|26 October 2010
GSA Release No. 10-56
Director of Education, Communication, & Outreach
November 2010 Highlights
Boulder, CO, USA - Geology includes two papers on the 2009 6.3 L'Aquila earthquake, one of which features a new analysis technique, Focal Mechanism Tomography; a study on the impact of the Three Gorges Dam on the middle Yangtze; three papers on methane hydrates; findings of "easily recognized signatures of life" in iron-oxide-rich spheres and pipes near Escalante, Utah; and examination of a ~300-million-year-old bivalve shell. GSA Today investigates what caused the Dead Interval after the Cambrian explosion.
Keywords: methane hydrates, Sri Lanka, monsoons, L'Aquila earthquake, UK North Sea oils, electron paramagnetic resonance spectrometry, Himalayan Range Front, North Anatolian fault, Focal Mechanism Tomography, Navajo sandstone, Gorgona komatiite, Aucanquilcha volcanic cluster, Mount St. Helens, Permian-Triassic marine biotic crisis, Early Permian seasonality, short-wavelength infrared spectroscopy, seawater boron, Akiyoshi karst dolines, Three Gorges Dam, Martian dune gullies, Swatch of No Ground, Cambrian Explosion, Great Ordovician Biodiversification Event.
Highlights are provided below. Representatives of the media may obtain complementary copies of GEOLOGY articles by contacting Christa Stratton at the address above. GSA TODAY articles are open access. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOLOGY or GSA TODAY in articles published.
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Access the GSA TODAY science article by clicking on the issue cover icon at http://www.geosociety.org/pubs/.
Abstracts for the November issue of GEOLOGY are available at http://geology.gsapubs.org/ under “pre-issue publication.”
Methane recycling between hydrate and critically pressured stratigraphic traps, offshore Mauritania
Richard J. Davies and Amy L. Clarke, Centre for Research into Earth Energy Systems (CeREES), Dept. of Earth Sciences, University of Durham, Science Labs, Durham DH1 3LE, UK. Pages 963-966.
Methane hydrates form naturally when gas and water found beneath the seabed freeze. They are important because methane is 25 times more effective than carbon dioxide as a greenhouse gas. Many scientists have suggested that if hydrates were to start to melt, methane could cause "runaway global warming," as new methane would further warm the planet, in turn causing additional hydrate melting. Methane release from hydrates has been linked to past major climatic warming events by many scientists and could be a trigger for future climatic change. By examining new three-dimensional seismic images of a methane hydrate in the Atlantic Ocean, Davies and Clarke propose a new model for the fate of released methane. It is not vented to the seabed and into the atmosphere where it could cause climate change, but instead is safely stored in underground rock strata in what are known as stratigraphic traps. Methane could be repetitively recycled between the frozen hydrate and the rock strata traps during phases of warming and cooling (when the methane could refreeze), without methane being released as a greenhouse gas. The geology of our oceans could in fact be adept at stopping methane emissions and storing the methane underground in such rock strata traps for significant periods of geological time.
Frontal ridge slope failure at the northern Cascadia margin: Margin-normal fault and gas hydrate control
Caroll López et al., School of Earth and Ocean Sciences, University of Victoria, Victoria, British Columbia V8W 3V6, Canada. Pages 967-970.
Submarine landslides are found on steep ridges at the foot of the continental slope off Vancouver Island, Canada. Lopez et al. examined a 2-km-wide slide with a 300-m-high headwall. The sidewalls of the slide are aligned with margin-perpendicular faults that are evident on the seafloor as prominent 15- to 75-m-high seafloor scarps. They thus interpret that the lateral extent of the slide is controlled by the faults. Gas hydrate, a form of methane ice, has been recovered beneath the ridge by the Integrated Ocean Drilling Program. Seismic data indicate that gas hydrate is widely distributed beneath the ridge, and that it extends downward from the seafloor to a well-defined depth of 255 m (plus or minus 15 m). From detailed bathymetry data, the submarine slide is clearly seen as a gouge in the ridge. By estimating the amount of missing material, López et al. determined that the slide thickness was also 255 m. Since the depth of the glide plane for the slide coincides with the base of gas hydrate, this suggests that the failure surface is controlled by gas hydrate. Below the gas hydrate, free gas weakens the sediments relative to the shallower, hydrate-cemented sediments. Strong earthquake shaking on this convergent margin likely provided the trigger for the slide.
