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September Geology and GSA Today media highlights

Geological Society of America

Boulder, CO, USA - GEOLOGY topics reach deep into Earth and far into space--from magma and plate tectonics to cosmic dust and asteroids--and touch on the intricate details of our planet, including a 1200-year record of corals and coral reef health and the wealth of climate change information found in both bat guano and Chinese loess. The GSA TODAY article studies the end-Permian mass extinction in the marine realm, examining long-term environmental stress and recovery.

Ups and downs of the Mississippi Delta

Michael Blum et al., Dept. of Geology and Geophysics, E235 Howe-Russell Geosciences Complex, Louisiana State University, Baton Rouge, Louisiana 70803, USA. Pages 675-678. NSF funding received.

Blum et al.'s recent research suggests the Mississippi delta has a complex history of vertical motion that reflects the response of Earth's crust to large-scale erosion during the ice age and deposition during more recent times. Erosion of the lower Mississippi valley and delta region during the last ice age removed a large mass of sediment--this would have resulted in large-scale uplift of the delta region and adjacent Gulf of Mexico shoreline--whereas deposition during the last few thousand years has resulted in replacement of that mass and in large-scale subsidence. The subsidence part of this cycle is ongoing and contributes to the modern-day land surface subsidence problem in the delta region. Recognition of this cyclical pattern of uplift and subsidence has a number of implications for understanding coastal evolution and sea-level change, as well as the evolution of large delta systems elsewhere.

Mantle upwelling, magmatic differentiation, and the meaning of axial depth at fast-spreading ridges

Douglas Toomey and Emilie Hooft, 1272 Geological Sciences, University of Oregon, Eugene, Oregon 97403-1272, USA. Pages 679-682. NSF funding received.

Since the first systematic mapping of the fast-spreading East Pacific Rise, changes in its depth below sea level have been used to infer regional-scale variations in magma supply or mantle temperature. Toomey and Hooft, however, show that segment-scale, rise-parallel undulations of both on- and off-axis seafloor depth result primarily from variations in the bulk density of oceanic crust. Seismic images of crustal and upper mantle structure are combined with gravity data and lava chemistry to demonstrate that variations in crustal density are caused by magmatic differentiation. This study concludes that the origin of rise-parallel changes in magmatic differentiation and seafloor depth is a newly discovered skew between the axes of mantle upwelling and plate spreading. The depth of fast-spreading ridges is thus controlled primarily by the geometry of mantle upwelling.

Stable carbon and hydrogen isotopes from bat guano in the Grand Canyon, USA, reveal Younger Dryas and 8.2 ka events

Christopher Wurster et al., School of Geography and Geosciences, University of St Andrews, Irvine Building, St Andrews, Fife KY16 9AL, Scotland, UK. Pages 683-686.

Free-tailed bats roost in huge populations in some caves in the southwestern United States. Over thousands of years, they collectively have left deep deposits of guano, once highly valued as fertilizer. These deposits also contain a record of past climate and plant life. Stable hydrogen and carbon isotope values of guano reflect local precipitation patterns and the relative amounts of succulents, such as cacti and warm-weather grasses, compared with trees, shrubs, and cool-weather grasses. A bat guano deposit from the Grand Canyon tells the story of the North American monsoon during times when global temperatures shifted rapidly. During the "globally" cool Younger Dryas, the Grand Canyon was cooler and possibly dry. However, contemporary climate patterns took another couple of millennia to set as the monsoon gradually gained strength and became established about 9000 years ago. The record also suggests a second "global" cool phase 8200 years ago that began and ended abruptly. Although packrat midden archives have provided much information on climate in arid regions such as the Grand Canyon, depositional archives are exceedingly rare. Rapid climate changes are difficult to discern without a continuous record, and Wurster et al.'s study shows that guano deposits can provide a wealth of such information, with many like deposits occurring throughout tropical and subtropical regions.

Koronis asteroid dust within Antarctic ice

Matthew Genge, Dept. of Earth Science and Engineering, Imperial College London, Exhibition Road, London SW7 2AZ, UK; Pages 687-690.

Cosmic dust is everywhere--on the streets, in our homes, even on our clothes. Each one of us is never more than a few feet from a dust particle from space. Despite their abundance, however, it is difficult to know exactly from where in space these particles come. Genge reports the discovery that a large proportion of cosmic dust particles found in Antarctic ice, known as micrometeorites, are from the Koronis asteroids of the main asteroid belt between Mars and Jupiter. The source of these dust particles was determined by observations of their mineralogy and chemistry (which closely match a type of meteorite known as ordinary chondrites) and from calculations that show that matching abundances of dust particles are delivered to Earth from Koronis. The Koronis asteroids are a family of rocky bodies formed approximately 2 billion years ago when a much larger asteroid broke into pieces. The most famous of these asteroids is 243 Ida, and its tiny moon Dactyl, which was imaged by the Galileo probe. Genge's work suggests that many micrometeorites are actually tiny pieces of about 20 smaller asteroids known as the Karin asteroid family, within the Koronis asteroid family, and for the first time confirm that these stony asteroids are similar to ordinary chondrites, the most common meteorites to fall on Earth.

