Boulder, CO, USA – The February issue of Geology is
online now. Articles cover Patagonian glaciations, the Younger
Dryas cold period, paleodiversity, submarine gullies, the
Transantarctic Mountain micrometeorite collection, the “fastest
glacier on Earth,” salt diapirs in the Nordkapp Basin,
reinterpretation of James Hutton’s historic discovery on the Isle
of Arran, a new tool to directly date dinosaur-bone fossils,
ancient megalakes in Australia, Egypt’s Kamil Crater, and more. GSA
TODAY examines seismic activity to gain insights into the Rio
Grande Rift.
Keywords: Ammonoids, Patagonia, Younger Dryas, map area,
Hikurangi subduction margin, Great Oxidation Event, Transantarctic
Mountain micrometeorite collection, shear zones, Jakobshavn
Isbræ, Nordkapp Basin, Merida Andes, bedrock terraces,
phreatic calcrete hardpan, James Hutton, North Atlantic drainage
basin, Western Europe, San Juan Basin, mammal faunas, Australia,
Arkansas River basin, Last Glacial Maximum, Siletzia accretion,
Kamil Crater, Bothnian Bay, seismic activity, Rio Grand rift
Highlights are provided below. Representatives of the media may
obtain complementary copies of Geology articles by
contacting Christa Stratton at the address above. Abstracts for
individual articles are available by searching the DOI number after
each article, or the complete issue’s abstracts are available at http://geology.gsapubs.org/.
Please discuss articles of interest with the authors before
publishing stories on their work, and please make reference to
Geology in articles published. Contact Christa Stratton for
additional information or assistance.
Non-media requests for articles may be directed to GSA Sales and
Service,
gsaservice@geosociety.org.
Ammonoid diversity and disparity track episodes of chaotic
carbon cycling during the early Mesozoic
Jessica H. Whiteside, Dept. of Geological Sciences, Brown
University, Box 1846, Providence, Rhode Island 02912, USA; and
Peter D. Ward. Pages 99-102; doi:10.1130/G31401.1.
Diversity loss may have wider-reaching effects than
conventionally believed, potentially driving long-term instability
in marine food webs. Jessica H. Whiteside of Brown University and
Peter Ward of the University of Washington draw together carbon
isotope data from sedimentary rocks and fossils of ammonoids
(carnivorous, octopus-like creatures with a coiled shell) across
two of life’s most severe mass extinction events (the end-Permian
about 250 million years ago and the end-Triassic about 200 million
years ago) to demonstrate that in the wake of mass extinctions,
major instability in the carbon cycle lasts for millions of years,
and that ecosystems do not recover from extinction until after
carbon cycle recovery. The post-extinction ecosystems have unstable
food webs, which cause boom/bust cycles that are reflected in the
fluctuation of the carbon cycle. Whiteside and Ward’s study
provides an important perspective on current global change:
Ecosystems may suffer detrimental effects even long after humans
cease carbon emissions and ecosystem alteration.
Exposure dating outwash gravels to determine the age of the
greatest Patagonian glaciations
Andrew S. Hein et al., School of Geosciences, University of
Edinburgh, Drummond Street, Edinburgh, Scotland EH8 9XP, UK. Pages
103-106, doi:10.1130/G31215.1.
Andrew S. Hein of the University of Edinburgh and colleagues
discuss application of the cosmogenic nuclide dating technique to
the dating of Quaternary glacial deposits in southernmost South
America. Specifically, Hein and colleagues demonstrate that broad
river terraces, which formed during previous ice ages when glaciers
in Patagonia were much larger than they are today, have remained
nearly unchanged since their formation (in some cases over one
million years earlier). This exceptional long-term stability means
that rocks on glacial surfaces can be used to determine the timing
of these ancient ice ages by deriving “exposure ages,” which are
based on the concentration of rare cosmogenic nuclides that have
accumulated in the rock through exposure to the atmosphere. Here,
Hein et al. derive 10Be and 26Al surface exposure ages from
sediment on two outwash terraces that give ages of around 600
thousand years and 1.2 million years, suggesting the glacial
advance that formed the terraces occurred near to this time. The
results of this study strongly suggest that well-preserved and
stable glacial outwash terraces can be directly dated to determine
the timing of past fluctuations of the Patagonian Ice Sheet over
the past 1.2 million years; this approach should improve
reconstructions of past climate changes at this key location in the
southern mid-latitudes.
