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Active faulting within the offshore western Gulf of Corinth, Greece: Implications for models of continental rift deformation
L.C. McNeill, University of Southampton, Southampton Oceanography Centre, Southampton, Hampshire SO14 3ZH, UK; et al. Pages 241-244.
The Gulf of Corinth, Greece, is a continental rift zone, where two parts of a continental plate move apart, and a region of high earthquake activity. Offshore high resolution geophysical data have been collected in the western rift where the rift margins move apart at 10-15 mm per year. The new data indicate that major steeply-dipping active faults (which accommodate extension and generate earthquakes) are located offshore as well as onshore. Deformation is therefore distributed across several faults rather than a single major fault. Total slip on these faults over geological time scales (hundreds of thousands of years) equals extension measured very recently by the global positioning system (GPS). These results constrain models of deformation within rifts and can be used to consider how these systems evolve to eventually produce new ocean basins.
An inland-directed base surge generated by the explosive interaction of pyroclastic flows and seawater at Soufrière Hills volcano, Montserrat Marie Edmonds, United States Geological Survey, Hawaiian Volcano Observatory, Hawaii Volcanoes National Park, Hawaii 96718, USA; and Richard A Herd, University of Hawaii, Hawaiian Volcano Observatory, Hawaii Volcanoes National Park, Hawaii 96718, USA. Pages 245-248.
In July 2003, a large eruption at Soufriere Hills volcano in Montserrat, West Indies, was accompanied by large-volume pyroclastic flows into the sea, 3.5 km from the lava dome. The rapid influx of such large volumes of hot material led to a hydrovolcanic explosion. A base surge created by the explosion column flowed radially from the sea's edge, back inland and up to 350 metres above sea level, impacting 7 km2 and killing many animals. This type of hydrovolcanic activity, with pyroclastic surges which affected areas on land not hit by the main eruption, has not been previously observed and documented at a recent eruption. The phenomenon represents a new hazard which should be taken into account when preparing risk maps for volcanoes near water.
Oxygen isotope compositions as a tracer for the origins of rubies and sapphires
Gaston Giuliani, IRD, UR154, CRPG/CNRS 15, Vandoeuvre-lès-Nancy Cedex, F-54501, France; et al. Pages 249-252.
The oxygen isotopic composition of rubies and sapphires worldwide is a good indicator of the geological environment of its formation. The consistently restricted 18O/16O ratio range found for each type of deposit is explained by host-rock buffering during fluid-rock interaction. The oxygen isotopic composition constrains the geological source of the major type of gem-quality rubies sold on the market and brings new insight to gems found in placers. High-quality blue sapphires from Kashmir, Andranondambo, and Sri Lanka have specific oxygen isotopic ranges, but they overlap those of Mogok in Myanmar. Combined with traditional gemology techniques, oxygen isotope analysis will contribute toward defining the origin of some commercial high-value blue sapphires, especially from Kashmir.
A 40,000 year unchanging seismic regime in the Dead Sea rift
Z.B. Begin, The Geological Survey of Israel, Geological Hazards, Jerusalem 95501, Israel; et al. Pages 257-260.
How old is an earthquake regime? Or, put otherwise, how stable are the rules that govern the frequency of occurrence of strong earthquakes in a fault zone? These questions have not been previously addressed, probably because of the lack of a suitable data set. The authors studied a unique archive of prehistoric earthquakes, one that is embedded in lake sediments that were deposited 70-14 thousand years before the present (B.P.) in the seismically active Dead Sea basin. Some 30 fragmented beds in these sediments mark ancient earthquakes. Within this sequence, the authors identified some fragmented beds that are overlain by gypsum. They suggest that the gypsum was formed as a result of mixing of the lake water column, due to tsunami waves accompanied by strong earthquakes that were caused by faulting within the ancient lake. Thus, 11 earthquakes with M > 7 were determined, and the record was extended to an additional earthquake that occurred about 5,000 years B.P., based on a study of damaged cave deposits. Of these 12 strong earthquakes, the 8 earlier ones were clustered within about 5,000 years (at 54-49 thousand years B.P.), while the later four earthquakes occurred at a recurrence interval of about 10,000-12,000 years, and may represent relaxation of the fault system. Since the 10,000-year interval of M > 7 earthquakes characterizes the present seismic regime in the Dead Sea basin, the authors concluded that this regime has been stable for the last 40,000 years.