Sensitive high-resolution ion microprobe U-Pb dating of prograde and retrograde ultrahigh-temperature metamorphism as exemplified by Sri Lankan granulites
K. Sajeev et al., Centre for Earth Sciences, Indian Institute of Science, Bangalore 560012, India. Pages 971-974.
The rocks of the central Highland Complex in Sri Lanka have been subject to some of the highest peak temperatures of crustal metamorphism known, over 1100 degrees Celsius. At such temperatures, most rocks would turn into molten magma, but K. Sajeev of the Indian Institute of Science and colleagues report rocks from near Kandy that not only survived the high temperatures, but that contain zircon crystals in which a U-Pb isotopic record of their provenance and thermal history has survived. Such survival is contrary to all predictions from experimental studies of the rate that lead is lost from zircon by thermal diffusion. From a study of the metamorphic minerals and thermodynamic modeling, and U-Pb isotopic analyses of zircon and monazite, Sajeev et al. show that the rocks near Kandy were sediments derived from sources ranging in age from 2500 to 830 million years. The sediments were heated to over 1100 degrees Celsius at a depth of about 25 km about 570 million years ago, then rapidly lifted toward the surface while still hot about 550 million years ago. These Sri Lankan rocks were probably trapped and buried in the violent collision between the two halves of the Gondwana supercontinent about 600 million years ago, superheated by basalt magmas rising from the Earth's interior, then forced to the near surface again as the tectonic pressures relaxed. The preservation of the isotopic record of these events is remarkable, and still remains to be fully explained.
Monsoon control of effective discharge, Yunnan and Tibet
Amanda C. Henck et al., Quaternary Research Center and Dept. of Earth and Space Sciences, 070 Johnson Hall, Box 351310, University of Washington, Seattle, Washington 98195, USA. Pages 975-978.
Analysis of suspended sediment and discharge data for rivers in Yunnan and Tibet shows that monsoon flows control suspended sediment transport. Amanda C. Henck of the University of Washington and colleagues calculated effective discharge, defined as the discharge that transports the most sediment, for 44 stations and found that the effective discharge is approximately the mean monsoon discharge for all stations. The correspondence of the effective discharge with the mean annual flow and monsoon discharge for all stations demonstrates that monsoon flow dominates suspended sediment transport in the region, rather than storm flow during discrete, short-duration storm events. In this region, the monsoon lasts for 4 months (June-September) and during that time, transports almost 90% of the suspended sediment load. In contrast to the general observation from temperate environments that infrequent, stochastic storm events dominate sediment transport (with 90% of the suspended sediment transport occurring in 10% of the time), Henck et al.'s findings show that the mean monsoon discharge dominates sediment transport in the rivers draining the southeastern Tibetan Plateau.
Fault-charged mantle-fluid contamination of United Kingdom North Sea oils: Insights from Re-Os isotopes
Alexander J. Finlay et al., Dept. of Earth Sciences, Durham University, Science Laboratories, Durham DH1 3LE, UK. Pages 979-982.
By analyzing two isotopes of osmium (Os-187 and Os-188), Alexander J. Finlay of Durham University and colleagues investigate how fluids sourced from the mantle can interact with oil systems. Fluids sourced from the mantle have a low Os-187/ Os-188 ratio, whereas oils generally have a high Os-187/ Os-188 ratio. Within the United Kingdom North Sea, some areas of oil fields contain oil that has a low Os-187/Os-188. Therefore, Finlay et al. show that these oils have interacted with fluids from the mantle, and so provide further insights on migration pathways within oil systems.
