Boulder, CO, USA - Topics include: imaging of impact strata on Mars' Holden Crater; Eocene-Oligocene conditions leading to the freezing of Antarctica; evidence that liquid water is not responsible for recent changes observed in Martian gullies; new insights into Cretaceous early Albian greenhouse conditions and implications for future atmospheric carbon dioxide levels; new estimates of Earth's copper supply; discovery of large nitrate pools under the Mojave's desert pavement; frequency of magnitude 9 earthquakes; and the pre-Grand Canyon Colorado River.
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HiRISE imaging of impact megabreccia and sub-meter aqueous strata in Holden Crater, Mars
John A. Grant et al., Center for Earth and Planetary Studies, National Air and Space, Museum, Smithsonian Institution, 6th at Independence SW, Washington, DC, 20560, USA. Pages 195-198.
Grant et al. use images of Holden crater from the High Resolution Imaging Science Experiment (HiRISE) on Mars Reconnaissance Orbiter (MRO) to resolve, for the first time, megabreccia deposits in the walls that were created during the formation of the crater. These impact deposits contain enormous blocks that are up to 50 meters across and are unconformably overlain by water-lain sediments deposited during two ancient phases (during the Noachian Era) of aqueous activity. A lighter-toned and typically finely bedded lower unit exhibiting clay minerals was probably deposited in a long-lived lake setting. An overlying darker-toned and often blocky upper unit drapes the lighter-toned deposits. The upper unit was emplaced during later high-magnitude flooding as a lake, impounded outside of Holden and within adjacent Uzboi Vallis, overtopped the crater rim, draining into the crater. The stratigraphy exposed in Holden crater provides the first geologic context for deposition of clays during persistent wet and perhaps habitable conditions on early Mars.
Identifying tsunami deposits using bivalve shell taphonomy
S.V. Donato et al., McMaster University, School of Geography and Earth Science, Hamilton, Ontario L8P 4A9, Canada. Pages 199-202.
On 28 November 1945, off the coast of what is now Pakistan, a large earthquake (8.1 Mw) produced a destructive tsunami. Despite news reports chronicling the tsunami in Pakistan, Iran, and India, no records describing the magnitude of the event exist for Oman. Donato et al.'s study documents the characteristics of a shell bed deposited by the 1945 tsunami in Sur Lagoon, Oman, and describes a new technique for identifying tsunami deposits in the geologic record using shell preservation. This shell bed provides the first physical evidence that the large tsunami impacted this region of Oman.
Rapid warming and salinity changes of Cretaceous surface waters in the subtropical North Atlantic
Thomas Wagner et al., School of Civil Engineering and Geosciences, Devonshire Building, Newcastle University, Newcastle upon Tyne NE1 7RU, UK. Pages 203-206.
Paleo records reveal occasional and rapid releases of greenhouse gases into the atmosphere. The regional consequences on the surface ocean processes and timing between the atmospheric changes and ocean changes, however, are still far from understood. Wagner et al. examined an exceptionally well-preserved section of the early Albian oceanic anoxic event 1b (OAE) (112 million years ago) occurring in the warm Cretaceous greenhouse period. By studying one distinct and relatively short climate perturbation in the geological past, Wagner et al. aimed to better understand the mechanism and feedbacks of short climate perturbations. Atmospheric carbon dioxide concentrations in the Cretaceous early Albian period were reconstructed to have ranged from 500 to 3000 parts per million. The consensus of climate scientists is that a 200 percent increase of modern levels of atmospheric carbon dioxide may occur by 2100 or later, placing future levels of atmospheric carbon dioxide within the range of early Albian greenhouse conditions.
From slow to ultraslow: A previously undetected event at the Southwest Indian Ridge at ca. 24 Ma
Heather Sloan et al., Environmental, Geographic and Geological Sciences, Lehman College, CUNY, 250 Bedford Park Boulevard West, Bronx, New York 10468, USA. Pages 207-210.
Using a newly compiled set of published and unpublished magnetic data, Sloan et al. reconstruct plate motion at the Southwest Indian Ridge. Findings indicate a 50 percent drop in spreading rate at circa 24 million years ago. This shift from slow to ultraslow spreading had remained undetected because it was not accompanied by a change in spreading direction which would have been observed in fracture zone trends. This change in relative motion appears to have been transmitted to two of the four adjoining plate boundaries. Occurring during a period of significant change in relative and absolute plate motion, the rate of change at the Southwest Indian Ridge is interpreted as part of a global event along the plate boundary system.
Recent bright gully deposits on Mars: Wet or dry flow?
