Boulder, CO, USA - Several papers in October's Geology
describe fossil records: a fungal disaster species; single-celled
sea-bottom-dwellers; trilobite soft tissues; fossil rainforests;
Archean microbial mats; and pollen and freshwater algae. These
records were recovered using varying methods: high sensitivity mass
spectrometer, detailed exploratory fieldwork, biochemical analysis,
and deep drill-coring. Additional papers cover geology and health
hazards, erosion and climate, and tsunamis and sedimentation. GSA
Today discusses what happens when increased atmospheric oxygenation
interacts with naturally enriched uranium deposits.
Highlights 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
articles published. Contact Christa Stratton for additional
information or assistance.
View abstracts for the complete issue of Geology at http://geology.gsapubs.org/
. Representatives of the media may obtain complementary copies of
Geology articles by contacting Christa Stratton at the
address above.
Chemical constitution of a Permian-Triassic disaster
species
Mark A. Sephton et al., Impacts and Astromaterials Research Centre,
Dept. of Earth Science and Engineering, South Kensington Campus,
Imperial College London SW7 2AZ, UK. Pages 875-878.
Around 250 million years ago, Permian forests were hit hard
during the world's greatest mass extinction. While many species on
land suffered at this time, others benefited from the event.
Sephton et al. imply that microscopic fossils in extinction-age
rocks are the remains of fungi that proliferated on dead wood
generated by the global catastrophe. It seems that in a dying and
decomposing world, the fungi felt entirely at home. The close of
the Permian saw the largest ever mass extinction, which was
accompanied by the greatest ever outpouring of flood basalt lavas
in present day Siberia. The gases from this massive volcanic event
acidified the land and sea and depleted Earth's protective ozone
shield. Trees were among the first casualties of the changing
end-Permian environment. The increased supply of decomposing woody
materials led to an opportunistic expansion of fungi; however, the
origin of these microfossils has recently been questioned, with
some researchers preferring to assign them to algae rather than
fungi. To settle the controversy, Sephton et al. used new
analytical approaches, including a high-sensitivity mass
spectrometer originally designed to detect interstellar grains in
meteorites. The data support the status of these enigmatic
end-Permian microfossils as a fungal disaster species. The feeding
frenzy at the end of the Permian reminds us that there will always
be winners and losers even during dramatic environmental changes.
The extinction allowed the fungi to gorge themselves on the remains
of fallen forests.
Geographic origin of species: The temperate-tropical
interchange
Martin A. Buzas and Stephen J. Culver, Dept. of Paleobiology,
National Museum of Natural History MRC-121, Smithsonian
Institution, Washington, D.C. 20013-7012, USA. Pages 879-882.
The geographic origin of modern marine species living on the
Atlantic continental margin of North America indicates the
existence of a vast temperate-tropical interchange. The excellent
fossil record of single-celled, bottom-dwelling organisms called
foraminifera enabled Buzas and Culver to pinpoint the geologic time
and geographic origin of 259 species currently residing off the
Atlantic seaboard. The results indicate that modern distributions
are assembled from a mixture of species originating in more than
one geographic region. Species living in temperate areas originated
from both temperate and tropical regions, and species living in the
tropics originated from tropical and temperate areas. Dispersal on
a global scale is essential for assembling modern communities. The
species pool supplying modern communities is much larger than
expected, and this study demonstrates the interdependence of the
communities inhabiting world oceans.
Turbulent dynamics of the 18 May 1980 Mount St. Helens
eruption column
Benjamin J. Andrews and James E. Gardner, Dept. of Geological
Sciences, Jackson School of Geosciences, The University of Texas at
Austin, Austin, Texas 78712, USA. Pages 895-898.
During explosive volcanic eruptions, columns of gas and ash
denser than the atmosphere erupt from volcanic vents. In buoyant
eruption regimes, those eruption columns turbulently mix in enough
air to lower their densities and rise to altitudes higher than 10
km. In collapsing eruption regimes, the columns entrain
insufficient amounts of air and collapse to generate devastating
pyroclastic flows. Andrews and Gardner analyze video of the 18 May
1980 eruption of Mount St. Helens and describe changes in the
turbulent structure of the eruption column margins that accompanied
a change from buoyant to partially collapsed eruption behavior.
