|26 January 2012
GSA Release No. 12-04
Director - GSA Communications & Marketing
New Articles Posted Ahead of Print - 23 January
Boulder, Colo., USA - New GEOLOGY articles posted ahead of print examine the role of climate warming in the Permian-Triassic mass extinction, documentation of one of the first examples of land-based magnetic lineations similar to those that characterize sea-floor spreading centers, evidence that the disappearance of the Indus Valley Civilization around 2000 BC may be linked to a rearrangement of river drainage systems, fossil trees from the Cretaceous that reveal the true magnitude of past climate warmth, and more.
Highlights are provided below. Representatives of the media may obtain complimentary copies of GEOLOGY articles by contacting Christa Stratton at the address above. Abstracts for the complete issue of GEOLOGY are available at http://geology.gsapubs.org/.
Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOLOGY in articles published. Contact Christa Stratton for additional information or assistance.
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Climate warming in the latest Permian and the Permian–Triassic mass extinction
Michael M. Joachimski et al., GeoZentrum Nordbayern, Universität Erlangen-Nuremberg, Schlossgarten 5, 91054 Erlangen, Germany. Posted online 23 Jan. 2012; doi: 10.1130/G32707.1.
The Permian-Triassic mass extinction (254 million years ago) represents the most severe biotic extinction in Earth history. The causes of this extinction event are discussed controversially. Catastrophic volcanism in Siberia and related climate warming were suggested as a potential trigger of the faunal crisis. Joachimski et al. document, for the first time, that the temperature in low-latitudinal seas was rising by up to 8 °C parallel to the extinction. This climatic warming is seen as direct consequence of Siberian trap volcanism and related processes culminating in higher greenhouse gas levels. Rapid climate warming in the latest Permian and high temperatures in the Early Triassic may not only have contributed to the faunal decline but also to the slow recovery in the aftermath of the crisis.
Rhenium-osmium isotope systematics and platinum group element concentrations in oceanic crust
Bernhard Peucker-Ehrenbrink et al., Woods Hole Oceanographic Institution, 360 Woods Hole Road, MS 25, Woods Hole, Massachusetts 02543-1541, USA. Posted online 23 Jan. 2012; doi: 10.1130/G32431.1.
Peucker-Ehrenbrink et al. attempt to assess the average concentration of platinum group elements and the isotope composition of osmium in the oceanic crust. They use new data from an ocean crust section exposed in the Oman Mountains together with previously published values for the upper portion of the oceanic crust that was based on cores from the International Ocean Drilling Program. They find that the inventory of platinum group elements is controlled by the lower, cumulate portions of the oceanic crust. These metals are mainly hosted in small accessory minerals, mostly sulfides and metal alloys. Their study confirms long-held (but unproven) views of the behavior of platinum group elements and rhenium during mantle melting and hydrothermal alteration of oceanic crust. It also shows that long storage times in the Earth’s mantle are required to generate isotope signatures observed in some ocean island basalts that are thought to be formed, in part, from contributions from old recycled oceanic crust.
Magnetic stripes of a transitional continental rift in Afar
David L. Bridges et al., Department of Geological Sciences and Engineering, Missouri University of Science and Technology, Rolla, Missouri 65409, USA. Posted online 23 Jan. 2012; doi: 10.1130/G32697.1.
Magnetic stripes parallel to mid-oceanic spreading centers are one of the most significant consequences of sea-floor spreading, and have played an essential role in the establishment of modern plate tectonics theory and the determination of sea-floor spreading rates. Similar magnetic anomaly patterns have not been well documented in continental rifts transitioning into proto-seafloor spreading centers. Here, using high-resolution magnetic data that was collected across the Tendaho Graben in the Afar Depression, Ethiopia, Bridges et al. document one of the first examples of land-based magnetic lineations similar in pattern and amplitude to those that characterize sea-floor spreading centers. Bridges et al. revise current thinking from the idea that magnetic stripes are initially produced at the onset of oceanic basin formation. Their findings suggest that these magnetic lineations can form as soon as continental rifting transitions to the rift axis by magma-assisted rifting.