North Atlantic Deep Water and climate variability during the
Younger Dryas cold period
Aurora C. Elmore and James D. Wright, Dept. of Earth and Planetary
Sciences, Rutgers University, 610 Taylor Road, Piscataway, New
Jersey 08854, USA. Pages 107-110, 10.1130/G31376.1.
The Younger Dryas, the last large millennial-scale climate
oscillation (12.9-11.6 thousand years ago), has been widely
attributed to a massive meltwater discharge event that disrupted
ocean circulation and plunged the circum-North Atlantic back into a
near-glacial state. Low-resolution deep-water reconstructions
indicate lower North Atlantic Deep Water (NADW) production during
the Younger Dryas, though the delta-C14 record requires some
deep-water production. Elmore and Wright reconstruct deep-water
mass variations using a southern Gardar Drift sediment core with an
expanded Younger Dryas section. Elmore and Wright show that
southern-sourced water invaded the deep North Atlantic to start the
Younger Dryas, but was replaced by NADW within 500 years.
Southern-sourced waters briefly reappeared at the end of the
Younger Dryas. These deepwater reorganizations to start and end the
Younger Dryas suggest that increased meltwater fluxes were limited
temporally and focused on regions where deep-water convection
occurred during the deglaciation.
Using remote sensing and a geographic information system to
quantify rock exposure area in England and Wales: Implications for
paleodiversity studies
Alexander M. Dunhill, Dept. of Earth Sciences, University of
Bristol, Wills Memorial Building, Queen’s Road, Bristol BS8 1RJ,
UK. Pages 111-114, doi:10.1130/G31503.1.
A key debate among paleontologists is about the quality of the
fossil record: Can we use the fossils as a reasonable guide to the
history of life, or is the record heavily biased by incomplete
preservation of ancient rocks? In recent studies, many geologists
have used map area as a measure of rock availability, reasoning
that the overall area of rocks of different ages on the geological
map will give a rough guide to the likely access and intensity of
collecting. However, access to rocks depends on the rocks appearing
at the surface, termed exposure, and whether they are located in a
quarry or on a coastline. If the rocks are there, but buried deep,
then they cannot yield any fossils. In this study, Alexander M.
Dunhill of the University of Bristol uses remote sensing and a
Geographic Information System to show that map area and exposure do
not match up, and so the current use of “map area” as a measure of
fossil sampling is probably not appropriate.
A topographic signature of a hydrodynamic origin for
submarine gullies
Aaron Micallef, University of Malta, Msida, MSD 2080, Malta; and
Joshu J. Mountjoy, National Institute of Water and Atmospheric
Research Ltd., Private Bag 14901, Wellington, New Zealand. Pages
115-118, 10.1130/G31475.1.
The dynamics of mesoscale seafloor erosional and depositional
processes are potentially susceptible to varying oceanic regimes
that may result from global climate change. Submarine gullies are a
ubiquitous feature of continental margin morphology whose detailed
form has only become apparent with the advent of high-resolution
seafloor mapping systems, such as multibeam sonar. The origin and
evolution of these seafloor features remains poorly understood.
Aaron Micallef of the University of Malta and Joshu J. Mountjoy
present evidence of a topographic signature of submarine gully
erosion in the Cook Strait sector of the Hikurangi subduction
margin, New Zealand. This signature indicates that gully initiation
is a threshold process driven by unconfined, directionally stable
fluid or sediment gravity flows accelerating downslope. Micallef
and Mountjoy propose cascading dense water, a type of current that
is driven by seawater density contrast, as the source of these
flows. The sensitivity of such ephemeral hydrodynamic events to
climate change raises questions regarding implications for future
variation of the distribution and magnitude of a significant
seafloor erosion process.
