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June 2009 Geology and GSA Today media highlights

Geological Society of America

Boulder, CO, USA - GEOLOGY includes details on the 12 May 2008 Wenchuan earthquake; a natural gas-hydrate system offshore of Korea; findings that abiogenic methane emissions may be more prevalent than originally thought; two studies on the nature of mud; and three fossil studies, one finding evidence for lush forests and rich animal life in the Eocene High Arctic, and two concentrating on bones in Montana, Madagascar, and Sharktooth Hill, California. GSA TODAY examines Greenland's contribution to sea-level change.

Bedload transport of mud by floccule ripples -- Direct observation of ripple migration processes and their implications
Juergen Schieber and John B. Southard, Dept. of Geological Sciences, Indiana University, Bloomington, Indiana 47405, USA. Pages 483-486.

It is a widespread assumption among geoscientists that muds accumulate by settling after suspension in quiescent waters. Recent flume work has shown, however, that mud can accumulate from swift-moving suspensions because floccules form that travel in bedload in the form of ripples. Observing the sediment movement in these ripples is hampered by the inherently turbid nature of muddy suspensions. Schieber and Southard have now succeeded in imaging sediment movement associated with these floccule ripples. Floccule ripples, although having a water content near 90% (by volume), have cross sections comparable to sand ripples, and the transport of sediment across these ripples is directly analogous to the processes observed for sandy ripples. Thus, even though the particles that compose floccule ripples are fragile and significantly less dense than sand grains, transport processes and morphologies are the same in both cases. This is the first direct observation of particle movement and migration processes in bedload ripples of flocculated mud.

Truncation and translation of Appalachian promontories: Mid-Paleozoic strike-slip tectonics and basin initiation
James Hibbard and John W.F. Waldron, Dept. of Marine, Earth and Atmospheric Sciences, Jordan Hall, Faucette Drive, North Carolina State University, Raleigh, North Carolina 27695, USA. Pages 487-490.

The wave-like structural trend, or "grain," of the Appalachian mountain belt was inherited from the original shape of the eastern North American margin after the break up of the supercontinent Rodinia. This shape was imprinted on multiple island arcs and microcontinents that accreted to eastern North America between about 500 and 300 million years ago to form the Appalachians. In the Carolinas and in Newfoundland, the wave-like trend of the mountain belt in the more easterly portions of the Appalachians is offset from that inland. Hibbard and Waldron propose that this offset is the result of more than 200 km of displacement along a fault system that was operative along the entire eastern margin of North America approximately 360 million years ago.

Drainage basin response to climate change in the Pisco valley, Peru
Damian Steffen et al., Institute of Geological Sciences, University of Bern, CH-3012 Bern, Switzerland. Pages 491-494.

The Quaternary development of the Pisco valley in central Peru has been characterized by multiple phases of sediment accumulation and erosion, which formed distinct levels of cut-and-fill terraces and alluvial fans. Luminescence dating shows that they were formed in response to at least two different stages of sediment accumulation and erosion during the last 60 thousand years, with the main phase of sediment aggradation occurring between ~54-38 thousand years ago. The ages show that sediment accumulation was contemporaneous with the time intervals of the Minchin (47.8-36 thousand years ago, with enhanced precipitation beginning around 54.8 thousand years ago) and Tauca (26-14.9 thousand years ago) paleolakes on the Altiplano, where the headwaters of the Pisco River are located. Steffen et al. conclude that sediment accumulation was triggered by shifts toward a more humid climate, whereas erosion is the response of the fluvial system to the depletion of the hillslope sediment reservoirs.

Excess methane in continental hydrothermal emissions is abiogenic
Jens Fiebig et al., Institut fur Geowissenschaften, Goethe-Universitat, Altenhoferallee 1, 60438 Frankfurt am Main, Germany. Pages 495-498.