Gas hydrate pingoes: Deep seafloor evidence of focused fluid flow on continental margins
Christophe Serié et al., School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester M13 9PL, UK. Posted online 23 Jan. 2012; doi: 10.1130/G32690.1.
Natural gas hydrates occur widely along deepwater continental margins and represent one of the largest carbon reservoirs on Earth. Methane seeps and associated features represent the venting points between methane sources in the shallow lithosphere and the hydrosphere/atmosphere. Serié et al. have discovered a number of seep-related seafloor mounds in the deepwater Kwanza Basin, offshore Angola, and have interpreted these as submarine gas hydrate pingoes, comparable to hydraulic pingoes commonly found in periglacial environments on land. Detailed seismic interpretation of their size, morphology, and geophysical signatures highlight different development stages associated with the formation and dissociation of shallow gas hydrate, as well as their link to deep-rooted plumbing systems that allow thermogenic fluid migration from several-kilometers-deep sedimentary basins. The occurrence of near-surface massive gas hydrate pingoes has important implications for the understanding of basin de-gassing, hydrate accumulation and dissociation dynamics, gas hydrate resource assessments, and the global carbon cycle.
U-Pb zircon dating evidence for a Pleistocene Sarasvati River and capture of the Yamuna River
Peter D. Clift et al., School of Geosciences, University of Aberdeen, Aberdeen AB24 3UE, UK. Posted online 23 Jan. 2012; doi: 10.1130/G32840.1.
Rivers are important to sustaining human societies, especially those that exist in desert environments where they form a critical part of the water supply. The disappearance of the Indus Valley Civilization from the northwestern edge of the Thar Desert around 2000 BC may be linked to such rearrangement of drainage systems. In this study, Clift et al. used the crystallization ages of zircon sand grains to fingerprint the origin of ancient river sands in the region. They show that around 50,000 years ago the Yamuna River, that now flows east into the Ganges, then flowed west into the Indus via the region of study. They further show that the Sutlej River flowed further south than present allowing the Beas River, now its tributary, to have its own separate course. Capture of the Yamuna to the east is shown to far predate the demise of the Harappan, suggesting a climatic trigger to their end. However, Clift et al. do show that a small river continued to flow through the settlement area until after 2500 BC and that this was abandoned around the time of Harappan demise.
New insights into Archean granite-greenstone architecture through joint gravity and magnetic inversion
Luis A. Gallardo and Nicolas Thebaud, Centre for Exploration Targeting, University of Western Australia, Crawley, WA 6009, Australia. Posted online 23 Jan. 2012; doi: 10.1130/G32817.1.
The study of Archean terranes has been largely bedevilled by (1) scarce outcrops, (2) a complex and overprinted geological structure, and (3) the emplacement of intrusives of varied suites that are incredibly difficult to map at depth. By selecting one of the best studied cratonic areas in the world as a test site, Gallardo and Thebaud demonstrate that these three aspects can be significantly overcome by the sole use of widely available gravity and magnetic data and joint geophysical inversion. This represents a fundamental advancement in tectonic studies and opens vast possibilities for similar tectonic scenarios anywhere in the world.
Cretaceous forest composition and productivity inferred from a global fossil wood database
Emiliano Peralta-Medina and Howard J. Falcon-Lang (corresponding), Department of Earth Sciences, Royal Holloway University of London, Egham, Surrey TW20 0EX, UK. Posted online 23 Jan. 2012; doi: 10.1130/G32733.1.
One hundred million years ago, the Earth was gripped by extreme global warming. A study of over 2000 fossil trees from this time period (known as the Cretaceous) now reveals the true magnitude of past climate warmth. Peralta-Medina and Falcon-Lang compared the width of the annual tree-rings of Cretaceous trees with their modern counterparts (a measure of tree-growth rate). For mid- and high-latitudes, they discovered that fossil trees were growing at twice the modern rate. Their findings show that, one hundred million years ago, the Arctic was forested and had a climate similar to that of present-day London.