Late Archean euxinic conditions before the rise of
atmospheric oxygen
Clinton T. Scott et al., Dept. of Earth Science, University of
California, Riverside, California 92521, USA. Pages 119-122,
doi:10.1130/G31571.1.
Life on Earth is thought to have coevolved with the chemistry of
the oceans and atmosphere, and the shift from an anoxic to an oxic
world across the Archean-Proterozoic boundary represents a
fundamental step in this process. In order to understand the
relative influence of biological and geological factors on this
transition, scientists must constrain key variables in seawater
chemistry before the Great Oxidation Event (circa 2500 million
years ago). Scott et al. present a multi-element (C-S-Fe-Mo)
biogeochemical study of circa 2662-million-year-old shales from the
Hamersley Province in Western Australia. Data obtained by Clinton
T. Scott of the University of California-Riverside and colleagues
reveal a sustained episode of iron-limited pyrite formation under
an anoxic and sulfidic (euxinic) water column. This is the oldest
known occurrence of euxinia in Earth’s history and challenges the
paradigm of persistently iron-rich Archean oceans. Bulk trace metal
chemistry and preservation of strong mass-independent sulfur
isotope fractionations in sedimentary pyrites indicate that ocean
euxinia was possible prior to oxidative weathering, suggesting that
sulfidic waters may have been common throughout the Achaean. C-S-Fe
systematics suggest that oxygenic photosynthesis was the primary
source of organic carbon in the basin, and the absence of Mo
enrichments highlights a potential link between inefficient
nitrogen fixation and the delayed arrival of the Great Oxidation
Event.
Constraining the terrestrial age of micrometeorites using
their record of the Earth’s magnetic field polarity
Clement Suavet et al., Museo Nazionale dell’Antartide, Universita
di Siena, Via Laterina 8, 53100 Siena, Italy. Pages 123-126,
10.1130/G31655.1.
Micrometeorites acquire a magnetization when they cool down in
the atmosphere. In polar regions, the local magnetic field is
almost vertical, which makes it possible for micrometeorites to
record Earth’s magnetic field polarity at the time of their fall.
Clement Suavet of the University of Siena and colleagues selected
micrometeorites from the Transantarctic Mountains (Antarctica) for
which the direction of fall could be inferred thanks to the
presence of gas vesicles or iron-nickel droplets, and measured
their magnetization. Half of the samples recorded a reverse
polarity, which indicates that they fell on Earth before the last
magnetic field inversion 0.78 million years ago. Suavet et al.’s
results confirm that the Transantarctic Mountain micrometeorite
collection comprises the oldest non-fossil micrometeorites
available. It is the first time that the paleomagnetic record of
flying objects has been retrieved.
Lithospheric shear zones as constant stress
experiments
J.P. Platt and W.M. Behr, Dept. of Earth Sciences, University of
Southern California, Los Angeles, California, 90089-0742, USA.
Pages 127-130, doi:10.1130/G31561.1.
On Earth’s surface, plate boundaries commonly form discrete
faults, such as the San Andreas fault in California. At depths
greater than about 15 km, however, rocks are hot enough to deform
in a ductile fashion, without breaking to form faults. J.P. Platt
and W.M. Behr of the University of Southern California ask what
controls the width of these zones of ductile deformation, which are
known as ductile shear zones, and how deep they penetrate into the
body of the Earth. They propose that the width is related to the
strength of the rocks on either side, as the shear zones form by
deformation and damage of those rocks. This means that at any given
depth, the shear zones form at a constant level of stress, equal to
the strength of the rocks around them. The stress controls the rate
of deformation within them, and hence their width because the shear
zones have to be wide enough to accommodate the relative motion of
the bounding plates. Under these conditions, the process that
causes the shear zones to form is likely to be the reduction in
grain size caused by deformation, which weakens rocks and allows
them to deform more rapidly.
