June GEOLOGY media highlightsBoulder, Colo. - Topics in the June issue include: seismic precursors of volcanic eruptions; use of fluid inclusions to identify biomarkers and conditions on early Earth; evidence for the earliest-known possible link between a large igneous province and a mass extinction event; the 2004 Sumatran earthquake and dynamics of major subduction zone earthquakes; use of fossil amphibians and reptiles in estimating paleoprecipitation; new earth systems model of element interactions during geological processes; and tectonic plate motion relative to Earth's deep interior.
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Siberian glaciation as a constraint on Permian–Carboniferous CO2 levels
William T. Hyde, Duke University, Department of Earth and Ocean Sciences, Durham, NC 27708, USA; et al. Pages 421-424.
The Permian–Carboniferous glaciation (~330–265 million years ago) was the last great glaciation before the late Cenozoic advances and a time in which the Siberian landmass was in middle to high latitudes, yet had little or no permanent land ice. In fact, the distribution of glacial deposits from this long-lived cold period convinced many southern hemisphere geologists that continental drift was real--long before their northern hemisphere colleagues accepted the idea. Previous work has suggested that a decrease in carbon dioxide (CO2) levels played a key role in the development of the Gondwanan glaciers, and this conclusion is supported by proxy CO2 data. Hyde et al. demonstrate that climate/ice sheet models can also be used to constrain magnitudes of CO2 levels during the Permian–Carboniferous, and their estimates in general agree well with estimates from geochemical models and data. Results further indicate that a northern drift of Pangea in the early Permian may have initiated Gondwanan ice reduction, but that substantially higher levels of CO2 were required to achieve the observed full deglaciation. Since this meltback occurred millions of years before the end Permian extinction, high CO2 levels are implied for the background environment leading up to the end-Permian events.
New method to estimate paleoprecipitation using fossil amphibians and reptiles and the middle and late Miocene precipitation gradients in Europe
Madelaine Böhme, Ludwig-Maximilians-University Munich, Geo-Bio-Centre and Section Paleontology, Department for Geo- and Environmental Science, Munich D-80333, Germany; et al. Pages 425-428.
Precipitation is an important geodynamic control factor coupled to tectonics, erosion, continental run-off, weathering, and oceanic circulation. But in practice this climate parameter is difficult to estimate. Existing methods for determining paleo-precipitation are either subject to large errors (± 350–400 mm or more using mammalian proxies) or restricted to wet climate systems due to their strong facies dependence (paleobotanical proxies). Böhme et al. describe a new tool to estimate paleo-precipitation based on an indexing of the frequency of eco-physiologic groups within fossil amphibian and reptile communities. In recent communities, these indices show a highly significant correlation to annual precipitation (r2 = 0.88) and will yield paleo-precipitation estimates with average errors of ± 250–280 mm. Knowledge about fossil amphibians and reptiles has increased dramatically over the last decade, and large distribution databases also exist. Therefore, the new method will widely extend the applicability of precipitation estimates in the geological past. It will contribute to the development of more comprehensive paleo-precipitation databases, which is crucial for understanding geodynamic, oceanographic, and atmospheric feedback mechanisms. In this way, the new method will help overcome a methodological barrier in modern geosciences by bridging marine and continental records.
Biomarkers from Huronian oil-bearing fluid inclusions: An uncontaminated record of life before the Great Oxidation Event
Adriana Dutkiewicz, University of Sydney, School of Geosciences, Sydney, NSW 2006, Australia; et al. Pages 437-440.
The development of an ultra-sensitive technique for extracting and analyzing oil from tiny fluid inclusions trapped inside mineral grains has allowed geologists to delve further into Earth's early biological past. The presence of diverse and abundant molecular fossils (biomarkers) inside oil inclusions in a 2450-million-year-old sandstone from the Great Lakes region in Canada confirms the antiquity of oxygenic photosynthesis. It also shows that bacteria and more complex organisms such as algae survived extreme environmental perturbations, including major glaciations associated with the so-called "Snowball Earth." Using fluid inclusions for such studies is advantageous because they represent closed systems that have been protected from contamination by younger fluids for millions and even billions of years; thus, they yield indisputable evidence of life and conditions on early Earth.
