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Find Your Science at GSA
3 December 2009
GSA Release No. 09-67
Contact:
Christa Stratton
Director - GSA Communications & Marketing
+1-303-357-1093
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Geology and GSA Today

December 2009 Highlights

Boulder, Colorado, USA - Hot topics from this month's Geology include the nature of Martian sediments and minerals, microorganisms that thrive and survive for thousands of years in extremely salty environments, river flood dynamics, relative sea-level change in the UK, volcanism, geothermal systems, atmospheric carbon dioxide and extinction, anthropogenic atmospheric carbon dioxide and ocean acidification, Chicxulub impact ejecta, and more. GSA Today celebrates the 200th anniversary of Darwin's birthday with an analysis of "Darwin's Boulders" in Tierra del Fuego.

Highlights are provided below. Representatives of the media may obtain complementary copies of GEOLOGY articles by contacting Christa Stratton at the address above. View abstracts for this issue of GEOLOGY at http://geology.gsapubs.org/.

GSA TODAY articles are open access. Access the GSA TODAY science article by clicking on the issue cover icon at http://www.geosociety.org/pubs/.

Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOLOGY or GSA TODAY in articles published. Contact Christa Stratton for additional information or assistance.

Non-media requests for articles may be directed to GSA Sales and Service, .

GEOLOGY

How do prokaryotes survive in fluid inclusions in halite for 30 k.y.?
Brian A. Schubert et al., Dept. of Geology and Geophysics, University of Hawai'i at Manoa, Honolulu, Hawaii 96822, USA. Pages 1059-1062.

Microorganism communities (prokaryotes and eukaryotes) that thrive in extremely salty water (about 8-10 times the salt content of seawater) exist within fluid inclusions in ancient salt crystals (22,000-34,000 years in age) from Death Valley, California, USA. Some of the prokaryotes from these crystals were cultured in the laboratory, indicating that certain microbes can survive for tens of thousands of years trapped in salt. Schubert et al. hypothesize that these extreme survivors are able to endure for millennia by slowing their metabolism and living off nutrients released by eukaryotes, notably dead algal cells that are trapped together with the prokaryotes inside the ancient salt. Support for their hypothesis includes: (1) prokaryotes cultured from ancient salt crystals were capable of growth using glycerol, a carbon compound produced abundantly by the algal cells; and (2) just one algal cell is calculated to provide enough glycerol for one prokaryote cell to survive for 12 million years. Results from this work will guide future searches for microbial life in chloride and sulfate evaporite deposits in more ancient deposits on Earth, and possibly on Mars.

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Sensitive high-resolution ion microprobe analysis of zircon reequilibrated by late magmatic fluids in a hybridized pluton
Monika A. Kusiak et al., Polish Academy of Sciences, Institute of Geological Sciences, Krakow-Warszawa, Poland. Pages 1063-1066.

The uranium-lead and thorium-lead isotopic systems are used for dating rocks with complex thermal histories, because these elements commonly reside in minerals, such as zircon, that are chemically robust, and that can grow episodically in response to multiple geological events that affect the host rock. In a study by Kusiak et al., developments in instrumentation have made it possible to measure the ages of growth stages on the scale of tens of micrometers. Zircon from a mafic enclave within post-collisional granite is characterized by clustered and skeletal morphologies and internal re-equilibration textures. Thorium-rich zircon grains were corroded internally and subsequently replaced by porous aggregates of zircon and other minerals. Corrosion and replacement are different mechanisms that produced different effects; the former involves alteration that enriches zircon in calcium and light rare earth elements, the latter regenerates altered zircon, ejecting these elements along with thorium, uranium, and lead. This allows both original and replaced zircon to be dated. Original growth occurred with the mingling of mafic magma into a cooler granitic melt; reequilibration was caused by late magmatic fluids several millions of years after the crystallization of the main granite.

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Do hyperpycnal-flow deposits record river-flood dynamics?
Michael P. Lamb and David Mohrig, Division of Geological and Planetary Sciences, California Institute of Technology, MC 170-25, 1200 E. California Blvd., Pasadena, California 91125, USA. Pages 1067-1070.

