Keywords: Greenland-Iceland-Norwegian (GIN) Seas, ocean anoxic event, Mayaguana, 2009, L'Aquila earthquake, Strabo, Piraeus, cartography, 1980, Mount St. Helens, modeling, Fifteenmile Group, Yukon, knickpoint, beryllium-10, Wenchuan earthquake, groundwater, subsidence, aquifer, Western Interior Basin, Farallon plate, 2004, Sumatra-Andaman earthquake, tsunami, Integrated Ocean Drilling Program, dolomite, Peru-Chile Trench, biosphere, micrometeorites, Latemar carbonate, Purujosa trilobite, Bering Sea, Aleutian Low Pressure System, salmon, marine metazoan ecosystems, logjams, Colorado, mountain streams, North China block, Gondwanaland, Cambrian, Cenozoic, nanoparticles, foraminifera, hydrocarbon seeps
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Persistent Nordic deep-water overflow to the glacial North Atlantic
K.C. Crocket et al., Earth Science and Engineering, Imperial College London, Royal School of Mines Building, Prince Consort Road, London SW7 2AZ, UK; doi: 10.1130/G31677.1.
Deep-water formation in the Greenland-Iceland-Norwegian (GIN) Seas is an important component of Atlantic overturning circulation and global climate. Due to ambiguities associated with interpreting established sedimentary records, it remains unclear how rates of deep-water formation, and hence strength of overturning circulation, varied across glacial-interglacial cycles. K.C. Crocket of Imperial College London and colleagues present a bottom-water neodymium isotope record from the Rockall Trough in the northeast Atlantic to investigate changes in the sources of circulating waters in the past 43,000 years. Today and throughout most of the Holocene, water from the GIN Seas, along with entrained water from the North Atlantic Current (NAC), controls the bottom-water neodymium isotope composition along the Rockall Trough. Crocket et al.'s results suggest the persistence of this scenario from the last glacial maximum (LGM) 24,000 to 18,000 years ago and beyond. Periodic deviations from this circulation regime punctuate the record at times of centennial-scale meltwater events, implying either continued GIN Seas overflow without NAC entrainment, or millennial-scale interruptions in the overflow and incursion of deep water of Southern hemisphere origin. Crocket and colleagues conclude that overflow was at least intermittently, if not continuously, present during the LGM, and that the GIN Seas have remained a source of deep water to the North Atlantic during the last glacial cycle.
Short-term euxinia coinciding with rotaliporid extinctions during the Cenomanian-Turonian transition in the middle-neritic eastern North Atlantic inferred from organic compounds
Masahiro Oba et al., Institute of Geology and Paleontology, Tohoku University, Sendai 980-8578, Japan; doi: 10.1130/G31805.1.
The ocean anoxic event (OAE) that occurred near the Cenomanian-Turonian boundary is the most extensive among the mid-Cretaceous OAEs and is characterized by stepwise extinctions in marine biota. Recently, reports from organic geochemical studies provided estimates of the dissolved oxygen changes in the photic zone using molecular fossils. But, there is no discussion in those studies concerning the relationship between the extinctions and the anoxic condition of the water column. Masahiro Oba of Tohoku University and colleagues report the coincidence of two peaks of organic compounds showing short-term OAEs and double planktonic foraminiferal extinctions in the late Cenomanian. This indicates that those extinctions can be attributed to short-term OAEs, which Oba and colleagues have discovered for the first time.
Discovery of Miocene to early Pleistocene deposits on Mayaguana, Bahamas: Evidence for recent active tectonism on the North American margin
Pascal Kindler et al., Section of Earth and Environmental Sciences, University of Geneva, Maraichers 13, 1205 Geneva, Switzerland; doi: 10.1130/G32011.1.
Recent geological investigations by Pascal Kindler of the University of Geneva and colleagues on Mayaguana (SE Bahamas) have revealed that this small (53 x 12 km) island was uplifted and tilted toward the south about 500,000 years ago. This tilting explains the exclusive occurrence of very old limestone rocks along the north coast of the island, whereas there is no exposure of these rocks along the southern shoreline. This is a new discovery. It has long been known that islands such as Barbados are being uplifted by processes related to continental motion, but until now, all Bahamian islands were considered to be slowly sinking. Tilting and uplift are usually related to the presence of large cracks in Earth's crust called faults. Movement along these faults generates earthquakes. Thus, there are probably some faults in the vicinity of Mayaguana, as shown by the linear shape of its coastlines. However, the Mayaguana faults are by no means as active as those near Haiti, and the probability of a major earthquake, and associated tsunami, occurring in this area remains fortunately low.
