Earth is (mostly) flat: Apportionment of the flux of continental sediment over millennial time scales Jane K. Willenbring et al., Dept. of Earth and Environmental Science, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA, and National Center for Earth-Surface Dynamics, 2 Third Ave. SE, Minneapolis, Minnesota 55414, USA. Posted online 4 Jan. 2013; http://dx.doi.org/10.1130/G33918.1.
Although churning smokestacks and gasoline-burning vehicles are contributing to greenhouse gas emissions, some natural processes do the reverse by pulling carbon dioxide out of the atmosphere. One of these processes is chemical weathering, which occurs when rock turns into soil. Carbon dioxide molecules and rain combine and dissolve rocks; the weathering products eventually become sequestered in the ocean. For years, geologists believed that mountains, due to their steep slopes and high rates of erosion, were large contributors to this "carbon drawdown" effect. Here, Jane K. Willenbring and colleagues suggest that mountains do not play a significant role in this activity. They analyzed published cosmogenic nuclide-derived erosion rates from around the world and extrapolated rates to unmeasured watersheds to determine where most sediment was produced. Their findings show that even the steepest, most erosion-prone slopes make up only a small fraction of Earth's surface. So, while mountains may have very high rates of erosion and soil production per unit area, they are far outstripped by the much more abundant expanses of gently sloping land. This result challenges previous studies, which suggested that mountainous rivers contribute most of the sediment to the oceans and that recent mountain uplift cooled the planet via weathering and carbon dioxide drawdown.
Mobilization of radiogenic Pb in zircon revealed by ion imaging: Implications for early Earth geochronology Monika A. Kusiak et al., Swedish Museum of Natural History, Box 50007, SE104 05 Stockholm, Sweden Dept. of Applied Geology, Curtin University, PO Box U1987, Perth, WA 6845, Australia. Posted online 4 Jan. 2013; http://dx.doi.org/10.1130/G33920.1.
The isotopic disturbance of zircons from ~3.4 billion-year-old Archean high-grade metamorphic rocks of the Napier Complex, Antarctica, has been known for many years. Here, Monika Kusiak and colleagues use a novel high spatial resolution ion imaging technique to generate Pb-isotopic "age maps" for these zircons. These record patchy variations in the isotopic ratios that result in spurious ages, including some that are Hadean (older than 4.0 billion years). These data provide the first unambiguous demonstration of the distribution of unsupported ancient radiogenic lead in zircon. The authors relate this feature to the ultrahigh temperature metamorphism (UHT) in this region which occurred about 2.5 billion years ago. Their results raise the possibility that other old ages from zircon subject to similar metamorphism may be erroneous. Given that zircon is the only direct sample we have of the Hadean (e.g. the well-known Jack Hills detrital zircons), many models for Earth's earliest evolution critically depend on the veracity of ages obtained using an ion microprobe.
Aseismic plate boundary in the Indo-Burmese wedge, northwest Sunda Arc Vineet K Gahalaut et al., CSIR-National Geophysical Research Institute, Uppal Road, Hyderabad 500606, Andhra Pradesh, India. Posted online 4 Jan. 2013; http://dx.doi.org/10.1130/G33771.1.
Plate motion, crustal deformation, and earthquake occurrence processes in the northwest Sunda Arc, which includes the Indo-Burmese wedge (IBW) in the forearc and the Sagaing fault in the backarc, are very poorly constrained. Plate reconstruction models and geological structures in the region suggest that subduction in the IBW occurred in the geological past, but whether it is still active and how the plate motion between the India and Sunda plates is partitioned between motion in the IBW and Sagaing fault is largely unknown. Vineet K. Gahalaut and colleagues report results from a dense GPS network in the IBW that has operated since 2004. Their analysis of these measurements and the seismicity of the IBW suggest that the steeply dipping Churachandpur-Mao fault in the IBW accommodates the remaining motion between the India and Sunda plates through dextral strike-slip motion, and this motion occurs predominantly through velocity strengthening frictional behavior, i.e., aseismic slip. The aseismic motion on this plate boundary fault significantly lowers the seismic hazard due to major and great interplate earthquakes along this plate boundary.