Dating large, infrequent earthquakes by damaged cave deposits
Elisa J. Kagan, Hebrew University of Jerusalem, Institute of Earth Sciences, Jerusalem 91904, Israel; et al. Pages 261-264.
The long-term recurrence pattern of past earthquakes is of considerable consequence for hazard assessments with implications for earthquake physics. Kagan et al. introduce a dated record of earthquakes from well-preserved earthquake-damaged cave deposits (e.g., stalactites and stalagmites). The study caves are located 15 km from the city of Jerusalem (Israel) and 40 km from the active Dead Sea transform fault. During an earthquake, cave deposits can be broken and may fall. Subsequent to an earthquake, deposition continues on top of the damaged material. The boundary between the pre- and post-earthquake material can be dated precisely, bracketing the date of the earthquake. Kagan et al. dated a large number of rock falls inside the caves using Uranium-Thorium radiometric dating methods and stable isotope correlation with well-dated climatic events previously recognized in the deposits. Cave deposits are advantageous to this kind of study since they are potentially well dated up to about half a million years. This presents the opportunity to recreate a long-term earthquake record and include the largest earthquakes to have hit a region, which is usually unavailable by other methods. Within the last 185,000 years covered, they dated 38 damaged samples. They determined that these deposits were damaged during 13-18 earthquakes. The average recurrence interval of earthquakes causing damage at the study location is 10,000-14,000 years. Kagan et al. show that there is a correlation of the earthquakes dated in the study caves with independent earthquake records (mainly deformed lake sediments) near the Dead Sea. The earthquakes dated in their study are most likely large (M > 8), infrequent events, given the distance from the active fault. This opens up a significant new avenue of earthquake research that will provide precise dating and observational constraints on large, infrequent earthquakes.
New evidence for 250 million-year age of halotolerant bacterium from a Permian salt crystal
Cindy Satterfield, Binghamton University (SUNY), Geological Sciences, Binghamton, NY 13902-6000, USA; et al. Pages 265-268. The 250 Ma age of the halotolerant bacteria cultured from a droplet of brine trapped in a halite crystal (Vreeland et al., 2000) has been challenged because of potentially ambiguous geologic evidence. Satterfield et al. provide new evidence for the 250 Ma age of the brine inclusion using halite from the same sampling area that produced the bacteria. Brine inclusions from these samples analyzed using microthermometric techniques show that the crystals grew at temperatures of 17-37 °C. The major ion composition of the brine inclusions indicates the crystals formed via evaporation of Late Permian seawater. This new work suggests the brine inclusions are composed of Permian seawater that was trapped in crystals at surface temperatures which is compatible with a syndepositional shallow groundwater origin. The results therefore support the long-term survivability of microorganisms.
Dauphiné twinning as evidence for an impact origin of preferred orientation in quartzite: An example from Vredefort, South Africa.
H.-R. Wenk, University of California, Department of Earth and Planetary Science, Berkeley, CA 94720-4767, USA; et al. Pages 273-276.
Several minerals undergo mechanical twinning if subjected to stress. Quartz is an example. Twinning produces a reorientation of the crystals. In this contribution, Wenk et al. interpret the orientation pattern of crystals in a quartzite from the Vredefort structure in South Africa as having originated from a shock wave that was produced during meteorite impact 2 billion years ago.
Decoding the dipstick: Thickness of Siple Dome, West Antarctica, at the Last Glacial Maximum
E.D. Waddington, University of Washington, Earth and Space Sciences, Seattle, WA 98195-1310, USA; et al. Pages 281-284.
