|13 May 2010
GSA Release No. 10-24
Director - GSA Communications & Marketing
GSA Bulletin Highlights
Boulder, CO, USA – The July-August GSA Bulletin includes several articles on the nature of the continental crust; mountain building; and landscape evolution as driven by tectonics, erosion, wind, and climate. Highlights include the complicated geochemical evolution of groundwater in a major east-central Illinois aquifer; the architecture of an ancient gas hydrate stability zone; microbes and coalbed methane in north-central Louisiana; the Permian-Triassic "death by anoxia" extinction scenario; and the largest asteroid break-up event known for the last billion years of Earth's history.
Highlights are provided below. View abstracts at http://gsabulletin.gsapubs.org/content/current. Representatives of the media may obtain complimentary copies of GSA BULLETIN articles by contacting Christa Stratton at the address above. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GSA BULLETIN in articles published. Contact Christa Stratton for additional information or assistance.
Non-media requests for articles may be directed to GSA Sales and Service, .
Slow-slip phenomena in Cascadia from 2007 and beyond: A review
Joan Gomberg et al., U.S. Geological Survey, University of Washington, Dept. of Earth & Space Sciences, Box 351310, Seattle, Washington 98195-1310, USA. Pages 963-978.
The serendipitous discovery and recognition of the significance of slow-slip phenomena have revolutionized scientists’ views of how the relative motions between tectonic plates are accommodated along major plate-boundary faults. Traditionally, these faults were thought either to slip continuously without stresses building, or to be stuck causing stresses to build to their breaking point when the fault would rupture in an earthquake. In earthquakes, all the accumulated motion occurs as fault slip within seconds to minutes and seismic wave radiate. We now know that this “either-or” model is too simple, as revealed in observations and models of slow-slip phenomena. Some of these phenomena are manifest as transient deformations of the Earth’s surface lasting days to months measured using GPS and other instrumentation. A previously unrecognized class of seismic signals recorded on seismograph networks often accompanies these slowly evolving deformations. The primary origins of slow-slip phenomena appear to be slow fault slip on a plate interface (a fault) that is stressed almost to failure levels or is very weak, probably facilitated by processes involving fluids. The Cascadia subduction zone, located in the Pacific Northwest of the coterminous United States and adjacent Canada, is an archetype of exploration and learning about slow-slip phenomena. In Cascadia and other plate boundaries, this transient slip occurs at depths below the locked region of plate interface that hosts great earthquakes (e.g. like the 2004 M9.1 Sumatra and 2010 M8.8 Chilean earthquakes), where information was previously elusive. Although not dangerous itself, slow-slip affects the energy budget of the entire subduction zone and bounds the locked, earthquake-producing regions of the plate interface. Ongoing studies continue to reveal new insights about how accrued plate motion stresses are relieved along the entirety of Cascadia, and at much smaller scales. Our new understanding provides key constraints on the probable characteristics of future great earthquakes in Cascadia. This team from the U.S. Geological Survey, University of Washington, Dept. of Earth & Space Science provides a comprehensive review of slow-slip phenomena in Cascadia, focusing on what has been learned since 2007, when major investments in monitoring and temporary deployments dramatically increased the quality and quantity of available data.
Presence of Permian extension- and arc-type magmatism in southern Tibet: Paleogeographic implications
Di-Cheng Zhu et al., State Key Laboratory of Geological Processes and Mineral Resources, and School of Earth Science and Resources, China University of Geosciences, Beijing 100083, China. Pages 979-993.
