May GEOLOGY media highlights

04/21/05

Boulder, Colo. - The May issue of GEOLOGY covers a wide variety of potentially newsworthy subjects. Topics include: new insights into Nova Scotia's Joggins fossil forests; a seismic tomography look underneath the British Isles; limits on potential earthquake magnitudes in northwest Los Angeles; earthquake potential of the Lake Tahoe region; analysis of post-impact heating at the Haughton crater in the Canadian high Arctic; newly identified features of the Chesapeake Bay impact crater; and evidence linking euxinic oceans and increased atmospheric methane with the end-Permian mass extinction.

Highlights are provided below. Representatives of the media may obtain complimentary copies of articles by contacting Ann Cairns at acairns@geosociety.org. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOLOGY in articles published. Contact Ann Cairns for additional information or other assistance.

Non-media requests for articles may be directed to GSA Sales and Service, gsaservice@geosociety.org.

Role of evaporite withdrawal in the preservation of a unique coal-bearing succession: Pennsylvanian Joggins Formation, Nova Scotia
John W.F. Waldron, University of Alberta, Department of Earth and Atmospheric Sciences, Edmonton, Alberta, Canada, and Michael C. Rygel, Dalhousie University, Department of Earth Sciences, Halifax, Nova Scotia, Canada. Pages 337-340.

The cliffs at Joggins, on the coast of the Bay of Fundy in Nova Scotia, are famous for fossil trees that are preserved in sandstone and shale layers known as the Joggins Formation. The trees stand upright, and are 5–6 m (16–20 ft) tall. The section in the coastal cliffs is widely regarded as the world's best exposure of Carboniferous-age, coal-bearing strata--distinctions that will feature prominently in the Canadian government's pending application for UNESCO World Heritage Site status. This section profoundly influenced the founders of the science of geology in the 19th century, and is mentioned in Darwin's Origin of Species. The fossil trees, known as lycopsids, grew in tropical wetlands, a little over 300 million years ago. The mud and sand in which they grew accumulated extremely rapidly; the Joggins Formation is about 1.5 km (just under a mile) thick, but was laid down in only about 1.5 million years, suggesting that the area was subsiding rapidly. The new results presented in this paper are based on seismic profiles, recently collected in the course of petroleum exploration for in the Cumberland Basin just east of Joggins. These profiles show that layers of salt were once present deep below the surface, but that the salt was able to flow because of its softness and low density, rising to the surface in structures known as diapirs. The profiles show that the salt was moving while the Joggins Formation was being deposited. The withdrawal of salt led to the subsidence of the low-lying area in which the Joggins trees grew, leading to the preservation of both the coal seams and these famous fossil forests.

Effusive to explosive transition during the 2003 eruption of Stromboli volcano
Maurizio Ripepe, Università di Firenze, Department of Earth Science, Firenze, Italy; et al. Pages 341-344.

Understanding magma dynamics is of primary interest for volcanology and for correct risk assessments, especially on persistent active volcanoes. Stromboli volcano, Italy, is extremely well-known for its persistent and moderate explosive activity. This activity was interrupted in December 2002 by a rare effusive eruption, which also triggered a tsunami. During a seven-month-long eruption, the typical explosive activity of Stromboli ceased, causing a high level of concern. Extensive real-time monitoring using seismology, infrasonic acoustics, ground-based thermometry, satellite remote sensing, and gas chemistry have revealed the dynamics of this eruption. The synergy of this integrated data set has allowed scientists to track the migration of magma in the conduit and moves researchers a significant step closer to the ultimate aim of volcanology: to confidently predict the behavior of an ongoing volcanic eruption.

Seismic imaging of a hot upwelling beneath the British Isles
Stephen J. Arrowsmith, Scripps Institution of Oceanography, San Diego, California 92093-0225, USA; et al. Pages 345-348.

