Boulder, CO, USA - Several papers in October's Geology describe fossil records: a fungal disaster species; single-celled sea-bottom-dwellers; trilobite soft tissues; fossil rainforests; Archean microbial mats; and pollen and freshwater algae. These records were recovered using varying methods: high sensitivity mass spectrometer, detailed exploratory fieldwork, biochemical analysis, and deep drill-coring. Additional papers cover geology and health hazards, erosion and climate, and tsunamis and sedimentation. GSA Today discusses what happens when increased atmospheric oxygenation interacts with naturally enriched uranium deposits.
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View abstracts for the complete issue of Geology at http://geology.gsapubs.org/ . Representatives of the media may obtain complementary copies of Geology articles by contacting Christa Stratton at the address above.
Chemical constitution of a Permian-Triassic disaster species
Mark A. Sephton et al., Impacts and Astromaterials Research Centre, Dept. of Earth Science and Engineering, South Kensington Campus, Imperial College London SW7 2AZ, UK. Pages 875-878.
Around 250 million years ago, Permian forests were hit hard during the world's greatest mass extinction. While many species on land suffered at this time, others benefited from the event. Sephton et al. imply that microscopic fossils in extinction-age rocks are the remains of fungi that proliferated on dead wood generated by the global catastrophe. It seems that in a dying and decomposing world, the fungi felt entirely at home. The close of the Permian saw the largest ever mass extinction, which was accompanied by the greatest ever outpouring of flood basalt lavas in present day Siberia. The gases from this massive volcanic event acidified the land and sea and depleted Earth's protective ozone shield. Trees were among the first casualties of the changing end-Permian environment. The increased supply of decomposing woody materials led to an opportunistic expansion of fungi; however, the origin of these microfossils has recently been questioned, with some researchers preferring to assign them to algae rather than fungi. To settle the controversy, Sephton et al. used new analytical approaches, including a high-sensitivity mass spectrometer originally designed to detect interstellar grains in meteorites. The data support the status of these enigmatic end-Permian microfossils as a fungal disaster species. The feeding frenzy at the end of the Permian reminds us that there will always be winners and losers even during dramatic environmental changes. The extinction allowed the fungi to gorge themselves on the remains of fallen forests.
Geographic origin of species: The temperate-tropical interchange
Martin A. Buzas and Stephen J. Culver, Dept. of Paleobiology, National Museum of Natural History MRC-121, Smithsonian Institution, Washington, D.C. 20013-7012, USA. Pages 879-882.
The geographic origin of modern marine species living on the Atlantic continental margin of North America indicates the existence of a vast temperate-tropical interchange. The excellent fossil record of single-celled, bottom-dwelling organisms called foraminifera enabled Buzas and Culver to pinpoint the geologic time and geographic origin of 259 species currently residing off the Atlantic seaboard. The results indicate that modern distributions are assembled from a mixture of species originating in more than one geographic region. Species living in temperate areas originated from both temperate and tropical regions, and species living in the tropics originated from tropical and temperate areas. Dispersal on a global scale is essential for assembling modern communities. The species pool supplying modern communities is much larger than expected, and this study demonstrates the interdependence of the communities inhabiting world oceans.
Turbulent dynamics of the 18 May 1980 Mount St. Helens eruption column
Benjamin J. Andrews and James E. Gardner, Dept. of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas 78712, USA. Pages 895-898.
During explosive volcanic eruptions, columns of gas and ash denser than the atmosphere erupt from volcanic vents. In buoyant eruption regimes, those eruption columns turbulently mix in enough air to lower their densities and rise to altitudes higher than 10 km. In collapsing eruption regimes, the columns entrain insufficient amounts of air and collapse to generate devastating pyroclastic flows. Andrews and Gardner analyze video of the 18 May 1980 eruption of Mount St. Helens and describe changes in the turbulent structure of the eruption column margins that accompanied a change from buoyant to partially collapsed eruption behavior. From those turbulent structures, they infer changes in the interior the eruption column and the development of rising and collapsing regions within the eruption column.
