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GEOLOGY
Sources of the large A.D. 1202 and 1759 Near East earthquakes
M. Daëron, IPGP, Tectonique, Cedex 05, Paris 75252, France; et al. Pages 529-532.
The sources of the May 1202 and November 1759, Mw ~7.5 Near East earthquakes remain controversial, because their macroseismal areas coincide, straddling subparallel active faults in the Lebanese restraining bend. Paleoseismic trenching in the Yammoûneh basin yields unambiguous evidence both for slip on the Yammoûneh fault in the twelfth–thirteenth centuries and for the lack of a posterior event. This conclusion is supported by comparing the freshest visible fault scarps, which imply more recent slip on the Râchaïya-Serghaya system than on the Yammoûneh fault. Daëron et al.'s results suggest that the recurrence of a 1202-type earthquake might be due this century, as part of a sequence similar to that of A.D. 1033–1202, possibly heralded by the occurrence of the 1995 Mw ~7.3 Aqaba earthquake. The seismic behavior of the Levant fault might thus be characterized by millennial periods of quiescence, separated by clusters of large earthquakes.
Tectonic consequences of Martian dichotomy modification by lower crustal flow and erosion
Francis Nimmo, University of California–Santa Cruz, Earth Sciences, Santa Cruz, CA 95064, USA. Pages 533-536.
Mars is a planet of two halves: one has thick crust, the other has thin. The dividing line between these two halves shows ancient tectonic features, some of which are caused by shortening, and others by extension. In this article Nimmo shows that if the thickened crust has oozed outwards slightly, then tectonic features very similar to those actually observed should result. The amount by which the crust has oozed depends on its temperature, and hence tells us about conditions within the crust of Mars four billion years ago.
Post-glacial carbonate production by cold-water corals on the Norwegian Shelf and their role in the global carbonate budget
Björn Lindberg, Statoil, N-4035 Stavanger, Norway, and Jürgen Mienert, University of Tromsø, Department of Geology, Tromsoe, Troms 9037, Norway. Pages 537-540.
Though only studied in detail the last couple of decades, cold-water corals on the Norwegian shelf have been known for a long time. In this paper, the authors perform a first attempt to estimate the amount of carbonate produced by these cold-water corals, both on the Norwegian shelf and worldwide. The main factors influencing the cold-water coral reef growth are also discussed. Even though the production figures are moderate compared to tropical reefs and the total marine carbonate production, the cold-water reefs should be included in calculations of carbonate budgets, and can be significant factors on a local and/or regional scale.
Active geothermal systems with entrained seawater as modern analogs for transitional volcanic-hosted massive sulfide and continental magmato-hydrothermal mineralization: The example of Milos Island, Greece
Jonathan Naden, British Geological Survey, Economic Minerals & Geochemical Baselines, Nottingham, Nottinghamshire NG12 5GG, UK; et al. Pages 541-544.
Geothermal systems associated with volcanic activity are the modern-day equivalents of many ancient mineral deposits. These fossilized hot-springs help to provide the world with metals such as gold, silver, copper, lead, and zinc. They can form on the land, where the springs are refreshed by rainwater or in the deep ocean, where seawater is the source of the mineralizing fluid. Examples are the hot springs seen in New Zealand and "black smokers" observed during deep-sea exploration. Naden et al.'s research, which used specialized analytical techniques to trace the source of hot waters on the dormant volcano of Milos in the Aegean Sea, reveals a new type of fossilized hot spring. It forms as a submarine volcano emerges from beneath the sea and is exposed to rainwater. In this fresh environment sea and rainwater mix and help form a new type of hybrid mineral deposit. These latest results will give geologists, who explore for metals, a better understanding of where new mineral deposits can be found. They may also help volcanologists get a better grasp of how the interaction of hot waters and magma can trigger volcanic eruptions.
Direct radiometric dating of the Devonian- Mississippian time scale boundary using the Re-Os black shale geochronometer
David Selby and Robert A. Creaser, University of Alberta, EAS, Edmonton, Alberta T6G 2E3, Canada. Pages 545-548.
Although extremely difficult, determining accurate and precise radiometric dates for boundaries of geologic time is important to understanding Earth's history, such as the times of mass extinctions. Selby and Creaser present a radiometric dating method that allows black shale rock units, which are commonly markers of geologic time boundaries, to be accurately and precisely dated. This methodology will allow for better constraints of geologic time.
Evidence from ophiolites, blueschists, and ultrahigh-pressure metamorphic terranes that the modern episode of subduction tectonics began in Neoproterozoic time
Robert J. Stern, University of Texas–Dallas, Geosciences Department, Richardson, TX 75083-0688, USA. Pages 557-560.