Large river floods can produce dense plumes of sediment-laden water where they enter the ocean. Pervious workers have suggested that the marine deposits of these plumes provide a robust sedimentary record of river dynamics (and potentially river response to climate change, tectonics, and other forcings) because of an assumed direct link between the river discharge and the offshore plume velocity. Lamb and Mohrig present a numerical simulation that reveals that transitional zones between the river and the offshore plume can break this link. The transitional zones themselves are dynamic and shift landward and seaward during the course of a river flood, which can result in the unintuitive finding that plume velocities decrease when river discharge increases. Sedimentary deposits of dense plumes might record the local complexities of shifting transitional zones rather than river discharge.

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Global dispersal of Pb by large-volume silicic eruptions in the Paraná-Etendeka large igneous province
David W. Peate, Dept. of Geoscience, University of Iowa, 121 Trowbridge Hall, Iowa City, Iowa 52242, USA. Pages 1071-1074.

Large igneous provinces are major episodic events in Earth's history in which huge volumes of magma (millions of cubic kilometers) are erupted in a short time interval (less than a few million years). Assessing their global environmental impact requires demonstration of synchronicity between the volcanism and sedimentary records of environmental change. Valanginian sediments at Ocean Drilling Project Site 1149B that span a delta-13C excursion have lead (Pb)-isotope compositions that suggest input of volcanic Pb from the Paraná-Etendeka large igneous province. Re-evaluation of Pb isotope data on Paraná-Etendeka lavas by Peate shows that this Pb came from large-volume explosive silicic eruptions rather than from degassing of aerosols from the effusive basaltic lavas. Although basaltic magmatism dominates the Paraná-Etendeka province, there is no evidence for Pb associated with basalt eruptions in the Site 1149B sediments on the other side of the globe, suggesting that eruption columns produced by these basaltic eruptions did not reach the stratosphere. The widespread global dispersal of Pb in this case is apparently linked to the ability of large-volume (greater than 1000 km3) silicic eruptions to loft aerosols into the upper stratosphere.

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Kinematic vorticity analysis and evolving strength of mylonitic shear zones: New data and numerical results
Scott E. Johnson et al., Dept. of Earth Sciences, University of Maine, Orono, Maine 04469, USA. Pages 1075-1078.

Estimating kinematic flow parameters in deformed rocks is of first-order importance to the structure and tectonics community, allowing them to test hypotheses in relation to such topics as lower-crustal channel flow and exhumation of ultrahigh-pressure rocks. The tools available for extracting kinematic parameters from deformed rocks are few, the most widely used being the microstructural determination of kinematic vorticity number. However, microstructural vorticity gauges commonly give a wide range of values from samples collected in close proximity to one another, and different methods can give markedly different values when applied to the same rock. Johnson et al.'s data and numerical experiments show that clast rotation methods for estimating kinematic vorticity are compromised by strain localization at the clast interfaces, and they show that localization increases with modal matrix mica content. They also show that lubrication of clast interfaces can have an important and previously unexplored effect on the bulk shear strength of mylonitic shear zones.

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Quantifying postglacial sediment storage at the mountain-belt scale
Ralph K. Straumann and Oliver Korup, GIS Division, Dept. of Geography, University of Zurich, CH-8057 Zurich, Switzerland. Pages 1079-1082.

Substantial advances in geological dating techniques and digital topographic modeling have greatly increased our knowledge about rates of erosion and sediment yield in mountain belts. However, quantifying long-term sediment storage, which is the key term linking these rates in any sediment budget, has remained a mystery in this regard, and thus largely absent from many state-of-the-art numerical models of landscape evolution. This is a major shortcoming, as formerly glaciated mountain belts such as the European Alps store large bodies of sediment deposited in lakes and deeply cut valleys. Here, Straumann and Korup present a new method for objectively extracting and quantifying such storage at the mountain-belt scale from a digital elevation models. Their results compare well with manually mapped areas of sediment storage, allowing unprecedented insights on not only the pattern of sediment storage with respect to topography, uplift, and precipitation, but also helping distinguish domains of bedrock, mixed bedrock-alluvial, and alluvial rivers. Comparison with sediment storage in more active mountain belts indicates that the extent and the spatial pattern of sediment storage are likely controlled by the rates of rock uplift and precipitation, rather than by differences in rock type or land cover.