Analysis of large, seismically induced, gravitational deformations imaged by high-resolution COSMO-SkyMed synthetic aperture radar
M. Moro et al., Istituto Nazionale di Geofisica e Vulcanologia, Centro Nazionale Terremoti, Rome, Italy; doi: 10.1130/G31748.1.
M. Moro of the Istituto Nazionale di Geofisica e Vulcanologia, Italy, and colleagues have studied large gravitational deformations triggered by the M6.3 L'Aquila earthquake, which occurred in the Abruzzi region of central Italy on 6 April 2009. Indeed, it is common knowledge that strong ground motions caused by large earthquakes can induce gravitational collapses. By means of the differential synthetic aperture radar interferometry (DInSAR) technique and using high-resolution COSMO-SkyMed (Constellation of Small Satellites for the Mediterranean Basin Observation) X-Band synthetic aperture radar (SAR) data acquired on 4 April 2009, and 12 April 2009, respectively, Moro and colleagues have obtained a spatially continuous measure of the coseismic ground displacement at 5 m resolution. Within the classical, low-frequency fringe pattern belonging to surface coseismic deformation caused by normal fault dislocation, they observed higher spatial frequency fringe patterns, suggesting localized deformation not related to the fault displacement field. In particular, they show two examples of subtle (4 to 5 cm) deformation patterns associated with long-term gravitational mass movements recognized through geological and geomorphological interpretation. Finally, Mora and colleagues give an interpretation of the deformation mechanisms by integrating geological analysis and simple analytical modeling of the DInSAR observations.
Piraeus, the ancient island of Athens: Evidence from Holocene sediments and historical archives
Jean-Philippe Goiran et al., Centre National de la Recherche Scientifique (CNRS), UMR 5133-Archeorient, MOM, 7 rue Raulin, 69007 Lyon, France; doi: 10.1130/G31818.1.
The famous Greek geographer Strabo wrote in the first century A.D. that "Piraeus was formerly an island and lay in front of the mainland, from which it got its name." To validate Strabo's hypothesis, cartographic and historical data were compiled with geoscience analyses and radiocarbon datings from a series of cores drilled in the Kephissos coastal plain, southwest of Athens, Greece. The results of this interdisciplinary research demonstrate the reliability of Strabo's text by revealing that Piraeus was indeed an island. In the early Holocene, the rocky hill of Piraeus was linked to the mainland of Attica. During the Late Neolithic to Chalcolithic period (4800-3400 B.C.), Piraeus became an island surrounded by a shallow marine bay (due to sea-level rise). Until 1550 B.C., Piraeus was separated from the mainland by a wide lagoon. In the fifth century B.C., Themistocles, and then Pericles, definitively connected Athens to Piraeus by two long walls partly built on a residual coastal marsh called the Halipedon. Jean-Philippe Goiran of France's CNRS and colleagues present an example of how palaeo-environmental studies have helped to confirm the accuracy of Strabo's descriptions regarding past marine conditions in and around Piraeus.
Multiphase-flow numerical modeling of the 18 May 1980 lateral blast at Mount St. Helens, USA
T. Esposti Ongaro et al., Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Pisa, Via della Faggiola 32, 56126 Pisa, Italy; doi: 10.1130/G31865.1.
Thirty years after the 1980 eruption of Mount St. Helens (Washington State, United States), computer simulations have allowed T. Esposti Ongaro of the Istituto Nazionale di Geofisica e Vulcanologia, Italy, and colleagues to reproduce and analyze the large-scale features of the 18 May blast, which devastated a wide area of 600 square kilometers north of the volcano and killed 57 people. Initial conditions, triggering the violent explosion of the magmatic mixture, have been derived from the wide geological dataset available for this eruption, which is one of the most studied and documented in volcanology. The three-dimensional computer model was then able to describe the propagation of the eruptive cloud, (made up of high-temperature gases, ash, pumice, and rock fragment) over the mountainous region surrounding the volcano, and to correctly reproduce the flow-front velocity, runout (the maximum distance reached by the cloud), and impact. The results demonstrate that, where detailed geological constraints are available and thanks to the availability of high-performance supercomputers, physical models can fairly accurately reproduce the large-scale features of blast scenarios. Such an improvement in modeling capability will make it possible to more effectively map potential blast flows at blast-dangerous volcanoes worldwide.
