Neoproterozoic stratigraphic comparison of the Lesser Himalaya (India) andYangtze block (South China): Paleogeographic implications
Ganqing Jiang, University of California, Earth Sciences, Riverside, CA 92521, U.S.A., et al. Pages 917-920.
India and south China are now separated by the Himalaya Mountains, but some 600 million years ago they may have been together as an intact continent. Sedimentary records and fossil contents presented by Jiang et al. in this volume indicate that the two continents share identical depositional history from ca. 600 Ma to 540 Ma, arguing against the previous views in which south China had a closer relationship with southeastern Australia during the period.
Impacts do not initiate volcanic eruptions: Eruptions close to the crater
Jay Melosh, University of Arizona, Department of Planetary Sciences, Room 901, Gould-Simpson Building, Building 77, Tucson, AZ 85721, U.S.A., and Boris Ivanov, Russian Academy of Sciences, Institute for the Dynamics of Geospheres, Leninsky Prospect, 38-6, Moscow 117334, Russia. Pages 869-872.
Many geologists have assumed that large meteorite impacts may cause volcanic eruptions. However, evidence for the actual occurrence of impact-caused volcanism is scarce, both on Earth and on the other planets, in spite of the common presence of impacts and volcanism. In this paper we show that the most commonly proposed mechanism for impact-caused volcanism, decompression melting, is ineffective even for impact craters as large as 350 km diameter into hot oceanic crust. We also show that the number of large volcanic eruptions on both Earth and Venus greatly exceeds the expected number of large impacts, again arguing that there is not causal connection between impacts and volcanic eruptions. The only case in which we find a possible connection is if a very large impact were to strike a volcanic center on the verge of eruption. However, the probability of this is very low-such an event may have occurred only once in Earth's history.
Formation of modern and Paleozoic stratiform barite at cold methane seeps on continental margins
Marta Torres, Oregon State University, College of Oceanic and Atmospheric Sciences, 104 Oceanography Administration Building, Corvallis, OR 97331-5503, U.S.A., et al. Pages 897-900.
Barite deposits of Paleozoic age occur as large bedded sequences and are the dominant source of industrial barite, commonly used as a weighting agent in drilling fluids by the oil and gas industry. Existing models for the origin of these deposits remain controversial as they fail to explain all their characteristics. A new mechanism for barite formation is based on discoveries of massive barite deposits on the seafloor at sites of non-hydrothermal fluid venting along continental margins. These venting sites, also known as cold seeps, discharge fluids rich in methane and barium at bottom water temperatures. Barite deposits at these sites reflect barite remobilization in barite-enriched, sulfate-depleted sediments, subsurface transport of Ba+2 by hydrologic and tectonic processes, and barite reprecipitation at sites of cold fluid discharge on the seafloor. The characteristics of these modern deposits clearly support a cold-seep origin for Paleozoic bedded barite that is relevant to economic geology. Furthermore, this new genetic mechanism provides a consistent model to aid reconstruction of tectonic and oceanographic conditions operating in the Paleozoic.
Ups and downs on spreading flanks of ocean-island volcanoes: Evidence from Mauna Loa and Kilauea
Peter Lipman, U.S. Geological Survey, Volcano Hazards Team, MS910, 345 Middlefield Rd, Menlo Park, CA 94025, U.S.A., et al. Pages 841-844.
Hawaiian volcanoes such as Mauna Loa and Kilauea are enormous edifices that have grown from the deep ocean floor, at depths of 15-20,000 feet below sea level; only small portions are above sea level. The underwater flanks of these huge volcanoes slowly spread laterally due to loading by continuing accumulation of lavas on land, and they also are subject to infrequent but catastrophic slope failures that generate giant underwater landslides and associated tsunamis. Recent exploration of the underwater flanks of these volcanoes, using research submersibles with support from the Japan Marine Research and Technology Center (JAMSTEC), has provided improved understanding of the eruption rates, recurrence intervals, and causative mechanisms for growth of Hawaiian volcanoes and landslide failure of their flanks.