Paleogeography, the study of geography in relation to geological history, is critical for the development of the concept of continental drift and the theory of plate tectonics. Reconstruction of the geographic locations of continents and oceans in geological history is not straightforward due to often incomplete preservation of paleogeographic record (such as geodynamic, paleomagnetic, paleontological, lithologic, and magmatic). For example, the Permian paleogeography of the Lhasa Terrane remains a subject of debate. It was traditionally placed adjacent to Greater India in popular models of Paleozoic reconstruction, depicting a vast shallow-marine sea from the Qiangtang Terrane via the Lhasa Terrane to the Greater India and Himalaya south of the Paleo-Tethyan Ocean. Zhue et al. describe the presence of Permian extension-related magmatism in the Tethyan Himalaya and continental arc-related magmatism in the Lhasa Terrane, the first comprehensive documentation in present-day southern Tibet. They thus suggest t
Authigenic carbonates in Upper Miocene sediments of the Tertiary Piedmont Basin (NW Italy): Vestiges of an ancient gas hydrate stability zone?
Francesco Dela Pierre et al., Dipartimento di Scienze della Terra, Universita di Torino, Via Valperga Caluso, 35-10125 Torino, Italy. Pages 994-1010.
Gas hydrates are solid compounds made up of water and gas (generally methane) that have been documented worldwide in marine sediments by a wealth of studies carried out with different methodologies. In the past few decades, interest in gas hydrates has increased due to their potential use as an energy source and their role in climate change and geologic hazard. Gas hydrate-associated authigenic carbonates have been sampled at present-day settings where they form in direct contact with gas hydrates exposed on the sea floor or a few meters below. These carbonates, recently defined as clathrites, represent natural archives of the processes responsible for the formation and dissociation of gas hydrates. By converse, very few examples of fossil clathrites have been documented. In this paper, Dela Pierre et al. describe some carbonate-rich rocks recently discovered in upper Miocene slope sediments of the Tertiary Piedmont Basin (northwest Italy), which, they propose, represent a solid evidence of the past formation of gas hydrates within the sedimentary column, of their destabilization, and of the migration of the resulting hydrocarbon-rich fluids toward the sea floor. The study of these rocks provides an opportunity to illustrate the architecture of an ancient gas hydrate stability zone, and to evaluate the processes that acted (and likely still act) within this zone.
Evaporitic paleosols in continental strata of the Carroza Formation, La Popa Basin, Mexico: Record of Paleogene climate and salt tectonics
Brenda J. Buck et al., Dept. of Geoscience, University of Nevada, Las Vegas, Nevada 89154, USA. Pages 1011-1026.
Scientists have found a new way to track the severity and extent of climate change in the rock record. Ancient soils from the Eocene Epoch in La Popa Basin (northern Mexico) show that very dry climates (where salty water evaporated quickly) have left behind telltale salt minerals and related unique features. Before this study, one of the main ways used to say that an area had been arid was to look for calcium carbonate. Buck et al. have proven that calcium carbonate may not always be present in arid environments and climate scientists can focus on salts and these unique features in ancient soils to measure climate change.
Calcite precipitation driven by the common ion effect during groundwater-surface-water mixing: A potentially common process in streams with geologic settings containing gypsum
Li Jin et al., Dept. of Earth Sciences, Syracuse University, Syracuse, New York 13244, USA. Pages 1027-1038.
Jin et al. report the results of a synoptic ("snapshot") sampling of inorganic and isotopic geochemistry of surface water and groundwater during base flow in Red Canyon Creek watershed (Wyoming, USA) to evaluate how mixing of waters and geochemical processes modify stream-water chemistry. Their synoptic approach of studying the geochemistry of a stream mimics and has the same strengths of that widely used to characterize geochemical processes in groundwater systems. Gypsum dissolution, carbonate precipitation, and the influx of tributary and groundwater all affect Red Canyon Creek stream-water chemistry. The results of this study suggest that calcareous accumulations in the fluvial sedimentary record, in association with gypsum, may possibly be paleohydrologic proxies for environments where mixtures of surface water with different chemical composition occurred.
The mid-Ordovician Osmussaar breccia in Estonia linked to the disruption of the L-chondrite parent body in the asteroid belt
Carl Alwmark et al., Dept. of Geology, University of Lund, Solvegatan 12, SE-22362 Lund, Sweden. Pages 1039-1046.
