|17 May 2010
GSA Release No. 10-25
Director - GSA Communications & Marketing
Boulder, CO, USA - Fossils, faulting, continent formation, river evolution, eolian sedimentation - the June Geology covers all this and more, with input from scientists around the world. Highlights include the first field evidence of neotectonic activity in the Hochschwab karst massif; in-situ sampling from the AlpTransit tunnel site; discovery in Mexico of the oldest known Bryozoan fossils; and fingerprinting of magmatic glasses and crystals to determine their "rock DNA." GSA Today examines microbial ecosystems in the Tibetan Plateau.
Highlights are provided below. Abstracts for the complete issue of GEOLOGY are available at http://geology.gsapubs.org/. 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. Access the GSA TODAY science article by clicking on the issue cover icon at http://www.geosociety.org/pubs/.
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. Contact Christa Stratton for additional information or assistance.
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Neotectonic extrusion of the Eastern Alps: Constraints from U/Th dating of tectonically damaged speleothems
Lukas Plan et al., Dept. for Karst and Caves, Natural History Museum Vienna, Museumsplatz 1/10 A-1070 Vienna, Austria. Pages 483-486.
This team of international scientists, lead by Lukas Plan from the Dept. for Karst and Caves at the Natural History Museum in Vienna, Austria, suggest that the Salzachtal-Ennstal-Mariazell-Puchberg (SEMP) fault, a major strike-slip fault system in the Alps, is tectonically still active. The main tectonic activity of this fault dates back to Oligocene and Miocene times, but it is not known if the fault is still moving. The team presents the first geologic field evidence of neotectonic activity from a cave in the Hochschwab karst massif (Styria, Austria), which is situated within the SEMP fault zone. Speleothems (dripstones, flowstone) in this cave are damaged by tectonic movement. Layers of younger flowstone that were deposited above the deformed flowstone allow constraining the time frame of the tectonic event(s), as the age of their formation can be dated by determining the ratio of radioactive uranium and thorium isotopes. The flowstone of the older, pre-damage generation formed about 118,000 years ago at the end of the last warm period during the ice age. The younger flowstone layer that formed after the deformation is about 9,000 years old (early Holocene). The tectonic event bracketed by these layers coincided with a growth interruption during the last glacial period, consistent with the high-alpine setting of the cave. These new data are consistent with vectors of continuous GPS measurements as well as the historic earthquake catalogue, and collectively suggest that the SEMP is an active fault.
Dating of shallow faults: New constraints from the AlpTransit tunnel site (Switzerland)
Horst Zwingmann et al., CSIRO Earth Science and Resource Engineering, P.O. Box 1130, Bentley, WA 6102, Australia. Pages 487-490.
This interdisciplinary team from Australia and Switzerland presents data from the deep AlpTransit tunnel site to describe shallow fault dating and provide new insight into methods and constraints on such dating. The AlpTransit tunnel site provides a unique opportunity to sample fault gouges in situ. Detailed ages are reported from fresh, non-weathered clay-rich fault gouges, which are consistent both internally and with tightly defined field constraints, demonstrating the applicability of this method for direct dating of brittle deformation. The team notes that the understanding of fault processes and the timing and extent of clay-rich fault gouge formation are important for many reasons, including (1) regional correlation of shallow fault activity, of critical importance in neotectonic studies; (2) civil engineering and the evaluation of earthquake hazards; (3) assessing the suitability of sites for waste storage including nuclear waste; and (4) hydrocarbon exploration, as faults may act as either a conduit zone or a seal for fluids and/or hydrocarbons.
Formation and stability of transverse and longitudinal sand dunes
E. Reffet et al., LESIA (Laboratoire d’études spatiales et d'instrumentation en astrophysique), UMR 8109, Observatoire de Paris, Section de Meudon, 5 place Jules Janssen, 92195 Meudon Cedex, France. Pages 491-494.