Anthropogenic lead as a tracer of rock varnish growth: Implications for rates of formation Michael N. Spilde et al., Institute of Meteoritics, MSC03-2050, University of New Mexico, Albuquerque, New Mexico 87131, USA. Posted online 4 Jan. 2013; http://dx.doi.org/10.1130/G33514.1.
Rock varnish is a thin dark coating best known from deserts, and is believed to grow extremely slowly. Varnish samples from near Socorro, New Mexico (United States), contain as much as 3.7% lead oxide, derived from nearby smelters operating from A.D. 1870 to 1931. Additional varnish indicates continued growth from 1931 to 2003. Comparison with other varnish confirms that the lead is not an artifact. Based on Pb layer thickness, and the period of smelter operation, these very young rock varnishes yield growth rates of 28 to 639 microns per thousand year, substantially higher than previously documented fastest rates. These rates imply that the average rate for older varnish is not the active growth rate. Rather, it is a long-term value including periods of nondeposition, erosion, and active growth. Therefore, models of rock varnish formation should be reevaluated with consideration of much faster maximum growth rates.
Construction of an oceanic island: Insights from the El Hierro (Canary Islands) 2011-2012 submarine volcanic eruption J. Rivera et al. (M. Canals, corresponding), Instituto Español de Oceanografía, Corazón de María 8, Madrid E-28002, Spain. Posted online 4 Jan. 2013; http://dx.doi.org/10.1130/G33863.1.
J. Rivera and colleagues present the first continuous monitoring of the growth of a newly born submarine volcano while erupting in an ocean island flank. The eruption occurred south of El Hierro Island in the Canaries, off Northwest Africa, from 10 October 2011 to 5 March 2012 and was marked by earth tremors, stained waters, and dead fish. An unprecedented high-frequency, high-resolution bathymetric monitoring allowed calculating changes in eruption rates. The first survey, 15 days after the eruption's onset, showed a cone topping at 205 m depth. 137 days later, the cone had developed into a fissure eruption with four associated vents, the shallowest topping at only 89 m of water depth. Despite the fact that the eruption added about 0.006% to the volume of the overall edifice only, the shallow depth reached by the growing volcano raised serious concern since shallow water volcanism may lead to explosive eruptions and tsunami generation. This small, geologically frequent growth episode contrasts with the destruction by large-scale catastrophic flank collapses that affect most volcanic islands. The most recent giant debris avalanche in El Hierro, which occurred 13 to 134-thousand ago, mobilized in a single event a volume equivalent to 450 to 550 eruptions similar in size to the one described. This seems to be the fate of ocean islands.
Synchronous vertical and horizontal tectonism at late stages of Archean cratonization and genesis of Hemlo gold deposit, Superior craton, Ontario, Canada Shoufa Lin, Dept. of Earth and Environmental Sciences, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada; and School of Resources and Environment, Hefei University of Technology, Hefei 230026, P.R. China; and Gary P. Beakhouse. Posted online 4 Jan. 2013; http://dx.doi.org/10.1130/G33887.1.
The tectonic processes in modern Earth are characterized by large scale horizontal motion (plate tectonics, a form of horizontal tectonism), whereas those at the early stages of Earth's evolution were probably very different and dominated by vertical motion (vertical tectonism). The transition from dominant vertical to dominant horizontal tectonism took place about 2.7 to 2.6 billion years ago in the Superior craton in Canada and is characterized by the synchronous and interactive operation of vertical and horizontal tectonism. Such a transitional process is important for gold mineralization. The world-class Hemlo gold deposit in Ontario (containing more than 700 tons of gold) formed as a result of the process.
High-precision temporal calibration of Late Permian vertebrate biostratigraphy: U-Pb zircon constraints from the Karoo Supergroup, South Africa Bruce S. Rubidge et al. (Jahandar Ramezani, corresponding), Bernard Price Institute for Palaeontological Research, School of Geosciences, University of the Witwatersrand, PO Wits, Johannesburg 2050, South Africa; and Dept. of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA. Posted online 4 Jan. 2013; http://dx.doi.org/10.1130/G33622.1.