This article presents new pictures of inside Earth, to a depth of around 400 km beneath the British Isles. The images were generated using a technique called seismic tomography, which images Earth in much the same way CAT scans can image inside the human body. The results show evidence for a hot upwelling mantle that correlates geographically with a region that has experienced a history of vertical motions of the crust. The upwelling mantle may be the cause of the majority of earthquakes in the British Isles.

Fire and the evolution of steep, soil-mantled landscapes
Joshua J. Roering and Molly Gerber, University of Oregon, Geological Sciences, Eugene, OR 97403-1272, USA. Pages 349-352.

Erosion associated with recent fires in the western United States highlights how climatic patterns affect soil sustainability and sediment delivery to channels. On steep, soil-mantled landscapes of the Oregon Coast Range, widespread erosion via dry ravel (whereby the incineration of vegetation induces particles to bounce, slide, or roll into adjacent channels) occurs within hours of fires. The magnitude of post-fire soil erosion, which occurs in the dry season without the aid of rainfall, is equivalent to decades of erosion associated with background or non-fire processes. At several sites, Roering and Gerber witnessed complete removal of the soil mantle, exposing underlying bedrock and profoundly altering the ecological regime. They used a physically based model and high-resolution topographic data generated via airborne laser swath mapping to simulate how topographic variations affect the pattern of hillslope erosion, enabling them to predict areas susceptible to rapid erosion and potential soil loss. Paleoclimate records in the study area suggest that fires were more frequent in the early Holocene (~9,000 years ago), such that the processes they describe here would have occurred more frequently. Fire-driven increases in erosion may explain valley aggradation observed throughout the region during that time. Given current climate change and predictions of increased fire frequency in the Pacific Northwest, their results suggest that the maintenance of soil in steeplands may become tenuous.

Cooling of the lower oceanic crust
John MacLennan, University of Edinburgh, School of GeoSciences, Grant Institute, Edinburgh, Scotland EH9 3JW, UK; et al. Pages 357-360.

Earth's oceans are floored by oceanic crust, which forms at chains of undersea volcanoes known as mid-ocean ridges. Molten rock is supplied from the deep Earth to these ridges, where it cools and solidifies to form the crust. The hot rock is able to heat seawater that is circulating through the ridges and drives hydrothermal activity. These hydrothermal fluids transfer chemicals from Earth's interior to the oceans and may have an important role in controlling the chemical composition of seawater. Results of MacLennan et al.'s new models show that seawater must circulate surprisingly deep in the crust at ridges, perhaps up to 7 km beneath the seafloor. Each pound of crustal rock is cooled by transferring its heat to an equal weight of circulating seawater.

60 k.y. record of extension across the western boundary of the Basin and Range province: Estimate of slip rates from offset shoreline terraces and a catastrophic slide beneath Lake Tahoe
G.M. Kent, Scripps Institution of Oceanography, IGPP/SIO, UCSD, San Diego, CA 92093, USA; et al. Pages 365-368.

For years, it was difficult for scientists and public officials to assess the seismic hazard of the Lake Tahoe region due to the deep cobalt waters that overlie several basin-forming faults. To study the activity of the faults that lie beneath some 500 meters of water, a novel combination of geophysical techniques, including high-resolution seismic chirp imagery, airborne laser and acoustic-multibeam bathymetry, and age-dated sediment cores, have enabled the first slip-rate estimates for several of the normal faults in the region. By melding these emergent technologies and data sources, Scripps Institution of Oceanography's Graham Kent and his colleagues have developed a 60,000-year record of fault slip beneath the lake. Using these new measurements and subsequent analyses, Kent and his colleagues calculated a potential recurrence interval for a large M7 earthquake at or near 3,000 years. Such an event would carry the potential for significant damage in the Lake Tahoe region, particularly through tsunami waves, up to 10 meters in height, that would emerge with little or no warning and slosh back and forth across the lake for an extended period of time. On-going and future work involves attempting to catalogue individual ruptures over the past 10 or 20 thousand years to enable an assessment of where each fault lies within its cycle of strain accumulation and rupture.