Dunes present many shapes that can be related to different wind regimes. In deserts where winds blow successively in two different directions, dunes are found to be linear ridges whose trend depends on the wind’s transport capacity and the angle between the two wind directions. When the two winds have comparable magnitude and period, dunes are either perpendicular (transverse dunes) or parallel (longitudinal dunes) to the average sand transport direction. Besides their different domain of existence, transverse and longitudinal dunes are morphologically different, respectively, to the small and large angles between the winds. In nature, it is hard to control the winds, so this team from France’s LESIA and Laboratoire MSC reproduced these structures experimentally and underwater, which reduces space and time scales by a factor of one thousand. Thanks to these controlled experiments, and to numerical simulations, they reveal the physical mechanisms at play in the formation, selection, and long-time evolution of these dunes widely observed in nature. They show in particular that transverse dunes grow slowly and are finally unstable toward barchans, while the longitudinal dunes grow quicker and are stable. This could explain why they are the most observed dunes on Earth, Mars, and Titan. Reffet et al. were also able to reproduce dunes with a chestnut shape, which were thought to be particular to Mars.
Do Cenozoic analogues support a plate tectonic origin for Earth's earliest continental crust?
Alan R. Hastie et al., School of Earth and Ocean Sciences, Cardiff University, Main Building, Park Place, Cardiff CF10 3YE, UK. Pages 495-498.
Plate tectonics is the dominant paradigm in geosciences, explaining how the Earth's crust was formed, how it evolves, and the processes by which it is recycled into the mantle. However, considerable controversy exists over when processes analogous to modern plate tectonics began. Some suggest Earth's earliest Archean (about 3.5 billion years old) trondhjemite, tonalite, and granodiorite/dacite (TTG/D) continental crust formed by plate tectonic processes, while others believe that plate tectonics started much later. This study reports the discovery that Cenozoic adakites on Jamaica are compositional analogues of the earliest TTG/D crust. These adakites are derived from partially melting metamorphosed underthrust (subducting) oceanic plateau crust in a setting that is rare today but was likely to have been common in the Archean. This discovery is direct evidence that formation of the earliest continents is entirely consistent with plate tectonic processes operating in the early Archean, and alternative Archean-specific processes are not required.
Detrital zircons from fluvial Jurassic strata of the Michigan basin: Implications for the transcontinental Jurassic paleoriver hypothesis
William R. Dickinson et al., Dept. of Geosciences, University of Arizona, Tucson, Arizona 85721, USA. Pages 499-502.
Geoscientists from the University of Arizona and Southern Illinois University present uranium-lead (U-Pb) isotopic ages for detrital grains of the mineral zircon in Jurassic stream deposits of the Michigan basin in central Michigan. These ages provide confirmation that the windblown sandstones forming picturesque cliffs in scenic national parks of the western United States, such as Zion and Canyonlands on the Colorado Plateau, are composed of sand grains that were transported westward for thousands of kilometers by paleorivers with headwaters in the eastern part of the continent, before being blown into the Jurassic dunefields where they finally came to rest. The evidence consists of a close match in the age spectra of zircon grains in stream deposits preserved in the subsurface of the Michigan basin and in eolian sandstones of the same depositional age on the Colorado Plateau. The U-Pb analysis was supported by U.S. National Science Foundation grant EAR-0443387 to the Arizona LaserChron Center at the University of Arizona.
Supereruptions of the Snake River Plain: Two-stage derivation of low-delta-18O rhyolites from normal-delta-18O crust as constrained by Archean xenoliths
Kathryn E. Watts et al., Dept. of Geological Sciences, University of Oregon, Eugene, Oregon 97403, USA. Pages 503-506.
Supereruptions in the Snake River Plain-Yellowstone Plateau volcanic province have produced an extreme volume (greater than 10,000 cubic kilometers) of rhyolitic magma depleted in 18O. Because oxygen is the most abundant element in rhyolitic magmas, understanding the cause of these 18O depletions is essential for understanding how rhyolites of the Snake River Plain-Yellowstone Plateau volcanic province were generated. This team, lead by Kathryn E. Watts of the University of Oregon, presents new oxygen isotopic data for a suite of Archean xenoliths from the Snake River Plain that show that the cratonic crust in this region does not share the 18O depletion of the rhyolites. Watts et al. synthesize O-Sr-Nd-Pb isotope data for Archean xenoliths and Snake River Plain magmas and construct isotopic mixing models that support a two-stage model of rhyolite magma genesis: (1) normal-18O rhyolites are generated by partial melting and hybridization of cratonic crust by mantle-derived basalt in proportions of ~30–50% crust and ~50–70% mantle; and (2) 18O-depleted rhyolites tap ~20–80% of hydrothermally altered (18O-depleted) portions of normal-18O rhyolitic rocks. The team proposes that this model of rhyolite genesis may be applicable to other caldera systems around the world, for which the characteristic oxygen isotope depletions are either less pronounced or undiscovered. This work was supported by U.S. National Science Foundation grant EAR/CAREER-844772.
