Relationship between mass extinction and iridium across the Cretaceous-Paleogene boundary in New Jersey
Kenneth G. Miller et al., Dept. of Earth and Planetary Sciences, Rutgers University, Piscataway, New Jersey 08854, USA. Pages 867-870.
The Cretaceous-Paleogene boundary is associated with the extinction of dinosaurs and the third largest mass extinction event in history. The link of the Cretaceous-Paleogene extinction to an impact in Chicxulub, Mexico, associated with a global iridium enrichment has been recently questioned. An iridium anomaly has been found below the extinction of Cretaceous fossils in Freehold, New Jersey. To test the relationship of iridium to the extinctions, Miller et al. conducted a campaign of shallow coring (100 km depth
Emma R. Humphreys et al., Dept. of Earth Sciences, University of Bristol, Wills Memorial Building, Bristol BS8 1RJ, UK. Pages 911-914.
Volcanoes erupting magma with very carbon dioxide-rich melts (carbonatites) are considered rare. Currently, there is only one active volcano producing carbonatites. How and where carbonatite melts form has been a topic of intense debate since their discovery. The presence of mantle-rock fragments and the mineralogy of such volcanism have suggested that they may form up to 70 km beneath Earth's surface. However, the style of eruption and the anticipated carbon dioxide content of such melts have indicated a volcanic style more akin to kimberlites (volcanism that entrains diamonds), which can originate several hundreds of kilometers deep. Emma R. Humphreys of the University of Bristol and colleagues describe the first reported example of igneous aragonite; a high-pressure form of calcite (a calcium-rich carbonate mineral) in an area of carbonatite volcanism. Aragonite's stability allows the team to constrain that melting occurred at depths greater than 100 km. They also show that the eruption must have happened very quickly, requiring high-volatile (carbon dioxide, water, fluorine, chlorine, etc.) content to provide the impetus for eruption. They conclude that melts such as these tell us more about the processes operating to form carbonatites in the mantle and also provide important insight on composition and processes operating in the deeper mantle.
Evidence from zircon U-Pb age spectra for crustal structure and felsic magma genesis at Taupo volcano, New Zealand
B.L.A. Charlier et al., Dept. of Earth and Environmental Sciences, Open University, Milton Keynes MK7 6AA, UK. Pages 915-918.
Age dating of zircon crystals extracted from rocks at Taupo volcano, New Zealand, sheds new light on processes operating below the volcano, and gives important clues on how long it might take for the volcano to reactivate from its currently dormant state. Bruce L.A. Charlier of Open University and colleagues show that a piece of the underlying basement rocks (greywacke) caught up in a 28,000-year-old eruption deposit exhibits ages matching a rock type that only occurs at the surface more than 75 km from the volcano. This sample shows that the types of greywacke forming the crust below Taupo are more complex than previously thought, and allows for more accurate modeling of the magma generation processes at depth. Zircon ages in a 20,000-year-old pumice deposit can be matched to two types of greywacke, as well as to intrusions mostly related to a gigantic eruption that occurred about 340,000 years ago, showing that wholesale crustal melting occurs during the magma generation processes. Most of the zircons in the pumice are still well-shaped, despite being immersed in melt of a composition that would have dissolved them in one to 10 years. This implies that the magma generation processes must be geologically fast, and that this giant volcano can move from dormancy to eruption in less than one to 10 years.
Pre-eruptive reheating during magma mixing at Quizapu volcano and the implications for the explosiveness of silicic arc volcanoes
Philipp Ruprecht and Olivier Bachmann, Dept. of Earth and Space Sciences, University of Washington, Box 351310, Seattle, Washington 98195, USA. Pages 919-922.
Volcanic eruptions of viscous dacite magma can vary in style from explosive Plinian columns, such as the Pinatubo eruption in 1991, to relatively quiet lava flows. While water in the magma has always been recognized as a key variable that controls the eruptive style, the effect of magma temperatures has received less attention. This study by Philipp Ruprecht and Bachmann of the University of Washington compares two large historic eruptions (1846-1847 and 1932) of opposite style but identical dacite magma composition, and pre-eruptive storage temperature (about 870 degrees Celsius) from the same volcano, Quizapu volcano (Chile). The major difference between these two eruptions is that lava flows from the 1846-1847 eruption show significant mingling with hot recharge magma and reheating from ~870 to ~1000 degrees Celsius. Mass and heat balancing show that conductive reheating and partial mixing of the dacite with the recharge magma, as well as latent heat release from crystallization as the recharge magma cools against the dacite, can account for the reheating of the 1846-1847 dacite. The authors argue that the 1846-1847 reheating led to enhanced, early degassing during ascent and early loss of the potentially explosive water vapor. Such processes are likely to occur at similar volcanoes and suggest that hot recharge magma may reduce the potential for explosive behavior. This study was funded in part by NSF grants EAR-0440391, EAR-0711551, and EAR-0809828.
