Boulder, CO, USA - GEOLOGY topics include confirmation that East Java's "Lusi" mud volcano was most likely triggered by a blowout in the Banjar Panji-1 well; evidence of the oldest human presence yet discovered along Egypt's northern Nile delta; analysis of Yellowstone's hotspot; and GPS measurements that show why no large earthquakes occur in the Snake River Plain. GSA Today's science article studies the impact of Glen Canyon dam in Grand Canyon on sandbar ecosystems along the Colorado River.
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Predynastic human presence discovered by core drilling at the northern Nile delta coast, Egypt
Jean-Daniel Stanley et al., Smithsonian Institution, Rm. E-205 NMNH, Paleobiology, 10th St. and Constitution Ave. NW, Washington, DC 20013, USA. Pages 599-602.
A small but significant find made during a geological survey provides evidence of the oldest human presence yet discovered along the northernmost margin of Egypt's Nile delta. A rock fragment carried by humans to the site was discovered in a sediment core section north of Burullus lagoon near the Mediterranean coast. Radiocarbon analysis of plant-rich matter in the mud surrounding the object provides a date of 3350 to 3020 B.C., the late Predynastic period. This long, thin object, formed of dolomite, had not been deposited by the Nile or the sea, but was collected and transported from an outcrop exposure positioned at least 160 kilometers south of the core site. The fragile object lay buried at a depth of 7.5 meters in dark mud deposited in a brackish lagoon setting close to a marsh. Stanley et al.'s fortuitous find documents an early human presence in the mid-Holocene wetlands along the delta's paleocoast, a sector where traditional excavation and augering are normally incapable of reaching zones of ancient human activity now at considerable subsurface depths.
Mantle earthquakes frozen in mylonitized ultramafic pseudotachylytes of spinel-lherzolite facies
T. Ueda and Masaaki Obata et al., Dept. of Geology and Mineralogy, Faculty of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan. Pages 607-610.
Ueda et al. present a new type of ultramafic pseudotachylyte from Balmuccia peridotite, a "fossil" of mantle earthquakes. They report a case in which a ductile shear localization led to a seismic rapture and frictional melting of spinel peridotite, and the thus-produced pseudotachylyte was subsequently mylonitized and recrystallized in spinel lherzolite facies. Ueda et al. hypothesize that the chemical evolution of fluid phases, from H2O-rich to CO2-rich, played a critical role for such a rheological transition.
Hydrothermal venting at pressure-temperature conditions above the critical point of seawater, 5°S on the Mid-Atlantic Ridge
Andrea Koschinsky et al., Jacobs University Bremen, P.O. Box 750561, D-28725 Bremen, Germany. Pages 615-618.
Volcanic activity on mid-ocean ridges produces very hot fluids circulating in the oceanic crust and emanating as black smokers at the seafloor. Koschinsky et al. document results from recent research cruises at the Mid-Atlantic Ridge, revealing the hottest fluids ever found, with a stable maximum temperature of 407 degrees Celsius and maximum temperatures that spike up to 464 degrees Celsius. For several years now, the phase of high-temperature activity has seemed to remain stable. These are the first hydrothermal fluids that could be sampled at temperature and pressure conditions at which seawater is a so-called supercritical fluid. The specific properties of a supercritical fluid--in terms of solution and transport of metals--make this hydrothermal field an exciting natural laboratory to investigate mineral deposit formation at conditions that previously could only be simulated in the laboratory.
Temperature of the plume layer beneath the Yellowstone hotspot
Derek Schutt and Ken Deuker, Dept. of Geosciences, Warner College of Natural Resources, Colorado State University, Colorado 80523-1482, USA. Pages 623-626.
The basalt-covered Snake River Plain and the geysers of Yellowstone National Park (Wyoming) are caused by the Yellowstone hotspot--a hot region deep below Yellowstone National Park. But how hot is this hotspot and what is causing it? Recently, Schutt and Deuker measured the temperature of the hotspot. They built on past work that found that earthquake energy travels much more slowly in the rocks 100 kilometers and more below southern Idaho and northwestern Wyoming than nearly anywhere else in Earth at this depth. They modeled the effects of temperature and other processes that affect the speed at which seismic energy travels, and found the Yellowstone hotspot to be 50-200 degrees Celsius hotter than its surroundings, perhaps as hot as 1450-1500 degrees Celsius at 100 kilometer depth. Combined with other work on the shape of the low-velocity region, Schutt and Deuker's study implies that the hotspot is an upwelling from deep within Earth, possibly coming from as deep as the boundary between Earth's core and mantle (located 2900 kilometers deep).
