|20 May 2013
GSA Release No. 13-32
Transition between glacial conglomerates to postglacial cap carbonates of once tropical Ediacaran Siberia exposed at shore of Lake Baikal. Rock hammer for scale.
Image courtesy Arkady Stanevich.
See related article by Ivanov et al.,
New in GEOLOGY: Gems, Darwin, Mars, Hemp, Snowball Earth, a Siberian Impact Crater, and More
Boulder, Colorado, USA – New Geology articles posted online ahead of print 9 and 16 May 2013 cover a wide swath of geoscience subdisciplines, including minerals exploration, archaeology, planetary geology, tectonics, oceanography, geophysics, and paleobotany. Locations studied include Siberia; the Sumatran subduction margin; the Monte Arsiccio mine at Alpi Apuane, Italy; Ukraine; Mars; and the Southeastern U.S. Atlantic Margin. Brief highlights follow:
1. Rubies, jadeite, and plate tectonics;
2. The clear fingerprint of ice ages left on coral reefs around the world;
3. The history of human activities related to hemp;
4. Glaciers on Mars;
5. The "mush model" and a deepening in geoscientists' understanding of silicic magma systems;
6. A call for a cautious use of ocean sediments in developing past histories of major earthquakes.
7. Analysis of orebodies of the Monte Arsiccio mine, Italy;
8. Major targets for mineral exploration;
9. High-frequency, climatically controlled sea-level changes;
10. The pollen record in lake sediments from the Boltysh meteorite impact crater in Ukraine;
11. Ediacaran glaciation in Siberia and snowball Earth;
12. The first evidence for widespread seabed methane venting along the southeastern U.S.; and
13. Nano- to micro-scale frictional processes.
GEOLOGY articles published ahead of print can be accessed online at http://geology.gsapubs.org/content/early/recent. All abstracts are open-access at http://geology.gsapubs.org/. Representatives of the media may obtain complimentary GEOLOGY articles by contacting Kea Giles at the address above. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOLOGY in articles published. Contact Kea Giles for additional information or assistance. Detailed highlights are provided below.
Non-media requests for articles may be directed to GSA Sales and Service, .
Plate tectonic gemstones
Robert J. Stern, Geosciences Dept., The University of Texas at Dallas, Richardson, Texas 75083-0688, USA. Posted online ahead of print 9 May 2013; http://dx.doi.org/10.1130/G34204.1.
Humans have used, sought, and traded gemstones for thousands of years, uniquely linking art, economics, and geology, from the dawn of civilization until now. In contrast, the theory of plate tectonics, which explains how Earth's crust and upper mantle is divided into independent plates, has only existed for about 50 years. Gemstones mostly form where special conditions of pressure, temperature, and chemical composition occur, and sometimes these conditions can be linked to plate tectonic processes. Plate boundaries are dynamic geologic environments where conditions for gemstone formation occur, especially at convergent plate boundaries, where plates are subducted, sinking back into Earth's interior. In this paper, a team of U.S., Japanese, and Canadian geoscientists identify two gemstones -- ruby and a type of jade (jadeite) -- that form at two types of convergent plate boundaries, in subduction zones and where continents collide. Jadeite forms where seafloor (oceanic crust) is subducted -- for example, around the Pacific Ocean, in Central America, New Zealand, and SE Asia. In contrast, ruby forms where two continents collide, for example now beneath the Himalayas, where India and Asia are colliding, and in east Africa, where east and west Gondwana collided 550 million years ago. This new understanding of "plate tectonic gemstones" provides a new perspective on how plate tectonics functions and also suggests fresh ways to look for new deposits of these gemstones.
Profiles of ocean island coral reefs controlled by sea-level history and carbonate accumulation rates
Michael Toomey et al., Geology and Geophysics Dept., Woods Hole Oceanographic Institution, 360 Woods Hole Road, MS#22, Woods Hole, Massachusetts 02543, USA; and Dept. of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA. Posted online ahead of print 9 May 2013; http://dx.doi.org/10.1130/G34109.1.
Coral reefs around islands develop a wide variety of forms, including narrow platforms fringing the coast, barrier reefs encircling deep lagoons, and flights of terraces that have been raised above or drowned deep below the sea surface. Charles Darwin, who was also a geologist, suggested that reefs take on a fixed sequence of forms as a volcanic island gradually sinks below sea level. Reefs on some island chains, such as the Society Islands, appear to follow this sequence, but Darwin’s idea cannot explain much of the diversity of reef forms found around the world, such as the reefs of the Hawaiian Islands. In addition to island sinking, reefs are shaped by the interaction of coral growth, wave erosion, and sea level changes. In this study, Michael Toomey and colleagues use a computer simulation of reef development to understand these interactions, and compare their results with a global compilation of rates of coral growth and island sinking or uplift. They find that large sea-level cycles driven by ice ages have left a clear fingerprint on coral reefs around the world, and that Darwin’s proposed sequence of reef forms only develops in a small subset of environments.