On the retrieval of lava flow surface temperatures from infrared satellite data
Robert Wright, Hawaii Institute of Geophysics and Planetology, University of Hawaii, 2525 Correa Road, Honolulu, HI 96822, U.S.A., and Luke Flynn, Hawaii Institute of Geophysics and Planetology, University of Hawaii, 2525 Correa Road, Honolulu, HI 96822, U.S.A. Pages 893-896.
Earth-orbiting satellites provide an attractive means for studying the surface temperatures of active volcanic lava flows, lava domes and lava lakes. Hyperion, the first orbiting hyper-spectral imaging sensor, was recently launched on-board NASA's experimental Earth Observing-1 satellite. This paper shows how best to approach the analysis of these new hyper-spectral data. We analyzed thermal images of active Hawaiian pahoehoe lava flows collected using a hand-held thermal imaging camera (of the type used by police forces to locate suspects in the dark) and found that although the surface of an active lava flow comprises an unwieldy continuum of temperatures (ranging from normal Earth surface temperature to 1100 ºC), the simpler and easily applicable models of lava flow surface temperature we present will allow the main statistical properties of this continuum to be determined. Application of the techniques we present will allow detailed calculation of lava flow temperature and cooling rates, from space, for the first time.
Depressed rates of origination and extinction during the late Paleozoic ice age: A new state for the global marine ecosystem
Steven Stanley, Johns Hopkins Univ,Dept Earth & Planetary Sciences, Earth & Planetary Sciences, Charles & 34th, Baltimore, MD 21218, U.S.A., and Matthew Powell, Johns Hopkins University, Earth and Planetary Sciences, Baltimore, Maryland 21218, U.S.A. Pages 877-880.
We have discovered that the global marine ecosystem shifted to a new state during the largest ice age of the past half billion years. Marine life suffered a mass extinction at the start of the ice age, in Late Paleozoic time, when continental glaciers spread throughout large areas of the Southern Hemisphere. Following this crisis, evolutionary change throughout the ocean was sluggish. For all major groups of marine animals, rates of origination and extinction of species declined to low levels. The result was that diversity was restored very slowly; there was no rapid recovery of the sort that followed other mass extinctions. Species of the ice age seas were broadly adapted, occupying a wide range of habitats and temperature regimes, hence their low rates of extinction and production of species. Just as the new evolutionary regime was initiated at the very start of the ice age, it ended precisely at its end. As massive glaciers melted, more than forty million years after they had formed, evolution sped up and marine diversity rose toward its pre-ice age level.
Extreme 34S depletions in ZnS at the Mike gold deposit, Carlin Trend, Nevada: Evidence for bacteriogenic supergene sphalerite
Thomas Bawden, Global Mineral Resources, Exploration, P.O. Box 193135, San Francisco, California 94119-3135, U.S.A. Pages 913-916.
We have documented the large-scale formation of supergene sphalerite determined to be of bacteriogenic origin, demonstrating that significant geochemical processes in Earth's history can be attributed to the action of bacteria. Formation of the mineral sphalerite (ZnS) in groundwater close to Earth's surface, known as "supergene" enrichment, is predicted by thermodynamic arguments to be impossible, and has never before been conclusively observed in nature. We have documented an anomalously large-scale concentration of supergene sphalerite (>400,000 metric tons of zinc) that formed in the Mike gold deposit, located on the Carlin Trend in northeastern Nevada, USA. The action of sulfate-reducing bacteria ca. 16-20 million years ago drove this large-scale formation of supergene sphalerite. The chemical purity of the sphalerite, along with its grain size and spherical shape, all point to a bacteriogenic origin, with stable isotope geochemical data providing additional conclusive evidence. This discovery is the first conclusive example of supergene sphalerite formation, illustrating not only that supergene sphalerite does exist, but also that bacteria can be responsible for large-scale geologic phenomena, as in this example where bacteria concentrated zinc in quantities large enough to be an economically viable natural resource.