Tracing past migrations of uranium in Paleoproterozoic basins: New insights from radiation-induced defects in clay minerals
E. Morichon et al., Universite de Poitiers, Laboratoire HydrASA, CNRS-UMR 6269, 40 avenue du Recteur Pineau, F-86022 Poitiers Cedex, France. Pages 983-986.
E. Morichon of Universite de Poitiers and colleagues present a study of three groups of clay minerals typical of the alteration halos surrounding the uranium orebodies in the Athabasca basin (Saskatchewan, Canada). In spite of their age (1.2-1.6 billion years old), the use of electron paramagnetic resonance (EPR) spectrometry allowed identification of natural radiation-induced defects in the clay minerals' structure, which appear similar in nature. However, their concentrations can vary over several orders of magnitude, particularly along the regional unconformity between the lower sandstones and the basement rocks, and close to crosscutting brittle structures as well in sandstones and in basement. Two major conclusions can be deduced from these results: (1) the proximity of mineralization can be revealed through defects that record the past presence of uranium in altered rocks at significant distances from the mineralized bodies, and (2) migrations of uranium-bearing fluids took place after the formation of clay minerals, according to the absence of correlation between defect concentrations and present dose rate. The EPR spectrometry, useful for exploration, seems to be a promising method to identify ancient pathways of uranium-bearing fluids.
The signature of an unbalanced earthquake cycle in Himalayan topography?
Brendan J. Meade, Dept. of Earth & Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, Massachusetts 02138, USA. Pages 987-990.
Fifty percent of the relative motion between the Indian and Asian plates is accommodated by active convergence at the Himalayan Range Front (HRF). Interseismic rock uplift rates reach a maximum north of the active main frontal thrust, and have been suggested to significantly influence the collocated convex bulge in HRF topography. Using geodetically constrained models of interseismic rock uplift rates and simple channel erosion rate laws, Harvard’s Brendan Meade shows that convex channel profiles are predicted when interseismic deformation outpaces coseismic deformation. Applying this model to the observed elevation profiles of 20 HRF-spanning channels in Nepal yields a minimum mean residual elevation (72 m) if interseismic deformation has outpaced coseismic deformation by a factor of four. The long-term earthquake deficit required for the application of this model is consistent with some estimates of historical moment imbalance but requires temporally variable fault system activity. The spatial correlation between nominally interseismic rock uplift and the HRF topographic bulge may be explained by (1) a non-causal geometric coincidence, (2) geodetic observations of significant deformation not directly related to earthquake cycle processes, or (3) an unbalanced earthquake cycle at the HRF.
Rupture characteristics of the A.D. 1912 Mürefte (Ganos) earthquake segment of the North Anatolian fault (western Turkey)
Murat Ersen Aksoy et al., Institut de Physique du Globe (UMR 7516) Strasbourg, 5 rue Rene Descartes, 67084 Strasbourg Cedex, France. Pages 991-994.
The North Anatolian fault is a major tectonic plate boundary that crosses Turkey and has generated several large magnitude earthquakes (greater than 7) in the 20th century, among them the 9 August 1912 Murefte (Ganos) earthquake (magnitude 7.4). This study by Aksoy et al. deals with the measurements of the 1912 earthquake fault slip coupled with an analysis of historical seismic records. The 1912 surface ruptures show 1.5-5.5 m right-lateral offsets measured at 45 sites on the onshore 45-km-long fault section. Because part of the fault zone is offshore, fresh fault scarps are visible in the Sea of Marmara and Saros Bay. An analysis of seismograms revealed that the 9 August shock was followed by the 13 September 1912 earthquake (magnitude 6.8). In addition to the offset measurements and fault structure observations, the seismic model indicates a 40-second rupture duration, in agreement with an estimated total 150 plus or minus 30-km-long rupture for the two earthquakes. The total fault zone results from a combination of onshore and offshore fault length. These parameters allow Aksoy et al. to better constrain the western limit of the Marmara Sea seismic gap, and the related potential for a large earthquake, which sharply increased after the devastating 1999 Izmit seismic event.
