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Solution of shallow water carbonates: An insignificant buffer against rising atmospheric CO2.
Andreas J. Andersson, Department of Oceanography, School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, Hawaii 96822, USA, et al. Pages 513-516.
Continuous globally rising atmospheric CO2 concentrations and mean atmospheric temperatures may inhibit the ability of calcareous organisms such as corals and coralline algae to calcify at rates similar to those of the recent past. This is due to the invasion of CO2 into the ocean and consequent increase in the acidity (decreasing pH) and decrease in the carbonate saturation state of seawater and to rising sea-surface temperatures. Thus, the overall development of reefs may be negatively impacted, making them more vulnerable to erosion and deterioration. Alternatively, it has been proposed that no future negative impacts of this nature will be observed on reef ecosystems because any changes in the inorganic carbon chemistry of seawater could be restored by dissolution of metastable carbonate minerals such as high magnesian calcites found in reef sediments. Our model numerical simulations show that such dissolution is likely to occur in the future, but it will not be sufficient to restore the imposed changes in seawater properties due to rising atmospheric CO2 and temperature. Thus, calcification rates of calcareous marine organisms and the development of carbonate reefs will likely be negatively affected as a consequence of these environmental changes. In addition, the imposed changes could alter the composition and rate of precipitation of carbonate cements in contemporary shallow-water marine sediments.
Absence of a runaway ice-albedo feedback in the Neoproterozoic.
Christopher J. Poulsen, Department of Earth Sciences, University of Southern California, Los Angeles, California 90089, USA. Pages 473-476.
Geologists are currently debating a fantastic hypothesis, that Earth was once completely covered by ice. As currently formulated, this idea, known as the snowball Earth hypothesis, is based on the existence of a climate instability that causes low-latitude ice to quickly grow to the equator. In this contribution, we show that such an instability does not occur in a Neoproterozoic climate model of the ocean and atmosphere. The climate model results show that sea ice could have existed at very low latitudes (10º north and south of the equator) without resulting in an ice-covered Earth. Although these results do not completely rule out a snowball Earth, they do indicate that the conditions necessary to initiate a snowball Earth are much more severe than previously thought.
A Statistically Significant Relation between Rn Flux and Weak Earthquakes in the Dead Sea Rift Valley.
Gideon Steinitz, et al., Geological Survey of Israel, 30 Malchei Israel Street, Jerusalem 95501, Israel et al. Pages 505-508.
Many seismologists maintain that earthquakes are not predictable. A major reason is the failure of scientists to produce rigorous examples in which "anomalies," claimed to precede earthquakes, are objectively defined and statistical evidence for their correlation with earthquakes is demonstrated. This study shows, for the first time, a highly significant statistical connection between earthquakes and anomalous flux of radon gas that precedes them. The amount of radon was measured every 15 minutes for eight years near the active western fault in the Dead Sea, Israel. It was found that weak earthquakes, whose epicenters lie within the Dead Sea rift valley, preferentially occur in the three days that follow the start time of 110 anomalies of exceptionally high radon flux. This phenomenon was not found in the two weeks preceding or following that time interval. Although this study does not provide a practical earthquake prediction tool, the results give room for optimism that in some cases radon flux can be developed as a tool indicating changes in crustal strain that precede earthquakes.
Isotopic composition of gypsum in the Macquarie Island ophiolite: Implications for the sulfur cycle and the subsurface biosphere in oceanic crust.
Jeffrey C. Alt, Department of Geological Sciences, 2534 C.C. Little Building, University of Michigan, Ann Arbor, Michigan 48109-1063, USA, et al. Pages 549-552.
Chemical analyses of gypsum occurring on Macquarie Island in the southern ocean provide compelling evidence for the existence of a subsurface biosphere in volcanic ocean crust, and that may not rely on organic carbon for its existence. Macquarie Island is a fragment of ocean crust that has been uplifted along the boundary between two oceanic plates. Analyses of the oxygen, sulfur, and strontium isotopic compositions of gypsum (calcium sulfate) indicate that sulfate-reducing bacteria were active in the volcanic basement where this mineral precipitated. This confirms textural evidence from oceanic basement for a subsurface microbial biosphere in volcanic ocean crust. The preservation of gypsum in ocean crust of Macqurie Island suggests that formation of calcium sulfate in ocean crust may be important for the geochemical cycle of sulfur in the oceans.
Volcanic fronts form as a consequence of serpentinite dehydration in the forearc mantle wedge.