Secular variation in seawater chemistry and the origin of calcium chloride basinal brines
Tim Lowenstein, State University of New York, Binghamton, Department of Geological Sciences, P.O. Box 6000, Binghamton, NY 13902, U.S.A., et al. Pages 857-860.
Global seawater has undergone two oscillations in chemical composition during the Phanerozoic Eon, in rhythm with other tectonically driven phenomena such as changes in sea level, seafloor spreading rates, and icehouse/greenhouse climate oscillations. Here we show that the CaCl2 basinal brines stored in the deeper parts of many sedimentary basins originated from evaporated paleoseawaters during geologic periods when the oceans were CaCl2 seas, enriched in Ca2+ and depleted in SO42-. The genetic link between ancient seawater and CaCl2 basinal brines is strengthened by the fact that (1) the world's oceans were CaCl2 seas for ~60% of the Phanerozoic and (2) many basinal brines are close in major ion chemistry to evaporated CaCl2 seawater. CaCl2 basinal brines from the Illinois basin, U.S.A., for example, chemically resemble evaporated CaCl2 seawater in their elevated Ca2+ concentrations and depleted Na+, SO42-, Mg2+ and K+. Processes of dolomitization of limestone and formation of dolomite and K-feldspar cements are required to chemically transform Silurian-Devonian seawater into the present-day basinal brines of the Silurian-Devonian formations in the Illinois basin. These results have implications for the connections in space and time between ancient seawater, basinal brines, dolomitized limestones, hydrocarbon resources, and sedimentary metallic (Pb-Zn) ore deposits.
Pt-Pd reefs in magnetitites of the Stella layered intrusion, South Africa: A world of new exploration opportunities for platinum group elements
Wolfgang Maier, University of Pretoria, Geology, Pretoria, Gauteng 0002, South Africa, RSA 12 4203316, et al. Pages 885-888.
Most of the world's platinum-group-element ores are hosted within the ultramafic-mafic, lower portions of large layered intrusions. The Stella intrusion of South Africa hosts significant reef-type PGE mineralization in magnetitites and magnetite gabbros. Whereas other examples of this type of mineralization have been described from elsewhere, the Stella reefs show for the first time that such mineralization may be economic. This opens up considerable new exploration opportunities for platinum-group elements.
Mid-continental magnetic declination: A 200-year record starting with Lewis and Clark
Robert E. Criss, Department of Earth & Planetary Sciences, Washington University, 1 Brookings Drive, Campus Box 1169, St. Louis, MO 63130-4899.
Thomas Jefferson gave many charges to Lewis and Clark for their journey of discovery two centuries ago, among them to make "with great pains and accuracy" astronomical measurements to determine latitude and longitude along the way and also to note variations in the compass in different locations. In this paper, Dr. Robert Criss of Washington University in St. Louis, Missouri, analyzes the careful observations that the explorers recorded and finds that their sextant and compass readings can be combined to yield the oldest determinations of the magnetic declination in the continental interior of the United States. Magnetic north varies from true north with both time and location, the complex wandering of its declination caused by processes in Earth's deep interior, where the magnetic field is generated. Abundant historic records of the deviations of the compass needle from true north are available from the logs of ships sailing the oceans, but the historic record in the interior of the continents is much sparser. From the Lewis and Clark data, Criss determined that over the past 200 years, the magnetic declination near St. Louis has changed from an azimuth of 7.7 degrees east to 0 degrees today. At Cape Disappointment on the Pacific Coast the declination has changed by only about one degree. The 1803-1806 declinations provide data for the North American interior that could be used to test and improve existing models of the historic variation of Earth's magnetic field. Further, the declinations help in interpreting the travel legs made by Lewis and Clark, because their detailed sketch maps turn out to be oriented to magnetic north rather than to Earth's spin axis.
To review the abstracts for these articles, go to www.gsajournals.org. To review the complete table of contents for the October issue of GEOLOGY, go to http://www.