High-pressure fluid at hypocentral depths in the L'Aquila region inferred from earthquake focal mechanisms
Toshiko Terakawa et al., Research Center for Seismology, Volcanology and Disaster Mitigation, Nagoya University, Nagoya 464-8601, Japan. Pages 995-998.
Toshiko Terakawa of Nagoya University and colleagues apply a new analysis technique using earthquake focal mechanisms to infer the three-dimensional fluid pressure field at depth in the source region of the 2009 L'Aquila earthquake and aftershock sequence. The technique, termed Focal Mechanism Tomography, inverts for absolute fluid pressure by examining the fault orientation relative to the regional tectonic stress pattern. They identify three large-scale pockets of high fluid pressure (up to 50 megapascals above hydrostatic pressure) at depths of 7-10 km that strongly correlate with an independent dataset of well-located foreshocks and aftershocks. The shape of over-pressured regions and the evolution of seismicity indicate a plausible scenario that this sequence is being driven in part by the poro-elastic response of trapped reservoirs of high-pressure fluid, presumably carbon dioxide, and post-seismic fluid flow initiated by the main shock.
Follow the water: Connecting a CO2 reservoir and bleached sandstone to iron-rich concretions in the Navajo Sandstone of south-central Utah, USA
David B. Loope et al., Dept. of Geosciences, University of Nebraska, Lincoln, Nebraska 68588-0340, USA. Pages 999-1002.
David B. Loope of the University of Nebraska and colleagues describe the Navajo Sandstone of southern Utah, which contains abundant iron-oxide-rich spheres and pipes. These are composed of a dense, black, outer rind, but inside, they contain light-colored, iron-poor sand. The Utah spheres -- outwardly similar to those imaged on the surface of Mars -- formed when groundwater first dissolved iron that was originally widely dispersed in the porous rock. The water then transported and precipitated the iron to form solid spheres cemented by iron carbonate. Iron-rich pipes over a broad area near Escalante, Utah, parallel the direction that groundwater and surface water now flow in the area. Iron was stripped from sandstones that held carbon dioxide and methane at high elevation, and was then carried southeastward to lower elevations by gravity. Later, after the carbon dioxide and methane had been flushed from the rock, iron-oxidizing microbes feasted on the iron carbonate. After the microorganisms colonized the surfaces of the spheres, the iron inside the spheres diffused outward to the colonies, where it combined with oxygen to form the iron-oxide rinds. The chemical species needed to make the spheres are widespread in our solar system. The iron carbonate cements provide energy and carbon sources for life, and the rinded structure of the spheres is a distinct, easily recognized signature of life.
Composition and temperature of komatiite melts from Gorgona Island, Colombia, constrained from olivine-hosted melt inclusions
Vadim S. Kamenetsky et al., ARC Centre of Excellence in Ore Deposits, and School of Earth Sciences, University of Tasmania, Hobart, Tasmania 7001, Australia. Pages 1003-1006.
Hot and magnesian komatiite magmas are represented among many Precambrian magmatic suites. The only known example of Phanerozoic (Late Cretaceous) komatiite is found on Gorgona Island (Colombia). This study by Vadim S. Kamenetsky of the University of Tasmania and colleagues demonstrates that melt inclusions, entrapped in primitive olivine in these unusual rocks, originated from a single parental melt with elevated abundances of volatiles elements (water, boron, and chlorine). A primary melt composition had about 17 wt% MgO, and given the presence of 0.2-1.0 wt% water, the initial crystallization temperature could be as low as 1330-1340 degrees Celsius. The result is quite surprising; it looks like the Gorgona komatiite magmas were not much hotter than primitive magmas of mid-ocean ridges. The magmatic origin of volatiles in the komatiite melt has significant implications for composition and melting of the mantle source. It is possible that the mantle source of the Gorgona komatiite was enriched in volatiles, although still depleted in incompatible trace elements.
