A topic of hot debate is whether plate tectonic processes operated as far back in time as the Archaean. The July GSA TODAY science article argues for active Archaean plate tectonics based on features in ultramafic blocks contained within the oldest known remnants of oceanic crust.
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 at acairns@geosociety.org for copies of articles and for additional information or other assistance.
GEOLOGY
Eukaryotic stromatolite builders in acid mine drainage: Implications for Precambrian iron formations and oxygenation of the atmosphere?
Sandra Brake et al., Department of Geography, Geology, and Anthropology, Indiana State University, Terre Haute, Indiana 47809, USA. Pages 599-602.
This article documents for the first time the contribution of Euglena mutabilis, an acidophilic, metal-fixing, photosynthetic protozoan, to the formation of iron-rich stromatolites in acid mine drainage systems. E. mutabilis is the dominant microbe in bright green benthic mats, or biofilm, that coats drainage channels at abandoned coal mine sites in Indiana. The biological activity of E. mutabilis impacts the formation of stromatolites in several ways. First, the microbe builds iron-rich layers by moving to the sediment-water interface for dissolved gases and/or for light when covered by iron and aluminum chemical sediments that form during rainfall events in response to pH changes associated with increased hydrologic input. Mats are often trapped in the precipitates and are preserved as patches of decaying green biofilm. Second, E. mutabilis stores iron obtained from acidic water as intracellular iron-rich granules that may be released after death, contributing to the solid iron-rich deposits in the channel and acting as nucleation sites for further precipitation of iron minerals. Third, the oxygen generated by E. mutabilis during photosynthesis facilitates precipitation of reduced iron in the acidic water, with any excess of oxygen not consumed by iron precipitation being released to the atmosphere. These biological activities raise a provocative hypothesis relating processes involved in the formation of iron-rich stromatolites by E. mutabilis to those responsible for development of ancient iron-rich deposits in the Precambrian and the oxygenation of the early atmosphere.
Current strain regime in the Western Alps from continuous GPS measurements, 1996–2001.
Eric Calais et al., Centre National de la Recherche Scientifique, Géosciences Azur, Valbonne, France. Pages 651-654.
Mountain ranges are usually thought of as areas where Earth's crust is under compression, as a result of convergence between two tectonic plates. These compressional stresses produce thickening of the crust, which in turn produces a high topographic relief, and then hold this relief in place. The Western Alps are one of the best-known mountain ranges in the world. Their geological evolution over the past 80 m.y. is clearly related to the African-Eurasia relative motion. However, using high-accuracy Global Positioning System measurements from permanent stations, the authors have found that the central part of the Western Alps is currently undergoing extension. The authors propose a model in which the Africa-Eurasia convergence is not the mechanism currently driving strain in the Alps, but rather a rapid counterclockwise rotation of the Adriatic microplate.
Ediacaran epifaunal tiering.
Matthew Clapham and Guy Narbonne, Department of Geological Sciences, Queen's University, Kingston, Ontario K7L 3N6, Canada. Pages 627-630.
Modern marine communities are tiered, with different organisms feeding at different heights above the sea bottom in order to use food resources most effectively. When did tiering begin? The authors' study of the oldest animal communities known anywhere in the world—565 Ma fossils of the Ediacara biota at Mistaken Point in Newfoundland, Canada—shows that tiering was present at the beginnings of animal evolution. The presence of well-developed tiering suggests that these organisms may have fed by filtering small food particles from the water, similar to well-known fossil and modern deep-water organisms. Although the Ediacaran communities at Mistaken Point have no species in common with younger animal communities, the tiering structure described here is strikingly similar to the vertical structure typical of those animal communities.
Origin of the Mount Pinatubo climactic eruption cloud: Implications for volcanic hazards and atmospheric impacts.
Sébastien Dartevelle et al., Department of Earth and Planetary Sciences, McGill University, 3450 University Street, Montreal, Quebec H3A 2A7, Canada, and BRUEGEL, Université Libre de Bruxelles, CP 160/02, 50 Avenue Roosevelt, 1050 Brussels, Belgium. Pages 663-666.