Middle Cambrian echinoderms from north Spain show echinoderms diversified earlier in Gondwana
Samuel Zamora, Área y Museo de Paleontología-IUCA, Departamento de Ciencias de la Tierra, Universidad de Zaragoza, E-50009 Zaragoza, Spain. Pages 507-510.
The Cambrian explosion is widely regarded as one of the most important points in the history of life, and it was in that period that the majority of metazoan groups appeared in the geological record, including echinoderms. Echinoderms (star fish and their relatives) are one of the most important components in recent seas, but their fossil record started slowly in the Lower Cambrian (520 million years ago). Zamora presents exquisitely preserved Middle Cambrian fossil echinoderms recently found in Spain. Showing detailed anatomic details, these taxa reveal a surprisingly higher diversity from Gondwana, pushing back some clades that were typical from later strata. This diversity, shown by articulated specimens, agrees with the hidden diversity recorded from isolated echinoderm plates, and changes our view of the first echinoderm diversification event. This result is probably not an exception, and indicates how poorly known Cambrian echinoderms are worldwide, except for a few other well-known examples from Europe and North America, and suggests locations where collector efforts should be concentrated in the near future.
Evidence of similar probability of intense hurricane strikes for the Gulf of Mexico over the late Holocene
Davin J. Wallace and John B. Anderson, Dept. of Earth Science, Rice University, Houston, Texas 77251, USA. Pages 511-514.
Predicting annual hurricane impacts along the Gulf of Mexico coast is quite difficult. In order to put recently observed trends in modern hurricane activity into context, we must establish the geologic record of hurricane impacts. Researchers Davin Wallace and John Anderson of Rice University determine an average probability of ~0.46% for late Holocene (~5000 to ~1000 years ago) intense hurricanes by studying sediment cores in Laguna Madre, Texas, USA. By combining their data with previously published records from the eastern Gulf Coast, they determined that, for the late Holocene, the intense hurricane landfall probability ranged between ~0.39% and ~0.46%. Despite centennial to millennial oscillations between cool and wet and warm and dry climate conditions over southwestern North America, the intense hurricane impact probability does not appear to have varied for the Gulf of Mexico through the late Holocene. Therefore, these oscillating climate conditions do not appear to have influenced the path of storms between the western and eastern Gulf Coast.
Late Oligocene-Miocene mid-latitude aridification and wind patterns in the Asian interior
Jimin Sun et al., State Key Laboratory of Cenozoic Geology and Environment, Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Beijing 100029, China. Pages 515-518.
Windborne deposits are widespread in the Asian interior. Such deposits provide information about climatic changes and wind patterns in the geological past. Sun et al. have found eolian deposits from as early as 24 million years ago in the giant Junggar inland basin of northwestern China. These are the oldest eolian deposits in Asia. Based on particle size and stable isotope analysis, the team concludes that these eolian deposits were transported by westerly winds, possibly from areas in Kazakhstan. This is different from the airborne dust transported by northwest winter monsoon winds from the deserts of Mongolia and northern China that accumulated on the Chinese Loess Plateau, which is about 2000 km to the southeast of the Junggar Basin. Their results further reveal that a climate pattern similar to that of the present has prevailed at least since 24 million years ago in Central Asia.
Isotopic evidence for Younger Dryas aridity in the North American midcontinent
J.A. Dorale et al., Dept. of Geoscience, University of Iowa, 121 Trowbridge Hall, Iowa City, Iowa 52242, USA. Pages 519-522.
