Fe and O isotope composition of meteorite fusion crusts: Possible natural analogues to chondrule formation?

Meteorite fusion crust formation is a brief event in a high‐temperature (2000–12,000 K) and high‐pressure (2–5 MPa) regime. We studied fusion crusts and bulk samples of 10 ordinary chondrite falls and 10 ordinary chondrite finds. The fusion crusts show a typical layering and most contain vesicles. All fusion crusts are enriched in heavy Fe isotopes, with δ⁵⁶Fe values up to +0.35‰ relative to the solar system mean. On average, the δ⁵⁶Fe of fusion crusts from finds is +0.23‰, which is 0.08‰ higher than the average from falls (+0.15‰). Higher δ⁵⁶Fe in fusion crusts of finds correlate with bulk chondrite enrichments in mobile elements such as Ba and Sr. The δ⁵⁶Fe signature of meteorite fusion crusts was produced by two processes (1) evaporation during atmospheric entry and (2) terrestrial weathering. Fusion crusts have either the same or higher δ¹⁸O (0.9–1.5‰) than their host chondrites, and the same is true for Δ¹⁷O. The differences in bulk chondrite and fusion crust oxygen isotope composition are explained by exchange of oxygen between the molten surface of the meteorites with the atmosphere and weathering. Meteorite fusion crust formation is qualitatively similar to conditions of chondrule formation. Therefore, fusion crusts may, at least to some extent, serve as a natural analogue to chondrule formation processes. Meteorite fusion crust and chondrules exhibit a similar extent of Fe isotope fractionation, supporting the idea that the Fe isotope signature of chondrules was established in a high‐pressure environment that prevented large isotope fractionations. The exchange of O between a chondrule melt and an ¹⁶O‐poor nebula as the cause for the observed nonmass dependent O isotope compositions in chondrules is supported by the same process, although to a much lower extent, in meteorite fusion crusts.     

Integration of optical and Synthetic Aperture Radar (SAR) imagery for delivering operational annual crop inventories

Agriculture plays a critical role within Canada’s economy and, as such, sustainability of this sector is of high importance. Targeting and monitoring programs designed to promote economic and environmental sustainability are a vital component within Canada’s agricultural policy. A hierarchy of land information, including up to date information on cropping practices, is needed to measure the impacts of programs on land use decision-making and to gauge the environmental and economic benefits of these investments. A multi-year, multi-site research activity was completed to develop a robust methodology to inventory crops across Canada’s large and diverse agricultural landscapes. To move towards operational implementation the methodology must deliver accurate crop inventories, with consistency and reliability. In order to meet these operational requirements and to mitigate risk associated with reliance on a single data source, the methodology integrated both optical and Synthetic Aperture Radar (SAR) imagery. The results clearly demonstrated that multi-temporal satellite data can successfully classify crops for a variety of cropping systems present across Canada. Overall accuracies of at least 85% were achieved, and most major crops were also classified to this level of accuracy. Although multi-temporal optical data would be the preferred data source for crop classification, a SAR-optical dataset (two Envisat ASAR images and one optical image) provided acceptable accuracies and will mitigate risk associated with operational implementation. The preferred dual-polarization mode would be VV–VH. Not only were these promising classification results repeated year after year, but the target accuracies were met consistently for multiple sites across Canada, all with varying cropping systems.     

Caribbean hydrological variability during the Holocene as reconstructed from crater lakes on the island of Grenada

Contemporary precipitation patterns in the Caribbean region are spatially variable, and the small number of Holocene paleoclimatic records may not adequately capture patterns of variation in the past. The hydrological history of Grenada was inferred from paleolimnological analyses of sediment cores from two crater lakes on the island. The basins were formed by volcanic activity some time during the Last Termination, but were dry between ca. 13 000 and ca. 7200 cal. a BP. After filling, the lakes were initially very shallow, and sedimentation was interrupted by a hiatus ca. 6300–5500 cal. a BP, followed by deposition of a thick tephra in both sites. After 5500 cal. a BP, lake level shows considerable multi‐centennial variability, superimposed upon a long‐term trend of generally higher lake level after 3200 cal. a BP. The pattern of lake‐level variation in Grenada shows some similarity with other Caribbean paleoclimatic records in terms of the timing of transitions, but differs from several classic studies in the sign of inferred precipitation change. The differences among records may reflect spatially variable precipitation patterns in the past in response to the position of the Intertropical Convergence Zone and to sea surface temperature influences on the trade winds and Caribbean low‐level jet. Copyright © 2011 John Wiley & Sons, Ltd.     

