminiferal species and was coincident with lowest sea level according to benthic isotope values. The ensuing stepwise SST decrease characterizes the overall climate deterioration preceding the increase in global ice volume by  3 ka. This cooling trend was followed by a more pronounced cold event that began at 388 ka, and that terminated in the recurrence of icebergs at the site around 382 ka. Because the water mass configuration of early MIS 11 evolved quite differently from that of the early Holocene, there is little evidence that MIS 11 can serve as an appropriate analogue for a future Holocene climate, despite the similarity in some orbital parameters.     

Formation of mesosiderites by fragmentation and reaccretion of a large differentiated asteroid

Abstract— We propose that mesosiderites formed when a 200–400 km diameter asteroid with a molten core was disrupted by a 50–150 km diameter projectile. To test whether impacts can excavate core iron and mix it with crustal material, we used a low‐resolution, smoothed‐particle hydrodynamics computer simulation. For 50–300 km diameter differentiated targets, we found that significant proportions of scrambled core material (and hence potential mesosiderite metal material) could be generated. For near‐catastrophic impacts that reduce the target to 80% of its original diameter and about half of its original mass, the proportion of scrambled core material would be about 5 vol%, equivalent to ～10 vol% of mesosiderite‐like material. The paucity of olivine in mesosiderites and the lack of metal‐poor or troilite‐rich meteorites from the mesosiderite body probably reflect biased sampling. Mesosiderites may be olivine‐poor because mantle material was preferentially excluded from the metal‐rich regions of the reaccreted body. Molten metal globules probably crystallized around small, cool fragments of crust hindering migration of metal to the core. If mantle fragments were much hotter and larger than crustal fragments, little metal would have crystallized around the mantle fragments allowing olivine and molten metal to separate gravitationally. The rapid cooling rates of mesosiderites above 850 °C can be attributed to local thermal equilibration between hot and cold ejecta. Very slow cooling below 400 °C probably reflects the large size of the body and the excellent thermal insulation provided by the reaccreted debris. We infer that our model is more plausible than an earlier model that invoked an impact at ～1 km/s to mix projectile metal with target silicates. If large impacts cannot effectively strip mantles from asteroidal cores, as we infer, we should expect few large eroded asteroids to have surfaces composed purely of mantle or core material. This may help to explain why relatively few olivine‐rich (A‐type) and metal‐rich asteroids (M‐type) are known. Some S‐type asteroids may be scrambled differentiated bodies.     

Unsuitability of TAC management within an ecosystem approach to fisheries: An ecological perspective

Fisheries management in European waters is gradually moving from a single-species perspective towards a more holistic ecosystem approach to management (EAM), acknowledging the need to take all ecosystem components into account. Prerequisite within an EAM is the need for management processes that directly influence the ecological effects of fishing, such as the mortality of target and non-target species. Up until recently, placing limits on the quantities of fish that can be landed, through the imposition of annual total allowable catches (TACs) for the target species, has been the principal management mechanism employed. However, pressure on non-target components of marine ecosystems is more closely linked to prevailing levels of fishing activity, so only if TACs are closely related to subsequent fishing effort will TAC management serve to control the broader ecosystem impacts of fishing. We show that in the mixed fisheries that characterise the North Sea, the linkage between variation in TAC and the resulting fishing effort is in fact generally weak. Reliance solely on TACs to regulate fishing activity is therefore unlikely to mitigate the impacts of fishing on non-target species. Consequently, we conclude that the relationship between TACs and effort is insufficient for TACs to be used as the principal management tool within an EAM. The implications, and some alternatives, for fisheries management are discussed.     

Minor element and Ca isotope composition of calcareous dinoflagellate cysts of cultured Thoracosphaera heimii

We investigated the influences of temperature, salinity and pH on the calcium isotope as well as trace and minor element (uranium, strontium, magnesium) to Ca ratios on calcium carbonate cysts of the calcareous dinoflagellate species Thoracosphaera heimii grown in laboratory cultures. The natural habitat of this species is the photic zone (preferentially at the chlorophyll maximum depth) of temperate to tropical oceans, and it is abundant in deep-sea sediments over the entire Cenozoic. In our experiments, temperatures ranged from 12 to 30 °C, salinity from 36.5 to 38.8 and pH from 7.9 to 8.4. The δ^44/40Ca of T. heimii cysts resembles that of other marine calcifiers, including coccolithophores, foraminifers and corals. However, its temperature sensitivity is considerably smaller and statistically insignificant, and T. heimii might serve as a recorder of changes in seawater δ^44/40Ca over geologic time. The Sr/Ca ratios of T. heimii cysts show a pronounced temperature sensitivity (0.016 mmol/mol °C^? 1) and have the potential to serve as a palaeo-sea surface temperature proxy. No clear temperature- and pH-dependences were observed for Mg/Ca. U/Ca seems to be influenced by temperature and pH, but the correlations change sign at 23 °C and pH 8.2, respectively.     

The Odra River Load of Heavy Metals at Hohenwutzen during the Flood in 1997

The Odra river flood of 1997 was a rare hydrological as well as an interesting sedimentological event. At Hohenwutzen (Lower Odra River) we observed the suspended particulate matter transport and the temporal development of water and solidsπ pollution with heavy metals and As. While the suspended particulate matter concentration decreased the trace element concentrations increased during the flood by fractionation of particles and solution processes. Because of a successive flooding of differently contaminated sedimentary sources and polluted regions the contents of heavy metals developed irregularly. Their median particulate concentrations did not exceed the values of older samples taken under mean discharge conditions between 1989 and 1995. The dissolved amounts correspond to those of the Elbe river in 1990. During the flood the dissolved share of all analyzed total element contents increased. The total loads increased 4fold (Cr) to 17fold (Cd).     

Last interglacial (MIS 5e) surface water conditions at the Vøring Plateau (Norwegian Sea), based on dinoflagellate cysts

Sediments from the last interglacial, marine isotope stage 5e (MIS 5e), have been studied for their dinoflagellate cyst content in a core retrieved from the Vøring Plateau, Norwegian Sea. Qualitative and quantitative analyses of the data, and comparison with the surface sample and published Holocene data from the core, reveal distinct differences in hydrological surface conditions between the late Holocene and MIS 5e. A higher number of co-dominant, subordinate species in the last interglacial samples suggests there was a more pronounced seasonality of the surface water at this time. This is supported by the significant presence of Bitectatodinium tepikiense , a species that was virtually absent from the area for most of the Holocene. The seasonality signal is further substantiated by transfer-function reconstructions, which also indicates a stronger stratification of the upper water column during MIS 5e. Moreover, the assemblage data clearly show that optimal, fully marine interglacial conditions prevailed only late in MIS 5e (between ca. 117.5 and 116.5 Kya), which is in contrast with the climatic optimum early in the Holocene. Stable oxygen isotope values from planktic foraminifera for this MIS 5e optimum are comparable with the average Holocene values, but are generally ca. 0.3‰ higher than those of the earlier part of the last interglacial ( sensu stricto ). These higher δ¹⁸O values are likely to be the result of the enhanced and prolonged influence of Saalian deglacial meltwater, thus corroborating the existence of a quite differently structured sea surface, as suggested by the dinocyst data.