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Compaction of Holocene strata and the implications for relative sea-level change on the east coast of England
Benjamin P. Horton and Ian Shennan, Dept. of Earth and Environmental Science, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA. Pages 1083-1086.

Horton and Shennan have employed a novel approach, using a quality-controlled database of sea-level observations over a 10,000-year period, to provide improved, quantitative estimates of subsidence along the east coast of England. Their findings indicate that glacial rebound explains regional-scale differences in relative sea levels. Current sea levels in north east England, the most northerly study area, have been receding to their present level for the past 4000 years, whereas the Tees Estuary, Humber Estuary, Lincolnshire Marshes, Fenlands, and north Norfolk area each reveal sea-level histories of an upward trend during the past 10,000 years. Researchers found that sediment compaction explained the variation in sea-level observations at every study area, revealing striking correlations between elevation residuals and the thickness of overlying sediment. Coastal subsidence causes sea-level rise, shoreline erosion, and threatens to permanently submerge socio-economically and environmentally valuable wetlands. Yet the causes of subsidence remain controversial, and estimates of subsidence rates vary widely. This collaborative study offers insight into the future behavior of these environmental systems and is an effort to inform policy and management decisions for coastal protection.

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Martian magmas contained abundant chlorine, but little water
Justin Filiberto and Allan H. Treiman, Lunar and Planetary Institute, Universities Space Research Association, 3600 Bay Area Blvd, Houston, Texas 77058, USA. Pages 1087-1090.

The martian water cycle, a key for Mars habitability, largely relies on the abundance of water in the mantle and magmas of Mars. Therefore, it is important to understand how much water was delivered to the surface during volcanic eruptions. Here, Filiberto and Treiman combine data from martian meteorites (bulk and mineral chemistry) and experimental work to show that martian magmas did not have very much water; instead they had abundant chlorine. Eruption and degassing of such chlorine-rich lavas would have increased the acidity of the surface of Mars and contributed only small amounts of water to Mars' surface, thereby limiting the availability of magmatic water for habitability.

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Impact-generated hydrothermal systems capable of forming phyllosilicates on Noachian Mars
Susanne P. Schwenzer and David A. Kring, Lunar and Planetary Institute, Universities Space Research Association (USRA), 3600 Bay Area Blvd., Houston, Texas 77058, USA. Pages 1091-1094.

The Mars Express spacecraft and Mars Reconnaissance Orbiter spacecraft made a surprising discovery: The ancient highlands of Mars contain abundant water-bearing minerals. Schwenzer and Kring demonstrate that those water-bearing minerals may have been produced during an early period of impact bombardment that generated vast subsurface hydrothermal systems. Those systems were long-lived and ubiquitous on Mars approximately 4 billion years ago. The same intense period of impact bombardment affected the Earth-Moon system and has previously been implicated in the early evolution of life on our planet. For that reason, ancient impact craters on Mars, which are better preserved than those on Earth, will be fascinating places to visit with future spacecraft landers.

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Oxygen isotope precipitation anomaly in the North Atlantic region during the 8.2 ka event
David Domínguez-Villar et al., School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston, B15 2TT Birmingham, UK. Pages 1095-1098.

For many years, an extreme cold and dry climate anomaly 8200 years ago has been identified as the most extreme in the last 11,000 years. The event was triggered by the sudden drainage of mega-lakes formed from large glaciers in North America into the North Atlantic Ocean. A new record from a northern Spain speleothem has revealed that the event produced a distinctive hydrological signal with two distinct pulses. The stalagmite record uses oxygen isotopes. After the mega-lake outburst, their particular oxygen isotope signature was transferred from the ocean surface to rainfall, and recorded in the continents. Domínguez-Villar et al. report that, contrary to previous research, climate anomalies in the Northern Hemisphere were not particularly significant, and the large oxygen isotope anomalies are due to the lake outbursts. This research has dated the two known outbursts with excellent precision, and also highlights the importance of changes in the ocean isotope composition on decadal time scales and its imprint on continental records.

