August Geology and GSA Today media highlights
Boulder, Colo. - Topics in the August issue include: discovery in Spain of fossilized bone marrow from 10 million-year-old amphibians; tectonic stability of the Mississippi delta; dietary influences on survival versus extinction in the late Pleistocene; use of fossilized buffalo teeth to reconstruct climate and vegetation of Great Plains grasslands; quantifying long-term landscape evolution; and proposed standard terminology for fragmental volcanic rocks. The GSA TODAY science article addresses the formation of desert landscapes.
Highlights are provided below.
Millennial-scale volcanic impact on a superhumid and pristine ecosystem
Rolf Kilian, Lehrstuhl für Geologie, Rheinlandpfalz 54286, Germany; et al. Pages 609-612.
Distal deposition of ash leeward of volcanic centers usually causes little mechanical destruction of vegetation and only limited effects on ecosytems, which generally recover within decades. However, sediment, stalagmite, and pollen records from the southernmost Andes indicate a 2000-year impact on forest and aquatic ecosystems after deposition of a thin ash layer. Sulfur released from altering volcanic ash produced intense soil and lake acidification in a 150,000 square kilometer area. Acidification led to nutrient leaching and affected soil microorganisms, causing plant decay and increased soil erosion in an area larger than 8000 square kilometers. Kilian et al. conclude that weakly buffered soils in humid environments are extremely vulnerable to volcanic and anthropogene acidification, causing long-lasting ecosystem damage and perturbations of paleoclimate proxy records.
Messinian climate change and erosional destruction of the central European Alps
Sean D. Willett, University of Washington, Department of Earth and Space Sciences, Seattle, Washington 98195, USA; et al. Pages 613-616.
Six million years ago, the Alps ceased their outward and upward expansion and began to decrease in size. By analysis of sediments and structures on the southern margin of the Alps, Willett et al. demonstrate that this reversal in growth rate was coincident with an increase in the volume of eroded sediment from the Alps. Furthermore, it occurred during the Mediterranean salinity crisis, when the Mediterranean Sea lost its connection to the global ocean system and desiccated. These observations suggest that erosion of the Alps outpaced its tectonic growth as a response to deep incision of the rivers flowing from the Alps into the Mediterranean basin. However, erosion rates remained high after the Mediterranean refilled, suggesting an additional cause. Willet et al. argue that the last stage of the salinity crises is marked by an influx of freshwater that represents a shift to a warmer, wetter European climate following the end of the Miocene glacial period. This wetter climate was responsible, not only for the freshwater in the Mediterranean during the salinity crises, but also for the enhanced erosion rates in the Alps, allowing erosion to outpace tectonic growth and reduce the size of the Alps.
Do the trace element compositions of detrital zircons require Hadean continental crust?
Laurence A. Coogan, University of Victoria, School of Earth and Ocean Sciences, Victoria, British Columbia V8W 3P6, Canada; and Richard W. Hinton, Edinburgh University, School of Geosciences, Edinburgh, Scotland EH9 3JW, UK. Pages 633-636.
The oldest pieces of Earth currently known are zircon crystals from Australia. These are up to 4.4 billion years old. The compositions of these crystals have been used to suggest that they formed under conditions not dissimilar to current conditions on Earth. This contrasts with previous ideas that Earth at that time was very different--dominated by basaltic volcanism and broadly "hell-like." A comparison of the composition of zircons that crystallized from modern basalts and the >4 billion-year-old zircons suggests that Earth may well have been "hell-like" at that time.
Quantified time scale for glacial valley cross-profile evolution in alpine mountains
Martin S. Brook, Massey University Geography Programme, School of People, Environment & Planning, Manawatu 11222, New Zealand; et al. Pages 637-640.
Time constraints - a seemingly intractable problem when studying long-term landscape evolution - are applied to the investigation of valley evolution in alpine mountains. This time constraint problem is addressed by a novel, multi-source approach combining an offshore oxygen isotope record with an onshore geological record of glacial advances. This is then superimposed on the well-constrained tectonic uplift and transport (advection) of topography in New Zealand's central Southern Alps, allowing Brook et al. to determine and quantify a timeline of the transition from unglaciated, fluvial topography to a U-shaped valley and arete glacial landscape.
High-fidelity organic preservation of bone marrow in ca. 10 Ma amphibians
Maria E. McNamara, University College Dublin, Geology, Dublin 4, Ireland; et al. Pages 641-644.