The Eltanin asteroid impact: possible South Pacific palaeomegatsunami footprint and potential implications for the Pliocene–Pleistocene transition

Large asteroid impacts are rare, and those into the deep ocean are rarer still. The Eltanin asteroid impact around 2.51 ± 0.07 Ma occurred at a time of great climatic and geological change associated with the Pliocene–Pleistocene boundary. Numerical models of the event indicate that a megatsunami was generated, although there is debate concerning its magnitude and the region‐wide extent of its influence. We summarise the existing evidence for possible Eltanin megatsunami deposits in Antarctica, Chile and New Zealand, while also examining other potential sites from several locations, mainly around the South Pacific region. In reviewing these data we note that these events were unfolding at the same time as those associated with the Pliocene–Pleistocene boundary and, as such, most of the geological evidence from that time has a climatic interpretation. The potential climatic and geological ramifications of the Eltanin asteroid impact, however, have failed to be considered by most researchers studying this time period. Although we are not advocating that all geological activity at that time is connected with the Eltanin asteroid impact, it raises interesting questions about the role potentially played by such catastrophic events in contributing to or even triggering epochal transitions. Copyright © 2012 John Wiley & Sons, Ltd.     

Biogeography of the deep-sea galatheid squat lobsters of the Pacific Ocean

We analyzed the distribution patterns of the galatheid squat lobsters (Crustacea, Decapoda, Galatheidae) of the Pacific Ocean. We used the presence/absence data of 402 species along the continental slope and continental rise (200–2000 m) obtained from 54 cruises carried out in areas around the Philippines, Indonesia, Solomon, Vanuatu, New Caledonia, Fiji, Tonga, Wallis and Futuna and French Polynesia. The total number of stations was ca. 3200. We also used published data from other expeditions carried out in the Pacific waters, and from an exhaustive search of ca. 600 papers on the taxonomy and biogeography of Pacific species. We studied the existence of biogeographic provinces using multivariate analyses, and present data on latitudinal and longitudinal patterns of species richness, rate of endemism and the relationship between body sizes with the size of the geographic ranges. Latitudinal species richness along the Western and Eastern Pacific exhibited an increase from higher latitudes towards the Equator. Longitudinal species richness decreased considerably from the Western to the Central Pacific. Size frequency distribution for body size was strongly shifted toward small sizes and endemic species were significantly smaller than non-endemics. This study concludes that a clear separation exists between the moderately poor galatheid fauna of the Eastern Pacific and the rich Western and Central Pacific faunas. Our results also show that the highest numbers of squat lobsters are found in the Coral Sea (Solomon-Vanuatu-New Caledonia islands) and Indo-Malay-Philippines archipelago (IMPA). The distribution of endemism along the Pacific Ocean indicates that there are several major centres of diversity, e.g. Coral Sea, IMPA, New Zealand and French Polynesia. The high proportion of endemism in these areas suggests that they have evolved independently.     

Metamorphism in oceanic layer 3, Gorringe Bank,eastern Atlantic

Gorringe Bank is an anomalously high structure of the eastern part of the north Atlantic, which was known to be composed of mantle-derived peridotites (layer 4) and gabbros (layer 3). During the submersible cruise CYAGOR II in 1981, the contact between layer 4 and layer 3 was observed on Mount Gettysburg and interpreted as tectonic. The overlying series of gabbro was extensively sampled on both mounts composing the bank, Gettysburg and Ormonde. Coarse-grained to pegmatoid clinopyroxene gabbros predominate and are associated with differentiated rocks (ferrogabbros and diorites). Cumulate gabbros are missing. The gabbroic section sampled is therefore interpreted as the upper part of the plutonic section. Most samples were strongly recrystallized during two distinct events. Metamorphism occurred close to the ridge axis, from interaction of a seawater-derived fluid with still hot gabbros. High temperature shear zones favoured fluid circulation, but the water/rock ratio — estimated from the sodium input — was very small in undeformed rocks (<1). The low W/R ratio explains the strong evolution of the fluid phase and therefore some particular compositions of secondary minerals. Low temperature alteration occurred when the gabbros were tectonically emplaced close to the sea bottom.