Where mountains poke through a polar ice sheet such as West Antarctica, trim lines, moraine limits, and exposure-age dating can show how high the ice sheet was in the past. However, the West Antarctic Ice Sheet has a central region of more than 2 million square kilometers where no mountain peaks are exposed; the traditional geologic methods cannot be used there. Because the thickness of annual layers in an ice sheet depend on snowfall rates in the past and on the amount of thinning that has occurred, researchers can use an ice-flow model to find combinations of accumulation and ice-sheet thickness histories that match the measured depth-age relation and the layer-thickness pattern in the Siple Dome ice core. When Waddington et al. reject unreasonable accumulation histories, the remaining histories indicate thinning of only 200 to 400 m in the past 20,000 years. Their estimate is lower than most previous reconstructions. During the last glacial period, a grounded ice sheet extended more than 1000 km farther. As a result, low surface elevations near Siple Dome imply that the ice had very low surface slope, requiring a very slippery bed. The fast-flowing ice streams in West Antarctica must have been much longer than they are today.
Mountains, monsoons, and megafans
Andrew L. Leier, University of Arizona, Geosciences, Tucson, AZ 85721, USA; et al. Pages 289-292.
In some parts of the world, the rivers that originate in mountain ranges like the Himalayas and the Andes spill out onto the adjacent lowlands and deposit huge quantities of cobbles and sediments in patterns that, when viewed from above, look like giant fans. These features, which are some of the largest landforms on the planet, are wide in the middle of the basin and narrow to a point at the foot of the mountains. Why certain rivers construct such "megafans" while others fail to do so has remained an enigma ever since they were first observed. This study shows that climatic factors, specifically monsoonal rain patterns, play a fundamental role in controlling which rivers will form these giant fans. The periodic deluge that rivers experience during monsoonal rains causes these rivers to overflow, during which time they commonly jump their banks, and commence flowing into another part of the basin. Repeated several times, over many years, this behavior will eventually produce a megafan. Understanding this process and these relationships will allow scientists that find ancient megafan deposits to reconstruct the precipitation patterns that existed on Earth millions of years ago.
Spinel-bearing spherules condensed from the Chicxulub impact-vapor plume
Denton S. Ebel, American Museum of Natural History, Earth & Planetary Sciences, New York, NY 10024, USA; and Lawrence Grossman, University of Chicago, Geophysical Sciences, Chicago, IL 60637, USA. Pages 293-296.
Spinel-bearing spherules precisely mark the Cretaceous-Paleogene (K-P) 65 Ma stratigraphic boundary, coincident with a severe mass extinction event and with the Chicxulub impact that produced a superheated vapor plume. Impact simulations and well log data yield the initial composition of the plume, which circled the globe and deposited the Ir-rich boundary layer. In this paper, thermodynamic models of condensation are used to produce the most detailed picture yet of the complicated chemistry of the expected vapor. Silicate liquid droplets would condense at high temperature. Spinel crystals with compositions very similar to those in K-P boundary spherules would crystallize from the liquid droplets. The highly oxidizing nature of the plume, due to the thick carbonate and sulfate layers in the impact target, is crucial to this outcome. Compositions of spinels found in the Atlantic and Europe are consistent with their crystallization at higher temperatures than spinels found in Pacific drill core.
Fine-grained debris flows and extraordinary vertebrate burials in the Late Cretaceous of Madagascar
Raymond R. Rogers, Macalester College, Geology Department, St. Paul, MN 55105, USA; et al. Pages 297-300.
During the Late Cretaceous (65-75 million years ago), the exotic island of Madagascar was home to a diverse assortment of animals, including dinosaurs, crocodiles, mammals, and birds. Bones and teeth of these ancient creatures literally spill from the easily eroded grasslands in the northwestern portion of the island, near the village of Berivotra. This study concludes that the extraordinarily abundant and well-preserved fossils from the Berivotra area of Madagascar were entombed by multiple fine-grained debris flows. The debris flows are linked to exceptional rainfall events that prompted erosion and flooded ancient riverbeds again and again with sediment-laden slurries rich with clay and sand. These extraordinary burial events shielded vertebrate remains from destructive surface processes and also afforded some protection for soft tissues. The remarkable and recurrent association of bone beds and debris flow deposits in ancient Madagascar likely reflects marked seasonality in this Late Cretaceous terrestrial ecosystem. It was clearly a tough place to make a living, and prolonged dry spells and their associated hardships prompted mortality. Animals big and small sought refuge in and around drying river courses as resources dwindled. There they died, and their carcasses were picked clean by both vertebrate and insect scavengers. Subsequent drenching rains set debris flows in motion to capture the accumulated death records. Today, the fossilized remains of these animals afford novel insights into life and death in the Late Cretaceous world.