It is commonly assumed that nature provides conditions suitable for hydrocarbon production through the abiogenic reduction of carbon oxides. The results of Fiebig et al. suggest that the overwhelming proportions of methane released from CO2-rich continental-hydrothermal systems may indeed be abiogenic in origin. The co-occurring hydrocarbons higher than methane, however, are not generated through the same abiogenic reaction. From hydrocarbon distribution patterns, it cannot be excluded that they originated from the thermal degradation of small amounts of organic matter instead. These findings suggest that abiogenic methane is more widespread than previously assumed. Its maximum contribution to the global scale of methane emissions is estimated at one percent. On the contrary, it remains unresolved if organic compounds other than methane can be generated abiogenically in nature.

Lower-latitude mammals as year-round residents in Eocene Arctic forests
J. Eberle et al., University of Colorado Museum and Dept. of Geological Sciences, Univ. of Colorado, Boulder, Colorado 80309, USA. Pages 499-502.

At present, Arctic regions are characterized by sparsely vegetated tundra, permafrost, ice sheets, and mostly small terrestrial fauna. These areas were much warmer during "hothouse" climate states, such as the early Eocene (~53 million years ago), and on Canada's Ellesmere Island (nearly 80 degrees north latitude), there is fossil evidence for lush forests, alligators, giant tortoises, and a variety of mammals including primates, tapirs, and hippo-like Coryphodon. Occurrence of these animals in the early Eocene High Arctic raises interesting questions: Did they live in polar regions year-round or did they migrate there seasonally? If they stayed, what did they eat during extended periods of winter darkness? To address these questions, Eberle et al. analyzed carbon and oxygen isotope ratios from the teeth of Coryphodon, tapirs, and rhinoceros-like brontotheres. They determined that these large land mammals probably overwintered in the Eocene Arctic by subsisting on a variety of foodstuffs ranging from leaf litter to evergreens to phytoplankton. Recognition that large land mammals could survive prolonged polar darkness is important because such behavior is a prerequisite to dispersal across northern polar land bridges (such as Beringia), the probable migratory routes into North America for many modern mammalian orders originating in Asia or Europe during the Eocene. It also provides a plausible "deep-time" model for how today's mammals will adapt to warming in the future.

Recycling detrital zircons: A case study from the Cretaceous Bisbee Group of southern Arizona
William R. Dickinson et al., Dept. of Geosciences, University of Arizona, Tucson, Arizona 85721, USA. Pages 503-506.

Uranium-lead ages of detrital zircon grains in sand and sandstone are commonly used as a guide to the locations of the igneous source rocks for the zircons because the isotopic ages are not reset by any processes in the sedimentary realm. Zircon crystals are so resistant to weathering and diagenesis, however, that erosional recycling of zircon grains from sedimentary rocks is a common phenomenon. Dickinson et al. show conclusively that the population of detrital zircons present in sandstone can be derived by recycling from older sedimentary strata without any change in the age signature inherited from the ultimate igneous source rocks for the zircons. Failure to allow for recycling of zircon grains through multiple sedimentary cycles of erosion and deposition can lead to misinterpretations of sand provinces.

Contourite depositional system on the Argentine Slope: An exceptional record of the influence of Antarctic water masses
F. Javier Hernandez-Molina et al., Facultad de Ciencias del Mar, Universidad de Vigo, 36200 Vigo, Spain. Pages 507-510.

Deep-water circulation generates contour-following currents that are sometimes strong enough to profoundly affect sedimentation, from winnowing of fine deposits to developing large-scale depositional or erosive features. The deposits generated by such along-slope currents are known as contourites or contourite drifts, and the depositional and erosive features associated may develop into a contourite depositional system (CDS). Hernandez-Molina et al. identify, for the first time, significant influence of Antarctic-sourced water masses, on the continental slope of the southern Argentine margin. Interaction of these water masses with the seafloor is complex, generating an exceptional example of a large CDS. This system started to develop at the time of the opening of the Drake Passage (Eocene-Oligocene boundary). However, the present margin morphology is much younger, developed after a major paleoceanographic change in the middle to late Miocene, due to major water masses reorganization on the Southern Hemisphere.