Organization and thermal maturation of long-lived arc systems: Evidence from zircons at the Aucanquilcha volcanic cluster, northern Chile
Barry Alan Walker et al., Oregon State University, Corvallis, Oregon 97331, USA. Pages 1007-1010.
This study by Barry Alan Walker of Oregon State University and colleagues explores long-term interaction between magma and Earth's continental crust to address current debates about the connection between plutonic (deeply cooled) and volcanic rocks. They document the age and temperature information stored in zircon crystals in lavas erupted intermittently for the last 11 million years from the Aucanquilcha volcanic cluster in northern Chile, where the arid climate has preserved a long history of Andean subduction volcanism. In particular, by using super high-resolution ion microprobe techniques, uranium-lead ages were obtained from individual spots about 25 microns in diameter for about 20 points each from 244 zircon grains spanning the range of eruption ages of the Aucanquilcha volcanic cluster. These ages were coupled with titanium analyses to obtain the temperatures of zircon crystallization. Walker and colleagues find that crystal ages and compositions within individual lava samples vary systematically with the magnitude of eruptive activity. The maximum eruption rate, the most restricted zircon age range per sample, and the magma temperature, as deduced from titanium-in-zircon thermometry, coincide and reflect thermal maturity of the magma system at about 5-2.5 million years ago, after about six years of thermal incubation.
Zircon reveals protracted magma storage and recycling beneath Mount St. Helens
Lily L. Claiborne et al., Dept. of Earth and Environmental Sciences, Vanderbilt University, Nashville, Tennessee 37240, USA. Pages 1011-1014.
Previous work at Mount St. Helens found that minerals and melts erupted from the volcano were relatively young, suggesting that magmas move relatively quickly from source to eruption at this volcano. Lily L. Claiborne of Vanderbilt University and colleagues explore the full magmatic history of Mount St. Helens by studying zircon from samples spanning the eruptive history of the volcano. Zircon is a resilient mineral that incorporates uranium (U), titanium (Ti), and hafnium (Hf) as it grows, and can therefore be used to determine the age of the crystal (U), and the temperature (Ti) and composition (Hf) of the magma from which it grew. The wide range of zircon ages found in rocks from Mount St. Helens indicates that magmas regularly stall in the crust, cool, and crystallize beneath the volcano, and are then rejuvenated and incorporated by hotter, young magmas on their way to the surface. Compositional changes can be tracked in the magmas in this storage zone that do not appear at the surface for tens of thousands of years. As a whole, the zircon data reveal the history of the hidden roots of Mount St. Helens, where melt and crystals are stored for up to hundreds of thousands of years and interact with fresh magmas that traverse the intrusive reservoir before erupting.
Temporal variation of seismic velocity and anisotropy before the 2009 MW 6.3 L'Aquila earthquake, Italy
Francesco Pio Lucente et al., Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy. Pages 1015-1018.
Through seismological observations of the foreshock sequence preceding the dramatic 6 April 2009 magnitude 6.3, L'Aquila earthquake in Italy, Lucente et al. give a detailed reconstruction of the preparatory phase of the main shock. Prior to the earthquake, clear variations of the seismic wave propagation properties in the fault region are observed. The elastic characteristic of rocks within the main shock nucleation volume undergoes a dramatic change about a week before the earthquake. The observations suggest that a complex sequence of dilatancy and fluid diffusion phenomena affected the rocks surrounding the nucleation area, with paroxysmal manifestations about a week before the main shock occurrence. Differences in the seismic wave behavior at various seismic stations in the epicenter area allow for retrieval of the directional properties of the fluid diffusion process, and the proposal of a physical model explaining the observations.
Massive methane release triggered by seafloor erosion offshore southwestern Japan
N.L. Bangs et al., The University of Texas at Austin Institute for Geophysics, J.J. Pickle Research Campus, Building 196, 10100 Burnet Road, Austin, Texas 78758-4445, USA. Pages 1019-1022.
