|19 June 2008
GSA Release No. 08-28
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JuLY Media Highlights
Boulder, CO, USA - GEOLOGY topics span the globe and beyond, covering southern Africa’s diamond kimberlites, sandstones and shales of Scotland, a "Greenhouse World" in the Wyoming badlands, glacial erosion in East Greenland, oxidation of methane in southern Namibia, preservation of woolly rhinoceroses in Ukraine, gold nanoparticles in Australia, and the possibility of a water-rich environment in Mars' past. GSA TODAY contains the first authoritative geological and geophysical treatment of the 12 May 2008 earthquake in Sichuan Province, China.
Highlights are provided below. Representatives of the media may obtain complimentary copies of articles by contacting Christa Stratton, . Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOLOGY in articles published. Contact Christa Stratton for additional information or other assistance.
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Ghosts of lithospheres past: Imaging an evolving lithospheric mantle in southern Africa
Alan F. Kobussen et al., GEMOC, Dept. of Earth and Planetary Sciences, Macquarie University New South Wales 2109, Australia. Pages 515-518.
Kimberlites--volcanic rocks that can carry diamonds to the surface--bring up samples of rocks and minerals from the ancient continental roots, 100-200 kilometers beneath Earth's surface. In southern Africa, two episodes of volcanism 10-15 million years apart provide "snapshots" in two different time slices of this otherwise inaccessible part of Earth. Chemical fingerprinting of minerals in these deep roots has revealed dramatic changes in the composition and structure of the continental root caused by the infiltration of magmas from deeper in the Earth over this short period. Continental roots like the one in southern Africa can remain stable for billions of years, but they also can evolve through time. Kobussen et al. provide new insights into the processes that can modify and potentially destabilize the continental roots.
Ancient Laurentian detrital zircon in the closing Iapetus Ocean, Southern Uplands terrane, Scotland
John W.F. Waldron et al., Dept. of Earth and Atmospheric Sciences, 1-26 Earth Sciences Building, University of Alberta, Edmonton, Alberta T6G 2E3, Canada. Pages 527-530.
The Southern Uplands of Scotland are built from sandstones and shales that were deposited during the Ordovician and Silurian periods, between about 450 and 420 million years ago, on the edge of an ocean known as Iapetus. Waldron et al. used isotopic dating to determine the ages of sand grains within the sandstones, so as to provide evidence for the sources of sediment in the adjoining continents. This method uses laser ablation to release uranium and lead atoms from grains of the mineral zircon, extracted from the sandstones, and a multicollector inductively coupled-mass spectrometer to measure the amounts of their isotopes. The results show that zircon grains were derived from sources characteristic of the Laurentian continent, comprising most of North America, together with the Scottish Highlands and Greenland. Many of the grains are much older than the sandstones in which they are now found. Grains as old as 3.6 billion years were recorded, older than any within the British Caledonides southeast of the Moine Thrust. The results support a model for the origin of the Southern Uplands in an ocean trench along the margin of Laurentia, where the floor of the Iapetus Ocean was being consumed under the adjacent continental margin.
Astronomical climate control on paleosol stacking patterns in the upper Paleocene-lower Eocene Willwood Formation, Bighorn Basin, Wyoming
Hayfaa Abdul Aziz et al., Utrecht University, Faculty of Geoscience, Budapestlaan4, Utrecht 3584 CD, Netherlands. Pages 531-534.
The Late Paleocene to Early Eocene, between 60 and 50 million years ago, was associated with high carbon dioxide concentrations, similar to those expected for the coming centuries due to fossil fuel burning. Superimposed on this "Greenhouse World" was an episode of dramatic, short-lived global warming caused by an enhanced greenhouse effect, called the Paleocene-Eocene Thermal Maximum (PETM). The PETM coincides with the evolution of modern mammals, including the early ancestors of humans. The sediments exposed in the badlands of Wyoming are famous for their fossil remains of ancient mammals, such as the earliest primates and horses that inhabited the plains of North America. The sediments include spectacularly red, orange, and purple color-banded soils that extend for kilometers throughout the landscape of Wyoming. The nature of these soils, however, has always been a matter of debate. Abdul Aziz et al. study the color records of these ancient soils, and show in this paper that the color banding was caused by cyclic climate changes. These climate changes are provoked by Earth's precession cycle of 21,000 years, which influences the distribution of solar energy on Earth's surface. Similar cycles are associated with glacial-interglacial cycles. The discovery of climate cycles in the sedimentary deposits in Wyoming is essential for several lines of research. For the first time, such cycles are shown to have influenced soil formation. These regular 21,000-year cycles provide an unparalleled time control for the sediments in Wyoming, enabling paleontologists to study the rate of mammal (including primate) evolution during the PETM and the long-term greenhouse phase in Earth's history.
