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May 2009 GEOLOGY European Research Highlights

05 May 2009 Geological Society of America, The

GEOLOGY topics include the "footprint" of a formerly much more extensive ice sheet on the Antarctic continental shelf; frozen nanoparticle schwertmannite that indicates the presence of transient geochemically active microenvironments deep within glacial ice; study in northern Portugal of the largest trilobites ever found, now part of the Arouca GeoPark there; and documentation of the first elliptical crater on Earth that provides insights into the mechanisms of crater formation at low angles.

Highlights are provided below.

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Subglacial bedforms reveal complex basal regime in a zone of paleo-ice stream convergence, Amundsen Sea embayment, West Antarctica
Robert D. Larter et al., British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK. Pages 411-414.

Larter et al. present the most extensive, continuous area of seafloor sonar imagery collected to date on the Antarctic continental shelf, which reveals the "footprint" of a formerly much more extensive ice sheet. The swath bathymetry data collected on expeditions by the British Antarctic Survey and the German Alfred Wegener Institute show details of former subglacial features on parts of the seabed that are now up to 1600-m deep. The trends and characteristics of these features allow reconstruction of past ice flow paths and provide clues about the processes that enabled fast ice flow. Understanding the processes that control fast ice flow in “ice streams” is important because they account for most ice discharge from the large ice sheets on Antarctica and Greenland, and knowledge of how large ice sheets responded to warming at the end of the last glacial period will help predict how the ice sheets will change in response to future warming. The new data indicate that conditions at the base of the former ice sheet were more complex than previously thought, suggesting much more research will be necessary to reliably predict the future behavior of modern ice sheets.

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Schwertmannite in wet, acid, and oxic microenvironments beneath polar and polythermal glaciers
R. Raiswell et al., Earth and Biosphere Institute, School of Earth and Environment, Leeds University, Leeds LS2 9JT, UK. Pages 431-434.

Chemical conditions beneath glaciers are difficult to observe and are usually studied by sampling meltwaters emerging from glacial oulets or drill holes. These waters average chemical signals over a large area and cannot record small-scale spatial and temporal variability. By contrast, subglacial sediments contain minute (nanometer-sized) particles of iron oxides that can only have formed in transient geochemical "hotspots." Raiswell et al. have found minute crystals of schwertmannite, an iron hydroxy-sulfate mineral, in glaciers from the Antarctic and Arctic. Schwertmannite is typically found in acid mine drainage, where it forms by the oxidation of pyrite at low pH. These conditions can also be created in subglacial environments, but only in minute hotspots or "microenvironments." There, schwertmannite forms rapidly but is very unstable, and its survival requires freezing into ice within 100 years. So, frozen nanoparticle schwertmannite indicates the presence of transient geochemically active microenvironments deep within glacial ice. The formation, preservation, and delivery of nanoparticles of schwertmannite (and iron oxides) into the Southern Ocean may partially relieve iron-limited photosynthesis and assist in the removal of manmade carbon dioxide from the atmosphere.

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Giant trilobites and trilobite clusters from the Ordovician of Portugal
Juan C. Gutierrez-Marco et. al., Departamento de Paleontologia, Instituto de Geologia Economica (CSIC-UCM), Facultad de Ciencias Geologicas, 28040 Madrid, Spain. Pages 443-446.

Fossils from 465 million years ago recently discovered in Portugal have revealed the huge size reached by trilobites, the most diverse group of extinct marine arthropods. Gutierrez-Marco et al. describe the largest trilobites ever found, which, in life, would have reached up to 90 centimeters (35 inches). This remarkable record suggests evidence of polar gigantism in an area of Gondwana close to the South Pole during the Ordovician. The Portuguese trilobites also show an astounding array of behavioral clustering -- with some patches reaching groups of over a thousand specimens -- revealing a very diverse social conduct, including hiding from predators and sexual aggregations. This could have played a major role in the undisputed success of this group through the Paleozoic Era. The original discovery site and its fossils are one of the main attractions of the recently established Arouca Geopark in northern Portugal.

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Low-angle collision with Earth: The elliptical impact crater Matt Wilson, Northern Territory, Australia
Thomas Kenkmann and Michael H. Poelchau, Museum fur Naturkunde, 10115 Berlin, Germany. Pages 459-462.

Nearly all meteorite impact craters on Earth are circular. Elongated crater structures are expected only at impacts at angles lower than 12 degrees from the horizontal. Kenkmann and Poelchau document the first elliptical crater on Earth that provides insights into the mechanisms of crater formation at low angles. The diameter of the Proterozoic Matt Wilson impact structure (Northern Territory, Australia) is 7.5 by 6.3 kilometers, with its long axis trending northeast-southwest. The exposed crater floor shows a preferred stacking of thrust sheets within the central uplift, indicating a material transport top-to-southwest. This is explained by remnant horizontal momentum transferred from the impacting projectile to the target rocks. The Matt Wilson structure provides evidence for the usefulness of structural asymmetries as a diagnostic tool to infer the direction of impact.

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