Volcanic biotite-sanidine 40Ar/39Ar age discordances reflect Ar partitioning and pre-eruption closure in biotite
John M. Hora et al., Geowissenschaftliches Zentrum, Universitat Gottingen, 37077 Gottingen, Germany. Pages 923-926.
The 40Ar/39Ar method of dating volcanic rocks assumes that 40Ar produced by radioactive decay escapes from minerals at high temperatures prior to eruption. Accordingly, all minerals in a given lava should record the same age -- that of eruption. John M. Hora of Universitat Gottingen and colleagues show that two commonly used minerals, biotite and sanidine, frequently do not, and that this discrepancy can be up to 600,000 years. They use an independent chronometer (uranium-thorium disequilibrium, which has no loss at high temperature) to show that the discrepancy in 40Ar/39Ar ages does not result from different pre-eruption ages of the crystals themselves. Instead, they find that even though both biotite and sanidine crystals are significantly older than the eruption, only biotite has anomalously old 40Ar/39Ar ages, whereas sanidine faithfully records eruption age. They propose that biotite both incorporates excess 40Ar present in the magma and also begins accumulating 40Ar prior to eruption (recording extra time). This record can be preserved if biotite violates the initial assumption and retains some of this Ar at temperatures higher than that of the eruption, as opposed to sanidine, wherein the clock starts only after the rock cools. Consequently, interpretations solely based on biotite ages without independent age control may require reevaluation, especially in the case of young samples. This study was funded in part by NSF grants EAR-0538159, EAR-0538206. and EAR-0710545.
Paleogeographic implications of non-North American sediment in the Mesoproterozoic upper Belt Supergroup and Lemhi Group, Idaho and Montana, USA
Eric D. Stewart et al., Dept. of Geosciences, Idaho State University, Pocatello, Idaho 83209, USA. Pages 927-930.
Eric D. Stewart of Idaho State University and colleagues present new isotopic data from the upper Belt Supergroup, a 1.4-billion-year-old sequence of sedimentary rocks found in Idaho and Montana, USA, that suggest that a rifted continent, located off the western coast of North America, contributed sediment to the ancient Belt basin. The SWEAT (southwestern United States and East Antarctica) paleogeographic model, which places Antarctica adjacent to the western margin of North America, provides a suitable source for this non-North American sediment component. Paleogeographic models that place Siberia, Australia, or South China next to western North America do not fit the new isotopic data as well. North America's western neighbor stayed attached for hundreds of millions of years after the Belt Supergroup was deposited, eventually rifting away sometime between 650 and 550 million years ago. This study was funded in part by NSF grant 08-19884 and a GSA graduate student research grant (no. 8836-08 to Stewart).
The Paleogene California River: Evidence of Mojave-Uinta paleodrainage from U-Pb ages of detrital zircons
Steven J. Davis et al., Dept. of Global Ecology, Carnegie Institution of Washington, Stanford, California 94305, USA. Pages 931-934.
Steven J. Davis of the Carnegie Institution of Washington and colleagues have found evidence that some 55 million years ago, a large river flowed northeast through Arizona into Utah along the path, but in the opposite direction, of the present-day Colorado River. They made the discovery by comparing the uranium and lead isotopes in samples of zircon collected from sedimentary deposits in Utah and southwest Arizona. Davis and team have named the river the California River after its inferred source in the Mojave region of southern California. This study was funded in part by NSF grants EAR-732436 and EAR-0443387.
Crinoids for lunch? An unexpected biotic interaction from the Upper Ordovician of Scotland
Stephen K. Donovan et al., National Centre for Biodiversity-Naturalis, Postbus 9517, NL-2300 RA Leiden, Netherlands. Pages 935-938.
The evidence for interactions between ancient organisms is an exciting aspect of the fossil record. Speculation is easy, but evidence is harder to find. A specimen in The Natural History Museum in London, collected over 100 years ago, has yielded fascinating data on who-ate-whom through new analytical techniques employed by Stephen K. Donovan of the Netherland's National Centre for Biodiversity-Naturalis and colleagues. A specimen of the mollusk Helminthochiton thraivensis Reed, a chiton from the Upper Ordovician of southwest Scotland (about 450 million years old), is an exceptional fossil in preserving the complete series of eight valves ("shells") in near life-position. Although the valves themselves have dissolved away, natural molds preserve these details. Application of high-resolution, X-ray microtomography (XMT) to this specimen has revealed the exceptional preservation of its last meal, including elements of the stalk of a crinoid in its intestine. Crinoids, also known as sea lilies because of their plant-like appearance, are related to extant sea urchins and sea stars. These animals were attached to the seafloor by a stalk and thus might be considered "a sea star on a stick." The interaction between the chiton and the crinoid was either predatory or scavenging; extant chitons are not known to eat crinoids. This is the earliest definite record of predation/scavenging on crinoids in the fossil record.