Earth is the only planet known to have plate tectonics, and it is controversial why and when this began. Some scientists argue that plate tectonics already operated as early as 4 billion years ago whereas others argue for a much later beginning. This paper answers the question by equating plate tectonics with its driving force--the sinking of lithosphere in subduction zones. This allows us to identify three geologic smoking guns that, when one or more is found in rocks of a certain age signify the functioning of subduction zones--and plate tectonics--at that time. These are (1) ophiolites (fragments of oceanic lithosphere) and two kinds of metamorphic assemblages that require the unusually cold environment of subduction zones, (2) blueschists, and (3) ultra-high pressure (UHP) metamorphic terranes. The geologic record is examined for the first appearance of these, yielding a consistent answer: ophiolites, blueschists, and UHP terranes first become important in the rock record in Neoproterozoic time, about 1000–542 Ma, implying that plate tectonics began operating on Earth at that time.
Subducting oceanic crust: The source of deep diamonds
Ralf Tappert, University of Alberta, Department of Earth and Atmospheric Sciences, Edmonton, Alberta T6G 2E3, Canada; et al. Pages 565-568.
For the first time, evidence has been found that ultradeep diamonds grow from remnants of living organisms in former oceanic crust. Ultradeep diamonds from the Jagersfontein diamond mine in South Africa formed at depths of more than 500 kilometers. This is much deeper than most other diamonds in the world. The composition of the minerals enclosed by these diamonds provides clear evidence that the diamonds grew in former ocean floor rocks, during their passage deep into Earth's mantle. In addition, the carbon forming the diamonds was derived from organisms, which may have inhabited the ocean floor rocks.
Phase-lagged amplitude modulation of hemipelagic cycles: A potential tool for recognition and analysis of sea level change
Jiri Laurin, Academy of Sciences of the Czech Republic, Institute of Geophysics, Prague, 14131, Czech Republic. Pages 569-572.
Two-dimensional computer modeling of marine depositional systems was used to explore possible distortions of orbitally driven climatic signals that may occur during their incorporation in the geological record. The results suggest that systematic changes in the amplitude of short-period cycles are inherent to multi-order climatic cycles transferred into the geological record via changes in sea level. This amplitude modulation provides a signature of sea-level change and represents a potential tool for detection and analysis of orbitally driven eustatic oscillations in poorly understood intervals of Earth's past, such as the greenhouse Cretaceous.
Phosphorus, nitrogen, and the redox evolution of the Paleozoic oceans
Matthew R. Saltzman, Ohio State University, Department of Geological Sciences, Columbus, OH 43210, USA. Pages 573-576.
The oxygenation of Earth's surface environment is believed to have taken place between about 2.2 and 0.6 billion years ago. However, anoxic conditions in the oceans returned during the Paleozoic era because greenhouse climates limited the supply of downwelling oxygen relative to demand from sinking organic matter. Oceanic anoxia can profoundly influence organic carbon production and burial (the biological pump) by enhancing recycling of the limiting nutrient phosphorus. The nitrogen cycle is also affected because anoxia promotes denitrification, which is the primary loss mechanism for biologically available (or "fixed") nitrogen. Saltzman reports on large changes in carbon isotopes that indicate episodes of enhanced organic carbon burial (million year time scales) during cool, glacial periods in the Paleozoic. These episodes were sustained by enhanced phosphorus recycling. In contrast, organic carbon production and burial was maintained at low levels during greenhouse climates, which may reflect negative feedbacks on productivity in a nitrogen limited ocean.
Conduit implosion during Vulcanian eruptions
B. Kennedy, McGill University, Earth and Planetary Sciences, Montréal, PQ H3A 2A7, Canada; et al. Pages 581-584.
Volcanoes can erupt for only a few seconds yet produce devastating consequences. To model and forecast eruptions, volcanologists must understand how magma inside a volcanic conduit responds to pressure changes. Exploded crystals within erupted rocks record evidence of pressure changes within a volcano. Kennedy et al. use these rocks and a novel set of fragmentation experiments to show that implosion occurred during recent eruptions on Montserrat. This mechanism explains many characteristics of eruptions from Montserrat, and has important implications for the modeling and hazard assessment of all explosive eruptions.
Fe-oxide spherules infilling Thalassinoides burrows at the Cretaceous-Paleogene (K-P) boundary: Evidence of a near contemporaneous macrobenthic colonization during the K-P event
Francisco Javier Rodríguez-Tovar, University of Granada, Estratigraphy and Paleontology, Granada 18071, Spain. Pages 585-588.
The Cretaceous-Paleogene boundary has been profusely studied, mainly due to the mass extinction bio-event registered at this time, being one of the most debated mass extinctions of the Phanerozoic. However, little evidence exits on the relationship between the Cretaceous-Paleogene event and the macrobenthic response. In this article a detailed trace fossil analysis shows the first record of spherules related to the Cretaceous-Paleogene boundary within trace fossil infilling material, revealing a rapid macrobenthic colonization during the event, nearly contemporary with the spherule layer deposition.
Charcoal reflectance as a proxy for the emplacement temperature of pyroclastic flow deposits
Andrew C. Scott and Ian J. Glasspool, Royal Holloway University of London, Geology, Egham, Surrey TW20 OEX, UK. Pages 589-592.