Stable isotope signals from brines in the Barents Sea: Implications for brine formation during the last glaciation
Tine L. Rasmussen and Erik Thomsen, Dept. of Geology, University of Tromso, Dramsveien 201, N-9037 Tromso, Norway. Pages 903-906.
Oxygen and carbon isotope values of benthic foraminifera from the cold stadials of the last glacial period in the Nordic seas are low, whereas they are high from the warm interstadials. The low values have been attributed to brine formation carrying a low stable isotope signal from the surface water into deep water, as it occurs around Antarctica today. Brines are often considered to have played a major role in the abrupt millennial-scale climate shifts during the last glaciation. However, very little is known of the isotopic composition of modern brines in the Northern Hemisphere, greatly hampering the interpretation of past data. Here, Rasmussen and Thomsen report on the oxygen and carbon isotope composition of benthic foraminifera in two cores from a brine-influenced shelf environment in Storfjorden, Svalbard, in the Barents Sea. The results indicate that brines with sufficient density to contribute significantly to intermediate and deep water are formed from cold, salty waters and have high oxygen and carbon isotope values. Brines with low oxygen isotope values formed from cold, fresher water have relatively low density and they are unable to penetrate the deeper parts of the Arctic and Nordic seas. This indicates that the low benthic oxygen and carbon isotope values obtained from the Nordic seas during stadials cannot be attributed to brines. The implication is that brines did not contribute significantly to the millennial-scale climate shifts.
Beyond Beecher's Trilobite Bed: Widespread pyritization of soft tissues in the Late Ordovician Taconic foreland basin
Una C. Farrell et al., Dept. of Geology and Geophysics, Yale University, New Haven, Connecticut 06520, USA. Pages 907-910.
The fossil record is greatly biased toward those organisms or parts of organisms that are most easily preserved (i.e., shells, bones, and teeth). Only with exceptional preservational conditions do we get a glimpse of the true morphological and biological diversity of the geological past. Beecher's Trilobite Bed, from the Ordovician of New York State, yields trilobites with soft tissues preserved in pyrite. Conditions in the sediment, which included high amounts of reactive iron, allowed the organic matter of the animals to be replaced by pyrite, preserving in detail the limbs and other soft parts. The original "Trilobite Bed" - a thin, rapidly deposited mudstone - has been known since the late 1800s, but only in the last decade has it become obvious that conditions for pyritization are more widespread in this sedimentary basin than previously imagined. Farrell et al. report the discovery of three new layers preserving pyritized trilobites at the original Beecher's Trilobite Bed site, as well as similar layers at multiple new sites around New York State. Although the majority of the pyritized fossils are trilobites, new organisms with pyritized soft parts have been discovered, including ostracods and other arthropods. This work highlights the importance of exploratory fieldwork, and clarifies the sedimentary and geochemical conditions required for pyritization. This new understanding will improve the chances of discovering exceptionally preserved fossils elsewhere in the basin.
Earthquake histories and Holocene acceleration of fault displacement rates
Andrew Nicol et al., GNS Science, P.O. Box 30368, Lower Hutt, New Zealand. Pages 911-914.
Displacement rates for normal and reverse faults are generally higher when averaged over the past 10,000 years than for time periods of hundreds of thousands of years or longer. This relatively recent acceleration of displacement rates could be accounted for by geological processes that produce increases of tectonic tempo. Nicol et al. propose an alternative explanation in which the observed rate changes arise from variability in the time between, and/or the slip of, successive earthquakes on individual faults, coupled with a sampling bias toward those faults that are best represented at Earth's surface and accrued displacement fastest during the last 10,000 years. This idea is supported by displacement rates measured over time intervals of up to about 300,000 years from the Taupo Rift in New Zealand, where variations in the time interval between earthquakes and their slip are attributed to fault interactions. These variations in earthquake parameters are widely observed and occur over longer time intervals for slower-moving faults. Nicol et al. suggest that complexity in earthquake histories arising from fault interaction is an intrinsic property of the majority of fault systems comprising numerous faults, and represents a severe impediment to earthquake prediction.
