May GEOLOGY media highlights
Boulder, Colo. – The May issue of GEOLOGY covers a wide variety of potentially newsworthy subjects. Topics include: discovery in the U.K. of Earth's earliest recorded wildfire; new perspectives on high levels of atmospheric oxygen in the late Paleozoic; new paleoclimatological and fossil record evidence documenting migration of snakehead ("killer") fish; insights from a study of 300 million-year-old rocks in Newfoundland regarding the emergence of upland forests; and dating of Neoproterozoic glaciations in south China.
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Massive collapse of volcano edifices triggered by hydrothermal pressurization
Mark Reid, U.S. Geological Survey, Menlo Park, CA 94025, U.S.A. Pages 373–376.
Catastrophic collapse of steep volcano flanks threatens lives at stratovolcanoes around the world. Collapses that occur without the eruption of magma can be difficult to anticipate; however, they are associated with shallow hydrothermal groundwater systems. This paper uses numerical models to evaluate hydrothermally driven volcano collapse. Heating from magma at depth can generate elevated groundwater pressures that propagate upward through the edifice. Stress models show that these pressures are capable of destabilizing the core of an edifice, resulting in massive, deep-seated collapse. Moreover, data from numerous hydrothermal systems illustrate that this process can transpire in realistic settings.
Charcoal in the Silurian as evidence for the earliest wildfire
Ian Glasspool, et al., Cardiff University, School of Earth Ocean and Planetary Sciences, Cardiff, United Kingdom. Pages 381-383.
Rare early land plant fossils with exceptional three-dimensional cellular preservation were isolated from Late Silurian sediments in the Welsh Borders of England. These fossils, which are preserved as charcoal, are evidence for the earliest recorded wildfire. The charring process that occurred ca. 419 million years ago rendered these small delicate plants chemically inert and physically resistant to compression. As such, once incorporated into the sediment, these fossils were able to survive the burial process and retained much of their cellular detail intact. The exceptional preservation of these charred fossils makes them key to our understanding of early land plant communities.
Migration history of air-breathing fishes reveals Neogene atmospheric circulation patterns
Madelaine Boehme, University Munich, Section Paleontology, Department for Geo- and Environmental Science, Munich, Germany D-80333, Germany. Pages 393-396.
"Killer fish," "Frankenfish," or "pit bull with fins" are terms that are given to snakehead fishes (Channidae) by the public, because of the predatory lifestyle and the high migration potential of this air-breeding fish. In summer 2002, snakeheads became a media superstar in the United States. Treated as an invasive, non-indigenous fish group in North America, it was feared that they seriously harm native ecosystems. An unexpected contribution to paleoclimatology gives the fossil record of snakeheads, reaching back to 50 Ma. The study shows that snakeheads are sensitive indicators of summer precipitation maxima in subtropical and temperate regions and occur regularly if the wettest month exceeds 150 mm precipitation and 20 °C mean temperature. The analysis of 515 fossil freshwater fish deposits of the past 50 m.y. from Africa and Eurasia shows two continental-scale migration events from the snakeheads' center of origin in the south Himalayan region, which can be related to changes in the Northern Hemisphere circulation pattern. The first migration at ca. 17.5 Ma into western and central Eurasia may have been caused by a northward shift of the Intertropical Convergence Zone that brought western Eurasia under the influence of trade winds that produced a zonal and meridional precipitation gradient in Europe. During the second migration, between 8 and 4 Ma into Africa and East Asia, snakeheads reached their present-day distribution. This migration could have been related to the intensification of the Asian monsoon that brought summer precipitation to their migratory pathways in East Africa–Arabia and East Asia.
Exceptionally fast growth rate of <100-year-old tufa, Big Soda Lake, Nevada: implications for using tufa as a paleoclimate proxy
Michael Rosen, US Geological Survey, Water Resources, Carson City, Nevada 89706, U.S.A., et al. Pages 409-412.
Large tufa mounds, chemical sedimentary rocks composed of calcium carbonate formed in lakes and rivers, that are greater than 3 m tall, with a basal circumference of 5 m, have been discovered on the margin of Big Soda Lake, Nevada, USA. These tufa mounds are rooted at a maximum of 4 m below the current lake surface and are actively forming from groundwater seepage, which can be seen emanating from the top of the tufa mounds. Big Soda Lake is a volcanic crater lake whose water level is maintained exclusively by groundwater. The age of the tufa mounds is well constrained because prior to the development of the Newlands Irrigation Project in 1907, the water level was ~18 m lower than the current lake level. Thus, the tufa mounds are <100 yr old and have grown at a rate of ~30 mm/yr. The fluids responsible for their precipitation are a simple mixture of modern groundwater and lake water and do not reflect a recent climate signature. The exceptionally fast growth of the tufa mounds indicates that large tufa deposits may form almost instantaneously in geologic time. Given this potential for rapid growth and the fact that tufa deposits have been interpreted in terms of changes in paleoclimate over thousands of years, care should be taken when trying to determine the significance of variations in the composition of tufas that may have been caused solely by mixing with groundwater.
