image: This photo-like image of South Africa was captured on April 12, 2010, by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA's Aqua satellite. view more
Credit: NASA
Boulder, Colo., USA - Earth's biosphere witnessed its greatest ecological catastrophe in the latest Permian, dated to about 251.9 million years ago. The current model for biodiversity collapse states that both marine and terrestrial animals were impacted simultaneously, as a consequence of global climate change.
On land, South African vertebrate fossils, and the stratigraphic record in which they are preserved, are reported to document the extinction and recovery associated with the crisis. The pattern -- reported as the end of the Dicynodon biozone and beginning of the Lystrosaurus biozones -- has been extrapolated to other continents and hemispheres and used to recognize the boundary event globally. Yet, to date, there has been no age constraint placed near the turnover in vertebrate fossils in this, or any, area. In this new study for Geology, Robert Gastaldo and colleagues present new multidisciplinary data from the Karoo Basin and call into question our current understanding of the terrestrial response to the End Permian Mass Extinction. Paleoecological evidence does not support the reported coincidence of climate aridification, floral collapse, and tetrapod turnover. Similarly, magnetostratigraphic and geochronometric data, when conservatively interpreted, indicate that the turnover between the biozones occurred in the early Changhsingian, more than 1.6 million years beforehand, and was not coeval with the marine mass extinction event.
FEATURED ARTICLE
Is the vertebrate-defined Permian-Triassic boundary in the Karoo Basin, South Africa, the terrestrial expression of the end-Permian marine event?
Robert A. Gastaldo et al., Dept. of Geology, Colby College, Waterville, Maine 04901, USA This article is OPEN-ACCESS online; http://dx.doi.org/10.1130/G37040.1.
Other recently published GEOLOGY articles are highlighted below:
South Atlantic opening: A plume-induced breakup?
Tanja Fromm et al., Alfred-Wegener-Institut für Polar- und Meeresforschung, Am Alten Hafen 26, D-27568 Bremerhaven, Germany. This article is OPEN-ACCESS online; http://dx.doi.org/10.1130/G36936.1.
Plate tectonics is a well-accepted concept to explain major geological processes like the earthquake distribution, volcanism, etc., on Earth. However, the driving forces for continent drift are not yet understood. The plume model predicts that hot material from Earth's core-mantle boundary rises to Earth's surface and causes the separation of continents and their subsequent drift. The opening of the South Atlantic is believed to be a textbook example for plume-related continental breakup. Here, large flood basalt provinces exist both in Brazil and Namibia. Off Namibia, the formation of the extensive, aseismic Walvis Ridge since the Cretaceous has been related to a mantle plume. In this open-access paper for Geology, Tanja Fromm and colleagues report on a large-scale geophysical project to test this model, including land-sea active and passive seismic investigations. The deep seismic sounding results indicate that the thermal mantle anomaly had only a very limited areal extend. The Namibian crust has been modified to a little extent by the breakup process. These new results do not support models that consider a plume as the driving factor alone for continent breakup in the South Atlantic. It is more likely that the combination of plate tectonic and mantle processes is responsible for the breakup.
A contourite drift system on the Baffin Bay-West Greenland margin linking Pliocene Arctic warming to poleward ocean circulation
Paul C. Knutz et al., Geophysics Dept., Geological Survey of Denmark and Greenland, Øster Voldgade 10, 1350 Copenhagen, Denmark. This article is online at http://dx.doi.org/10.1130/G36927.1.
Vast boreal forests covered the present-day Arctic regions of Canada and northern Greenland during the Pliocene period 2.5 to 5 million years ago, indicating that the climate there was warmer and more humid compared to present day. To explain this warmth, geoscientists have focused on two possible mechanisms: A greenhouse effect related to past atmospheric CO2 concentrations, and/or an increased intensity of North Atlantic circulation conveying more ocean heat to polar regions. However, neither of these mechanisms have yet provided clear answers to the Pliocene enigma. In this study for Geology, Paul C. Knutz and colleagues have analyzed the sedimentary record of Baffin Bay over the last 15 million years using a dense grid of seismic data tied to borehole information. The results document kilometer-thick deposits of late Miocene and Pliocene age that were shaped by strong ocean currents flowing northwards along the West Greenland margin. Their main conclusion is that this extinct ocean-current system kept Greenland and the Canadian Arctic warm, akin to the influence that the present-day Gulf Stream has on Northern European climate. Regional tectonic changes in ocean-landmass configurations may have turned off the poleward heat transport and thus provided an important precondition for full-scale growth of the Greenland Ice Sheet.