Analyses of carbon isotopes in organic matter in a well-dated buried soil sequence at the Big Eddy archeological site in southwestern Missouri reveal that a major increase in the abundance of warm-season prairie grasses coincides with the Younger Dryas climate episode at the end of the Pleistocene. Instead of indicating warmer conditions, J.A. Dorale of the University of Iowa and colleagues interpret this record to indicate an eastward shift in the prairie-forest border, driven largely by an increase in aridity. This interpretation is consistent with other Midwestern lines of evidence that indicate somewhat cooler temperatures during this time, and could be accomplished by an increased zonal flow of the atmosphere that changed the frequency of cool, dry air masses originating in the Pacific, versus the frequency of warm, humid air from the Gulf of Mexico. This study was supported in part by U.S. National Science Foundation grant ATM-0402482.
Recovery of the forest ecosystem in the tropical lowlands of northern Guatemala after disintegration of Classic Maya polities
Andreas D. Mueller et al., Geological Institute, Swiss Federal Institute of Technology, ETH, 8092 Zurich, Switzerland. Pages 523-526.
Little is known about the recovery of the tropical environment in Petén, Guatemala, after the Classic Maya Collapse (~A.D. 900), when a reduction of anthropogenic stress occurred on this highly sensitive ecosystem. Questions remain about when, why, and how quickly the tropical forest and soil recovered over the past millennia. Lithological, geochemical, magnetic, and palynological data from sediment cores of Lake Petén Itzá in the Maya Lowlands of northern Guatemala were used by this international team to investigate tropical forest recovery and soil stabilization. Results indicate that the tropical forest ecosystem in the watershed of Lake Petén Itzá had been reestablished by the early Postclassic Period (A.D. 1000-1200). This study provides important insights into tropical forest ecosystem recovery following a prolonged period of human vegetation disturbance. This work was funded in part by U.S. National Science Foundation grant ATM-0502030.
Eocene clocks agree: Coeval 40Ar/39Ar, U-Pb, and astronomical ages from the Green River Formation
M. Elliot Smith et al., Dept. of Geology, Sonoma State University, 1801 E Cotati Ave, Rohnert Park, California 94928, USA. Pages 527-530.
Zircon crystals from ash beds in the Green River Formation give concordant U-Pb ages that are indistinguishable from sanidine 40Ar/39Ar ages adjusted to the 28.201-million-year age for the Fish Canyon sanidine 40Ar/39Ar standard. Applying these new ages, M. Elliot Smith of Sonoma State University and colleagues find that the ~400 meters of strata that comprise the Green River Formation in southwest Wyoming equate to about 3 million years of the Early Eocene. Scaled in time relative to astronomical models for insolation variations, the pronounced vertical repetitions of lacustrine to alluvial sedimentary facies present within the Green River Formation compare well to oscillations in eccentricity. This study was funded in part by U.S. National Science Foundation grant EAR-0230123.
Isostatic uplift driven by karstification and sea-level oscillation: Modeling landscape evolution in north Florida
Peter N. Adams et al., Dept. of Geological Sciences, University of Florida, Gainesville, Florida 32611, USA. Pages 531-534.
Elevated, ridge-shaped landforms in north-central Florida have puzzled geologists for some time. The landforms’ sediments contain fossil evidence for young, nearshore marine origin, but their elevation is well above modern sea level. A previous hypothesis attempted to resolve these seemingly conflicting lines of data. As precipitation drives dissolution of limestone, Florida erodes from the inside out. This mass loss is balanced by pressure equilibration in the mantle driving uplift of Earth's crust, raising these ridges well above modern sea level. This study out of the University of Florida presents a numerical model using this hypothesis, combining sea-level history and a precipitation-karstification function to calculate uplift in coastal carbonate landscapes. Results provide estimates for the ages of the north Florida ridges that are in agreement with fossil evidence. Additionally, this work suggests that void space creation is occurring at a rate approximately 3.5 times faster than previously estimated, resulting in an uplift rate that is approximately twice as high as previously thought. This process has implications for other carbonate landscapes worldwide and may play an under-appreciated role within the global carbon cycle.
Significance of channel-belt clustering in alluvial basins
Elizabeth A. Hajek et al., University of Wyoming Dept. of Geology and Geophysics, 1000 E. University Avenue, Laramie, Wyoming 82071, USA. Pages 535-538.
