image: Late Devonian brachiopod fossils were preserved as molds in a fine-grained sandstone (Big Creek, Hornell, New York, N 42.364361, W 77.645760). Individual fossil samples like this are the basis for testing the effects of biases on the global record of biodiversity history. From "Sustained Mesozoic-Cenozoic diversification of marine Metazoa: A consistent signal from the fossil record," by A.M. Bush and R.K. Bambach. view more
Credit: Geology and A.M. Bush and R.K. Bambach
Boulder, Colo., USA - A new analysis of the fossil record by paleontologists at the University of Connecticut and the Smithsonian Institute demonstrates that the number of animal species in the world's oceans has skyrocketed during the past 200 million years, despite mass extinctions like the one at the end of the Cretaceous Period (66 million years ago).
The history of diversification has been controversial due to concerns about data quality, but these biases were controlled in this analysis of a large Internet database of paleontological data. The analyses demonstrate that modern oceans are uniquely diverse -- never before in the history of the Earth have so many species coexisted. These results provide context for concerns about escalating extinction rates -- humans have been fortunate to live at an exceptional time of unprecedented biodiversity, but human impacts could tip the Earth from a state of increasing biodiversity to one of decline.
FEATURED ARTICLE
Sustained Mesozoic-Cenozoic diversification of marine Metazoa: A consistent signal from the fossil record
Andrew M. Bush and Richard K. Bambach, Department of Ecology and Evolutionary Biology and Center for Integrative Geosciences, University of Connecticut, 75 North Eagleville Road, Storrs, Connecticut 06269-3043, USA and Department of Paleobiology, MRC-121, National Museum of Natural History, Smithsonian Institution, P.O. Box 37012, Washington, D.C. 20013-7012, USA. This article is online at http://dx.doi.org/10.1130/G37162.1.
Other recently posted GEOLOGY articles are highlighted below:
A continental-weathering control on orbitally driven redox-nutrient cycling during Cretaceous Oceanic Anoxic Event 2
Simon W. Poulton, et al., School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK. This paper is OPEN ACCESS online at http://dx.doi.org/10.1130/G36837.1.
During periods of elevated global temperatures in the past, associated increases in weathering often resulted in higher inputs of nutrients, such as phosphorus, to the ocean. This, in turn, stimulated enhanced primary productivity, ultimately leading to increased production of organic matter, which removed oxygen from the ocean as it decomposed. At several points in Earth history this led to global episodes of oxygen depletion in the ocean - termed Oceanic Anoxic Events (OAEs). However, the precise chemistry of the ocean during these events, and ensuing implications for the bioavailability of phosphorus and hence the longevity of OAEs, are poorly understood. This study presents a millennial-scale record of ocean chemistry during one of the most significant of these events, which occurred around 95 million years ago. Using novel geochemical techniques applied to marine rocks from northwest Morocco, the study shows that the ocean remained devoid of oxygen, but ocean waters changed from containing high concentrations of toxic hydrogen sulfide, to high concentrations of iron. This occurred repetitively, and was linked to natural changes in the intensity of global weathering and the hydrologic cycle. Surprisingly, however, during both chemical states phosphorus remained bioavailable, thus maintaining the low oxygen state of the global ocean.
Melt inclusion shapes: Timekeepers of short-lived giant magma bodies
Ayla S. Pamukcu et al., Department of Earth, Environmental and Planetary Sciences, Brown University, 324 Brook Street, Providence, Rhode Island 02912, USA. This paper is online at http://dx.doi.org/10.1130/G37021.1.
Supereruptions evacuate giant volumes of magma from the Earth's crust swiftly and violently. Consequently, these volcanic eruptions have the potential to be devastating to life, climate, and infrastructure. Given the risks these events pose, it is crucial that we establish and implement useful hazards assessment, preparation, and mitigation programs. To do this, it is critical that the timescales over which supereruptive magma bodies accumulate and erupt be constrained. In this work, we describe a new method that uses textures (sizes, shapes, positions) of quartz-hosted melt inclusions (blebs of crystal-free magma), determined using quantitative propagation phase-contrast x-ray tomography, to assess the longevity of these giant, melt-rich magma bodies in the crust. This approach also provides important information on crystal growth rates. To test this method, we compare results to those determined using titanium compositional zoning and diffusion chronometry in the same quartz crystals. Our results indicate that (1) the melt inclusion faceting method can be used to assess timescales, (2) quartz growth rates are ~10-12 m/s, and, most critically, (3) supereruptive magma bodies accumulate and erupt quite rapidly, over notably short timescales of 10s-100s of years.
