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Lower crustal H2O controls on the formation of adakitic melts G.F. Zellmer et al., Institute of Earth Sciences, Academia Sinica, 128 Academia Road Section 2, Nankang, Taipei 11529, Taiwan, and Lamont Doherty Earth Observatory, 61 Route 9W, Palisades, New York 10964, USA. Posted online ahead of print 10 April 2012; doi: 10.1130/G32912.1.
G.F. Zellmer and colleagues study mafic volcanics from the SW Japan arc and show that they indicate constant lower crustal temperatures (about 1080 deg. Celsius) but variable water contents (about 2-6 wt%). As water contents in the mafic melts decrease northeastward, Sr/Y ratios in spatially associated intermediate melts increase; i.e. the intermediate rocks become adakitic in composition. Our data indicate that adakites in SW Japan are not related to melting of an eclogitic subducting slab at unusually high temperatures, but instead to melt evolution in the lower overriding crust in the presence of residual garnet, the stability of which increases as water contents of parental melts decrease. Lower crustal melt evolution at reduced water contents likely represent a unifying process for generating adakitic signatures in all tectonic settings that have previously been considered to enhance slab melting (subduction of young slabs, shallow subduction, or subduction of slab edges or windows). The results presented here by Zellmer and colleagues demonstrate that magmatic water plays a key role in the differentiation of arc magmas in modern and ancient subduction settings.
Stromatolite lamination frequency, Walker Lake, Nevada: Implications for stromatolites as biosignatures Victoria A. Petryshyn et al., Dept. of Earth Sciences, University of Southern California, Los Angeles, California 90089, USA. Posted online ahead of print 10 April 2012; doi: 10.1130/G32675.1.
Lamination in stromatolites (considered some of the oldest fossils on Earth) is commonly interpreted to record the periodic response of a microbial community to daily, seasonal, or perhaps yearly environmental forcing. This study by Victoria A. Petryshyn and colleagues uses high-resolution carbon-14 dating of Holocene stromatolites from Walker Lake (Nevada, USA) to construct a record of lamination rate over the course of accretion. The stromatolite lamination rates are found to form on a variable, multi-year time scale that closely tracks lake level and local climate factors (e.g., El Niño Southern Oscillation), suggesting that climate and lake geochemistry had a larger influence on lamination formation than did microbial communities. While it is clear that some modern and ancient stromatolites are built in response to a microbial-paced forcing, this study demonstrates that caution must be used when interpreting the meaning of stromatolites and stromatolitic lamination, especially with respect to ancient Earth.
Methane seeps as ammonite habitats in the U.S. Western Interior Seaway revealed by isotopic analyses of well-preserved shell material Neil H. Landman et al., American Museum of Natural History, Division of Paleontology (Invertebrates), New York, New York 10024, USA. Posted online ahead of print 10 April 2012; doi: 10.1130/G32782.1.
Ammonites are an important component of the marine biota during the Cretaceous. They are abundant at cold methane seeps, but their role in seep ecosystems is unknown. Neil H. Landman of the American Museum of Natural History and colleagues explore a seep from the Upper Cretaceous Pierre Shale of southwestern South Dakota that contains original shell material, allowing them to investigate the isotopic composition of the ammonite shells. The ammonites consistently display light values of delta-13C ranging to approx. 14 parts per mil. In specimens sampled along the shell, light values occur throughout ontogeny. These values are generally lighter than those in non-seep specimens from age-equivalent rocks elsewhere in the basin and suggest that ammonites incorporated isotopically light methane-derived carbon in their shells because they lived in close proximity to the vent fluids and methane-oxidizing bacteria. Thus, even though they were mobile animals, ammonites were not transient visitors but long-term residents. In addition, the values of 87Sr/86Sr in the limestone and well-preserved fossils at the seep are higher than that of the open ocean at that time. These elevated values suggest that the seep fluids were imprinted with a radiogenic Sr signature, perhaps derived from equilibration with granitic deposits at depth during the initial uplift of the Black Hills.
Storm and fair-weather wave base: A relevant distinction? Shanan E. Peters and Dylan P. Loss, Dept. of Geoscience, University of Wisconsin, Madison, Wisconsin 53706, USA. Posted online ahead of print 10 April 2012; doi: 10.1130/G32791.1.