Nature is a great recycler. It is the fate of almost every organism to be broken back down into its constituent molecules and reused. Fossils, the primary source of data on the history of life on Earth, represent those organisms that have escaped this process and, through the processes of preservation, are incorporated into the fossil record. It is, however, a biased record--walk along any beach and this point is obvious; there are the familiar shells, maybe the occasional bone and tooth, but none of the internal organs, musculature, etc. that would have been present in the organisms during life. Such 'soft tissues' are rapidly broken down during decay and are therefore fossilized only rarely and under exceptional environmental conditions. McNamara et al. describe the first examples of fossilized bone marrow ever to be discovered, the remains of which occur in 10 million-year-old amphibians from Spain. The bone marrow is preserved in three dimensions with much of its original biological detail, including, remarkably, the original coloration. This find, together with the reports last year of fossilized blood vessels in Tyrannosaurus rex, sets a new lower threshold for just what sorts of tissues we can expect the fossil record to yield. The model McNamara et al. propose suggests that such tissues owe their preservation to the protective micro-environments inside the bone that shielded the material from the effects of the decay processes until they could be preserved. Such tissues are likely to be much more commonplace in the fossil record than has hitherto been expected.
Denali fault slip rates and Holocene-late Pleistocene kinematics of Central Alaska
A. Matmon, Hebrew University of Jerusalem, The Institute of Earth Sciences, Jerusalem, Israel 91904; et al. Pages 645-648.
The Denali fault is the principal intracontinental strike-slip fault accommodating deformation of interior Alaska. Matmon et al. obtained the first quantitative late Pleistocene-Holocene slip rates on the Denali fault system from dating offset geomorphic features. Analysis of cosmogenic isotopes in boulders and sediment collected at seven sites offset 25–170 meters gives average ages that range from 2.4-0.3 to 17.0-1.8 thousand years. These offsets and ages yield late Pleistocene–Holocene average slip rates of 9.4-1.6, 12.1-1.7, and 8.4-2.2 millimeters per year along the western, central, and eastern Denali fault, respectively, and 6.0-1.2 millimeters per year along the Totschunda fault. Our results suggest a westward decrease in the mean Pleistocene-Holocene slip rate. This westward decrease likely results from partitioning of slip from the Denali fault system to thrust faults to the north and west.
Reconstructing grassland vegetation and paleotemperatures using carbon isotope ratios of bison tooth enamel.
Kathryn A. Hoppe, University of Washington, Earth and Space Science, Seattle, Washington 98195-1310, USA; et al. Pages 649-652.
American buffalo (Bison bison) provide a record of climate and vegetation change on the Great Plains, which are currently the most productive agricultural region in North America. Hoppe et al. demonstrate that chemical analyses of teeth from modern buffalo can be used to reconstruct the type of grass each animal ate during its life, which in turn can be used to reconstruct changes in local temperatures. The fossil record of buffalo teeth stretches back more than 100,000 years in North America. Thus, analyses of buffalo teeth may help reconstruct past climatic conditions and determine how the productivity of Great Plains grasslands changed through time.
In situ oxygen isotope analysis of monazite as a monitor of fluid infiltration during contact metamorphism: Birch Creek Pluton aureole, White Mountains, eastern California
John C. Ayers, Vanderbilt University, Earth and Environmental Sciences, Nashville, Tennessee 37235-0105, USA; et al. Pages 653-656.
High-temperature fluids are capable of causing extensive changes in the composition and mineralogy of rocks, a process that often leads to formation of ore deposits. However, it is often difficult to characterize the process of hydrothermal alteration because the fluids are rarely preserved in rocks, and the remaining solid products are not suitable for dating the alteration event. Ayers et al. describe a method that uses the mineral monazite, a rare earth phosphate, to detect and date fluid-induced alteration events. The age of monazite grains can be measured because monazite incorporates radioactive thorium (Th) during growth, and the measured amount of Th that has decayed to lead (Pb) since growth gives the age of the monazite. Monazite grains over ~500 million years old existed in a quartzite that was intruded by the Birch Creek granite about 80 million years ago in what is now the White Mountains of eastern California. Monazite grains more than half of a kilometer from the intrusion are more than 500 million years old and have oxygen isotope compositions different from monazites in the granite, suggesting that they were unaffected by intrusion of the granite. In contrast, monazite grains less than half of a kilometer from the intrusion are the same age and have the same oxygen isotope composition as monazites in the granite, suggesting that they dissolved in, and then were reprecipitated from, fluids released from the granite. This suggests that monazite can be used to identify rocks that have been altered by high-temperature fluids and to date alteration events, which will help in understanding how ore deposits form and where they might be found.