Stratigraphers often use the distribution of channel deposits in sedimentary basins to interpret past changes in climate, tectonics, and sea level. This study by Elizabeth Hajek and colleagues presents evidence that some of these stratigraphic patterns may form spontaneously due to long time-scale (~1000-100,000 years) self-organization in depositional systems, rather than as a consequence of climate, tectonic, or sea-level changes. A physical experiment and an ancient deposit both show stratigraphic patterns in which clusters of closely spaced channel deposits are separated by expanses dominated by floodplain deposits. Statistical analysis shows that both basins exhibit a similar degree of channel clustering. In the experiment, external controls (base level, subsidence rate, and sediment/water supplies) were not varied, and channel clustering arose due to internal dynamics of the depositional system. Likewise, the ancient deposits lack stratigraphic and sedimentologic evidence of external controls on channel clustering. Channel clusters may reflect a scale of fluvial self-organization that is not usually recognized in sedimentary basins. This type of internally generated stratigraphy is important to consider when reconstructing tectonic, climate, and sea-level changes from ancient deposits and when correlating between outcrop belts or subsurface wells.
Using the Sr isotope compositions of feldspars and glass to distinguish magma system components and dynamics
Victoria M. Martin et al., Dept. of Earth Sciences, University of Durham, Durham DH1 3LE, UK. Pages 539-542.
This team out of the UK notes that magmas may be made up of crystals and melts (now solidified to volcanic glass) mixed from a variety of sources. A technique has been developed to fingerprint the glasses and crystals using their isotopic compositions. Such compositions serve as rock “DNA”. Using this “rock DNA,” Victoria Martin and colleagues can tell, for instance, when crystals have been contributed from two different magmas that were then mixed. Tracing such components and their ancestry has helped this team to understand how magma systems work in the subsurface prior to delivering magmas via eruptions. This, in turn, helps in predicting volcanic eruptions so that geoscientists will be able to tell what types of mixing produce a certain type of eruption and constrain the time scales of mixing by studying the crystals identified as have been exchanged.
Evidence for middle Miocene uplift of the East African Plateau
Henry Wichura et al., Institut fur Erd- und Umweltwissenschaften, Universitat Potsdam, 14476 Potsdam, Germany. Pages 543-546.
Uplift of the East African Plateau is related to the development of the East African Rift System. While the far-reaching climatic and environmental influences of the plateau uplift are widely accepted, the timing and the magnitude of uplift are ambiguous. This team from Universität Potsdam shows, for the first time, evidence for the existence of significant relief along the East African Plateau prior to the formation of the rift valleys, as inferred from modeling the paleo-topography (slope angle) of one of the longest lava flows on Earth. This lava flow is 13.5 million years old and originated in the region that presently corresponds to the eastern Kenya Mountains. Upon eruption, the lava filled an old river valley that once routed runoff away from the eastern rim of the plateau. Their results imply that the lava flow must have originated at a minimum elevation of 1400 m. Thus, high topography in the region of the present-day great rift valleys of East Africa must have existed 13.5 million years ago. Importantly, this inferred uplift is at least 8 million years earlier than previously assumed and coincides with the two-step expansion of grasslands, as well as important radiation and speciation events in tropical Africa that may have ultimately affected hominin evolution.
Cambrian origin of all skeletalized metazoan phyla -- Discovery of Earth’s oldest bryozoans (Upper Cambrian, southern Mexico)
Ed Landing et al., New York State Museum, 222 Madison Avenue, Albany, New York 12230, USA. Pages 547-550.
Definitive proof that all major animal groups with skeletons appeared in the Cambrian geological period (543-489 million years ago) is provided by this team from the New York State Museum, the Chevron Gulf of Mexico Business Unit, and Universidad Nacional Autónoma de Mexico. They record Earth's oldest known bryozoan fossils from Late Cambrian rocks of Oaxaca State in southern Mexico. The fossils show that bryozoans, which are important contributors to modern marine reefs, had appeared by about 490 million years ago. The Mexican bryozoan fossils are approximately eight million years older than specimens described in 2003 from south China as Earth’s oldest bryozoans. The exquisite preservation of the Mexican bryozoans suggests that highly mineralized, twig-like bryozoan colonies evolved from soft-bodied colonies that were attached to shells and pebbles on the sea floor. The twig-like form elevated the colony above the sea floor, and allowed feeding in higher-energy and more food-rich waters above the sea floor. Ed Landing and colleagues propose that the almost simultaneous first-occurrences of bryozoans, cephalopods (ancient squid relatives), polyplacophorans (chitons), and euconodonts (fish-like vertebrates) about 490 million years ago marked a key stage in the origin of complex marine animal communities that resemble those of modern oceans.