Mechanism of instantaneous coal outbursts
Ping Guan et al., MOE Key Laboratory of Orogenic Belts and Crustal Evolution, School of Earth and Space Sciences, Peking University, Beijing 100871, China. Pages 915-918.
Coal outbursts are spontaneous ejections of coal and gas from an underground coal seam during mining, especially when excavation exposes a new layer of coal. Such outbursts are a major hazard for miners. Hundreds, if not thousands, of miners die every year due to such accidents. The most deadly outbursts occurred in Turkey in 1992, killing 263 people. On 30 May 2009, a coal outburst in Chongqing, China, killed about 30 workers and injured many more. Underground coal outbursts have been documented for over 170 years and have been investigated by scientists for more than 150 years, but the precise mechanism of coal outbursts is still elusive. Guan et al. hypothesize that gas-rich coal can erupt when suddenly decompressed and carry out experiments to verify this hypothesis. Potentially, this work could lead to the development of methods to predict and prevent future coal outbursts in underground coal mines, which would save numerous lives.
Increased sediment transport via bioturbation at the last glacial-interglacial transition
Matthew W. Hughes et al., Soil and Physical Sciences Group, P.O. Box 84, Lincoln University, Lincoln 7647, Canterbury, New Zealand. Pages 919-922.
Vegetation is often used to stabilize soils and reduce erosion in terrain that has been subject to land-use practices, such as cultivation or grazing. As a result, it is commonly assumed that climate-driven variations in vegetation have influenced past erosion rates in a fairly straightforward fashion: The more extensive and deeply-rooted the vegetation, the more resistant it would have been to erosion. Working on the South Island of New Zealand, Hughes et al. documented changes in vegetation and erosion over the past 27,000 years and found the opposite relationship: Rates of erosion increased as the vegetation regime switched from grassland to forest about 10,000 years ago, with the glacial-interglacial transition. They attribute the increase to vigorous soil mixing and transport associated with tree rooting activity, such as root growth and tree turnover. In the absence of landsliding (the gentle, soil-mantled slopes of their study area are not prone to slope instability), these results suggest that over long time scales, increases in vegetative cover can promote bioturbation and increase downslope soil transport and erosion. These results contrast with the dominant climate-erosion paradigm for steep, landslide-dominated mountains, which associates rapid erosion with sparse vegetation during glacial conditions. Interpreting the role of climate on landscape evolution thus requires explicit consideration of topographic properties and geomorphic processes.
Incised channel fills containing conifers indicate that seasonally dry vegetation dominated Pennsylvanian tropical lowlands
Howard J. Falcon-Lang et al., Dept. of Earth Sciences, Royal Holloway, University of London, Surrey TW20 0EX, UK. Pages 923-926.
New fossil discoveries show that ice ages repeatedly decimated the first tropical rainforests to evolve on our planet. However, rainforests bounced back when the climate returned to normal. These findings are based on studies of 300-million-year-old fossils found in coal mines in Illinois, USA. Spectacular fossil rainforests have been reported from this region for several years, attracting media attention. A new study investigates how these earliest rainforests responded to climate change. Falcon-Lang et al. have found evidence for ice age cycles in the rocks of fossil rainforests. Results show that rainforests flourished when the tropical climate was warm and wet. However, they collapsed when the tropics became cool and dry at the height of ice ages. This work transforms our understanding of the first rainforests to evolve on our planet. It shows that climate change wreaked havoc on these tropical ecosystems, but rainforests could recover from the brink of disaster. The research suggests that ancient rainforests were resilient to climate fluctuations and may help us understand how ecosystems will respond to changes in the future.