The origin of intracellular structures in Ediacaran metazoan embryos
James D. Schiffbauer et al., Nanoscale Characterization and Fabrication Laboratory, Institute for Critical Technology and Applied Science, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, USA. Posted online 23 Jan. 2012; doi: 10.1130/G32546.1.
The exceptionally preserved animal embryo–like fossils of the Ediacaran Doushantuo Formation, South China, have been the center of recent phylogenetic debate. While the prominent interpretation identifies these fossils as the earliest representatives of the Metazoa, significantly predating the radiation of macroscopic animals during the Ediacaran-Cambrian transition, others have suggested alternative placement within non-metazoan Holozoa or even as giant prokaryotes. These interpretations, however, are complicated by their complex fossilization histories. These fossils show exquisite cellular and potential intracellular details, which provide tantalizing glimpses into cytological structures of early multicellular organisms, but dissecting their preservation is a necessity to distinguish biological features from preservational artifacts. To address this issue, Schiffbauer et al. utilized a micro-to-nanoscale investigatory approach to provide the most detailed views of these fossils to date. Their findings support a two-stage fossilization history, directly impacting the interpretive power of the intracellular structures. The new data show that the intracellular structures are preserved as later-stage void-filling cements and not direct replicas of membrane-bound organelles, thus casting doubt on the non-metazoan Holozoa interpretation based on the behavior of nuclear membrane during nuclear division. Phylogenetic interpretations of these microfossils, then, should be based on more robust features such as cleavage patterns, cell-cell adhesion, and ornamented envelopes. Individually, these features may not constrain the affinity of these fossils, but their combination can be found only in modern animals.
Detecting hydrate and fluid flow from bottom simulating reflector depth nomalies
Matthew J. Hornbach et al., Southern Methodist University, Huffi ngton Department of Earth Sciences, Dallas, Texas 75275-0395, USA. Posted online 23 Jan. 2012; doi: 10.1130/G32635.1.
For more than two decades, researchers have debated the resource potential of methane hydrate (a solid consisting of ice and methane) and whether it is responsible for global warming and ocean acidification. At the crux of this debate is how much methane hydrate truly exists. Unfortunately, detecting and quantifying methane hydrate is difficult, and as a result, estimates of methane hydrate vary by orders-of-magnitude. Here, using a newly developed 3D thermal modeling technique, Hornbach et al. reveal a new approach for detecting and quantifying methane hydrate in unprecedented detail. This study, located at Hydrate Ridge, offshore Oregon, suggests that heat flow and hydrate concentrations are coupled, and that 3D heat-flow analysis can be used to constrain hydrate quantities in 3D seismic data with meter-scale resolution. Hydrate estimates using this technique are consistent with 1D drilling results, but also reveal new, previously unrecognized swaths of hydrate-rich sediments that have gone undetected due to spatially limited drilling and sampling techniques used in previous studies. The analysis indicates that previous global hydrate estimates based on drilling at this site are low by a factor of ~3, and that we may therefore be significantly underestimating the amount of methane trapped below the world’s oceans.
Boron- and magnesium-rich marine brines at the origin of giant unconformity-related uranium deposits: 11B evidence from Mg-tourmalines
Julien Mercadier et al., Géologie et Gestion des Ressources minérales et énergétiques (G2R), Nancy-Université, CNRS, CREGU, Boulevard des Aiguillettes, B.P. 70239, F-54506, Vandoeuvre-lès-Nancy, France. Posted online 23 Jan. 2012; doi: 10.1130/G32509.1.
In the giant unconformity-related uranium deposits from the Athabasca Basin (Canada), boron- and magnesium-bearing minerals are typically associated with the uranium oxides. Existing models suggest that the boron and the magnesium were leached from surrounding sedimentary or basement rocks, or carried by a basement-derived fluid. In this paper, by Mercadier et al., stable boron isotopic composition of boron- and magnesium-bearing tourmalines from these deposits challenge the previous models by showing that brines of marine origin were the only fluids involved in the formation of the deposits and that they brought most of the boron and magnesium to the ore system.
Seismic constraints on the water flux delivered to the deep Earth by subduction
Brian Savage, Department of Geosciences, University of Rhode Island, 317 Woodward Hall, 9 East Alumni Ave, Kingston, Rhode Island 02881, USA. Posted online 23 Jan. 2012; doi: 10.1130/G32499.1.