The continental sedimentary rocks deposited in the Karoo Basin of South Africa preserve a remarkable fossil record of reptile and mammal ancestors that elucidate the evolutionary and biodiversity changes in the crucial geologic period between the Permian and the Jurassic (270-180 million years ago). Stratigraphic zonations based on the South African tetrapod fossils have global applicability in correlating continental deposits throughout the Pangean supercontinent. A major limiting factor, however, has been a lack of independent and reliable age constraints for the Karoo biozones. Bruce S. Rubidge and colleagues report on the discovery of volcanic ash deposits interbedded with four lower Permian biozones of the Karoo Supergroup and their geochronology based on high-precision U-Pb analyses by the CA-TIM method. The calibrated biostratigraphy improves understanding of basin development, as well as rates of vertebrate evolution at a time when the earliest reptile lineages were being established. It also allows correlation to the marine realm with enormous implications for global patterns of faunal evolution, including mass extinction events of the Permian Period and their possible driving mechanisms.
Oxygen isotope composition of meltwater from a Neoproterozoic glaciation in South China Yongbo Peng et al. (Huiming Bao, corresponding), Dept. of Geology and Geophysics, Louisiana State University, Baton Rouge, Louisiana 70803-4101, USA; and Dept. of Geological Sciences, Indiana University, 1001 East 10th Street, Bloomington, Indiana 47405-1405, USA. Posted online 4 Jan. 2013; http://dx.doi.org/10.1130/G33830.1.
Climate information associated with a glaciation, such as temperature or moisture source, is recorded by the oxygen isotope composition of its ice. Unfortunately, few glacial ices on Earth have survived for more than 1 million years. The oxygen isotope composition of older glacial ices must be found indirectly via oxygen-bearing minerals that formed in the glacial meltwater. This is difficult because of the scarcity of suitable minerals and deposits formed and preserved in meltwater environments. Yongbo Peng and colleagues show that sulfate minerals (barite and malachite) in the immediate vicinity of chalcocite clasts in a Neoproterozoic diamictite from South China bear extremely negative oxygen isotope values. The low oxygen isotope sulfate minerals are best explained by formation via chalcocite oxidation in glacial meltwater. The data suggest that the diamictite was deposited on land in Neoproterozoic glacial meltwater that had almost the same oxygen isotope value as modern glaciers in Antarctica. This study opens up a new possibility for retrieving glacial meltwater isotope signatures in deep Earth history.
Northeast African vegetation change over 12 m.y. Sarah J. Feakins et al., Dept. of Earth Sciences, University of Southern California, 3651 Trousdale Parkway, Los Angeles, California 90089-0740, USA. Posted online 17 Jan. 2013; http://dx.doi.org/10.1130/G33845.1.
This study by Sarah J. Feakins and colleagues reports vegetation change over 12 million years in northeast Africa (specifically Somalia, Ethiopia and Kenya). Nearby marine sediments act as a trap for dust, including waxy molecules from plant leaves and pollen grains, blowing off the continent. The waxy molecules record the types of plants that were present -- based on signals locked in when the plant synthesized its first sugar with either three (C3) or four carbon (C4) atoms. The C4 plants are found only amongst grasses and shrubs that like warm-season precipitation and low carbon dioxide levels and are also commonly drought tolerant; the C3 category includes most other plants. The authors added pollen evidence for proportions of grasses versus trees to build a complete picture of the changing landscape. They found that rainforests were not present in the source region at any time in the last 12 million years. Instead, the landscape was surprisingly open, with C4 grasses and shrubs increasing at 10 million years, replacing earlier C3 grasslands. These ecosystem changes may have shaped faunal evolution: landscapes were open much earlier than previously thought and the timing of the C4 rise confirms the fast uptake of this new foodstuff by the ancestors of modern horses.