From those turbulent structures, they infer changes in the interior
the eruption column and the development of rising and collapsing
regions within the eruption column.
Stable isotope signals from brines in the Barents Sea:
Implications for brine formation during the last
glaciation
Tine L. Rasmussen and Erik Thomsen, Dept. of Geology, University of
Tromso, Dramsveien 201, N-9037 Tromso, Norway. Pages 903-906.
Oxygen and carbon isotope values of benthic foraminifera from
the cold stadials of the last glacial period in the Nordic seas are
low, whereas they are high from the warm interstadials. The low
values have been attributed to brine formation carrying a low
stable isotope signal from the surface water into deep water, as it
occurs around Antarctica today. Brines are often considered to have
played a major role in the abrupt millennial-scale climate shifts
during the last glaciation. However, very little is known of the
isotopic composition of modern brines in the Northern Hemisphere,
greatly hampering the interpretation of past data. Here, Rasmussen
and Thomsen report on the oxygen and carbon isotope composition of
benthic foraminifera in two cores from a brine-influenced shelf
environment in Storfjorden, Svalbard, in the Barents Sea. The
results indicate that brines with sufficient density to contribute
significantly to intermediate and deep water are formed from cold,
salty waters and have high oxygen and carbon isotope values. Brines
with low oxygen isotope values formed from cold, fresher water have
relatively low density and they are unable to penetrate the deeper
parts of the Arctic and Nordic seas. This indicates that the low
benthic oxygen and carbon isotope values obtained from the Nordic
seas during stadials cannot be attributed to brines. The
implication is that brines did not contribute significantly to the
millennial-scale climate shifts.
Beyond Beecher's Trilobite Bed: Widespread pyritization of
soft tissues in the Late Ordovician Taconic foreland
basin
Una C. Farrell et al., Dept. of Geology and Geophysics, Yale
University, New Haven, Connecticut 06520, USA. Pages 907-910.
The fossil record is greatly biased toward those organisms or
parts of organisms that are most easily preserved (i.e., shells,
bones, and teeth). Only with exceptional preservational conditions
do we get a glimpse of the true morphological and biological
diversity of the geological past. Beecher's Trilobite Bed, from the
Ordovician of New York State, yields trilobites with soft tissues
preserved in pyrite. Conditions in the sediment, which included
high amounts of reactive iron, allowed the organic matter of the
animals to be replaced by pyrite, preserving in detail the limbs
and other soft parts. The original "Trilobite Bed" - a thin,
rapidly deposited mudstone - has been known since the late 1800s,
but only in the last decade has it become obvious that conditions
for pyritization are more widespread in this sedimentary basin than
previously imagined. Farrell et al. report the discovery of three
new layers preserving pyritized trilobites at the original
Beecher's Trilobite Bed site, as well as similar layers at multiple
new sites around New York State. Although the majority of the
pyritized fossils are trilobites, new organisms with pyritized soft
parts have been discovered, including ostracods and other
arthropods. This work highlights the importance of exploratory
fieldwork, and clarifies the sedimentary and geochemical conditions
required for pyritization. This new understanding will improve the
chances of discovering exceptionally preserved fossils elsewhere in
the basin.
Earthquake histories and Holocene acceleration of fault
displacement rates
Andrew Nicol et al., GNS Science, P.O. Box 30368, Lower Hutt, New
Zealand. Pages 911-914.
Displacement rates for normal and reverse faults are generally
higher when averaged over the past 10,000 years than for time
periods of hundreds of thousands of years or longer. This
relatively recent acceleration of displacement rates could be
accounted for by geological processes that produce increases of
tectonic tempo. Nicol et al. propose an alternative explanation in
which the observed rate changes arise from variability in the time
between, and/or the slip of, successive earthquakes on individual
faults, coupled with a sampling bias toward those faults that are
best represented at Earth's surface and accrued displacement
fastest during the last 10,000 years. This idea is supported by
displacement rates measured over time intervals of up to about
300,000 years from the Taupo Rift in New Zealand, where variations
in the time interval between earthquakes and their slip are
attributed to fault interactions. These variations in earthquake
parameters are widely observed and occur over longer time intervals
for slower-moving faults. Nicol et al. suggest that complexity in
earthquake histories arising from fault interaction is an intrinsic
property of the majority of fault systems comprising numerous
faults, and represents a severe impediment to earthquake
prediction.
