GEOLOGY
End-Permian catastrophe by a bolide impact: Evidence of a gigantic release of sulfur from the mantle. Kunio Kaiho et al. Institute of Geology and Paleontology, Tohoku University, Sendai 980-8578, Japan. Pages 815-818.
The authors’ studies in southern China have revealed a remarkable sulfur and strontium isotope excursion at the end of the Permian, along with a coincident concentration of impact-metamorphosed grains and kaolinite and a significant decrease in manganese, phosphorous, calcium, and microfossils (foraminifera). These data suggest that an asteroid or a comet hit the ocean at the end of the Permian and caused a rapid and massive release of sulfur from the mantle to the ocean-atmosphere system, leading to significant oxygen consumption, acid rain, and the most severe biotic crisis in the history of life on Earth.
Role of biomineralization as an ultraviolet shield: Implications for Archean life. V.R. Phoenix et al. School of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK. Pages 823-826.
It is believed that during the Archean, Earth’s surface was bathed in harmful UV light due to a lack of ozone in the atmosphere. Thus, early life forms were required to develop an effective screening mechanism to protect themselves from UV. This article describes how cyanobacteria may have been shielded from UV by acting as nucleation sites for the precipitation of iron-enriched silica biominerals. These iron-enriched silica biominerals absorb significant amounts of UV, while still allowing the passage of light required for photosynthesis, thus affording the microbe significant UV protection. Iron-silica minerals precipitated onto the surfaces of the cyanobacteria because the shallow-water environments they inhabited were likely highly enriched in silica and iron.
Lacustrine isotopic evidence for multi-decadal natural climate variability related to the circumpolar vortex over the northeast United States during the past millennium. Matthew E. Kirby et al. Department of Earth Sciences, Syracuse University, Syracuse, New York 13244, USA. Pages 807-810.
The heavily populated and economically important northeast United States is an excellent region in which to develop new climate records with annual resolution. The authors’ research presents a unique climate record from world-famous Fayetteville Green Lake in upstate New York. Using annually layered sediments from the bottom of the lake, the authors observed a unique climatic relationship between winter season atmospheric circulation (jet stream position) and precipitation source. Information on precipitation source was extracted from chemical analysis of the lake sediments. From studying this relationship, the authors examined 1000 yr of winter jet stream variability and its associated influence on storms across the northeast United States. From these data, the authors propose that winter jet stream position is strongly controlled by natural variability, perhaps linked to oceanic processes in the North Atlantic. Most intriguing, however, is that the winter jet stream position seems relatively unaffected by the recent trend of increasing global temperature.
Coseismic Hydrologic response of an alluvial fan to the 1999 Chi-Chi earthquake, Taiwan Chi-yuen Wang et al. Department of Earth and Planetary Science, University of California, Berkeley, California 94720, USA. Pages 831-834.
Alluvial fans around the world occur along active mountain belts where streams emerging from steep mountains lose velocity and discharge their sediment load. Because they are close to active thrust faults, these fans are often subjected to severe earthquake shaking, which has led to some of the worst historical landslides, even on very gentle slope. Despite the suggestion that these earthquakes may have been caused by rising pore pressure in sediment, direct evidence from systematic field measurements has been lacking.
Widespread coseismic change in pore-water pressure occurred across a large alluvial fan in central Taiwan during the 1999 Chi-Chi (Mw = 7.5) earthquake. This change was captured for the first time by a dense network of hydrologic monitoring wells. The complex, yet systematic, pattern in the water-pressure change appears inconsistent with the existing models, and requires one that is based on the nonlinear mechanical behavior of sediments under seismic shaking. This paper presents, for the first time, direct field evidence that earthquake shaking causes rising pore pressure in alluvial fans, which in turn may lead to landslide, even on very gentle slope.
Large and rapid climate variability during the Messinian salinity crisis: Evidence from deuterium concentrations of individual biomarkers. Nils Andersen et al. Department of Earth Sciences, ETH-Z Zurich, CH-8092 Zurich, Switzerland. Pages 799-802.
During the Messinian, ca. 6 Ma, massive sea-level fall and widespread deposition of evaporites occurred in the Mediterranean Sea when it became isolated from the world oceans. The stable hydrogen isotope composition of individual organic compounds (biomarkers) was analyzed for the first time in ancient sediments, providing a record of climatically driven hydrographic changes in response to extreme evaporation during the Messinian salinity crisis. The source waters of the biomarkers were in some cases extremely enriched in deuterium, producing isotopic values that are only known from desert climates today. This work shows that the stable hydrogen isotope composition of biomarkers is preserved in sediments as old as the Miocene, and that these compounds can be used for paleoenvironmental and climatic reconstructions.
Visual observation of gas hydrate formation and dissociation in synthetic porous media by means of glass micromodels. Bahman Tohidi et al. Department of Petroleum Engineering, Heriot-Watt University, Edinburgh EH14 4AS, Scotland, UK. Pages 867-870.