Tunnelling trilobites: Habitual infaunalism in an Ordovician carbonate seafloor
Lesley Cherns, Cardiff University, School of Earth, Ocean and Planetary Sciences, Cardiff, CF10 3YE, UK; et al. Pages 657-660.
Outcrop evidence from Swedish Ordovician limestones (~460 million years old) is interpreted as showing truly infaunal tunneling behavior by trilobites, an extinct group of arthropods. Exposed across a wide bedding surface in a disused quarry, the numerous complete trilobite fossils preserved within extensive branching burrow trace fossil networks suggest that trilobites were the tunnel constructors. Early firming of seafloor lime sediments supported tunnel construction. A blanketing thin dark shale layer over the bed with tunnels and trilobites indicates that a catastrophic episode of low oxygen conditions may have suffocated the trilobites in place. Comparison with living arthropods suggests that the trilobites tunneled to avoid predators, in this case large, typically straight-shelled nautiloid cephalopods (Orthoceras Limestone), and possibly also to enhance breathing, feeding, and mating strategies. Trilobites are identified here as constructing ancient trace fossil burrow networks like those made today by callianassid shrimps, but which pre-date considerably the first fossil record of such decapod crustaceans.
Revised age of Aleutian Island Arc formation implies high rate of magma production
Brian R. Jicha, University of Wisconsin, Geology and Geophysics, Madison, Wisconsin 53706, USA; et al. Pages 661-664.
Radioisotopic dating of subaerial and submarine volcanic and plutonic rocks provides insight into the timing of Aleutian Island Arc formation. Twenty-eight new 40Ar/39Ar ages coupled with existing K-Ar ages suggest that Aleutian Arc volcanism began at ~46 million years ago, not at 55 million years ago as previously thought. Aleutian Arc growth occurred in three main pulses from 38-29, 16-11, and 6-0 million years ago, which coincide with periods of intense magmatism in other western Pacific island arcs. When interpreted in the context of new-generation seismic data, the 40Ar/39Ar ages imply an astonishingly high time-averaged magma production rate of 89-182 km3/km/my. Determining the cause of extraordinarily high rates of arc magma production is critical to understanding the past and present geodynamics of the entire Pacific region.
Fissure eruptions at Mount Vesuvius (Italy): Insights on the shallow propagation of dikes at volcanoes
Valerio Acocella, Universita degli Studi Roma Tre, Dipartimento di Scienze Geologiche, Roma, Italy 00146; et al. Pages 673-677.
Eruptive fissures along the flanks of a volcano consist of aligned vents fed by a common source, typically a dike, at depth. Understanding the development of these fissures may provide important information on the shallow propagation of magma through dikes at volcanoes. Historical chronicles at Vesuvius (Italy) are considered to define the development of the 37 fissure eruptions that occurred between 1631 and 1944. The 1631 fissure re-opened the magmatic conduit after a long quiescence: the fissure migrated upward on the volcano slope and appears to have been triggered by the subvertical upward propagation of a dike. When the conduit was open, after 1631, 25 fissure eruptions migrated downward; these appear to be have been fed by the downslope lateral propagation of the dikes. Acocella et al. suggest two scenarios for the development of the fissures. When the summit conduit is closed, the fissures are fed by vertically propagating dikes. When the summit conduit is open, the fissures are fed by laterally propagating dikes along the volcano slopes. Therefore, the mechanism of shallow emplacement of magma in the Vesuvius cone seems to be largely controlled by dikes whose upward/downward sense of propagation depends upon the closure/opening of the conduit. Consistent behavior is found at other composite volcanoes, suggesting a general agreement with our model, independently of the tectonic setting and composition of magma.
Primary volcaniclastic rocks
J.D.L. White (corresponding author), Otago University, Geology Department, Dunedin 9015, New Zealand; and Bruce Houghton, University of Hawaii, Geology and Geophysics, Honolulu, Hawaii 96822, USA. Pages 677-680.
This paper addresses the words that volcanologists and other geologists use to describe fragmental deposits formed by volcanic eruptions. Using consistent terminology is critical to effective communication. At present, there are many volcanic deposits that are given different names by different researchers and others that have no agreed upon descriptive name at all. White and Houghton propose an integrated suite of clear volcanic terms that can be consistently applied to the entire range of fragmental volcanic rocks, and they suggest that their scheme should become standard terminology.
Dietary controls on extinction versus survival among avian megafauna in the late Pleistocene
Kena Fox-Dobbs, University of California-Santa Cruz, Earth Sciences, Santa Cruz, California 95064, USA; et al. Pages 685-689.