Explosive volcanic clouds and their products are considered an important threat to the environment (e.g., ozone depletion, forcing of climate, and fluorine ground poisoning), to human activities (e.g., harvest and building destructions, displaced people, and the disruption of airport and aircraft operations), and to human health (e.g., asthma, silicosis, and lung cancer). In large-scale eruptions, ash clouds can be fed by an upward-directed eruption column (i.e., plinian column) or by extensive pyroclastic flows from fountain collapse (i.e., coignimbrite ash cloud), but there is considerable uncertainty about which mechanism is dominant. In this article, the authors focus on the climactic eruption of Mount Pinatubo, Philippines, on 15 June 1991, which was one of the largest eruptions of the twentieth century. Although this eruption has been widely studied, the ash cloud origin remains unclear. Until now, evaluations of atmospheric impacts and health hazards of the Pinatubo ash cloud assumed a plinian eruption column. Here, based on previous field, grain-size, and satellite data and on a new grain-size approach in volcanology, the authors demonstrate that the Pinatubo giant ash cloud was substantially fed from pyroclastic flows and that the resulting widespread coignimbrite ash falls were rich in breathable <10 mm ash (5–25 wt% at most distances from the source volcano), possibly posing greater health risks than previously thought. It is also shown that coignimbrite ash clouds, as at Pinatubo, are expected to be more water-rich than plinian clouds, leading to removal of more SO2 and particularly HCl prior to stratospheric injection, thereby reducing their atmospheric impact.
Large-scale fractures related to inception of the Yellowstone hotspot.
Jonathan Glen, U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA, and Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, California 94709, USA, and David Ponce, U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA. Pages 647-650.
Long, arcuate magnetic features crossing the northern Great Basin are caused by mafic dikes formed in the middle Miocene, coincident with flood-basalt eruptions on the Columbia River Plateau. Together with the trends of dike swarms, faults, topography, and fold axes, they produce a radial pattern converging on a point near the Oregon-Idaho border at lat 44°N. The source of stress responsible for this radial pattern of structures was likely the inception of the Yellowstone hotspot.
Evidence of synchronous climate change across the Northern Hemisphere between the North Atlantic and the northwestern Great Basin, USA.
Mladen Zic et al., Department of Physics and Geology, California State University, Bakersfield, California 93311, USA. Pages 635-638.
From ca. 50 to 20 ka, Summer Lake, Oregon, rose and fell in tune with oscillations between warm and cold temperatures, respectively, in the North Atlantic. The phase relationship of these climate-change records (high Summer Lake during warm North Atlantic, low Summer Lake during cold North Atlantic) is demonstrated at thousand-year resolution by the relative positions of two aborted reversals of Earth's magnetic field recorded in both records. These results, in conjunction with comparisons of historical climate records also presented in this paper, imply a direct temporal connection at the sub-century scale between climate in the North Atlantic and the northwestern Great Basin via atmospheric circulation processes.
Quantified rates of geomorphic change on a modern carbonate tidal flat, Bahamas
Eugene Rankey, Department of Geologic and Atmospheric Sciences and Virtual Reality Applications Center, Iowa State University, Ames, Iowa 50011, USA, and Jim Morgan, Department of Geologic and Atmospheric Sciences, Iowa State University, Ames, Iowa 50011, USA. Pages 583-586.
Most coastlines are currently in a state of change. In this paper, the authors compared historical aerial photographs and ultrahigh-resolution remote sensing imagery to evaluate and quantify changes that occurred on a carbonate tidal flat in the Bahamas between 1943 and 2001. The authors found that tidal channels had extended, the shoreline had eroded, marshes were contracting, and ponds were expanding. All of these changes were interpreted to reflect the tidal flat's response to a relative rise in sea level, although the rates of change are apparently highly variable.
Sublacustrine mud volcanoes and methane seeps caused by dissociation of gas hydrates in Lake Baikal. Pieter Van Rensbergen et al., Renard Centre of Marine Geology, University of Ghent, Krijgslaan 281-S8, B-9000 Gent, Belgium. Pages 631-634.
The article reports the discovery of methane seeps and mud volcanoes at the lake floor of Lake Baikal, Siberia. Lake Baikal is one of the largest and deepest freshwater lakes in the world, and it is the only freshwater lake with a demonstrated occurrence of gas hydrates in the lake floor sediments. Gas hydrates are solids formed of water and gas, commonly methane, caged in a crystal lattice. They occur naturally in Earth's shallow subsurface, within the pores of sedimentary rocks under specific conditions of pressure (high), temperature (low), and gas composition and concentration. As the hydrate structure has very high gas-storage capacity (1 volume of methane hydrate may hold 150–180 volumes of free gas), enormous amounts of methane (CH4) are stored as natural hydrates. The methane seeps and mud volcanoes in Lake Baikal were found in water depths between 1300 m and 1400 m, in an area where the gas hydrates appear to be locally unstable and actively dissociating at 200 m to 400 m below the sediment surface. Hydrate dissociation is probably due to subsurface circulation of thermal fluids. The methane seeps and mud extrusion at the lake floor in Lake Baikal are interpreted as gas outbursts caused by the release of free gas by hydrate dissociation within the subsurface sediments.