Did incision of the Three Gorges begin in the Eocene?
N.J. Richardson et al., Maersk Oil North Sea UK Ltd, Crawpeel Road, Altens, Aberdeen AB12 3LG, UK. Pages 551-554.
The evolution of the Yangtze River is a long-standing puzzle in Asian geomorphology. It has been surmised since the 1930s that the modern Yangtze is made up of several segments that appear to have started as separate rivers, and that later amalgamated to form the third-longest river in the world. While the geomorphic evidence for this amalgamation is clear, there is little or no evidence for the time at which it occurred. Two of the original segments, the Lower and Middle Yangtze, are now joined at the Three Gorges, a steep-walled series of gorges up to 3000 m deep. In this paper, an international team of scientists shows that rocks in the Three Gorges region experienced localized cooling beginning about 40-45 million years ago. The team attributes this cooling to the onset of river incision, as the east-flowing Lower Yangtze River captured the Middle Yangtze and began to cut headward into the Sichuan Basin upstream. Gorge incision provided an outlet for sediment in the basin and triggered widespread (but previously unexplained) erosion of several kilometers of basin sediments beginning about 40 million years ago. Thus, the simple capture of the Middle Yangtze by the Lower Yangtze had profound geological consequences for a large region of southwestern China and the eastern Tibetan Plateau margin.
Quasi-periodic recurrence of large earthquakes on the southern San Andreas fault
Katherine M. Scharer et al., Dept. of Geology, Appalachian State University, Boone, North Carolina 28608, USA. Pages 555-558.
The southern San Andreas fault has not ruptured in a large earthquake since 1857. The purpose of this study, headed by Katherine Scharer of Appalachian State University, was to examine the record of prehistoric earthquakes near Wrightwood, California, USA, about 50 miles northeast of Los Angeles, to look for patterns in the length of time between large earthquakes. Large earthquakes rupture the ground surface, so large prehistoric earthquakes can be seen in trenches excavated across the fault as disrupted prehistoric ground surfaces, and can be dated using radiocarbon methods. The Wrightwood record contains 29 paleo-earthquakes that occurred over 3000 years, making it one of the longest in the world. Scharer and colleagues used statistical tests to examine regularity, randomness, or clustering in the record. The tests incorporate uncertainty in the age and quality of evidence for each earthquake directly into the test. They found that although not perfectly periodic (like clockwork), large earthquakes on the San Andreas fault at Wrightwood are more regular than random, and thus contain some temporal predictability which can be used in forecasting future earthquakes. Only four of the 28 intervals in the record are longer than the 153 years that have elapsed since the fault last ruptured. These results emphasize the hazard posed by this fault. This work was primarily supported by internal and external support from the U.S. Geological Survey National Earthquake Hazards Reduction Program (grants 02HQGR0005, 04HQGR0042, and 05HQGR0071) and the Southern California Earthquake Center (SCEC), which is funded by National Science Foundation Cooperative Agreement EAR-0106924 and USGS Cooperative Agreement 02HQAG0008. This is SCEC contribution 1291.
From passive margins to orogens: The link between ocean-continent transition zones and (ultra)high-pressure metamorphism
Marco Beltrando et al., Dipartimento di Scienze Mineralogiche e Petrologiche, Università di Torino, Via Valperga Caluso 35, 10125 Turin, Italy. Pages 559-562.
