July 2011 Geology highlights -- articles posted ahead of print May 24

May 26th, 2011
Locations studied include Alligator Point, Cat Island, Bahamas; Rice Lake, Ontario, Canada; Liverpool Land, east Greenland; Mount Rainier, Washington, USA; the Yangtze Gorges area, South China; the Moresby Seamount detachment, Woodlark Basin (east of Papua New Guinea); Hilo Ridge, Hawaii, USA; the Isua supracrustal belt, southern West Greenland; the northern Bohemian Massif; the Lonar crater, Deccan traps, India; the Rhone Glacier; and the Mersa/Wadi Gawasis along the Egyptian Red Sea coast.

Keywords: Jurassic Tank, Cat Island, sediment transfer, ground-based laser scanners, geodetic slip-rate estimates, Great Lakes, Rice Lake, archaeology, Liverpool Land, Greenland, LiDAR, Mount Rainier, Ediacaran, Burgess Shale, Yangtze Gorges, end-Permian mass extinction, Moresby Seamount, Woodlark Basin, bathymetry, Hilo Ridge, Hawaii, JAMSTEC, Isua, Greenland, Eoarchaean, Bohemian Massif, Bohemian diamond, Lonar crater, India, Great Ordovician Biodiversification Event, Poland, Rhone Glacier, Alps, Eocene-Oligocene Transition, Mersa/Wadi Gawasis, Egyptian Red Sea coast, Pine Island Bay, West Antarctica, Vesuvius, Pompeii, circum-Chryse Planitia region, Mars

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Mass-balance control on the interaction of axial and transverse channel systems

Wonsuck Kim et al., Dept. of Geological Sciences and Institute for Geophysics, Jackson School of Geosciences, University of Texas at Austin, Austin, Texas 78712, USA; doi: 10.1130/G31896.1.

Sediment flows from multiple sources create dynamic boundaries that migrate. Wonsuck Kim of The University of Texas at Austin and colleagues investigate the dynamic moving boundaries in a large tank at St. Anthony Fall Laboratory, Minneapolis, Minnesota, USA. The tank (nicknamed "Jurassic Tank") has a mobile floor, mixed sediment-water flows from multiple sources, and exhibits a small depositional landscape. In the tank experiment, a central, axially flowing river and fans that are transversely oriented to the river were developed. Boundaries between the axial river and the transverse fans advance or retreat and show a self-organizing nature, arranging the sedimentary system and balancing differential sediment supplies from various locations with the subsiding basement. Kim and colleagues report experimental measurement of the dynamic responses of moving boundaries in a range of sediment supply and tectonic conditions.


Polygonal sandcracks: Unique sedimentary desiccation structures in Bahamian ooid grainstone

Bosiljka Glumac et al., Dept. of Geosciences, Smith College, Northampton, Massachusetts 01063, USA; doi: 10.1130/G31973.1.

Bosiljka Glumac of Smith College and colleagues describe an unusual type of sedimentary structure - polygonal cracks in sand-dune and beach carbonate rocks that are less than 6000 years old on Alligator Point, Cat Island, Bahamas. These structures resemble cracks that typically form when mud dries (a feature geologists use to identify ancient sediments that were initially exposed to the air), but on Cat Island, such cracks formed in carbonate sand without mud. Glumac and colleagues collected sand from a modern Cat Island beach, which is composed of tiny sphere-shaped grains called ooids, and conducted a series of experiments to determine how these cracks form in the sand. The experiments showed that cracks can easily form in the absence of mud or sticky microbial films when ooid sand with common large irregular pores between grains dries at room temperature. These types of cracks have been documented rarely from mud-free sandy deposits because they are not easily preserved, and their formation requires well-rounded spherical grains of relatively uniform size and shape, a feature of Cat Island sand. Documentation of this unique sedimentary structure can help geologists constrain ancient environmental conditions.


Terrestrial source to deep-sea sink sediment budgets at high and low sea levels: Insights from tectonically active Southern California

Jacob A. Covault et al., U.S. Geological Survey, Eastern Energy Resources Science Center, Reston, Virginia 20192, USA; doi: 10.1130/G31801.1.

