Carbon isotopic composition of Neoproterozoic glacial carbonates as a test of paleoceanographic models for Snowball Earth phenomena.
Martin J. Kennedy et al. Department of Earth Science, University of California, Riverside Riverside, CA 92521-0423, USA. Pages 1135-1138.
The Snowball Earth hypothesis of P.F. Hoffman and colleagues proposes that during times of especially widespread glaciation ca. 600-750 Ma, the surface ocean became completely covered with ice, resulting in the mass extinction of marine life. The prospect that such events occurred more than once is intriguing for its implications about Earth's climate system and the possibility that snowball conditions acted as evolutionary bottlenecks, preventing the appearance of complex organisms. The authors test this idea by measuring the ratio of the carbon isotopes 13C and 12C within carbonate sediments that precipitated during ice ages in the supposedly ice-covered ocean. This ratio preserves a record of the ocean's organic productivity. According to the snowball Earth hypothesis, during times of exceedingly low productivity, the isotopic ratio ought to have approached mantle values of ~-5 to -6 per mil. The new data, obtained from multiple locations on three continents and for two different ice ages, show that the isotopic ratio is actually enriched with respect to 13C, dropping off sharply to negative values only after the end of each glacial event. These data challenge the core of Hoffman et al.'s hypothesis, and call for a different explanation for observed carbon isotopic trends. One possibility is that the post-glacial values reflect the widespread destabilization of methane hydrates, as proposed earlier this year by the authors.
Seismic character of volcanic activity at the ultraslow-spreading Gakkel Ridge.
Maya Tolstoy et al. Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964-8000. Pages 1139-1142.
Beginning in January 1999, an ~7 month period of intense seismic activity was observed on the ultraslow-spreading Gakkel Ridge in the Arctic ocean, with earthquake magnitudes as high as 5.4 and a total of 252 events recorded on global seismic networks. Analysis of these data, and sonar images from the U.S.S. Hawkbill suggest that the seismicity was associated with a seafloor eruption. This marks the first time an eruption has been observed at an ultraslow-spreading ridge. The event was unprecedented in duration and magnitude for a mid-ocean ridge eruption and indicates a negative power-law relationship between these parameters and spreading rate. The triggering of large events on the bounding-wall faults suggests that magma for this eruption may have been tapped directly from the mantle.
High-resolution analyses of an early Holocene climate event may imply decreased solar forcing as an important climate trigger.
Svante Bjorck et al. Lunds Universitet, Department of Quaternary Geology, Lund University, SE-223 63 Lund, Sweden. Pages 1107-1110.
Early Holocene lacustrine, tree-ring, ice-core, and marine records reveal that the Northern Hemisphere underwent a short cooling event ca. 10 k.y. The event coincides with a distinct Holocene thermohaline disturbance recognized in the North Atlantic Ocean. The authors found that the onset of the cooling coincides with an exceptionally distinct rise in Be-10 flux. This rise was most likely caused by markedly decreased solar forcing; the resultant question is: can such cooling be triggered by solar wind variations and changes in UV-radiation?
Namacalathus-Cloudina assemblage in Neoproterozoic Miette Group (Byng Formation), British Columbia-Canada's oldest shelly fossils.
Hans J. Hofmann Redpath Museum and Department of Earth and Planetary Sciences, and Eric W. Mountjoy, Department of Earth and Planetary Sciences, McGill University, Montreal, Quebec H3A 2A7, Canada. Pages 1091-1094.
Animals with shells first appeared ~550 Ma, before the Cambrian Period. They are represented by thin, tubular and globular fossils millimeters to centimeters in size, which were originally described from Namibia. The newly found occurrence in western Canada extends their known range halfway around the globe; it also provides additional insight into the diversity of life at a time prior to the explosive evolution in the Cambrian, and assists in the correlation of rock sequences between distant areas. The Canadian fossils are preserved in shallow-water, stromatolitic limestone that was left relatively unaltered.
Anatomy and origin of carbonate structures in a Miocene cold-seep field.
Ivano W. Aiello et al. Departments of Ocean Sciences and Earth Sciences, University of California, Santa Cruz, California, 95064, USA. Pages 1111-1114.