Mechanism of instantaneous coal outbursts
Ping Guan et al., MOE Key Laboratory of Orogenic Belts and Crustal
Evolution, School of Earth and Space Sciences, Peking University,
Beijing 100871, China. Pages 915-918.
Coal outbursts are spontaneous ejections of coal and gas from an
underground coal seam during mining, especially when excavation
exposes a new layer of coal. Such outbursts are a major hazard for
miners. Hundreds, if not thousands, of miners die every year due to
such accidents. The most deadly outbursts occurred in Turkey in
1992, killing 263 people. On 30 May 2009, a coal outburst in
Chongqing, China, killed about 30 workers and injured many more.
Underground coal outbursts have been documented for over 170 years
and have been investigated by scientists for more than 150 years,
but the precise mechanism of coal outbursts is still elusive. Guan
et al. hypothesize that gas-rich coal can erupt when suddenly
decompressed and carry out experiments to verify this hypothesis.
Potentially, this work could lead to the development of methods to
predict and prevent future coal outbursts in underground coal
mines, which would save numerous lives.
Increased sediment transport via bioturbation at the last
glacial-interglacial transition
Matthew W. Hughes et al., Soil and Physical Sciences Group, P.O.
Box 84, Lincoln University, Lincoln 7647, Canterbury, New Zealand.
Pages 919-922.
Vegetation is often used to stabilize soils and reduce erosion
in terrain that has been subject to land-use practices, such as
cultivation or grazing. As a result, it is commonly assumed that
climate-driven variations in vegetation have influenced past
erosion rates in a fairly straightforward fashion: The more
extensive and deeply-rooted the vegetation, the more resistant it
would have been to erosion. Working on the South Island of New
Zealand, Hughes et al. documented changes in vegetation and erosion
over the past 27,000 years and found the opposite relationship:
Rates of erosion increased as the vegetation regime switched from
grassland to forest about 10,000 years ago, with the
glacial-interglacial transition. They attribute the increase to
vigorous soil mixing and transport associated with tree rooting
activity, such as root growth and tree turnover. In the absence of
landsliding (the gentle, soil-mantled slopes of their study area
are not prone to slope instability), these results suggest that
over long time scales, increases in vegetative cover can promote
bioturbation and increase downslope soil transport and erosion.
These results contrast with the dominant climate-erosion paradigm
for steep, landslide-dominated mountains, which associates rapid
erosion with sparse vegetation during glacial conditions.
Interpreting the role of climate on landscape evolution thus
requires explicit consideration of topographic properties and
geomorphic processes.
Incised channel fills containing conifers indicate that
seasonally dry vegetation dominated Pennsylvanian tropical
lowlands
Howard J. Falcon-Lang et al., Dept. of Earth Sciences, Royal
Holloway, University of London, Surrey TW20 0EX, UK. Pages
923-926.
New fossil discoveries show that ice ages repeatedly decimated
the first tropical rainforests to evolve on our planet. However,
rainforests bounced back when the climate returned to normal. These
findings are based on studies of 300-million-year-old fossils found
in coal mines in Illinois, USA. Spectacular fossil rainforests have
been reported from this region for several years, attracting media
attention. A new study investigates how these earliest rainforests
responded to climate change. Falcon-Lang et al. have found evidence
for ice age cycles in the rocks of fossil rainforests. Results show
that rainforests flourished when the tropical climate was warm and
wet. However, they collapsed when the tropics became cool and dry
at the height of ice ages. This work transforms our understanding
of the first rainforests to evolve on our planet. It shows that
climate change wreaked havoc on these tropical ecosystems, but
rainforests could recover from the brink of disaster. The research
suggests that ancient rainforests were resilient to climate
fluctuations and may help us understand how ecosystems will respond
to changes in the future.
Impact melt sheet zircons and their implications for the
Hadean crust
J. Darling et al., Dept. of Earth Sciences, University of Bristol,
Bristol BS8 1RJ, UK. Pages 927-930.