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The effects of flank collapses on volcano plumbing systems
Andrea Manconi et al., GFZ German Research Centre for Geoscience, Telegrafenberg, 14473 Potsdam, Germany. Pages 1099-1102.

The growth of large volcanoes is commonly interrupted by episodes of flank collapse. While most of the attention devoted to this topic has focused on their great destructive potential, flank collapses likely have important repercussions on the internal magma plumbing systems of the volcanoes they affect. Field observations and petrological data on volcanic rocks of El Hierro Island, Canary Archipelago, suggest that after a large Pleistocene flank collapse (the El Golfo landslide), the erupted products are characterized by higher proportions of denser, crystal-rich and less-evolved magmas. Manconi et al.'s numerical simulations indicate that an El Golfo-sized landslide would have generated substantial pressure gradients down to upper mantle depths beneath El Hierro, providing an explanation for the observed changes in magmatism. Indeed, at El Hierro and other similar volcanoes, landslide-induced disturbances in the state of stress of a magmatic system at depth are likely to affect storage, transport, mixing, differentiation, and degassing of magma batches. This work thus presents a model that accounts for changes in the eruptive and geochemical regimes, until now unexplained, at volcanoes that have experienced large-scale flank collapse.

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Metals in deep liquid of the Reykjanes geothermal system, southwest Iceland: Implications for the composition of seafloor black smoker fluids
V. Hardardottir et al., Earth Sciences, University of Ottawa, Ottawa, K1N 6N5 Ontario, Canada. Pages 1103-1106.

The Reykjanes geothermal system is located where the Mid-Atlantic volcanic ridge comes on land in Iceland, within 1 km of the Atlantic Ocean. In a study by Hardardottir et al., a specially designed titanium sampler was lowered into three wells to 1350-1500 meters depth with a temperature of 284 to 295 degrees Celsius, where high pressures prevent boiling. These samples of deep seawater-dominated hydrothermal liquid, collected prior to boiling, are the first of this kind, and indicate much higher metal concentrations than those in hot fluids sampled at the surface of Reykjanes. The low metal concentrations in the surficial samples are due to boiling during ascent, causing precipitation of metals. The concentrations of copper, zinc, and lead in the deep liquid samples are similar to those in the highest-temperature black smoker discharges, sampled on the ocean floor, whereas gold and silver concentrations are 10 to 100 times higher at Reykjanes. This indicates that gold and silver may be depositing below the ocean-floor black smokers due to boiling.

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Salt as a fluid driver, and basement as a metal source, for stratiform sediment-hosted copper deposits
Lyudmyla Koziy et al., ARC Centre of Excellence in Ore Deposits, University of Tasmania, Hobart, Tasmania 7001, Australia. Pages 1107-1110.

In the Zambian Copper Belt, one of the principal copper provinces in the world, the volume of clastic rocks, generally considered to be a source of copper, is insufficient to account for the copper contained there. The only viable alternative is the metal-rich crystalline basement, which is not easy penetrable for fluids. Numerical modeling by Koziy et al. suggests that the solution to the problem may be associated with halite-containing evaporites ubiquitous in sediment-hosted copper districts. The model demonstrates that dense brine plumes generated beneath the salt seal can cross the basin-basement interface and penetrate to a great depth. Buoyancy-induced convection is an efficient mechanism driving saline fluid into the metal reservoir and then focusing metal-enriched fluid to the sites where mineralization can be expected.

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A new approach to risk assessment of lava flow at Mount Etna
Massimiliano Favalli et al., Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Pisa, Via della Faggiola, 32, 56126 Pisa, Italy. Pages 1111-1114.

Destruction of human property by lava flow invasion is a significant volcanic hazard at Mount Etna (Italy), where reliable risk maps are important for risk mitigation. Favalli et al. present novel high-resolution quantitative risk maps of Mount Etna which are based on lava flow simulations starting from more than 70,000 different vents, a probability distribution of vent opening, a law for the maximum length of lava flow, and a database of buildings. Standard risk maps classify areas according to the expected damage at each point. Reversing the perspective, Favalli et al. classify each point of the volcano according to either the expected damage produced once a vent opens at that point or to the damage expected in a given time interval for a vent opening at that point. The resulting maps should help local authorities in making the correct decisions to deal with ongoing eruptions and to plan long-term land use.