Sediment transfer across Earth's surface influences landscape evolution, soil distribution, and geochemical cycling of particulate and dissolved material. A comprehensive assessment of sediment fluxes from land to the deep sea over thousands of years of climate change is lacking, but important for separating anthropogenic influences from natural processes. Jacob A. Covault of the U.S. Geological Survey and colleagues pair cosmogenic radionuclide-derived erosion rates from river catchments with sediment accumulation rates from their corresponding deep-sea depositional systems in tectonically active Southern California. These measurements allow Covault's team to calculate sediment fluxes over thousands of years, the range over which global sea-level changes operate. During periods of both low and high sea level, land and deep-sea sediment fluxes do not show imbalances that might be expected in the wake of major sea-level changes. Sediment-transfer processes in small, tectonically active systems might be fundamentally different from those of larger systems that drain entire continents, in which sediment storage in flood plains and continental shelves can exceed millions of years. Thus, in small systems, depositional changes offshore can reflect onshore changes when viewed over thousands of years.


Estimating surface roughness of terrestrial laser scan data using orthogonal distance regression

Ryan M. Pollyea and Jerry P. Fairley, Dept. Geological Sciences, University of Idaho, Moscow, Idaho 83844-3022, USA; doi: 10.1130/G32078.1.

The availability of inexpensive, easy-to-operate ground-based laser scanners has opened up a multitude of possibilities for the collection of data about the character of Earth's surface. The current rush to apply this new tool to a wide array of geological problems has resulted in an exponentially increasing number of studies that utilize the technology, but no consensus currently exists among geologists regarding the techniques and methods used to analyze the vast amounts of data collected. Ryan M. Pollyea and Jerry P. Fairley of the University of Idaho at Moscow discuss shortcomings of some of the existing ad hoc methods for evaluating surface roughness, and describe a consistent and robust methodology that can be applied to data regardless of outcrop orientation and morphology. Surface roughness is one of the most common metrics used to extract information from large terrestrial laser-scanned datasets; the new method of analysis promises a higher level of accuracy and a wider range of applicability for this promising technology.


Reconciling geologic and geodetic model fault slip-rate discrepancies in Southern California: Consideration of nonsteady mantle flow and lower crustal fault creep

Ray Y. Chuang and Kaj M. Johnson, Dept. of Geological Sciences, Indiana University, Bloomington, Indiana 47408, USA; doi: 10.1130/G32120.1.

An important parameter in probabilistic earthquake forecasting is long-term fault slip rate. The measured age of offset geologic features across faults and the amount of offset can be used to obtain geologic slip-rate estimates. The number of these measurements in Southern California is limited. Ray Y. Chuang and Kaj M. Johnson of Indiana University, however, inferred slip rates on faults at more locations using GPS measurements of surface motions and numerical models. These so-called "geodetic" slip-rate estimates sometimes disagree with the geologic estimates. In Southern California, the largest discrepancy occurs on the Mojave section of the San Andreas fault. The geodetic slip-rate estimates may be biased by oversimplified model assumptions. Chuang and Johnson developed a new model that accounts for viscous flow, a process that was previously neglected in models. They show that deformation at the ground surface and inferred fault slip rates depend on the mantle viscosity and the elapsed time since the last large earthquake on each fault segment. If the last earthquake occurred relatively recently, the surface deformation rates will be higher than if the last earthquake occurred long ago. Chuang and Johnson demonstrate that this time-varying flow in the lower crust and mantle can reconcile the discrepancies between geologic and geodetic slip rates.


Quartz flakes in lakes: Microdebitage evidence for submerged Great Lakes prehistoric (Late Paleoindian-Early Archaic) tool-making sites

Elizabeth P. Sonnenburg et al., School of Geography and Earth Sciences, McMaster University, 1280 Main Street West, Hamilton, Ontario L8S 4K1, Canada; doi: 10.1130/G31964.1.

