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GEOLOGY
Archaeology, history, and geology of the 749 AD earthquake, Dead Sea Transform
Shmuel Marco, Tel Aviv University, Department of Geophysics and Planetary Sciences, Ramat Aviv, Tel Aviv, Israel, et al. Pages 665-668.
Historical records expand our knowledge and understanding of natural hazards such as earthquakes. However, pre-twentieth century historians did not know that earthquakes are associated with geological faults and occur when Earth's crust breaks. Consequently, historical texts usually report the time and damage caused by earthquakes but not the associated faults. Conversely, observed fault ruptures are often difficult to date. We combined archaeological and geological observations from recent excavations in the ancient city of Tiberias with interpretation of historical accounts on earthquakes. Tiberias was founded in A.D. 19 by King Herod on the western shore of the Sea of Galilee. We discovered that Herod's stadium, exposed for the first time, as well as later buildings, dated up to the early eighth century, are damaged and displaced by an earthquake fault, whereas buildings from the late eighth century are intact. The only strong earthquake that occurred between these dates was the earthquake of 18 January 749. We realized that in this case the geological observations in Tiberias agree with the historical records. Both data sets indicate that the earthquake was associated with a 100-km-long rupture between the Sea of Galilee and the Dead Sea basins, indicating a Richter magnitude of at least 7.
Granite recrystallization: the key to the nuclear waste problem?
Fergus Gibb, University of Sheffield, Engineering Materials, Mappin Street, Mappin Street, Sheffield, South Yorkshire S1 3JD, U.K., and Philip G. Attrill, Parexel International Ltd., Sheffield S2 5SU, U.K. Pages 657-660.
Many of the well-known problems with conventional mined repositories for disposal of high-level radioactive waste arise from the small proportion of long-lived and heat-generating radionuclides in the waste. An alternative disposal strategy is suggested whereby this problematic fraction is removed from the waste prior to disposal, thus avoiding the necessity for an impossibly long-lived repository. Implementation of such a strategy requires a safe means of disposing of the relatively small volumes of problematic radionuclides separated from the waste. The paper describes such a scheme in which these radionuclides are disposed of in a deep borehole, encased in a sarcophagus of recrystallized granite, and demonstrates that this is technically feasible.
Extreme southern margin of Late Quaternary glaciation in North America: Timing and controls
Lewis Owen, University of California, Riverside, Department of Earth Sciences, University of California, Riverside, CA, USA, et al. Pages 729-732.
Using the newly developing technique of cosmogenic radionuclide surface exposure dating, Lewis Owen and his colleagues show that small glaciers existed in southernmost California, just west of Los Angeles, during the last glacial (between ~22,000 and 11,000 years ago) and in the early part of the present interglacial (several thousand years ago). Their work has implications for reconstructing past climate to aid in understanding the future climate change.
A Cross-section of a Magma Conduit System at the Margin of the Colorado Plateau
Keith Putirka, California State University, Fresno, Earth and Environmental Sciences, Fresno, CA, USA, and Christopher D. Condit, Geoscience Department, University of Massachusetts, Amherst, MA, USA. Pages 701-704.
There is some controversy regarding the controls of magma chamber formation, and the general shape and depth extent of magma conduits. Mineral compositions can be used to probe the depths at which magma chambers form, and thereby test models of magma transport. In the Springerville volcanic field, mineral compositions indicate that most magmas stall at depths of 0–30 km--well above the crust/mantle boundary (39–45 km). This observation contradicts the commonly held view that magma chambers form at the base of the crust and supports the view of Bruce Marsh, that the conduit is a "magma mush column," or a plexus of dikes and sills. However, the distribution of dikes and sills beneath the Springerville volcanic field is not entirely random. Low crystallization temperatures and evolved lava compositions derive exclusively from two depth intervals: 0–12 and 23–30 km. Both of these intervals coincide with regional depth intervals that are seismically reflective. Mineral compositions thus suggest that seismic reflectors at the margins of the Colorado Plateau are magmatic sills, related to recent volcanic activity. Density and rock strength relationships also suggest that the 0–12 km stagnation level is a level of neutral buoyancy for Springerville magmas, while a weak layer at the base of the middle crust controls ponding within the 23–30 km interval. It thus appears that liquid evolution and wall rock partial melting occur where density or rheology contrasts impede the upward movement of magma.
Fulvic acid-like organic compounds control nucleation of marine calcite under suboxic conditions
Fritz Neuweiler, Georg-August-Universität Göttingen, GZG, Abteilung Geochemie, Goldschmidtstrasse 1, Göttingen - D-37077, Germany, et al. Pages 681-684.
Marine deep-water carbonate mud mounds, unlike shallow-water reefs, are devoid of skeletal frame builders and are instead accreted by in-situ precipitation of microcrystalline calcite and mechanical accumulation of fine-grained carbonate sediment. Traditionally considered to be essentially microbial in origin, so far no site-specific microbial process has been demonstrated to explain the in-situ precipitated microcrystalline calcite. By contrast, during recent years another view has arisen stressing the role of natural organic matter as a possible catalyst of mineral nucleation acting within an environment of fluctuating oxygen levels, e.g., near oxygen minimum zones. Based on a well-preserved example of a Cretaceous carbonate mud mound, Neuweiler and co-workers were able to characterize spectroscopically the organic matter enclosed within the in-situ precipitated calcite as marine, fulvic acid-like organic compounds. As a mineral-locked phase the authors assume that this mixture is still similar to the original composition, thus correlating mineral nucleation with the very early stages of geopolymer formation (humification). In order to crosscheck a catalytic role of the natural organic matter instead of representing a simple inclusion the rare earth element geochemistry was analyzed. The calcite exhibits a relative enrichment of middle-weight rare earth elements, a feature considered to occur due to preferential uptake via organic complexation. Furthermore, a positive Cerium anomaly indicates suboxic conditions during precipitation. By merging results of field observations, carbonate microfabrics, and organic and inorganic geochemistry Neuweiler and co-workers conclude that humification sourced by a local benthic biomass represents the key process for the formation of carbonate mud mounds and that this process becomes geologically significant under fluctuating oxygen levels near oxygen minimum zones. The Neoproterozoic rise of carbonate mud mounds is consistent with this view as there is molecular evidence for early metazoan divergence at that time, but not for a major evolutionary episode of microorganisms.