Gas hydrates are ice-like crystalline compounds formed when gas and water molecules are combined under low-temperature and high-pressure conditions. In the natural environment, such conditions can be found in deep marine sediments and in the subsurface of arctic permafrost regions, where hydrates can form in the presence of a suitable gas source (usually methane). The large volumes of methane trapped as gas hydrates are a potential strategic energy reserve, but could become a major contributor to global climatic change if the stability of hydrates is compromised. In addition, researchers believe that hydrates are closely related to subsea landslides, and that they pose a significant hazard to deep-water hydrocarbon drilling operations, undersea pipelines, and cables. Current methods for locating and calculating the volume of gas hydrates in sediments are based largely on the interpretation of seismic data. Such methods require speculation about how hydrate crystals, water, and gas are distributed in sediment pore spaces. Glass micromodels provide a novel approach to the study of gas hydrates in sediments, allowing the visual observation of hydrate behavior at the microscopic scale in synthetic sediment models. Experiments have provided valuable visual information on the mechanisms of gas hydrate formation, crystal growth, and the distribution of hydrates, water, and gas within sediment pore spaces. The results have particular significance with regard to the potential cementing of hydrates on sediments, and have important implications for the relationship between gas hydrates and the stability of subsea slopes.
Volcano collapse promoted by hydrothermal alteration and edifice shape, Mount Rainier, Washington Mark E. Reid et al. U.S. Geological Survey, 345 Middlefield Road, MS 910, Menlo Park, California 94025, USA. Pages 779-782.
The catastrophic collapse of steep volcano flanks threatens many populated regions worldwide, and understanding the factors that promote collapse could save lives and property. Large collapses of hydrothermally altered parts of Mount Rainier, Washington, have generated far-traveled debris flows; future flows would threaten densely populated parts of the Puget Sound region. The authors evaluated volcano-collapse hazards at Mount Rainier using a new, three-dimensional slope stability method that incorporates detailed geologic mapping and subsurface geophysical imaging to identify distributions of strong (fresh) and weak (altered) rock. The authors’ slope stability calculations reveal that large flank collapse is promoted by voluminous, weak, hydrothermally altered rock situated high on steep slopes. These conditions exist only on Mount Rainier’s upper west slope, consistent with the past 5000 yr of debris-flow activity.
First survey of Antarctic sub-ice shelf sediments reveals mid-Holocene ice shelf retreat. C.J. Pudsey and J. Evans. British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK. Pages 787-790.
The northern Antarctic Peninsula has warmed by 2.5°C in the past 50 yr. This is significantly greater than the global mean temperature rise over the same period, and has been interpreted by some as an indication of anthropogenic climatic warming, which is expected to be amplified in the polar regions. As a result of the warming, several ice shelves have retreated or disintegrated. By retrieving marine sediments from locations that were previously covered by ice shelves, it can be determined whether the current retreat is unique within the Holocene, which spans the past 10 k.y., or whether it has earlier analogues. The authors collected cores from previously ice-covered areas of the northern Larsen ice shelf. Analysis of rock fragments and organic material found in the cores revealed that in the past, icebergs transported different rock types freely through the area. At such times the ice shelf could not have been present. Carbon-14 dating indicates that the ice shelf was absent from ca. 5000–2000 yr ago, but has probably been a stable feature for the past 1800 yr. Other evidence suggests the Antarctic Peninsula was relatively warm at the time the ice shelf was absent. Thus, the present loss of ice shelves cannot be taken as a "fingerprint" of anthropogenic climate change, since such events occur naturally.
GSA TODAY
Stream Piracy Revisited: A Groundwater Sapping Solution. Darryll T. Pederson, University of Nebraska, Lincoln.
Stream piracy describes a water-diversion event where water from one stream is captured by another stream, with a lower base level. Stream piracy has been reported on all time and size scales and has had major importance in determining drainage patterns through time. This process thus affects such things as migration patterns for aquatic animals, rates of erosion in upland areas, and stream chemistry changes, as well as landscape evolution. Two “villains” have been identified as main suspects in causing stream piracy: surface water and groundwater. While both likely operate as cooperators, and the effects of groundwater sapping can often be destroyed by surface water erosion, this paper summarizes the importance of groundwater sapping as a primary mechanism for stream piracy. The author cites numerous studies that show that groundwater sapping is effective in rock and cohesive sediment, is focused by the intersection of the extending channel with the water table, and is effective in hillslope processes. Further, the persistence of groundwater-flow systems as a pirating stream approaches a divide can cause breaching of the divide by groundwater-sapping processes. Unlike surface-water energy, which decreases near divides, groundwater systems maintain erosive energy because the position of the groundwater divide is independent of the topographic divide and can migrate with channel extension where streams in adjacent drainage basins are at different elevations. Wetter climatic periods can add energy to the system because increased recharge causes groundwater levels to rise, accelerating stream piracy. Hence, past climates may have been more conducive to piracy than today’s climate.
To view abstracts for the current issues of GEOLOGY and GSA TODAY, go to http://www.gsajournals.org. To obtain a complimentary copy of any GEOLOGY article, contact Ann Cairns, acairns@geosociety.org.