Methane hydrate is an ice-like material that forms from mixing methane gas and water under high-pressure and low-temperature conditions. Vast amounts of methane hydrate exist beneath the seafloor along continental margins around the world, but it is unclear if this methane ever releases on a large enough scale to impact the oceans, and potentially the atmosphere. N.L. Bangs of the University of Texas at Austin and colleagues analyzed new three-dimensional seismic reflection data that show a huge V-shaped notch, 400 m deep, 4 km wide, and about 35 km long, along the southern margin of the Kumano Basin 100 km offshore of southwest Honshu, Japan. Their analysis shows that the notch formed by rapid seafloor erosion, which was caused by deep currents and by unroofing highly pressured methane that vented and further eroded the seafloor. This process rapidly released massive quantities of methane (estimated at about 1.5 E-11 cubic meters) from the notch.
Anomalous Early Triassic sediment fluxes due to elevated weathering rates and their biological consequences
Thomas J. Algeo, Dept. of Geology, University of Cincinnati, Cincinnati, Ohio 45221, USA; and Richard J. Twitchett. Pages 1023-1026.
The Permian-Triassic boundary mass extinction is the largest such event in the geologic record, during which about 90% of all marine species died out. The underlying causes of this biotic crisis, as well as of the subsequent delayed recovery of marine ecosystems, remain poorly understood. Thomas J. Algeo of the University of Cincinnati and colleague Richard Twitchett have determined for the first time that sedimentation rates in epicontinental and coastal marine areas increased at the time of the mass extinction event, and remained elevated for up to 2 million years during the Early Triassic. This increase, which averaged 700% globally, was due to a combination of high surface temperatures and acid rainfall, which accelerated weathering rates, as well as reduced landscape stability following the destruction of terrestrial ecosystems. The surge in sediment from the land to the seas may have contributed to the Permian-Triassic marine biotic crisis owing to the harmful effects of siltation and eutrophication on the feeding activity, osmoregulation, growth rate, larval recruitment, development, and survival of many marine organisms. Patterns of differential survival and re-radiation among clades of marine organisms during the Early Triassic are consistent with the effects of a massive sediment pulse into the sea.
Early Permian seasonality from bivalve delta-18O and implications for the oxygen isotopic composition of seawater
L.C. Ivany, Dept. of Earth Sciences, Syracuse University, Syracuse, New York 13244-1070, USA; and B. Runnegar. Pages 1027-1030.
The oxygen isotopic composition of marine carbonates is widely used to infer paleotemperatures in Earth's distant past. The older the carbonate is, however, the more skeptical scientists become that it still retains its original isotope value and thereby preserves the signal of ancient climate. L.C. Ivany of Syracuse University and colleague B. Runnegar present data from an ~300-million-year-old bivalve shell sampled at high resolution across growth bands reflecting the years over which the clam was alive. They show that oxygen isotope values increase and decrease systematically with age, reflecting the original seasonal variation in temperature experienced during the clam's lifetime. This is the oldest record of seasonal temperature variation yet recovered, and Ivany and Runnegar argue that preservation of this seasonal cycle proves that the carbonate chemistry is still original. In addition, the fossil was collected from what was a high-latitude site during an interval of glaciation, and co-occurs with glacial deposits and a mineral that only forms at near-freezing temperatures. In combination with this independent evidence for cold conditions, the bivalve isotope values suggest that ocean chemistry was different in the past, and offer support for a controversial hypothesis of secular change in the oxygen isotopic composition of the oceans over Earth's history.
Short-wavelength infrared spectroscopy: A new petrological tool in low-grade to very low-grade pelites
Michael P. Doublier et al., Geological Survey of Western Australia, P.O. Box 1664, Kalgoorlie, WA 6433, Australia. Pages 1031-1034.