More than a trace of oxygen: Ichnological constraints on the formation of the giant Zn-Pb-Ag +/- Ba deposits, Red Dog district, Alaska
Merilie A. Reynolds et al., Dept. of Earth and Atmospheric Sciences, 1-26 Earth Sciences Building, University of Alberta, Edmonton, Alberta T6G 2E3, Canada. This article is online at http://dx.doi.org/10.1130/G36954.1.
Sediment-hosted massive sulfide (SHMS) deposits are an important source of global zinc resources, and the Red Dog Pb-Zn-Ag +/- Ba district in Alaska (USA) contains giant deposits of this type. The existing model for ore formation at Red Dog involves early diagenetic replacement of sediment deposited in a restricted basin with stratified suboxic bottom waters. In this article for Geology, Merilie Reynolds and colleagues present new observations of trace fossils Schaubcylindrichnus ichnospecies (isp.) and Chondrites isp. in several Red Dog deposits. The presence of the trace fossils, the size of the largest burrows, and the pervasiveness of the ichnofabric indicate that at least some intervals of the host sediment were deposited in an oxygenated middle to outer shelf environment. The burrow linings and infill are replaced by barite, hydrothermal quartz, and sulfide minerals, and the lack of compaction suggests that mineralization was diagenetically early. To reconcile these data with those from previous regional sedimentological and lithogeochemical studies, Reynolds and colleagues propose a new model whereby the ore-hosting sediment was deposited in a shelfal setting in which redox conditions were affected by a fluctuating oxygen minimum zone. The strong spatial correlation between bioturbation and Red Dog SHMS deposits suggests that the presence of trace fossils may have played an important role in controlling the flow of ore-forming fluids by increasing host sediment permeability.
Seafloor cratering and sediment remolding at sites of fluid escape
Matt O'Regan et al., Dept. of Geological Sciences, Stockholm University, Stockholm SE-106 91, Sweden. This article is online at http://dx.doi.org/10.1130/G36945.1.
Submarine sites of focused fluid flow are found in most oceanographic settings, and are prolific in hydrocarbon and gas hydrate bearing regions. Focused fluid escape from marine sediments results from overpressure development and pressure release, and can occur slowly through geologic time or catastrophically. Conceptual and numerical models describing the genesis of focused fluid flow systems are largely built upon interpretations of remotely acquired seismic data. In the absence of direct sampling, many questions concerning the underlying geologic processes, the nature of the internal sediments, and subsurface migration pathways for fluids are unresolved. In this study, Matt O'Regan and colleagues present a numerical subsidence model to quantify the potential volumetric contraction of sediments that have been remolded at sites of focused fluid escape. The model describes a novel mechanism to explain changes in seafloor and subsurface topography in areas of fluid escape while highlighting a potentially important but overlooked interplay between focused fluid flow systems and sediment geomechanics.
Experimental evidence for rock layering development by pressure solution
Jean-Pierre Gratier et al., ISTerre, Université Grenoble Alpes, 38041 Grenoble Cedex 9, France. This article is online at http://dx.doi.org/10.1130/G36713.1.
Natural deformation of rocks is commonly associated with development of mineralogical layering, leading to irreversible transformations of their microstructure. The mechanisms of such chemical differentiation processes during diagenesis, tectonics, metamorphism, or fault differentiation remain poorly understood, as they are difficult to reproduce experimentally due to the very slow kinetics involved. This paper shows that development of differentiated layering, similar to that observed in natural deformation, is stress driven and can be obtained from indenter experiments. Samples of (1) gypsum plaster mixed with clay, and (2) natural diatomite loosely interbedded with volcanic ash, saturated with aqueous solutions in equilibrium, were subjected to loading for several months at 40 °C and 150 °C, respectively. X-ray microtomography and scanning electron microscopy observations show that layering develops by a self-organized pressure solution process. Stress-driven dissolution of the soluble minerals (either gypsum or silica) is initiated in the areas initially richer in insoluble species (clay or volcanic ash), as diffusive mass transfer along the interface between soluble and insoluble minerals is much faster than along the healed boundaries of the soluble minerals. The passive concentration of the insoluble minerals amplifies the dissolution along layers oriented perpendicularly to the maximum compressive stress. Conversely, in areas with an initial low content of insoluble minerals and clustered soluble minerals, dissolution is slower. Consequently, these areas are less deformed; they host the re-deposition of the soluble species and act as rigid objects that concentrate both stress and dissolution near their boundaries, thus amplifying the differentiation and the development of layered microstructures.
