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GEOLOGY
Balancing crustal thickening in arcs by tectonic vs. magmatic means
Michael Haschke, GFZ Potsdam, Telegrafenberg, C 220, Potsdam, Germany 14473, Germany; and A. Günther, Institut für Umweltgeologie, Technische Universität Braunschweig, Braunschweig 38106, Germany. Pages 933–936.
Our paper provides an estimate on how much continents are thickened by orogenic processes such as contraction and shortening versus addition of magmatic material from the underlying mantle. Our study attempts to resolve the question on how the central Andes gained their present thickness of 70 km. How much thickening was achived by shortening and how much was due to underplating of newmafic crust? This ratio is about 2:1 in our sample area in North Chile.
Contribution of La Niña and El Niño to extreme mid-Holocene drought and late Holocene moisture in the American southwest
Kirsten Menking, Vassar College, Geology and Geography, Box 59, 124 Raymond Ave., Poughkeepsie, New York 12604, U.S.A., and Roger Y. Anderson, Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, U.S.A. Pages 937–940.
A little-known valley in the middle of New Mexico has captured an 8000 yr history of El Niño and its climatic opposite, La Niña. Today, during El Niño years, heavy snows fall on the 10,000 ft Manzano Mountains that border the valley, more snow than during an ordinary winter. During La Niñas, snowfall tends to be light. Each year, after the snow melts, and as water seeps deep underground, it rises up and soaks the floor of the broad Estancia Valley at the foot of the mountains. Ca. 8000 yr ago, the heavy snows from El Niños virtually disappeared as La Niñas became more frequent. Water stopped rising in the valley, and powerful winds scooped deep bowls, some more than a mile wide, from the sand and clay of the dry valley floor, piling up dunes as high as 100 ft. Then, almost as suddenly, El Niños replaced La Niñas and the snows returned. Water once again soaked the valley floor, and the bowls began filling up with water and mud. For the past 6400 yr, the water in the Estancia Valley has slowly continued to rise, suggesting that El Niño may be stronger today than at anytime in the past 8000 yr.
Paleoecology of the familiar trilobite Elrathia kingii: An early exaerobic zone inhabitant
Robert Gaines and Mary L. Droser, University of California, Riverside, Department of Earth Sciences, Riverside, California 92521, U.S.A. Pages 941–944.
Anyone who has ever held a trilobite has likely handled Elrathia kingii, the most widely commercially distributed trilobite in the world. New work demonstrates that E. kingii lived a bizarre lifestyle, previously unknown despite its ubiquity in museum and personal collections. The arthropod lived in extreme environments hostile to other animals, and likely fed from a food web that was not rooted in photosynthetic primary production. Almost exclusively, photosynthesis sustains Earth's ecosystems today, with the exception of hydrothermal vent and seep communities of the deep sea. Modern arthropods living in these unusual environments have a symbiotic relationship with the sulfur bacteria that are the primary producers there (chemoautotrophic productivity), cultivating them in bacterial "gardens" on the insides of their exoskeletons. Previous work has suggested that some trilobites, shaped similarly to E. kingii, may have possessed exoskeletons adapted for the same purpose. E. kingii led a lifestyle common to these vent faunas, with one important difference: instead of living near vents, it sought its food source in low-oxygen environments prohibitive to other animals, but ideally suited for proliferation of sulfur bacteria. While this ecological strategy is known from younger parts of the fossil record, these findings push the minimum age for the establishment of such unusually-rooted ecosystems back considerably, placing the origin of non-photosynthesis-based communities and their alternative lifestyles close in time to the origin of animal groups.
The Permian-Triassic boundary interval as a model for forcing marine ecosystem collapse by long-term atmospheric oxygen drop
Oliver Weidlich, Christian-Albrechts-Universität zu Kiel, Institut für Geowissenschaften, Olshausenstrasse 40-60, Kiel, D-24118, Germany; et al. Pages 961–964.