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Spatial variability of late Holocene and 20th century sea-level rise along the Atlantic Coast of the United States
Simon E. Engelhart et al., Dept. of Earth and Environmental Science, University of Pennsylvania, 240 South 33rd Street, Philadelphia, Pennsylvania 19104, USA. Pages 1115-1118.

Engelhart et al. have shown that land is subsiding along the Atlantic Coast of the United States as the planet continues to respond to the huge weight of ice sheets during the last glacial period. Research found significant spatial variations in land movement, with the mid-Atlantic coastlines of New Jersey, Delaware, and Maryland subsiding twice as much as areas to the north and south. Coastal subsidence enhances sea-level rise, which leads to shoreline erosion, loss of wetlands, and threatens coastal populations. Values of coastal subsidence show that the 20th century rate of relative sea-level rise is two millimeters higher than the background rate of the past 4,000 years. Furthermore, the magnitude of the sea-level rise increases from Maine to South Carolina. This is the first demonstrated evidence of this phenomenon from observational data alone. Researchers believe this may be related to either the melting of the Greenland Ice Sheet, and/or ocean steric effects.

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Imaging the source region of Cascadia tremor and intermediate-depth earthquakes
Geoffrey A. Abers et al., Lamont-Doherty Earth Observatory of Columbia University, Rte 9W, P.O. Box 1000, Palisades, New York 10964, USA. Pages 1119-1122.

Regular earthquakes rupture faults in a few seconds or less, and are found in many tectonic environments. Some of the most inexplicable are those within subducting plates, at pressures where normal frictional behavior seems unlikely but where metamorphic reactions release water. This release may sufficiently weaken faults. Recently, a second class of slow-to-silent earthquakes has been discovered in subduction zones, generating a phenomenon termed Episodic Tremor and Slip (ETS). In a study by Abers et al., to better understand how these two classes of earthquakes could occur, a 62-element seismograph array was deployed in the Cascadia subduction zone of Washington to record seismic signals, and to use these signals to image subsurface structures. These data provide detailed images of the subducting plate that are for the first time co-registered with recordings of both earthquake types. The images show that ETS is confined to a region where high fluid pressures seem likely, trapped at the plate interface at depths less than 45 km. Regular earthquakes nucleate always below the plate interface and continue to greater depths, near the places where metamorphic fluids are released. Thus, fluid release and high fluid pressures produce both kinds of earthquakes, but in different ways.

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Controls on carbonate skeletal mineralogy: Global CO2 evolution and mass extinctions
Andrey Yu. Zhuravlev and Rachel A. Wood, Area y Museo de Paleontologia, Departamento de Ciencias de la Tierra, Facultad de Ciencias, Universidad de Zaragoza, c/Pedro Cerbuna, 12, E-50009 Zaragoza, Spain. Pages 1123-1126.

A quantitative compilation of the mineralogy of skeletons through the last 550 million years shows a progressive replacement of low-Mg calcite by aragonite. The replacement of low-Mg calcite by aragonite was, however, achieved episodically at mass extinction intervals. In particular, the end-Permian extinction both preferentially removed species bearing "unfavorable" low-Mg calcite, and allowed the selective radiation of biota with “favorable” aragonite. This demonstrates the importance of "incumbency" in the evolution of skeletal mineralogy. Zhuravlev and Wood suggest that the broad increase of aragonitic biota has been controlled by changes in atmospheric carbon dioxide partial pressure (pCO2). Through the last 550 million years, broadly decreasing pCO2 levels led to decreasing total alkalinity and dissolved inorganic carbon, and increasing oceanic pH. Superimposed upon this general trend were cyclic episodes of relatively high pCO2 and saturation state combined with a lower ratio of magnesium to calcium ions in seawater driven by the relatively slow changes in mid-ocean ridge expansion rates. Mass extinction events, many of which may be caused by rapid global changes in temperature and/or pCO2, represent major intervals of turnover.