For the first time, large quantities of water are identified by Savage throughout the entire length of a descending tectonic plate and an efficient pathway is defined for injecting water into deep Earth, more than 600 km, by the incorporation of sea water. Water is thought to facilitate plate tectonics, but a quantification of the amount of water and thus its effect are poorly known. Taking advantage of an anomalous seismic arrival recorded in the Tonga-Fiji subduction zone, the presence, volume, water content, and flux of water in the descending plate is quantified to be many times larger than previous inferences based on surface observations. Water fluxes of this magnitude into the deep Earth will substantially increase the role of water in hypotheses of plate tectonic movements, mantle plumes, and flood basalt activity.
3.8 Ga zircons sampled by Neogene ignimbrite eruptions in Central Anatolia
Jean-Louis Paquette (corresponding) and Jean-Luc Le Pennec, Clermont Université, Université Blaise Pascal, Laboratoire Magmas et Volcans, BP 10448, F-63000 Clermont-Ferrand, France. Posted online 23 Jan. 2012; doi: 10.1130/G32472.1.
Dating the oldest terrestrial rocks and minerals allows constraining the age and formation conditions of the ancient basement of our present-day continents. The oldest rocks, with ages ranging from 3.8 to 4.0 billion years old, have been identified in Canada, while Australian zircon crystals dated at 4.4 billion years old represent the oldest dated minerals on Earth. The exceptional resistance of zircons grains to repeated geological cycles has allowed dating 3.3 to 3.9 billion year old crystals in several Archean cratons worldwide. Identifying new, scarce, but essential witnesses to this primitive Earth is still a challenging goal. In Central Anatolia (Turkey), thick volcanic deposits were produced by large-scale explosive eruptions. During magma residence and ascent, basement enclaves may be scavenged and mixed with the magma. Here, Paquette and Pennec show that an explosive eruption from Central Anatolia dated at 5.4 million years ago has sampled very old zircon grains ranging from 2.3 to 3.8 billion years old. This is the first documented occurrence of Early Archean zircon crystals sampled by a recent volcanic event. These Central Anatolian zircons are among the oldest discovered on Earth and imply the presence at depth of remnants of an Early Archean basement which was hitherto unknown in Eurasia.
Splay faults imaged by fluid-driven aftershocks of the 2004 Mw 9.2 Sumatra-Andaman earthquake
Felix Waldhauser et al., Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York 10964, USA. Posted online 23 Jan. 2012; doi: 10.1130/G32420.1.
ABSTRACT: High-precision teleseismic double-difference locations and focal mechanisms of aftershocks of the A.D. 2004 magnitude 9.2 Sumatra-Andaman earthquake illuminate an active imbricate fault system in the accretionary prism offshore northern Sumatra. They reveal repeated failure of a shallow northeast-dipping thrust fault above the megathrust, which Waldhauser et al. interpret to be the reactivation of a splay fault that rises from the megathrust at ~55 km depth and cuts through the overriding Sunda plate. The projected intersection of the splay fault with the seafloor correlates with a recently active thrust fault seen in postseismic bathymetry data west of the Aceh basin. A spatiotemporal analysis of the streaky aftershock distribution indicates that ascending fluids released from the subducting oceanic crust along inherited seafloor fabric may control brittle fracture in the overriding plate. Waldhauser et al. speculate that if the splay fault was active coseismically, it may have led to amplified vertical uplift of the forearc ridge and contributed to generating the cataclysmic near-field tsunami that struck the northwest Sumatra coast following the 2004 rupture.
Spatiotemporal analysis of channel wall erosion in ephemeral torrents using tree roots—An example from the Patagonian Andes
Markus Stoffel et al., Laboratory of Dendrogeomorphology, Institute of Geological Sciences, University of Bern, Baltzerstrasse 1+3, CH-3012 Bern, Switzerland. Posted online 23 Jan. 2012; doi: 10.1130/G32751.1.