Jon D. Pelletier et al., University of Arizona, Department of Geosciences, 1040 E. Fourth Street, Tucson, Arizona 85721, USA. Pages 211-214.
Flow deposits laid down on Mars within the last few years look like they were laid down by flowing water. In this paper, Pelletier et al. test this hypothesis by computer modeling of water-based versus dry landsliding. Dry landslides can "fluidize" under certain conditions and appear as though they were laid down by water. Model results show that the observed deposit on Mars was most consistent with a dry landsliding process. This has important implications in the debate about the presence/absence of liquid water on the surface of Mars today.
Nanometer-scale complexity, growth, and diagenesis in desert varnish
Laurence Garvie et al., Center for Meteorite Studies, School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287, USA. Pages 215-218.
Finely layered coatings, rich in manganese and iron and commonly called desert varnish, are common on rocks in desert environments worldwide. These coatings have been the subject of intense scientific debate and extensive research, owing to their potential for indicating past climates, for dating geological surfaces, and, via artwork carved in varnish, for providing information about ancient cultures. The full scientific potential of desert varnish can only be realized through a rigorous probing of the physico-chemical variables and fundamental properties of varnish components, especially its mineralogical components. Determining the mineralogy of the manganese- and iron-bearing materials is challenging because the minerals are extremely fine grained, generally down to nanometer-sized, and often poorly crystalline. In addition, the thin film-like nature of varnish on rock makes separating and studying it difficult. Garvie et al. used novel sample preparation methods, high-resolution electron microscopy, and spectroscopic imaging to provide novel insights into desert varnish structure, mineralogy, and chemistry. The spectroscopic imaging shows nanometer-scale separation of manganese- and iron-bearing phases, possibly reflecting differing degrees of chemical oxidation. A suite of late-grown manganese and iron phases commonly occur also, together with sparse barium and strontium sulfates, and rare, entrained, carbonaceous particles. These data demonstrate that varnish remains a mineralogically and structurally active system. They furthermore suggest that there must be strict climate controls on varnish growth.
Forearc diamond from Japan
Simon Wallis et al., Department of Earth and Planetary Sciences, Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8602, Japan. Pages 219-222.
Most diamonds are brought to the Earth's surface in explosive volcanic eruptions in geologically old continental regions. Volcanism in convergent margins is generally thought to be too shallow in origin to be a host for diamond. Micro-diamonds found in xenoliths within a lamprophyre dike in southwest Japan show this assumption is incorrect, and, furthermore, that diamonds occur in a wider range of geological settings than previously realized. This may help explain the origin of some known anomalous diamond deposits. Studies of associated minerals show the newly recognized diamond-bearing rocks rose from depths of around 160 km and cooled from temperatures of ~1500 C. This result implies that mantle flow in convergent plate boundaries occurs on a larger scale that previously recognized.
First exposure ages from the Amundsen Sea Embayment, West Antarctica: The Late Quaternary context for recent thinning of Pine Island, Smith, and Pope Glaciers
Joanne Johnson et al., British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK. Pages 223-226.
Dramatic acceleration and thinning of major ice streams in the Amundsen Sea sector of the West Antarctic Ice Sheet have been observed during the last two decades, and raise concerns that a major retreat of the West Antarctic Ice Sheet is in progress. Until now, however, there has been little evidence that would allow a comparison of recent rates of change with those that have arisen naturally since the end of the last glacial period. Johnson et al. present the first "surface exposure ages" from rock outcrops in the Amundsen Sea region of the West Antarctic Ice Sheet. The ages indicate the mean rates of thinning of Pine Island, Smith, and Pope Glaciers since 14.5 thousand years ago. The average rate of thinning varies from 2.3 to 3.8 centimeters per year; this is an order of magnitude slower than the average rate of thinning of Pine Island Glacier (1.6 meters per year) seen in satellite altimetry data between 1992 and 1996. The new data also record a general trend of progressive thinning throughout the past 14.5 thousand years, suggesting that currently observed changes are not simply a long-delayed response to conditions at the end of the glacial period. Further data are required to determine whether there have been short-lived rapid thinning events since the end of the last glacial period, but the data presented in this paper provide the first long-term evidence that current rates of ice sheet thinning in this part of the West Antarctic Ice Sheet may be unusually rapid.
Upheaval Dome, Utah, USA: Impact origin confirmed
Elmar Buchner, Institut für Geologie, Universität Stuttgart, Herdweg 51, 70174 Stuttgart, Germany; Thomas Kenkmann, Humboldt-Universität zu Berlin, Museum für Naturkunde, Institut für Mineralogie, Berlin D-10115, Germany. Pages 227-230.
