Boulder, CO, USA - GEOLOGY studies include some curious associations: air hockey and plate tectonics; calcite and Earth's orbit; Columbia River Flood Basalts and the Congo Fan; and rock hyrax middens and global climate change. Also in Geology: briny eruptions on Mars; the only basin known to have both a marine and terrestrial P-T boundary; a major Silurian fish die-off; and mapping naturally occurring asbestos in the Sierra Nevada. GSA TODAY illustrates the Klondike goldfields and megafauna of Beringia.
High Cu grades in porphyry Cu deposits and their relationship to emplacement depth of magmatic sources
John M. Proffett, P.O. Box 772066, Eagle River, Alaska 99577, USA. Pages 675-678.
In porphyry copper (Cu) deposits, which produce most of the world's copper, Cu-sulfides are disseminated along small veins and fractures associated with small bodies of porphyritic-textured granitic rock. These are emplaced above larger bodies of granitic magma, which are the sources of both mineralizing fluids and the porphyritic rock. Average copper grades of these deposits are relatively low, but zones of higher grades contained within them are important economically, and for an understanding of their origin. The paper by Proffett identifies two types of higher-grade copper mineralization, one or the other of which may be dominant in different deposits. He shows that the type that is dominant correlates with the depth of emplacement of the underlying magmatic source. Deposits in which most high Cu grades are associated with small, early, granular quartz veins are found above magmatic sources emplaced to depths of about 4 km or less. Those in which most high Cu grades occur in narrow halos of altered rock along early fractures are located above magmatic sources emplaced to depths of about 5 km or more. Fluids with water, salt, and sulfur concentrations typical for magmatic sources would exist as separate brine and vapor phases at pressures corresponding to the shallower depths, and proportions of sulfur occurring as hydrogen sulfide should be relatively low. Such fluid behavior appears to be consistent with characteristics of the quartz vein-related high-grade zones. At greater depths, fluids would exist as one supercritical phase, with a higher proportion of hydrogen sulfide, which appears to be consistent with characteristics of high-grade zones related to alteration halos.
Elevated fluid pressure and extreme mechanical weakness of a plate boundary thrust, Nankai Trough subduction zone
Harold J. Tobin and Demian M. Saffer, Dept. of Geology and Geophysics, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA. Pages 679-682.
Global observations indicate that the faults that lie between tectonic plates slip far more easily than is predicted based on measurements of rock strength. The cause of this mechanical weakness may be a key to understanding earthquake rupture processes and tsunami generation. The presence of trapped, pressurized fluids is one hypothesis widely invoked to explain these "weak faults," yet it has been notoriously difficult to test. Tobin and Saffer have detected such pressurized fluids and quantified their extent, using acoustic images of a plate boundary fault system from a subduction zone under the Pacific Ocean offshore Japan. Their results show that the strength of the plate boundary fault zone remains very low and almost constant for at least 20 km as it slides down into the subduction zone. Like a puck on an air-hockey table, the upper plate glides over the lower one on a cushion of high-pressure pore water trapped in sediments thrust beneath the fault zone.
Mid-Cretaceous seafloor spreading pulse: Fact or fiction?
M. Seton et al., EarthByte Group, School of Geosciences, Madsen Building F09, University of Sydney, NSW 2006, Australia. Pages 687-690.
Profound changes occurred on our planet during the mid-Cretaceous (80-120 million years ago): sea level was higher, continents were inundated, the climate was warmer, ocean basins were oxygen poor, voluminous volcanism occurred, and Earth's magnetic field underwent a period of relative stability. In the past, the cause of these substantial changes was believed to be due to a rapid pulse of seafloor spreading. However, it has also been suggested that new mid-oceanic ridges, created after the breakup of the supercontinent Pangea, were responsible. To resolve this debate, Seton et al. have reconstructed the world's ocean basins and obtained estimates of seafloor spreading rates and mid-oceanic ridge lengths from the Cretaceous to the present day. They have found that these planetary-wide changes were not related to a sudden change in global mid-oceanic ridge length, but rather were related to higher seafloor spreading rates and the changing age-area distribution of oceanic lithosphere.