Structures that occur in 8 Ma sedimentary rocks (Miocene age) in Santa Cruz, California, are identical to structures that are formed on the present seafloor at sites where natural gas flows from sediments into the overlying ocean water. In recent years, submarine gas seepage has been recognized as a widespread phenomenon that may affect the chemistry of ocean water. The California structures are among the most completely exposed fossil seep sites known, and they provide unusually clear insights into the manner in which gases and fluids flow from sediments on the seafloor into the ocean.
Bunger Hills, East Antarctica: Ice free at the Last Glacial Maximum.
Damian B. Gore et al. Department of Physical Geography, Macquarie University, Sydney, New South Wales, 2109, Australia. Pages 1103-1106.
Bunger Hills, located on the coast of East Antarctica, was previously thought to have been covered by more than a half mile of ice during the Last Glacial Maximum 20 ka, a time when ice sheets covered much of Canada and the northernmost United States. However, new research using a technique known as Optically Stimulated Luminescence, which can date when quartz grains are buried and shielded from sunlight, has revealed that instead of being covered by a half mile of ice, the 100-square-mile Bunger Hills were much as they are today. This finding is at odds with the current understanding of Antarctic ice sheet characteristics during the Last Glacial Maximum, and forces a reconsideration of where the ice was distributed at that time.
Cosmogenic 3He and 10Be chronologies of the late Pinedale northern Yellowstone ice cap, Montana, USA.
Joseph Licciardi et al. Department of Geosciences, Oregon State University, Corvallis, Oregon 97331, USA. Pages 1095-1098.
New results suggest that the age of the last major advance of the northern Yellowstone ice cap occurred 16.5 k.y., or several thousand years earlier than previously considered. The ice cap then underwent rapid deglaciation so that by ca. 14 ka, it had largely disappeared.
Nutrient-triggered bioerosion on a giant carbonate platform masking the postextinction Famennian benthic community.
Arndt Peterhänsel and Brian R. Pratt, Department of Geological Sciences, University of Saskatchewan, Saskatoon, Saskatchewan, S7N 5E2, Canada. Pages 1079-1082.
The 365-million-year-old Palliser Formation of western Canada constitutes one of the largest carbonate platforms (150,000 cubic kilometers of limestone) to have ever developed on Earth. The authors' study revealed that a large part of the limestone particles have been destroyed by bioeroding microorganisms, thereby concealing the true diversity and abundance of skeleton-secreting sedentary fauna in this postextinction community. Profound changes in the biosphere at this time, such as increased soil formation accompanying the spread of deep-rooting seed plants, may have resulted in a disturbance of the ecological balance in inland seas. Analogous to modern tropical environments, enhanced nutrient mobilization and riverine nutrient flux originating from areas colonized by land plants in the north of the continent (today's Canadian Arctic) may have caused the proliferation of bioeroders on the Devonian western Canadian shelf. The dominant bioclasts identifiable after the alteration process are sea lily fragments (crinoids), and it is likely that the broad Palliser platform was characterized by meadows of these filter feeders. The authors conclude that the effects of these destructive processes are often overlooked, and poorly fossiliferous, fine-grained limestones may hide a more prolific and diverse community than is initially apparent. In addition, the degree of particle obliteration by bioerosion may serve as a proxy for nutrient availability in shallow carbonate environments.
Evidence of organic structures in Ediacara-type fossils and associated microbial mats.
Michael Steiner, Technische Universität Berlin, 13355 Berlin, Germany, and Joachim Reitner, Georg-August-Universität, Institut und Museum für Geologie und Paläontologie, Goldschmidtstrasse 3, 37077 Göttingen, Germany. Pages 1119-1122.
The authors investigated organic remains from late Precambrian Ediacara fossils. The remains of colonies of bacteria, some of which may have been sulfur-oxidizing and /or myxobacterial, show that they formed biofilms in close relationship with larger, possibly metazoan organisms. The main portions of the biofilms are preserved as pyrite formed by sulfate reduction and have a wrinkle structure called "elephant skin." In addition to the Ediacara biofilms, other larger prokaryotic structures were described and interpreted.
Rooted Brooks Range ophiolite: Implications for Cordilleran terranes.
Richard W. Saltus et al. U.S. Geological Survey, Denver, Colorado 80225, USA. Pages 1151-1154.