Many of the world's largest and most violent volcanic eruptions have resulted in major devastation and the deposition of vast quantities of ash over a wide area. The sudden overwhelming of the landscape buries buildings, people, and livestock, as well as large amounts of vegetation. Geologists have found that, unlike magnetic or mineral data, temperature data from charcoalified woods can be obtained not only for primary but also from reworked deposits providing a valuable means of validating observations made about the style of eruption of volcanoes in ancient settings. The Soufrière Hills volcano on the Island of Montserrat in the British West Indies, which began to erupt in July 1995, has caused major devastation to the island. The eruption gave rise to devastating hot ash clouds, or pyroclastic flows, which travel at high speeds and are particularly dangerous. Studies of pyroclastic flows are important for understanding some of the more dangerous volcanic eruptions, but until now there has been a lack of substantial data on their emplacement temperatures. Much of the ash ejected during such eruptions is hot, often 200–400 °C, but possibly >600 °C and the incorporated plant material may be charcoalified. When studied in a polished section, this charcoal is highly reflective. Novel data from experimentally charred woods demonstrate that reflectance increases with both temperature and time. At temperatures above 250 °C, reflectance rises rapidly for the first hour, effectively stabilizing after 4 hours for temperatures below 400 °C. However, for higher temperatures, reflectance only stabilizes after ~24 hours. Charcoalified woods, from block and ash flow deposits generated by the Soufrière Hills Volcano in Montserrat, have yielded interpreted deposit temperatures, based on reflectance data, of 325–525 °C. These temperatures compare favourably with directly measured data (365–640 °C) from the same deposits indicating that charcoal reflectance is useful as a temperature proxy for ancient pyroclastic flow deposits, particularly where magnetic or mineral data are absent for this purpose.
Seawater chemistry, coccolithophore population growth, and the origin of Cretaceous chalk
Steven M. Stanley, Johns Hopkins University, Earth & Planetary Sciences, Baltimore, MD 21218, USA; et al. Pages 593-596.
Coccolithophores are very small, single-celled floating algae that play a significant role in food webs of the modern ocean. These algae secrete coccoliths, which are shieldlike elements of calcite. Coccoliths are extruded from the interior of the nearly spherical coccolithophore cell to cover its surface. Coccolithophores flourished during Late Cretaceous time, producing the voluminous chalk deposits that gave the Cretaceous period its name. Stanley et al. have conducted experiments that strongly support the hypothesis that the high level of calcium and low level of magnesium estimated for Cretaceous seawater permitted coccolithophores to flourish because these ionic concentrations favor precipitation of calcite, the form of calcium carbonate of which coccoliths are composed. Calcification stimulates coccolithophore population growth by contributing carbon dioxide to photosynthesis. In their experiments, three extant coccolithophore species multiplied much faster as the composition of ambient seawater was shifted toward that estimated for Cretaceous seas and thus favored coccolith production. They conclude not only that the ionic composition of Cretaceous seawater enabled coccolithophores to produce massive chalk deposits, but also that, conversely, the ionic composition of modern seawater inhibits population growth for most extant coccolithophore species, which are confined to nutrient-poor waters and fail to respond to fertilization by nitrate, phosphate, or iron in the manner of most other photosynthetic plankton.
Deformed symmetrical volcanoes
G. Norini, Università degli Studi di Milano, Dipartimento di Scienze della Terra - A. Desio, Milano 20133, Italy; and A.M. F. Lagmay, University of the Philippines, National Institute of Geological Sciences, Quezon City, Metro Manila 1104, Philippines. Pages 605-608.
Analog experiments of volcanic cones and analysis of their digital elevation models reveal that symmetrical volcanoes can have well-developed fractures in their interior. These findings pose a subtle but significant point in the assessment of volcanic landslide hazards: not all perfect cones are undisturbed. The detection of concealed deformation in symmetrical volcanoes is important because fractures induce further instability and act as sliding planes during volcano-collapse events. Over the past four centuries, such collapse events resulted in 20,000 fatalities. There are many examples of symmetrical volcanoes in nature. The faultless appearance of these volcanoes can be misleading in the conduct of hazards assessment.
GSA TODAY Science Article
The subduction factory: How it operates in the evolving Earth.
Yoshiyuki Tatsumi, Institute for Research on Earth Evolution (IFREE), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokosuka 237-0061, Japan.
What happens to thick slabs of ocean crust from beneath the seafloor when they sink below deep ocean trenches? This sinking, a process called subduction, has long been related to arcuate lines of earthquakes and volcanoes such as those that make up the Pacific ring of fire. In a new synthesis of the subduction factory, Yoshiyuki Tatsumi concisely analyzes ways in which the raw materials of the subducting slab, as it sinks and heats, are processed. The processing includes dehydration, which drives off fluids to interact with and melt overlying rocks, and direct melting of the raw materials, a process that may have prevailed in the first half of Earth's history. New products from the subduction factory ultimately rise and build eruptive volcanoes and over time have built much of the crust of the continents. Leftovers, or solid "waste" from the factory, end up deep in Earth's mantle, where they may eventually be largely recycled as raw materials for the hotspot factory that builds volcanoes such as those in Hawaii.
To view the complete table of contents for the July issue of GEOLOGY, go to http://www.gsajournals.org/gsaonline/?request=get-current-toc&issn=0091-7613.
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