Salt tectonics and collapse of Hebes Chasma, Valles Marineris, Mars
J.B. Adams et al., Dept. of Earth and Space Sciences, University of Washington, Seattle, Washington 98195, USA. Pages 691-694.
Planetary scientists have debated the origin of the great canyons of Valles Marineris on Mars ever since their discovery. Hypotheses have included erosion by giant floods, tectonic rifting, and collapse. Intense interest has been focused on the huge flood channels that drained some of the canyons, and on the materials that are exposed to depths of over 8 km. At stake in the debate is whether large volumes of water shaped the early history of Mars, and how much of that water might still be trapped below the surface. This study by Adams et al. presents evidence that Hebes Chasma, a 310-km-long canyon that has no surface outlet, was formed by collapse and draining of brines and entrained solids into sink holes. The study was based on scaled laboratory models of Hebes and on satellite images and spectra. The authors conclude that a salt and ice-rich regolith at Hebes dissolved and melted when the region was heated early in the planet's history. They suggest that a similar geologic history may apply to other parts of Valles Marineris. The study supports the conclusions of previous studies that brines derived from hydrous salts and ice moved through a pressurized aquifer and erupted in the giant outflow channels. The emerging picture is that the Martian regolith still may be rich in water.
A lacustrine carbonate record of Holocene seasonality and climate
Chad A. Wittkop et al., Dept. of Chemistry and Geology, Minnesota State University, Mankato, Minnesota 56001, USA. Pages 695-698.
Sediments from a small lake in Michigan contain millimeter-sized layers of organic matter and the mineral calcite, deposited in pairs each year for the past 9000 years. The amount of the mineral calcite in these layers varies through time, and Wittkop et al. show how these variations are linked to long-term changes in the Northern Hemisphere growing season as controlled by dynamics of Earth's orbit around the sun. Short-term variations in calcite abundance also occur on a scale of decades. While the cause of the short-term variations is more difficult to determine, comparison with other climate records from the same region and time period suggest changes in precipitation and temperature may be the cause. These short-term cycles share similarities with other records documenting the pattern of El Niño events over the past 5000 years, which suggest that such events have become more frequent and intense in this time.
Middle Miocene oxygen minimum zone expansion offshore West Africa: Evidence for global cooling precursor events
S. Kender et al., British Geological Survey, Keyworth, Nottingham NG12 5GG, UK. Pages 699-702.
The Middle Miocene Climatic Optimum (16 to 14 million years ago), considered generally to be the warmest period in Earth's history within the past 25 million years, saw a major shift in climate from global warming to cooling. This reversal of climatic warming has proven difficult to understand because of the limited range of available records. The first records of this time interval from the Congo Fan, West Africa, may shed light on the events preceding the major cooling at 14 million years ago. The results of Kender et al. reveal two acidification events in the deep sea off West Africa about 16 and 15.5 million years ago, coincident with massive flood basalt release in the Columbia River basin, USA. The Columbia River Flood Basalts represent the largest group of eruptions to occur on Earth since the Paleogene, over 50 million years ago, and Kender et al. now suggest that they had a significant effect on the climate at this time. The acidification events, which dissolved carbonate organisms on the seafloor, probably resulted from the accumulation of dead organic matter on the seafloor. Stronger offshore winds and increased primary productivity in the surface ocean off the west coast of Africa is likely to account for the accumulation and burial of this organic matter, which ultimately removed CO2 from the atmosphere, leading to long-term cooling. The implications of this study are that Columbia River volcanism likely triggered climatic cooling by sulphuric acid-induced volcanic winters, which would have increased the tradewinds off West Africa and, in turn, fertilized the ocean by transporting more nutrients such as sulphates to the ocean. Large extinctions in deep-sea organisms (benthic foraminifera) associated with this time interval imply that increasing ocean productivity may have dramatic consequences for ocean ecology.
