Pearl oysters Pinctada margaritifera grazing on natural plankton in Ahe atoll lagoon (Tuamotu archipelago, French Polynesia)

In atoll lagoons of French Polynesia, growth and reproduction of pearl oysters are mainly driven by plankton concentration. However, the actual diet of black-lip pearl oysters Pinctada margaritifera in these lagoons is poorly known. To fill this gap, we used the flow through chamber method to measure clearance rates of P. margaritifera in Ahe atoll lagoon (Tuamotu Archipelago, French Polynesia). We found: (i) that pearl oysters cleared plankton at a rate that was positively related to plankton biovolume, (ii) that nanoflagellates were the main source of carbon for the pearl oysters, and (iii) that the quantity and origin of carbon filtrated by pearl oysters was highly dependent on the concentration and composition of plankton. These results provide essential elements for the comprehension of growth and reproduction variability of pearl oysters in atoll lagoons of French Polynesia.     

Changes in water table level influence solute transport in uniform porous media

Changes in the water table level result in variable water saturation and variable hydrological fluxes at the interface between the unsaturated and saturated zone. This may influence the transport and fate of contaminants in the subsurface. The objective of this study was to examine the impact of a decreasing and an increasing water table on solute transport. We conducted tracer experiments at downward flow conditions in laboratory columns filled with two different uniform porous media under static and transient flow conditions either increasing or decreasing the water table. Tracer breakthrough curves were simulated using a mobile–immobile transport model. The resulting transport parameters were compared to identify dominant transport processes. Changes in the water table level affected dispersivities and mobile water fractions depending on the direction of water table movement and the grain size of the porous media. In fine glass beads, the water flow velocity was similar to the decline rate of the water table, and the mobile water fraction was decreased compared with steady‐state saturated conditions. However, immobile water was negligible. In coarse glass beads, water flow was faster because of fingered flow in the unsaturated part, and the mobile water fraction was smaller than in the fine material. Here, a rising water table led to an even smaller mobile water fraction and increased solute spreading because of diffusive interaction with immobile water. We conclude that changes of the water table need to be considered to correctly simulate transport in the subsurface at the transition of the unsaturated–saturated zone. Copyright © 2014 John Wiley & Sons, Ltd.     

A spatial analysis of neolithic chambered cairns on the island of Arran,Scotland

The island of Arran, located to the west of central Scotland, has a dense concentration of Neolithic chambered cairns. The aim of this study is to determine by using a simulation analysis the extent to which thirteen factors influenced the distribution of cairns. Overall, proximity to arable land is shown to be of particular importance but the influence of stream order, coastal type, amplitude of relief, and aspect are also demonstrated to have been of relevance. Since there is little evidence from absolute dating, subdivision of cairns on typological grounds provides the only means for analyzing cairn distributions at three different phases. Results from simulation and cluster analysis indicate variation through time in importance of particular factors. Early cairns are only associated with low relief areas. For cairns of intermediate and later age, the factors as listed above are shown to have been of importance, with proximity to arable land becoming particularly relevant in the last phase. The paper discusses these results in terms of an increasing agricultural component to the Neolithic economy of Arran.     

Origins of meteoritic nanodiamonds investigated by coordinated atom‐probe tomography and transmission electron microscopy studies

To constrain the origins of meteoritic nanodiamonds, the abundance ratios of stable C isotopes in acid residues from the carbonaceous chondritic meteorite Allende CV3 were measured using coordinated atom‐probe tomography (APT) and transmission electron microscopy (TEM). We combined our data with previously published APT data. A statistical analysis of this combined data set suggests an upper bound of 1 in 10² on the subpopulation that could have a large isotopic enrichment in ¹³C relative to ¹²C, consistent with the possible detection by secondary ion mass spectrometry of a similar enrichment in a 1 in 10⁵ fraction, abundant enough to account for the Xe‐HL anomalous isotopic component carried by the acid residues. Supernovae are believed to be the source of Xe‐HL, leading to the mystery of why all other supernova minerals do not carry Xe‐HL. The lack of Xe‐HL in low‐density disordered supernova graphite suggests that the isotopically anomalous component is the nanodiamonds, but the disordered C in the residue is not ruled out. We discuss possible origins of the disordered C and implications of our results for proposed formation scenarios for nanodiamonds. At least 99% of the meteoritic acid residue exhibits no unambiguous evidence of presolar formation, although production with solar isotope ratios in asymptotic giant branch stars is not ruled out. Comparison of TEM and APT results indicates that a minority of the APT reconstructions may preferentially sample disordered C rather than nanodiamonds. If this is the case, a presolar origin for a larger fraction of the nanodiamonds remains possible.     

