||June 27, 2003
GSA Release No. 03-17
Boulder, Colo. — The Geological Society of America's July issue of GEOLOGY contains several potentially newsworthy items. Topics include: new evidence from Brazil in support of the Snowball Earth hypothesis; large cold-based water glaciers on Mars that may hold the record of ancient Martian climate data; global changes during the Carboniferous-Permian glaciation that argue for global climate control mechanisms over local forcing factors; and the relationship of iron seeding of the oceans to carbon dioxide depletion and glaciation. The GSA TODAY science article, a geology and astrophysics collaborative effort, addresses a possible link between Earth's climate and celestial processes such as solar activity, supernovas, and spiral galaxies.
Highlights are provided below. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOLOGY in stories published. Contact Ann Cairns for copies of articles and for additional information or other assistance.
- Soft-sediment deformation at the base of the Neoproterozoic Puga cap carbonate (southwestern Amazon craton, Brazil): Confirmation of rapid icehouse to greenhouse transition in snowball Earth
Afonso Cesar Rodrigues Nogueira, Departamento de Geociencias, Universidade Federal do Amazonas, Av. Gal. Rodrigo O. J. Ramos 3000, Manaus, AM 69.077-000, Brazil, and Programa de Pos-Graduacao em Geologia Sedimentar, Instituto de Geociencias, Universidade de Sao Paulo, Rua do Lago, 562, Sao Paulo, SP 05508-080, Brazil; et al. Pages 613-616.
- Relatively deep-water pinkish carbonates of the Araras Group lie directly upon marine glacial deposits of the Puga Formation on the southeastern margin of the Amazon Craton, Brazil. These have been interpreted as a late Neoproterozoic cap carbonate, the first to be documented in detail in South America. Much like other similarly aged caps worldwide, the Puga cap exhibits a strong negative carbon isotopic signature as well as unusual structures (tepee-like features and tube rock) and anomalous facies (microbialites and, higher in the section, crystal fans and cement-rich limestone). But the most striking discovery associated with the cap is the soft-sediment deformation of its contact with the underlying glaciogenic rocks. Hence, both sediments must have been largely unconsolidated at the time of deformation. This then is highly suggestive evidence of the abrupt transition from extremely cold (icehouse) to extremely warm (greenhouse) conditions postulated by the snowball Earth model for Neoproterozoic glaciation. The deformation is interpreted as the result of seismic shaking induced by glacial unloading.
- Cold-based mountain glaciers on Mars: Western Arsia Mons
James W. Head, Department of Geological Sciences, Brown University, Providence, Rhode Island 02912, USA, and David R. Marchant, Department of Earth Sciences, Boston University, Boston, Massachusetts 02215, USA. Pages 641-644.
- Surface environmental conditions on Mars are presently extremely cold and hyper-arid, most equivalent to polar deserts on Earth. In the equatorial regions of Mars, solar energy is sufficient to cause surface water ice to sublime, while it can accumulate in the polar regions. Coupling newly acquired Mars Global Surveyor data with field-based observations regarding the flow, surface morphology, and depositional history of polar glaciers in Antarctica, this paper presents evidence that large cold-based mountain glaciers existed on the flanks of the large Tharsis Montes equatorial volcanoes in the recent geological past. For example, the multiple facies of an extensive fan-shaped deposit on the western flanks of Arsia Mons, Tharsis Rise are consistent with deposition from cold-based mountain glaciers. An outer ridged facies is interpreted as drop moraines formed at the margin of an ablating and predominantly receding cold-based glacier. Inward of the ridges lies a knobby facies interpreted as a sublimation till derived from in situ downwasting of ash-rich glacier ice. A third facies likely represents rock-glacier deposits, some of which may still be underlain by a core of glacier ice. Taken together, these surficial deposits show that the western flank of Arsia Mons was occupied by an extensive mountain glacial system accumulating on, and emerging from, the upper slopes of the volcano and spreading downslope to form a piedmont-like fan. These deposits may still contain glacial ice below the surface and thus hold the record of ancient climatic conditions on Mars. Such deposits may provide water resources for future human explorers.
- Waxing and waning volcanism along the East Pacific Rise on the millennium time scale
Marie-Helene Cormier, Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York 10964, USA, et al. Pages 633-636.
- Most of the volcanism on Earth occurs hidden from sight along the mid-ocean ridges at 2000-4000 m water depth. The southern East Pacific Rise is the fastest-spreading mid-ocean ridge and frequent eruption is expected along its length. To gain a better understanding of its volcanic activity, the autonomous underwater vehicle ABE was deployed as a nighttime program to a submersible expedition. ABE is a robot capable of surveying along pre-programmed tracks only meters from the seafloor, yielding continuous bathymetric coverage. The micro-bathymetry precisely outlines field relations between subtle volcanic features. In one area, it reveals a system of elongated drained lava lakes and their associated networks of lava channels and lava tubes. In another area, the seafloor subsided and formed a broad tectonized trough, with late-stage volcanism limited to small volcanic mounds. This new type of data makes it possible to carry out underwater volcanology at the same resolution as on land.