1200 year paleoecological record of coral community development from the terrigenous inner shelf of the Great Barrier Reef

Chris Perry et al., Dept. of Environmental and Geographical Sciences, Manchester Metropolitan University, John Dalton Building, Chester Street, Manchester M1 5GD, UK. Pages 691-694.

Perry et al.'s study presents a long-term (approximately 1200 year) record of reef accretion and coral community composition from a turbid-zone, nearshore reef on the inner shelf of the Great Barrier Reef, Australia, and examines long-term interactions between coral communities and terrigenous sediment accumulation. This has direct relevance to contemporary issues of marine water quality and land-use change because increased terrestrial sediment and nutrient yields, linked to changes in catchment land use practices, are regarded as significant threats to coral reef health. In the central Great Barrier Reef lagoon, water quality has reportedly declined since European settlement (since approximately A.D. 1850), and inner-shelf reef conditions have purportedly deteriorated. However, the link between reef decline and water quality change remains controversial, primarily because of a lack of ecological baseline data, encompassing the pre- and post-European interval, against which contemporary ecological states can be assessed. Perry et al.'s long-term records of coral community composition from Paluma Shoals reef demonstrate the potential for coral communities to initiate and persist in settings dominated by fine-grained terrigenous sediment accumulation. Furthermore, reef-building has been dominated by a temporally persistent (but low-diversity) suite of corals for at least the past millennium, and the coral assemblages exhibit no evidence of community shifts attributable to post-European water quality changes at this site. While extrapolation of these findings to other turbid-zone and inner-shelf reefs must remain tentative, the study raises important questions about the resilience of inner-shelf reefs that have been under terrestrial sediment influence and subject to elevated turbidity conditions.

Cyrenaican "shock absorber" and associated inversion strain shadow in the collision zone of northeast Africa

William Bosworth et al., Apache Egypt Companies, 11, Street 281, New Maadi, Cairo, Egypt. Pages 695-698.

The Mediterranean Sea is closing; Europe is moving south to collide with Africa. This process began more than 80 million years ago and will take millions of years to complete. Bosworth et al. describe some of the geologic events that occurred during the beginning of this closure and how they impacted the development of sedimentary basins in northeast Africa, which affected how oil and gas were formed and trapped in the prolific Sirt basin of Libya and the Western Desert of Egypt.

The Pangea conundrum

Brendan Murphy and R. Damian Nance, Dept. of Earth Sciences, St. Francis Xavier University, P.O. Box 5000, Antigonish, Nova Scotia, B2G 2W5, Canada. Pages 703-706.

The existence of the supercontinent Pangea, which formed about 300 million years ago and broke up about 200 million years ago, is a cornerstone of plate tectonics, and processes resulting in its assembly and fragmentation have governed the evolution of Earth's crust for 500 million years. Over the past 20 years, evidence has been amassing that Pangea is just the latest in a series of supercontinents that formed repeatedly since the Archean, only to break up and reform again. Although the mechanisms responsible are controversial, many geoscientists agree that repeated cycles of supercontinent amalgamation and dispersal have had a profound effect on the evolution of Earth's crust, atmosphere, climate, and life. The geological record for the past one billion years is sufficiently well documented that we have a first-order picture of the changing positions of continents. Using these reconstructions in combination with other data, Murphy and Nance show that supercontinents form by different mechanisms and that many current geodynamic models cannot explain the processes that led to the amalgamation of Pangea. These models suggest that plate tectonics is primarily driven by subduction and that supercontinents break up and migrate from sites of mantle upwelling to reassemble at sites of mantle downwelling where subduction zones exist. Such models would predict that the young oceans created by the breakup of a supercontinent some 600 million years ago would have continued to expand as the continental fragments migrated toward sites of mantle downwelling that existed in the older ancestral Pacific Ocean. Instead, Pangea assembled as a result of the closure of the young oceans. The geologic record suggests that there are geodynamic linkages between the younger and older oceans that deserve more detailed study; it also suggests that, in the case of Pangea, the reversal in continental motion may have coincided with emergence of a superplume 460? million years ago that produced mantle upwelling in the ancestral Pacific. If so, the top-down geodynamics driven by subduction, which accounts for the assembly of the supercontinent that broke up 600 million years ago, may have been overpowered by bottom-up geodynamics involving large-scale mantle upwelling that led to the amalgamation of Pangea.

