A climatic control on the accretion of meteoric and super-chondritic iridium–platinum to the Antarctic ice cap

Meteoric smoke particles (MSPs) form through the vaporization of meteoroids and the subsequent re-condensation of metallic species in the mesosphere. Recently, iridium and platinum enrichments have been identified in Greenland ice layers and attributed to the fallout of MSPs supplying polar latitudes with cosmic matter during the Holocene. However, the MSP fallout to Antarctica during the Earth's climatic history remains essentially unknown.

We have determined iridium and platinum in deep Antarctic ice from Dome C and Vostok dated back to 240 kyrs BP. We find high super-chondritic fluxes during warm periods and low meteoric accretion during glacial times, a pattern that is opposite to any known climatic variation in dust fallout to polar regions. The proposed explanation of this accretion regime is a weaker polar vortex during warm periods, allowing peripheral air masses enriched in volcanic iridium and platinum to penetrate inland to Antarctica. The MSP signal emerges only during cold phases and is four times lower than in the Greenland ice cap where more snow accumulates. This suggests that wet deposition is an important route of cosmic material to the Earth's surface.     

Nutrient and microbial dynamics in eutrophying shrimp ponds affected or unaffected by vibriosis

A field survey was conducted on two intensive shrimp farms using similar technical practices: one (DF) historically affected by a vibriosis, the other (HC) in which the pathogen has been observed although no mortality event has occurred. Because historical data suggest that eutrophication process may directly or indirectly play a role in the disease outbreak, we focussed our research on its dynamics. A higher variability of the phytoplanktonic compartment linked to an imbalance in the molar N:P ratio was observed in farm DF compared to farm HC, implying a modification on the linkage between the bacteria and phytoplankton compartments at DF. The beginning of the mortality outbreak at DF followed a shift from pico- to nanophytoplankton. The organic matter mineralization process at the water-sediment interface may explain the disturbance observed in the water column during eutrophication. The consequences of this disturbance on shrimps’ health status and pathogen ecology are discussed.     

Contribution à l'étude des litages pegmatitiques: l'exemple de la pegmatite de Covide (Nord-Portugal)

Résumé La pegmatite de Covide (Nord-Portugal) fournit un nouvel exemple de litage à quartz-albite-feldspath potassique. Son étude conduit à proposer un modèle de pétrogenèse magmatique faisant intervenir des processus de double diffusion dans un gradient de température. Ces processus provoquent la formation simultanée et périodique de niveaux millimétriques d'albite, en »planchers flottants« horizontaux. Chaque cellule, phénomène-unité du litage compris entre deux niveaux planchers successifs, évolue ensuite en système clos par cristallisation orientée centripète. L'organisation litée est acquise en milieu magmatique liquide.

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The Covide pegmatite, (Northern Portugal), exhibit a new example of K-feldspar, Albite, quartz layering. The present study leads to propose a magmatic petrogenetit model, with double diffusive processe in temperature gradient allowing the simultaneous growth of millimetric and horizontal »floating layers« of albite. Each cell, unit-phenomenon between two consecutive boundary layers, evolves in closed system with a centripetal, oriented crystallization. The layered organisation is obtaind in the magmatic liquid medium.

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Zusammenfassung Der Pegmetit aus Covide (Nordportugal) stellt ein neues Beispiel für Quarz-Albitfeldspat Wechsellagerung dar. Seine Untersuchung führt zu einem magmatischen Petrogenesemodell, bei dem Prozesse der Doppeldiffusion in einem Temperaturgefälle intervenieren. Diese Prozesse bewirken die gleichzeitige und periodische Bildung von millimeterdicken Albitflächen in horizontalen »schwimmenden Flächen«. Jede Zelle, die ein Phänomen der Einheit der Anlagerungsfläche zwischen zwei aufeinanderfolgenden Flächenlagen ist, entwickelt sich schlie\lich durch zentripetale Kristallisierung zu einem geschlossenen System. Die schichtweise Anordnung erfolgt im flüssigen magmatischen Milieu.