Alwmark et al. present findings of extraterrestrial material, in the form of chromite, in the ~466 million-year-old Osmussaar Breccia. The L-chondritic composition and angular morphology of the chromite implies that the breccia is not, as previously suggested, reworked material from the nearby Neugrund structure but instead is the result of a coeval impact of an L-chondrite body. This is an important finding because it shows that the breccia most likely is linked to the disruption of the L-chondrite parent body in the asteroid belt at about 467 million years ago. This is the largest asteroid break-up event known for the last billion years of Earth's history. The findings by this team from the University of Lund and the University of Tartu strengthen the hypothesis that the influx of larger bodies to Earth was dramatically increased during the time following the breakup of the L-chondrite parent body and this in turn gives support to the recent suggestion that abundant coeval megabreccias worldwide are impact triggered. The presence of extraterrestrial chromite also strengthens the fact that physical pieces of a large celestial body can survive upon impact with Earth.
Chemical and isotopic indicators of groundwater evolution in the basal sands of a buried bedrock valley in the Midwestern United States: Implications for recharge, rock-water interactions, and mixing
Keith C. Hackley et al., Illinois State Geological Survey, Institute of Natural Resource Sustainability, University of Illinois, 615 E. Peabody Dr., Champaign, Illinois 61820, USA. Pages 1047-1066.
Isotopic and chemical data collected from a major aquifer in east-central Illinois indicate a fairly complicated geochemical evolution for the groundwater. The geochemical data vary substantially across the aquifer, indicating influences of various biogeochemical reactions as well as groundwater mixing. Some parts of the aquifer show little obvious microbial influence, whereas others show strong evidence of microbial activity, including sulfate reduction and methanogenesis. Combining the carbon-14, tritium, stable isotope, and chemical data for the various components of the groundwater allow for a more complete understanding of the reactions that control the bulk chemistry and helped delineate locations of recharge. A conceptual model helps delineate the areas of major recharge represented by younger, more dilute waters compared to other areas where the aquifer is more isolated relative to surface recharge and where older groundwater containing greater dissolved constituents from bedrock units have impacted the aquifer. Buried bedrock valley aquifers exist throughout North America and as population and industry increase, it is important know the locations of major recharge zones and to understand what controls the chemistry of these valuable water resources.
Fjords as temporary sediment traps: History of glacial erosion and deposition in Muir Inlet, Glacier Bay National Park, southeastern Alaska
Ellen A. Cowan et al., Dept. of Geology, Appalachian State University, Box 32067, Boone, North Carolina 28608, USA. Pages 1067-1080.
Cowan et al. combine glacial geology, sedimentology, and seismic stratigraphic data to interpret the Quaternary to recent history of Muir Inlet. The sequence stratigraphy within this Alaskan fjord is interpreted from two sets of high-resolution seismic reflection profiles with nested resolution collected over twenty years apart. The first set, collected by the U.S. Geological Survey (USGS) in 1978–1980, imaged sediment over 100 m thick deposited during post-Little Ice Age (LIA) glacial retreat over the past 125 years. In 2004, a GI-gun profile collected down the axis of Muir Inlet imaged up to 300 m of previously unknown sediment below the acoustic basement on the USGS profiles. These sediments are interpreted to have been deposited during retreat from the Last Glacial Maximum, the largest known advance in the region prior to the LIA. This paper contributes to the ongoing debate of the effects of glacial erosion in the active tectonic margin of Coastal Alaska. Proglacial fjord basins are known to be effective sediment traps for the products of glacial erosion and the LIA sediment within fjords in Southern Alaska has been used as a proxy for bedrock erosion. Cowan et al. caution that a significant volume of LIA deposits are eroded from LGM sediment stored within the basin. In addition, they are able to document for the first time in Alaska evidence for a complete glacial cycle where large ice streams advance onto the continental shelf and scour fjord basins to bedrock, whereas smaller advances (i.e., LIA) did not evacuate the fjord fill.
A 48 m.y. history of fracture opening, temperature, and fluid pressure: Cretaceous Travis Peak Formation, East Texas basin
S.P. Becker et al., Bureau of Economic Geology, University of Texas at Austin, University Station Box X, Austin, Texas 78713-8924, USA. Pages 1081-1093.