Contrasting strike-slip motions on thrust and normal faults: Implications for space-geodetic monitoring of surface deformation Andrea Hampel et al., Institut für Geologie, Leibniz-Universität Hannover, Callinstrasse 30, 30167 Hannover, Germany. Posted online 17 Jan. 2013; http://dx.doi.org/10.1130/G33927.1.
Global positioning system (GPS) measurements are important tools for monitoring seismically active faults in Earth's crust. Both the abrupt surface motions during the earthquake and the slower postseismic movements over years to decades after the earthquake can be determined. Among the first intra-continental earthquakes recorded by GPS are the 2009 L'Aquila (Italy) and 2003 Chengkung (Taiwan) earthquakes. In contrast to the common notion, these dip-slip faults showed hitherto unexplained differences in the direction and velocity of the coseismic and postseismic surface movements parallel to the fault trace. Using three-dimensional numerical models, Andrea Hampel and colleagues demonstrate that the observed contrasting fault-parallel slip patterns are characteristic features of the two different fault types and that they are also evident in field data of coseismic and long-term cumulative fault slip. During the postseismic phase, viscous flow in the lower crust induces fault-parallel surface displacements, which have the same direction as the coseismic displacements but are distributed over a much larger area. In order to detect this signal, future studies require GPS stations to be placed in the prolongation of the fault trace, in addition to those positioned near the fault.
Delamination in the Betic Range: Deep structure, seismicity, and GPS motion Flor de Lis Mancilla et al., Instituto Andaluz de Geofísica, Universidad de Granada, 18071-Granada, Spain. Posted online 17 Jan. 2013; http://dx.doi.org/10.1130/G33733.1.
A significant portion of Earth's lithosphere is recycled into the deeper mantle. The two principal mechanisms for recycling are subduction and delamination. While the areas where subduction processes are taking place are well recognize and its mechanisms well understood, delamination processes are not completely revealed mainly because they are more difficult to detect. In this study, Flor de Lis Mancilla and colleagues provide important evidence of crustal delamination in the Western Mediterranean region from different datasets. The whole underthrusted Iberia crust is delaminated beneath part of the Betic mountain range, still connected to the foreland lithosphere on one side, and displaced along a tear fault along the other side.
Retrodeforming the Arabia-Eurasia collision zone: Age of collision versus magnitude of continental subduction Nadine McQuarrie (corresponding) and Douwe J.J. van Hinsbergen, Dept. of Geology and Planetary Science, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, USA. Posted online 17 Jan. 2013; http://dx.doi.org/10.1130/G33591.1.
Two first-order questions seem to defy consensus along the Alpine-Himalayan orogen: When did continents initially collide? Where is deformation partitioned as continents continue to converge? These that these questions remain because of difficulty in identifying geologic markers that require collision. N. McQuarrie and van Hinsbergen examine the medial portion of this collision zone where Arabia has collided with Eurasia. Sedimentary rocks from Eurasia, deposited on the Arabian plate are indisputable evidence of Eurasia-Arabia collision; however, several authors have argued for a collision age that is 10-15 million years earlier. This 10 to 15 million-year difference is important because the Arabia-Eurasia collision has been linked to global cooling, the slowing of Africa, Mediterranean extension, the rifting of the Red Sea, an increase in exhumation and sedimentation on the Eurasian plate, and the slowing and deformation of the Arabian plate, features that require a collision about 35 million years ago. Collision at this time would require subduction of ~400-600 km of unaccounted for Arabian continental crust. McQuarrie and van Hinsbergen argue that remnants of oceanic crust preserved on Arabia (similar to the Semail Ophiolite in Oman) limit the amount of subduction of additional continental crust to 180 km. This suggests collision is no older than 27 million years.
Mw 8.6 Sumatran earthquake of 11 April 2012: Rare seaward expression of oblique subduction Miaki Ishii et al., Dept. of Earth & Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, Massachusetts 02138, USA. Posted online 17 Jan. 2013; http://dx.doi.org/10.1130/G33783.1.