The origin of Upheaval Dome in Utah has been discussed for decades; it has been interpreted as a crypto volcanic feature, a salt diapir, a pinched-off salt diapir, and as an eroded impact crater. Buchner and Kenkmann present new and unambiguous evidence for the impact origin of Upheaval Dome.
Molybdenum isotope evidence for global ocean anoxia coupled with perturbations to the carbon cycle during the Early Jurassic
Anthony Cohen et al., Department of Earth and Environmental Sciences, The Open University, Milton Keynes MK7 6AA, UK. Pages 231-234.
What happens to the Earth during and after abrupt global warming, over time scales of hundreds or thousands of years? One way of addressing this question is to use computer models to try to predict the course of future climate and environmental conditions. However, the uncertainty of longer-term predictions may be substantial because of our relatively poor understanding of the great complexity of the Earth's behavior. Adopting a complementary approach in their study, Pearce et al. examine and de-code the geological record of extreme, but infrequent, events that occurred in the distant past. These records have the potential to provide quantifiable information about precisely how Earth has actually responded to severe environmental change in the longer term. Pearce et al. show that there was widespread reduction in the oxygen content of the oceans during an abrupt period of global warming 183 million years ago in the Early Jurassic period, and that these conditions persisted for approximately 200,000 years. The authors also demonstrate that the changes in seawater oxygenation at that time were periodic and were coupled with regular, large-scale fluctuations in the global carbon cycle. The precise relationships between the various expressions of this environmental crisis in the Early Jurassic, which also involved a significant mass extinction of marine and terrestrial species, may be able to provide valuable constraints that could help to validate our predictions about environmental change in the future.
The nature of shallow-water carbonate lithofacies thickness distributions
Peter Burgess, Shell International E&P, EPT-RXF, PO Box 60, Rijswijk, Rijswijk 2280 AB, Netherlands. Pages 235-238.
Carbonate lithofacies thickness distributions are of fundamental importance to understanding shallow-water carbonate deposystems because they record evidence of the lateral distribution and migration of lithofacies elements, as well as evidence for the various other intrinsic and extrinsic controls on stratal geometries and accumulation rates. Previous analyses of lithofacies thickness data led to the suggestion that exponential distributions are ubiquitous in the ancient record. This has been interpreted to indicate deposition by stochastic Poisson process lithofacies mosaics. To further investigate these ideas, Burgess performed a statistical analysis of 56 outcrop and core examples. The Kolmogorov-Smirnov test was used to identify the degree to which measured lithofacies thicknesses are well represented by a theoretical exponential distribution. Results from this analysis show that 16 of the 56 examples can be confidently shown to be exponential, while 28 are probably not exponential. This indicates that stochastic Poisson processes are a plausible explanation for many carbonate successions, but they do not explain all of those tested here, suggesting that other non-Poisson processes, either stochastic or deterministic in nature, or both, must also be important. Thus lithofacies planform geometries, and the processes controlling vertical stacking in ancient carbonate platform top deposystems, were likely more diverse than has been suggested, requiring significant further quantitative analysis and numerical forward modeling.
Creation of a continent recorded in zircon zoning
Desmond Moser et al., Biology and Geological Sciences Building, Room 1070, Department of Earth Sciences, University of Western Ontario, London, Ontario N6A 5B7, Canada. Pages 239-242.
Moser et al. discovered ancient microcrystals, about as wide as a human hair, that grew over a period of 200 million years. The approximately 3-billion-year-old crystals of the mineral zircon were found in a metamorphosed bedrock outcrop of originally sedimentary rock in northern Ontario, Canada, where the lower crust has been pushed to the surface from depths of 30 kilometers. Like trees in old growth forests, the crystals were found to have roughly circular growth zones that Moser et al. were able to date and analyze with specialized ion probes. From core to rim, the zones track the formation of the early North American continent from its beginning as a series of volcanic island chains, to its eventual fusion into a large, thick continental plate. While crystals of this age were previously known, the longevity of the crystal growth that has been detected is truly remarkable and holds the promise that other crystal micro-records tracking planet evolution exist on Earth and rocky planets including Mars.
Tectonic burial and "young" (<10 Ma) exhumation in the southern Apennines fold-and-thrust belt (Italy)
S. Mazzoli et al., Dipartimento di Scienze della Terra, Università di Napoli 'Federico II', Largo San Marcellino 10, 80138 Napoli (NA), Italy. Pages 243-246.