Response of Jakobshavn Isbræ, Greenland, to Holocene
climate change
Nicolas E. Young et al., Dept. of Geological Sciences, University
at Buffalo, Buffalo, New York 14260, USA. Pages 131-134,
10.1130/G31527.1.
Jakobshavn Isbræ, located on west Greenland, is considered
the fastest glacier on Earth, and is the Greenland Ice Sheet’s
single largest contributor to present-day sea-level rise. Young et
al. determine the history of ice margin changes over the past
10,000 years for the Greenland Ice Sheet’s largest outlet glacier.
This is the first time that fluctuations of a Greenland outlet
glacier have been determined at such a fine spatial and temporal
resolution from before the modern satellite-based record. Nicolas
E. Young of the University of Buffalo and colleagues discovered
that Jakobshavn Isbræ responded rapidly to relatively modest
centennial-scale temperature changes – changes similar to those
predicted to occur over the next century. Specifically, between
8,000 and 7,500 years ago, Jakobshavn Isbræ retreated through
its fjord at the rate of 100 m per year, likely in response to
increasing regional and local temperatures. The Jakobshavn
Isbræ margin remained behind its current position for 7,000
years, during a past warm period that was about 2 degrees Celsius
warmer than today. Thus, throughout the past 10,000 years,
Jakobshavn Isbræ underwent large and rapid adjustments in
response to relatively modest Holocene temperature changes, which
may foreshadow the Greenland Ice Sheet response to future
warming.
Magnetic expression of salt diapir-related structures in the
Nordkapp Basin, western Barents Sea
Laurent Gernigon et al., Geological Survey of Norway (NGU),
Geophysics, Leiv Eirikssons vei 39, Trondheim, N-7491, Norway.
Pages 135-138, doi:10.1130/G31431.1.
Salt diapirs are atypical geological features formed when a
thick bed of evaporite minerals intrudes overlying sedimentary
rocks. The importance of salt structures in the understanding of
worldwide sedimentary basins is widely recognized and the knowledge
of such geological structures has considerably increased over the
past 20 years. Recent geophysical investigation of the Nordkapp
Basin, Western Barents Sea, demonstrates the capability of modern,
high-resolution aeromagnetic surveys to provide an efficient and
promising tool for mapping shallow features related to salt
diapirism. Authors Laurent Gernigon of the Geological Survey of
Norway and colleagues point out that salt diapirs are clearly
visible by small, low-amplitude negative round-to-ellipsoidal
magnetic pattern. This pattern coincides with sedimentary layers
deformed by the rising salt during active and passive
diapirism.
Asynchronous Miocene-Pliocene exhumation of the central
Venezuelan Andes
Mauricio A. Bermudez et al., Institut des Sciences de la Terre
CNRS, Universite Joseph Fourier, BP53, 38041 Grenoble, France.
Pages 139-142, doi:10.1130/G31582.1.
The Merida Andes form the highest mountain belt in Venezuela,
with elevations rising to nearly 5000 m at Pico Bolívar, the
nation’s highest point. The mountains were formed by the oblique
collision of the South American and Caribbean plates. Their rise
had a profound impact on river patterns and ecosystems in northern
South America, as they confined the Amazon rainforest to the north
and forced major rivers like the Orinoco to follow an easterly
course. However, the timing of uplift and exhumation of the
Venezuelan Andes have not been precisely constrained before.
Mauricio A. Bermudez of Universite Joseph Fourier, France, and
colleagues present new apatite fission-track data from a profile up
Pico Bolívar that allow for the cooling of rock samples
below about 120 degrees Celsius as they are exhumed toward the
surface. The data show that this region was uplifted and exhumed
between 10-4 million years ago, whereas similar data from the
Sierra La Culata just to the north show much younger rapid uplift
and exhumation. The two massifs are separated by the Bocono fault,
a major continental strike-slip fault comparable to the San Andreas
fault in California. The data presented by Bermudez and colleagues
imply that this fault not only accommodated large-scale lateral
motions of two crustal blocks, but also differential uplift.
