Following are highlights from the October issue of GEOLOGY and a summary of the science article from the October issue of GSA TODAY, published by the Geological Society of America. Stories written regarding these articles are embargoed until October 1. We ask that you discuss articles of interest with the authors before publishing stories on their work, and that reference be made to GEOLOGY or GSA TODAY in stories published. Please contact Ann Cairns at GSA to request advance copies of articles and for additional information or assistance.
GEOLOGY
Oceanside and Thirtymile Bank blind thrusts: Implications for earthquake hazards in coastal southern California. Carlos Rivero et al.
This article defines a new blind-thrust system in coastal southern California that threatens the Los Angeles and San Diego metropolitan regions. An integrated approach documents recent activity in the blind thrusts by combining geophysical, seismological, and coastal tectonics studies with three-dimensional structural modeling. These thrusts originated as Miocene normal faults, suggesting that positions of pre-existing structures may control modern blind-thrust systems. The new fault system yields plausible scenarios for large (M 7.1–7.6) but infrequent earthquakes.
Grenville-age basement provinces in East Antarctica: Evidence for three separate collisional orogens. I.C.W. Fitzsimons.
The continents on Earth have periodically amalgamated to form supercontinents as they drift across Earth's surface, and each of these amalgamation events has produced a network of collision zones marked by elongate belts of deformed and metamorphosed rock. The supercontinent of Rodinia is widely believed to have assembled some 1000 million years ago along the so-called "Grenville-age" metamorphic belts, including the Grenville Province of eastern North America and the Circum-East Antarctic Mobile Belt. These Grenville-age belts are commonly regarded as a continuous network of time-equivalent collision zones developed during a simple amalgamation event. However, careful consideration of recent age data from East Antarctica indicates that the Circum-East Antarctic Mobile Belt actually comprises three different crustal fragments recording three distinct collision events at approximately 1300, 1100, and 1000 million years ago, and that these three fragments were juxtaposed during the amalgamation of
Gondwana 500 million years ago. It follows that the concept of a single Grenville-age metamorphic belt in East Antarctica is invalid, and that Grenville-age events in Antarctica span over 450 million years of Earth history and do not reflect a simple period of amalgamation, but rather a protracted multi-stage history of supercontinent assembly.
Microscale observations of sulfate reduction: Correlation of microbial activity with lithified micritic laminae in modern marine stromatolites. Pieter T. Visscher et al.
Although stromatolites are the oldest record of life on Earth, the role of biological processes in their formation is poorly understood. The authors use microbiological techniques to better understand which bacterial processes facilitate precipitation of calcium carbonate crystals. The challenge addressed in this paper is to find ways to observe microbial activity on a micrometer scale and compare that with geological structure of modern marine stromatolites. Of particular interest is the population of sulfate-reducing bacteria (that respire with sulfate rather than with oxygen) because these organisms precipitate calcium carbonate (in contrast to aerobic heterotrophs, which dissolve calcium carbonate). The main findings were that lithification of stromatolites coincides with maximum rates of sulfate reduction and that this process takes place at the surface and shallow subsurface of the stromatolite. The notion of sulfate reduction that proceeds in the presence of oxygen, which has been reported by several other authors previously, seems to play a crucial role in stromatolite formation.
Remote sensing of CO2 and H2O emission rates from Masaya volcano, Nicaragua. Michael R. Burton et al.
Measurements of the composition and emission rates of volcanic gases are crucial to interpret and understand volcanic activity. At active volcanoes, remote sensing techniques provide obvious advantages over direct sampling of potentially hazardous vents. Unfortunately, the two most abundant volcanic gases, steam and carbon dioxide, are difficult to measure remotely because of the high atmospheric background levels of these gases. The authors report here precise field determinations of both steam and CO2 emissions from the active Masaya volcano, Nicaragua, using an infrared spectrometer. Masaya is amongst the world’s most prodigious point sources of sulfur and halogens to the atmosphere and its low summit elevation results in severe environmental degradation downwind as the gases reach the ground. It is also situated close to the Nicaraguan capital and has a prehistoric record of major explosive eruptions, unusual for its basaltic magma composition. In addition, the authors measured sulfur, chlorine, and fluorine emissions from the volcano, providing a near-complete picture of the gas composition. The technique described could be applied to monitoring efforts at other volcanoes.
New chronology for the late Paleocene thermal maximum and its environmental implications. U. Röhl et al.