Mazzoli et al. examine the recent tectonic evolution of the southern Apennines, a key Italian area in terms of geological hazards, as well as for petroleum exploration and production. Integrated structural and thermochronological data indicate that, over the past one to five million years, the mountain belt underwent gravitational collapse. This process involved the shallow part (upper 5 kilometers) of the orogen, producing rock exhumation in the axial zone of the chain. Large horizontal extension dismembering the mountain belt was accompanied by forward (eastward) motion of thrust sheets. Linked low-angle extensional faults and frontal thrusts shared a common, shallow detachment. Below the detachment, coeval crustal shortening, though limited (less than 15 percent), led to the growth of large subsurface structures that form the hydrocarbon traps for the significant oil discoveries in the area. These subsurface structures also created gravity disequilibria in the overlying tectonic wedge, triggering further gravitational readjustments. Subsequently, as shortening ceased even at depth, crustal extension became dominant (from the Middle Pleistocene to the present).
Neogene extension and basin deepening in the West Antarctic rift inferred from comparisons with the East African rift and other analogs
Wesley LeMasurier, Institute of Arctic and Alpine Research, University of Colorado at Boulder, 1560 30th St., 450 UCB, Boulder, Colorado 80309-0450, USA. Pages 247-250.
The West Antarctic rift is a region of volcanic activity and crustal stretching that is roughly the size of the western United States (from Salt Lake City to the Pacific Ocean). About 98 percent of it is buried beneath glacial ice, up to 2.5 miles thick, and bedrock beneath the ice is 2000-3000 feet below sea level over large areas. All of this makes it a difficult region to study. It is interesting nevertheless, because volcanic eruptions beneath the ice could destabilize the ice sheet, leading to as much as 25 feet of sea-level rise. How likely is it that this could happen is a question scientists have debated for over a decade. LeMasurier addresses the question by comparing the West Antarctic rift with similar areas of crustal stretching elsewhere in the world. The comparison shows that volcanic activity in rifts is most common where the land is a mile or more above sea level, and rising, which can readily be seen in Antarctica along the Transantarctic Mountains, and in the Pacific coast mountains of Marie Byrd Land. The large sub-sea-level interior of the rift does not, therefore, seem to be a likely place for present-day volcanic activity. This is good news, because the sub-sea-level base of the West Antarctic ice sheet is already especially vulnerable to warming of the atmosphere and surrounding seas. However, this study also shows that the land in West Antarctica has been rising beneath the ice sheet in some areas and subsiding beneath it in others, over roughly the past 25 million years. Some areas have subsided to as much as 8500 feet below sea level. This tectonic restlessness contrasts markedly with the stability of the regions that lay beneath the northern hemisphere ice sheets of the recent geologic past, and its affect on the history of the West Antarctic ice sheet has not yet been evaluated.
Cooling and ice growth across the Eocene-Oligocene transition
Caroline Lear et al., School of Earth, Ocean and Planetary Sciences, Main Building, Park Place, Cardiff CF10 3YE, UK. Pages 251-254.
One of the most important climatic changes in Earth's history happened at the Eocene-Oligocene boundary (approximately 34 million years ago) when a thick ice sheet grew in East Antarctica. Previous temperature records found no evidence of ocean cooling at this time, which presented a confusing picture of the climate system. Lear et al. present new temperature records from the geochemistry of preserved fossils that show, for the first time, that the oceans did cool (by about 2.5 °C) across the Eocene-Oligocene climate transition. These new records help resolve a long-standing puzzle regarding the extent of ice sheet growth versus global cooling, and bring the climate proxy records into line with the climate model predictions.
Earth's copper resources estimated from tectonic diffusion of porphyry copper deposits
Stephen Kesler, Department of Geology, University of Michigan, Ann Arbor, Michigan 48109, USA; Bruce Wilkinson, Department of Earth Sciences, 222 Heroy Geology Laboratory, Syracuse University, Syracuse, New York 13244-1179, USA. Pages 255-258.
Kesler and Wilkinson provide the first estimate of the total copper resources in Earth's crust. The estimate is derived from a computational model that generates mineral deposits with the same ages as those at Earth's surface. For a likely depth of 3.3 kilometers for future mining, the calculation indicates that Earth's copper deposits can supply about 5,500 years of copper at present consumption rates.
Large near-surface nitrate pools in soils capped by desert pavement in the Mojave Desert, California
Robert Graham et al., University of California-Riverside, Environmental Sciences, Soil & Water Sciences Program, Riverside, California 92521-0424, USA. Pages 259-262.