Oxygen isotope evidence for the late Cenozoic development of an orographic rain shadow in eastern Washington, United States
Akinori Takeuchi, Washington State University, Department of Geology, Pullman, WA 99164-2812, USA; and Peter B. Larson, Washington State University, Department of Geology, Physical Science 1228, Pullman, WA 99164-2812, USA. Pages 313-316.
The present-day topography of the southern Washington Cascade Range was developed by tectonic rock uplift, and the topographic development of the range has been intimately linked to the regional climate change as described by Takeuchi and Larson in this paper. Evidence for the topographic development of the range is based on an investigation of the timing and extent of the development of the rain shadow in eastern Washington. The authors suggest that the elevation of the southern Washington Cascades has increased by approximately 1,200-1,700 m in the last 15.6 m.y. The magnitude of this surface uplift is almost identical with the change in elevation of strata from the Columbia basin to the southern Washington Cascades. However, topographic development of the other part of the Cascade Range as well as the timing and extent of rain shadow development in the Pacific Northwest is still poorly understood so that further investigation is needed. This study will be of great interest to other scientists who study the relationship between uplift of mountains and climate change.
Oligocene-Miocene age of aridity in the Atacama Desert revealed by exposure dating of erosion sensitive landforms
Tibor J. Dunai, Vrije Universiteit Amsterdam, Faculty of Earth and Life Sciences, Amsterdam, Holland 1081HV, Netherlands; et al. Pages 321-324.
In this study, areas in the Atacama Desert, northern Chile, are identified as the oldest landscape surfaces on Earth. With 20-25 Million years of continuous, unaltered exposure, this landscape is about twice as old as the Dry Valleys region in Antarctica, which was previously considered the oldest landscape on Earth. The findings of the study indicate that an arid to hyper-arid climate must have prevailed in the last 20-25 million years in the Atacama Desert. This has important implications for the verification of a recent hypothesis that suggests that the aridity in the Atacama Desert is the cause, rather than the reason, for the uplift of the Andes in this region.
Nappe stacking resulting from subduction of oceanic and continental lithosphere below Greece
Douwe Jacob Jan van Hinsbergen, Utrecht University, Faculty of Earth Sciences, Stratigraphy and Paleontology, Utrecht 3584 CD, Netherlands; et al. Pages 325-328.
In Greece, a pile of nappes is exposed that developed since the late Mesozoic era during the Alpine orogeny. Such nappes consist of laterally continuous, internally more or less coherent sedimentary rocks of comparable age and sedimentary facies. Occasionally, underneath these sediments the pre-Alpine basement has been preserved, allowing to identify whether the nappe is of continental or oceanic ("ophiolitic") origin. Van Hinsbergen et al. tested a model in which these nappes were "scraped off" a continuously subducting plate. To this end, they balanced the nappes, i.e., they placed the nappes next to each other, instead of on top of each other, to estimate the amount of convergence associated with their stacking. Additionally, they used paleomagnetism and plate tectonic reconstructions to assess the amount of (oceanic) crust that entirely subducted without leaving a nappe. This led them to conclude that following a phase of 700 km of convergence in the Jurassic, 2400 km of convergence occurred continuously since the Early Cretaceous, at least 900 km of which involved continental crust Then, they compared the geological reconstruction with images of the subducted slab in the mantle beneath Greece. From this image follows that a deep, poorly resolved anomaly occurs south of a well-resolved, continuous anomaly from the surface into the lower mantle. The latter anomaly corresponds to 2100-2500 km of subducted lithosphere. The deeper anomaly may correspond to the 700 km of lithosphere that subducted in the Jurassic, the shallow anomaly is most likely the lithosphere that once underlain the nappes of Greece. As a consequence of the positive test, van Hinsbergen et al. conclude that, in Greece, at least 900 km of continental crust subducted since the Early Cretaceous.