While scientists are questioning the severity of mass extinctions, the non-scientific audience remains fascinated by their disastrous impact. Focusing on the biggest mass extinction of Earth, the Permian-Triassic boundary event, abrupt mechanisms are in favor, including supergiant volcanic eruptions, a bolide impact, CO2 poisoning and/or anoxia of oceans, and methane expulsion. Our novel observation predicts that long-term atmospheric oxygen decrease can cause oceanic anoxia and, therefore, is in concert with the above-mentioned mechanisms; this is a key factor, which causes the devastative character of mass extinctions.
Biomarkers, brines, and oil in the mesoproterozoic, Roper Superbasin, Australia
Adriana Dutkiewicz, The University of Sydney, School of Geosciences, Building FO5, Sydney, NSW 2006, Australia; et al. Pages 981–984.
This paper reports first results of a new tool for looking at Earth's ancient ecosystems and environments, which are otherwise only sketchily known from sparse and equivocal microfossils and geochemical data. We report the discovery of biomarkers (molecular fossils) preserved in oil trapped more than 1000 million yr ago as tiny inclusions in quartz crystals, in much the same way that gas bubbles are trapped in ice. Such oil inclusions promise to provide a more complete picture of life on early Earth. Oil inclusions fluoresce under excitation by ultraviolet light and were found in ~1400 million-yr-old sandstone in the Roper Superbasin, Australia. The molecular composition of the oil determined by careful gas chromatography–mass spectrometry revealed a complex composition that has remained protected from the outside environment since trapping. Biomarkers show that the oil was derived from the remains of bacteria, in particular cyanobacteria. Colonies of these aquatic and photosynthesizing microbes also form stromatolites, which are some of the oldest fossils in the world, and are responsible for having generated an oxygen-rich atmosphere. Future work on even older samples could lead to a richer and more robust record of early microbial life.
Onset of major Pleistocene glaciations in the Alps
Giovanni Muttoni, Universita di Milano, Dipartimento di Scienze della Terra, via Mangiagalli 34, Milan, n/a 20133, Italy; et al. Pages 989–992.
Since alligators patrolled Greenland swamps some 40 millions years ago, Earth's climate underwent significant cooling, which culminated about one million years ago in the ice ages with recurring glaciations in vast regions of the Alps, Eurasia, and North America, and overgrowth of polar icecaps in Antarctica and Greenland. The Po Basin of northern Italy is a natural collector of past biological and climatic changes involving the waxing and waning of the nearby Alpine icecap. Muttoni et al. (2003) found evidence for one such major glacial pulsation, which occurred in correspondence to a regional unconformity associated with a major reorganization of vegetation cover and Alpine drainage pattern. The age of the unconformity was dated using magnetostratigraphy to the first major Pleistocene glacio-eustatic low-stand at 0.87Ma (Oxygen Isotope stage 22). This corresponds to the end of the "Mid-Pleistocene Revolution" (MPR), a marked reorganiztion of Northern Hemisphere glaciation pattern, which took place in the late Early Pleistocene. Muttoni et al. (2003) suggest that the MPR/MIS 22 event determined the onset of the first major Pleistocene glaciation in the Alps, whose age was, up to now, very poorly constrained.
New light on the age of the White Nile
Martin Williams, University of Adelaide, Geography and Environmental Studies, North Terrace, Adelaide, South Australia 5005, Australia; et al. Pages 1001–1004.
Without the waters from the White Nile, the main Nile would cease to flow during very dry years. We are now able to extend the demonstrated age of the White Nile from 15,000 yr to well over 250,000 yr. In addition, we can account for the remarkable flatness of the present river, with its flood gradient of only one centimeter per kilometer. It flows across an ancient lake floor whose existence, long suspected, we have now mapped for the first time using high-resolution satellite imagery. The ancient lake existed ca. 400, 000 yr ago.
A tropical view of Quaternary sequence stratigraphy: Siliciclastic accumulation on slopes east of the Great Barrier Reef since the Last Glacial Maximum
Michael Page, James Cook University, Earth Sciences, Townsville, Queensland 4811, Australia; et al. Pages 1013–1016.
