||August 24, 2001
GSA Release No. 01-35
September Media Highlights:
Geology and GSA Today
BOULDER, Colo. The Geological Society of America's September issue of GEOLOGY
contains a number of newsworthy items. Highlights from GEOLOGY and a summary of
the science article for the September GSA TODAY are provided below. Please discuss
articles of interest with the authors before publishing stories on their work,
and please make reference to GEOLOGY or GSA TODAY in stories published. Contact
Ann Cairns for copies of articles
and for additional information or assistance.
- 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
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
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,
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,
- 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.
- 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 www.gsajournals.org.
To obtain a complimentary copy of any GEOLOGY article, contact Ann