Thermal alteration of organic matter in an impact crater and the duration of postimpact heating
John Parnell, University of Aberdeen, Department of Geology and Petroleum Geology, Kings College, Aberdeen, Grampian, Scotland AB24 3UE, UK; et al. Pages 373-376.

The Haughton meteorite impact structure in the Canadian High Arctic is exceptionally well preserved and exposed. This allows detailed sampling across the crater to determine the degree to which the rocks were heated by the impact. By looking at the distribution of temperature-sensitive organic compounds, researchers can estimate the time over which heating occurred, which turns out to be about 5000 years for the 24 km-diameter crater. This geologically short time scale suggests that in comparative craters on Mars, heating would not be so extensive that evidence for organic remains would be destroyed.

Links between the onset of modern Walker circulation and the mid-Pleistocene climate transition
Erin L. McClymont, University of Bristol, School of Chemistry, Bristol, Avon BS8 1TS, UK; and Antoni Rosell-Melé, Universitat Autònona de Barcelona, ICREA and Institut de Ciència i Tecnologia Ambientals, Bellaterra, Catalonia 08193, Spain. Pages 389-392.

New evidence shows that the strength of the tropical Pacific Ocean–atmosphere circulation system (the Walker circulation) reached its modern intensity by 0.9 million years ago. This occurred as part of the mid-Pleistocene climate transition, when the northern hemisphere ice-sheets expanded and the climate system shifted toward longer (100 k.y.) and more intense glaciations. The authors use the El Niño–Southern Oscillation phenomenon, which dominates the modern tropical Pacific, as an analogue for the longer-term trends identified during the mid-Pleistocene. They propose that the strengthening of the Walker circulation played a key role in driving the Northern Hemisphere ice-sheet expansion, by altering the transfer of heat and moisture to the high-latitudes. Significantly, the data reveal the potential for tropical climate change to drive and/or modulate global climate change.

Massive release of hydrogen sulfide to the surface ocean and atmosphere during intervals of oceanic anoxia
Lee R. Kump, Pennsylvania State University, Department of Geosciences, University Park, PA 16802, USA; et al. Pages 397-400.

Kump et al. argue that a likely consequence of Siberian volcanism and the attendant global warming was an oceanic anoxia so severe that the world's oceans became euxinic (rich in hydrogen sulfide) in the latest Permian. Toxic hydrogen sulfide permeated the ocean's surface layer and escaped to the atmosphere, destroying the ozone layer and permitting a rapid increase in the atmospheric methane level. These changes, together with sulfide toxicity itself, provide an attractive mechanism for Late Permian mass extinction. Moreover, persistent euxinia in the Proterozoic may have forestalled the development and spread of metazoan life in surface waters and on land.

Paleomagnetism-based limits on earthquake magnitudes in northwestern metropolitan Los Angeles, California, USA
Shaul Levi, Oregon State University, College of Oceanic and Atmospheric Sciences, Corvallis, OR 97331, USA; et al. Pages 401-404.

Recent studies suggest that prehistoric earthquakes in the San Gabriel Valley of metropolitan Los Angeles and the Piru area of eastern Ventura County might have released up to ten times more energy than the 1971 and 1994 earthquakes in the San Fernando Valley. Shaul Levi and his colleagues at Oregon State University tested this idea in the region affected by the 1971 San Fernando and 1994 Northridge earthquakes and concluded that this area is not likely to suffer earthquakes larger than those experienced in 1971 and 1994. These conclusions are based on the orientation of magnetic minerals in sediments, which recorded the direction of Earth's magnetic field when the sediments were originally deposited. The measured magnetic orientations of the sediments indicate that the crust underlying the San Fernando and Santa Clarita areas is fragmented into four independent blocks. Two tectonic blocks are rotated clockwise, and two are unrotated. The limits on earthquake magnitude are based on the small size of the blocks in this area.

Late Ordovician global warming--The Boda event
Richard A. Fortey and L. Robin M. Cocks, The Natural History Museum–London, Palaeontology, London, SW7 5BD, UK. Pages 405-408.