Visualizing fossilization using laser ablation-inductively coupled plasma-mass spectrometry maps of trace elements in Late Cretaceous bones
Alan E. Koenig et al., U.S. Geological Survey, Box 25046, MS-973, Denver Federal Center, Denver, Colorado 80225, USA. Pages 511-514.

Elemental maps generated by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) provide a previously unavailable high-resolution visualization of the complex physicochemical conditions operating within individual bones during the early stages of diagenesis and fossilization. In this study by Koenig et al., a selection of LA-ICP-MS maps of bones collected from the Late Cretaceous of Montana and Madagascar graphically illustrate diverse paths to recrystallization, and reveal unique insights into geochemical aspects of taphonomic history. Some bones show distinct gradients in concentrations of rare earth elements and uranium, with highest concentrations at external bone margins. Other bones exhibit more intricate patterns of trace element uptake related to bone histology and its control on the flow paths of pore waters. Patterns of element uptake as revealed by LA-ICP-MS maps can be used to guide sampling strategies, and call into question previous studies that hinge upon localized bulk samples of fossilized bone tissue. LA-ICP-MS maps also allow for comparison of recrystallization rates among fossil bones, and afford a novel approach to identifying bones or regions of bones potentially suitable for extracting intact biogeochemical signals.

Coseismic reverse- and oblique-slip surface faulting generated by the 2008 Mw 7.9 Wenchuan earthquake, China
Xiwei Xu et al., Institute of Geology, China Earthquake Administration, Beijing 100029, China. Pages 515-518.

The Mw 7.9 Wenchuan earthquake occurred at 2:28 PM LT (6:28 UTC) on 12 May 2008, striking Wenchuan, Beichuan, and Qingchuan counties, located in the northwestern part of the Sichuan province. This earthquake was the most devastating earthquake in China in the past three decades. As of 29 September 2008, 69,227 deaths had been confirmed, with 374,643 people injured and 17,823 people missing. The maximum meizoseismal intensity (MMI) reached XI around Yingxiu Town, Wenchuan County, and Beichuan Town, Beichuan County, where 80% of homes were destroyed during the quake. Field investigations by Xu et al. reveal that the Mw 7.9 Wenchuan earthquake ruptured an imbricated thrust fault system, activating different faults of the system at the same time. The earthquake produced a 240-km-long rupture along the Beichuan fault. The rupture is characterized by oblique faulting, mixing reverse motion and horizontal motion. An additional 72-km-long rupture, along the Hanwang segment, part of the Pengguan fault, is characterized by almost pure reverse faulting. The average vertical offset of the surface rupture zone along the Beichuan rupture zone reaches 3 to 4 meters. Along the 135-kilometer-long northern section, a maximum vertical offset of 6.5 plus or minus 0.5 meters has been measured at Beichuan Town, and a maximum right-lateral offset of 4.9 plus or minus 0.5 meters at Pingtong Town. The maximum vertical offset on the 105-kilometer-long southern section is 6.2 plus or minus 0.5 meters at Shenxi Village, near Yingxiu Town. A maximum vertical offset of 3.5 meters was measured along the Hanwang rupture zone. A 3-D model for rupture geometry shows that the Beichuan and Hanwang rupture zones appear to merge at depth. The total crustal shortening accommodated by the Wenchuan earthquake may reach 8.5 meters, and the total vertical uplift may reach 7.5 meters. The oblique thrusting accomplished by the earthquake indicates that the east-southeastward extrusion of the Tibetan Plateau is transformed into crustal shortening and uplift along its eastern margin that is responsible for the growth of high topography in the region.

Origin of a widespread marine bonebed deposited during the middle Miocene Climatic Optimum
Nicholas D. Pyenson et al., Museum of Paleontology and Dept. of Integrative Biology, University of California-Berkeley, Berkeley, California 94720-4780, USA. Pages 519-522.