The largest known strike-slip earthquake of magnitude 8.6 occurred on 11 April 2012 off the northwestern coast of Sumatra. This earthquake ruptured within the Indo-Australian plate, and the slip propagation is shown to be primarily southeast to northwest using seismological and tsunami data. This trend is almost parallel to the nearby trench where the 2004 magnitude 9 Sumatran earthquake occurred. These megathrust events are known to promote subsequent failures within the subducting plate, suggesting that the great earthquake in 2012 is a consequence of the devastating 2004 Sumatran earthquake. Furthermore, this region is characterized by oblique plate convergence, and the trench-parallel component is thought to be taken up by the Great Sumatran Fault on the island of Sumatra. There have been rising concerns that the expected slip from the plate convergence rate is much larger than the observed slip on the Great Sumatran Fault, which has been interpreted as a sign of an impending large earthquake. We propose that the missing slip is mostly accounted for by the magnitude 8.6 earthquake in 2012. If such partitioning of slip between two plates occurs along other oblique subduction zones, the hazard potential of the faults on over-riding plates must be reevaluated.
Eoarchean within-plate basalts from southwest Greenland F.E. Jenner et al., Dept. of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road Northwest, Washington, DC 20015, USA; and Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia. Posted online 17 Jan. 2013; http://dx.doi.org/10.1130/G33787.1.
Previous geochemical studies have used the similarities in geochemistry of magmas erupted 3.8 billion years ago and modern subduction zone magmas to argue for subduction-style tectonics early in the Earth's history. However, the lack of evidence for non-subduction related volcanic rocks older than 3 billion years, such as mid-ocean ridge and/or oceanic island magmas, may instead indicate that the subduction-like compositions of these ancient volcanic suites resulted from early chemical differentiation of the Earth and/or crustal contamination instead of modern-style tectonic processes. New analyses of a suite of 3.8-billion-year-old volcanic rocks from the island of Innersuartuut, southwest Greenland, show no evidence for crustal contamination or subduction-related magmatism. Instead, their compositions are comparable to those erupting at modern oceanic islands, such as Hawaii. These new data can be used to demonstrate that the range in tectonic settings 3.8 billion years ago was comparable to present. This new finding strengthens previous arguments that subduction of oceanic crust into the mantle has been taking place for at least 3.8 billion years.
Strong rocks sustain ancient postorogenic topography in southern Africa Taryn E. Scharf et al., Earth Surface Geochemistry, Deutsches GeoForschungsZentrum (GFZ), Potsdam D-14478, Germany; and Africa Earth Observatory Network (AEON), Nelson Mandela Metropolitan University, Port Elizabeth 6031, South Africa. Posted online 17 Jan. 2013; http://dx.doi.org/10.1130/G33806.1.
The Cape Mountains of southern Africa exhibit an alpine-like topography in conjunction with some of the lowest denudation rates in the world. This presents an exception to the often-cited coupling of topography and erosion and suggests that steep slopes alone are not sufficient to incite the high erosion rates with which they are commonly associated. Taryn E. Scharf and colleagues attribute the maintenance of rugged topography and suppression of erosion rates primarily to the presence of physically robust and chemically inert quartzites that constitute the backbone of the mountains. The low erosion rates they determine using cosmogenic isotopes are in agreement with the low erosion rates estimated for southern Africa during the late-Cenozoic using a variety of other techniques. Accumulatively, these data suggest that the coastal hinterland of the subcontinent may have experienced relative tectonic stability throughout the Cenozoic.
A sagging-spreading continuum of large volcano structure P.K. Byrne et al., Dept. of Terrestrial Magnetism, Carnegie Institution of Washington, 5240 Broad Branch Road NW, Washington, D.C. 20005, USA; and Laboratoire Magmas et Volcans, Université Blaise-Pascal, 5 Rue Kessler, 63038 Clermont-Ferrand Cedex, France. Posted online 17 Jan. 2013; http://dx.doi.org/10.1130/G33990.1.