Short-wavelength infrared spectroscopy is a quick method of mineral physico-chemistry analysis, and is particularly applied in economic geology and mineralization studies. Michael P. Doublier of the Geological Survey of Western Australia and colleagues show that spectrally derived mineral data can also be used to quantitatively describe metamorphic parameters in pelites, based on the spectral features related to potassic white mica. For the estimation of metamorphic grade, three different spectral parameters are presented and applied to a regional example from the southern French Massif Central, where they allow differentiation between diagenetic and anchi/epizonal metamorphic grade areas. Another parameter differentiates the metamorphic pressure facies and related metamorphic thermal gradients. An application example of this parameter includes samples from New Caledonia and the French Massif Central. An interesting aspect of the development of these parameters in the short-wavelength infrared spectral range is that there is application potential in remote sensing studies.
Geological evolution of seawater boron isotopic composition recorded in evaporites
G. Paris et al., Equipe Geochimie et Cosmochimie, Institut de Physique du Globe de Paris, Universite Paris-Diderot, UMR CNRS 7154, 4 place Jussieu, 75252 Paris, France. Pages 1035-1038.
Understanding the evolution of carbon dioxide content in the atmosphere through geological times will help us to better understand the consequences of the anthropogenic carbon dioxide input within the atmosphere. Oceanic pH, which interestingly reflects atmospheric carbon dioxide levels, can be estimated from the abundance of boron isotopes in ancient marine carbonates. This powerful proxy requires, however, that we know the boron isotopic composition of seawater at any geological time. The past chemical composition of seawater is trapped in fluid inclusions inside sea-salt crystals, such as halite. Here, G. Paris, J. Gaillardet, and P. Louvat of the Institut de Physique du Globe de Paris report the secular variation of boron isotopic composition measured in ancient marine halite, expressed as per mil (1/1000). Phanerozoic halites reveal a clear evolution of the boron isotopic composition of seawater (up to 8 per mil) during the past 40 million years, which can be compared with that of other seawater constituents, such as the ratio between magnesium and calcium concentration in seawater. High consistency between these tracers should permit a better understanding of the drivers of seawater chemical composition in future studies. These data will also allow us to reevaluate the evolution of atmospheric carbon dioxide levels during the past 40 million years.
Evolution of solution dolines inferred from cosmogenic 36Cl in calcite
Yuki Matsushi et al., Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan. Pages 1039-1042.
It takes 0.3-0.6 million years to develop solution dolines in Akiyoshi karst, southwest Japan. A new approach reveals the ages of solution dolines, dimple-like closed depressions commonly observed in karst landscape, using a rare isotope accumulated in carbonate rock surfaces by cosmic ray irradiation. Development of such dolines has been explained by a relatively rapid lowering of land surfaces near the center of a depression, owing to the enhanced carbonate dissolution by shallow groundwater convergence. This report by Yuki Matsushi of Kyoto University and colleagues demonstrates initial verification of this hypothesis and presents a simple modeling of the evolution of doline side-slopes for determining formative ages of the depressions.
Implications of flow control by the Three Gorges Dam on sediment and channel dynamics of the middle Yangtze (Changjiang) River, China
Zhongyuan Chen et al., State Key Laboratory for Estuarine and Coastal Research, East China Normal University, Shanghai 200062, China. Pages 1043-1046.
China's Three Gorges Dam, completed in 2003, was built to help mitigate floods in the Yangtze River, generate hydroelectricity, and facilitate shipping. The dam retains the majority of sediment supplied from the upper catchment, leading to scour below the dam. Prior to the building of the dam, there was a seasonal pattern of scour and erosion in the channel of the Middle Yangtze, and Dongting Lake has a long history of sedimentation from summer high flows. Regulation of flows for hydroelectricity generation will maintain flows downstream of the dam at erosive levels for longer time periods. River channel erosion will extend downstream from the dam toward the coast, but the time frame for the development of a new equilibrium channel is not yet known. The middle Yangtze is becoming a new source area of sediment, having previously been a net sink of sediment. It is predicted that the river channel of the middle-lower Yangtze will experience morphological adjustment in response to these changes that will take many decades. On the other hand, the historic shrinkage of Dongting Lake (from more than 6000 square kilometers 150 years ago to presently about 2000 square kilometers) in the middle Yangtze River will cease due to sediment reduction and changes to flow patterns.