A record of rapid Holocene climate change preserved in hyrax middens from southwestern Africa
B.M. Chase et al., Arid Environmental Systems Research Group, School of Geography and the Environment, University of Oxford, Oxford OX1 3QY, UK. Pages 703-706.
The paper by Chase et al. details the discovery of sensitive indicators of past environmental change in the stratified fecal deposits (known as middens) of rock hyraxes in the Namib Desert of Namibia. The records they have obtained, derived from stable carbon and nitrogen isotope signals, show that hyrax middens have extraordinary potential to preserve high-resolution records of rapid climate change, and that over the last 11,700 years, the Namib Desert has experienced highly variable climates, marked by abrupt shifts in moisture availability. These shifts include events such as the Little Ice Age, the Medieval Warm Period, and the 2700 B.P. event, indicating strong links with atmospheric and oceanic circulation dynamics as well as cycles of orbital and solar forcing. These data are critical to the evaluation of processes and feedbacks within the global climate system, particularly the relative importance of high- vs. low-latitude insolation forcing of tropical systems. Their findings indicate that the long-held hypothesis that the Inter-Tropical Convergence Zone has migrated southward during the Holocene following precessional variations in summer insolation cannot be applied to southern Africa. The implications of this in terms of understanding regional- to global-scale climate dynamics and developing general circulation models that can adequately predict future climate scenarios cannot be overstated.
Decoupled geochemical behavior of As and Cu in hydrothermal systems
Artur P. Deditius et al., Commonwealth Scientific and Industrial Research Organisation (CSIRO), Exploration and Mining, 26 Dick Perry Avenue, Perth, WA 6151, Australia. Pages 707-710.
Deditius et al. describe alternating zones of copper- and arsenic-rich pyrite from gold-deposits at Pueblo Viejo (Dominican Republic) and Yanacocha (Peru). Arsenic-rich zones are enriched in gold, silver, antimony, tellurium, and lead, while copper-rich zones are depleted in these elements. These zones are interpreted to result from invasion of the pyrite-forming fluid by magmatic vapors. The close association of arsenic with gold and silver strongly suggests that these three elements may be transported by hydrothermal vapor separately from copper-rich vapor. Comparison of the analytical data to experimental studies of elemental partitioning, and analyses of fumaroles and fluid inclusions from magmatic-hydrothermal systems, suggests that the arsenic-rich vapor formed at high and possibly magmatic temperatures, whereas the copper-rich vapor formed at lower temperatures, possibly during migration of the original magmatic vapor.
Paradox of late Paleozoic glacioeustasy
Daniel E. Horton and Christopher J. Poulsen, Dept. of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109, USA. Pages 715-718.
The ice age of the late Paleozoic (326-267 million years ago) was a period of dynamic climatic fluctuation, a time period in which southern Pangea saw the waxing and waning of massive continental ice sheets while low-latitude Pangean sedimentary sequences recorded multiple 100-m eustatic fluctuations. This cyclicity is thought to have been driven by changes in Earth's orbit with relation to the sun. Horton and Poulsen employ a coupled ice sheet-atmospheric general circulation model in an attempt to simulate both observed glacioeustatic changes and inferred ice sheet volumes. Their simulations present paradoxical results. While ice volumes of similar size to those implied by proxy evidence are simulated, sea-level changes brought about by cyclic variations in Earth's orbit are minuscule. It is only with large perturbations to atmospheric greenhouse gas concentrations that observed sea-level changes are simulated. Current geological evidence does not support such drastic greenhouse gas fluctuations, leaving the driving factors of dynamic late Paleozoic climate change up for debate.
U-Pb zircon ages from the southwestern Karoo basin, South Africa -- Implications for the Permian-Triassic boundary
Andrea Fildani et al., Chevron Energy Technology Company, 6001 Bollinger Canyon Rd, San Ramon, California 94583, USA. Pages 719-722.