Keiko H. Hattori, Ottawa-Carleton Geoscience Centre and Department of Earth Sciences, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada, and Stephane Guillot, Laboratoire de Dynamique de la Lithosphere, CNRS, UCB-Lyon et ENS-Lyon, 2 rue Dubois, 69622 Villeurbanne, France. Pages 525-528.
Arc volcanoes form chains parallel to ocean trenches, such as the Pacific "Ring of Fire." Between the volcanic chains and trenches, volcanoes are conspicuous by their absence. Why do volcanoes form such well-aligned chains, without some occurring close to the trench? Our paper addresses this question. Water is needed as a flux to melt mantle rocks to form volcanic magmas. Water is continuously released from the subducting slab as it descends. We propose that water initially released from the subducting slab is stored in a layer of serpentinite at the base of the mantle wedge, keeping the interior of the wedge dry. Eventually, as the serpentinite is dragged downward to higher temperatures and pressures, water and water-soluble elements are released, causing melting of the overlying mantle wedge. This occurs at a well-defined depth for the slab, directly below the volcanic arc.
Demise of one volcanic zone and birth of another--12 Ma marine record of major rhyolitic eruptions from New Zealand.
Lionel Carter, National Institute of Water and Atmospheric Research, Private Bag 14901, Wellington, New Zealand, et al. Pages 493-496.
Sediment cores, recovered from the SW Pacific Ocean during Leg 181 of the Ocean Drilling Program, contain a record of major volcanic eruptions from New Zealand. The Taupo Volcanic Zone (TVZ), known as the most frequently erupting and productive rhyolitic system on Earth for the past 2 million years, now appears to have been continuous with the older, but poorly known Coromandel Volcanic Zone (CVZ), which commenced rhyolitic eruptions around 12 million years ago. From the frequency and thickness of the ash deposits preserved in the marine sediments, it now appears that the CVZ eruptions were equally as violent and frequent as their TVZ counterparts. Importantly, many of these large eruptions appear to be contemporaneous with other large eruptions around the Pacific Rim as far away as Alaska. With such widespread volcanism, the question arises as to its impact on global climate change, in particular as a trigger for ice ages.
No frictional heat along the San Gabriel fault, California: Evidence from fission-track thermochronology.
M.A. d'Alessio, Department of Earth and Planetary Science, University of California, Berkeley, California 94720-4767, USA, et al. Pages 541-544.
Faults live and die by frictional properties, so understanding earthquakes and other geologic processes requires us to understand the frictional behavior of faults. Fault friction is well understood in the laboratory, where experiments are repeatable and scientists can often predict fault behavior in the lab. However, if we try to apply our laboratory knowledge to the behavior of large faults in nature, things don't work very well. For example, frictional sliding should generate heat (much like the heat generated when you rub your hands together on a cold day), but investigators over the past three decades have looked for evidence of that heat at the surface and have never been able to find it. This observation implies that either something is washing out the frictional heat signal, or that natural faults are weaker than laboratory faults--they have significantly less friction than previously thought. Our study helps resolve this mystery. We look for evidence of frictional heat on ancient faults using a technique called "fission-track thermochronology" that allows us to read the thermal history naturally recorded in the rock (just like you can tell something about the thermal history of a Hershey's bar that was left inside a hot car, even days after you put it in the refrigerator). The faults we examine were deep within the earth several million years ago, but uplift and erosion have brought them to the surface where we can collect samples from them today. We do not see any evidence of frictional heat, which suggests that faults in nature are indeed much weaker than faults studied in laboratory tests.
Uniquely extensive seismite from the latest Triassic of the United Kingdom: Evidence for bolide impact?
Michael J. Simms, Department of Geology, Ulster Museum, National Museums and Galleries of Northern Ireland, Botanic Gardens, Belfast BY9 5AB, Northern Ireland. Pages 557-560.
At sites across >250,000 km2 of the UK a one- to four-meter-thick sequence of thin sandstone and mudstone beds, about 200 million year old, show extraordinary convolutions. This deformation has been interpreted as the result of seismic shaking of soft sediments soon after deposition, while rippled sands immediately above this "seismite" are interpreted as deposits from a tsunami triggered by the same seismic event. The geographic extent of this seismite is unique in the geological history of the UK and suggests an earthquake of a magnitude too great to be attributable to fault or volcanic activity. It is suggested that this seismic event may instead have been caused by the impact, to the west of Ireland, of a meteorite several kilometers across. Further confirmatory evidence has yet to be found, although similar "megaseismites" may prove useful in the search for ancient impact sites.