Phosphate biomineralization in mid-Neoproterozoic protists
Phoebe A. Cohen et al., NASA Astrobiology Institute, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA; doi: 10.1130/G31833.1.
Fossils from the Neoproterozoic Era (1000-542 Ma) are essential to our understanding of the history and origins of eukaryotic groups. Phoebe A. Cohen of MIT and colleagues add to this record by describing exquisitely preserved microfossils from mid-Neoproterozoic (811-717 million years old) rocks of the Fifteenmile Group, Yukon. These fossils are interpreted as biomineralized plates that covered the surface of a single-celled alga much like the plates of a modern coccolithophore. While these fossils were previously documented in chert thin sections, Cohen and colleagues document new three-dimensional specimens recovered from carbonate rocks. These new specimens are preserved in sub-micron scale detail, revealing previously undocumented structures. Both fossils found in chert and carbonate are composed of calcium phosphate and organic carbon, which is unusual in modern protist groups. This suggests that the minerals used by marine eukaryotic groups have changed through time, perhaps linked to changing ocean chemistry. While the relationship of these fossils to modern organisms is difficult to determine, Cohen and colleagues argue that it is likely that these unique fossils are the plates of an organism most closely related to green algae.
Does decreasing paraglacial sediment supply slow knickpoint retreat?
John D. Jansen et al., School of Geographical and Earth Sciences, University of Glasgow, Glasgow G12 8QQ, UK; doi: 10.1130/G32018.1.
Knickpoint (waterfall) erosion in bedrock rivers is a key means by which climatic and tectonic signals are communicated to the whole landscape; hence, knowing the factors that control the rate of knickpoint retreat is an important contribution to understanding how landscapes evolve through time. In their study of four rivers in the western Scottish Highlands, John D. Jansen of the University of Glasgow and colleagues quantified knickpoint speed by measuring the concentration of in situ-produced cosmogenic beryllium-10 on bedrock terraces formed in the wake of retreating knickpoints. Jansen et al.'s results show that the rate of retreat has decreased sharply over the Holocene in all four rivers, such that knickpoints are now retreating at a similar rate of less than 20 m per thousand years, irrespective of the size of each river. The researchers find that this regional decrease in erosional capacity is not related to changes in either discharge or stream power. Rather, the decrease appears to reflect the depletion of sediments available to act as erosional "tools," as widespread evidence indicates an overall drop in supply from slopes and sediment storage since deglaciation 10 thousand years ago. Their results imply that major episodes of fluvial erosion may be in tune with glacial cycles, and that sediment depletion following glacial-interglacial transitions may be an important cause of bedrock erosion rate variations in rivers draining glaciated landscapes.
Seismic slip recorded by fluidized ultracataclastic veins formed in a coseismic shear zone during the 2008 Mw 7.9 Wenchuan earthquake
Aiming Lin, Graduate School of Science and Technology, Shizuoka University, Shizuoka 422-8529, Japan; doi: 10.1130/G32065.1.
A. Lin of Shizuoka University, Japan, reports findings of fluidized ultracataclastic veins produced by the 12 May 2008 Mw 7.9 Wenchuan (China) earthquake. The findings support earlier suggestions that ultracataclastic veins, which resemble pseudotachylyte veins in appearance, can be generated by crushing and rapid injection accompanied by fluidization of fine-grained material during seismic faulting, with little or no melting. Ultracataclastic veins formed in this way may record seismic slip events in seismogenic fault zones, acting as earthquake fossils in much the same way as pseudotachylyte veins.
Drought-driven transient aquifer compaction imaged using multitemporal satellite radar interferometry
Pablo J. Gonzalez and Jose Fernandez, Dept. of Earth Sciences, University of Western Ontario, Biological and Geological Sciences Building, London, Ontario N6A 5B7, Canada; doi: 10.1130/G31900.1.