Global warming is not a new phenomenon. New evidence suggests that near the end of the Ordovician period, about 445 million years ago, Earth entered a brief period of exceptional global warmth, which encouraged formerly tropical animals to move toward the poles, where limestones were deposited for the first and last time for millions of years. This warm phase was followed shortly by a massive glaciation--one that contributed toward one of the great mass extinction events in the history of the biosphere.

Low-latitude glaciation in the Neoproterozoic of Oman
Ben Kilner, University of Oxford, Earth Sciences, Parks Road, Oxford, Oxfordshire OX1 3PR, UK; et al. Pages 413-416.

The Late Precambrian marks a time of rapid climatic fluctuations, with the intimate, and widespread, juxtaposition of cold-water and warm-water deposits leading to the proposal that Earth experienced a number of runaway icehouse to greenhouse episodes at this time. This has lead to the so-called "Snowball Earth" theory, which argues for a frozen ocean and dramatic global cooling during the Late Precambrian. Kilner et al. present new palaeomagnetic data from Oman, Arabia, that reveal, for the first time, that both glacial and the overlying warm-water carbonates were deposited at low-latitudes, within the tropics. This rules out the possibility that plate motions are responsible for the close association of these contrasting deposits. The presence of glacial/interglacial sedimentary cycles within the glacial units, however, indicates that areas close to the equator did not experience continuous ice-cover, a result inconsistent with the classic Snowball Earth model.

New surveys of the Chesapeake Bay impact structure suggest melt pockets and target-structure effect
Anjana K. Shah, Naval Research Laboratory, Marine Physics, Washington, DC 20375, USA; et al. Pages 417-420.

Shah et al. conducted shipboard and land-based geophysical surveys over Chesapeake Bay and the Delmarva Peninsula to learn more about an ~45-mile wide buried impact structure located there. Maps of Earth's magnetic field suggest that the impact, which occurred ~35 million years ago, created up to ~10 cubic kilometers of melt that ponded near the structure center beneath the town of Cape Charles, Virginia. This volume is several thousand times that of the Great Pyramid at Giza, but is actually very small when compared to the amount of melt observed at other impact structures of comparable size. The low amount of melt observed may be due to the fact that the area was a shallow water environment at the time of impact. Such an environment could respond with an explosive release of water and carbon dioxide gasses as the bolide struck, sending melt into widely scattered directions and leaving little coherent evidence of its presence. From the gravity field, Shah et al. observe that the shape of several of the structure features is oval rather than circular. These features are aligned with older, larger-scale geologic units. The shape of an impact structure can be influenced by variations in target rock strength, or by the angle of impact. The relative contributions of each of these factors has yet to be established, but upcoming deep drilling efforts by the International Continental Drilling Program may shed light on these questions.

Physical mingling of magma and complex eruption dynamics in the shallow conduit at Stromboli volcano, Italy
Nicole C. Lautze and Bruce F. Houghton, University of Hawaii, Geology and Geophysics, Honolulu, HI 96822, USA. Pages 425-428.

Strombolian eruptions are relatively low intensity, single explosions each representing the bursting of a slug of large gas bubbles that rise rapidly through the volcanic conduit and break through at the magma's free surface. Until now, the magma residing in the uppermost part of the conduit was thought to be more or less stationary and stagnant. This study investigates the physical properties of this shallow magma via a detailed analysis of vesicularity textures in fragments that were ejected in 2002 at Stromboli volcano, Italy, the archetype location for this style of volcanism. These show clear evidence that the majority of the erupted material is the product of a late-stage, dynamic mingling of magmas with variable residence time in the shallow conduit. Lautze and Houghton propose a model in which actively vesiculating magma rises with the gas slug and mingles with more mature, stagnant magma en route to the surface. They suggest that the shallow conduit at Stromboli is a dynamic environment, and that the complex rheology of the mingled magma feeds back to strongly influence eruption dynamics.

Source: Eurekalert & others

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