Open fractures are essential for the commercial production of hydrocarbons from deeply buried sandstone reservoirs. Using scanning-electron microscope-cathodoluminescence, Becker and colleagues examined the quartz cement partially filling such fractures at 3 km depth in the East Texas basin. This technique reveals the layered incremental growth texture of cement, which is inferred to grow during slow fracture opening. The quartz cement layers contain 1-10 micrometer-sized inclusions of methane and aqueous pore fluid, providing a record of temperature and fluid pressure evolution during fracture opening. Correlation of these temperatures with the burial history of the basin indicates that fractures opened over a 48-million-year time interval. This corresponds to a fracture opening rate of about 20 micrometers/million years or 0.00000002 mm/year. These rates are some of the slowest observed rates for geologic processes.
Thickness and composition of the Bonanza arc crustal section, Vancouver Island, Canada Dante Canil et al., School of Earth and Ocean Sciences, University of Victoria, Victoria, British Columbia V8W 3P6, Canada. Pages 1094-1105.
Earth is unique in the solar system as having a continental crust, about 30 km thick, that is distinctly different in origin and composition from the interior of the planet. Though studies of the crust are numerous, its actual composition and how it is built from base to top is a matter of debate. Most crust is thought to be built and evolved at plate margins along volcanic arcs. This study investigates a cross section of crust from beneath a 180-million-year-old volcanic arc, turned over on its side and exposed on Vancouver Island, Canada. This team from the University of Victoria use mineral chemistry to investigate the depth of arc exposure and the chemistry of the different levels exposed to estimate the bulk composition of the entire crust. They find that if the volcanic arc originates by magma coming from the mantle, some of the crust in the exposed section must now be missing. The missing crust must somehow have been removed from below the arc and was returned or “recycled” into the mantle, leaving an evolved crustal composition.
Deuterium and oxygen isotopes, paleoelevations of the Sierra Nevada, and Cenozoic climate
Peter Molnar, Dept. of Geological Sciences, Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, Colorado 80309-0399, USA. Pages 1106-1115.
Two schools of thought dominate opinions about the topographic evolution of the Sierra Nevada of California. One school has argued for decades that the range tilted in the past 3 to 10 million years, so that its crest rose 1000-2000 m. The other suggests that little change in elevations of the Sierra has occurred for tens of millions of years. One leg in the support for the latter view comes from similarities of stable isotopes of hydrogen and oxygen in today’s precipitation and ancient precipitation, which depends strongly on the presence of the Sierra and its role as a rain shadow. However, CIRES scientist Peter Molnar suggests that the precipitation and its isotopic content depend comparably strongly on climate and that different climates earlier in the Sierra’s history allow for a recent rise of the range that is consistent with the ancient isotopes.
Relationships between displacement and distortion in orogens: Linking the Himalayan foreland and hinterland in central Nepal
Kyle P. Larson et al., Dept. of Geological Sciences and Geological Engineering, Queen’s University, Kingston, Ontario K7L 3N6, Canada. Pages 1116-1134.
Leading theories recently proposed to explain the evolution of the Himalayan-Tibetan orogen can be separated into two basic end members: those that envisage the orogen developing as a typical thrust and fold belt and those that place an emphasis on the importance of an extruding weak middle crust. Using structural, thermochronologic, and pressure-temperature data gathered from exhumed mid-crustal rocks exposed in the central Nepalese Himalaya, this team from Queen’s University in Kingston, Ontario, concludes that both models may be applicable for different portions of the orogen at any one time during its development.
Composition and age of the East Antarctic Shield in eastern Wilkes Land determined by proxy from Oligocene-Pleistocene glaciomarine sediment and Beacon Supergroup sandstones, Antarctica
John W. Goodge and C. Mark Fanning, Dept. of Geological Sciences, University of Minnesota, Duluth, Minnesota 55812, USA; Pages 1135-1159.
