October 2010 Geology and GSA Today Highlights
The October Geology includes a study using fish teeth to understand ocean circulation; discussion of the “Dead Clade Walking” taxa; description of the first reported example of igneous aragonite; discovery of a Paleogene California River, flowing in similar location but opposite direction to the Colorado River; a report of the earliest definite record of predation on pelagic sea lilies; and discovery of the only known active drumlin field in the world. GSA Today examines calderas.
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.
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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 (<25 m) at eight New Jersey localities in 2008 and 2009 and show that iridium anomalies are associated with the marine extinctions at three other clayey New Jersey sections. Thus, they attribute the anomaly at Freehold to the downward movement of iridium, and reaffirm the link between impact and mass extinction.
Formation of “Southern Component Water” in the Late Cretaceous: Evidence from Nd-isotopes
Stuart A. Robinson et al., Dept. of Earth Sciences, University College London, Gower Street, London WC1E 6BT, UK. Pages 871-874.
During periods of extreme warmth in the geologic past, the oceans may have circulated in a way very different from the present day, with implications for how heat was transferred around the planet and how the deep ocean was oxygenated. In this study, Stuart A. Robinson of University College London and colleagues reconstruct ocean circulation in the Late Cretaceous (about 100 to 65 million years ago), a period of extreme warmth and, occasionally, very low oxygen conditions in the oceans. They measured isotopes of the element neodymium (Nd) in fish teeth and bones from deep-sea sediments. When fish die and settle on the seabed, their teeth and bones start to incorporate a range of elements from seawater, including neodymium. Nd-isotopes can be used to reconstruct ocean circulation patterns because they are principally controlled by the local geology where a water mass forms. Robinson and colleagues show that different mechanisms of ocean circulation occurred during the Cretaceous and that a change in the style of circulation about 80 million years ago was likely caused by a combination of changing geographies, allowing different oceans to mix, and Southern Hemisphere climate cooling, allowing dense surface waters to sink to the deep ocean.
CO2-driven ocean circulation changes as an amplifier of Paleocene-Eocene thermal maximum hydrate destabilization
Daniel J. Lunt et al., School of Geographical Science, University of Bristol, Bristol BS8 1SS, UK. Pages 875-878.
Earth was much warmer 50 to 60 million years ago than today, with higher greenhouse gas concentrations in the atmosphere and little or no permanent ice on Antarctica or Greenland. In addition, a series of rapid global warming events occurred, the strongest of which was at the Paleocene-Eocene boundary, around 55 million years ago. Yet, the trigger for these events has remained a mystery. Daniel J. Lund of the University of Bristol and colleagues, using a version of the United Kingdom climate model similar to that used to predict future climate change, throw new light on the trigger. They show how the circulation of the ocean could have switched in response to slow changes in the background climate, bathing intermediate depths in the Atlantic Ocean with much warmer waters. In turn, this ocean warming could have released methane gas trapped in icy hydrates in the sediments and, hence, strongly contributed to the transient warming. If their model scenario for ancient greenhouse warming is true, an important question is whether similar triggers and feedbacks exist in the modern system.
Lizardite versus antigorite serpentinite: Magnetite, hydrogen, and life(?)
Bernard W. Evans, Dept. of Earth and Space Sciences, Box 351310, University of Washington, Seattle, Washington 98195-1310, USA. Pages 879-882.
In this study, Bernard Evans of the University of Washington argues that serpentinization of Earth’s mantle peridotite, leading to the growth of antigorite, takes place at temperatures sufficiently elevated to allow magnesium-iron diffusion in olivine. This reduces or eliminates the need to precipitate magnetite and evolve hydrogen — as is typical of low-temperature lizardite serpentinites, such as those on the ocean floor. Serpentinization of mantle-wedge peridotite most likely corresponds to the first case. These differences have geophysical implications, as well as implications for hypotheses concerning the origin of life.
Roveacrinids (Crinoidea, Echinodermata) survived the Cretaceous-Paleogene (K-Pg) extinction event
Mariusz A. Salamon et al., Faculty of Earth Sciences, University of Silesia, PL-41-200, Sosnowiec, Poland. Pages 883-885.