Stepwise extinction of larger foraminifers at the Cenomanian-Turonian boundary: A shallow-water perspective on nutrient fluctuations during Oceanic Anoxic Event 2 (Bonarelli Event)

Mariano Parente et al., Dipartimento di Scienze della Terra, Universita "Federico II" Napoli, Largo San Marcellino 10, 80138 Napoli, Italy. Pages 715-718.

The middle Cretaceous was a period of extreme climate changes and repeated perturbations of the global carbon cycle, which resulted in widespread deposition of organic carbon-rich sediments. High atmospheric pCO2 has been invoked as the main cause of middle Cretaceous super greenhouse Oceanic Anoxic Events (OAEs) and biocalcification crises. Most of what we know about climate and global change in the middle Cretaceous was derived from the study of geochemical proxies and biotic change in deep-water sequences. Comparatively much less is known from shallow-water carbonate systems, although they are an important part of the global carbon cycle and are very sensitive to oceanographic and climatic forcing. Larger foraminifers were among the most conspicuous carbonate producers in the late Cenomanian carbonate platforms. They were almost completely wiped out by extinction during the late Cenomanian OAE (about 94-93.5 million years ago). Parente et al. show how the pattern of extinction parallels the ecological succession along a gradient of increasing nutrient input predicted by model studies and observed in field studies of nutrient-polluted habitats of larger foraminifers. Correlation by high-resolution carbon-isotope stratigraphy shows that extinction on shallow-water platforms is contemporaneous with an episode of thermal instability in the open ocean. They suggest that enhanced water mass mixing delivered high-nutrient loads to the oligotrophic surface waters of the subtropical Tethys, triggering the stepwise extinction of larger foraminifers and the drowning of many carbonate platforms.

Anhydrite-bearing andesite and dacite as a source for sulfur in magmatic-hydrothermal mineral deposits

Isabelle Chambefort et al., CODES, ARC Centre of Excellence in Ore Deposits, University of Tasmania, Private Bag 79, Hobart, TAS 7001, Australia. Pages 719-722. NSF funding received.

Magmatic anhydrite from andesites and dacites occurs as inclusions in high- and low-aluminum amphibole and pyroxene, and indicates that sulfate-saturated magmas spanned a period of six million years at Yanacocha, Peru. Magmatic anhydrite from Yanacocha and other sites is characterized by light rare earth element-enriched patterns and elevated strontium contents distinct from magmatic-hydrothermal anhydrite. Petrologic arguments suggest that the hydrous and oxidized Yanacocha magmas contained more than approximately 1000 ppm sulfur, both dissolved in the melt and as a separate sulfate phase, which is sufficient to provide all the sulfur for the genetically related giant sulfur-rich Yanacocha epithermal gold deposits. High-aluminum amphiboles contain unusual anhydrite with wormy and amoeboidal textures, which are tentatively interpreted to represent trapping of an immiscible CaSO4-water melt, together with sulfur-rich apatite at a temperature of ~950 degrees Celsius and a water pressure greater than 3 kbar. Chambefort et al. claim that such unusually sulfate-rich magmas may be required to produce sulfur-rich magmatic-hydrothermal mineral deposits.

Particle size separation and evidence for pedogenesis in samples from the Chinese Loess Plateau spanning the past 22 m.y.

Frank Oldfield et al., Dept. of Geography, University of Liverpool, Liverpool L69 7ZT, UK. Pages 727-730.

The deep accumulations of wind-blown silt (loess) that form the Loess Plateau of China provide a record of past climate changes spanning the past 22 million years. They therefore comprise one of the longest continuous terrestrial archives of environmental change anywhere in the world. The record is encoded in alternating bands of loess blown from the dry lands of the continental interior and buried soils formed during periods when warmer, moister climatic conditions prevailed. Decoding the signatures of past changes involves a mix of techniques, including particle-size analysis and magnetic measurements. These yield evidence of the climate at each stage in accumulation. Oldfield et al. provide a much more reliable basis for isolating the products of weathering and soil formation and analyzing their magnetic and geochemical properties. In so doing, they point the way to shedding new light on the environmental history of a key region in the evolution of global climate.

Long recurrence interval of faulting beyond the 2005 Kashmir earthquake around the northwestern margin of the Indo-Asian collision zone

Hisao Kondo, AIST, Active Fault Research Center, Site7, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567, Japan. Pages 731-734.