Mountain belts like the European Alps formed at the expense of two converging continental plates and of the oceanic basin that was originally located between them. In such geodynamic settings, rocks can be subducted to depths exceeding 100 km and then be carried back to the surface. Very often, rock units that have undergone this process are characterized by the complex association of widely different rock types, ranging from granitoids to serpentinites, which are generally considered representative of continental and oceanic plates, respectively. As a result, mountain building is often regarded as a chaotic process leading to mixing of originally unrelated rock types. To the contrary, this study by scientists from Italy, Australia, and France finds that such associations of different rock types may be related to sampling and preservation of so-called ocean-continent transition zones, which are transitional areas located between continental and oceanic plates, observed nowadays offshore Newfoundland and Portugal. During plate convergence, such areas may be carried to considerable depth and then be exhumed back to the surface. Their preservation at the scale of mountain belts indicates that the process of mountain building is significantly less chaotic than hitherto recognized and opens new perspectives on the investigation of oceanic basins sampled in mountain belts.
Normalized velocity profiles of field-measured turbidity currents
J.P. Xu, U.S. Geological Survey, Menlo Park, California 94025, USA. Pages 563-566.
This study by J.P. Xu of the U.S. Geological Survey records multiple in-situ turbidity currents with acoustic Doppler current profilers on moorings deployed in two submarine canyons. Normalization of the high-resolution velocity profiles show that parameters from lab experiments, where turbidity currents are readily obtained in controlled environments, can be used to characterize the normalized velocity profile of field turbidity currents whose temporal and spatial scales are several orders of magnitude larger.
Northward intrusions of low- and mid-latitude storms across the Saharo-Arabian belt during past interglacials
Nicolas Waldmann et al., Dept. of Earth Science, University of Bergen, Allégaten 41, 5007-Bergen, Norway. Pages 567-570.
The Dead Sea is widely known as one of the saltiest lakes in the world. The lake, which is located in an extreme hyperarid environment with less than 50 mm/year of precipitation, is mostly fed by moisture carried by the Subtropical Jet Stream in association with mid- to high-latitude cyclones. Past lake level changes show that the region experienced episodic wet and dry intervals during past glacial and interglacials cycles, respectively. This study, which includes the photo featured on this month’s cover, shows that sporadic deposition of speleothems and travertines in the Dead Sea basin region during past interglacials indicate intrusions of humidity from southern sources, probably in association with enhanced activity of mid-latitude Red Sea synoptic troughs and/or low latitude tropical plumes. These southerly incursions of wetness might have provided optimal short-term climatic corridors, allowing early human migrations out of Africa, and strongly influencing speciation and later cultural development of mankind.
New York-Alabama lineament: A buried right-slip fault bordering the Appalachians and mid-continent North America
Mark G. Steltenpohl et al., Dept. of Geology and Geography, Auburn University, Auburn, Alabama 36849, USA. Pages 571-574.
The New York-Alabama (NY-AL) lineament is a magnetic anomaly between the Appalachian region and mid-continent North America that has remained a mystery after 30 years of scrutiny and debate. The source of the anomaly lies buried beneath thousands of feet of younger sedimentary rocks, is nowhere exposed, and has never been penetrated by a bore hole. Detailed geophysical surveys were not available for Alabama when the lineament was first discovered in 1978, so, ironically, the southern termination for which it was named remained undefined. New aeromagnetic maps presented by this team from Auburn University and the U.S. Geological survey constrain the southern termination of the lineament in Alabama and indicate that the source is a continental-scale, strike-slip fault with about 220 km of right-slip displacement. The eastern Tennessee seismic zone, which has the second highest frequency of earthquakes in the eastern United States, is localized in a crustal block adjacent to the magnetic lineament with the largest magnitude earthquakes focused near to it. The modern earthquake stress field is compatible with the ancient one that initiated right-slip motion along the NY-AL lineament as early as a billion years ago, making it stand out as an unusual and long record of continental dynamics.
Impacts of environmental change and human activity on microbial ecosystems on the Tibetan Plateau, NW China
Hailiang Dong, Key Laboratory of Biogeology and Environmental Geology of Ministry of Education, China University of Geosciences, Wuhan, 430074, China; Geomicrobiology Laboratory, State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing, 100083, China; and Dept. of Geology, Miami University, Oxford, Ohio 45056, USA.
Recent research on biological responses to global climate change has focused mainly on plants and animals. However, microorganisms in soils and aquatic systems also play important roles in maintaining ecosystems. Through a thorough review of data and current literature focused the on the Tibetan Plateau, this team from China and the U.S. demonstrates that the microbial record preserved in lake sediments and ice cores can be a reliable indicator of past environmental change and human activity.