Wind-generated surface waves are an important mechanism for coastal erosion and the redistribution of sediment on shallow marine shelves and are commonly portrayed as comprising two distinct size classes: fair-weather waves and larger storm waves. Here, Shanan E. Peters and Dylan P. Loss combine more than two million observations of wave sizes taken over 12 years from 32 NOAA buoys located in the Caribbean, the Gulf of Mexico, and the western Atlantic to characterize wave sizes in the modern ocean. They find no evidence for two distinct types of waves subdivided by wave size. Instead, there is evidence for significant geographic variability in wave size, with locations in the relatively protected Gulf of Mexico and Caribbean regions having waves about 50 m smaller in wavelength than locations in the western Atlantic. Time-integrated estimates of wave size provide empirical constraints on the paleo-water depths of ancient sedimentary deposits and highlight differences between sheltered shelf environments, such as those that characterized many ancient seas, and open-ocean-facing, narrow continental shelves, such as those that predominate today.
Oxygen isotopic evidence for Late Triassic monsoonal upwelling in the northwestern Tethys M. Rigo et al., Dept. of Geosciences, University of Padova, Via Gradenigo 6, 35131 Padua, Italy. Posted online 10 April 2012; doi: 10.1130/G32792.1.
M. Rigo and colleagues use the oxygen isotopic composition of conodonts from the Sicani Basin (Sicily, Italy), on the northwestern Tethys margin, to reconstruct regional paleo-seawater conditions. Values determined by sensitive high-resolution ion microprobe (SHRIMP II) are consistent with previously published data from the contemporaneous marginal Hallstatt-Meliata Basin, located farther north, but are distinctly higher than those previously reported from the open ocean setting of the nearby Lagonegro Basin. The ~2 parts per mil offset represents a temperature differential of about 8 deg. Celsius, suggesting that sea surface temperatures along the northwestern margin (Sicani and Hallstatt Meliata basins) of the Tethys Ocean were lower than in the open ocean (Lagonegro Basin) during the lower part of Late Triassic. The cooler waters of the marginal basins have been interpreted to reflect coastal upwelling driven by strong monsoonal systems, found in existing paleoclimate models for the region.
Convection of North Pacific deep water during the early Cenozoic Ashley M. Hague et al., Texas A&M University, Dept. of Oceanography, MS 3146, College Station, Texas 77843-3146, USA. Posted online ahead of print 10 April 2012; doi: 10.1130/G32886.1.
The ocean's deep-water (or "meridional overturning circulation" [MOC]), contributes significantly to transport of heat and nutrients and ventilation of the deep oceans, and is a crucial component of the climate system. In the modern mode, deep-water formation occurs in the North Atlantic as well as near Antarctica. However, a different mode likely operated during warm intervals of the past. The hypothesized modes of MOC include those characterized by high latitude Pacific sources, sinking of warm and salty waters in the subtropics, or widespread mixing across an overall weaker thermocline. Thus, determining how the deep ocean basins were ventilated in the past is necessary to address the role of deep-ocean circulation during fundamentally warm climate states. New proxy data, presented here by Ashley M. Hague and colleagues, from the North Pacific spanning the Late Cretaceous-Paleogene (~70-30 million years ago) combined with the results of coupled climate model simulations reveal a circulation mode characterized by deep convection in the North Pacific, and possibly the South Pacific. Convection in both regions was most intense during the relatively "cool" portion of the Late Cretaceous and Early Paleocene and waned with the approach of the peak global warmth of the Early Eocene (~52 million years ago).
No causal link between terrestrial ecosystem change and methane release during the end-Triassic mass extinction Sofie Lindström et al., GEUS (Geological Survey of Denmark and Greenland), Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. Posted online ahead of print 10 April 2012; doi: 10.1130/G32928.1.
The end-Triassic mass extinction event (201 million years ago) severely affected animals and plants both in the sea and on land. Carbon isotope records across the Triassic-Jurassic (T/J) boundary show at least two prominent negative peaks interpreted to reflect injection of light carbon to the atmosphere, either from massive volcanic emissions of carbon dioxide or from release of methane through contact metamorphism during the initial stages of the breakup of the supercontinent Pangea. In particular, the most prominent negative carbon isotope peak has been suggested as the trigger of the end-Triassic event. Sofie Lindström and colleagues present a new, extended organic carbon isotope record across the T/J boundary in the Danish Basin, which correlates with reference records from the UK, providing new evidence that the major biotic changes, both on land and in the oceans, commenced prior to the most prominent negative carbon isotope excursion. If massive methane release was involved, it did not trigger the end-Triassic mass extinction, at least not on land. Instead, the highest negative C-isotope excursion is contemporaneous with the onset of floral recovery on land, whereas marine ecosystems remained disturbed for millions of years after the event.