Antarctica, the last frontier of continental geology, is one of Earth’s oldest continents (up to about 3.8 billion years old). Less than 2% of the geology of Antarctica is exposed through the modern ice cap for direct observation, yet eroding ice streams and glaciers deliver crustal material to the continental margin. Geoscientists from the University of Minnesota sampled marine glacial deposits off the Wilkes Land coast to trace geologic materials of particular compositions and ages into the ice-covered interior, providing information on its age and composition. Uranium-lead zircon dating of glacial rock clasts and the sandy matrix that contains them shows that inland Wilkes Land is dominated by igneous and metamorphic crust with discrete ages of 670-780, 900-1300, 1740-2300, and more than 2700 million years old, including distinctive Paleoproterozoic rocks of granitoid, charnockite gneiss and granulite gneiss with ages of 1720-1740 million years old. Comparison of clast ages and compositions with detrital-zircon age profiles from glacial-marine sedimentary deposits thus provides a first-order representation of the character and age of shield rocks underlying the adjacent ice sheet. These data support geological correlation with Australia, even though distinctive volcanic rocks there are not exposed in Antarctica, which helps to refine models of past supercontinents such as Gondwana and Rodinia.
Thermochronologic evidence for orogen-parallel variability in wedge kinematics during extending convergent orogenesis of the northern Apennines, Italy
Stuart N. Thomson et al., Dept. of Geology and Geophysics, Yale University, New Haven, Connecticut 06511, USA. Pages 1160-1179.
The northern Apennines is an unusual mountain belt in that it exhibits widespread ongoing extension despite forming at a convergent plate boundary. Such synconvergent extension has been recognized elsewhere, including the Himalaya, Aegean, and the European Alps. Numerous models have been proposed in an attempt to explain this apparent paradox. However, no single geodynamic model yet explains all the features found in such orogens. This work by Thomson et al. forms part of a large, multidisciplinary NSF-funded Continental Dynamics project with the ultimate goal to develop a self-consistent geodynamic crustal and mantle model of synconvergent extension. A critical aspect of such models is that they should adequately predict local and regional long-term patterns of erosion. Thomson et al. present an extensive, new low-temperature data set that establishes a comprehensive long-term erosion record across the northern Apennines. The data reveal a previously unrecognized sharp east-to-west transition in upper crustal kinematics over the past 8 million years. The east is characterized by migration of the orogen to the northeast in a manner that can be likened to a wave, with enhanced uplift and erosion on its frontal flank, and extension and reduced erosion on its rear flank. By contrast, the west has shown little migration over the same time-span, with highest uplift and erosion rates restricted to the crest of the orogen. The team attributes this difference to either a switch to material being accreted beneath, rather than at the front of the orogen, or a slow-down or cessation of convergence here altogether. Their findings emphasize that no single kinematic model is likely appropriate to explain long-term northern Apennine orogenesis and synconvergent extension, but rather that different lithospheric geodynamic processes have acted at different times in different lateral segments of the orogen.
A complete magnetic-polarity stratigraphy of the Miocene continental deposits of Mae Moh Basin, northern Thailand, and a reassessment of the age of hominoid-bearing localities in northern Thailand
Pauline Coster et al., Institut International de Paléoprimatologie et Paléontologie Humaine: Evolution et Paléoenvironments (IPHEP, UMR CNRS 6046), Bâtiment Sciences Naturelles, 40 Avenue de Recteur Pineau, F86022 POITIERS Cedex, France. Pages 1180-1191.
This team of geoscientists from France, Mexico, and Thailand report the first high-resolution magnetic polarity sequence for the Tertiary continental deposits of northern Thailand. This detailed magnetostratigraphic and biostratigraphic study conducted in the Middle Miocene sequence of the Mae Moh basin allows correlations with other basins of the region and provides a precise temporal framework for the numerous Neogene Thai localities, which have yielded hominoid fossils and rich mammal faunas. These results are crucial for the understanding of the evolution of Asian mammalian lineages, the radiation among south Asian Miocene hominoids, and the timing of structural development of the intermontane basin in northern Thailand.