Reconstructing a 180 yr record of natural and anthropogenic induced low oxygen conditions from Louisiana continental shelf sediments
Lisa E. Osterman, U.S. Geological Survey, EESPT, Reston, VA 20192, USA; et al. Pages 329-332.
During summer months a large area of water on the Louisiana continental shelf becomes depleted in oxygen when the uptake of oxygen by respiration exceeds its resupply. When oxygen concentration is less than 2 mg/L, it is defined as hypoxic. The likelihood of hypoxia occurring is increased with nutrient loading or greater stratification. A new source of nutrients can stimulate marine surface phytoplankton blooms whose organic matter upon death may sink to the bottom and decay. Oxygen is consequently removed from the water column and marine organisms become distressed, evacuate the area, or die. The modern recurrent hypoxia on the Louisiana shelf is attributed to increased use of nitrogen-rich fertilizer in the Mississippi River Basin. Variations in the relative abundance of low-oxygen-tolerant benthic foraminifers in four sediment cores from the Louisiana shelf provide a proxy record of low oxygen events during the last ~180 years. Core chronologies are obtained using 210Pb dating techniques. The foraminiferal data are consistent with previous studies, indicating that the intensity of hypoxic events has increased over the past 50 yr owing to the higher nutrient loading associated with the use of commercial fertilizer, and also reveal several low-oxygen events between A.D. 1817 and 1910, prior to the widespread use of fertilizer. The pre-1910 low-oxygen events are associated with high Mississippi River discharge rates, indicating that these low-oxygen episodes are related to natural variations in river drainage that enhance transport of nutrients and fresh water to the continental shelf. Their data show that the low-oxygen events of the last few decades were more extreme than any that occurred in the previous ~180 yr and support the interpretation that the increased use of fertilizer has amplified an otherwise naturally occurring process.
Pedogenic carbonate proxies for amount and seasonality of precipitation in paleosols
Gregory J. Retallack, University of Oregon, Department of Geological Sciences, Eugene, OR 97403-1272, USA. Pages 333-336.
Buried soils have long been known as sources of evidence of past rainfall, but they can also be used to determine the seasonality of rainfall as well as amount of rainfall. In modern soils, the spread within the profile of carbonate nodules is related to the difference in precipitation between the driest and wettest month. This feature in ancient soils can be used to reinterpret past climates. During the transient carbon dioxide greenhouse at the end of the Paleocene epoch, some 55 million years ago, paleosols indicate that Utah was not only wetter but more seasonal. Such increases in rainfall seasonality and storminess are also likely during ongoing global warming.
GSA TODAY Science Article
Old landscape images record landscape change through time, Paul R. Bierman, Department of Geology and Natural Resources, University of Vermont, Burlington, Vermont 05405, USA, et al.
Historical photographs are a powerful tool for examining and understanding the distribution of geologic, ecologic, and human-triggered processes that change the landscape over decades and centuries. Taking advantage of a large, searchable, web-based image archive of landscape images, Paul Bierman and co-authors were able to document that erosion in Vermont is more common in clear-cut areas than in partially or wholly forested sites. Once the old roots rot or the stumps are pulled, root strength is gone and the treeless hillslopes are primed for failure, awaiting only a storm large enough to saturate the ground and trigger landsliding. Image analysis by the scientists indicates that erosion peaked in Vermont in the 1860s just before maximum land clearance, again in 1927 at the time of a region's biggest recorded flood, and in the 1960s coincident with the time of construction of the interstate highway system. A steady rise in the frequency of erosion images from 1900 to 1930 probably reflects the advent of road building and associated development. The researchers report that the quality of riparian buffers along streams has increased slowly over the past 150 years, with a dramatic improvement after 1980 that may reflect changes in land management practices including establishment of tree planting programs and the decline of the Vermont dairy industry. The readily searchable image archives provide a promising new tool for scientists, planners, and land managers; for more information, go to www.uvm.edu/~pbierman/landscape/.
To view the complete table of contents for the April issue of GEOLOGY, go to http://www.
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