Origin of the copper-cobalt deposits of the Zambian Copperbelt: An epigenetic view from Nchanga.
Ross R. McGowan, School of Ocean and Earth Science, University of Southampton, Southampton Oceanography Centre, Southampton SO14 3ZH, UK, et al. Pages 497-500.
The Zambian Copperbelt is arguably one of the most significantly mineralized Neoproterozoic basins on Earth. In total, in excess of 1 billion tonnes of ore at ~2.7% copper have been extracted to date, and there are major cobalt accumulations, yet the genesis of these world-class deposits is poorly understood. A study of the Nchanga deposits has revealed that mineralization may have been introduced via copper-rich hydrothermal fluids that found their way into suitable host rocks during structural evolution. This model represents a significant reinterpretation of the formation of these major ore deposits and may impact not only on our understanding of similar copper accumulations in the crust, but also on future exploration for Copperbelt-style mineralization in Zambia.
Magnetoclimatology: Teleconnection between the Siberian loess record and North Atlantic Heinrich events.
M.E. Evans, Institute for Geophysical Research, University of Alberta, Edmonton, Alberta T6G 2J1, Canada, et al. Pages 537-540.
Abrupt stratigraphic fluctuations in the magnetic content of windblown deposits in southern Siberia record climatic events that can be correlated to the so-called Heinrich layers seen in North Atlantic deep-sea sediments. These, in turn, represent the increased mineral flux generated by armadas of icebergs launched from the North American continent during sharp cold pulses. Such teleconnections not only link together the continental and marine sedimentary records, but also provide evidence for the hemispheric--perhaps even global--nature of millennial-scale climate events during the last glacial-interglacial cycle.
Acid-neutralizing scenario after the Cretaceous-Tertiary impact event.
Teruyuki Maruoka and Christian Koeberl, Department of Geological Sciences, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria. Pages 489-492.
Acid rain has been proposed to account for some observations at the Cretaceous-Tertiary (K-T) boundary. The acid rain would be predicted to have more seriously affected freshwater environments than marine environments, as is observed today in highly polluted areas; however, only a minor extinction of freshwater species at the K-T boundary is evident. The extinction selectivity implies that either the acid rain was not severe enough to damage freshwater environments, or that an acid-neutralization mechanism existed only right after the K-T impact event.
Maruoka and Koeberl propose a mechanism to neutralize the acid using larnite (Ca2SiO4), produced as a result of the specific lithology at the Chicxulub impact site. The acid-neutralizing capacity of the larnite grains is high enough to consume acid produced after the K-T event within several hours to a level at which freshwater life would not be affected. This scenario can explain some of the extinction selectivity at the K-T boundary.
GSA TODAY Science Article
High-resolution lidar topography of the Puget Lowland, Washington--A bonanza for earth science.
Ralph A. Haugerud, US Geological Survey, c/o Department of Earth and Space Sciences, Box 351310, University of Washington, Seattle, Washington 98195, USA, et al.
High-resolution topography using the lidar (light detection and ranging) technique is revolutionizing investigations of active faulting, continental glaciation, landslides, and surface geologic processes in the seismically active Puget Lowland. Lidar is based on laser ranging from an aircraft, and has the ability to see through forest cover and study the detailed physiography of heavily forested areas such as this. Heavy forest cover earlier had hidden physiographic details, which illuminate geologic processes that build and modify the landscape and point to geologic hazards. The Puget Lowland--the population and economic center of the Pacific Northwest--presents special problems for geologic hazards investigations, with its young glacial topography, dense forest cover, and urbanization. Lidar mapping by the Puget Sound Lidar Consortium now has led to a detailed digital model of the landscape beneath the forest canopy. The surface thus revealed contains a rich and diverse record of previously unknown surface rupturing faults, deep-seated landslides, uplifted beaches, and subglacial features formed beneath the last ice sheet to invade the region. More than half a dozen suspected postglacial fault scarps have been identified to date. Five scarps that since have been studied by trenching show evidence of large, surface rupturing earthquakes within the last few thousand years. Future earthquakes on these these faults threaten the urban areas of Seattle and Tacoma. The fault scarps discovered with lidar strongly support a recent tectonic model in which reverse and oblique-slip movements along multiple faults accommodate north-south shortening of the Puget Lowland.
To review the abstracts for these articles, go to www.gsajournals.org. To review the complete table of contents for the June issue of GEOLOGY, go to http://www.