The Cretaceous-Paleogene extinction event represents one of the most spectacular mass extinctions, in which many animal and plant species became extinct within a geologically short period of time. It has been generally accepted that these dramatic changes were caused by a massive asteroid impact that struck the earth about 65.5 million years ago. However, it has been suggested recently that some organisms (such as certain dinosaurs and ammonites), previously assumed as extinct near the Cretaceous-Paleogene boundary, survived into the earliest Paleocene but, immediately following, became extinct without descendants. These organisms have been referred to as the “Dead Clade Walking” taxa. Though echinoderms appear to have suffered no great change in diversity across the Cretaceous-Paleogene extinction event, it has long been assumed that the collapse of the planktonic food chain at the end of the Cretaceous was a crucial phase for pelagic sea lilies (crinoids), which might have led to their complete extinction. A new discovery by Mariusz A. Salamon of the University of Silesia and colleagues of well-preserved fossils from the early Paleocene of Poland strongly demonstrates that these crinoids survived into the earliest Cenozoic. Therefore, this find qualifies these pelagic crinoids as being a Dead Clade Walking.
Seismic anisotropy associated with continental lithosphere accretion beneath the CANOE array, northwestern Canada
Anna M. Courtier et al., Dept. of Geology and Environmental Science, James Madison University, Harrisonburg, Virginia 22807, USA. Pages 887-890.
Using data recorded during the Canadian Northwest Experiment (CANOE), Anna M Courtier of James Madison University and colleagues examine upper-mantle fabrics beneath the cordillera and continental interior of northwestern Canada. The CANOE array traverses a wide variety of continental settings, allowing the study of mantle variability associated with the formation of continental cratons and continental assembly over a time span of nearly four billion years. The close spacing of CANOE instruments provides a detailed view of the mantle and lithosphere across these transitions. Over broad portions of the array, fabrics align with plate-motion directions, suggesting that coherent asthenospheric fabric underlies much of the region. Within this framework, variability over smaller-length scales is generally correlated with surface features. An anomalous region is observed across an ancient suture zone, apparently associated with complex dipping fabric produced during continental assembly. Courtier and colleagues find that the front of the Canadian Rockies correlates with a significant change in behavior, consistent with the front range, demarking a major transition within the mantle. Fabric is weaker or less coherent beneath the cordillera than beneath the continental interior. In the western cordillera, results rotate abruptly to parallel the plate boundary, implying that fabric associated with plate-boundary deformation extends about 200 km into the North American continent.
Small, isolated glacial catchments as priority targets for cosmogenic surface exposure dating of Pleistocene climate fluctuations, southeastern Pyrenees
Raimon Pallàs et al., Depto. Geodinamica i Geofisica, Universitat de Barcelona, 08028 Barcelona, Spain. Pages 891-894.
The Pyrenees form a mountain range in western Europe that links the Atlantic Ocean and the Mediterranean Sea. Because they are highly sensitive to variability in the North Atlantic atmosphere-ocean circulation system, the Pyrenees are ideally located for retrieving valuable climatic information of the past. However, the age of Pyrenean glacier fluctuations is not well established. A combination of dating methods suggests a long period of maximum and near-maximum ice extent ranging from more than 30,000 to 20,000 years ago. However, this interpretation is challenged by ecological reconstructions based on pollen-bearing lake sediments that indicate increased aridity in the Mediterranean region during the global Last Glacial Maximum (the period between 23,000 and 19,000 years ago). Raimon Pallas of the University of Barcelona and colleagues showcase the potential of small, isolated, low-gradient glacial catchments to improve the chronology of peak glacial fluctuations using specific isotopes that accumulate at Earth’s surface under the effect of cosmic radiation. Focusing on a small catchment in the southeastern Pyrenees, they present a new data set that introduces unprecedented coherence to the Pyrenean chronology while also narrowing discrepancies between glacial and ecological reconstructions in the Mediterranean region.
The Yakutat terrane: Dramatic change in crustal thickness across the Transition fault, Alaska
Gail L. Christeson et al., Institute for Geophysics, Jackson School of Geosciences, University of Texas at Austin, J.J. Pickle Research Campus, Austin, Texas 78758-4445, USA. Pages 895-898.