Impact melt sheet zircons and their implications for the Hadean crust
J. Darling et al., Dept. of Earth Sciences, University of Bristol, Bristol BS8 1RJ, UK. Pages 927-930.
Impacts may have been important mechanisms of crustal redistribution and differentiation, particularly during intense post-accretionary bombardment between 4.5 and 3.9 billion years ago. Evidence of crustal processes during this period is largely provided by detrital zircons from the Yilgarn Craton. Trace element compositions, crystallization temperatures, and inclusion populations of these ancient zircons have been taken as evidence for predominantly granitic source magmas, implying widespread felsic continental crust on the early Earth. There is, however, little knowledge of zircons formed in impact melt sheets, a potential source for the Hadean zircons. Here, Darling et al. present titanium-in-zircon thermometry, trace elements, and inclusion populations of zircons from the 1.85-billion-year-old Sudbury impact melt sheet. Their results demonstrate that large variations in zircon crystallization temperature and composition will be an inevitable consequence of the evolution of such magmatic systems. It is also shown that zircons in mafic rocks crystallize in residual liquids of granitic composition, producing inclusion assemblages that are remarkably similar to those reported for the ancient Yilgarn grains. Thus, they conclude that the trace element compositions and inclusion populations of the Hadean zircons are consistent with crystallization from more mafic melts than previously recognized, although high crystallization temperature distributions of Sudbury zircons indicate that impact melt sheets were not a dominant source for the more than 3.9-billion-year-old grains.
An early ecosystem of Archean tidal microbial mats (Moodies Group, South Africa; ca. 3.2 Ga)
Christoph Heubeck, Dept. of Geological Sciences, Freie Universitat Berlin, 12249 Berlin, Germany. Pages 931-934.
The search for traces of life on other planets has its parallels to the search for the beginning of life on Earth itself because it teaches us in which environments we should look for suspicious shapes and signs of biological activity. It is therefore encouraging that the oldest well-preserved shallow-water strata, in a mountain range in South Africa, show evidence that microbial life not only existed microscopically and in isolated patches but already occupied tracts that are mappable and continuous over several kilometers. The sandstone strata studied in the Barberton Greenstone Belt by Heubeck are densely permeated by abundant carbon-rich laminations. These wavy-crinkly bands, interpreted as remains of microbial mats, apparently interacted with considerable currents that transported sand and gravel in a coastal or tidal environment. The laminations show deformation patterns that suggest a flexible but sturdy mechanical behavior, a surface morphology of at least a few centimeters, and very rapid growth. In addition, the microbial mats were apparently cohesive enough to trap fluids and gases, occasionally giving rise to small sand volcanoes. The mechanical strength of the biomats was likely derived from very early silicification, similar to what can be observed today in active hot spring fields. Thus, shallow-water environments at elevated temperatures may constitute a promising and readily detectable habitat in the search for early life.
Particle sizes of andesitic ash fallout from vertical eruptions and co-pyroclastic flow clouds, Volcan de Colima, Mexico
Jason R. Evans et al., Michigan Technological University, Houghton, Michigan 49931 USA. Pages 935-938.
Evans et al. discuss differences between two types of eruption deposits at Volcan de Colima in Mexico. One deposit type is formed by eruptions that release a relatively small amount of energy (vertical eruptions), where the ash particles interact less and tend to be larger in size. The other deposit type (from co-pyroclastic flow clouds) is from eruptions with relatively higher amounts of energy where the ash particles interact more, likely causing breakage, and therefore smaller particles to be produced. Small particles are more dangerous to living creatures as they can cause health problems and may be inhaled directly, rather than being filtered by the body. In areas where these eruptions occur, there could be a greater potential for human harm if the co-pyroclastic ash particles are present.
Sulfate-reducing ammonium oxidation: A thermodynamically feasible metabolic pathway in subseafloor sediment
Heather N. Schrum et al., Graduate School of Oceanography, University of Rhode Island, Narragansett, Rhode Island 02882, USA. Pages 939-942.
Schrum et al. have discovered evidence for a previously unknown biochemical pathway that may fuel life in subseafloor sediment throughout the world. They have shown that in marine sediment, microorganisms can derive energy by reacting with ammonium and sulfate. The process is called sulfate-reducing ammonium oxidation. Their evidence is based on observations made during research cruises in the Indian Ocean and Narragansett Bay. Microbes that live in marine sediment comprise a significant fraction of the life on Earth. This previously unsuspected process may remove a significant amount of biologically accessible nitrogen from the ocean and recycle it back to the atmosphere.