Sedimentary cannabinol tracks the history of hemp retting
Marlène Lavrieux et al., Institut des Sciences de la Terre d’Orléans, Université d’Orléans, ISTO, UMR 7327, 45071 Orléans, France; CNRS/INSU, ISTO, UMR 7327, 45071 Orléans, France; and BRGM, ISTO, UMR 7327, BP 36009, 45060 Orléans, France. Posted online ahead of print 9 May 2013; http://dx.doi.org/10.1130/G34073.1.
Hemp (Cannabis sp.) has been a fundamental plant for the development of human societies. Its fibers have long been used for textiles and rope making, which requires prior stem retting. This process is essential for extracting fibers from the stem of the plant but can adversely affect the quality of surface waters. The history of human activities related to hemp (its domestication, spread, and processing) is frequently reconstructed from seeds and pollen detected in archaeological sites or in sedimentary archives, but this method does not always make it possible to ascertain whether retting took place. Hemp is also known to contain phytocannabinoids, a type of chemicals that is specific to the plant. One of these chemicals, cannabinol (CBN), was discovered in a sediment record from a lake in the French Massif Central and was shown to be related to retting. This molecule tracks the hemp retting history in the area during the last 800 years and brings information about its induced water pollution. These findings, supported by pollen analyses and historical data, show that this novel sedimentary tracer can help to better constrain past impacts of human activities on the environment.
Evidence for Hesperian glaciation along the Martian dichotomy boundary
Alfonso F. Davila et al., SETI Institute, Mountain View, California 94043, USA. Posted online ahead of print 9 May 2013; http://dx.doi.org/10.1130/G34201.1.
Alfonso F. Davila and colleagues analyzed images and topographic data from the Aeolis Mensae region of Mars. Their analyses indicate that these terrains were eroded by glaciers emanating from the Martian dichotomy boundary. Collectively, our observations suggest that glacial activity could have been an important mechanism of modification of equatorial regions on Mars for three billion years.
The longevity of large upper crustal silicic magma reservoirs
Sarah E. Gelman et al., Dept. of Earth and Space Sciences, University of Washington, Seattle, Washington 98195, USA. Posted online ahead of print 9 May 2013; http://dx.doi.org/10.1130/G34241.1.
Using numerical heat transfer models, Sarah E. Gelman and colleagues simulated the incremental assembly of upper crustal silicic magma reservoirs, the source of Earth's largest volcanic eruptions. Incorporating reasonable magma emplacement rates, complexity in thermal properties, and appropriate igneous phase diagrams, they demonstrate that these large reservoirs can remain continuously active for more than a million years in highly productive magmatic environments, while remaining more transient in lower flux regions. These results are consistent with volcanological, geochronological, and geophysical data obtained from various provinces (e.g., Taupo Volcanic Zone and the Southern Rocky Mountain Volcanic Field as long-lived, productive regions, while the Cascades arc hosts lower flux stratovolcanoes). This work supports recent models emphasizing the role of in situ upper crustal magma storage and differentiation in a crystal-rich environment ("mush zones"). This is a particularly provocative deepening in geoscientists' understanding of silicic magma systems because previous thermal modeling studies, which have incorporated fewer complexities than those addressed in this study, have been used as primary evidence against the "mush model." The results presented here are consistent with natural observations from multiple techniques and represent an important contribution toward predicting how and where large reservoirs can grow.
Can turbidites be used to reconstruct a paleoearthquake record for the central Sumatran margin?
Esther J. Sumner et al., Ocean and Earth Science, National Oceanography Centre Southampton, University of Southampton, Southampton, Hampshire SO14 3ZH, UK. Posted online ahead of print 9 May 2013; http://dx.doi.org/10.1130/G34298.1.