Graham et al. present exceptionally high nitrate levels (up to 12,750 kilograms per hectare) that have been found at shallow depths (less than 1 meter) in soils mantled by desert pavement, a common land surface feature in arid regions. Nearby soils without desert pavement had nitrate contents that were one to two orders of magnitude lower. The soil conditions coincident with desert pavement (i.e., stability, antiquity, and virtually no leaching) favor the retention and accumulation of nitrate delivered by atmospheric deposition or fixed in situ. The nitrate stored in soils under desert pavement is a previously unrecognized pool of nitrogen, with the potential to increase the global nitrogen inventory for near-surface desert soils to five times the previous estimates. Its near-surface occurrence makes this labile nitrogen pool particularly susceptible to mobilization by climate change or human disturbance, risking contamination of surface and ground waters.
Global frequency of magnitude 9 earthquakes
Robert McCaffrey, GNS Science, EVT, 1 Fairway Drive, PO Box 30368, Lower Hutt, New Zealand. Pages 263-266.
McCaffrey reports on the expected frequency of great earthquakes of the type that struck Sumatra in 2004. Even though we have experienced four or five in the past 50 years, the expected number is one to three per 100 years. The recent past has seen a clustering of these improbable events that may have misled scientists in the quest for the causes of massive earthquakes.
Trans-Alaska Crustal Transect and continental evolution involving subduction underplating and synchronous foreland thrusting
Gary S. Fuis et al., M/S 977, U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, California 94025, USA. Pages 267-270.
There is perhaps no better place on Earth to study the origin of continents than Alaska. Alaska is composed of fragments of crust that were transported northward on a plate-tectonic conveyor belt and scraped off against the North American continent as the conveyor belt (oceanic plates) descended beneath it. These fragments included volcanic islands and plateaus that formed on the oceanic plates, parts of the continental shelf of North America, and even parts of the oceanic plates. During the last several hundred million years of Alaskan history, oceanic plates slid beneath North America in subduction zones, generating volcanoes inland above places where the descending plates dehydrated and generated melts in overlying mantle and crust. At least twice, fragments of the oceanic plates became doubled up, and, in attempting to subduct, uplifted and compressed the Alaskan crust landward of the subduction zone. One instance was 40-60 million years ago when doubled oceanic plates uplifted and compressed the Pacific coastal region of Alaska. This compression was felt as far away as northern Alaska and Canada, where the Brooks Range and Canadian Rockies were uplifted. The second instance was more recent, beginning only a few million years ago, when a piece of abandoned oceanic plate, known as the Yakutat terrane was underthrust by the Pacific plate and, together with the Pacific plate, entered the subduction zone. According to Fuis et al.'s and other studies, it appears that the western half of this terrane is being subducted atop the Pacific plate at a very shallow angle, and this doubled thickness of oceanic crust has shut off volcanoes in the central part of Alaska that would normally accompany subduction. The eastern half of the terrane is too buoyant to subduct and is being offscraped beneath southern Alaska, beneath the St. Elias Range, leading to some of the highest uplift rates in the world. The effects of this offscraping are being felt as far inland as the Brooks Range and Canadian Rockies - again. North of the St. Elias Range, the Pacific plate, freed of its piggyback Yakutat terrane, is sinking (or subducting) at a steep angle, and has torn away from the part of the Pacific plate to the west that is subducting at a shallow angle with the overlying Yakutat terrane. The Great Alaskan earthquake, M 9.2, of 1964, nucleated near this tear and ruptured southwestward along the upper interface of the subducting Yakutat terrane.
GSA TODAY Science Article
The mystery of the pre-Grand Canyon Colorado River - Results from the Muddy Creek Formation
Joel L. Pederson, Department of Geology, Utah State University, Logan, Utah 84322-4505, USA.
The Colorado River's integration off the Colorado Plateau remains a classic mystery in geology, despite its pivotal role in cutting Grand Canyon and in the region's landscape evolution. The upper paleodrainage apparently reached the southern plateau during Miocene time (between 23 and five million years ago), and recent work supports the longstanding idea that the river was superimposed over the Kaibab uplift by this time. Once off the plateau, the lower river integrated with the Gulf of California by downstream basin spillover starting about five or six million years ago. An unknown link remains: the history of the river in the Grand Canyon region in Miocene time. One of the viable hypotheses put forward by previous workers - that the Muddy Creek Formation represents the terminal deposits of the paleo-Colorado River in the Basin and Range northwest of Grand Canyon - is tested in this paper. Results indicate instead that local drainages and the paleo-Virgin River are the likely sources of this sediment. The remaining hypothesis - that the paleo-upper Colorado River dissipated and infiltrated in the central-western Grand Canyon area - has modern analogs, provides a potential source for extensive Miocene spring and evaporite deposits adjacent to the southwestern plateau, and implies a groundwater-driven mechanism for capture of the upper drainage.
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