Impacts may have been important mechanisms of crustal
redistribution and differentiation, particularly during intense
post-accretionary bombardment between 4.5 and 3.9 billion years
ago. Evidence of crustal processes during this period is largely
provided by detrital zircons from the Yilgarn Craton. Trace element
compositions, crystallization temperatures, and inclusion
populations of these ancient zircons have been taken as evidence
for predominantly granitic source magmas, implying widespread
felsic continental crust on the early Earth. There is, however,
little knowledge of zircons formed in impact melt sheets, a
potential source for the Hadean zircons. Here, Darling et al.
present titanium-in-zircon thermometry, trace elements, and
inclusion populations of zircons from the 1.85-billion-year-old
Sudbury impact melt sheet. Their results demonstrate that large
variations in zircon crystallization temperature and composition
will be an inevitable consequence of the evolution of such magmatic
systems. It is also shown that zircons in mafic rocks crystallize
in residual liquids of granitic composition, producing inclusion
assemblages that are remarkably similar to those reported for the
ancient Yilgarn grains. Thus, they conclude that the trace element
compositions and inclusion populations of the Hadean zircons are
consistent with crystallization from more mafic melts than
previously recognized, although high crystallization temperature
distributions of Sudbury zircons indicate that impact melt sheets
were not a dominant source for the more than 3.9-billion-year-old
grains.
An early ecosystem of Archean tidal microbial mats (Moodies
Group, South Africa; ca. 3.2 Ga)
Christoph Heubeck, Dept. of Geological Sciences, Freie Universitat
Berlin, 12249 Berlin, Germany. Pages 931-934.
The search for traces of life on other planets has its parallels
to the search for the beginning of life on Earth itself because it
teaches us in which environments we should look for suspicious
shapes and signs of biological activity. It is therefore
encouraging that the oldest well-preserved shallow-water strata, in
a mountain range in South Africa, show evidence that microbial life
not only existed microscopically and in isolated patches but
already occupied tracts that are mappable and continuous over
several kilometers. The sandstone strata studied in the Barberton
Greenstone Belt by Heubeck are densely permeated by abundant
carbon-rich laminations. These wavy-crinkly bands, interpreted as
remains of microbial mats, apparently interacted with considerable
currents that transported sand and gravel in a coastal or tidal
environment. The laminations show deformation patterns that suggest
a flexible but sturdy mechanical behavior, a surface morphology of
at least a few centimeters, and very rapid growth. In addition, the
microbial mats were apparently cohesive enough to trap fluids and
gases, occasionally giving rise to small sand volcanoes. The
mechanical strength of the biomats was likely derived from very
early silicification, similar to what can be observed today in
active hot spring fields. Thus, shallow-water environments at
elevated temperatures may constitute a promising and readily
detectable habitat in the search for early life.
Particle sizes of andesitic ash fallout from vertical
eruptions and co-pyroclastic flow clouds, Volcan de Colima,
Mexico
Jason R. Evans et al., Michigan Technological University, Houghton,
Michigan 49931 USA. Pages 935-938.
Evans et al. discuss differences between two types of eruption
deposits at Volcan de Colima in Mexico. One deposit type is formed
by eruptions that release a relatively small amount of energy
(vertical eruptions), where the ash particles interact less and
tend to be larger in size. The other deposit type (from
co-pyroclastic flow clouds) is from eruptions with relatively
higher amounts of energy where the ash particles interact more,
likely causing breakage, and therefore smaller particles to be
produced. Small particles are more dangerous to living creatures as
they can cause health problems and may be inhaled directly, rather
than being filtered by the body. In areas where these eruptions
occur, there could be a greater potential for human harm if the
co-pyroclastic ash particles are present.
Sulfate-reducing ammonium oxidation: A thermodynamically
feasible metabolic pathway in subseafloor sediment
Heather N. Schrum et al., Graduate School of Oceanography,
University of Rhode Island, Narragansett, Rhode Island 02882, USA.
Pages 939-942.
Schrum et al. have discovered evidence for a previously unknown
biochemical pathway that may fuel life in subseafloor sediment
throughout the world. They have shown that in marine sediment,
microorganisms can derive energy by reacting with ammonium and
sulfate. The process is called sulfate-reducing ammonium oxidation.