Hf-Nd isotope variation in Mariana Trough basalts: The importance of "ambient mantle" in the interpretation of subduction zone magmas
Jon Woodhead et al., School of Earth Sciences, University of Melbourne, Parkville VIC 3010, Australia. Posted online ahead of print 10 April 2012; doi: 10.1130/G32963.1.
Defining the magnitude and nature of the elemental mass transfers occurring during the subduction process is a key component in our aspiration to understand the geochemical evolution of the planet itself. In this study, Jon Woodhead and colleagues explore a potential new tool for such investigations. By analyzing the isotopic composition of the chemically immobile elements Hf and Nd in basalts from the Mariana Trough back-arc basin, they are able to define the "ambient mantle" composition. This baseline then forms a starting point from which to interpret the many and varied volcanic products of the Mariana Arc itself, ranging from depleted tholeiites to highly enriched shoshonites. While much research to date has concentrated on the influence of the subducting slab assemblage in controlling the composition of arc magmas, this study highlights the equally important role played by Earth's mantle in this process.
The invisible hand: Tectonic triggering and modulation of a rhyolitic supereruption
Aidan S.R. Allan et al., School of Geography, Environment and Earth Sciences, Victoria University of Wellington, P.O. Box 600, Wellington 6140, New Zealand. Posted online ahead of print 10 April 2012; doi: 10.1130/G32969.1.
Sorting out what happens during prehistoric volcanic eruptions on timescales relevant to modern society is challenging due to the limits of what can be understood from the geological record. Discovery of minor volumes of "foreign" magma in early eruption deposits shows that the outbreak of the world's youngest super-eruption (the Oruanui eruption, Taupo volcano, New Zealand, 27 thousand years ago) was intimately controlled by tectonic stresses. Chemical fingerprinting of that foreign magma shows that it came from an independent magmatic system some 10-15 km away, yet it was erupted at Taupo volcano from the same vent as the early Oruanui magma. Eruption was triggered by a rifting-related event that unclamped faults above the magma chambers and diverted the foreign magma laterally to collide with the evacuating Oruanui magma. Relaxation of stresses associated with the rifting event then led to premature shutdown of explosive activity for some months, despite only a trivial volume of magma being released. Renewed rifting then restarted the eruption, again with the two magmas. Tectonics played a crucial role in controlling the onset and vigor of this large eruption and, because of its invisibility, the role of tectonics may currently be under-recognized in many other eruptions.
Origin of contrasting Devonian supradetachment basin types in the Scandinavian Caledonides
Vegard V. Vetti and Haakon Fossen, Dept. of Earth Science, University of Bergen, Postboks 7803, 5020 Bergen, Norway. Posted online ahead of print 10 April 2012; doi: 10.1130/G32512.1.
Extensional detachments (faults) in collapsing orogens tend to generate strongly asymmetric sedimentary basins with beds dipping against the tectonic transport direction, in fault contact with the low-angle detachment. A series of such basins occur in the SW Norway Caledonides atop a W-dipping detachment zone. One of these Devonian basins shows consistently E-dipping beds that rest unconformably on the substrate over a lateral distance of more than 11 km. Restoring everything by rigid rotation gives a very steep and ~10 km tall paleoslope, which is clearly unrealistic. Instead, Vegard V. Vetti and Haakon Fossen explain these peculiar unconformable relations by deposition in a hanging-wall ramp basin that opens due to ~10-km-tall ramp in the underlying detachment. Forward modeling of this situation shows that the model is realistic. The authors suggest that this model has application to other sedimentary basins associated with detachments.
Fluid-mineral reactions and trace metal mobilization in an exhumed natural CO2 reservoir, Green River, Utah
Max Wigley et al., Dept. of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK. Posted online ahead of print 10 April 2012; doi: 10.1130/G32946.1.
Natural CO2 reservoirs are an important tool for understanding the long-term processes that will occur in geological carbon storage sites. Max Wigley and colleagues show that an area of alteration near the town of Green River, Utah (United States), where originally red sandstones have been bleached white, is due to a CO2-bearing fluid. Reactions between the CO2 and the rock have resulted in growth of secondary carbonate minerals, and there is a zone of increased secondary mineral growth at the CO2-promoted reaction front. This is similar to predictions made using simple modeling techniques. The CO2-bearing fluid has also mobilized trace metals from the formation and deposited them in high concentrations at the reaction front. This area of alteration can be used as an analogue for processes in geological carbon capture and storage reservoirs. Trace metals mobilized by injected CO2 are likely to be rapidly redeposited.
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