- Archean zircons from the mantle: The Jormua ophiolite revisited
Petri Peltonen, Geological Survey of Finland, PO Box 96, FIN-02151 Espoo, Finland, et al. Pages 645-648.
- This paper reports the discovery of Archean (up to 3100 million years old) mineral grains in ancient fragments of seafloor exposed in eastern Finland. The dated mineral grains are called zircon, which contains a small amount of uranium. This uranium slowly decays into lead and measurement of the isotopic ratios enables scientists to determine ages for such grains. What makes these analyzed grains so unique is that they are derived directly from Earth's mantle and not from the crust. In the mantle they probably occur in veins, and their presence implies that Earth's mantle had already gone through a complex history 3 billion years ago. Such mantle evolution also suggests that thick continental crust-similar to that in modern Earth-existed already at that time. This finding is of great interest to those earth scientists who study how and when first continents grow and break-up in the earliest history of Earth.
- New gas-hydrate phase: Synthesis and stability of clay-methane hydrate intercalate
Stephen Guggenheim and August F. Koster van Groos, Department of Earth and Environmental Sciences, University of Illinois at Chicago, Chicago, Illinois 60607, USA. Pages 653-656.
- Methane hydrate belongs to a family of minerals in which water molecules form a cage around a guest (gas) molecule, such as methane, propane, argon, or carbon dioxide. These minerals are stable near Earth's surface at low to sub-zero temperatures and elevated pressures. Although these types of compounds have been recognized for almost two centuries, it was established only recently that methane hydrates are common in ocean-floor sediments associated with the continental shelf. Methane hydrates may represent the main methane repository on Earth. They are being studied intensely because of the possibility that (1) the methane repository may be an economic resource, (2) sudden methane release from these hydrates may be involved in short-duration high-temperature excursions of the climate, and (3) methane hydrates are geologic hazards. In our research, we discovered that methane can become incorporated within a smectite clay. These clays are a common mineral group present on Earth's surface and ocean floor. We report on the synthesis and the stability relations of this new methane hydrate-clay phase, where methane hydrate complexes occur between the clay-silicate layers. Our study shows that the stability of this new methane-bearing phase is very similar to that of methane hydrate sensu stricto. Our next phase of research is to search for a natural occurrence of the clay-methane hydrate phase synthesized here. We conclude that at conditions similar to where methane hydrate is stable, clay (smectite) may provide an additional sink for methane.
- Increase of human over natural erosion rates in tropical highlands constrained by cosmogenic nuclides
Tilak Hewawasam, Isotope Geology, University of Berne, Erlachstrasse 9a, 3012 Berne, Switzerland, et al. Pages 597-600.
- Tropical highlands conserve their soil layers by developing thick forest covers. If this forest is removed, soil erosion is inevitable, together with negative side effects such as loss of nutrients and silting in reservoirs. This is exactly what has happened in the tropical highlands of Sri Lanka, an island in the Indian Ocean. Hills have been deforested during colonization for coffee and tea plantations. Today, landslides and gullies scar the Sri Lankan landscape, but the actual degree of the damage remained unquantified. A team led by T. Hewawasam and F.von Blanckenburg of the Universities of Berne, Switzerland, and Hannover, Germany have now chosen rare cosmogenic nuclides, measured in a handful of river sediment, to quantify the natural, pre-development rates of erosion and have compared these to today's erosion rates. Erosion of these tropical highlands has increased up to 100 times! This means that soil is now being lost up to 100 times faster from agriculturally-utilized areas than it is being produced by weathering of rocks.
- Fate of the subducted Farallon plate inferred from eclogite xenoliths in the Colorado Plateau
Tomohiro Usui, Pheasant Memorial Laboratory for Geochemistry and Cosmochemistry, Institute for Study of the Earth's Interior, Okayama University at Misasa, Tottori 682-0193, Japan, et al. Pages 589-592.
- Based on detailed mineralogical, chemical, and chronological observations, we demonstrate that eclogite xenoliths from the Colorado Plateau represent fragments of the subducted Farallon Plate entrained in the upper mantle since the Late Cretaceous. This is the first conclusive evidence that any eclogite xenoliths can be directly linked to a known subducted plate. Our study also includes the first report of the high-pressure mineral assemblage coesite + lawsonite in natural eclogites, which has been forecasted by recent experimental studies. Our study provides physical and chemical constraints on the subducted oceanic lithosphere and, thus, can provide enormous insight to any field of earth science in terms of the formation of island-arc magma, material recycling between the crust and mantle, and the geometry of subducted oceanic lithosphere in the mantle.
- Iron isotope constraints on Fe cycling and mass balance in oxygenated Earth oceans
Brian L. Beard, Department of Geology and Geophysics, University of Wisconsin, 1215 West Dayton Street, Madison, Wisconsin 53706, USA, et al. Pages 629-633.