Pennsylvanian uplands were forested by giant cordaitalean trees
Howard Falcon-Lang, University of Bristol, Department of Earth Sciences, Bristol, Avon U.K., and Arden Bashforth, Memorial University, Earth Sciences, St Johns, Newfoundland, Canada. Pages 417-420.
When did mountains first become covered by forests? This is an important question because upland vegetation influences rates of silicate weathering, a process that removes carbon dioxide from the atmosphere, and drives climate change. This paper answers this question by describing the earliest known upland forests from 300 m.y. old rocks in Newfoundland, Canada.
Gigantic SO2 emission from Miyakejima volcano, Japan, caused by caldera collapse
Kohei Kazahaya, National Institute of Advanced Industrial Science and Technology, Geological Survey of Japan, Ibaraki, Japan, et al. Pages 425-428.
Extremely large amounts of volcanic sulfur dioxide have been emitting from Miyakejima volcano, Japan, since mid-2000, after a silent caldera formation. The peak emission rated twice the global volcanic sulfur flux, which is very important not only as a peculiar volcanological phenomenon, but also as impact on atmospheric environment. Understanding the mechanism of this phenomenon will contribute evaluations of future degassing activity and magmatic processes in the crust.
New constraints on the age of the Neoproterozoic Nantuo glaciation in South China
Shuhai Xiao, Virginia Polytechnic Institute, Department of Geological Sciences, Blacksburg, VA, U.S.A., et al. Pages 437-440.
Earth experienced several global glaciations (or "snowball" Earth events) in the Neoproterozoic, ~750–580 million years ago. However, the exact age and duration of these glaciations are uncertain. Geological record indicates that at least two Neoproterozoic glaciations occurred in South China, with the Nantuo glaciation being the youngest and most severe. Stable carbon isotope patterns of the cap carbonate that immediately overlies the Nantuo glacial deposit suggest that it may be equivalent to the Neoproterozoic Marinoan glaciation in Australia. Radiometric dates from above and below the Nantuo glacial deposit suggest that this glaciation started after 663 ±4 Ma and ended before 599 ±4 Ma. Thus, one of the "snowball" Earth events must have occurred within this 64 m.y. window.
Bypassing and diachronous deposition from density currents: Evidence from a giant regressive bedform in the Poris ignimbrite, Tenerife, Canary Islands
Richard Brown and Michael Branney, Geology Department, University of Leicester, University Road, Leicester, Leicestershire, U.K. Pages 445-448.
Exceptional exposure through an ignimbrite in Tenerife, Canary Islands, reveals a complex internal regressive structure and indicates that the parent pyroclastic density current experienced prolonged periods of patchy deposition and bypassing during which it conveyed most of its pyroclastic load out to the Atlantic Ocean (over 20 km from source). The recognition of such behavior is critically important in understanding volcanic hazard and in estimating eruption durations. However, the evidence may go unrecorded in other less well-exposed density-current deposits.
Burning of forest materials under late Paleozoic high atmospheric oxygen levels
Richard Wildman, California Institute of Technology, Geological and Planetary Sciences, Pasadena, CA, U.S.A., et al. Pages 457-460.
Definitive results on the history of oxygen as a component of Earth's atmosphere are hard to find. One notion that has been widely accepted is that O2 levels have not gone much above the present value of 21%, and have certainly not exceeded 25%. This notion rests on the argument that O2 levels above 25% would produce a fire catastrophe for the land biota and there is no sign of such a catastrophe in the fossil record. The 25% figure is justified by a series of experiments in which paper was used as a fuel. This article presents results from burning various plant materials under elevated O2. They show that O2's effect on fire is quite variable--depending on the fuel type and moisture content--and suggests that there is nothing absolute about a value of 25%. Rather, the results support the notion that plant life could have survived fires in an atmosphere with 35% oxygen, the maximum value deduced in theoretical geochemical models. The paper also notes that both the sedimentary deposition and various biological oddities of the Late Paleozoic are consistent with an atmosphere that is far richer in oxygen than the present atmosphere.
To view the complete table of contents for the May issue of GEOLOGY, go to http://www.gsajournals.org/gsaonline/?request=get-current-toc&issn=0091-7613.
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