Kondo's successful paleoseismic trench survey in Pakistan reveals that the 2005 Kashmir earthquake occurred on the relatively minor intra-plate active fault, not on the plate boundary mega-thrust of the Indo-Asian collision zone, in contrast with the damage and loss of the earthquake.

Origin of microbiological zoning in groundwater flows

Craig Bethke, Dept. of Geology, 1301 West Green St, Urbana, Illinois 61801, USA. Pages 739-742.

Freshwater aquifers contain large numbers of various types of bacteria and other microbes that control the chemistry and composition of the water in the aquifer. Microbes, for example, degrade pollution into harmless chemicals and dictate whether natural groundwater contains undesirable levels of iron and arsenic. Rather than being distributed evenly through the subsurface, microbial communities in an aquifer are commonly segmented into zones that appear to be dominated by a single type of microbe, and scientists have been studying the origin of this phenomenon. Two working explanations, one based on thermodynamics and the other on chemical kinetics, have been proposed, but neither has proved fully satisfactory. Bethke's study shows that by integrating principles of population dynamics with the equations of thermodynamics and kinetics, the microbial zoning can be explained as an ecological effect in which one group of microbes maintains conditions under which other groups die more quickly than they can reproduce.

Tourmaline B-isotopes fingerprint marine evaporites as the source of high-salinity ore fluids in iron oxide copper-gold deposits, Carajas Mineral Province (Brazil)

Roberto Xavier et al., Instituto de Geosciencias, Universidade Estadual de Campinas, Rue Pandia Calogeras, Campinas, Sao Paolo 13083-970, Brazil. Pages 743-746.

Xavier et al.'s paper describes the first application of tourmaline boron isotope data to iron oxide copper-gold deposits, using as a case study three world-class deposits of the Carajas Mineral Province, northern Brazil. The data essentially indicate that marine evaporites, and not magmatic fluids, are the source of boron and high salinity in these deposits.

Biogeochemical controls on photic-zone euxinia during the end-Permian mass extinction

Katja Meyer et al., Dept. of Geosciences, 801 Deike Building, Pennsylvania State University, University Park, Pennsylvania 16802, USA. Pages 667-670. NSF funding received.

The largest Phanerozoic mass extinction marks the end of the Permian period and coincides with evidence of environmental change in the oceans and on land. Multiple lines of evidence suggest the oceans may have become anoxic and sulfidic, and previous work hypothesized that hydrogen sulfide release to the atmosphere could link the marine and terrestrial crises. Meyer et al. used an earth-system model to investigate the conditions necessary for hydrogen sulfide buildup and release to the atmosphere. They found that greater than three-fold increases in oceanic phosphate, a key nutrient derived from rock weathering, leads to hydrogen sulfide accumulation in the deep ocean and in upwelling zones of the surface ocean. Unique hydrogen sulfide-consuming bacteria thrive in sulfidic photic zones today, consuming hydrogen sulfide that upwells into the photic zone. Accounting for the presence of these organisms in the surface ocean reduces the amount of hydrogen sulfide released to the atmosphere but does not prevent hydrogen sulfide eruption from the ocean. The end-Permian geologic record is consistent with the modeled distribution of hydrogen sulfide, supporting the notion that hydrogen sulfide and carbon dioxide toxicity may have provided the kill mechanism for this mass extinction.

GSA Today Science Article

Understanding mechanisms for the end-Permian mass extinction and the protracted Early Triassic aftermath and recovery

David J. Bottjer, Dept. of Earth Sciences, University of Southern California, Los Angeles, California 90089-0740, USA. Pages 4-10.

The Permo-Triassic (P-T) extinction is generally recognized as the largest mass extinction in Earth's history, with estimates ranging upwards of 90% species extinction. Identifying a cause for this extinction has proven elusive, as historically various agents from sea level fall, to volcanism, to global oceanic anoxia, to deadly oceanic burps of H2S have been hypothesized. New paleobiological studies on the environmental distribution and ecological importance of brachiopods, benthic molluscs, and bryozoans across the P-T boundary support the hypothesis that stressful ocean conditions, primarily elevated H2S levels (euxinia), but also heightened CO2 concentrations, were the prime causes of the end-Permian mass extinction. These studies also further demonstrate that both the Late Permian interval preceding and the Early Triassic interval following this mass extinction were times of similarly aggravated environmental stress. In the low-diversity, long term Early Triassic recovery, huge numbers of benthic mollusks typically covered the seafloor. That a relatively few marine molluscs thrived during this time indicates a tolerance for some combination of marine anoxia, elevated CO2, and/or increased H2S concentrations. The survival of organisms with such attributes is likely a reflection of the P-T extinction cause.


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