Evolution of recycled crust within the mantle: Constraints from the garnet pyroxenites of the External Ligurian ophiolites (northern Apennines, Italy)
Alessandra Montanini and Riccardo Tribuzio, DIFEST, Università di Parma, Parco Area delle Scienze 157a, 43100 Parma, Italy, Dipartimento di Scienze della Terra e dell'Ambiente, Università di Pavia, Via Ferrata 9, 27100 Pavia, Italy. This article is online at http://dx.doi.org/10.1130/G36877.1.
The pyroxenite-peridotite sequence from the External Ligurian (northern Apennines, Italy) ophiolites is evidence of the evolution of recycled crust within the mantle. In this paper for Geology, Alessandra Montanini and Riccardo Tribuzio present new major and trace element and Nd-Hf isotopic compositions of garnet clinopyroxenites and websterites from this mantle section. The garnet clinopyroxenites display clinopyroxene and bulk-rock rare earth element patterns with distinct positive Eu anomalies, which argue for the involvement of plagioclase-rich precursors in their origin. They propose that the garnet clinopyroxenites formed by crystallization of eclogite-derived melts that underwent negligible interaction with the host peridotites. The garnet websterites are interpreted to have been produced by reactions between the eclogite-derived melts and peridotites, thereby giving rise to hybrid, second-stage pyroxenites with a crustal geochemical fingerprint. In our petrogenetic scenario, a rifting-related event about 220 million years ago caused melting of eclogites originating from a mid-oceanic ridge basalt-type gabbroic sequence. These mafic protoliths underwent a long-lived evolution of recycling in the mantle (1.5 to 1.0 billion years ago). They show that heterogeneity of crustal protoliths, age of recycling, and interaction with the host peridotites may lead to a significant compositional and isotopic diversity of crust-derived mantle pyroxenites.
Focused fluid transfer through the mantle above subduction zones
Cassian Pirard and Joerg Hermann, Research School of Earth Sciences, The Australian National University, Mills Road, Building 61, Canberra, ACT 0200, Australia; Earth and Oceans, James Cook University, James Cook Drive, Building 34, Townsville, QLD 4811, Australia. This article is online at http://dx.doi.org/10.1130/G37026.1.
Volcanic arcs such as the Cascades, Japan, or the Caribbean produce lavas that contain a specific geochemical signature that is inherited from the melting oceanic plate subducting under the arc. In this study for Geology, Pirard and Hermann have designed a set of high-pressure experiments to investigate how magmas carrying the geochemical signature reach the surface unmodified. Experimental runs and geochemical data show that the transfer of magmas 100 km below arc volcanoes cannot be done using the porosity between mineral grains of the mantle, nor through the detachment of a piece of subducted oceanic crust. Their experiments show that a transfer through a network of veins is the only process minimizing the modification of the subducted plate signature found in volcanic arc lavas.
Tracking changes in crustal thickness during orogenic evolution with Sr/Y: An example from the North American Cordillera
James B. Chapman et al., Dept. of Geosciences, University of Arizona, 1040 E. 4th Street, Tucson, Arizona 85721, USA. This article is online at http://dx.doi.org/10.1130/G36996.1.
Global compilations indicate that the geochemistry of arc magmatism is sensitive to Moho depth. Magmatic products are prevalent throughout the history of Cordilleran orogenesis and can be employed to constrain the timing of changes in crustal thickness as well as the magnitude of those changes. In this study for Geology, James Chapman and colleagues investigate temporal variations in crustal thickness in the United States Cordillera using Sr/Y from intermediate continental arc magmas. Their results suggest that crustal thickening began during the Late Jurassic to Early Cretaceous and culminated with 55-65-km-thick crust about 85 to 95 million years ago. Crustal thicknesses remained elevated until the mid-Eocene to Oligocene, after which time crustal thicknesses decreased to 30-40 km in the Miocene. The results are consistent with independent geologic constraints and suggest that Sr/Y is a viable method for reconstructing crustal thickness through time in convergent orogenic systems.
Fault strength in thin-skinned tectonic wedges across the smectite-illite transition: Constraints from friction experiments and critical tapers
Telemaco Tesei et al., Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, 00143 Rome, Italy. This article is online at http://dx.doi.org/10.1130/G36978.1.
When tectonic plates converge, slices of the topmost part of crust (the sedimentary cover) are commonly off-scraped and stacked in wedge-shaped bodies. This commonly occurs in subduction zones and at the edge of mountain ranges. The understanding of the physical processes going on in these wedges is critical, since they host intense seismicity. The stacking of crustal slices commonly occurs along weak clay-rich horizons, which are thought to become progressively stronger with increasing depth, causing the inception of seismicity.