Persistent monsoonal forcing of Mediterranean Outflow Water dynamics during the late Pleistocene
A. Bahr et al., Institute of Geosciences, University of Frankfurt, Altenhöferallee 1, 60438 Frankfurt, Germany. This paper is online at http://dx.doi.org/10.1130/G37013.1.
Climate models predict enhanced meltwater input into the high-latitude North Atlantic Ocean as a consequence of anthropogenic warming. Ultimately, this process can cause a significant reduction of the global oceanic overturning circulation by obstructing the sinking of dense surface waters into the deep Atlantic basin. We investigated whether this freshening might be counteracted by the discharge of highly saline Mediterranean Outflow Water (MOW) entering the North Atlantic via the Strait of Gibraltar. Using marine drill cores recovered off southern Spain, we reconstructed MOW variability over the past 150,000 years. Our results indicate distinct changes in MOW production, with an unexpected persistent coupling to the intensity of the African monsoon. Intense monsoonal rainfall over eastern Africa enhanced runoff into the Eastern Mediterranean Sea, thereby reducing the production of dense and saline waters feeding the MOW. In contrast, aridity over northern Africa, as it is also predicted for global warming scenarios, promoted MOW generation. These results indicate that enhanced MOW discharge into the North Atlantic has stabilized the oceanic overturning circulation in the past, and they lead us to conclude that this mechanism may likewise counteract meltwater-induced freshening in the future, thereby helping to maintain oceanic heat transport into the high latitudes.
Hydrothermal fluid flow disruptions evidenced by subsurface changes in heat transfer modality: The La Fossa cone of Vulcano (Italy) case study
Tullio Ricci et al., Istituto Nazionale di Geofisica e Vulcanologia, via di Vigna Murata 605, 00143 Rome, Italy. This paper is online at http://dx.doi.org/10.1130/G37015.1.
Detecting volcanic unrest is of primary importance for volcanic eruption forecasting and hazard assessment and, ultimately, volcanic risk mitigation. We present a simple and innovative analysis of shallow vertical temperature profiles to depths of 70 cm. These data were recorded at La Fossa cone of Vulcano (Aeolian Islands, Italy) during an episode of increased seismic activity occurred on 2009. This new technique permits to identify changes in heat transfer modality occurring between the hydrothermal system and the surface. In fact, our results indicate that monitoring the heat transfer changes in the near surface is an efficient tool for the identification of volcanic unrest associated with disruption of a hydrothermal system. Such a low-cost device associated with easy real-time data processing could constitute a very promising, yet deceptively simple, technique to monitor hydrothermal systems, in order to assess the hazard posed by high-energy eruptions for populations living close to active volcanoes.
Principal slip zones: Precursors but not recorders of earth-quake slip
Matt J. Ikari, MARUM, Center for Marine Environmental Sciences, University of Bremen, D-28359 Bremen, Germany. This paper is online at http://dx.doi.org/10.1130/G37028.1.
Most of the motion on faults occurs in a very narrow part of the fault called principal slip zones (PSZs), which are commonly used as evidence that a particular fault has experienced earthquakes. In this study, laboratory experiments are performed to re-create fault motion using two types of powdered rock similar to what would naturally be found in faults (shales and slates). The slate gouges develop a black, narrow, PSZ composed of very small particles of less than 1/1000 of a millimeter. These PSZs formed at sliding speeds about 100,000 times slower than earthquake speeds. Earthquake generation requires the fault to become weaker when it is slid faster, this only occurs for slate samples that have a PSZ. Shale gouges, on the other hand, do not develop a PSZ and become stronger when slid faster, which is not compatible with earthquake occurrence. Therefore, a fault might have to have PSZ before it can host an earthquake, but the PSZ is not necessarily evidence that an earthquake happened on the fault in the past.
To see, or not to see? Rifted margin extension
Ken McDermott and Tim Reston, School of Geography, Earth and Environmental Sciences, University of Birmingham, Birmingham B15 2TT, UK. This article is online at http://dx.doi.org/10.1130/G36982.1.
When continents break apart and separate to form the ocean basins, the edge of the continental crust is broken into pieces which are slid apart along faults, producing a continental margin that is both thinner and wider than before. The amount of stretching that can be measured is however generally far less than needed to explain the thinning of the crust: it appears as if some of the crust has somehow been removed. We show that much of the extension is simply very difficult to recognize using the standard seismic imaging techniques: as the crust is broken into smaller and smaller pieces, it becomes difficult to identify all the faults, and much of the extension goes unnoticed.
Age and geochemistry of magmatism on the oceanic Wallaby Plateau and implications for the opening of the Indian Ocean
Hugo K.H. Olierook et al., Department of Applied Geology, Curtin University, GPO Box U1987, Perth, WA 6845, Australia. This article is online at http://dx.doi.org/10.1130/G37044.1.