The late Pleistocene extinction decimated terrestrial megafaunal communities in North America, but did not affect marine mammal populations. In coastal regions, marine megafauna may have provided a "buffer" that allowed some large predators or scavengers, such as California condors, to survive into the Holocene. To track the influence of marine resources on avifaunas, Fox-Dobbs et al. analyzed the carbon, nitrogen, and hydrogen isotopic composition of collagen from late Pleistocene vultures and raptors, including species that survived the extinction (condor, bald eagle, golden eagle) and extinct species (teratorn, black vulture). Their results suggest that dependence upon terrestrial megafaunal carrion as a food source led to the extinction of inland California condor populations and coastal populations of teratorns and black vultures at the Pleistocene-Holocene boundary, whereas use of marine foods allowed coastal condor populations to survive.
How stable is the Mississippi Delta?
Torbjörn E. Törnqvist, Tulane University, Department of Earth and Environmental Sciences, New Orleans, Louisiana 70118-5698, USA; et al. Pages 695-700.
Hurricane Katrina has placed the international spotlight on the loss of Louisiana's coastal wetlands, one of the United States' most severe environmental problems of the past decades. Land subsidence exacerbates this problem and increases the impact of sea-level rise and storm surge. Unfortunately, there is a great deal of controversy about the main causes of subsidence in this area. A recent study has suggested that subsidence of the deeper crust may be 10 millimeters per year or more, which could have major consequences for the rebuilding of New Orleans and other communities in the Mississippi Delta. However, a new study that compares the rate of sea-level rise in various sections of the Mississippi Delta shows very little variation between these sections, indicating similar subsidence rates. Surprisingly, comparison of these data with sea-level data from the Caribbean, a region that is widely believed to be very stable, also shows little if any difference. This would indicate that the basement, at a 10–15 meter depth underneath the Mississippi Delta, has been highly stable for the past 8000 years, with subsidence rates of only fractions of millimeters per year. The main cause of land subsidence, therefore, appears to be compaction of the shallowest and most recent sediments in the delta. Future work will measure subsidence rates within the New Orleans metropolitan area using this sea-level method.
Seamount subduction erosion in the Nankai Trough and its potential impact on the seismogenic zone
Nathan L.B. Bangs, University of Texas-Austin, Institute for Geophysics, Austin, Texas 78759, USA; et al. Pages 701-704.
The world's largest earthquakes and tsunamis are generated when one tectonic plate thrusts over another in subduction zones. Bangs et al. conducted a three-dimensional seismic reflection survey along the Nankai Trough subduction zone, offshore southwestern Japan, to image the bumps and irregularities of the plate contacts and their effects on seismogenesis. Bangs et al. reveal in 3-D the geometry of a significant irregularity along the plate-boundary thrust fault, a 1-kilometer-high basement ridge on the subducting oceanic crust (a seamount) subducted to ~8 kilometers below seafloor. This seamount plowed through the overriding plate, immediately above the fault, and tectonically removed a ~1-kilometer-thick layer of material from the base of the overriding plate. Approximately 25 cubic kilometers of material has been removed via tectonic erosion by this seamount. The eroded material is presumably carried deep into the subduction zone, possibly still in front of the subducting seamount, where it may now become intimately involved in controlling rock strength and stress accumulation that leads to major earthquakes. These data reveal previously unseen details of irregularities caused by seamounts along the plate contact, and the erosional effects of seamounts that will help understand subduction zone structures, as well as earthquakes and tsunamis off Japan and other similar and potentially dangerous settings around the world.
GSA TODAY Science Article
Dates and rates of arid region geomorphic processes Kyle K. Nichols et al., Department of Geosciences, Skidmore College, Saratoga Springs, New York 12866, USA.
How fast does desert scenery form? The dramatic landscapes of deserts are well known throughout the world. For example, the rocky Alabama Hills of southeastern California are known from John Wayne movies, commercials for Subaru Outbacks, and the subterranean monsters of the movie Tremors. How long does it take these unusual landscapes to form? In a recent paper in GSA Today, Kyle Nichols of Skidmore College and colleagues utilized a novel type of atomic clock, one that is set by the exposure of elements common in rocks, such as beryllium, chlorine, and aluminum, to cosmic rays. As fresh material is exposed by weathering, it is possible to date the rates of erosion and landscape formation for such common desert landscape components, such as rock slides, alluvial fans, and sculpted boulder topography. These data provide an important calibration of the rates of surface processes in the arid Southwest, which are increasingly important because of the burgeoning human population and associated development.
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