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Solving Speed and Memory Issues in Multiple-Point Statistics Simulation Program SNESIM

In the last 10 years, Multiple-Point Statistics (MPS) modeling has emerged in Geostatistics as a valuable alternative to traditional variogram-based and object-based modeling. In contrast to variogram-based simulation, which is limited to two-point correlation reproduction, MPS simulation extracts and reproduces multiple-point statistics moments from training images; this allows modeling geologically realistic features, such as channels that control reservoir connectivity and flow behavior. In addition, MPS simulation works on individual pixels or small groups of pixels (patterns), thus does not suffer from the same data conditioning limitations as object-based simulation. The Single Normal Equation Simulation program SNESIM was the first implementation of MPS simulation to propose, through the introduction of search trees, an efficient solution to the extraction and storage of multiple-point statistics moments from training images. SNESIM is able to simulate three-dimensional models; however, memory and speed issues can occur when applying it to multimillion cell grids. Several other implementations of MPS simulation were proposed after SNESIM, but most of them manage to reduce memory demand or simulation time only at the expense of data conditioning exactitude and/or training pattern reproduction quality. In this paper, the original SNESIM program is revisited, and solutions are presented to eliminate both memory demand and simulation time limitations. First, we demonstrate that the time needed to simulate a grid node is a direct function of the number of uninformed locations in the conditioning data search neighborhood. Thus, two improvements are proposed to maximize the ratio of informed to uniformed locations in search neighborhoods: a new multiple-grid approach introducing additional intermediary subgrids; and a new search neighborhood designing process to preferentially include previously simulated node locations. Finally, because SNESIM memory demand and simulation time increase with the size of the data template used to extract multiple-point statistics moments from the training image and build the search tree, a simple method is described to minimize data template sizes while preserving training pattern reproduction quality.     

Rare gas systematics: formation of the atmosphere, evolution and structure of the Earth's mantle

To explain the rare gas content and isotopic composition measured in modern terrestrial materials we explore in this paper an Earth model based on four reservoirs: atmosphere, continental crust, upper mantle and lower mantle.This exploration employs three tools: mass balance equations, the concept of mean age of outgassing and the systematic use of all of the rare gases involving both absolute amount and isotopic composition.The results obtained are as follows: half of the Earth's mantle is 99% outgassed. Outgassing occurred in an early very intense stage within the first 50 Ma of Earth history and a slow continuous stage which continues to the present day. The mean age of the atmosphere is 4.4 Ga.Our model with four main reservoirs explains quantitatively both isotopic and chemical ratios, assuming that He migrates from the lower to the upper mantle whereas the heavy rare gases did not.Noble gas fluxes for He, Ar and Xe from different reservoirs have been estimated. The results constrain the K content in the earth to 278 ppm. Several geodynamic consequences are discussed.     

Nankai Trough and Zenisu Ridge: a deep-sea submersible survey

Eight submersible dives between 3000 and 4200 m water depth were made off southern Japan in the eastern Nankai subduction zone. Benthic communities associated with chemosynthetic processes were discovered along the 800 m wide active tectonic zone, at the toe of the accretionary prism. A benthic community was also discovered along a zone of active compression, at the foot of Zenisu Ridge, 30 km south of Nankai Trough. Temperature measurements within the sediments below the benthic communities confirm that upward motion of interstitial water occurs there. Studies of water samples indicate advection of methane and light hydrocarbons. Specimens of the benthic community have been shown to have included in their shells carbonate resulting from methane consumption. Thus the benthic communities are related to overpressure-driven fluid advection along tectonic zones with active surface deformation. A 300 m high active scarp at the toe of the accretionary prism is related to relative motion in a 280° direction which is close to the 305° average direction of subduction in this area. The dives establish further that compressive deformation is presently occurring at the foot of Zenisu Ridge. The previous interpretation of the Zenisu Ridge as a zone of recent north-south intraplate shortening, 40 km south of the Nankai Trench, is confirmed. We conclude that tectonic evolution might well lead to future detachment of the Zenisu Ridge and overthrusting of this large piece of oceanic crust over the continental margin. Such a process might be an efficient one to emplace ophiolites over continents.     