- Dissolved Fe is the limiting nutrient in parts of the world's oceans. Past climatic conditions may have been controlled by seawater-iron contents by a mechanism in which high Fe contents promote increases in biologic activity. The increase in biologic activity causes atmospheric carbon dioxide depletion, which leads to global cooling. Iron isotope investigations may be able to determine if iron seeding in the oceans by delivery of atmospheric particulate matter was a mechanism that promoted carbon dioxide depletion and glaciation. Iron sources delivered to the world's oceans have different Fe isotope compositions, and it is possible to identify the relative mixture of these sources by analysis of Fe seawater proxies such as Fe-Mn crusts that grow by the slow accumulation of Fe and Mn oxides from seawater. Glaciation events are predicted to increase the Fe isotope composition of seawater because the relative flux of atmospheric particulate matter that has a high 56Fe/54Fe isotope ratio relative to the other major hydrothermal Fe flux that has a low 56Fe/54Fe ratio.
- Subsurface combustion in Mali: Refutation of the active volcanism hypothesis in West Africa
Henrik Svensen, Physics of Geological Processes, University of Oslo, PO Box 1048 Blindern, 0316 Oslo, Norway, et al. Pages 581-584.
- Surface heat anomalies have been known from the Timbuktu region (Mali) in West Africa for more than a century. Since the 1960s, several authors have argued that these heat anomalies are caused by incipient volcanic and hydrothermal activity. This is surprising, as West Africa is regarded as geologically stable. Surface temperatures as high as 765 °C were measured during fieldwork in January 2002, and smoke emanated from holes and fractures in the ground. When trying to determine the cause of the extreme heat, a trench dug into the hot ground revealed a combusting peat layer. We suggest that the phenomena previously attributed to volcanism in the region are caused by subsurface combustion of peat. Several square kilometer-large areas are currently combusting, or have been affected by subsurface fires since 2001. We propose that the subsurface combustion started as a result of self-ignition, and that these fires have a long history in the sub-Sahara region.
- Global changes during Carboniferous-Permian glaciation of Gondwana: Linking polar and equatorial climate evolution by geochemical proxies
K. Scheffler, Mineralogical-Petrological Institute, Bonn University, Poppelsdorfer Schloss, 53115 Bonn, Germany, et al. Pages 605-608.
- During the time interval from about 310 to 290 million years ago, when all continents were agglomerated in a supercontinent called Pangaea, a very long and pronounced ice age occurred. In the Southern Hemisphere, land areas were positioned close to the southern pole, which favored large ice accumulation on the continent. Previous studies were able to show the effects of ice accumulation by investigating the isotopic composition of carbonates. Occurrence of these carbonates is restricted to warm waters regions of the equatorial belt. A connection between these sediment deposits and those of the polar region was previously hampered by lack of precisely dated sediments. In our study we used well-dated sediment sequences from the south African Karoo basin for analysis. Geochemical methods show that the shift between glacial phases, identified by sedimentological and geochemical means and interglacial phases, is accompanied by a pronounced change in the isotopic signature of non-carbonate organic matter. The fluctuations in organic matter isotopes are in parallel with those determined on equatorial carbonate. This argues against local forcing factors but indicates the existence of a global climate-control mechanism.
- Celestial Driver of Phanerozoic Climate?
Nir J. Shaviv, Racah Institute of Physics, Hebrew University of Jerusalem, Jerusalem, 91904, Israel, and Ján Veizer, Institut für Geologie, Mineralogie und Geophysik, Ruhr Universitšt, 44780 Bochum, Germany, and Ottawa-Carleton Geoscience Centre, University of Ottawa, Ottawa, ON K1N 6N5, Canada.
- Is there a link between the climate on planet Earth and celestial processes associated with solar activity, supernovas ands spiral galaxies? In a uniquely cross-disciplinary approach to this problem, Nir Shaviv, an astrophysicist at Racah Institute of Physics (Hebrew University, Israel) and Jan Veizer a geochemist at the University of Ottawa (Canada) and Ruhr University (Germany), have collaborated to address this question from the perspective of astrophysics and geology. In this paper, the authors examined the periodicity in the Earth's climate over the past 600 m.y. that is apparent from the analysis of isotopes of oxygen in fossil material. This was compared to the periodicity predicted in the variation in the flux of cosmic rays (CRF) reaching the Earth (and observed in the CRF recorded in iron meteorites) as a function of the periodic passage of our solar system through the spiral arms of the Milky Way. Cosmic rays are interpreted to influence cloud formation on our planet and hence affect the planetary albedo. The authors demonstrate a tantalizing correlation between celestial and geological processes and add to the proposition that celestial processes may be an important, perhaps even dominant influence, on climate change. If this is the case, then the role of carbon dioxide becomes even more critical to understand as it may well amplify the signals forced by celestial processes. This is one of a number of new studies that are shaking up our traditional understanding of the link between climate and atmospheric gases.
To review the abstracts for these articles, go to www.gsajournals.org.
To review the complete table of contents for the current issue of GEOLOGY, go to www.gsajournals.org/gsaonline/?request=get-current-toc&issn=0091-7613.
Representatives of the media may obtain a complimentary copy of any GEOLOGY article by contacting Ann Cairns.
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