Submarine fans at all sea-level stands: Tectono-morphologic and climatic controls on terrigenous sediment delivery to the deep sea
Jacob A. Covault and Stephan A. Graham, Chevron Energy Technology Company, Clastic Stratigraphy R&D, San Ramon, California 94583, USA. Pages 939-942.
Submarine fans, located at the ends of continental-margin, sediment-routing conduits, are the largest sediment deposits on the deep seafloor, and are composed of significant volumes of ancient sedimentary rocks exposed in mountain belts and buried beneath Earth's surface. Thus, they record past climate and changes to landward sedimentary systems and can be prolific hosts for petroleum resources. Sediment delivery to submarine fans is hypothesized to occur during sea-level fall and lowstand, when space for sediment deposition is predominantly located in the deep sea. Jacob A. Covault and Stephen A. Graham of Chevron Energy Technology Company employ a global database of submarine-fan sediment deposition from 35,000 years ago to the present, a period of significant global sea-level rise and fall, to test this hypothesis. Results show that deposition rates are larger during sea-level lowstand and subsequent rise. Additionally, deposition of land-derived sediment in the deep sea can occur at any sea level as a result of tectonic and climatic characteristics of the continental margin. These results underscore the importance of a holistic understanding of the tectonic and climatic characteristics of a land-to-deep sea sediment-routing system to accurately predict timing and magnitude of submarine-fan deposition and place it in the context continental-margin evolutionary models.
Active drumlin field revealed at the margin of Múlajökull, Iceland: A surge-type glacier
Mark D. Johnson et al., Dept. of Earth Sciences, University of Gothenburg, Box 460, SE-405 30 Goteborg, Sweden. Pages 943-946.
Fields of drumlins characterize many regions of the world that were covered by continental glaciers during the last ice age. These streamlined, elliptical hills form underneath ice sheets, and they have long captured the attention of geologists and geomorphologists; over 1500 scientific articles have been written about drumlins in the past century. The origin of drumlins has been hotly debated, and many theories have been proposed, but there is no consensus. Frustratingly, drumlins are rare along present-day glacier margins, and fields of drumlins were unknown – until the discovery by Mark D. Johnston of the University of Gothenburg and colleagues of a field of over 50 drumlins in front of Múlajökull, Iceland, during the summer of 2009. Because this glacier is a surging glacier, and because the deposits within the drumlin can be tied to surge events, the team considers this to be an active drumlin field. The drumlins formed likely within the past 200 years or so. Every surge event (occurring once every few decades) further shapes them, and, thus, future surges will continue to shape them. Drumlins likely lie underneath the glacier waiting to be exposed as the ice recedes. As far as Johnson et al. can tell from the literature (and recent Google Earth images), the field at Múlajökull represents the only known active drumlin field in the world. However, with continued warming and melt-back of glaciers, other glaciers may reveal similar drumlin fields.
Wave-enhanced sediment-gravity flows and mud dispersal across continental shelves: Reappraising sediment transport processes operating in ancient mudstone successions
Joe H.S. Macquaker et al., Dept. of Earth Sciences, Memorial University of Newfoundland, Saint John's, Newfoundland A1C 5S7, Canada. Pages 947-950.
Recent studies of marine shelf sediment dispersal show that wave-enhanced sediment gravity flows are widespread phenomena that can transport large volumes of fluid mud rapidly across low-gradient shelves. Flow evolution is controlled by sediment supply, seabed gradient, and spatial distribution of wave energy at the seabed. Using existing flow models, Joe H.S. Macquaker of Memorial University of Newfoundland and colleagues predict that such flows in mud-dominated sediments are characterized by a three-part microstratigraphy produced by these changing flow conditions: (1) a largely homogenous lower lamina set that may exhibit curved lamina (particularly subtle after compaction), which downlap on to the underlying erosion surface; (2) an abrupt contact with a middle lamina set composed of thin intercalated silt/clay laminae, which are typically continuous and planar to wavy; and (3) an uppermost homogenous clay-rich drape that may contain burrow mottles. Petrographic examination of recent flow deposits collected from the Eel Shelf reveals that resultant beds possess a microstratigraphy consistent with the team's hypothesis. Further analyses of ancient mud-rich outer-shelf and basinal successions (Cleveland Ironstone, Jurassic, UK; Mowry Shale, Cretaceous, United States) reveals that they too contain beds with this three-part organization, suggesting that such flows were active in these ancient settings as well. Identification of these recognition criteria now allows the products of this newly recognized sediment dispersal mechanism to be identified in other shale-dominated successions and requires that mud-dominated successions not be routinely attributed to low-energy suspension settling from buoyant plumes.