Episodic bedrock strath terrace formation due to meander
migration and cutoff
Noah J. Finnegan and William E. Dietrich, Dept. of Earth and
Planetary Science, University of California-Berkeley, 307 McCone
Hall, Berkeley, California 94720, USA, 10.1130/G31716.1.
In many actively incising river canyons, gravel-capped bedrock
terraces form topographic steps adjacent to rivers. Long recognized
as recording the former river bed elevation, bedrock terraces, when
dated, provide a direct means of quantifying rates of vertical
bedrock river incision and are therefore essential to studies of
geomorphology and active tectonics. Although it is widely
hypothesized that bedrock terraces result from changes in tectonic
uplift rates or in climate, little is actually known about
mechanisms of terrace formation. In order to explore terrace
formation processes, Noah J. Finnegan and William E. Dietrich of
the University of California-Berkeley created a physically based
numerical model that couples vertical incision and lateral channel
motion, the latter driven by river meandering. A surprising result
of the modeling is that bedrock terraces in river canyons, widely
argued to form from climatic or tectonic perturbations to rivers,
can be explained entirely from the dynamics of meandering.
Reinterpretation of James Hutton’s historic discovery on the
Isle of Arran as a double unconformity masked by a phreatic
calcrete hardpan
Pierre Jutras et al., Dept. of Geology, Saint Mary’s University,
Halifax, Nova Scotia B3H 3C3, Canada. Pages 147-150,
doi:10.1130/G31490.1.
Because it is partly masked by a phreatic calcrete hardpan
(PCH), a rare and poorly-known type of rock that can transgress
stratigraphic boundaries, there has been ongoing controversy
concerning the exact position of James Hutton’s first discovered
unconformity on the Isle of Arran in southwest Scotland. The
unconformity separates folded Neoproterozoic-to-lower Paleozoic
(Dalradian) metasedimentary rocks from upper Paleozoic redbeds. The
massive PCH developed in Upper Devonian red conglomerate above the
unconformity, but it also assimilated some of the underlying
basement rocks, thus giving the false impression that the
unconformity lies at a lower position, as both host materials are
almost entirely replaced by calcrete. At Hutton’s discovery site,
only a small remnant of the deeply calcretized Upper Devonian
conglomerate was preserved from erosion prior to being
disconformably overlain by Lower Carboniferous red conglomerate and
sandstone. Thus, according to Pierre Jutras of St. Mary’s
University and colleagues, there are two unconformities at Hutton’s
historical site, but the younger has previously gone unnoticed, and
the two redbed successions on each side of the disconformity were
previously thought to belong to the same unit.
Orbital, ice sheet, and possible solar controls on Holocene
moisture trends in the North Atlantic drainage basin
Bryan Shuman and Colin Plank, Dept. of Geology and Geophysics,
University of Wyoming, Dept. 3006, 1000 University Avenue, Laramie,
Wyoming 82071, USA. Pages 151-154, doi:10.1130/G31387.1.
The sensitivity of water resources to past climate changes need
to be understood to provide a context for potential future changes.
This survey by Bryan Shuman and Colin Plank (University of Wyoming)
of the geological evidence for past changes in the water levels in
eastern North America and western Europe shows that recent
centuries were likely wetter than any time in the past 12,000
years. The increase in fresh water probably resulted from slow
changes in Earth’s orbit and the extent of continental ice sheets,
as well as possible changes in solar activity. The changes may have
been important for influencing riverflow into the North Atlantic
Ocean and could in this way have also led to additional climate
changes.
Lithospheric delamination in the core of Pangea: Sm-Nd
insights from the Iberian mantle
Gabriel Gutierrez-Alonso et al., Departamento de Geología,
Universidad de Salamanca, Salamanca 37008, Spain. Pages 155-158,
doi:10.1130/G31468.1.