Incision and channel morphology across active structures along the Peikang River, central Taiwan: Implications for the importance of channel width
Brian J. Yanites et al., Dept. of Geological Sciences, University of Colorado, 2200 Colorado Avenue, Boulder, Colorado 80309, USA. Pages 1192-1208.
Recent movement along potential earthquake-generating faults in central Taiwan is revealed through the analysis of river valleys and erosion along them. This group from the University of Colorado at Boulder, USA, and National Taiwan University documents an increase in erosion as the river passes over the Shuilikeng Fault, indicating that this fault actively accommodates shortening and rock uplift in this region. The increase in erosion rate across this fault correlates with the rate at which the river dissipates energy per unit area on the river bed, suggesting that energy dissipation is a valid proxy for the erosive potential of a river. This increase in erosive potential is accomplished primarily through narrowing the width of the river channel, which focuses the erosive energy of the flowing river on the underlying bedrock; however, along the reach that flows over the Tili and Meiyuan faults, it is proposed that the river has reached a minimum width and thus steepens to further increase the erosive potential of the river. Changes in both river width and slope can influence the erosive potential of a river and indicate possible fault activity in this region of Taiwan.
Displacement profiles and displacement-length scaling relationships of thrust faults constrained by seismic-reflection data
Kristian J. Bergen and John H. Shaw, Dept. of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, Massachusetts 02138, USA. Pages 1209-1219.
Relationships between fault length and displacement provide insights into the mechanics of fault growth by helping constrain fault displacement magnitudes and rates. These are useful in studying regional tectonics, assessing seismic hazards, constraining structural restoration and fault growth models, and defining the evolution of hydrocarbon traps that often lie adjacent to these fault systems. Many researchers have studied displacement profiles on normal faults and have developed scaling laws that relate maximum displacement to fault length using power law relationships. Few studies, however, have looked into the displacement profiles of thrust faults. Thrust faults form when the maximum compressive stress is horizontal and the least confining stress is vertical, conditions commonly found in convergent plate margins, including accretionary wedges, toe thrust belts in passive margins, and restraining bends in strike slip fault systems. In this study, Harvard University geoscientists Bergen and Shaw use high-quality seismic reflection data to precisely map thrust faults and their displacements in the subsurface. This study shows that thrust faults, regardless of tectonic setting or surrounding lithology, have a wide array of displacement profile shapes and maximum displacement-length ratios. Bergen and Shaw propose conceptual fault growth models that may explain some of the observed variations in profile shape and scaling relationships.
Bimodal volcanism as evidence for Paleozoic extensional accretionary tectonism in the southern Appalachians
Christopher S. Holm-Denoma, U.S. Geological Survey, Denver, Colorado 80225, USA; and Reshmi Das. Pages 1220-1234.
The Appalachian mountain belt is a culmination of over 200 million years of active tectonics and has reached vast heights comparable to the modern Himalayas. Closure of an ocean, or oceans, between two large continents, Laurentia and Gondwana, and subsequent collision resulted in the supercontinent Pangaea. It has been shown that during the early parts of the Appalachian orogeny various island arc terranes and continental fragments were accreted onto Laurentia forming collisional tectonic features. However, in modern active tectonic plate boundaries (i.e. the western Pacific margin), backarc extension and accretion is an important process in adding juvenile material to the evolving lithosphere. Researchers Holm-Denoma and Das have identified in the southernmost Appalachians Ordovician age bimodal volcanic rocks that exhibit characteristics of erupting in a backarc basin, and hence signify extension, during a time traditionally linked to the early Paleozoic Taconic orogeny. This model of continental margin extension is evidence for a fundamental tectonic boundary between the southernmost and more northerly Appalachians during this time period.
Late Pleistocene landscape evolution in south-central Chile constrained by luminescence and stable cosmogenic nuclide dating
Katrin Rehak et al., Institut für Geowissenschaften, Universität Potsdam, Karl-Liebknecht-Strasse 24, 14476 Golm, Germany. Pages 1235-1247.