With the acceptance of the plate tectonics theory in the 1970s, geologic interpretation style was transformed. More and more often, rocks that were difficult to correlate tectonically with neighboring rocks were assumed to be out of place, and in many cases were postulated to have traveled great distances from their places of tectonic origin. For example, many packages of dark igneous (mafic) rocks throughout the western Cordillera have been interpreted as displaced pieces of oceanic crust. In this paper the authors examine gravity and magnetic data that show that one such package of mafic rocks, located in the Noatak region of the western Brooks Range, Alaska, is deeply rooted within the crust. In addition, the authors suggest that these rocks may have geologic affinities to the neighboring crustal rock section. Therefore, they argue that these rocks are not far-traveled as previously interpreted. They speculate that other appropriate analogs elsewhere in the western Cordillera may not be far-traveled either.
Sediment and rock strength controls on river incision into bedrock.
Leonard S. Sklar and William E. Dietrich, Department of Earth and Planetary Science, University of California, Berkeley, California 94720-4767, USA. Pages 1087-1090.
Rivers cut vertically through solid rock, making valleys and ultimately wearing away entire mountain ranges. How quickly this happens is generally thought to depend on how much water rivers carry, how steep they are, and the strength of the underlying bedrock. New theories suggest that the amount and size of the sediment that rivers carry also strongly influence how quickly they cut through rock. In a series of unique experiments, using rock taken from a wide variety of rivers, the authors show that sediment can either increase erosion rates by providing abrasive tools or inhibit erosion rates by burying and insulating underlying bedrock from the erosive forces of the flow. Furthermore, the experiments show that large sediment grains are more efficient at eroding rock than smaller grains, because small grains tend to travel suspended in the water column and impact the riverbed less frequently. Experimental erosion rates were also compared to rock strength measurements to reveal, for the first time, a quantitative relationship between rock strength and rock resistance to erosion. These findings suggest new ways of understanding how climate, tectonic uplift, and erosional processes interact to create the topography of Earth's surface.
Pattern of vertebrate extinctions across an event bed at the Permian-Triassic boundary in the Karoo Basin of South Africa.
Roger M.H. Smith, Division of Earth Sciences, South African Museum, Cape Town, 8000, South Africa, and Peter D. Ward, Department of Geological Sciences, University of Washington, Seattle, WA 98195, USA. Pages 1147-1150.
The extinction of vertebrates around the Permian-Triassic boundary has long been regarded as a gradual event occurring over hundreds of thousands to millions of years. The authors' new field investigations of fluvial strata in the central and southern Karoo Basin of South Africa have revealed the presence of an event bed coinciding with a mass extinction of terrestrial fauna and flora. This occurs within a sedimentary sequence that is marked by a reddening of floodplain mudrocks and a change from high- to low-sinuosity river channel systems. The authors show that the pattern of vertebrate taxa disappearing below this boundary, and the subsequent appearance of new taxa above the boundary, are consistent with a relatively sudden, possibly catastrophic event that lasted 50 k.y or less.
Thick-Structured Proterozoic Lithosphere of the Rocky Mountain Region.
Ken Dueker, Huaiyu Yuan, and Brian Zurek, University of Wyoming.
This paper provides a fundamentally new image of earth's mantle beneath the Rocky Mountains and leads to new insights into both how continental lithosphere was assembled in the Precambrian and how the old assembly structures have influenced subsequent tectonism, including ongoing reorganization of small-scale convection in the asthenosphere. It presents two especially important results. (1) The chemical lithosphere under the Rockies is still very thick (>200 km) in spite of the fact that the thermal lithosphere is thinning due to younger tectonism related to the present North American plate margin. Thus, we can study the process of thinning of lithosphere and we find that the old anisotropies are very important in controlling melt migration and the beginnings of asthenospheric convection as lithosphere is converted to asthenosphere. 2) One of the main paleosutures of the Rockies, the Cheyenne belt suture between Archean and Proterozoic lithospheres, has a north-dipping, high-velocity zone under it that the authors interpret to be a paleosubduction zone. This offers an important reinterpretation of the collision process involving flipping subduction zones during accretion of arcs into the growing North American continent. Together, these data are an important step in better understanding the evolution of the continental lithosphere in North America.
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