Seasonality of present-day Martian dune-gully activity
Serina Diniega et al., University of Arizona Tucson, Arizona 85721, USA. Pages 1047-1050.
Since their discovery in 2001, gullies found on Martian slopes have been argued to be evidence for liquid water flow on the surface of Mars, due to their resemblance to features carved by streams of water on Earth. However, Serina Diniega of the University of Arizona and colleagues use high-resolution images of Mars' surface (taken by the HiRISE camera aboard the Mars Reconnaissance Orbiter spacecraft) to show that changes in gullies on the slopes of sand dunes occur only during the winter, when temperatures are too cold for liquid water to exist at the surface. They present observations that imply that within the present-day Martian environment it is the seasonal accumulation and sublimation of carbon dioxide frost, not water, that is responsible for changing dune gullies: Both initiating material transport and extending the channels.
Mass failures associated with the passage of a large tropical cyclone over the Swatch of No Ground submarine canyon (Bay of Bengal)
Kimberly G. Rogers and Steven L. Goodbred, Jr., Dept. of Earth and Environmental Sciences, Vanderbilt University, Nashville, Tennessee 37235, USA. Pages 1051-1054.
Submarine mass failures are important mechanisms for transporting sediment and sequestered carbon across the continental margin to the deep sea. While most studies have centered on failures at the continental slope, Rogers and Goodbred present a pre- and post-storm comparison of morphologic features at the shallow-water head of the Swatch of No Ground submarine canyon, which incises the Bengal shelf and prograding delta front of the Ganges-Brahmaputra River. Following the passage of Cyclone Sidr in 2007, storm-induced mass wasting features around the canyon head ranged from liquefied mass flows with limited down-slope displacement of material, to kilometer-wide valleys resulting from the full or partial removal of sediment. Steep-walled gullies present before the storm resisted failure and may define the stability threshold of cohesive sediments that are prograding across the canyon. Kimberly G. Rogers and Steven L. Goodbred of Vanderbilt University suggest that subsurface controls, such as groundwater sapping or preexisting deformation, predispose certain areas of the canyon head to repeated failure. Such recurrent mass wasting may explain the relatively stable position of the canyon head despite being actively infilled with deltaic sediments.
Tropical Shoreline Ice in the Late Cambrian: Implications for Earth's climate between the Cambrian Explosion and the Great Ordovician Biodiversification Event
Anthony C. Runkel et al., Minnesota Geological Survey, University of Minnesota, 2642 University Avenue W, St. Paul, MN 55114-1057, USA. Pages 4-10.
What happened after the Cambrian "explosion" of multicellular life? Instead of an ever-expanding array of fauna, Earth experienced a 25-million-year period of subdued diversification, sometimes referred to as the Dead Interval. But just what caused the Dead Interval? That is the question addressed by Anthony Runkel of the Minnesota Geological Survey and his colleagues based on their mapping of Cambrian equatorial beach deposits preserved in southern Minnesota. What Runkel et al. demonstrate is that the beach deposits are characterized by features indicative of temperatures below freezing, including the presence of clasts that preserve evidence of having been cemented by ice during deposition. These findings require freezing temperatures at the equator during deposition. Such conditions were previously only thought to have existed during Neoproterozoic Snowball Earth events that predate and heralded the Cambrian Explosion, and suggest that the Dead Interval may have been characterized by one or more extraordinary global climate events involving extreme cold. The Dead Interval, and the subsequent great biodiversification of life on Earth, may therefore reflect the survival and subsequent success of a select few species that were able to survive yet another extreme climate event involving global freezing that developed after the Cambrian explosion of life.