Bonebeds are unusual rock layers that are, in large part, composed of vertebrate remains, and they can form in both terrestrial and marine settings. One particular example, the Sharktooth Hill bonebed, covers over 15 km (>9 miles) in the southern part of the Central Valley, California, preserving perhaps millions of bones from fossil marine mammals, sharks, and turtles in a thin, dense layer. How did such an expansive and rich marine bonebed form? Pyenson et al. draw together data from fossil localities, museum collections, and earth history to test different ideas about the bonebed's origin. In contrast to previous arguments for a one-time, catastrophic event, Pyenson et al. argue that the bonebed formed over thousands of years, during a prolonged period of exceptionally warm global temperatures between 17 and 14 million years ago. These changes in temperature were associated with major changes in global sea level, which may have directly led to the entombing and preservation of the Sharktooth Hill bonebed in the geologic record.

Simultaneous generation of Archean crust and subcratonic roots by vertical tectonics
C.M.I. Robin and R.C. Bailey, Departments of Physics and Geology, 60 St. George Street, Toronto, Ontario, M5R1A7, Canada. Pages 523-526.

The theory of plate tectonics, proposed by Tuzo J. Wilson in 1965, explains many large-scale features we see on Earth as the result of the horizontal motion of cold, hard surface plates over a hot convecting mantle. It has shaped our current understanding of how most of the present Earth's geology came to be. However, plate tectonics as we know it may not have operated in the hot early days of the Earth (during the early Archean eon, more than 2.5 billion years ago), when warm but dense mantle-derived volcanic rocks would have been softer and capable of sinking back down into the crust. Indeed, the geological record left by early Archean tectonic events show structures rarely observed in modern geological environments, such as diapir-like granite-greenstone belt structures. The numerical simulations of these structures by Robin and Bailey show that under thermal conditions appropriate for that time, vertical motion of crustal material could have been much more important than horizontal plate-dominated motion. In turn, such rapid and efficient vertical return of upper-crustal volcanic rocks to the lower crust by this diapiric process, followed by their subsequent partial re-melting, may help explain some of the enigmatic chemical differences between continental crust and the deeper Earth's mantle, without requiring the large-scale horizontal forces associated with modern plate tectonics.

Subtropical coral reveals abrupt early-twentieth-century freshening in the western North Pacific Ocean
Thomas Felis et al., MARUM-Center for Marine Environmental Sciences, University of Bremen, Bremen, Germany. Pages 527-530.

Sea-surface temperature and salinity play key roles in ocean circulation and therefore are major factors in global climate. Instrumental observations are scarce, particularly on salinity, complicating the identification of twentieth-century climate change. Felis et al. present a reconstruction based on the chemical composition of a 130-year-old coral from the western North Pacific Ocean that suggests a major shift in salinity took place between A.D. 1905 and 1910. Annually banded corals have previously been used to reconstruct climate in the tropics. However, at higher latitudes the application of this method is often impossible -- stony corals do not survive prolonged exposure to cool temperatures. But this new coral material has been recovered off the Ogasawara Islands (Japan), where warm waters are provided by the northward-flowing Kuroshio Current. Observations from weather stations reveal a shift in the atmospheric circulation over Asia and the North Pacific at about the time of the salinity change, suggesting that the ocean responded to changing winds. The new information on the interaction of ocean and atmosphere will expand our knowledge of how climate works, and may also improve the predictability of future climate changes.

Migration and venting of deep gases into the ocean through hydrate-choked chimneys offshore Korea
R.R. Haacke et al., Geological Survey of Canada, Sidney, British Columbia V8L 4B2, Canada E-mail: Pages 531-534.