Wetting facilitates late-stage segregation of precious metal-enriched sulfosalt melt in magmatic sulfide systems
Andrew G. Tomkins, School of Geosciences, Monash University, PO Box 28E, Victoria 3800, Australia. Pages 951-954.
Magmatic sulfide ore deposits are our most important source of platinum group elements (PGE), yet we currently have a relatively poor understanding of processes that drive ore genesis. This is because these metals are dominantly contained in minerals with the semi-metals arsenic, bismuth, antimony, and/or tellurium, and only a small amount of research has been conducted on how these elements behave within magmatic systems or how they control PGE distribution. Andrew G. Tomkins of Monash University conducted high temperature-pressure experiments showing that during the final stages of sulfide melt crystallization, an interconnected melt drainage network of arsenic-rich sulfosalt melt can form from low proportions of the semi-metals. Platinum, palladium, gold, and silver are shown to preferentially occur in this arsenic-rich sulfosalt melt, which is able to migrate through the drainage network to accumulate and form concentrations of these precious metals. This late-stage fractionation model is consistent with the observed mineral distribution in many magmatic sulfide deposits.
Quaternary origin of the inverse latitudinal diversity gradient among southern Chilean mollusks
Steffen Kiel and Sven N. Nielsen, Institut fur Geowissenschaften, Christian-Albrechts-Universitat, Ludewig-Meyn-Strasse 10, 24118 Kiel, Germany. Pages 955-958.
Fossils reveal a short history of marine biodiversity in Chile. Biodiversity decreases toward the poles almost everywhere in the world, except along the South American Pacific coast. Previous suggestions about the cause of this unusual diversity pattern included that southern Chile is a museum of diversity where species survived for millions of years in addition to new arrivals, or that Antarctic species colonized this area from the south. Investigating fossil clams and snails from that region, Steffen Kiel and Sven N. Nielsen of Christian-Albrechts-Universitat show that neither of these hypotheses can be maintained, but instead that this unusual pattern has its roots in the last ice age. The retreating glaciers created a mosaic landscape of countless islands, bays, and fiords in which new species evolved rapidly, geologically speaking. The ancestors of the species survived the ice age in the warmer Chilean north.
GSA Today Science Article
Caldera collapse -- Perspectives from comparing Galapagos volcanoes, nuclear-test sinks, sandbox models, and volcanoes on Mars
Keith Howard, U.S. Geological Survey, 345 Middlefield Road, MS 973, Menlo Park, CA 94025-3591, USA. Pages 4-10.
In this article, volcanologist Keith Howard of the U.S. Geological Survey explores the applicability of the caldera collapse behavior of the Galapagos volcano Fernandina to a variety of other collapse features across a huge range of scales (over 16 orders of magnitude). Calderas are great circular features left in volcanoes when they erupt; commonly, with the eruptive emptying of the hot magma chamber in the interior of the volcano, calderas collapse because nothing is left to support them. Here, a coherent style of subsidence is discussed, in which size and material strength scale together. These collapse structures may vary in shape from saglike to pistonlike and from symmetrical to trap-door-like. Coherent collapse contrasts with failure by chaotic piecemeal spalling, which characterizes most pit craters and other small collapses in rock that is strong relative to size. This underscores the influence of material strength relative to size. Howard demonstrates that the nature of the caldera collapse is not only consistent across an astounding range of sizes, but also across a variety of different collapse features: from small sandbox models, to collapse features resulting from underground nuclear tests, to huge extraterrestrial calderas, such as the Olympus Mons caldera on Mars.
###Keywords: Extinction, ocean circulation, ocean warming, serpentinite, Dead Clade Walking, CANOE array, Pyrenees, Yakutat terrane, astronomical tuning, glacial meltdown, ENVISAT satellite images, igneous aragonite, Taupo volcano, Quizapu volcano, biotite and sanidine, upper Belt Supergroup, California River, crinoids, submarine fans, drumlins, mud-flow models, platinum group elements, marine biodiversity in Chile, caldera, Galapagos, Fernandina, Olympus Mons, Mars, nuclear test sinks, sandbox models.
Highlights are provided above. Representatives of the media may obtain complementary copies of GEOLOGY articles by contacting Christa Stratton at the address above. GSA TODAY articles are open access. Abstracts for the complete issue of GEOLOGY are available at http://geology.gsapubs.org/.
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, gsaservice@geosociety.org.
END