A tropical view of Quaternary Sequence Stratigraphy: siliciclastic accumulation on slopes east of the Great Barrier Reef since the Last Glacial Maximum, by M.C. Page, G.R. Dickens and G.B. Dunbar discusses the sedimentary patterns observed along the northeast Australian continental margin over the past 25,000 yr, a period during which global sea levels have risen by ~120 m. Sediment on the northeast Australian margin is composed of both terrestrial siliciclastic sediments, sourced from mainland Australia, and biogenic carbonate sediments, sourced from the Great Barrier Reef shelf and Coral Sea, and is the world's largest example of a mixed siliciclastic-carbonate system. The flux of siliciclastic sediment from shelf to deep sea was found to be greatest during sea level transgression, 12–7 ka, a time when rapidly rising sea level was beginning to flood the previously exposed continental shelf, and lowest before this time during sea level lowstand. These results are wholly inconsistent with sedimentary patterns predicted for mixed siliciclastic-carbonate margins, which suggest that fluxes should be lowest during transgression and highest during the sea level lowstand. The presence of an extinct precursor to the modern Great Barrier Reef may have restricted the passage of rivers across the exposed shelf, resulting in siliciclastic deposition on the outer shelf during lowstand. This sediment was most likely reworked during transgression once rising sea level reached the shelf edge.
A significant southern ocean warming event in the late middle Eocene
Steven Bohaty and James C. Zachos, University of California, Santa Cruz, Earth Sciences Department, 1156 High Street, Santa Cruz, California 95064, U.S.A. Pages 1017–1020.
Long-term global cooling through the middle and late Eocene (ca. 49–34 Ma) ended with the sudden glaciation of East Antarctica in the early Oligocene (ca. 33 Ma). Previous work suggests that middle-to-late Eocene cooling was progressive and unidirectional. A new study of the stable oxygen isotope stratigraphy from cores in the Antarctic region, however, indicates the presence of a significant warming event in the late middle Eocene (ca. 41 Ma) that interrupts the long-term cooling pattern. Both surface and deep waters are interpreted to have warmed 4 °C during this short-term warming event in the southern Indian and Atlantic oceans near Antarctica. The cause and extent of warming is currently unknown. A rise in atmospheric CO2 levels is suspected during this period, which may have given rise to the observed high-latitude warming through greenhouse effects.
To view the complete table of contents for the November issue of GEOLOGY, go to http://www.gsajournals.org/gsaonline/?request=get-current-toc&issn=0091-7613.
GSA TODAY Science Article
An Alternative Earth
Warren B. Hamilton, Department of Geophysics, Colorado School of Mines, Golden, Colorado 80401, USA.
Do hot "plumes" rising from Earth's deepest mantle operate in addition to plate tectonics, as widely assumed by earth scientists? Celebrated geologist Warren Hamilton of the Colorado School of Mines proposes an alternative Earth model wherein no such plumes exist, and that departs radically from many other standard ideas of Earth's evolution. In contrast to the popular concept that continents have grown through time, he provocatively suggests that near-planetwide continental-like crust formed early in Earth's 4.6-billion-year history, was too hot and weak to stand high like modern continents, and has since been mostly recycled into the mantle. Plate tectonics began only about 2 billion years ago, and as key to understanding plate motions and their drive, Hamilton spotlights the rollback of hinges of downgoing (subducting) ocean-floor plates. As these plates, made dense from top-down cooling by seawater, sink into the upper mantle, this alternative Earth model avers that they pull the approaching continents toward them, while pushing mantle back toward plate divergences behind the hinges. Subduction drives plates, which are not propelled from beneath. Hamilton contends that this circulation involves only the upper part of the Earth, above an uncrossable barrier at a depth of 660 km. He argues forcefully against the popular notion that subducting slabs sink deeper into the lower mantle.
Geological Society of America
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