Elizabeth P. Sonnenburg of McMaster University and colleagues document the discovery of new submerged prehistoric finds at Rice Lake, Ontario, Canada. This is the first time underwater prehistoric human-made material has been identified using microscopic stone tool fragments (microdebitage) in lake sediments in North America or elsewhere. Quartz fragments were extracted from sediment cores in the McIntyre basin in Rice Lake and analyzed under a scanning electron microscope. Fragments with high angularity, flake scars, and lack of weathering were identified as microdebitage, providing clear evidence that they are human-made stone tool fragments. Microdebitage was restricted to a distinctive organic mud and peat layer which was identified as a wetland environment through microfossil (testate amoebae) analysis. The layer that contained the microdebitage was radiocarbon dated to the Late Paleoindian-Early Archaic period (10-8 thousand years B.P.) of Great Lakes prehistory, and demonstrates how stone tool fragments can identify submerged archaeological sites.


Crustal emplacement of exhuming (ultra)high-pressure rocks: Will that be pro- or retro-side?

J.P. Butler et al., Dept. of Earth Sciences, Dalhousie University, Halifax, Nova Scotia B3H 4J1, Canada; doi: 10.1130/G32166.1.

Metamorphism of continental crust at (ultra)high-pressure [(U)HP] conditions is thought to require the subduction of continental lithosphere. Yet in Liverpool Land, east Greenland, (U)HP metamorphism and subsequent exhumation took place in the Devonian, when Greenland (Laurentia) is widely considered to have been the overriding plate of the Caledonian orogen. One explanation, supported by recent geochronology, is that the Liverpool Land eclogite terrane is an exotic fragment derived from the deeply subducted lower plate (Baltica), similar in origin to the (U)HP rocks in western Norway, but its location in the upper plate remains problematic. Using numerical models, Butler et al. show how (U)HP rocks formed by one-sided continental subduction can then be transported horizontally into the lower (pro-wedge) or upper (retro-wedge) plate of an orogen, depending on relative strength of the upper plate crust. Horizontal transport of some exhuming (U)HP Baltica crust into the upper plate offers a simple and testable explanation for why Devonian (U)HP rocks exist on both sides of the Greenland-Norwegian Caledonides.


Whole-edifice ice volume change A.D. 1970 to 2007/2008 at Mount Rainier, Washington, based on LiDAR surveying

T.W. Sisson et al., U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA; doi: 10.1130/G31902.1.

Retreating alpine glaciers are visually compelling sentinels of climate change, but few studies precisely quantify ice volume changes over an entire mountain massif. Mount Rainier, Washington, USA, supports the largest concentration of glacial ice in the conterminous United States, and due to the broadly conical form of the volcano, its glaciers face all exposure directions. T.W. Sisson of the U.S. Geological Survey and colleagues use a high-resolution LiDAR topographic survey, differenced against an earlier digital elevation model derived from 1970 aerial images, to measure the changes in ice thickness and area over the entire Mount Rainier massif between 1970 and 2007/2008. Overall, the mountain has lost ~14 vol-percent of the ice estimated by earlier workers to have been present in the 1970s and early 1980s. Ice losses are greatest at lower elevations (< 2000 m) and higher elevations on the south flank. unexpectedly, glaciers on the east flank have expanded slightly.


Taphonomic study of Ediacaran organic-walled fossils confirms the importance of clay minerals and pyrite in Burgess Shale-type preservation

Evan P. Anderson et al., Dept. of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, USA; doi: 10.1130/G31969.1.

Potentially the most famous Cambrian deposit, the Burgess Shale, exceptionally preserves fossils as carbonaceous compressions, where organic tissues are preserved as carbon films providing extraordinary views into early animal anatomy. This taphonomic window, known as Burgess Shale-type preservation, is responsible for preservation of soft tissues outside of the Burgess Shale, including organic-walled microfossils. Understanding what leads to this preservational regime has been a subject of recent research and debate. Because of their abundance, organic-walled microfossils offer unique opportunities for destructive analyses that would be otherwise frowned upon with the exceptional fossils of the Burgess Shale. Using a combination of techniques, Evan P. Anderson of Virginia Tech and colleagues analyzed two Ediacaran organic-walled taxa from the Yangtze Gorges area, South China: Chuaria (an acritarch) and Vendotaenia (a ribbon-shaped fossil). The data suggest that these fossils are remarkably similar to Burgess Shale-type fossils, closely associated with both clay minerals and pyrite. The comparative taphonomy of organic-walled microfossils and Burgess Shale-type macrofossils indicates that the preservation of acritarchs may be aided by clay and pyrite mineralization, and that acritarchs may serve as proxies for uncovering the taphonomic histories of fossils preserved as carbonaceous compressions.