Researchers from Australian institutions have shed some light on the origin of massive volcanism when continents break apart using rocks collected from the Wallaby Plateau. This is the first time that rocks have been dated from the Wallaby Plateau, a 70,000 square km structure off the northwest coast of Australia. Although once at Earth's surface and positioned between Australia and India, the Wallaby Plateau now sits underneath >2 km of water. Previous studies have estimated the plateau's lava volume is 10,000 to 100,000 cubic km, but it was uncertain how these lava flows formed. In this study, chemical data shows that lava flowed out on a continent rather than on the ocean floor. It is only since this eruption that the Wallaby Plateau has sunk underwater. Radioactive dating has accurately constrained to age of at least part of the eruption to 124 million years ago. When piecing back together the Australian and Indian continents, it becomes clear that magma could not erupt before 124 million years ago. This space was only created when India broke away from Australia, allowing lava to flow out freely. Future expeditions have been proposed adjacent to the Wallaby Plateau to discover more about volcanism during continental breakup.
A terrestrial perspective on using ex situ shocked zircons to date lunar impacts
Aaron J. Cavosie et al., TIGeR (The Institute for Geoscience Research), Department of Applied Geology, Curtin University, Perth, WA 6102, Australia. This article is online at http://dx.doi.org/10.1130/G37059.1.
Reconstructing the meteorite impact history of the Moon is critical for constraining the timing, duration, and flux of the early impact bombardment, which played a first-order role in establishing when Earth became habitable. The U-Pb ages of zircons in lunar breccias have been interpreted as dating impacts on the Moon, however, this new study by Cavosie et al. questions these ages, as most lunar zircons do not show evidence of shock deformation caused by impact. To test the validity of dating impacts using zircons from unknown source rocks, the authors studied shocked zircons in modern sediments eroding from the giant 2.0 billion year old Vredefort impact in South Africa as a terrestrial analog for lunar breccia zircons, as both zircon populations have been separated from their hosts rocks. Electron backscatter diffraction was used to identify diagnostic shock microstructures, such as microtwins, however, these features did not result in age-resetting during impact. Only neoblasts, regions of newly recrystallized zircon, yielded the impact age. These results demonstrate that most lunar zircons record crystallization ages, rather than impact ages, and thus can't be used to reconstruct an accurate impact chronology for the Moon.
The ocean-continent transition in the mid-Norwegian margin: Insight from seismic data and an onshore Caledonian field analogue
Mansour M. Abdelmalak et al., Centre for Earth Evolution and Dynamics (CEED), University of Oslo, Oslo N-0315, Norway. This article is online at http://dx.doi.org/10.1130/G37086.1.
Exposed Ocean-Continent Transition (OCT) have contributed significantly to understanding hyper-extended margin development. Several studies have been addressed to the magma-poor margin analogues, but less is known about the magma-rich margin analogues. Most of the present-day magma-rich margins are submerged offshore and are therefore difficult to study by direct observation. Furthermore, the thick accumulation of extrusive and intrusive rocks presents a major challenge for seismic imaging of deeper levels. We use new and reprocessed seismic data and drill core 642E information from the Mid-Norwegian margin to establish better constraints of the OCT nature. These observations are compared to the field analogue in the Seve Nappe Complex (SNC) of the Scandinavian Caledonides. The studied areas of the SNC comprise some large areas of mostly contact metamorphosed sedimentary and intrusive rocks. The ~800-km-long SNC constituted an OCT zone of a magma-rich margin segment. Parts of the SNC represent an onshore analogue to the deeper level of the Mid-Norwegian margin permitting direct observation, sampling and providing an improved understanding, particularly of the deeper levels, of present-day magma-rich margins.
Pulsed Vulcanian explosions: A characterization of eruption dynamics using Doppler radar
Lea Scharff et al., Institut für Geophysik, CEN, Universität Hamburg, Bundesstrasse 55, 20146 Hamburg, Germany. This article is online at http://dx.doi.org/10.1130/G36705.1.
When a volcano erupts, it ejects a mixture of gases, fine ash and coarser material into the atmosphere. To date volcanic hazard assessment, especially the prediction of volcanic ash transport and dispersion, is based on numerical modelling. These numerical models use input from the so-called classic buoyant plume theory that assumes a steady ejection of volcanic ejecta. The ejection of volcanic particles into the atmosphere is, however, by no means a steady or constant process, as recently revealed by new measurement techniques. In our article, we analyze the temporal variability of volcanic ejecta velocities measured by a Doppler radar at two dome building volcanoes - Volcán de Colima in Mexico and Santiaguito volcanic complex in Guatemala. The measured trends suggest that the Vulcanian eruptions at both volcanoes consist of individual explosions or pulses that follow each other with gaps of 2-5 seconds. Using statistical analysis we propose that these pulses are generated by two competing processes within the volcano, for example, a short duration gas pressure release through a shallow fracture network competing against long term (tens of seconds) closing of fractures due to compaction of the edifice.