Tectonic controls on the nature of large silicic calderas in volcanic arcs
Gwyneth Hughes and Gail Mahood, Geological and Environmental Sciences, Stanford University, 450 Serra Mall, Building 320, Stanford, California 94301, USA. Pages 627-630.
At convergent margins, silicic magma chambers help to stabilize the continents by adding low-density, high-silica material to the upper crust. Silicic calderas are the volcanoes associated with these typically large magma chambers; the calderas result from the catastrophic collapse of the chamber roof following the explosive eruption of viscous magma. The primary goal of Hughes and Mahood's study was to determine which tectonic and crustal factors control the occurrence and composition of silicic calderas in volcanic arcs. They present the results of a global compilation study that examines the attributes of 91 Quaternary silicic calderas in 18 volcanic arcs. Hughes and Mahood show that the number of calderas in a given arc depends on the associated plate convergence rate, and that the age, thickness, and stress regime of the underlying crust determine the composition of caldera-forming eruptions. Better understanding of where and why these types of volcanoes occur will increase our understanding of how continental crust forms at margins and aid in interpreting the geological settings of ancient arcs.
Neoarchean lithospheric strengthening and the coupling of Earth's geochemical reservoirs
Patrice Rey and Nicolas Coltice, School of Geosciences, Madsen Building F09, The University of Sydney, Sydney, NSW 2006, Australia. Pages 635-638.
Rey and Coltice present a working hypothesis that claims that circa 2.8 to 2.5 billion years ago, Earth's continents became progressively stronger, allowing for the formation of high mountain belts. The erosion of these mountains acted as a coupling agent between the continental geochemical reservoir, the ocean/atmosphere geochemical system, and Earth's mantle. One of the outcomes of this coupling is that phosphorus (a key element for DNA), which occurs mainly in the continental crust, was made available in great abundance to the living world courtesy of Himalaya-like mountains.
Triggering of the Lusi mud eruption: Earthquake versus drilling initiation
Mark Tingay et al., Dept. of Applied Geology, Curtin University of Technology, Western Australia 6102, Australia. Pages 639-642.
The Lusi mud volcano in East Java is a unique geological disaster that has erupted unabated for two years, displacing over 25,000 people and causing approximately 420 million dollars (US) in damage. However, there is still significant scientific and political debate about whether the Lusi eruption was naturally initiated by a remote earthquake two days prior to the eruption or was triggered by a drilling accident ("blowout") in the nearby Banjar Panji-1 gas well. Tingay et al. conduct the first quantitative analysis into the proposed triggering mechanisms--man-made and natural. The authors examined all known mechanisms for remote triggering of the mud eruption by an earthquake and determined that the earthquake hypothesis is mechanically implausible. Tingay et al. then examined the drilling-induced theory and determined that the nearby Banjar Panji-1 well was drilled under unsafe conditions and with inadequate protective casing, which would have made drilling problems difficult to control. Furthermore, they found that the drilling problems that occurred in Banajr Panji-1 generated pressures sufficient to extensively fracture the subsurface rocks and create pathways for mud to erupt to the surface. Hence, Tingay et al. concluded that there are no known mechanisms that support the earthquake hypothesis and that the Lusi mud eruption was most likely triggered by a blowout in the Banjar Panji-1 well.
Continental stretching preceding the opening of the Drake Passage: Evidence from Tierra del Fuego
Matias Ghiglione et al., Depto. Ciencias Geologicas, Facultad Ciencias Exactas Naturales, Universidad de Buenos Aires, Ciudad Universitaria, Pabellón 2 C1428EHA, Ciudad de Buenos Aires, Argentina. Pages 643-646.
Due to the impact of global warming, it has become essential to understand the causes and processes involved in past climate changes. One of the most prominent events in Earth's climatic evolution was the transition from the global warmth of the Eocene "greenhouse" to the Oligocene "icehouse" glacial conditions. It is widely believed that the separation of South America from Antarctica and the subsequent formation of Drake Passage have influenced Cenozoic global cooling because these events enabled the development of the Antarctic Circumpolar Current. This wind-driven current facilitates inter-ocean exchange of seawater and is speculated to have reduced heat transport to Antarctica, triggering the Oligocene global cooling. Alternatively, it has been proposed that the opening of the Drake Passage influenced circulation-induced productivity increases that may have sequestered atmospheric CO2, contributing to global cooling and Antarctic glaciation. While the former theory is based on the presumption that the marine connection was coeval with initiation of Antarctic glaciation at the Eocene-Oligocene boundary, the latter requires an earlier (middle Eocene) ocean water exchange. Unfortunately, age estimates for the onset of a seaway through the Drake Passage range from middle Eocene to early Miocene, complicating interpretations of the relationship between ocean circulation and global cooling. Studying the southeast tip of Tierra del Fuego, a region that was once attached to the Antarctic Peninsula, Ghiglione et al. discovered evidence for the opening of widespread early Eocene extensional depocenters. The succession of events described in their study show that the opening of a seaway through the Drake Passage was early enough to contribute to global cooling through lowering levels of atmospheric CO2. Their data bolster interpretations of an early (Eocene) marine gateway by confirming the prediction of a continental extensional regime in the region during the period prior to the opening of small basins ca. 42 million years ago. Ghiglione et al.'s findings will have a broad impact, adding key pieces to the puzzle of the opening of the Drake Passage, and allowing a new field of investigation to continue unveiling its effects on global circulation and climate.