Gravity can cause large volcanoes to deform under their own weight. Such volcanoes can either spread outward along their underlying "basement" rocks, or they can sag downward into their basement. These deformation processes strongly affect the structural stability and eruptive behavior of the volcano. Using scaled "sand-box" simulations, P.K. Byrne and colleagues identify a continuum of deformation styles, from pure spreading to pure sagging. They explore how volcano-basement coupling and basement rigidity control the position of a given volcano along this continuum. For example, this study shows how a combination of spreading and sagging can explain many enigmatic features of Olympus Mons on Mars, the largest volcano in the Solar System. More broadly, their results can help volcano hazard studies by showing how spreading promotes, but sagging reduces, the risk of catastrophic landslides or sector collapses.
Isolation and characterization of the earliest taxon-specific organic molecules (Mississippian, Crinoidea) Christina E. O'Malley et al. (William I. Ausich, corresponding), School of Earth Sciences, The Ohio State University, 155 South Oval Mall, Columbus, Ohio 43210, USA. Posted online 17 Jan. 2013; http://dx.doi.org/10.1130/G33792.1.
The oldest biomarker molecules extracted directly from individual fossils are reported from Lower Mississippian (340-million-year-old) crinoids from Indiana. Here, as in many other well-preserved crinoid occurrences, crinoid specimens of the same species are preserved in calcite of different colors. Thus, the isolated biomarkers represent some attribute intrinsic to each species. Biomarker molecules are reported from three species that occur in the same sedimentary bed. Extracted molecules preserve a phylogenetic signal, with the two more closely related species having more similar biomarkers compared with a third, more distantly related taxon. These biomarker molecules resemble aromatic or polyaromatic quinones based upon ultraviolet visible light spectroscopy (UV-Vis), fluorescence excitation emission matrix fluorescence spectroscopy (EEMs), and electrospray ionization time-of-flight mass spectrometry (ESI-TOF-MS), which are similar in structure to echinochromes, spinochromes, and gymnochromes that occur in living echinoderms. Results suggest that the preservation of diagnostic organic molecules is much more common that previously realized and that preserved organic molecules may provide an independent method to unravel phylogenetic relationships among echinoderms and, perhaps, other fossilized organisms.
Long-term east-west asymmetry in monsoon rainfall on the Tibetan Plateau Adam M. Hudson (corresponding) and Jay Quade, Dept. of Geosciences, University of Arizona, Tucson, Arizona 85721, USA. Posted online 17 Jan. 2013; http://dx.doi.org/10.1130/G33837.1.
Adam Hudson and Jay Quade compare the modern lake areas of 130 closed basin lake systems on the Tibetan Plateau to the areas they occupied during the past warm period of the early Holocene. Past lake areas were calculated using abandoned shoreline features deposited around the lakes. Closed basin lakes have no outflowing rivers, so their surface area is directly related to the amount of precipitation and evaporation happening in their drainage basins. Hudson and Quade use this relationship to reconstruct the magnitude of rainfall increase associated with the intensification of the Asian summer monsoon in the region ~10,000 years ago. By comparing the past lake areas to the modern lake areas for lakes across the Tibetan Plateau, they take a regional look at how monsoon rainfall differed relative to today. They found that lakes located at the far western edge of the plateau increased fourfold to sixfold in area, but this expansion decreased to only about twofold in the center and eastern edge. This shows that rainfall in western Tibet increased more than to the east. This is in contrast to today, where the western plateau is much drier than the east. This difference indicates that monsoon rainfall may change asymmetrically during warmer climate.
Very high-resolution seismic stratigraphy of Pleistocene eustatic minima markers as a tool to reconstruct the tectonic evolution of the northern Latium shelf (Tyrrhenian Sea, Italy) S. Fraccascia et al., MARUM, University of Bremen, 28359 Bremen, Germany. Posted online 17 Jan. 2013; http://dx.doi.org/10.1130/G33868.1.
S. Fraccascia and colleagues explore the characteristics of paleo-shelf break and lowstand submerged depositional terraces along the continental shelf offshore Latium (Tyrrhenian Sea, Italy). Using very high resolution seismic stratigraphy, the study reconstructed the distribution of uplift and subsidence in the area for the past 500 thousand years, in good agreement with earlier studies on land.