Formation of gas hydrate in the seabed acts as a barrier to rising natural gas, inhibiting its expulsion through the seafloor and into the ocean. In some cases, however, the rising gas can penetrate the gas hydrate zone and enter the ocean, where it affects the chemistry of both the ocean and the atmosphere. In these cases, the gases form chimneys that are choked with, but not plugged by, high concentrations of gas hydrate. This hydrate is close to the seabed and is thus accessible to energy prospectors interested in the large amounts of natural gas held in the hydrate. It is also an important factor in the global carbon cycle because its proximity to the seabed means it is affected relatively quickly by ocean warming. Haacke et al. provide images of the complete gas-venting system, including the hydrate-choked chimney itself and the underlying zone of source gases. They compare two adjacent gas vents that appear to have high and low fluxes, respectively, and show how the seismic character of these two vents differ. They suggest that ocean warming vigorous enough to thin the regional gas hydrate stability zone will act to reduce the height of the barrier presented to rising gases by the formation of hydrate, enabling deep gas reservoirs to more efficiently vent gases at the seabed and alter the chemistry of the ocean-atmosphere system. This is the key response of the natural gas-hydrate system to climate change rather than a melting of the hydrate itself.

When do black shales tell molybdenum isotope tales?
G.W. Gordon et al., School of Earth & Space Exploration, Arizona State University, Tempe, Arizona 85287-1404, USA. Pages 535-538.

Molybdenum isotopes in ancient black shales have been used to tell us about the amount of oxygen in the world's oceans in the past. Lighter isotopes preferentially go into oxic sediments, while sediments laid down in water columns with high sulfide reflect the isotopic composition of the water. By looking at two similar sequences of Devonian rocks (about 380 million years old), Gordon et al. constrain what types of organic-rich sediments record the signal from ancient seas. The evidence is consistent with larger areas of the seafloor having low oxygen in the middle Devonian.

Tide- and wave-generated fluid mud deposits in the Tilje Formation (Jurassic), offshore Norway
Aitor. A. Ichaso and Robert W. Dalrymple, Dept. of Geological Sciences and Geological Engineering, Queen's University, Kingston Ontario K7L 3N6, Canada. Pages 539-542.

"Mud" is the most abundant sediment in the world. Many people think that the very fine-grained particles that make up mud travel suspended in the water column, disperse widely once a river reaches the sea, and accumulate slowly by settling from suspension. However, recent research in a range of coastal settings has discovered that mud commonly accumulates rapidly at the bottom of channels and on the continental shelf in the form of dense, viscous bodies called "fluid mud" -- something akin to a runny chocolate mousse in texture -- thicker than dirty water, but not as thick as a solid mud deposit. Ichaso and Dalrymple report the occurrence of such fluid-mud deposits in ancient sedimentary rocks where they have been "fossilized." They provide ways to recognize such deposits, and guidelines for using them to create more refined interpretations of sedimentary successions, some of which contain significant quantities of oil and gas.

Water in enstatite from Mid-Atlantic Ridge peridotite: Evidence for the water content of suboceanic mantle?
Jurgen Gose et al., Universitat Erlangen-Nurnberg, GeoZentrum Nordbayern, Schlossgarten 5a, D-91054 Erlangen, Germany. Pages 543-546.

An ocean in the suboceanic mantle? It has been known that water has the ability to enter the structure of nominally anhydrous minerals, where it forms OH defects. If defect water in mantle minerals amounts to several hundred weight-parts per million, Earth's mantle, because of its volume, may accommodate an amount of water similar to the hydrosphere. Gose et al. present the first data on the water content in the suboceanic Earth's mantle. The mantle-derived rocks are exposed on the ocean floor close to the Mid-Atlantic Ridge and were sampled by drilling during the Ocean Drilling Program, Leg 153. Gose et al.'s investigations yielded water content in enstatite in the range between 160 and 270 weight-parts per million. These new data reflect relatively high water content in the suboceanic mantle.

Cenozoic tectonic and topographic evolution of the northern Sierra Nevada, California, through stable isotope paleoaltimetry in volcanic glass
Elizabeth J. Cassel et al., Dept. of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA. Pages 547-550.