This paper presents absolute ages for flash floods and related sediment entrainment in headwater catchments to construct a spatio-temporal framework of process dynamics and location of hotspots of channel-wall erosion. The most reliable method for dating erosion is through dendrogeomorphic studies of exposed tree roots. Based on the analysis of erosion signals in root-ring records, Stoffel et al. documented a time series of channel-wall erosion and successfully dated 21 erosive flash flood events since AD 1870 in an ephemeral gully in the Patagonian Andes. The study was performed with roots from Austrocedrus chilensis, Nothofagus dombeyi and Pseudotsuga menziesii. Results demonstrate the potential of root analyses for the determination of hotspots of sediment entrainment. In addition, Stoffel et al. show that the position of damage within individual root rings allows inferences about the seasonal timing of impact and thus an assessment of possible meteorological triggers of erosive events—short intense storms occurring primarily in austral Fall and late Winter in the present case. The approach presented adds significantly to the documentation of sediment entrainment and facilitates identification of areas of rapid erosion in small, remote headwater catchments with ephemeral flash flood activity.
Splay fault slip during the Mw 8.8 2010 Maule Chile earthquake
Daniel Melnick et al., Institut für Erd- und Umweltwissenschaften and DFG Leibniz Center for Surface Process and Climate Studies, Universität Potsdam,14415 Potsdam, Germany. Posted online 23 Jan. 2012; doi: 10.1130/G32712.1.
Splay faults are thrusts that emerge from the plate boundary of subduction zones. Such structures have been mapped at several convergent margins and their activity commonly ascribed to large earthquakes. However, the behavior of splay faults is poorly constrained because typically these structures are located offshore and are difficult to access. Here we use geologic mapping combined with space and land geodesy, as well as offshore sonar data from Isla Santa María in south-central Chile (37°S) to document deformation during the magnitude 8.8 Maule earthquake of February 27, 2010. The earthquake tilted the island and normal faults ruptured the surface and adjacent ocean bottom. Melnick et al. associate these deformation features with a blind thrust fault rooted in the Nazca–South America plate boundary, which slipped during the Maule earthquake. Their field observations support the notion that splay faulting may frequently complement and influence the rupture of subduction-zone earthquakes.
Onset of North Atlantic Deep Water production coincident with inception of the Cenozoic global cooling trend
Michael W. Hohbein et al., Hess Corporation, Houston, Texas 710, USA. Posted online 23 Jan. 2012; doi: 10.1130/G32461.1.
Thermohaline circulation within the global oceans is intimately linked to the global climate. These ocean currents deposit and rework sediments, forming contourite sediment drifts, and analysis of these sediment bodies provides a means of reconstructing the history of the water mass circulation. This study, by Hohbein et al., utilizes seismic and borehole data to identify a large contourite drift body in the Faeroe Shetland Basin, a key gateway for present day circulation of cold deep water (North Atlantic Deep Water) from the high latitude oceans into the North Atlantic. Borehole calibration of the drift dates the onset of a southerly flowing modern style North Atlantic Deep Water mass through the Faeroe Shetland Basin at close to the early to middle Eocene boundary. This timing of onset for deep water circulation coincides closely with independent evidence for major changes in global deep water circulation patterns and biological productivity, and suggests that initiation of North Atlantic Deep Water production had a strong link to Cenozoic global climate evolution.
A 2400 yr Mesoamerican rainfall reconstruction links climate and cultural change
Matthew S. Lachniet et al., Department of Geoscience, University of Nevada−Las Vegas, 4505 Maryland Parkway, Las Vegas, Nevada 89154, USA. Posted online 23 Jan. 2012; doi: 10.1130/G32471.1.