The forces that continuously shape and reshape Earth’s surface
are vivid evidence of our dynamic planet. However, fundamental
processes also take place in Earth’s interior that are hidden from
direct observation, the effects of which can only be observed if
the overlying crust is removed by uplift and erosion. Recent
evidence has shown that in the late stages of mountain building, a
significant volume of the solid Earth can founder into the
underlying mantle, causing a short-lived, but extensive transfer of
heat from the mantle into the crust. Such a transfer results in
widespread melting and magma generation. Gabriel Gutierrez-Alonso
of the University of Salamanca and colleagues report an example of
this process, which occurred in Western Europe about 300 million
years ago in the core of Earth’s latest supercontinent, Pangea.
Radiogenic isotopes of the rare elements samarium and neodymium
were used, which are sensitive to the removal of old and the
creation of new mantle. Data presented here show mantle replacement
occurred at the same time as buckling of the upper crust during
continental collisions and Pangea formation. Gutierrez-Alonso and
colleagues propose a cause-effect relationship between
mountain-belt bending and mantle replacement, providing a robust
explanation of many previously unexplained features that
characterize Western Europe geology.
Direct U-Pb dating of Cretaceous and Paleocene dinosaur
bones, San Juan Basin, New Mexico
James E. Fassett et al., 552 Los Nidos Drive, Santa Fe, New Mexico
87501, USA. Pages 159-162, doi:10.1130/G31466.1.
Dinosaur fossils are relatively rare throughout the world,
although high concentrations do occur in a few localities.
Moreover, the endemic nature of dinosaurs, in even closely spaced
localities, has hindered the ability of vertebrate paleontologists
to confidently determine the biogeographic diversity, evolution,
and radiation of these animals. These problems have been
exacerbated by the fact that precise age determinations of
dinosaur-bearing rocks have generally not been possible, due to a
lack of precisely dateable rock layers, such as altered volcanic
ash beds, in dinosaur-bone bearing strata. The San Juan Basin
(SWB), of northwest New Mexico and southwest Colorado is one of the
few places where a series of radiometric ages through Upper
Cretaceous strata provides precise age constraints for the abundant
and diverse dinosaur fossils found in these rocks. In addition, the
ages of dinosaurs from Paleocene strata in the SWB have been
tightly bracketed by fossil pollen and paleomagnetic data. James E.
Fassett and colleagues present data that, for the first time,
directly date SWB Cretaceous and Paleocene dinosaur-bone samples
themselves based on laser-ablation, U-Pb methodology. The use of
this new tool to directly date dinosaur-bone fossils may well
revolutionize our understanding of the worldwide evolution and
radiation of these wondrous animals.
Flat latitudinal gradient in Paleocene mammal richness
suggests decoupling of climate and biodiversity
Peter J. Rose et al., Dept. of Earth Sciences, University of
Minnesota, Minneapolis, Minnesota 55455, USA. Pages 163-166,
oi:10.1130/G31099.1.
According to Peter J. Rose of the University of Minnesota and
colleagues, mammal faunas from western North America, during a very
warm climatic interval of Earth’s history about 60 million years
ago, exhibit no predictable change in numbers of species with
latitude. In contrast, mammal faunas in the same region today show
a strong latitudinal gradient in numbers of species. Proxy
indicators of past climate suggest that the latitudinal temperature
gradient during the time interval of interest was similar to the
modern one. The flat species diversity gradient in the past
indicates either different responses to climatic gradients by
faunas dominated by extinct mammals or distinct ecological
processes during the diversification of mammals following a mass
extinction event about 65 million years ago.
Continental aridification and the vanishing of Australia’s
megalakes
Tim J. Cohen et al., Dept. of Environment and Geography, Macquarie
University, Sydney, NSW 2109, Australia. Pages 167-170,
doi:10.1130/G31518.1.
On the Australian continent, humans have been shown to have
arrived between 60 and 51 thousand years ago, coincidental with the
last mass extinction of Australia’s megafauna. The debate
surrounding this extinction has been hampered by the lack of a
climate record for much of the continental interior. Tim J. Cohen
of Macquarie University and colleagues show that Australia’s
megalake system experienced a dramatic change (the biggest recorded
change over 100 thousand years) where Lake Mega-Eyre became
separate from Lake Mega-Frome. Cohen and colleagues demonstrate
that Australia has become increasingly dry since 45 thousand years
ago, with megalakes becoming continually smaller through to the
arid present. The authors suggest varying moisture sources for the
Australian continent over the last glacial cycle.