Vertical motions of the fjord regions of central East Greenland: Impact of glacial erosion, deposition, and isostasy
Sergei Medvedev et al., Physics of Geological Processes, University of Oslo, P.O. Box 1048, Blindern, 0316 Oslo, Norway. Pages 539-542.
Plate tectonics has successfully attributed many uplifts around the world to tectonic collisions. However, the vertical motions of the fjord regions of East Greenland cannot be explained by this mechanism because they occur at a time of regional tectonic quietness. Why are 100 to 200 million-year-old marine sedimentary rocks now located up to 1.2 km above sea level? Medvedev et al. address this long-standing enigma by analyzing field data and using numerical models. This uplifted area is cut by some of the world's biggest fjords. Scoresbysund fjord alone is up to 60 km wide and cuts 400 km into the land and more than 4 km down from the peaks of the region. Glacial erosion and fjord formation has thus removed significant loads from the upper crust, which are compensated by uplift due to buoyancy forces, which act from the deep earth. These forces, however, are smoothed out by the flexural rigidity of the lithosphere. Therefore, the isostatic readjustment results not only in smaller subsidence of topography in the eroded fjord, but also in the uplift of the surrounding non-eroded area. To quantify this effect, Medvedev et al. construct a numerical model and analyze the erosional process backward in time. They estimate that an average of almost 1.2 km of bedrock was eroded in the region during past 20 million years. Their model demonstrates that rocks in the central Fjord Mountains may be uplifted up to 1.1 km due to erosional unloading and flexural isostasy. Thus, these effects should be a main mechanism responsible for the uplift in central East Greenland and probably represent a model applicable to other deeply eroded and recently uplifted coastal ranges.
A new constraint on the antiquity of anaerobic oxidation of methane: Late Pennsylvanian seep limestones from southern Namibia
Daniel Birgel et al., MARUM - University of Bremen, Leobener Strasse, D-28359 Bremen, Germany. Pages 543-546.
Molecular fossils enclosed in Late Pennsylvanian seep limestones from southern Namibia reveal that prokaryotes oxidized methane by the same process that is still dominant at modern marine seeps. Birgel et al. show that the anaerobic oxidation of methane was performed by methanotrophic archaea and associated sulfate-reducing bacteria. Birgel et al.'s finding extends the record of a major biogeochemical process by approximately 150 million years.
Crustal evolution of southern Laurentia during the Paleoproterozoic: Insights from zircon Hf isotopic studies of ca. 1.75 Ga rocks in central Colorado
M.E. Bickford et al., Dept. of Earth Sciences, 204 Heroy Geology Laboratory, Syracuse University, Syracuse, New York 13244-1070, USA. Pages 555-558.
In the Gunnison-Salida region of central Colorado, there are exposures of 1750 million-year-old (Paleoproterozoic) volcanic rocks, including both basalt and rhyolite, and associated granite bodies. These rocks have been thought to represent ancient island arcs that were accreted to the growing North American continent at this time. The bimodal (basalt-rhyolite) composition, however, suggests instead that they were derived from preexisting older continental crustal rocks. In two earlier studies authors Bickford and Hill used the uranium-lead method to date zircons (ZrSiO4) from these rocks, finding that, in addition to the 1750 million-year-old zircons that date the crystallization age, there are numerous "inherited" older zircons whose ages are mostly about 1850-1870 million years. These presumably were derived from preexisting crust that was melted to produce the rhyolite and granite, as hot basaltic magma was introduced from the mantle during crustal rifting. In the current study, the same zircons were analyzed for their hafnium (Hf) isotopic composition. The results of analyses reported by Bickford et al. yielded Hf model ages of 1850 to 2000 million years, significantly older than the crystallization age of the rocks, and confirming their derivation from melting of older crustal sources. This result suggests that in the Gunnison-Salida area, and likely across much of the southwestern United States where similar bimodal volcanic assemblages are known, volcanic and related magmatic activity was related to a period of extension, rifting, and re-melting of previously formed crust. This scenario has important implications for the Proterozoic growth of North American continental crust and Proterozoic geodynamics in general.