Landscapes develop in a complex response to tectonics and climate. However, in many settings it is not clear which parts of the landscape are mainly shaped by tectonics and which by climate. In order to decipher the evolution of these landscapes, it is important to quantify geomorphic processes and rates on different time scales. This team from Germany and Switzerland used two different techniques, cosmogenic nuclide dating and optically stimulated luminescence, to quantify the age of various geomorphic surfaces in the forearc of south-central Chile. They show that major deposition events in this area broadly coincide with Marine Isotope Stages (MIS) 6 and 8, and hence appear to be climatically driven.
Coupled hydrology and biogeochemistry of Paleocene-Eocene coal beds, northern Gulf of Mexico
Jennifer C. McIntosh et al., Dept. of Hydrology and Water Resources, and U.S. Geological Survey Adjunct Research Geologist, University of Arizona, Tucson, Arizona 85721, USA. Pages 1248-1264.
McIntosh et al. show that microbes have generated economic accumulations of coalbed methane in north-central Louisiana within the last about 50,000 years. High salinities (above about 1.7 moles/L chloride) at depth in the Gulf of Mexico inhibit microbial methane generation. Microbes consume H2 and CO2, in addition to acetate, to produce methane. Interestingly, CO2 injection into sandstone reservoirs for enhanced oil recovery in the 1980s may have stimulated methanogenesis in adjacent coalbeds. These results have important implications for CO2 sequestration in organic-rich formations, such as coalbeds, and potential generation of new energy resources.
Pyrite framboid study of marine Permo-Triassic boundary sections: A complex anoxic event and its relationship to contemporaneous mass extinction
David P.G. Bond, School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK; and Paul B. Wignall. Pages 1265-1279.
The greatest mass extinction ever recorded occurred near the Permian-Triassic boundary, wiping out approximately 96% of life in the oceans, and fundamentally changing the course of life on earth. One of the leading contenders for the “smoking gun” is the eruption of vast quantities of lava in Siberia, leading to a cascade of environmental changes such as global warming and marine anoxia. It was the development of marine anoxia (the absence of oxygen) that led to such high casualties in the oceans. Geoscientists from the University of Leeds sampled boundary sections worldwide to build a picture of conditions in different oceans and test just how bad it got. Their study of pyrite framboids (like microscopic raspberries made from fools’ gold) and sediments from the U.S., Greenland, Spitsbergen, Hungary, Italy, China, and Australia reveals a complex and unstable history of oxygen levels. Unusually (because warm waters usually dissolve less oxygen), oxygen restriction was the most persistent and intense in cooler water at mid-latitude — in such locations, even very shallow waters became impossible to live in. In equatorial locations, anoxia developed in pulses, separated by a period of improved oxygen levels. This study is the first to undertake such widespread and detailed sampling through the crisis interval and has pinpointed the timing of anoxic events worldwide. Detailed timing is crucial to understanding extinction events. Bond and Wignall have shown that the vast majority of extinctions in the oceans coincide with periods of intense oxygen restriction, lending strong support to the “death by anoxia” scenario.
Record of mega-earthquakes in subduction thrusts: The black fault rocks of Pasagshak Point (Kodiak Island, Alaska)
F. Meneghini et al., Dipartimento di Scienze della Terra, Università di Pisa, via S. Maria, 53, 56126 Pisa, Italy. Pages 1280-1297.
The largest earthquakes in the world occur in tectonic environments known as subduction margins, where tectonic plates collide toward each other. This convergence between plates is accommodated by consumption of one plate below another one. At such plate boundaries, the friction between plates during subduction generates heat and storing of energy that can be instantaneously released in the form of destructive earthquakes. Seismogenic processes represent a relevant societal problem, so much so that the international earth-science community has invested most of its human and economic resources to the study of these processes. Because seismic activity profoundly affects rocks, one way to unravel the mechanisms of seismic deformation and the characteristics of the seismic cycle is to study ancient subduction plate boundaries and describe the features of the rocks preserved there. This team from Italy, South Africa, the United States, and Japan provide detailed structural analyses of a fossil plate boundary cropping out in the Island of Kodiak, Alaska, previously interpreted as a boundary active in the depth range typically associated with subduction seismicity. Structural and geochemical analyses have revealed the occurrence of peculiar rocks referred to as pseudotachylytes. Pseudotachylytes are the result of melting of rocks due to a friction-related temperature increase, and, therefore, are unambiguously related to seismic activity.