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Tracing exhumation and orogenic wedge dynamics in the European Alps with detrital thermochronology
Barbara Carrapa, Dept. of Geology and Geophysics, University of Wyoming, 82071 Laramie, Wyoming, USA. Pages 1127-1130.

Modeling the impacts of climate changes on coastal settings is a challenging objective of numerous research projects. The study of past changes and of their record in sediment successions is one key for unraveling the impacts and their recurrence time scale. Such an approach is common in deep-sea or lacustrine successions but still remains rare in high-energy coastal settings. In a study by Carrapa, the sediment infilling of the Mont-Saint-Michel Bay (northwest France), known worldwide for its very high tidal range (15 meters), was studied using very high-resolution seismic and core data. It is demonstrated that during the past 6000 years, 1500-year-periodicity climate changes are recorded in the sedimentary successions that constitute the infill. Sedimentary expressions of these climate changes vary according to the different sub-environments within the bay, and cycles, a few meters-thick, can be correlated throughout the coastal wedge. The various changes reflect an increase in wave dynamics. Radiocarbon dating suggests that these cycles have a millennial time scale, and the wave climate deterioration they record matches the time of the North Atlantic climate fluctuations known as the Bond cold events. In conjunction with these climate changes, long-term (1800-year-periodicity) tidal cycles possibly play a significant role in such macrotidal setting.

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Marine calcifiers exhibit mixed responses to CO2-induced ocean acidification
Justin B. Ries et al., Dept. of Marine Sciences, 333 Chapman Hall, Campus Box 3300, University of North Carolina-Chapel Hill, Chapel Hill, North Carolina 27599, USA. Pages 1131-1134.

Anthropogenic elevation of atmospheric carbon dioxide (pCO2) is making the oceans more acidic, thereby reducing the saturation state of seawater with respect to calcium carbonate (CaCO3). There is mounting concern over the impact that future CO2-induced ocean acidification will have on marine organisms that build their shells from this mineral. Ries et al. conducted 60-day laboratory experiments to investigate the effects of CO2-induced ocean acidification on calcification within a broad range of ecologically and economically important marine organisms. They found that 10 of the 18 species studied exhibited reduced calcification (oysters, scallops, temperate corals, tube worms) and, in some cases, net dissolution (hard and soft clams, conchs, periwinkles, whelks, tropical urchins) of their shell under elevated pCO2. Surprisingly, they also found that calcification increased for seven of the investigated species (crabs, shrimp, lobsters, calcifying red and green algae, limpets, temperate urchins) under elevated pCO2, with one species (mussel) showing no response at all. These varied responses may reflect differences in the organisms’ ability to regulate pH at their site of calcification, their ability to generate an organic layer that protects their shell from dissolution, the solubility of their shell mineral (aragonite vs. low-high Mg calcite), and their ability to utilize CO2 directly via photosynthesis. These results suggest that the impact of elevated atmospheric pCO2 on marine calcification is more varied than previously thought.

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Self-shielding of thermal radiation by Chicxulub impact ejecta: Firestorm or fizzle?
Tamara J. Goldin and H. Jay Melosh, Lunar and Planetary Lab, University of Arizona, Tucson, Arizona 85721, USA. Pages 1135-1138.

65 million years ago, a large meteorite impact led to a global environmental catastrophe, including the dinosaurs’ demise. During this impact event, material was ejected at high speeds and transported around the globe. As this ejecta reentered the Earth’s atmosphere at high speeds and decelerated, the particles heated up and radiated their heat. It has been postulated that the thermal radiation emitted as ejecta rained back through the atmosphere was sufficient to ignite global wildfires and kill exposed terrestrial animals. Goldin and Melosh used a numerical model to calculate the ejecta's descent through the atmosphere. The modeled ejecta reach temperatures exceeding 1000 degrees Celsius and radiate their heat, but not all the downward-radiated energy reaches the surface: some is absorbed by greenhouse gases and an increasing proportion is absorbed by previously-entered ejecta. This "self-shielding" effect grows stronger with time as more ejecta reenter the atmosphere and leads to a weaker and shorter pulse of thermal radiation at the Earth's surface. Their models suggest a thermal pulse equivalent to an oven set to "broil" for several minutes. This may not have been sufficient to ignite the world’s forests, but was likely still lethal to many flora and fauna that could not survive the cooking.