Strain rates and contemporary deformation in the Snake River Plain and surrounding Basin and Range from GPS and seismicity
Suzette Payne et al., Idaho National Laboratory, PO Box 1625, MS 2025, Idaho Falls, Idaho 83415-2025, USA. Pages 647-650.
Global positioning system (GPS) measurements collected in Idaho from 1994 to 2007 tell us why the Snake River Plain, unlike adjacent regions, does not have large earthquakes. New observations show that the Snake River Plain is not stretching as fast as the Basin and Range (mountains and valleys) northwest of it. Payne et al. explain that large magnitude (M 7+) earthquakes, such as the one in 1983 in central Idaho, occur along the Basin and Range faults northwest of the Snake River Plain. In the Snake River Plain, however, volcanism and earthquakes occur much more infrequently because of its low stretching rate. A zone of shear is present along the northern margin of the Snake River Plain to accommodate the different rates of stretching.
Middle Eocene climate cyclicity in the southern Pacific: Implications for global ice volume
Catherine Burgess et al., School of Earth and Ocean Sciences, Cardiff University, Main Building, Cardiff CF10 3YE, UK. Pages 651-654.
Cliffs at Hampden Beach on the east coast of South Island, New Zealand, have been found to contain the fossilized remains of microscopic ocean plankton, which can be used to reconstruct the climate of 42 million years ago. Burgess et al. use these past climate indicators to shows that the water around this coast, then located at approximately 55 degrees S, reached temperatures of 23 to 25 degrees Celsius, more than 10 degrees Celsius warmer than similar latitudes today. The fossils also record a cyclic variation in temperature, with warming and cooling by approximately 1.5 degrees Celsius about every 18,000 years, which is thought to be caused by variations in Earth's orbit around the sun. These climate swings are similar to those that caused the more recent ice ages, but Burgess et al. show that, at that time, there was little or no ice present on Earth.
Three-dimensional morphology of magmatic sulfides sheds light on ore formation and sulfide melt migration
Stephen Barnes et al., CSIRO Exploration and Mining, P.O. Box 1130, Bentley 6102, Western Australia, Australia. Pages 655-658.
Due to high demand for nickel from China and a prolonged period of high prices, the nickel mining industry is currently experiencing a major boom. Previously sub-economic, low-grade nickel deposits, such as those found at Mount Keith and Black Swan in Western Australia, are now being mined profitably. The nickel ores take the form of dispersed aggregates of nickel-iron sulfide minerals, which originally formed as immiscible liquid droplets within spectacularly hot, fluid magmas called komatiites. Barnes et al. have applied the technique of X-ray computed tomography (CAT or CT scanning), common in medical applications, to investigate the size and shape of these sulphide liquid droplets, and they have come to unexpected conclusions about the origins of the ore deposits. The application of a new generation of high-resolution CT instruments to rock textures is a promising development in the earth sciences.
Anomalous cold in the Pangaean tropics
Gerilyn Soreghan et al., University of Oklahoma, Geology and Geophysics, 100 E. Boyd St., SEC 810, Norman, Oklahoma 73019, USA. Pages 659-662.
Today, and for the past several million years, Earth's climate has been characterized by large-scale glaciation, although we currently live in an interglacial of this geologically rare state. Earth's last major glaciation, the Late Paleozoic Ice Age, occurred 300 million years ago. During this time, the continents assembled into the supercontinent Pangaea; sediments from the southern high latitudes preserve abundant evidence for large-scale glaciation. Geoscientists have long presumed that, like today, the tropics remained warm throughout this glaciation, but new evidence from the western United States -- paleo-tropical Pangaea -- indicate that cold temperatures gripped the low latitudes. Soreghan et al. found this evidence in the remarkable preservation of a 300 million-year-old glacial landscape and associated glacial sediments in western Colorado. Owing to the close proximity of the glacier to ancient sea level, the toe of the ice is estimated to have reached within less than 500 meters of elevation -- much lower than tropical glaciers of Earth's recent glacial states. Moreover, the Late Paleozoic preserves the only known occurrence of widespread tropical loess (windblown silt) in Earth's history, which is apparently of glacial derivation. Climate model simulations are unable to replicate such cold tropical conditions, even after incorporating the low atmospheric CO2 and reduced solar luminosity of the time. We are left with the prospect that what has been termed our "best-known" analogue to Earth's "modern" glaciation is poorly known, and Earth's climate system works in ways that remain elusive.