Their evidence is based on observations made during research
cruises in the Indian Ocean and Narragansett Bay. Microbes that
live in marine sediment comprise a significant fraction of the life
on Earth. This previously unsuspected process may remove a
significant amount of biologically accessible nitrogen from the
ocean and recycle it back to the atmosphere.
Tsunami waves generated by the Santorini eruption reached
Eastern Mediterranean shores
Beverly N. Goodman-Tchernov et al., Haifa University, Leon Charney
School of Marine Sciences, Haifa, Israel. Pages 943-946.
Goodman-Tchernov et al. have extracted a series of underwater
core samples from the seabed offshore of the Herodian harbor, in
Caesarea Maritima, Israel, with the primary aim of documenting
deposits produced by a historically attested tsunami associated
with the famous earthquake of A.D. 115. While consistent traces of
the A.D. 115 event were in fact found, a number of the cores
contained unexpected and exciting evidence of both a more recent
event (A.D. 551) and a much more ancient event - a tsunami deposit
as much as 40 cm thick carbon- and optically stimulated
luminescence (OSL)-dated to ca. 1600 B.C., when a cataclysmic
eruption obliterated much of the island of Santorini (Thera) and
the thriving Cycladic culture that flourished there. The eruption
of Santorini and its effects on surrounding civilizations, notably
that of Minoan Crete, has long been one of the most discussed
questions in Aegean Prehistory. While traces of deposits created by
waves stemming from the eruption have been identified in Crete and
Asia Minor, the results from Caesarea provide the first conclusive
evidence that tsunami waves generated by the eruption reached the
farmost eastern shores of the Mediterranean, and indeed in
considerable force. These findings indicate that the waves
generated by the Santorini eruption were powerful and far-ranging
enough to have had potentially profound effects on coastal
communities throughout the eastern Mediterranean. Moreover, they
demonstrate the potential of underwater core sampling as a means of
identifying similar evidence of tsunamis on shallow coastal shelves
throughout the world. A new window onto the study of past tsunamis
has been opened that may provide an invaluable gauge of the
destructive potential of future events.
Dune mobility and aridity at the desert margin of northern
China at a time of peak monsoon strength
J.A. Mason et al., Dept. of Geography, University of Wisconsin, 550
N. Park St., Madison, Wisconsin 53706, USA. Pages 947-950.
Wind-blown sands cover large areas at the margin of deserts in
northern China and are sensitive to climate change, becoming more
active during dry periods and more stable under vegetation cover
when the climate is wet. Mason et al. show that there was
widespread dune activity indicating aridity, from about 11,500 to
8000 years ago, which is surprising given the growing evidence that
a strong monsoon at that time made southern China wetter than it is
today. As monsoon precipitation in southern China decreased after
8000 years ago, the dunes stabilized, suggesting a wetter climate
at the desert margin. Climate modeling studies help explain these
contrasting patterns of climate change, showing that atmospheric
circulation associated with a strong monsoon may actually reduce
precipitation in northern China's drylands. Warm summer
temperatures from 11,500 to 8000 years ago may also have increased
aridity at the desert margin. These past links between changes in
the monsoon climate and aridity at the desert margin may help
predict environmental response to future climate change
A major drop in seawater 87Sr/86Sr during the Middle
Ordovician (Darriwilian): Links to volcanism and climate?
Seth A. Young et al., Dept. of Geological Sciences, Indiana
University, 1001 E 10th St., Bloomington, Indiana 47405, USA. Pages
951-954.
The rise of the Appalachian Mountains may have caused a major
ice age beginning approximately 450 million years ago. The
weathering of these mountains sequestered carbon dioxide
(CO2) from the atmosphere, causing the opposite of a
greenhouse effect, an "icehouse" effect. Scientists have suspected
that our current ice age, which began approximately 35 million
years ago, was caused by the rise of the Himalayas. This new study
by Young et al. links a much earlier major ice age, one that
occurred during the Ordovician Period, to the uplift of the early
Appalachians. It also reinforces the notion that CO2
levels in the atmosphere are a major driver of Earth's climate.