Earth's Earliest Microbial Mats in a Siliciclastic Marine Environment (Mozaan Group, 2.9 Ga, South Africa)
Nora Noffke, Old Dominion University, OEAS, 4600, Elkhorn Avenue, Norfolk, VA, USA, et al. Pages 673-676.
Microbial mats are organic, carpet-like layers that cover large parts of coastal sediments. Today, microbial mats occur worldwide along the modern coasts, and are constructed exclusively by bacteria and cyanobacteria. But also fossil (ancient) microbial mats are known. In rocks such as sandstones they create a specific group of "microbially induced sedimentary structures," detected and defined as their own category only a few years ago. Whereas many studies on early life focus on carbonate rocks or cherts, the authors Nora Noffke, Robert Hazen, and Noah Nhleko followed a new approach by investigating siliciclastic ("sandy") rock successions. The results provide evidence for the existence of filamentous bacteria forming microbial mats in shelf environments at 2.9 Ga--the oldest-known occurrence of microbial mats in siliciclastic rocks. The fossils resemble the cells of modern cyanobacteria, chloroflexi, or sulfur-oxidizing bacteria, and could indeed hint at very early cyanobacteria. Mineralogical, geochemical, and isotopic analyses are consistent with a biological origin of the structures.
A 725-yr cycle in the climate of central Africa during the late Holocene
G19449 Russell et al., p. 677–680
James Russell, University of Minnesota Duluth, Large Lakes Observatory, 10 University Drive, 202 R.L.B., Duluth, MN, USA, et al. Pages 677-680.
Climate records of the last ice age indicate a strong link between tropical African rainfall and climate variability in polar regions on glacial-interglacial time-scales. However, the patterns and causes of centennial to millennial-scale hydrologic variability during the past few thousand years in the tropics are poorly known, and are of crucial importance to sound water resource management practices in drought-prone regions such as tropical Africa. Russell et al. present a paleoclimatic record from equatorial Africa based upon the stable isotopic and geochemical composition of inorganic calcite from the sediments of Lake Edward, Uganda/Congo. Their record spans the past 5,400 years and documents a persistent drought cycle of ~725 years during this time. This cycle does not appear linked to high-latitude variability; rather, Russell et al. suggest that the 725 year-drought cycle may result from changes in the circulation and surface temperature of the Indian and Pacific Oceans. This work highlights the important role the tropics may play in driving century-scale climate variability.
GSA TODAY Science Article
Landslides and liquefaction triggered by the M 7.9 Denali Fault earthquake of 3 November 2002 Edwin L. Harp, U.S. Geological Survey, Golden, Colorado, USA. The Denali Fault earthquake in Alaska in November 2002 was, at a magnitude of 7.9, one of the biggest in U.S. history. The earthquake began 100 miles south of Fairbanks and unzipped a strike-slip fault eastward in a 200-mile-long rupture. In magnitude and rupture length, the temblor resembled the 1906 and 1857 quakes on California's San Andreas Fault and may hold important new lessons about the way such earthquakes work. The violent shaking triggered thousands of landslides from the steep slopes of the Alaska Range and surrounding areas. The most impressive were several giant rock avalanches that swept across glaciers at high speed and ran out as far as six miles. Another effect of the shaking was to cause liquefaction of sediments. Liquefaction occurs where unconsolidated, water-saturated sediments settle during prolonged shaking to occupy less space and force water out of pore spaces. Overlying firmer layers find themselves floating on the displaced water. Liquefaction from the quake caused lateral spreading, fracturing, and settlement within susceptible deposits in valley bottoms over a wide region, extending from Fairbanks eastward for hundreds of miles. An airstrip was damaged and ground under the Alaska Pipeline locally settled. A team of U.S. Geological Survey and University of California scientists report that the landslides and liquefaction effects offer one of the best guides to patterns of shaking intensity and duration in this area where seismic instruments were sparse. The team found that the landslides were uncommonly few and narrowly distributed compared to comparable large quakes, being mostly confined to within 10 miles of the fault rupture. The large rock avalanches all clustered near the epicenter and the early part of the rupture, and none were triggered near an eastern part of the rupture where the largest fault slip occurred. Liquefaction effects, however, increased eastward in severity and concentration and extended well beyond the zone of landslides, which is unusual for earthquake-produced effects. From the distribution patterns of landslides, the scientists deduced that the early part of the earthquake rupture produced the highest accelerations, but that overall the earthquake produced fewer high frequencies and attendant high accelerations than usual for such a large quake. Prolonged ground-motion, especially from eastern parts of the rupture, likely explains the wider pattern of liquefaction effects.
To review the abstracts for these articles, go to http://www.gsajournals.org. To review the complete table of contents for the August issue of GEOLOGY, go to http://www.gsajournals.org/gsaonline/?request=get-current-toc&issn=0091-7613. Representatives of the media may obtain a complimentary copy of any GEOLOGY article by contacting Ann Cairns at acairns@geosociety.org.