Testing the Cretaceous greenhouse hypothesis using glassy foraminiferal calcite from the core of the Turonian tropics on Demerara Rise.
Paul Wilson et al., Southampton Oceanography Centre, School of Ocean and Earth Science, European Way, Southampton SO14 3ZH, UK. Pages 607-610.
Geologists have been worrying about global warming for more than a century. Sediments deposited during the middle of the Cretaceous Period (ca. 100 Ma) contain convincing evidence that Earth was significantly warmer then than it is today. Breadfruit trees grew in Greenland, turtles swam in waters north of the Arctic Circle, and Antarctica was free of the large ice sheets that we see today. Why was Earth so warm during the Cretaceous? What can we learn that might help us today? This article provides new evidence to support the idea that life in the Cretaceous was hot because of high levels of greenhouse gases (such as carbon dioxide) in Earth's atmosphere. Scientists at Southampton Oceanography Centre and Woods Hole Oceanographic Institution report beautifully preserved fossils recovered from the floor of the tropical Atlantic that contain chemical evidence of the warmest surface ocean temperatures ever recorded for the past 150 m.y. of geological time. Crucially, although high past temperatures at the poles can be explained in many different ways, high past temperatures in the tropics are difficult to explain except by high levels of "greenhouse gas forcing." The researchers dated the fossils that they have analyzed very precisely. Interestingly, the dates that they obtained for this interval of peak warmth are significantly younger (by 20–30 m.y.) than the dates that geologists predict for peak levels of past atmospheric carbon dioxide. This mismatch can be interpreted in a number of different ways. One possibility is that the researchers have brought to light a previously "hidden" ancient pulse in tectonic processes that control natural global carbon dioxide emissions to the atmosphere.
GSA TODAY
Archean Podiform Chromitites and Mantle Tectonites in Ophiolitic Mélange, North China Craton: A Record of Early Oceanic Mantle Processes.
Jianghai Li, Department of Geology, Peking University, Beijing 100871 China, Timothy M. Kusky (corresponding author), Department of Earth and Atmospheric Sciences, St. Louis University, St. Louis Missouri 63103, USA, and Xiongnan Huang, Department of Geology, Peking University, Beijing 100871 China.
This paper documents features of an Archean (2.50 billion-year-old) oceanic mantle podiform chromitite and mantle tectonite in ophiolitic mélange in the North China craton. These are the oldest known remnants of oceanic crust associated with mantle tectonites and podiform chromite deposits on the planet. Textures in the ultramafic blocks provide a window into igneous and structural processes active in Archean suboceanic mantle. Ophiolites are remnants of oceanic lithosphere that have been tectonically emplaced onto continents. They provide valuable information on the nature of seafloor processes, global heat loss, and paleogeographic reconstructions of the continents through ancient times. The question of whether ophiolites are present in the earliest rock record (> 2.0 Ga) is one of the most hotly debated scientific questions in early Earth history. Many authors have cited the perceived absence of Archean ophiolites as evidence that plate tectonics did not operate in the Archean. However, these authors report a complete ophiolite with remarkably well preserved delicate magmatic and deformational textures that provide a glimpse into igneous and structural processes active in the suboceanic mantle in the Archean. Additionally, the tectonic setting of the ophiolite and ophiolitic fragments in mélange argues for plate collisions in the Archean. Research on the North China chromitites, harzburgite tectonites, and ophiolitic blocks and sheets, and their relationships to surrounding units and Archean plate tectonic style, is at an early stage. These authors have organized a team of interdisciplinary scientists who will be examining numerous aspects of these unique occurrences of Archean oceanic lithosphere and will be leading two field trips to the region following a GSA Penrose Conference on the Hengshan granulites in September and October 2002. Other meetings and research collaborations are being planned for the future. Researchers interested in the Dongwanzi, Zunhua, and other rocks described here should contact T. Kusky (kusky@eas.slu.edu) or J.H. Li (jhil@pku.edu.cn) for additional information.
To review the abstracts for these articles, go to www.gsajournals.org. To obtain a complimentary copy of any GEOLOGY article, contact Ann Cairns at acairns@geosociety.org. To review 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
Journal
Geology