High-pressure single-crystal X-ray diffraction and synchrotron Mössbauer study of monoclinic ferrosilite

The phase and spin transitions in single-crystal monoclinic ferrosilite, FeSiO₃, were investigated using X-ray diffraction and Mössbauer spectroscopy up to lower-mantle pressures and room temperature in a helium pressure medium. Using single-crystal X-ray diffraction, we measured the equation of state of ferrosilite up to about 43 GPa. We observed a P2₁/c-to-C2/c phase transition between 1.5 and 1.7 GPa and a phase transition from C2/c to a distinct P2₁/c structure between 30 and 34 GPa. With time-domain Mössbauer spectroscopy, we determined the hyperfine parameters of ferrous iron up to 95 GPa. The phase transitions were correlated with discontinuities in Mössbauer spectral features. We observed the onset of high-spin-to-low-spin transitions in the M1 and M2 sites at ～37 GPa and ～74 GPa, respectively. Understanding the electronic structure of iron in a well-characterized single crystal of ferrosilite may help interpret the behavior of iron in complex dense silicate phases.     

Brecciation and chemical heterogeneities of CI chondrites

Fragments in the size range from 40 μm to several hundred μm in the CI chondrites Orgueil, Ivuna, Alais, and Tonk show a wide range of chemical compositions with variations in major elements such as iron (10.4-42.4 wt% FeO), silicon (12.7-42.2 wt% SiO₂), and sulfur (1.01-15.8 wt% SO₃), but also important minor elements such as phosphorous (up to 5.2 wt% P₂O₅) or calcium (up to 6.6 wt% CaO). These variations are the result of the varying mineralogical compositions of these fragments. The distribution of phyllosilicates, magnetites, and possibly ferrihydrite, in particular, control the abundances of these elements. High REE contents—up to 150 times the solar abundances—were observed in phosphates, while matrix and sulfates are REE-depleted. The studied 113 fragments were subdivided into eight lithologies with similar mineralogical and thus chemical properties. The most common is the CGA lithology, consisting of a groundmass of Mg-rich, coarse-grained phyllosilicates and varying abundances of inclusions such as magnetite. The second most abundant lithology is the FGA lithology, consisting of a groundmass of fine-grained Fe-rich phyllosilicates. A rare, but important lithology consists of fragments with high contents of phosphates and other minerals. The proposed model for the evolution of these lithologies is based on a closed system alteration, where mineralogical differences in the lithologies reflect heterogeneities in the starting material. Comparison of our results with literature data indicates a general similarity of the four CI chondrites analyzed. Further comparison of bulk analyses suggests that the mass ‘threshold’ for chemical heterogeneities in CI chondrite samples is smaller than ∼1-2 g.     

Geochemistry and metamorphic evolution of the Pohorje Mountain eclogites from the easternmost Austroalpine basement of the Eastern Alps (Northern Slovenia)

Kyanite-rich and quartz-rich eclogites occur as lenses within amphibolite-facies quartzo-feldspathic gneisses in the Pohorje Mountains, Northern Slovenia, that form the easternmost Austroalpine basement. Major and trace elements indicate that the kyanite-rich eclogites were derived from plagioclase-rich gabbroic cumulates, whereas the quartz-rich eclogites represent more fractionated basaltic compositions. Both varieties are characterized by a LREE-depleted N-MORB type REE signature. Geothermobarometry and P-T pseudosections indicate that eclogites equilibrated at 1.8-2.5 GPa and 630-700 °C, consistently with the lack of coesite and with equilibration conditions of the chemically similar eclogites from the adjacent basement units at Koralpe and Saualpe type localities. Decompression reaction textures include (i) clinopyroxene-plagioclase intergrowths after omphacite, (ii) replacement of kyanite by corundum-plagioclase-spinel±sapphirine symplectites, (iii) breakdown of phengite to biotite-plagioclase sapphirine symplectites. The results of this study indicate that Koralpe, Saualpe and Pohorje high-pressure rocks represent former MORB-type oceanic crust that was subducted in the course of the late Cretaceous (approximately 100 Ma ago) collision between the European and the Apulian plates.     

Nonlinear precipitation response to El Niño and global warming in the Indo-Pacific

Precipitation changes over the Indo-Pacific during El Niño events are studied using an Atmospheric General Circulation Model forced with sea-surface temperature (SST) anomalies and changes in atmospheric CO₂ concentrations. Linear increases in the amplitude of the El Niño SST anomaly pattern trigger nonlinear changes in precipitation amounts, resulting in shifts in the location and orientation of the Intertropical Convergence Zone (ITCZ) and the South Pacific Convergence Zone (SPCZ). In particular, the maximum precipitation anomaly along the ITCZ and SPCZ shifts eastwards, the ITCZ shifts south towards the equator, and the SPCZ becomes more zonal. Precipitation in the equatorial Pacific also increases nonlinearly. The effect of increasing CO₂ levels and warming SSTs is also investigated. Global warming generally enhances the tropical Pacific precipitation response to El Niño. The precipitation response to El Niño is found to be dominated by changes in the atmospheric mean circulation dynamics, whereas the response to global warming is a balance between dynamic and thermodynamic changes. While the dependence of projected climate change impacts on seasonal variability is well-established, this study reveals that the impact of global warming on Pacific precipitation also depends strongly on the magnitude of the El Niño event. The magnitude and structure of the precipitation changes are also sensitive to the spatial structure of the global warming SST pattern.