Tsunami waves generated by the Santorini eruption reached Eastern Mediterranean shores
Beverly N. Goodman-Tchernov et al., Haifa University, Leon Charney School of Marine Sciences, Haifa, Israel. Pages 943-946.
Goodman-Tchernov et al. have extracted a series of underwater core samples from the seabed offshore of the Herodian harbor, in Caesarea Maritima, Israel, with the primary aim of documenting deposits produced by a historically attested tsunami associated with the famous earthquake of A.D. 115. While consistent traces of the A.D. 115 event were in fact found, a number of the cores contained unexpected and exciting evidence of both a more recent event (A.D. 551) and a much more ancient event - a tsunami deposit as much as 40 cm thick carbon- and optically stimulated luminescence (OSL)-dated to ca. 1600 B.C., when a cataclysmic eruption obliterated much of the island of Santorini (Thera) and the thriving Cycladic culture that flourished there. The eruption of Santorini and its effects on surrounding civilizations, notably that of Minoan Crete, has long been one of the most discussed questions in Aegean Prehistory. While traces of deposits created by waves stemming from the eruption have been identified in Crete and Asia Minor, the results from Caesarea provide the first conclusive evidence that tsunami waves generated by the eruption reached the farmost eastern shores of the Mediterranean, and indeed in considerable force. These findings indicate that the waves generated by the Santorini eruption were powerful and far-ranging enough to have had potentially profound effects on coastal communities throughout the eastern Mediterranean. Moreover, they demonstrate the potential of underwater core sampling as a means of identifying similar evidence of tsunamis on shallow coastal shelves throughout the world. A new window onto the study of past tsunamis has been opened that may provide an invaluable gauge of the destructive potential of future events.
Dune mobility and aridity at the desert margin of northern China at a time of peak monsoon strength
J.A. Mason et al., Dept. of Geography, University of Wisconsin, 550 N. Park St., Madison, Wisconsin 53706, USA. Pages 947-950.
Wind-blown sands cover large areas at the margin of deserts in northern China and are sensitive to climate change, becoming more active during dry periods and more stable under vegetation cover when the climate is wet. Mason et al. show that there was widespread dune activity indicating aridity, from about 11,500 to 8000 years ago, which is surprising given the growing evidence that a strong monsoon at that time made southern China wetter than it is today. As monsoon precipitation in southern China decreased after 8000 years ago, the dunes stabilized, suggesting a wetter climate at the desert margin. Climate modeling studies help explain these contrasting patterns of climate change, showing that atmospheric circulation associated with a strong monsoon may actually reduce precipitation in northern China's drylands. Warm summer temperatures from 11,500 to 8000 years ago may also have increased aridity at the desert margin. These past links between changes in the monsoon climate and aridity at the desert margin may help predict environmental response to future climate change
A major drop in seawater 87Sr/86Sr during the Middle Ordovician (Darriwilian): Links to volcanism and climate?
Seth A. Young et al., Dept. of Geological Sciences, Indiana University, 1001 E 10th St., Bloomington, Indiana 47405, USA. Pages 951-954.