The most extreme biological crisis on Earth, the end-Permian mass extinction, is recorded in terrestrial vertebrate fossil assemblages and paleosol horizons of the spectacularly preserved Karoo Basin, South Africa. By analyzing and dating 205 zircon grains extracted from deep-marine ash beds of the western Karoo (using an high-resolution microprobe [SHRIMP-RG] at Stanford University), Fildani et al. have discovered that the important Permian-Triassic boundary is marine in the southwestern sector of the basin, making the Karoo the only basin known to have a Permian-Triassic boundary identified in both marine and terrestrial sections. This discovery offers the unique opportunity to study, in close vicinity, impacts of the end-Permian crisis across an environmental spectrum.
Geological and archaeological implications of strontium isotope analysis of exposed bedrock in the Chicxulub crater basin, northwestern Yucatan, Mexico
Adrian Gilli et al., Geological Institute, ETH Zurich, Sonneggstrasse 5, 8092 Zurich, Switzerland. Pages 723-726.
The existence of a large buried impact crater beneath the Yucatan Peninsula of Mexico has been known for almost two decades. This crater, named after the town Chicxulub, was formed by a giant meteorite impact about 65 million years ago. The impact may have been responsible for the great extinction event that occurred at the Cretaceous-Paleogene boundary, including the extinction of dinosaurs. Although a several-kilometer-thick cover of sedimentary rocks buries the crater, the outline of the crater is still visible at the surface by a semicircular ring of water-filled sinkhole lakes, known by the Maya as cenotes. Older geologic maps showed no surface expression of the buried crater. Gilli et al. present new strontium isotope (87Sr/86Sr) dates of outcropping limestone, revealing a change in the age of the sediment inside and outside the ring of cenotes, with the youngest sediment deposited inside the ring between about 6 and 2.3 million years ago. The new strontium isotope map of northwestern Yucatan compiled by Gilli et al. shows a terrain of near uniform strontium isotope ratios inside the cenote ring that will offer new possibilities for both geochronology and archaeology. The study also constructed detailed strontium isotope maps around the archaeological sites of Mayapán and Chichén Itzá. These base maps are valuable for documenting local strontium isotope ratios and will be used to unravel the migration history to and from these ancient Maya cities.
Natural and anthropogenic sources of East Asian dust
Gaojun Li et al., State Lab of Mineral Deposits Research, Institute of Surficial Geochemistry, Dept. of Earth Sciences, Nanjing University, Nanjing 210093. Pages 727-730.
Densely populated East Asia is often hit by severe dust storms. It has been suggested that the dust particles are blown from the arid lands of northern China. However, scientists know little about the accurate source regions and the role of anthropogenic activities in the production of these dust particles. Li et al. constrain the origin of dust-storm particles by the isotopic tracers of neodymium and strontium. They collected samples of loess -- a natural eolian deposit of East Asia dust that has accumulated over the past few million years -- spanning northern China.
They analyzed the isotopic composition of the samples and found that the natural background of East Asia dust is dominated by binary sources from the arid lands around the northern boundary of China and the deserts on the northern margin of the Tibetan Plateau. However, the modern spring dust in Beijing is characterized by totally different isotopic compositions, which indicate an additional contribution (about 50%) from the desertified lands in its adjacent north and west, due to anthropogenic activities. The new finding is not only critical in planning long-term strategies for dust control but also is a significant benefit to researches of environmental change-based eolian deposits.
Vertebrate extinctions and reorganizations during the Late Silurian Lau Event
Mats E. Eriksson et al., Dept. of Geology, GeoBiosphere Science Centre, Lund University, Solvegatan 12, SE-223 62 Lund, Sweden. Pages 739-742.
During the Silurian Period (about 443-416 million years ago) fish faunas were struck hard by extinctions caused by the so-called Lau Event. This globally recognized event had significant effects on marine life, oceanic chemistry, and the sea-floor sediments deposited. Eriksson et al. show that the Lau Event had a profound impact on the early evolutionary history of vertebrates, wiping out almost two-thirds of the fish species and causing major ecological reorganizations over an estimated time span of 200,000 years. Immediately prior to the event, the jawed acanthodians dominated the fish faunas, whereas the event led to a diverse fauna and a brief but marked dominance of the jawless thelodonts. Both these groups of fish are now extinct but were common inhabitants in the Silurian seas. The stepwise changes observed among the fish faunas resemble those of conodonts, another extinct group of vertebrates, suggesting a similar mode of life and response to the Lau Event.