Large earthquakes are known to sometimes trigger huge underwater slides and flows of sediment in the oceans. An increasingly widespread technique is to sample these sediments, date them, and thereby infer the recurrence times of past major earthquakes that can be used in hazard assessment for future earthquakes. In this new study, Esther J. Sumner and colleagues tested whether large earthquakes reliably generate the types of deposit needed to reconstruct a complete and accurate earthquake record. They did this by analyzing the seafloor sediment record on the Sumatran subduction margin, in a region that experienced the third largest earthquake yet recorded in 2004 and numerous other large magnitude earthquakes in historical times. The seafloor sediment record offshore Sumatra reveals surprisingly little evidence for sediment slides and flows related to known large magnitude earthquakes. Therefore, large earthquakes on the Sumatran margin do not always trigger the large slides and flows of sediment necessary for reconstructing a complete and accurate history of major earthquakes. Until a better understanding is reached about why some seafloor slopes are more prone to widespread failure in the event of an earthquake, we suggest cautious use of ocean sediments in developing past histories of major earthquakes.
Mobilization of Tl-Hg-As-Sb-(Ag, Cu)-Pb sulfosalt melts during lowgrade metamorphism in the Alpi Apuane (Tuscany, Italy)
C. Biagioni et al. (M. D’Orazio, corresponding), Dipartimento di Scienze della Terra, Università di Pisa, Via Santa Maria 53, 56126 Pisa, Italy. Posted online ahead of print 16 May 2013; http://dx.doi.org/10.1130/G34211.1.
C. Biagioni and colleagues have discovered an exceptional assemblage of Tl-Hg-As-Sb-(Ag,Cu)-Pb sulfosalt minerals, showing textural evidence for their mobilization as melts in the barite-pyrite-iron oxide orebodies of the Monte Arsiccio mine (Alpi Apuane, Tuscany, Italy). The relative abundance of rare thallium sulfosalts (including three new mineral species), their peculiar textural features within the orebodies (e.g., migration along matrix grain boundaries, drop-like internal textures, low interfacial angles between sulfosalts and matrix minerals), and the overall high thallium content in pyrite from the entire mining district (up to ~900 ppm), make the barite-pyrite-iron oxide deposits of the Alpi Apuane a reference locality for studying very low-temperature sulfosalt melts in low-grade metamorphic complexes (greenschist facies). This study reveals how sulfosalt melting during low-grade regional metamorphism controls the redistribution of economically valuable and environmentally critical elements such as thallium (a highly toxic element) in sulfide orebodies containing significant amounts of low-melting-point chalcophile elements. The increase in local concentration combined with the change in thallium speciation (from trace level substituting ion to essential constituent element) could significantly influence the environmental release of thallium during weathering of such complex ore deposits.
Magmatic-hydrothermal processes within an evolving Earth: Iron oxide-copper-gold and porphyry Cu ± Mo ± Au deposits
Jeremy P. Richards, Dept. of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada; and A. Hamid Mumin, Dept. of Geology, Brandon University, 270 18th Street, Brandon, Manitoba R7A 6A9, Canada. Posted online ahead of print 16 May 2013; http://dx.doi.org/10.1130/G34275.1.
Copper and gold commonly occur together in two deposit types known as porphyry copper (plus or minus gold and molybdenum) and iron oxide-copper-gold (IOCG) deposits. Porphyry deposits are characterized by abundant iron-sulfide minerals, whereas IOCG deposits are characterized by abundant iron-oxide minerals. Both deposit types are major targets for mineral exploration. The origin of porphyry deposits is relatively well established (they are formed from hydrothermal fluids exsolved from magmas emplaced at shallow levels in the crust, in tectonic settings related to subduction of oceanic plates). The origin of IOCG deposits is more widely debated, but several aspects of their nature and mode of formation are similar to porphyry deposits, including their key metal contents, formation by hydrothermal fluids arguably exsolved from magmas, and broad tectonic association. However, IOCG deposits are more common in ancient (Precambrian, older than or about equal to 550 million years old) rocks, whereas porphyry deposits are most abundant in younger (Phanerozoic, younger than or about equal to 550 million years old) rocks. Study authors Jeremy P. Richards and colleagues propose that this temporal distribution is related to established oxygenation of the deep oceans at the end of the Precambrian, which for the first time introduced abundant seawater-derived sulfur into subduction zones, and led to the predominance of sulfide-rich porphyry deposits in the Phanerozoic.
Orbital-scale climate change and glacioeustasy during the early Late Ordovician (pre-Hirnantian) determined from δ18O values in marine apatite
M. Elrick et al., Earth & Planetary Sciences Dept., University of New Mexico, Albuquerque, New Mexico 87131, USA. Posted online ahead of print 16 May 2013; http://dx.doi.org/10.1130/G34363.1.
Results from this study by M. Elrick and colleagues suggest that 10 million years before the well-documented Latest Ordovician glaciation and mass extinction event, there were significant continental glaciers growing and melting and causing global sea levels to rise and fall on 104-105 year time scales. These high-frequency, climatically controlled sea-level changes resulted in the development of widespread subtropical sedimentary cycles and changes in the oxygen isotope values of marine apatite occurring within the cycles. These high-frequency climate and sea-level oscillations support the interpretation of a dynamic and prolonged Ordovician greenhouse to icehouse transition.