Assessing climatic and nonclimatic forcing of Pinedale
glaciation and deglaciation in the western United States
Nicolas E. Young et al., Dept. of Geological Sciences, University
at Buffalo, Buffalo, New York 14260, USA. Pages 171-174,
doi:10.1130/G31527.1.
In the western United States, it has been suggested that the
timing of when glaciers achieved, and retreated from, their maximum
positions at the termination of the last ice age (about 20,000
years ago) differs from region to region. These differences in
timing are routinely used to decipher patterns of regional climate
change during the Last Glacial Maximum. Nicolas E. Young of the
University of Buffalo and colleagues reconstruct a history of
glaciation, deglaciation, and the catastrophic draining of an
ice-dammed lake that occurred 19,000 and 17,000 years ago in the
upper Arkansas River basin located in central Colorado. These data,
when combined with previously published histories of glaciation,
indicate that the timing of when glaciers retreated from their
maximum positions at the end of the Last Glacial Maximum is
generally synchronous across the western United States. This study
suggests that the near-synchronous demise of western U.S. glaciers
was the result of the first major Northern Hemisphere warming
beginning 15,000 years ago.
Seismically imaged relict slab from the 55 Ma Siletzia
accretion to the northwest United States
Brandon Schmandt and Eugene Humphreys, Dept. of Geological
Sciences, University of Oregon, Eugene, Oregon 97403, USA. Pages
175-178, doi:10.1130/G31558.1.
Generally, when oceanic and continental plates collide, the
oceanic plate is more dense and gets thrust beneath the continent,
where it sinks deep into Earth’s mantle. This process is referred
to as subduction. New seismic images of the mantle beneath the
northwest United States reveal a large, curtain-shaped body
extending vertically from about 100 km to as much as 600 km depth
beneath an area thought to be just inland of the west coast of
North America prior to about 55 million years ago. The geometry and
seismic properties of this curtain are consistent with that
expected for the ocean plate that was subducting at this time, and
a short-lived volcanic trend lies directly above the imaged
curtain. Brandon Schmandt and Eugene Humphreys of the University of
Oregon suggest that subduction stalled 55 million years ago when an
ocean seamount chain was accreted near the modern coast and that
the crust of the stalled slab was quickly melted, accounting for
the overlying volcanic episode and leaving the remaining slab
curtain sufficiently buoyant to avoid its sinking deep into the
mantle. Other recent research suggests such circumstances may not
be uncommon, thus a more diverse range of subducted slab behavior
needs to be considered.
Kamil Crater (Egypt): Ground truth for small-scale meteorite
impacts on Earth
Luigi Folco et al., Museo Nazionale dell’Antartide, Universita di
Siena, Via Laterina 8, 53100 Siena, Italy. Pages 179-182,
doi:10.1130/G31624.1.
Small impact craters (more than 300 m in diameter) are rare on
Earth and deeply eroded, so that knowledge of their formation
mechanism, and the hazard small impactors constitute to human
populations, is largely based on physical models. Luigi Folco of
the University of Siena and colleagues report on the geophysical
investigation of the Kamil Crater they recently discovered in
southern Egypt. The Kamil Crater is a less-than-5000-year-old
impact crater 45 m in diameter with a pristine rayed structure.
Such well-preserved structures have been previously observed only
on extraterrestrial rocky or icy planetary bodies. This feature and
the association with an iron meteorite impactor and shock
metamorphism provide a unique picture of small-scale hypervelocity
impacts on the Earth’s crust. Contrary to current models, ground
data indicate that iron meteorites with masses on the order of tens
of tons can penetrate the atmosphere without substantial
fragmentation.
Zincian dolomite: A peculiar dedolomitization case?