Super-heavy pyrite (34Spyr > 34SCAS) in the terminal Proterozoic Nama Group, southern Namibia: A consequence of low seawater sulfate at the dawn of animal life
Justin B. Ries et al., Dept. of Marine Sciences, 333 Chapman Hall, CB# 3300, University of North Carolina-Chapel Hill, Chapel Hill, North Carolina 27599, USA. Pages 743-746.
Sulfur isotope ratios (34S/32S) in marine limestones tell us a great deal about sulfur and oxygen cycles in the ocean at the time that those limestones were deposited. Sulfur isotopes in marine limestones are recorded in two ways: (1) as carbonate-associated-sulfate, or sulfate that is substituted for the carbonate ion in calcium carbonate; and (2) as pyrite. Carbonate-associated sulfate is an oxidized form of sulfur and is interpreted as a proxy for sulfate dissolved in seawater at the time of limestone deposition, while pyrite is a reduced form of sulfur that is used as a proxy for sulfide in sedimentary porewaters at the time of limestone deposition. The primary way that sulfate is reduced to sulfide in seawater is through bacteria, a process known as bacterial sulfate reduction. Because bacteria are more efficient at converting isotopically lighter sulfate (32SO4) to sulfide, sulfide -- and the pyrite that it forms -- tends to be lighter, on average, than the original sulfate. Additionally, experiments have shown that the more sulfate there is in seawater, the greater the isotopic offset between the sulfate and sulfide will be. In modern seawater, which has relatively high sulfate concentrations compared with the geologic past, the offset will typically be between 20 and 50 parts per thousand. As sulfate concentrations in seawater decline, the isotopic offset between sulfate and sulfide declines toward zero. Geologists use the amount of this offset, as recorded in carbonate-associated sulfate and pyrite in marine limestones, to reconstruct sulfate levels in the ancient ocean and, therefore, oxygen levels in the ancient atmosphere (since seawater sulfate is formed primarily from the oxidation of sulfides on land). Ries et al present a very intriguing record of sulfur isotopes from carbonate-associated sulfate and pyrite within a well-preserved, 10 million year limestone sequence from the Precambrian-Cambrian boundary in southwest Namibia. Surprisingly, this 10 million year sequence of limestones contains sulfur isotopes in pyrite that are consistently heavier than the sulfur isotopes in co-deposited sulfate -- which is the inverse of the normally observed trend resulting from bacterial sulfate reduction. This is the first documented occurrence of sulfide that is consistently isotopically heavier than coeval sulfate, and challenges our entire paradigm for interpreting the geologic record of sulfur isotopes. Ries et al. interpret this inverted isotopic offset as evidence for extremely low sulfate concentrations at the Precambrian-Cambrian boundary, most likely driven by very low levels of atmospheric oxygen. Apparently, these low levels of atmospheric oxygen persisted tens of millions of years beyond the origin of animals -- much later than previously reported -- but probably rose prior to the major diversification of animals in Early Cambrian time.
Pore-fluid Fe isotopes reflect the extent of benthic Fe redox recycling: Evidence from continental shelf and deep-sea sediments
W.B. Homoky et al., National Oceanography Centre, University of Southampton, Southampton SO14 3ZH, UK. Pages 751-754.
New research by Homoky et al. highlights the potential utility of iron isotopes for addressing important questions in ocean and Earth science. Different biological and chemical processes can leave behind characteristic isotopic "fingerprints." Of specific interest here, iron isotopes in sediment pore fluids may be a unique tracer of bacterial processes that contribute to the input of iron to seawater; an important process contributing to natural carbon sequestration in the oceans. The research shows that iron cycling in the deep sea produces a different iron isotopic fingerprint in the pore fluids compared with previous studies on the coastal shelves. This study confirms the unique isotopic fingerprint of pore-fluid iron in coastal shelf settings, highlighting the potential for future research to use iron isotopes as a tracer of shelf-derived iron inputs to seawater. The findings will also aid future interpretations of ancient ocean iron cycling processes in the rock record.