In a study by Cassel et al., the past elevation of the northern Sierra Nevada (California) was determined based on the isotopic composition of ancient rainwater preserved in volcanic deposits. Ignimbrites (volcanic ash-flow tuffs) deposited 30-28 million years ago were sampled from the eastern edge of the California Great Valley east across the crest of the range into Nevada. A significant decrease in the isotopic composition of hydrogen in ancient rainwater preserved in these deposits reflects the effect of past high elevations on precipitation. These data show that 30-28 million years ago the northern Sierra Nevada had elevations similar to the present and likely has been a high topographic feature for the past 60 million years. In addition, these ignimbrites flowed across what is now the crest of the range, indicating that there was no drainage divide between central Nevada volcanic sources and the California Great Valley.

Clastic and core lava components of a silicic lava dome
G.Wadge et al., Environmental Systems Science Centre, University of Reading, Reading RG6 6AL, UK. Pages 551-554.

Lava domes are dangerous features of some volcanoes. Very viscous lava stays close to the eruptive vent and tends to break up into fragments during rockfall. When larger pieces of still-hot and gas-rich lava break away, they form hazardous pyroclastic flows. Wadge et al. report that the lava domes produced over several years at Soufriere Hills volcano in Montserrat convert between 50% and 90% of the lava extruded into these fragmental products, leaving behind a near-cylindrical core of lava that retains its energy for a long time.

Pyroxenite-rich mantle formed by recycled oceanic lithosphere: Oxygen-osmium isotope evidence from Canary Island lavas
James M.D. Day et al., Dept. of Earth Sciences, University of Durham, Durham DH1 3LE, UK. Pages 555-558.

Although materials recycled into Earth's mantle during subduction can explain much of the geochemical variations observed in oceanic islands such as the Canary Islands and Hawai'i, it has been shown that direct melting of these components is unlikely. Instead, it has been argued that oceanic islands inherit their geochemical character from complex melting interactions between subducted materials in Earth's mantle. New evidence presented by Day et al. from oxygen and osmium isotope analysis of lavas from the western Canary Islands of El Hierro and La Palma strongly supports this view. Furthermore, careful assessment of oxygen and osmium isotope variations in lavas from Hawai'i and the Azores show that they also conform to the addition of distinct subducted components. These observations provide powerful evidence for the importance of subducted materials in explaining the geochemical diversity and existence of many oceanic islands.

Melt inclusions track pre-eruption storage and dehydration of magmas at Etna
S.J. Collins et al., Dept. of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK. Pages 571-574.

Mount Etna, Sicily, is a persistently degassing volcano responsible for a large flux of sulfur dioxide and carbon dioxide to the atmosphere. Recent eruptions, which show marked changes in style, were investigated by Collins et al. using small pockets of melts trapped within crystals. Gases released at depth travel through the plumbing system of the volcano and effect the magma's volatile composition, driving out water and leaving magmas which appear enriched in carbon dioxide. Metals such as copper also act volatile in this system, and are transported through the system by vapor bubbles, and this process may release copper to the atmosphere.

GSA Today Science Article

Back to the future: Greenland's contribution to sea-level change
Antony J. Long, Dept. of Geography, Durham University, Science Site, South Road, Durham DH1 3LE, UK. Pages 4-10.

Global sea level is rising, quickly. The current rate of sea-level rise is greater than 3 mm per year, up from an average rise of less than 2 mm throughout much of the past century. Will the rate of sea-level rise continue to increase? If so, what is driving the increase? Will sea level rise by half a meter by the end of the century, or by much more? These are some of the questions addressed by Dr. Anthony Long of Durham University. Professor Long focuses his search for answers on a key suspect: the Greenland Ice Sheet. Satellite data now provide for detailed calculations of the annual change in the mass of the ice sheet and suggest that the Greenland Ice Sheet is indeed a major contributor to current sea-level rise. However, geological evidence constraining the evolution of the ice sheet over the past few millennia paints a more complex picture. The ice sheet has been, and will continue to be, a dynamic entity that responds sensitively to changes in the North Atlantic. Understanding the interplay between the ice sheet and its surrounding environs is crucial for longer term predictions of sea level rise.


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