Maria Boni et al., Dipartimento di Scienze della Terra, Universita
di Napoli “Federico II,” Via Mezzocannone 8, 80134 Naples, Italy.
Pages 183-186, doi:10.1130/G31486.1.
According to Maria Boni and colleagues from the University of
Naples, newly formed zinc-dolomite can be produced during
weathering of zinc sulfide ores. A peculiar dedolomitization
phenomenon is associated with the supergene alteration of Zn-Pb
sulfide ores, resulting in the precipitation of newly formed
carbonate phases. In addition to the deposition of calcite and
several metal carbonates, this phenomenon results in a widespread
replacement of host-rock dolomites by zincian dolomite phases. This
phase may lead to an incorrect evaluation of the metallic resources
contained in the supergene nonsulfide deposits.
Late Holocene freshening of the Baltic Sea derived from
high-resolution strontium isotope analyses of mollusk
shells
Anders Widerlund and Per S. Andersson, Division of Geosciences,
Lulea University of Technology, SE-971 87 Lulea, Sweden. Pages
187-190, doi:10.1130/G31524.1.
Strontium isotopic composition in up-to-7000-year-old mollusk
shells occurring in raised-beach sediments was used by Widerlund
and Andersson to measure postglacial salinity variations of the
brackish Baltic Sea. The salinity data show that the largest
surface-water freshening during the past 3000 years, from ten to
eleven parts per mil down to one to three parts per mil occurred in
Bothnian Bay. The decrease in salinity was caused by rapid
postglacial land uplift restricting the inflow of saline waters
into this northernmost subbasin of the Baltic Sea. The pronounced
freshening of Bothnian Bay is consistent with the absence of a
permanent salinity stratification resulting in well-oxygenated
surface sediments acting as an efficient phosphorus trap in the
present-day Bothnian Bay, where primary production is low and
phosphorus limited. A predicted future increase in precipitation of
10%-20% in the Baltic catchment could lead to accelerated
freshening of the Baltic Sea, resembling what has already occurred
in Bothnian Bay. Studies of environmental changes in Bothnian Bay,
involving historical salinity data, may be crucial to improving our
understanding of the possible effects of any future climate-induced
freshening of the Baltic Sea.
GSA Today Science Article
Alternative perspectives of crustal and upper mantle phenomena
along the Rio Grande rift
Marshall Reiter and Richard M. Chamberlin, New Mexico Bureau of
Geology and Mineral Resources, New Mexico Institute of Mining and
Technology, Socorro, New Mexico 87801, USA. Pages 49 doi:
10.1130/GSATG79AR.
Naturally occurring seismic activity can provide important
insights into Earth’s structure. Here, Marshall Reiter and Richard
M. Chamberlin of the New Mexico Bureau of Geology and Mineral
Resources have used naturally occurring seismic activity to attempt
to gain insights into the Rio Grande Rift, a north-trending zone of
lithospheric extension expressed in the upper crust by a series of
north-south-trending aligned Cenozoic basins that extend more than
1000 km from Colorado through New Mexico into west Texas. The
velocity of measured seismic waves gives insight into the amount of
heat flow in the mantle and lithosphere, itself related to the
tectonic activity of the rift. To gain insight into the nature of
the rift, Reiter and Chamberlin acquired data from La Ristra, a
linear series of seismic velocity measurements taken at
approximately 45 degrees to the orientation of the Rio Grande Rift.
The data reveal that the cold, strong crust and upper mantle of the
Great Plains has resisted extension, as focused in the Rio Grande
rift, while the high-viscosity mantle under the Great Plains tends
to restrain convection. Upward advection occurs in regions of
extension and lithosphere thinning as along the Rio Grande rift.
The oblique orientation of the La Ristra data provides a unique
perspective on the nature and morphology of the Rio Grande Rift
demonstrating, among other qualities, that apparent broadening of
the transition boundary between the Rio Grande rift and the Great
Plains along La Ristra is only a matter of perspective; the
tectonically active Rift and the stable craton to the east remain
sharply demarcated.