Groundwater drainage from clay-rich aquifers can cause delayed compaction and surface deformation (subsidence). Pablo J. Gonzalez and Jose Fernandez of the University of Western Ontario studied surface deformation affecting a sedimentary basin in southeast Spain, analyzing 15 years of radar satellite images. This technique allows monitoring of the temporal evolution of the ground surface. Gonzalez and Fernandez discovered the largest subsidence rates affecting a large basin (more than 200 square kilometers) in Europe (more than 10 cm per year). Subsidence is known to be driven by intense groundwater pumping since the 1960s. Time series ground displacements indicate a temporal nonlinear behavior. Gonzalez and Fernandez prove that subsidence is better characterized by a compound model with a linear and an exponential decay. The timing of the exponential decay is well constrained, with onset coincidence at the end of a drought period and lasting for eight years. Gonzalez and Fernandez, using remote sensing data alone, to suggest that the transient ground deformation was triggered by nonlinear temporal rates of pore-pressure diffusion of additional applied effective stresses (drought), assuming a constant background pumping rate (linear trend). This analysis method could represent a stimulating way to study nonlinear soil mechanics and groundwater flows at aquifers, which potentially can help the management of groundwater resources.
Migration of dynamic subsidence across the Late Cretaceous United States Western Interior Basin in response to Farallon plate subduction
Shaofeng Liu et al., State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences (Beijing), Beijing 100083, China; doi: 10.1130/G31692.1.
Quantitative differentiation of subsidence across central Utah, Colorado, and southern Wyoming in the Western Interior Basin during the Late Cretaceous reveals a component of continuously evolving long-wavelength residual subsidence, in addition to subsidence driven by the Sevier thrust belt and associated sediment loads. The loci of maximum rates of this residual subsidence moved eastward from approximately 98 to 74 million years ago in phase with the west-to-east passage of the Farallon slab beneath the northern United States. These new backstripped subsidence data allow for rigorous testing of existing subduction models and reveal that the fundamental driver for Western Interior Basin subsidence was the dynamic pull of the sinking slab. Furthermore, regional variations in subsidence rates suggest a possible deficit of mantle loading inside the slab beneath Colorado, supporting the hypothesis that the thickened slab represents a subducted oceanic plateau. Interpretation of these data also provides insight into the commencement of the Laramide tectonic events in the western United States. Shaofeng Liu of the China University for Geoscience and colleagues, for the first time, document how the Cretaceous stratigraphy records the timing, patterns, and position of underlying mantle processes during Farallon slab subduction, and that the combination of such data leads to important geophysical discoveries.
Geologic evidence for two pre-2004 earthquakes during recent centuries near Port Blair, South Andaman Island, India
Javed N. Malik et al., Dept. of Civil Engineering, Indian Institute of Technology, Kanpur 208016, India; doi: 10.1130/G31707.1.
The 26 December 2004 Mw 9.3 Sumatra-Andaman earthquake, followed by a tsunami, was devastating to the inhabitants of countries bordering the Indian Ocean. This mega-subduction zone earthquake resulted in a sudden change of the land level and was marked by uplift and subsidence, strong ground shaking in a few places, and a tsunami. The signature of such events usually is preserved in sediments or landforms. To identify older (historical and pre-historical) earthquakes and tsunami signatures, Javed N. Malik of the Indian Institute of Technology and colleagues studied the sediment record around Port Blair, South Andaman Island. Their study revealed that at least two major earthquakes have occurred along the Andaman trench during the past 400 years. The older earthquake that occurred around A.D. 1600 resembled the 2004 Sumatra-Andaman earthquake and was accompanied with subsidence (~1 m), but no strong shaking was recorded near Port Blair. The second event probably points to an A.D. 1762 Arakan earthquake accompanied by a tsunami.
Dolomite formation within the methanogenic zone induced by tectonically driven fluids in the Peru accretionary prism
Patrick Meister et al., Max-Planck-Institute for Marine Microbiology, Celsiusstrasse 1, 28359 Bremen, Germany; doi: 10.1130/G31810.1.