Microbes produce nanobacteria-like structures, avoiding cell entombment
Tomaso Bontognali et al., Geologisches Institut-CHN E 65, Universitatstrasse 16, 8092 Zurich, Switzerland. Pages 663-666.
Particles shaped like fossils of small bacteria have been found in old sedimentary rocks and in a meteorite from Mars. These structures were interpreted as possible evidence for extraterrestrial life, leading to an animated and still unsolved debate. Since then, many studies have been carried out to exclude or to demonstrate without doubt that such objects are, indeed, fossils of an unknown species of bacteria, whose size is smaller than 200 nanometers. Bontognali et al. were able to produce similar structures in a laboratory experiment with living bacteria. Their observations suggest that these particles are probably not fossils of microbes. However, the process leading to their formation requires the presence of living bacteria that produce and release organic molecules in the environment. Therefore, the ancient rocks and the Martian meteorite containing these structures may indicate the existence of very old terrestrial and extraterrestrial life.
Quantitative estimates of glacial and Holocene temperature and precipitation change in lowland Amazonian Bolivia
Surangi W. Punyasena et al., Smithsonian Tropical Research Institute, Center for Tropical Paleoecology and Archeology, Unit 0948, Balboa, Ancon, Panama. Pages 667-670.
Punyasena et al.'s study reconstructs temperature and precipitation changes in the lowland tropics of northeastern Bolivia from the late Pleistocene (circa 50,000 calendar years before present [cal yr B.P.]) through the present. In order to reconstruct paleotemperature and paleoprecipitation, they used published pollen records from two lakes at the southern boundary of Amazonian moist forest in Noel Kempff Mercado National Park to quantify vegetation change and apply a probabilistic climate-vegetation model based on the modern abundance distributions of 154 Neotropical plant families. Punyasena et al.'s results suggest that this area of the lowland tropics experienced cooling (1-3 degrees Celsius) and drying (400 mm/yr) during the late Pleistocene (50,000-12,000 cal yr B.P.), relative to present. Their data show the occurrence of a distinct transition from cooler temperatures and variable precipitation prior to the Last Glacial Maximum (LGM, ~21,000 cal yr B.P.) to a period of warmer temperatures and relative dryness that extends to the mid-Holocene (5000-3000 cal yr B.P.). This prolonged reduction in precipitation occurs against the backdrop of increasing atmospheric CO2 concentrations, suggesting that the mixed savanna and dry forest communities in northeastern Bolivia during the LGM were not just the result of low CO2 levels, as suggested previously, but also of lower precipitation. The results of Punyasena et al.'s analysis demonstrate the potential for using the distribution and abundance structure of modern Neotropical plant families to infer paleoclimate from the fossil pollen record.
GSA Today Science Article
Is there enough sand? Evaluating the fate of Grand Canyon sandbars
Scott A. Wright et al., U.S. Geological Survey, California Water Science Center, 6000 J Street, Sacramento, California 95819, USA. Pages 4-10.
Large dams are spectacular feats of engineering, and few are as impressive as the 216-m-high Glen Canyon Dam that sits astride Colorado River and supplies electricity to more than 650,000 households. By trapping the entire incoming sediment load in Lake Powell and eliminating spring snowmelt floods, the dam has, however, significantly impacted the Colorado River and the ecosystems that depend upon the river. It was the recognition of the imperilled state of the river that prompted this past spring's 60 hour "high-flow experiment" during which 41,000 cubic feet of water were released per second into the river. The question posed by Scott Wright of the U.S. Geological Survey and colleagues is whether the dam can be operated such that sandbars, which are crucial to the riparian ecosystem, be rebuilt and maintained over extended periods with the existing sediment supply (i.e., without augmentation from the upstream reservoir). By adopting a best-case scenario, they demonstrate that the sandbars could be rebuilt, albeit slowly. The viability of operating the dam according to the best-case scenario remains, however, an open question.
To review the abstracts for these articles, go to www.gsajournals.org. View the complete table of contents for the current issue of GEOLOGY at www.gsajournals.org/perlserv/?request=index-html&issn=0091-7613.