Are granites and granulites consanguineous?
Fawna Korhonen et al. (Corresponding author: Michael Brown); Geological Survey of Western Australia, East Perth, WA 6004, Australia; (Laboratory for Crustal Petrology, Department of Geology, University of Maryland, College Park, Maryland 20742, USA). This article is online at http://dx.doi.org/10.1130/G37164.1.
An important question is whether the heat necessary to melt the crust is internally generated by radioactive decay of U, Th and K or introduced from the mantle via thinning of the lithosphere and intrusion of basalt. As rocks are buried they follow different paths depending on the rates of heating versus burial. For burial faster than heating, the maximum depth occurs prior to the thermal maximum and heating is due to high concentrations of U, Th and K and low erosion rates. For heating faster than burial, the thermal maximum occurs prior to maximum depth due to an incursion of mantle heat, which raises the question of whether there is also an input of mass to the resulting granites. This is commonly addressed by inverting geochemical data from granites, but a complementary approach is to assess kinship among residual granulites and granite remnants in the source. In their Geology article, Korhornen et al. use geochemical data from a suite of granulites and granites from Eastern India to demonstrate an increasingly important mass input from the mantle to granite genesis from southwest to northeast. The spatial variation in mantle input was related to changing feedback between the rates of extension and flux of mantle melt.
Onset of aridification by 34 Ma across the Eocene-Oligocene transition in Central Asia
Jimin Sun and Brian F. Windley, State Key Laboratory of Cenozoic Geology and Environment, Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Beijing 100029, China; CAS (Chinese Academy of Sciences) Center for Excellence in Tibetan Plateau Earth Sciences, Beijing 100101, China; Department of Geology, University of Leicester, University Road, Leicester LEI 7RH, UK. This article is online at http://dx.doi.org/10.1130/G37165.1.
Since 65 million years ago, Earth's climate has changed markedly from warm and humid to cold and dry. The most significant climate cooling occurred 34 million years ago. It is interesting to know how the ecosystem responded to this climatic change in different parts of the world. Now, Sun and Windley have found wind-blown dust in the southwestern Mongolia. They used multiple geological methods to constrain the age of the wind-blown dust, their results suggest that the earliest wind-blown deposits can be as old as 34 million years in the Asian interior. This is about 10 million years older than the previously reported wind-blown dust in Asia. This oldest eolian dust deposit provides direct evidence for enhanced aridification since 34 million years ago. This can be explained by reduced moisture transport by prevailing westerlies from the westward retreated of a past ocean related to the India-Asia collision and the global sea level drop.
Chemosymbiont-dominated seafloor communities in modern and Cretaceous upwelling systems support a new, high-productivity variant of standard low-oxygen models
Yael Edelman-Furstenberg and Susan M. Kidwell, Geological Survey of Israel, 30 Malchei Yisrael Street, Jerusalem 95501, Israel; Department of Geophysical Sciences, University of Chicago, 5734 S. Ellis Avenue, Chicago, Illinois 60637, USA. This article is online at http://dx.doi.org/10.1130/G37017.1.
Faunal analysis of modern (Benguela upwelling system, Namibia) and ancient (upper Cretaceous, Israel) sedimentary records rich in organic matter, biogenic silica, carbonate, and phosphate indicates that, contrary to stereotypes of upwelling systems as dead zones, macrobenthic communities are present but highly variable. Biofacies vary with distance from the upwelling core and as a function of both the supply of food to the seafloor and the oxygen demand it creates there: under a high organic supply, large-bodied chemosymbiotic bivalves dominate exaerobic and lower dysaerobic seafloors, and deposit feeders are abundant in upper dysaerobic and aerobic zones. Macrobenthic biofacies under upwelling thus contrast strongly with the overwhelmingly filter feeder-dominated biofacies encountered in the dark siliciclastic shales on which standard oxygen-restricted biofacies models have been based, and argue that, mechanistically, the low-oxygen conditions characterizing those shales reflect ordinary levels of water-column productivity and arise largely from water-column stratification. Our biofacies model requires testing in other upwelling records. However, we expect it to be robust and useful given the long evolutionary histories of the chemosymbiotic and deposit-feeding guilds, providing a new means for discriminating the relative roles of high organic flux and low ventilation in creating low-oxygen conditions at the seafloor.
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