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Spatial variations in catchment-averaged denudation rates from normal fault footwalls
Alexander L. Densmore et al., Institute of Hazard and Risk Research and Dept. of Geography, Durham University, Durham, UK. Pages 1139-1142.

It is well established that tectonic activity, in the form of earthquakes on active faults, leads to uplift and erosion of mountains. The actual relationships between tectonics and erosion, however, are difficult to study, in large part because it is hard to find places on the Earth's surface where the rate and pattern of tectonic uplift is known well enough to compare directly with the rate of erosion. Densmore et al. chose to look at normal faults, which have a simple, symmetric variation in both slip rate and the total rock uplift -- highest in the middle of the fault, and tapering to zero at the fault tips. By measuring erosion rates at different positions along the fault, they can directly compare rock uplift and erosion rates. They used the concentration of cosmogenic beryllium to estimate how erosion rates varied along two different faults in the Basin and Range Province, western USA. Their results show that erosion rates are largely decoupled from rock uplift rates, but for two very different reasons: for small faults, erosion rates are set largely by topography inherited from the pre-faulting landscape, while for larger faults, erosion rates depend strongly on rare, stochastic events like landslides.

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Homologous skeletal secretion in tommotiids and brachiopods
Uwe Balthasar et al., Dept. of Earth Sciences, Uppsala University, Villavagen 16, 75236 Uppsala, Sweden. Pages 1143-1146.

The origins of animal phyla in the course of the Cambrian explosion of animal life represent a major challenge to biology as the bizarre body plans of the deep ancestors of animal phyla escape straightforward taxonomic categorization. One of the first phyla to emerge from the Cambrian explosion is Brachiopoda, a group of marine shellfish with either calcareous or phosphatic shells. It has been claimed that the typical two-shelled construction of brachiopods has evolved through the successive shortening of the multi-element phosphatic tubes formed by tommotiids, a group of still enigmatic Cambrian fossils. One of the implications of this hypothesis is that, unlike other groups with calcareous shells (e.g., clams and mussels), calcitic brachiopod shells evolved from phosphatic ancestors. Balthasar et al., analyzed the microstructure of tommotiids and some of the oldest known brachiopods and show that not only is there independent support for the above scenario, but in the course of the transition from tommotiids to brachiopods the shells successively reduce their degree of phosphate mineralization. They interpret this trend as an adaptation to the switch from a vagrant to sessile mode of life that must have occurred among basal tommotiids and its associated impact on maintaining a high phosphorous budget.

GSA TODAY

Enigmatic boulder trains, supraglacial rock avalanches, and the origin of "Darwin's Boulders" - Tierra del Fuego
Edward B. Evenson et al., Dept. of Earth and Environmental Sciences, Lehigh University, Bethlehem, Pennsylvania 18015, USA. Pages 4-10.

It is 200 years since the birth of Charles Darwin and 150 years since his publication of "On the Origin of Species." Although renowned for establishing evolution as the explanation of the diversity of life on Earth, Darwin considered himself first and foremost as a geologist. In celebration of this, Darwin's anniversary year, Edward Evenson of Lehigh University and his colleagues have mined Darwin's extensive library of geological publications to aid in their interpretation of the enigmatic, gigantic granite boulders that characterize the Atlantic coast of Tierra del Fuego. Darwin developed a life-long fascination with erratic boulders that stemmed from his very first geological expedition as an assistant to Adam Sedgewick mapping in Wales. Based on mapping undertaken during the now famous voyage of the Beagle, Darwin interpreted the boulder trains of eastern Tierra del Fuego in terms of ice rafting above a subsided continental margin. It is a testament to Darwin's abilities as a geologist that his interpretation of the boulders that now bear his name continues to merit testing, some 173 years after the Beagle's return to England.

www.geosociety.org
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