The Southern Alaskan continental crust was formed by the addition of multiple terranes, pieces of crust formed elsewhere that ultimately travel and accrete to the margin. Currently, the Yakutat terrane is adjacent to southern Alaska, where it is subducting at a low angle beneath the North America plate. The southern edge of the Yakutat terrane is bounded by the Transition fault. Gail L. Christeson of UT-Austin’s Jackson School of Geosciences and colleagues present the results of an offshore seismic experiment, conducted during the St. Elias Erosion and Tectonics Project (STEEP), that demonstrates the nature of the Transition fault and its relationship to the Yakutat terrane basement. The Transition fault is imaged as a near-vertical fault zone ~1 km wide. This fault is coincident with a dramatic change in Moho depth from 32 km for Yakutat oceanic plateau crust to 11.5 km for Pacific Ocean crust occurring over a horizontal distance of 0-5 km. This is proposed to be the largest change in crustal thickness across a fault observed anywhere in the world.
Astronomical tuning of the Aptian Stage from Italian reference sections
Chunju Huang et al., Dept. of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, Maryland 21218, USA. Pages 899-902.
Chinju Huang of Johns Hopkins University and colleagues collected a high-resolution grayscale series of the Aptian Fucoid Marls (Piobbico core, central Italy) for astronomical forcing. Tuning to orbital eccentricity, and extension to the Aptian-Barremian boundary by correlation to the Cismon core (northern Italy), indicates a duration of 13.42 million years for the Aptian Stage. The combined Aptian-Albian astronomical tuning of the entire 77-m-long Piobbico core (and part of the Cismon core) provides a 25.85-million-year-long astronomically calibrated time scale for Earth history.
Timing the deposition of 17O-depleted barite at the aftermath of Nantuo glacial meltdown in South China
Chuanming Zhou et al., State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, China. Pages 903-906.
Earth’s surface experienced extremely rapid changes following the meltdown of a Neoproterozic snowball Earth. The timeline of these rapid changes, if established, could distinguish the cause or consequence of the event and provide insights into the resiliency of our Earth system after an extreme perturbation. Many of the changes could have happened in less than 100,000 years and, thus, none of the current dating techniques have the resolution to piece together a timeline for what happened about 635 million years ago. Chuanming Zhou of the Chinese Academy of Sciences and colleagues demonstrate that classical field geology and sedimentology are our best bet. Built upon years of extensive fieldwork, petrographic observation, and context-specific isotope analysis in South China, the team has put together a sequence of events immediately following the meltdown of the global glaciations. The sequence not only pins the deposition of the enigmatic 17O-depleted sedimentary barite to a time after the deposition of the cap dolostone, it also assigns the precipitation of methane-related calcite to a much later stage, negating the argument that methane was the trigger of the great meltdown.
Poroelastic triggering in the 9-22 January 2008 Nima-Gaize (Tibet) earthquake sequence
Jiankun He and Gilles Peltzer, Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100085, China. Pages 907-910.
Earthquakes are generally followed by a sequence of smaller earthquakes called aftershocks. Earth scientists have studied these sequences for decades, because they provide clues about post-seismic relaxation and earthquake triggering processes. In January 2008, a cluster of three moderate earthquakes occurred within two weeks in southern Tibet. Jiankun He and Gilles Peltzer of the Chinese Academy of Sciences examine radar images acquired by the ENVISAT satellite that reveal surface displacement associated with the two largest events and indicate slip on two normal faults separated by 10 km. Numerical modeling shows that the stress change produced by the first event brought the second fault closer to failure. When coupled with pore fluids in the shallow crust, the elastic solution indicates that stress continues to increase near the hypocenter of the second event during the weeks after the first event as a result of the relaxation of pore pressure gradients. These findings illustrate the effect of the diffusion of pore fluids in the shallow crust after an earthquake on the state of stress of nearby faults, supporting the idea that poroelastic relaxation contributes to the temporal evolution of the near-field seismicity after an earthquake.
Aragonite in olivine from Calatrava, Spain — Evidence for mantle carbonatite melts from >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.
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.
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.
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.
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.