Detrital zircons indicate no drainage link between southern California rivers and the Colorado Plateau from mid-Cretaceous through Pliocene Raymond V. Ingersoll et al., Dept. of Earth and Space Sciences, University of California, Los Angeles, 595 Charles Young Dr. East, Los Angeles, California 90095-1567, USA. Posted online 25 Jan. 2013; http://dx.doi.org/10.1130/G33807.1.
Upper Cretaceous to Pliocene (two million to 100 million year old) sandstone in coastal southern California contains almost no detrital zircon with the distinctive age signature of Colorado Plateau strata. These results presented by Raymond V. Ingersoll and colleagues conflict with the hypothesis that a major river draining the Colorado Plateau (and cutting an ancestral Grand Canyon) flowed to coastal California during the Paleogene (23 to 65 million years ago). Their results are consistent with the hypothesis that Grand Canyon was cut during the latest Miocene to present (0 to 6 million years ago) by the integrated Colorado River draining into the Gulf of California.
Fractionation of Nb and Ta by biotite and phengite: Implications for the "missing Nb paradox" Aleksandr S. Stepanov (corresponding) and Jörg Hermann, Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia. Posted online 25 Jan. 2013; http://dx.doi.org/10.1130/G33781.1.
The subchondritic niobium/tantalum in both the continental crust and the depleted mantle remains enigmatic and is called the "missing Nb paradox." Aleksandr Stepanov and Jörg Hermann present partitioning data between biotite and granitic melt for experimental and natural samples that provide evidence that Nb is compatible in biotite and phengite. Nb can thus be enriched in the residue during partial melting of crustal rocks. Additionally, biotite and phengite in equilibrium with granitic melts preferentially incorporate Nb over Ta. Therefore, incipient partial melting of biotite-rich crustal rocks produces restites with high Nb/Ta. Progressive melting of such rocks leads to the consumption of biotite and the formation of peritectic rutile or ilmenite, which retain the high-Nb/Ta signature. Stepanov and Hermann suggest that such mid to lower crustal granulites could represent an important Nb-rich reservoir with high Nb/Ta. Similarly, high-Ti phengite that is present in deeply subducted sediments preferentially incorporates Nb over Ta. High-pressure incipient partial melting in the presence of residual phengite thus also produces restites with high Nb/Ta that could be subducted to the deeper mantle.
Video observations inside conduits of erupting geysers in Kamchatka, Russia, and their geological framework: Implications for the geyser mechanism A. Belousov et al., Institute of Volcanology and Seismology, Piip Boulevard 9, Petropavlovsk 683006, Russia. Posted online 25 Jan. 2013; http://dx.doi.org/10.1130/G33366.1.
Several models have been proposed to explain periodic eruptions of geysers. In essence, the models all use two principally different types of geyser plumbing configurations, dealing with two different physical mechanisms. Here A. Belousov and colleagues present data on direct video observations of interior conduit systems for four erupting geysers in Geyser Valley, Kamchatka, Russia. The video footage reveals highly contorted water-filled conduits that periodically discharge voluminous parcels of steam bubbles during eruptions. These observations do not favor the models that use the most popular long vertical conduit type of plumbing, where eruptions are caused by sudden flashing of superheated water into steam. In contrast, these data fit the models using the less-explored type of plumbing, where pressurized steam gradually accumulates in an underground cavity (bubble trap) and periodically erupts through a water-filled, highly contorted conduit with the configuration of an inverted siphon. Conduits of the studied geysers were developed from erosion by ascending geothermal water in landslide deposits having chaotic internal structures. Belousov and colleagues suggest that geyser fields are rare on Earth because they require the combination of hydrothermal discharge and geological formations having specific mechanical properties and structures (that facilitate the generation of highly contorted conduits).