Until a few decades ago, the general opinion prevailed that life forms did not exist 4 meters beneath the seafloor. Recently, a major initiative to study the deep biosphere, utilizing the drilling vessels operated by the Integrated Ocean Drilling Program, has revealed the existence of an extensive microbial community flourishing hundreds of meters below the seafloor. A special mineral called dolomite -- considered to be a product of microbial activity -- was discovered more than 230 meters beneath the Peru-Chile Trench in a zone where microorganisms produce large amounts of methane and carbon dioxide. This finding by Patrick Meister of the Max-Planck-Institute for Marine Microbiology and colleagues is surprising because carbon dioxide is acidic, and dolomite, a carbonate, should dissolve rather than precipitate under such conditions. Signatures of the isotopes strontium-87 and oxygen-18 measured in the dolomite may provide an explanation for this longstanding problem: The carbon dioxide acidification was buffered by a fluid flowing upward from great depth within Earth's crust. Such isotope signatures preserved in dolomites can now be used by geologists to reconstruct past deep biosphere processes from rocks that formed hundreds of millions of years ago.
Composition and accretion rate of fossil micrometeorites recovered in Middle Triassic deep-sea deposits
Tetsuji Onoue et al., Dept. of Earth and Environmental Sciences, Kagoshima University, Kagoshima 890-0065, Japan; doi: 10.1130.G31866.1.
Micrometeorites, submillimeter-sized extraterrestrial particles that survive atmospheric entry, originate from dust-producing objects such as comets and asteroids. Although ancient micrometeorites found in sedimentary rocks are of key interest as an historical record of meteoroid populations in the solar system, they are rare and prone to severe chemical weathering. Tetsuji Onoue of Kagoshima University and colleagues present the discovery of micrometeorites in 240 million-year-old sediments (Middle Triassic). The excellent state of preservation of this micrometeorite collection shows strong evidence of extraterrestrial origin. These micrometeorites are much older than any previous micrometeorite collection, and their existence provides a new means of tracing long-term changes in the composition and flux of incoming extraterrestrial matter, as well as changes in the solar system's meteoroid population.
Formation of dolomite at 40-80 degrees Celsius in the Latemar carbonate buildup, Dolomites, Italy, from clumped isotope thermometry
John M. Ferry et al., Dept. of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, Maryland 21218, USA; doi: 10.1130/G31845.1.
The common carbonate rock dolomite usually forms by chemical reaction of limestone with fluid, but the details remain poorly understood. Two important constraints on the source of the fluid are the temperature of formation and the oxygen isotope composition of the fluid. Conventional measurements of the oxygen isotope composition of dolomite, however, do not uniquely define both variables. The first independent determinations of temperature and the fluid's isotopic composition, specifically for formation of dolomite in the Latemar carbonate buildup (Dolomites, Italy), have been made by John M. Ferry of Johns Hopkins University and colleagues using clumped isotope thermometry. Both the estimated temperatures of 42 to 72 (plus or minus 9 to 11 degrees) Celsius and the oxygen isotope composition of the fluid are similar to modern diffuse flow fluid at mid-ocean ridges, a fluid never before implicated in the formation of dolomite. Concentrations of iron, manganese, zinc, and copper in the dolomite confirm that the fluid involved in formation of dolomite was like diffuse flow fluid in trace element chemistry as well. From a broader perspective, Ferry and colleagues demonstrate that clumped isotope thermometry has great potential for application to studies of sedimentation and sedimentary rocks by retrieving estimates of temperature and the oxygen and carbon isotope composition of fluids from a single stable isotope analysis of a carbonate mineral.
Purujosa trilobite assemblage and the evolution of trilobite enrollment
Jorge Esteve et al., Area de Paleontología-IUCA (Instituto Universitario de Investigacion en Ciencias Ambientales), Departamento Ciencias de la Tierra, Universidad de Zaragoza, 50009, Zaragoza, Spain; doi: 10.1130/G31985.1.
Jorge Esteve of the Universidad de Zaragoza, Spain, and colleagues describe a new and exceptional assemblage of trilobites (Purujosa trilobite assemblage) from northern Spain that sheds new light on one of the clearest examples of adaptive evolution in the fossil record: the trend in trilobite groups toward more efficient enrollment. This assemblage shows how different trilobites could enroll even the trilobites that had slightly different bodies. Esteve and colleagues also analyze broad-scale trends of this behavior throughout the Paleozoic. The knowledge of examples of adaptive evolution is vital for constraining the likely controls of evolutionary patterns. Trilobite enrollment, which is clearly a defense adaptation, provides one of the clearest cases of independent trends in different lineages toward a common solution.