Analyzing a set of rock samples from Nevada, by comparing the ratio
of two isotopes of the element strontium, it was found that,
immediately before geologic evidence for cooling appeared in the
stratigraphic record, the strontium ratio dropped dramatically
within 8 to 10 million years. The likely cause: A vast amount of
young volcanic rock was being eroded away, and the resulting
sediment was being deposited in the world oceans. The timing of the
strontium decline matches the rise of the Appalachian Mountains
that lifted volcanic rock up from island arcs onto the North
American continent. This kind of silicate rock weathers quickly,
reacting with CO2 and water, resulting in carbon from
the CO2 being trapped in the resulting sediment and
dissolved ions. The chemical reactions that weathered away part of
the Appalachians would have consumed large amounts of
CO2 from the atmosphere. A numerical model simulation
also presented in this study supports the strontium isotope data
and geologic evidence for uplift and weathering of these terranes
in the Appalachian Mountains as a likely cause of the Late
Ordovician icehouse period. The Ordovician period started out warm,
with relatively high sea levels worldwide, and ended cold, with low
sea levels as glaciers covered the landmasses over the South Pole.
The transition between greenhouse conditions and icehouse
conditions set the stage for major mass extinctions around the
planet at the end of the Ordovician.
Palynomorphs from sediment core reveal a sudden remarkably
warm Antarctica during the mid Miocene
Sophie Warny et al., Dept. of Geology and Geophysics, and Museum of
Natural Science, E235 Howe-Russell, Louisiana State University,
Baton Rouge, Louisiana 70803, USA. Pages 955-958.
Warny et al. have discovered algae and pollen grains in ANDRILL
AND-2A (a multinational collaboration between the Antarctic
Programs of the United States, New Zealand, Italy, and Germany)
samples that provide evidence of a remarkably warm period about 15
million years ago. Among the 1,107 meters of sediment recovered and
analyzed for microfossil content, a two-meter-thick layer in the
core displayed extremely rich content. This is unusual because the
Antarctic ice sheet was formed about 35 million years ago, and the
frigid temperatures there impede the presence of woody plants and
blooms of dinoflagellate algae. According to the researchers, these
fossils show that land temperatures reached a January average of 10
degrees Celsius - equivalent to about 50 degrees Fahrenheit - and
that estimated sea-surface temperatures ranged between zero and
11.5 degrees Celsius. The presence of freshwater algae in the
sediments suggests to researchers that an increase in meltwater,
and perhaps also in rainfall, produced ponds and lakes adjacent to
the Ross Sea during this warm period, which would obviously have
resulted in some reduction in sea ice. These findings most likely
reflect a poleward shift of the jet stream in the Southern
Hemisphere, which would have pushed warmer water toward the pole
and allowed a few dinoflagellate species to flourish under such
ice-free conditions. Warny et al. believe that shrub-like woody
plants might also have been able to proliferate during an abrupt
and brief warmer time interval.
GSA Today
Did natural reactors form as a consequence of the emergence
of oxygenic photosynthesis during the Archean?
Laurence A. Coogan and Jay T. Cullen, School of Earth and Ocean
Sciences, University of Victoria, P.O. Box 3055 STN CSC, Victoria,
British Columbia V8W 3P6 Canada
In order to get uranium (U) to undergo nuclear fission, for
example in power stations, it is artificially enriched. However,
2.5 billion years ago, fissile enriched U was abundant, enough so
that even small concentrations of it would have undergone
spontaneous nuclear fission. As is pointed out by authors Jay
Cullen and Laurence Coogan of the University of Victoria, British
Columbia, 2.5 billion years ago also happens to be about the time
that oxygen slowly started building up in Earth's atmosphere. They
demonstrate that, because of the way U behaves, the increase in
atmospheric oxygen would have led to U being dissolved out of
rocks, forming concentrated U deposits. Once more than about one
cubic meter of U was concentrated in a given place, it would have
"gone critical," forming a natural fission reactor. The implication
is that Earth's continental crust would have been pocked with
natural nuclear reactors, contaminating the environment with
hazardous, radioactive byproducts. Cullen and Coogan discuss the
potentially significant and severe impacts that these natural
reactors had on the evolution of life and the further oxygenation
of our atmosphere.
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