Late Holocene cyclic glaciomarine sedimentation in a subpolar fjord of the South Shetland Islands, Antarctica, and its paleoceanographic significance: Sedimentological, geochemical, and paleontological evidence
Ho Il Yoon et al., Korea Polar Research Institute of Korea Ocean Research and Development Institute, Songdo Techno Park, Incheon, 406-840, Korea. Pages 1298-1307.
In contrast to the records of long-term (glacial-interglacial) paleoclimatic oscillations from Antarctic ice cores, results of this study of sedimentologic, geochemical, and micropaleontological parameters from the Antarctic Peninsula’s marine sediment cores have revealed striking patterns of multi-century climatic variations. Ho Il Yoon and his team of Korean researchers find that the pattern of 550-year cycles of enhanced organic carbon content is consistent with unprecedented high C/N ratio and decreased abundance of diatom valves per gram sediments in marine cores. Given the understanding of unusually high C/N ratio and decreased diatom valves in sediment, the well-preserved organic carbon is interpreted not to reflect enhanced paleoproductivity but to reflect massive delivery of ice-rafted macroalgal materials from coast under cold and maximal sea ice conditions (low productivity). This record represents a unique cold climatic signal at 550-year cyclicity for the coastal environment of the Antarctic, although cold climate with 550-year cyclicity was recorded in marine sediment of the North Atlantic region. Given the established chronology, in order of timing, the top of core may document a return to cold climate. Potentially, the recent unprecedented warming in the Antarctic Peninsula may be responsible for dampening the effect of the natural climatic cooling.
Time Scales of Metamorphism, Deformation, and Crustal Melting in a Continental Arc, North Cascades, USA
Stacia M. Gordon et al., Dept. of Earth Science, University of California, Santa Barbara, Santa Barbara, California 93106, USA. Pages 1308-1330.
Gordon et al. investigate the timing and relationship between processes, including metamorphism, deformation, and melting, occurring within the mid-crust of a continental arc located in Washington State. The results reveal that metamorphism and melting occurred over an extensive time period from 68 to 46 million years ago. Furthermore, previously published results from sedimentary basins located adjacent to the exposed mid-crustal rocks yield dates that are the same or older than some of the mid-crustal rock dates, indicating that extension of the upper crust was occurring while there was still melting, deformation, and metamorphism in the mid-crust. Therefore, the timing results suggest a direct coupling between processes at different crustal levels.
Assessing the provenance of loess and desert sediments in northern China using U-Pb dating and morphology of detrital zircons
Thomas Stevens et al., Centre for Quaternary Research, Dept. of Geography, Royal Holloway, University of London, Egham, Surrey TW20 0EX, UK. Pages 1331-1344.
Windblown dust (loess) deposits on the Chinese Loess Plateau form some of the most detailed and complete archives of environmental change over the last 22 million years. However, there is still significant controversy over the deposit’s origin. This limits interpretation of the preserved paleoclimatic record and prevents detailed reconstruction of dust storm pathway in the past. This team of geoscientists from the UK and China use geochemical and morphological properties of single grains of the heavy mineral zircon extracted from loess and desert sand deposits in China to address these limits and track likely individual sources of the sediment. Significantly, the loess shows no single affinity to any one source, although a significant proportion is eroding directly from the Qilian Mountains to the west of the Loess Plateau. These findings show that dust-transporting storms tracked from the west to some degree during the last glacial maximum but that there must also have been multiple sources and significant storm track variations over the depositional period.