225 yr of Bering Sea climate and ecosystem dynamics revealed by coralline algal growth-increment widths
J. Halfar et al., Dept. of Chemical and Physical Sciences, University of Toronto, 3359 Mississauga Road N., Ontario L5L 1C6, Canada; doi: 10.1130/G31996.1.
Bering Sea climate and ecosystems have recently undergone major changes affecting sea ice and marine life, including commercially important salmon fisheries. Unfortunately, long-term Bering Sea dynamics are poorly understood due to an absence of high-resolution marine climate archives. J. Halfar of the University of Toronto and colleagues present the first record compiled from measuring the widths of annual growth bands of calcified marine algae that can exhibit century-scale life spans and grow on the shallow seafloor. Algal growth is controlled by changes in sunlight reaching the seafloor. Therefore, it provides an unusual archive of light variability, which in the Bering Sea is attributed to changes in strength of the Aleutian Low Pressure System, an important feature influencing North Pacific climate. When the Aleutian Low is strong, Bering Sea weather is cloudy and stormy, and water near the sea surface is well mixed. Mixing brings more nutrients to the ocean surface from deeper water and stimulates growth of plankton, a major food source of Alaskan salmon. Enhanced clouds and plankton production increase shading on the shallow seafloor and reduce algal growth. The light-driven algal growth rates compare well to historic Bering Sea salmon abundance since the 18th century, and suggest that changes in salmon stock abundance are closely related to the changing strength of the Aleutian Low.
Early Cambrian metazoans in fluvial environments, evidence of the non-marine Cambrian radiation
Martin J. Kennedy, School of Earth and Environmental Science, University of Adelaide, Adelaide, SA 5005, Australia; and Mary L. Droser; doi: 10.1130/G32002.1.
Invasion of non-marine environments by marine bilaterians is regarded by biologists as a fundamental evolutionary step that involved overcoming significant physiological barriers, such as maintaining osmotic balance, and environmental limitations like the absence of a terrestrial food supply. The fossil record has suggested that the bilaterian colonization of freshwater environments took place long after (more than 80 million years) the establishment of Phanerozoic-style marine metazoan ecosystems. Martin J. Kennedy of the University of Adelaide and colleague Mary L. Droser demonstrate that, in contrast to this basic understanding, animals adapted to freshwater conditions early in the Cambrian marine radiation and thus, for at least early animals, this threshold was not a considerable barrier. This indicates that the terrestrial realm sustained an active ecosystem. It adds to an increasing body of evidence that terrestrial environments played an important role in the critical biospheric events that allowed the transition from the single-celled microbial world of the Precambrian to the multicellular world of the Phanerozoic.
Threshold-induced complex behavior of wood in mountain streams
Ellen Wohl, Dept. of Geosciences, Colorado State University, Fort Collins, Colorado 80523-1482, USA; doi: 10.1130/G32105.1.
Logjams in streams create areas of slower flow in which sediment can accumulate. As sediment fills the channel, water spills over the stream banks and flows across the valley bottom, eroding smaller channels parallel to the main stream. Logjams can also form in these smaller channels, creating a complex accumulation of wood, sediment, and fine organic matter across the valley bottom. This process has been described previously for wide, lowland streams, but can also occur in mountain valleys. Ellen Wohl of Colorado State University describes the occurrence of multiple channels in mountain streams of the Colorado Front Range (United States). Two criteria are necessary to develop multiple channels: relatively wide valley bottoms and old-growth forest. Sections of wider valley alternate with steep, narrow sections along many streams of the Front Range. Old-growth forest is much more limited in extent, but only this type of forest produces wood of sufficient diameter and in sufficient volume to create large, closely spaced logjams and multiple channels. These sections of stream provide good fish habitat and store much larger volumes of carbon and other nutrients than other portions of the stream network, and are thus likely to disproportionately influence nutrient cycling and habitat along mountain streams.
Trilobites and zircons link north China with the eastern Himalaya during the Cambrian
N. Ryan McKenzie et al., Dept. of Earth Sciences, University of California, Riverside, California, 92521, USA; doi: 10.1130/G31838.1.