Geochemistry of the Tenryu Canyon deep-sea fan biological community (Kaiko)

The Tenryu Canyon deep-sea fan biological community is characterized by both reduced and oxidized sediments in the immediate vicinity of the pore water vents. The upper sediments in contact with the clams are reduced, the organic matter is enriched in sulfur, and inorganic sulfides (Fe, Cu, Zn) are forming. Towards the outer fringes of the communities the sediment is oxidized and metals generally associated with ferro-manganese oxides are concentrated. Several metals, Cd, Pb, Mo show distributions which are strongly influenced by the metabolism of the clam colony. Comparison of water and sediment geochemistry leads to the conclusion that there should be a downward flux of oxygenated seawater on the boundaries of the colony and an upward flux of chemically more reduced deep pore water at the location of the colony. Trace metals anomalies as well asδ¹⁵N anomalies of organic matter may be useful to prospect for extinct venting areas in ancient subduction zones.     

Lead isotope study of basic-ultrabasic layered complexes: Speculations about the age of the earth and primitive mantle characteristics

In order to explore mantle heterogeneity through geological time, Pb isotopic compositions have been determined for 8 differentiated-layered intrusions whose ages are between 2.7 and 0.05 b.y.. The Pb-Pb ages of these intrusions and the U-Pb characteristics of their parent sources are discussed.The Pb-Pb dating method is found to be applicable for this type of basic or ultrabasic rocks and agrees satisfactorily with other available methods. Significant differences are found between the calculated values for the parent bodies of these intrusions. This could reflect either mantle heterogeneity since Archean times, or contamination of some of the bodies by continental crust. Discrimination between these hypotheses can be proposed from the positions of the initial Sr ratios of these massives in respect to the supposed “terrestrial” evolution line. The two intrusions which plot on that closed system Sr evolution line (Muntsche Tundra, U.S.S.R., and Skaergaard, Greenland) belong also to a simple two-stage evolution model for Pb, with a low first-stage μ value of 7.8. If these two bodies are considered as pieces of a “primitive” closed-system mantle, a4.55 ± 0.01 age of the earth can be calculated from their Pb initial ratios.     

Nickel isotopic studies in meteorites

We have analyzed the nickel isotopic composition of meteoritic materials by high-precision mass spectrometry. The samples analyzed include almost all meteorite types for which large isotopic anomalies have been reported for oxygen, silver, magnesium and titanium. These samples are C₁, C₃, L, LL, H and E chondrites, IVB irons, Eagle Station pallasite and inclusion, matrix and “whole rock” samples of the Allende meteorite. The result is that we have not found any anomaly for nickel isotopic compositions within our accuracy of 0.7‰ for⁶¹Ni/⁶⁰Ni, 0.4-0.08‰ for⁶²Ni/⁶⁰Ni and 1–1.5‰ for⁶⁴Ni/⁶⁰Ni.     

Cold-season atmospheric response to the natural variability of the Atlantic meridional overturning circulation

The influence of the natural variability of the Atlantic meridional overturning circulation (AMOC) on the atmosphere is studied in multi-centennial simulations of six global climate models, using Maximum Covariance Analysis (MCA). In all models, a significant but weak influence of the AMOC changes is found during the Northern Hemisphere cold-season, when the ocean leads the atmosphere by a few years. Although the oceanic pattern slightly varies, an intensification of the AMOC is followed in all models by a weak sea level pressure response that resembles a negative phase of the North Atlantic Oscillation (NAO). The signal amplitude is typically 0.5?hPa and explains about 10% of the yearly variability of the NAO in all models. The atmospheric response seems to be due primarily due to an increase of the heat loss along the North Atlantic Current and the subpolar gyre, associated with an AMOC-driven warming. Sea-ice changes appear to be less important. The stronger heating is associated to a southward shift of the lower-tropospheric baroclinicity and a decrease of the eddy activity in the North Atlantic storm track, which is consistent with the equivalent barotropic perturbation resembling the negative phase of the NAO. This study thus provides some evidence of an atmospheric signature of the AMOC in the cold-season, which may have some implications for the decadal predictability of climate in the North Atlantic region.