The rise of the Appalachian Mountains may have caused a major ice age beginning approximately 450 million years ago. The weathering of these mountains sequestered carbon dioxide (CO2) from the atmosphere, causing the opposite of a greenhouse effect, an "icehouse" effect. Scientists have suspected that our current ice age, which began approximately 35 million years ago, was caused by the rise of the Himalayas. This new study by Young et al. links a much earlier major ice age, one that occurred during the Ordovician Period, to the uplift of the early Appalachians. It also reinforces the notion that CO2 levels in the atmosphere are a major driver of Earth's climate. Analyzing a set of rock samples from Nevada, by comparing the ratio of two isotopes of the element strontium, it was found that, immediately before geologic evidence for cooling appeared in the stratigraphic record, the strontium ratio dropped dramatically within 8 to 10 million years. The likely cause: A vast amount of young volcanic rock was being eroded away, and the resulting sediment was being deposited in the world oceans. The timing of the strontium decline matches the rise of the Appalachian Mountains that lifted volcanic rock up from island arcs onto the North American continent. This kind of silicate rock weathers quickly, reacting with CO2 and water, resulting in carbon from the CO2 being trapped in the resulting sediment and dissolved ions. The chemical reactions that weathered away part of the Appalachians would have consumed large amounts of CO2 from the atmosphere. A numerical model simulation also presented in this study supports the strontium isotope data and geologic evidence for uplift and weathering of these terranes in the Appalachian Mountains as a likely cause of the Late Ordovician icehouse period. The Ordovician period started out warm, with relatively high sea levels worldwide, and ended cold, with low sea levels as glaciers covered the landmasses over the South Pole. The transition between greenhouse conditions and icehouse conditions set the stage for major mass extinctions around the planet at the end of the Ordovician.
Palynomorphs from sediment core reveal a sudden remarkably warm Antarctica during the mid Miocene
Sophie Warny et al., Dept. of Geology and Geophysics, and Museum of Natural Science, E235 Howe-Russell, Louisiana State University, Baton Rouge, Louisiana 70803, USA. Pages 955-958.
Warny et al. have discovered algae and pollen grains in ANDRILL AND-2A (a multinational collaboration between the Antarctic Programs of the United States, New Zealand, Italy, and Germany) samples that provide evidence of a remarkably warm period about 15 million years ago. Among the 1,107 meters of sediment recovered and analyzed for microfossil content, a two-meter-thick layer in the core displayed extremely rich content. This is unusual because the Antarctic ice sheet was formed about 35 million years ago, and the frigid temperatures there impede the presence of woody plants and blooms of dinoflagellate algae. According to the researchers, these fossils show that land temperatures reached a January average of 10 degrees Celsius - equivalent to about 50 degrees Fahrenheit - and that estimated sea-surface temperatures ranged between zero and 11.5 degrees Celsius. The presence of freshwater algae in the sediments suggests to researchers that an increase in meltwater, and perhaps also in rainfall, produced ponds and lakes adjacent to the Ross Sea during this warm period, which would obviously have resulted in some reduction in sea ice. These findings most likely reflect a poleward shift of the jet stream in the Southern Hemisphere, which would have pushed warmer water toward the pole and allowed a few dinoflagellate species to flourish under such ice-free conditions. Warny et al. believe that shrub-like woody plants might also have been able to proliferate during an abrupt and brief warmer time interval.
GSA Today
Did natural reactors form as a consequence of the emergence of oxygenic photosynthesis during the Archean?
Laurence A. Coogan and Jay T. Cullen, School of Earth and Ocean Sciences, University of Victoria, P.O. Box 3055 STN CSC, Victoria, British Columbia V8W 3P6 Canada
In order to get uranium (U) to undergo nuclear fission, for example in power stations, it is artificially enriched. However, 2.5 billion years ago, fissile enriched U was abundant, enough so that even small concentrations of it would have undergone spontaneous nuclear fission. As is pointed out by authors Jay Cullen and Laurence Coogan of the University of Victoria, British Columbia, 2.5 billion years ago also happens to be about the time that oxygen slowly started building up in Earth's atmosphere. They demonstrate that, because of the way U behaves, the increase in atmospheric oxygen would have led to U being dissolved out of rocks, forming concentrated U deposits. Once more than about one cubic meter of U was concentrated in a given place, it would have "gone critical," forming a natural fission reactor. The implication is that Earth's continental crust would have been pocked with natural nuclear reactors, contaminating the environment with hazardous, radioactive byproducts. Cullen and Coogan discuss the potentially significant and severe impacts that these natural reactors had on the evolution of life and the further oxygenation of our atmosphere.
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Journal
Geology