Establishing pre-Pangean paleogeographic reconstructions proves to be a difficult task and requires multidisciplinary studies and integrative datasets. N. Ryan McKenzie of the University of California at Riverside and colleagues provide new constraints for the Cambrian paleogeography of tectonic blocks of Asia, specifically the North China block, through an investigation of its relationship with south China, northern India, and Australia. McKenzie and colleagues' species-level biogeographic analysis of Cambrian trilobites demonstrates strong faunal similarities between north China, south China, and northern India, much more so than between north China and Australia, as traditionally assumed. Analysis of age populations of detrital zircons from Cambrian sandstones of north China are remarkably similar to Cambrian sandstones from northern India, which suggests that north China was receiving clastic sediment from similar sources as India. This requires north China to have been physically attached to the Gondwanan margin during the Cambrian, and not isolated as it appears in most paleogeographic reconstructions. This is supported by evidence of Cambrian-Ordovician tectonism in both north China and north India, which likely represent a contiguous event. From these data, McKenzie et al. offer a new Cambrian paleogeographic model of eastern Gondwanaland, which places north China along the eastern Gondwanan margin in proximity to northern India.
Climate change imprinting on stable isotopic compositions of high-elevation meteoric water cloaks past surface elevations of major orogens
Christopher J. Poulsen and M. Louise Jeffrey, Dept. of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109-1005, USA; doi: 10.1130/G32052.1.
The elevation of ancient mountains has been inferred by Christopher J. Poulsen and M. Louise Jeffrey of the University of Michigan using the oxygen and hydrogen stable isotopic compositions of ancient minerals. This paleoaltimetry technique generally assumes that any change in the isotopic composition of these minerals can be related to surface elevation change. Poulsen and Jeffrey demonstrate, using climate models, that long-term climate change has a substantial effect on the isotopic composition of meteoric water at high elevations. In particular, climate cooling through the Cenozoic caused isotopic compositions over high-elevation regions to become lower due to increased isotopic fractionation and downward mixing of vapor from the upper troposphere. This climate imprinting on high-elevation isotope compositions has likely led to an underestimate of past surface elevations and an overestimate of Cenozoic surface uplift on the order of 1 kilometer.
Granular nanoparticles lubricate faults during seismic slip
Raehee Han et al., Geologic Environment Division, Korea Institute of Geoscience and Mineral Resources (KIGAM), Daejeon 305-350, South Korea; doi: 10.1130/G31842.1.
The study of nanoparticles (microscopic particles with at least one dimension less than 100 nm) in nature is becoming a fast-evolving field in the geosciences because nanoparticles may have very different physico-chemical properties from those of larger grains, often exhibiting unexpected unique characteristics. Recent observation indicates that nanoparticles can form in slipping zones of natural and experimental faults, where shear displacement occurs. However, little attention has been paid to how nanoparticles may affect the mechanical behavior of faults. Raehee Han of the Korea Institute of Geoscience and Mineral Resources and colleagues conducted friction experiments on granular nanoparticles (e.g., magnesium oxide, magnetite, and calcium oxide) at up to 1.3 m/s, a typical speed of earthquake fault slip, to investigate the mechanical role of nanoparticles. They observed that the tiny particles dramatically reduce friction on a fault and thus make the fault very slippery, in particular when the fault slips fast. Han and colleagues suggest that nanoparticles may act as a solid lubricant in tectonic faults during earthquakes.
An extinct foraminifer endemic to hydrocarbon seeps?
Bruce W. Hayward et al., Geomarine Research, 49 Swainston Road, St. Johns, Auckland, New Zealand; doi: 10.1130/G31974.1.
Bruce W. Hayward of Geomarine Research, New Zealand, and colleagues have discovered what they believe to be the first record of foraminifera (shelled amoeba) that only lived in association with seafloor seeps of methane. A few modern species of shellfish and worms are known to have adapted to live only around these seeps, but no modern foraminifera have been found to do so. This discovery is of a now-extinct fossil species that occurs as millions of specimens concentrated around a long-dead, 20-million-year-old methane seep. The only other known occurrences of this species are in ancient asphalt-impregnated seafloor rocks in Indonesia, also inferred to have been ancient hydrocarbon seeps.
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