Unique Quaternary environment for discoveries of woolly rhinoceroses in Starunia, fore-Carpathian region, Ukraine: Geochemical and geoelectric studies
Maciej J. Kotarba et al., AGH - University of Science and Technology, Faculty of Geology, Geophysics and Environmental Protection, Mickiewicza Ave. 30, 30-059 Krakow, Poland. Pages 567-570.
In 1907, remnants of a mammoth and a woolly rhinoceros were discovered in the Pleistocene clays of an earth-wax mine in Starunia village in the Ukraine. Then, in 1929, a near-fully preserved woolly rhinoceros was found in the same mine. A unique combination of clays, oil, and brine, into which the animals had sunk, is responsible for their almost perfect preservation. The herbivorous mammals migrating in search of food and water became trapped in such places and drowned in the clayey substance that was saturated with brines and oils. It seems possible that during the Pleistocene tundra winters, when a thick ice and snow cover was present in the tundra, “paleoswamps”--the areas of inflow of brines, oils, helium, and thermogenic hydrocarbon gases--had a higher temperature, which resulted in melting and cracking of the cover. Kotarba et al. performed geoelectric measurements, as well as molecular and stable isotope analyses of gases in the near-surface zone within the paleoswamp to reveal a few places within the paleoswamp where thermogenic gases occur, indicating sites favorable to the burial and preservation of Pleistocene large mammals, and most likely human remains as well.
Naturally occurring gold nanoparticles and nanoplates
R.M. Hough et al., ARRC, CSIRO Exploration and Mining, 26 Dick Perry Avenue, Kensington, Perth, Western Australia 6151, Australia. Pages 571-574.
Gold nanoplates and nanoparticles are important materials with unique properties defined by their size and shape. They are actively being pursued in the manufacturing industry for applications in drug delivery, including cancer therapy, new optical instrumentation, and as superconductors and catalysts. Hough et al. report, for the first time, a natural occurrence of such gold particles that show identical features to their manufactured counterparts. They form where gold deposits in the vast southern part of Western Australia are actively weathering today in the presence of salty groundwaters. The gold crystals are so thin (20 nanometers) that they are transparent to electron beams, and the finest population of nanoparticles are triangular, less than 100 nanometers in size, only 6 nanometers thick, and were previously invisible. These are the first observations of this natural nanoparticulate gold.
Olivine dissolution by acidic fluids in Argyre Planitia, Mars: Evidence for a widespread process?
Joshua L. Bandfield et al., Earth and Space Sciences, Box 351310, University of Washington, Seattle, Washington 98195-1310, USA. Pages 579-582.
Although there is ample evidence for compositions that formed in a water-rich environment at some point in the Martian past, these compositions are limited in areal extent. New results presented by Bandfield et al. combine results from Mars satellite and rover datasets to show that aqueous alteration has been much more extensive than has been presumed by many researchers. Bedrock outcrops show differences in composition with the surfaces and soils derived from these rocks. These differences are the same as would be expected with exposure to small amounts of liquid water. Bandfield et al.'s results indicate that water, in limited amounts, may have a large effect on the formation of the Martian surface, even in regions without obvious water-related features, such as stream valleys or evaporite minerals.
A geological and geophysical context for the Wenchuan earthquake of 12 May 2008, Sichuan, People’s Republic of China
B.C. Burchfiel et al., Dept. of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA. Pages 4-11.
Here is the first authoritative treatment of the geological and geophysical context of the 12 May 2008 magnitude 7.9 earthquake that shook the eastern Tibetan Plateau, killing more than 65,000 people. The paper is by a coalition of MIT geologists and geophysicists who have worked in the region for many years, together with their long-time Chinese colleagues from the Chengdu Institute of Geology and Mineral Resources, Sichuan, China. The setting for the earthquake is that of large-scale mountain-building, surface uplift, and right-lateral movement of the eastern Tibetan Plateau relative to southeast China. Several faults are likely candidates for movement, including the northeast-trending Beichuan and Wenchuan faults. Crustal shortening in this region is very slow, around 1 mm/yr (plus or minus 1 mm/yr) and, with its low seismicity, this area was considered to be of low seismic risk compared to surrounding areas. However, the slow shortening, ductile thickening in the deep crust, and the large elevation difference between the Tibetan plateau and the adjacent lowlands probably created large stresses in the upper crust near the edge of the plateau. GPS-determined convergence rates in the vicinity of the 12 May event suggest an average earthquake recurrence interval of about 2,000-10,000 years.
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