Predicting delta avulsions: Implications for coastal wetland restoration
D.A. Edmonds et al., The Pennsylvania State University, Dept. of Geosciences, University Park, Pennsylvania 16802, USA. Pages 759-762.
Avulsions are the natural process by which flow is diverted out of an established river course and onto the adjacent floodplain. Avulsions are significant natural hazards because they produce extensive flooding as water is diverted over the floodplain. In other areas, like river deltas, avulsions are the natural process that constructs wetlands. Currently there is considerable interest in wetland restoration, especially in the Mississippi deltaic plain, where rapid disappearance of wetlands may have exacerbated damage caused by Hurricanes Katrina and Rita. Until now, the timing and location of avulsions has been notoriously difficult to predict. Using scaled-physical experiments of deltas at the Exxon Mobil Research Lab, Edmonds et al. determined that the avulsion location can be predicted by finding the location on the levee that has experienced the greatest stress for the longest period of time. For five different experimental deltas, they were able to predict the avulsion location with 93% accuracy. These results represent the first mechanistic understanding of avulsions that allows for accurate prediction of their location.
Mapping potentially asbestos-bearing rocks using imaging spectroscopy
G.A. Swayze et al., U.S. Geological Survey, Denver, Colorado 80225, USA. Pages 763-766.
Rock and soil that may contain naturally occurring asbestos (NOA), a known human carcinogen, were mapped in the Sierra Nevada, California, using an airborne imaging spectrometer, to determine if these materials could be uniquely identified with reflectance spectroscopy. Such information can be used to prepare or refine maps of areas that may contain minerals that can be asbestiform, such as serpentine and amphiboles, which were the focus of the study by Swayze et al. Although thick vegetation can conceal underlying rock and soil, use of modeled spectra of dry grass and serpentine allowed detection of serpentine in some parts of the study area with up to about 80% dry-grass cover. Chaparral vegetation, which was dominantly, but not exclusively, found in areas underlain by serpentinized ultramafic rocks, was also mapped. Practical applications of this technology for mapping areas that may contain NOA include locating roads that are surfaced with serpentine aggregate, identifying sites that may require enhanced dust control or other safety measures, and filling gaps in geologic mapping where field access is limited.
GEOLOGY abstracts are available for review at http://geology.gsapubs.org/content/current.
GSA Today Science Article
The Klondike goldfields and Pleistocene environments of Beringia
Duane Froese et al., Dept. of Earth and Atmospheric Sciences, Univ. of Alberta, Edmonton, Alberta, T6G 2E3, Canada
Declining sea level during the last great glacial event resulted in the exposure of a continuous land bridge extending from Siberia in the west to Yukon in the east. Lying in the rain shadow of the St. Elias Mountains, this great region, now known as Beringia, was cold but too dry for the development of glaciers. Instead, the region consisted of a great Arctic steppe, bound by mountains to the south and continental glaciers to the north, west, and east. This vast plain was a veritable Serengeti, host to megafauna like the steppe bison, woolly mammoth, Yukon horse, western camel, American mastodon, American lion, the short-faced bear and the helmeted musk-ox. But how did this remote Arctic region support such a diverse array of grazing megafauna? This is the question addressed by Duane Froese of the University of Alberta and his international team of colleagues. They focus on the Klondike region of Yukon, one of North America's most productive localities for the recovery of late Pleistocene mammal fossils. By using numerous volcanic ash beds that characterize the fossil-rich strata to determine the age and correlations of disparate rock layers, they have been able to demonstrate that, unlike today's Arctic tundra, Beringia consisted of a well-drained, vegetated steppe that was continuously replenished by wind-blown loess. Having now developed a well-constrained stratigraphic record of Beringia, Froese and his team hope to further resolve questions concerning the relationships between mammals, glacial vegetation, and Pleistocene climate.
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