A high-resolution nonmarine record of an early Danian hyperthermal event, Boltysh crater, Ukraine
Iain Gilmour et al., Planetary and Space Sciences, Dept. of Physical Sciences, The Open University, Milton Keynes MK7 6AA, UK. Posted online ahead of print 16 May 2013; http://dx.doi.org/10.1130/G34292.1.
Lake sediments from the Boltysh meteorite impact crater in Ukraine record geochemical and pollen evidence for a brief period of global warming a short time after the boundary that marks the extinction of the dinosaurs. The 24-km-diameter Boltysh crater, which formed a few thousand years before its much larger and more famous cousin at Chicxulub, which is thought to be responsible for the mass extinction at the end of the Cretaceous, quickly filled with lake sediments. These sediments provide the first record on land of a brief period of warming and major perturbation of the carbon cycle previously found only in marine sediments. This indicates a global change in the Earth’s climate. Plant pollens record a change in the flora around the crater that shows an increasingly warm and dry climate over a period of up to 340 thousand years, while carbon isotopes show that this coincides with a major perturbation in Earth’s carbon cycle. Together they reveal that profound environmental change continued to occur on a global scale shortly after the dramatic events surrounding the Chicxulub impact.
Testing the snowball Earth hypothesis for the Ediacaran
Alexei V. Ivanov et al., Institute of the Earth’s Crust, Siberian Branch, Russian Academy of Sciences, Lermontov Street 128, Irkutsk 664033, Russia. Posted online ahead of print 16 May 2013; http://dx.doi.org/10.1130/G34345.1.
Earth experienced ultimately cold climates several times in its history, such as glaciers reaching the tropical latitudes, and the ocean probably stayed completely frozen. This type of climate stage is known as snowball Earth. Siberia was once tropical during the Ediacaran period about 580-570 million years ago, yet it was glaciated. If Earth was at the snowball stage and thus completely covered by ice, the ice prevented accumulation of cosmic dust and micrometeorites at oceanic floor. The dust and micrometeorite particles would have accumulated rapidly on the ocean floor at the ice-melting event, providing a geochemical signal. However, unlike a previous Cryogenian glaciation at about 635 million years ago, which is a classic example of the snowball Earth conditions, Alexei Ivanov and colleagues find this signal to be at a background level, suggesting that Ediacaran glaciation recorded in Siberia did not reach the snowball Earth stage. The severe cold climate and open oceans probably were prerequisites for evolution of metazoan and a later burst of life on Earth.
Evidence for extensive methane venting on the southeastern U.S. Atlantic margin
L.L. Brothers et al., U.S. Geological Survey, 384 Woods Hole Road, Woods Hole, Massachusetts 02543, USA. Posted online ahead of print 16 May 2013; http://dx.doi.org/10.1130/G34217.1.
Results reported here by L.L. Brothers and colleagues show the first evidence for widespread seabed methane venting along the southeastern U.S. Atlantic Margin beyond the well-known Blake Ridge Diapir Seep. While it was suspected that such seeps existed, there was little direct evidence until now. Data collected from recent ship and autonomous underwater vehicle surveys discovered multiple water-column gas plumes (>1000 m height and made up of bubbles). Brothers and colleagues also mapped extensive new chemosynthetic seep communities (communities of biological organisms that directly use methane and/or sulfide for life processes) at the Blake Ridge and Cape Fear Diapirs. Flow along these systems is both more dynamic (more active) and more widespread than previously believed.
Dynamic weakening by nanoscale smoothing during high-velocity fault slip
Xiaofeng Chen et al. (Ze’ev Reches, corresponding), School of Geology and Geophysics, University of Oklahoma, Norman, Oklahoma 73019, USA. Posted online ahead of print 16 May 2013; http://dx.doi.org/10.1130/G34169.1.
While slip during large earthquakes occurs along faults that are hundreds of kilometers long, the dynamic weakening that drives these earthquakes is controlled by nano- to micro-scale frictional processes. Xiaofeng Chen and colleagues analyzed the nano- to micro-scale friction processes along experimental faults that slipped at high slip-velocity. Their analysis showed that the experimental faults became very smooth and developed shiny, mirror surfaces. The nanoscale friction coefficient dropped on these highly smooth surfaces and demonstrated, for the first time, that slip-smoothing at high slip-velocities can be an effective mechanism of dynamic weakening.