Lachniet et al. used stalagmites to unveil a 2500-year climate history from southwestern Mexico. This discovery, coupled with archaeological evidence, links the rise and fall of ancient Mesoamerican civilizations to changing rainfall. Lachniet et al. analyzed a stalagmite from Juxtlahuaca Cave in the Mexican state of Guerrero. The cave is in the core region of the North American Monsoon, a climate phenomenon that is primarily responsible for rain in most of Mexico. The cave was also a great source for stalagmites; common cave formations formed over thousands of years as minerals from water droplets crystallize and accumulate. Much like tree rings, they can accurately record the rainfall history. Above average rainfall between the first and third centuries, for example, coincided with the rise of the largest early Mesoamerican city of Teotihuacan. At its peak, more than 125,000 people lived around the massive pyramids in the highly developed city. Conversely, a 500-year drying trend, including a drought of more than 150 years, coincided with rapid population decline in Teotihuacan around 550 CE. The drought likely impacted spring-fed agriculture practices in the semi-arid Mexican Highlands.
Global decline in ocean ventilation, oxygenation, and productivity during the Paleocene-Eocene Thermal Maximum: Implications for the benthic extinction
Arne M.E. Winguth et al., Department of Earth and Environmental Sciences, University of Texas at Arlington, 500 Yates Street, Arlington, Texas 76019, USA. Posted online 23 Jan. 2012; doi: 10.1130/G32529.1.
The Cenozoic era, the last 65 million years of Earth’s history, is an ideal geologic time interval to understand key relationships between the climate and the global carbon cycle. One of the most prominent global warming events in the Cenozoic occurred at the Paleocene-Eocene boundary (55 million years ago), referred to as the Paleocene-Eocene Thermal Maximum (PETM), and is characterized by rapid perturbations of the global carbon cycle and a wide-spread extinction of deep-sea foraminifera. In this study, Winguth et al. explored the environmental changes caused by a massive carbon input at the PETM by comparing sedimentary records with results from a comprehensive climate-carbon cycle model. The climate simulations indicate that the extinction of deep-sea foraminifera at the PETM was probably caused by multiple environmental changes, including ocean acidification, lowered oxygen levels in the deep-sea, and a globally reduced food supply.
Near-tropical Early Eocene terrestrial temperatures at the Australo-Antarctic margin, western Tasmania
Raymond J. Carpenter et al., School of Earth and Environmental Sciences, University of Adelaide, SA 5005, Australia. Posted online 23 Jan. 2012; doi: 10.1130/G32584.1.
This paper, by Carpenter et al., presents strong evidence from well-dated plant fossils that land temperatures akin to those now experienced in tropical latitudes occurred in Tasmania, Australia in the Early Eocene global greenhouse climate of about 50 million years ago. This is remarkable given that during the Early Eocene the region lay at approximately 65 degrees south, near the edge of the current Antarctic ice sheet. The fossil leaves were typically very large, and warm climate specialists such as cycads, mangroves, and broad-leaved conifers were identified. The fossils also typically belong to lineages that are now well-represented in Australasian tropical and subtropical rainforests. Overall, the Tasmanian plant fossils provide important independent support for recent conclusions derived from marine fossils that nearby ocean temperatures were extremely warm, perhaps warmer than those experienced anywhere else at high latitudes in the Early Eocene. Moreover, the Tasmanian study represents another example of how profound global climate changes can be over time.
Sea-level fall, carbonate production, rainy days: How do they relate? Insight from Triassic carbonate platforms (Western Tethys, Southern Alps, Italy)
Fabrizio Berra, Università degli Studi di Milano, Dipartimento di Scienze della Terra “A. Desio”, Via Mangiagalli 34, 20133 Milan, Italy. Posted online 23 Jan. 2012; doi: 10.1130/G32803.1.
In a study by Berra, the interplay between sea-level, climate, and environmental changes can be deciphered in favorable settings of the geological record. In the Western Tethys the effects of two major sea-level falls are recorded on Triassic carbonate platform top and basins. These sea-level falls are coupled with a shift from carbonate to siliciclastic deposits which reflect different climate sensitive facies (arid before and humid after) that can be traced up to Central Europe. The association of sea-level fall, demise of carbonate factories and climate change supports that a global cooling event triggered a shift from arid/semi-arid conditions during warm periods to semi-humid/humid conditions. Climate changes markedly affect precipitation patterns between 25° and 35° latitude, corresponding to the paleolatitude of the continental area to the north of the studied platforms. Sea-level fall and increased rainfall document a global cooling event which caused the shift of the depositional environment and changes in sediment composition. The control of global climate changes on sea-level fall, rainfall distribution, and demise of carbonate production highlights how, (1) climate changes may interact with sedimentation and (2) the sequence stratigraphic evolution of a sedimentary succession can be affected by climate-driven sedimentological changes interacting with the shift of base level.