Evolution of Atlantic thermohaline circulation: Early Oligocene onset of deep-water production in the North Atlantic
Rachael K. Via and Deborah J. Thomas (corresponding author), Texas A&M University, Oceanography, College Station, TX 77834-3146, USAPages 441-444.
The flow of deep-water masses is a key component of heat transport in the modern climate system; however, the role of deep-ocean heat transport during periods of extreme warmth is poorly understood. Presently, deep waters form in both the North Atlantic and the Southern Ocean. However, a different mode of deep-water circulation operated during the extreme "greenhouse" warmth of the early Cenozoic, in which the Southern Ocean was the dominant region of deep-water formation. The combination of general global cooling and tectonic evolution of the Atlantic basins over the past ~55 million years ultimately led to the development of a mode of overturning circulation characterized by both Southern Ocean and North Atlantic deep water sources. The change in deep-water circulation mode may, in turn, have impacted global climate; however, unraveling the causes and consequences of this transition requires a better understanding of the timing of the transition. New data from the southeastern Atlantic Ocean indicates that the initial transition to a bipolar mode of deep-water circulation occurred in the early Oligocene (ca. 33 Ma). The likely cause of significant deep-water production in the North Atlantic was the opening and deepening of the sill separating the Greenland-Norwegian seas from the North Atlantic.
Submarine volcanoes and high-temperature hydrothermal venting on the Tonga arc, southwest Pacific
Peter Stoffers, Institut fuer Geowissenschaften, University of Kiel, Kiel 24118, Germany; Mark D. Hannington (corresponding author), University of Ottawa, Earth Sciences, Ottawa, Ontario K1N 6N5, Canada; et al. Pages 453-456.
The first submarine hydrothermal vents and associated seafloor mineralization have been found in the summit calderas of two submarine volcanoes on the Tonga arc in the southwest Pacific. The highest temperature vents (245–265 ºC) occur at water depths of 385–540 meters and are related to basaltic dike swarms at a summit cone and in the caldera walls of the volcano. Clusters of large (to 10 meters high) barite, anhydrite, and sulfide chimneys on the summit cone are vigorously discharging clear hydrothermal fluids with temperatures on the seawater boiling curve. Flame-like jets of steam (H2O vapor) occur at the chimney orifices. At shallower depths, voluminous streaming of CO2 bubbles occurs through sulfur-cemented ash along a chain of recent explosion craters. These discoveries greatly extend the known areas and diversity of seafloor hydrothermal activity in the western Pacific Ocean.
The origin of volcano-tectonic earthquake swarms
Diana C. Roman, University of Leeds, School of Earth and Environment, Leeds, West Yorkshire LS2 9JT, UK; and Katharine V. Cashman, University of Oregon, Geological Sciences, University of Oregon, Eugene, OR 97403-1272, USA. Pages 457-460.
Swarms of low-magnitude, high-frequency earthquakes known as volcano-tectonic (VT) earthquakes typically precede and accompany volcanic eruptions. Because these earthquakes are generally the earliest seismic precursor to an eruption, understanding their origin and what information they can provide about magma migration would be advantageous for forecasting volcanic activity. A review of the characteristics of these earthquakes as documented in published case studies suggests that they can result from one of two processes: faulting of rock ahead of a propagating magma-filled crack (or 'dike'), or faulting of rock around an inflating dike. The first process is marked by VT earthquakes with locations that propagate systematically through time and by mechanisms that reflect stresses generated around a propagating crack tip. The second process is marked by VT earthquakes that are randomly distributed throughout a rock volume and by mechanisms that reflect stresses generated by the inflation of a dike (perpendicular to the orientation of that dike). Roman and Cashman suggest that one of these two processes may dominate in a particular volcanic system depending on one or more factors such as the rheology of the ascending magma, the nature of faulting in the rock surrounding the volcanic conduit, and/or the tectonic setting of the volcano. Thus, relatively basic analyses of VT earthquakes recorded prior to eruptions may be used to gain information about the nature and dynamics of ascending magma, which may form a basis for more sophisticated forecasts of volcanic activity.
The Kalkarindji continental flood basalt province: A new Cambrian large igneous province in Australia with possible links to faunal extinctions
Linda Glass, Pterodia Pty Ltd, Perth, Western Australia 6931, Australia; and David Phillips (corresponding author), The University of Melbourne, School of Earth Sciences, Melbourne, Victoria 3010, Australia. Pages 461-464.