The impact of dynamic topography change on Antarctic ice sheet stability during the mid-Pliocene warm period
Jacqueline Austermann et al., Dept. of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138, USA. This article is online at http://dx.doi.org/10.1130/G36988.1.
During the mid-Pliocene, temperatures were slightly elevated and atmospheric CO2 concentrations were similar to conditions in our present post-industrial world, making this time period particularly important for understanding ice volume and sea level changes in a warmer world. While geological data and ice sheet modeling indicate that most of the Greenland and West Antarctic Ice Sheets had melted during this warm period, debates about the fate of the much larger East Antarctic Ice Sheet continue. The stability of a grounded, marine-based ice sheet depends critically on the elevation of the bedrock it rests on. In this article for Geology, Jacqueline Austermann and colleagues use a state-of-the-art numerical model of mantle convective flow, constrained by a suite of geological and geophysical data, to reconstruct bedrock elevation across the Antarctic during the mid-Pliocene. They then adopt this reconstruction in a numerical simulation of the Antarctic Ice Sheet forced by mid-Pliocene climate conditions. Their mantle flow calculations indicate that the Wilkes Basin was at lower elevation in the mid-Pliocene, and they predict significantly more ice sheet retreat in the region compared to simulations that do not incorporate a geodynamic reconstruction of bedrock topography. A more retreated ice sheet in the Wilkes Basin has previously been inferred from geochemical analysis of offshore sediment cores but has, until now, been difficult to capture in ice sheet simulations.
Cracking the olivine zoning code: Distinguishing between crystal growth and diffusion
Thomas Shea et al., Dept. of Geology and Geophysics, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, Hawaii 96822, USA. This article is online at http://dx.doi.org/10.1130/G37082.1.
Olivine composition and zoning patterns are widely used to reconstruct the evolution of mafic magmas from source to surface and to extract time scales of magmatic processes. Deciphering the olivine zoning code is challenging because the contributions of growth and diffusion may overlap. A detailed multi-element (Fe, Mg, P, Al, and Ni) examination of zoning in an exceptional olivine with skeletal morphology allows unequivocal discrimination between these two processes using simple but powerful geometrical arguments. Olivine crystals initially grow rapidly and diagonally from corner locations, whereas diffusion effectively tracks mutually perpendicular crystal lattice orientations. Generating the zoning patterns for our case-study olivine required at least four to five months of diffusive reequilibration of Fe-Mg, further demonstrating that crystal morphologies produced by rapid growth can survive at magmatic temperatures for extended periods. No significant major element zoning is preserved after rapid growth, lending further credibility to time scales retrieved via diffusion modeling. Extending multi-element approaches to decoding olivine zoning patterns can help determine whether the kinetic relationship between growth- and diffusion-induced zoning recognized herein is widely applicable. Such studies will improve our understanding of time scales of magma storage, solidification, mixing, and/or transit toward the surface.
Crystallization of platinum-group minerals from silicate melts: Evidence from Cr-spinel-hosted inclusions in volcanic rocks
Vadim S. Kamenetsky et al., School of Physical Sciences, University of Tasmania, Hobart, TAS 7001, Australia. This article is online at http://dx.doi.org/10.1130/G37052.1.
The formation of platinum-group minerals (PGM) during magma differentiation has been suggested to be an important process in primitive magma evolution, but decisive textural evidence is difficult to obtain because PGM tend to be very small and very rare. Vadim Kamenetsky and colleagues have investigated Cr-spinel phenocrysts from two oxidized magmas (Siberian meimechite and Vanuatu [Ambae Island] arc picrite) and one reduced magma (Uralian [Russia] ankaramite) for PGM inclusions and their platinum-group element (PGE) contents. They observed Os-Ir and Pt-Fe alloys entrapped as inclusions in Cr-spinel in all three suites of lava. The alloys may occur in association with PGE-bearing sulfides and co-trapped silicate melt. Cr-spinel crystals also contain measurable amounts of Os, Ir, Ru, and Rh, which are at concentrations two times to 100 times higher than mantle values. Thermodynamic models indicate that the arc picrite and ankaramite melts were probably both saturated with the observed PGM phases, whereas the Os-Ir alloy grain observed in the meimechite is not in equilibrium with the "bulk" melt. These results demonstrate that PGM (alloys and sulfides) occur as liquidus phases in primitive (unfractionated) melts at high temperature and at a variety of redox conditions, and that Cr-spinel is a significant host of PGE, either in the crystal structure or as PGM inclusions.
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