CO2-depleted warm air venting from chrysotile milling waste (Thetford Mines, Canada): Evidence for in-situ carbon capture from the atmosphere
Julie Pronost et al. (Georges Beaudoin, corresponding), Département de Géologie et Génie Géologique, Université Laval, 1065 Avenue de la Médecine, Québec, Québec G1V 0A6, Canada. Posted online 23 Jan. 2012; doi: 10.1130/G32583.1.
Diffuse, warm air vents at the surface of a chrysotile mining waste heap at the Black Lake Mine, Thetford Mines, Québec, Canada. The venting areas form snow-free areas of unfrozen ground. The temperature and chemical composition of the warm air vents have been monitored for more than one year, from March 2009 to July 2010. The temperature of the warm air and ground surface at the venting sites ranged from 6.6 to 20.0 °C. In winter, the warm air has low CO2 content, between 10 and 18 ppm CO2 but has otherwise normal atmospheric gas composition. In summer, the venting areas are more diffuse, and less depleted in CO2 (260–370 ppm). Frozen ground is likely focusing airflow in winter compared to summer. Air enters the steep flanks of the chrysotile mining waste heap, into which CO2 reacts with Mg-rich minerals, stripping CO2 from air by exothermic mineral carbonation reactions. Pronost et al. estimate that the Black Lake Mine heap passively captures at least 0.6 kt CO2 per year.
Energetics of normal earthquakes on dip-slip faults
David Dempsey et al., Department of Engineering Science, University of Auckland, Private Bag 92019, Auckland Mail Centre, Auckland, New Zealand. Posted online 23 Jan. 2012; doi: 10.1130/G32643.1.
Earthquakes occurring on faults in regions of extension are accompanied by a loss of elevation along one side of the fault. This paper, by Dempsey et al., explores the role of gravity in assisting, or resisting, displacement of the crust during these types of earthquakes. A computational model describes seismic deformation around a hypothetical fault, including storage and release of elastic and gravitational energy, and dissipation of energy by fault friction and other mechanisms. The model demonstrates that, during an earthquake, uplift on one side of the fault is driven by a source of elastic energy at depths of 6 to 8 km. In contrast, on the other side of the fault, elastic energy is stored as the subsiding crust compresses the rock below. Associated with these localized fault phenomena is a low-level, but widespread, compression of the near surface of the crust, which might be associated with increased spring or stream flow following an earthquake.
Burgess shale−type biotas were not entirely burrowed away
Robert R. Gaines et al., Geology Department, Pomona College, 185 E. Sixth Street, Claremont, California 91711, USA. Posted online 23 Jan. 2012; doi: 10.1130/G32555.1.
Burgess Shale–type fossil assemblages provide the foundation for our understanding of the Cambrian Explosion, and therefore comprise a particularly valuable piece of the fossil record. Curiously, this type of preservation, once widespread, suffered a drastic decline after the Cambrian period. Until now, conventional wisdom has held that these exquisitely preserved fossil assemblages were literally “burrowed away” from the fossil record by increasing activity of burrowing organisms. In a study by Gaines et al., new drill cores through the Chengjiang as well as data from the Burgess Shale and other principal deposits from around the world demonstrate that Burgess Shale–type fossil assemblages were not simply burrowed away. Escalating bioturbation would have greatly limited the geographic and stratigraphic ranges of Burgess Shale–type biotas, however, increasing burrowing activity would not have affected the most richly fossiliferous portions of the Burgess Shale and the Chengjiang, the two most important deposits. These findings emphasize the role of widespread oxygen deficiency in offshore environments of the Cambrian oceans in promoting exceptional fossil preservation, in part, by restricting burrowers from the sites of deposition. While widespread oxygen deficiency may have assisted in extraordinary preservation, it also may have placed limits on animal evolution during the Cambrian period.