Glass and Phillips present new geochemical and geochronological data that link scattered Cambrian basalt suites across northern and central Australia into a single large igneous province (LIP). This newly identified province, named the Kalkarindji Continental Flood Basalt Province (CFBP), is Australia's largest and oldest Phanerozoic igneous province. The province covers an area of at least 1 million square kilometers with an estimated original volume exceeding 0.5 million cubic kilometers, which ranks the province with other CFBP's such as the Deccan, Parana, and Karoo LIP's, in terms of size. New high-precision 40Ar/39Ar ages suggest that the newly identified Kalkarindji CFBP may be linked to the early Toyonian mass extinction event at the Early/Middle Cambrian boundary, thus providing the oldest-known possible link between a LIP and a mass extinction event.
Moving hotspots or reorganized plates?
Shimin Wang and Mian Liu, University of Missouri, Columbia, Department of Geological Sciences, Columbia, MO 65211, USA. Pages 465-468.
The theory of plate tectonics describes how tectonic plates, the dozen or so pieces of Earth's broken outer shell, move relative to each other. While observations in the past decades have greatly refined the relative motion of these plates, their motion relative to Earth's deep interior remains uncertain. Two propositions suggested more than 30 years ago provide the framework for relating plate tectonics to the deep mantle: 1) that hotspots, caused by upwelling plumes from the deep mantle, have remained fixed relative to each other, and 2) that the direction and rate of plate motions have not changed significantly in the past 40 million years. The former allows hotspots to be the most commonly used reference framework for studying plate motion relative to Earth's deep interior, and the later permits relative plate motion established from young marine magnetic anomalies to be extrapolated to the geological past. In this study, Wang and Liu re-examined hotspot data with an up-to-date relative plate motion model and found that these two propositions cannot be tenable simultaneously. Their statistical analyses indicate that plate motion did not reorganize significantly in the past 40 million years, but hotspots may have moved systematically, in the direction opposite to plate motion.
Seafloor morphology of the Sumatran subduction zone: Surface rupture during megathrust earthquakes?
Timothy J. Henstock and Lisa C. McNeill (corresponding author), National Oceanography Centre, Southampton, University of Southampton, Southampton, Hampshire SO14 3ZH, UK; et al. Pages 485-488.
The effect of major subduction zone earthquakes, such as the 2004 Sumatra event, on the seabed is the subject of much debate. For example, does the slip on the fault extend directly to the seabed, producing identifiable fault scarps? A survey of the region affected by the 2004 Sumatran earthquake suggests that during these large events, the fault slip on the plate boundary can extend to the seabed at the edge of the deformation zone. The swath bathymetry data collected by HMS Scott revealed for the first time the detailed seabed morphology of this rarely explored region, showing the effects of both gradual long-term deformation over tens to hundreds of thousands of years and infrequent, but rapid, processes such as faulting and rare submarine landslides. Large-scale ridges in the sediments are formed due to underlying faults that unusually dip in the opposite direction from the main plate boundary. Small seafloor scarps were observed on the seaward side of these sediment folds, near the edge of the deformation zone; these scarps are in the wrong position to form as the folds develop, suggesting a link instead to the sudden movement between the two plates during large earthquakes. Because seafloor slip occurs at the seaward edge of the deformation zone, these results suggest that the whole width of deformed sediments may move during very large earthquakes, with potential impacts on the mode and magnitude of tsunami generation and associated hazards.
Redox decoupling and redox budgets: Conceptual tools for the study of earth systems
K.A. Evans, Australian National University, Research School of Earth Sciences, Canberra, ACT 0200, Australia. Pages 489-492.
Evans presents a new way to examine how elements such as iron, carbon, oxygen, sulfur, and hydrogen interact and move during geological processes. The new method presents immediate opportunities for application to a wide range of geologic topics, including the formation of ore deposits, volcanism, the evolution of Earth, environmental problems, and biotic systems. The method is used to demonstrate that mid-oceanic-ridge basalts, the most common lava type, lose hydrogen as they crystallize. This loss could affect the balance of elements in the atmosphere and oceans.
To view the complete table of contents for the June issue of GEOLOGY, go to http://www.gsajournals.org/gsaonline/?request=get-current-toc&issn=0091-7613.
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