About 55 million years ago, in the late Paleocene epoch, one of the most abrupt climatic events in the geological record transformed Earth. A brief but intense interval of global warming heated up the oceans and atmosphere. The magnitude and abruptness of the event can be explained by a series of catastrophic releases of the greenhouse gas methane from the seabed, which perturbed the global carbon cycle and led to a 2–3 ºC increase in average global temperatures. The subsequent oxygen deficiency in the water column may have been responsible for the extinction of 30% to 50% of deep-sea benthic foraminiferal species. The late Paleocene global warming may also have led to the migration of mammalians from Eurasia to North America. By geochemically analyzing deep-sea sediments, a group of German and American scientists developed a new chronology for the global warming event. "Although the event spans about 210,000 years, two thirds of the methane release occurred within only two steps, each of which lasted less than 1,000 years," according to Dr. Ursula Röhl from Bremen University, lead author of the investigation. The new chronology may be significant in understanding the long-
term fate of human input of greenhouse gases. According to these findings, it would take about 170,000 years for carbon dioxide, at its current rate of input, to be removed naturally from the surface biosphere.
Extremely fast supercooling of water in the anti-Jovian hemisphere of Europa: A speculative model for the fracturing pattern and ascent of brines along cracks. Roberto Oyarzun et al.
Europa, the icy moon of Jupiter, displays a complex array of fractures, including roughly concentric and arcuate fractures. The geometry of this intriguing pattern resembles that of the cracks formed in perlite, a volcanic glass. The authors argue that fast cooling of felsic magma (e.g., under submarine conditions) may serve as an analogue for fast supercooling of water leading to formation of amorphous ice ("glassy water"). The authors suggest two possible scenarios for ice melting and subsequent extremely fast supercooling: (1) massive subglacial volcanic activity, or (2) a large impact. Fast supercooling of water would have initially led to brine retention in the ice phase; however, after some time these brines would have been laterally rejected toward the fractures. As solidification progressed downward, brine concentration must have increased in the resulting shrinking ocean. These brines would have also escaped to the surface via the fracture network. High-pressure crystallization of salts within fractures may provide an efficient mechanism for differential plate separation, horizontal movement, and formation of new fractures on the surface of Europa.
Did two massive earthquakes in the Holocene induce widespread landsliding and near-surface deformation in part of the Ottawa Valley, Canada? J. M. Aylsworth et al.
Although the section of the Ottawa–St. Lawrence seismic zone near Ottawa, Canada, is relatively quiet currently, new geological and radiocarbon evidence suggests that in the past this area may have been the site of two of the most geologically destructive earthquakes in eastern Canada. About 7060 years B.P. a strong earthquake shook a thick deposit of geotechnically sensitive clay filling a deep bedrock basin, resulting in ground subsidence and severe sediment deformation. At approximately 4550 years B.P. another strong earthquake triggered many large landslides of sizes unequaled in historical time.
Evidence for periodicity and nonlinearity in a high-resolution fossil record of long-term evolution. Andrea Prokoph et al.
This article deals with the problem of whether evolution is controlled by external forces or more likely by internal forces (so-called self-organization). The fossil record of planktic foraminifera is probably the best fossil record available and shows a ~30 m.y. periodicity over the past 128 m.y. In addition, a significant determinism is found by wavelet and nonlinear analysis methods, while randomness is suppressed in the long-term evolution.
Major Neogene fluctuations of the East Antarctic ice sheet: Stratigraphic evidence from the Lambert Glacier region. Michael J. Hambrey and Barrie McKelvey.
Recent research by the authors in one of the most remote parts of Antarctica, the Prince Charles Mountains, has yielded a unique 20-million-year record of fluctuations of the world's largest glacier, the Lambert Glacier. Expansions and contractions of the ice front over a distance of up to 500 km suggest that the East Antarctic ice sheet was subject to major volume changes until as recently as 2 million years ago, and only then did it become stable. This evidence is locked up in uplifted glacial marine sediments, now uplifted by previously unknown tectonic processes to elevations of up to 1400 m above sea level. The issue concerning the stability of the ice sheet has been vigorously debated on the basis of conflicting evidence from the Transantarctic Mountains. This new area of investigation provides evidence that those who support a dynamic ice sheet are correct.
GSA TODAY
Evaluating global warming: A post-1990s perspective.
David Gutzler synthesizes what presently is known and not known about climate change. He succinctly lays out the problems with definitely attributing global warming to human activity and predicting future change with certainty, and discusses the implications of the problems for development of policies to deal with climate change. He concludes that greater emphasis on observational data would frame the discussion more meaningfully and concretely, and would facilitate development of a coherent policy for the future.
*To view the complete table of contents of GEOLOGY, as well as that of the GEOLOGICAL SOCIETY OF AMERICAN BULLETIN, see http://www.geosociety.org/pubs/cattract.htm.
*Full text GEOLOGY articles and the science article from GSA TODAY are available on the first of each month on the GSA Web site, http://www.geosociety.org/pubs/journals.htm.