70 Ma chemical zonation of the Tristan-Gough hotspot track Joana Rohde et al., GEOMAR Helmholtz Centre for Ocean Research Kiel, Wischhofstrasse 1-3, 24148 Kiel, Germany. Posted online 25 Jan. 2013; http://dx.doi.org/10.1130/G33790.1.
Hotspot tracks are age-progressive chains of volcanoes that form as lithosphere moves over a mantle plume. Some Pacific hotspot tracks display a spatial geochemical zonation with distinct isotopic domains attributed to their location above the margin of the Pacific superplume, a large-scale low-shear velocity province in the lower mantle beneath the southern Pacific. Joana Rohde and colleagues investigate geochemical variations along the Tristan-Gough hotspot track in the South Atlantic to evaluate if such spatial compositional heterogeneities also exist in the Atlantic. They observe that the isotopic compositions display a bilateral zonation with two distinct mantle source compositions, making Tristan-Gough the first chemically zoned plume to be recognized overlying the African superplume. Moreover, the zonation is traced for at least 70 million years, four times longer than recognized for any zoned hotspot track in the Pacific thus far. The southeastern Gough subtrack is chemically more enriched and may reflect the composition of the African superplume, whereas the more depleted northwestern Tristan subtrack could be derived through mixing of the ambient depleted mantle with the enriched material. Finally it appears that seismic anomalies are not simply thermal but also chemical in nature and that chemical zonation of hotspots could map out chemical heterogeneities in the lower mantle.
Refining the Mediterranean "Messinian gap" with high-precision U-Pb zircon geochronology, central and northern Italy Domenico Cosentino et al., Dipartimento di Scienze Geologiche, Università degli Studi Roma Tre, 00154 Rome, Italy; and Istituto di Geologia Ambientale e Geoingegneria-CNR, 00138 Rome, Italy. Posted online 25 Jan. 2013; http://dx.doi.org/10.1130/G33820.1.
At the end of Miocene (5.96-5.33 million years ago) the progressive closure of the Atlantic-Mediterranean connection induced a major evaporative event known as the Messinian Salinity Crisis (MSC), which lead to the near complete desiccation of the Mediterranean Sea and the deposition of thick evaporite layers. Astronomical tuning of the precessionally modulated sedimentary cycles deposited during the MSC has indicated and approx. 90-thousand-year "Messinian gap," corresponding to the evaporative drawdown of the Mediterranean following complete closure of the Mediterranean-Atlantic gateway.
Evidence for large-scale methane venting due to rapid drawdown of sea level during the Messinian Salinity Crisis Claudia Bertoni et al. (Joseph Cartwright, corresponding), School of Earth and Ocean Sciences, Cardiff University, Cardiff CF10 3AT, UK. Posted online 25 Jan. 2013; http://dx.doi.org/10.1130/G33987.1.
Claudia Bertoni and colleagues report the discovery, from three-dimensional seismic mapping, of a field of pockmarks buried at present depths of ~4 km in the Levant Basin (southeastern Mediterranean). The pockmarks cover an area of ~1000 square kilometers, have diameters as great as 2 km, and erode as much as 200 m into their substrate of deep-water clastic sediments, which immediately predate the Messinian Salinity Crisis (MSC, about 5.5 to 6.2 million years ago). These craters are filled by the basal units of the Messinian evaporites, thus implying they formed at or close to the time of the earliest major drawdown of sea level. All the pockmarks are developed at a single, regionally correlatable surface, Horizon N, which is observed throughout the Mediterranean to coincide with the onset of the MSC. Bertoni and colleagues propose that a rapid basinal drawdown led to a dramatic increase of the shallow subsurface pore-fluid overpressure regime in the mostly fine-grained pre-Messinian sediments. Subsequent pressure release then resulted in high flux fluid venting, sediment remobilization, and pockmark formation. The spatial association of the craters with an underlying canyon suggests that their formation is linked to biogenic methane generation. The model of drawdown-induced overpressuring and remobilization may be applicable to many other evaporitic basins, and represents a novel mechanism for inducing large-scale sediment remobilization of shallow buried depositional systems at the earliest stages of salt basin development.