Enhanced nitrogen fixation in the immediate aftermath of the latest Permian marine mass extinction

Shucheng Xie et al., Key Laboratory of Biogeology and Environmental Geology of Ministry of Education, China University of Geosciences, Wuhan 430074, China; doi: 10.1130/G32024.1.

Based on corresponding organic carbon and nitrogen isotopic records, Shucheng Xie of China University of Geosciences and colleagues suggest that nitrogen fixation was the main source of nutrient nitrogen in the ocean directly after the end-Permian mass extinction, the largest extinction since the beginning of animal life on Earth. Therefore, the ocean was deficient in available biotic nitrogen owing to high energy demand for nitrogen fixation. This would be caused by widespread ocean anoxia that enhanced denitrification and/or anaerobic ammonium oxidation at this time. The widespread ocean anoxia would also be the direct mechanism of the end-Permian mass extinction. On the other hand, perturbation of the marine nitrogen cycle might have contributed to high temperatures following the main marine mass extinction through the release of the greenhouse gas nitrous oxide. Cyanobacteria and/or other anaerobic heterotrophic nitrogen-fixing organisms were supposed to be the main nitrogen fixers, which might be different from those in synchronous deep-water settings.


Splitting a continent: Insights from submarine high-resolution mapping of the Moresby Seamount detachment, offshore Papua New Guinea

Romed Speckbacher et al., Marine Geodynamics, IFM-GEOMAR, 24148 Kiel, Germany; doi: 10.1130/G31931.1.

The Moresby Seamount detachment in the Woodlark Basin (east of Papua New Guinea) is arguably the best exposed active detachment fault in the world. Romed Speckbacher of IFM-GEOMAR, Germany, and colleagues present the results of a high-resolution autonomous underwater vehicle survey of bathymetry, bottom water temperature, and turbidity. In combination with dredging and existing drillhole data, a synthesis of the tectonic geomorphology, kinematics, and mechanics of the detachment is provided. Two major smooth areas are tectonically created, and megascopic (kilometer scale) slickensides indicate downdip direction of movement. The detachment is transected by a major sinistral strike-slip fault, suggesting deformation partitioning in the detachment zone in response to the 500-thousand-year change in plate kinematics. The mainly gabbroic protoliths and cataclasites from the fault show pervasive syntectonic alteration, leading to large increases in abundance of quartz and, more important, calcite. Resulting quartz-rich and calcite-rich mylonites play a crucial role as weak fault rocks. A kilometer-sized anomaly in bottom water temperature and turbidity is found at the downdip end of the detachment zone, indicating that it hosts an active hydrothermal system, probably fed by overpressured fluids from a deep crustal source.


A rich Ediacaran assemblage from eastern Avalonia: Evidence of early widespread diversity in the deep ocean

Philip R. Wilby et al., British Geological Survey, Keyworth, Nottingham NG12 5GG, UK; doi: 10.1130/G31890.1.

The late Precambrian was one of the most turbulent periods in Earth's history, yet it was against this backdrop that the first large, complex creatures appeared. Collectively referred to as the "Ediacara biota," almost everything about them is enigmatic, not least where they lie on the tree of life and how they made their living. The Ediacara biota is only known from a handful of sites worldwide, most of which record shallow water environments. Philip R. Wilby of the British Geological Survey and colleagues describe remarkably diverse and abundant fossil communities from the United Kingdom in rocks that are more than 560 million years old and that were deposited on the deepwater slopes of an active volcanic island. Each community was smothered by a catastrophic underwater mudflow and was preserved intact, with bizarre bush-like species and meter-tall fronds lying next to unimposing limpet-like domes. Now virtually invisible on the rock surface, these fossils have been revealed by an extensive program of silicon rubber molding. Collectively, they provide one of our clearest windows on late Precambrian life, and provide new insight into how widespread these creatures were in the world's oceans, how they might have dispersed, and how they interacted with their environment.


Early growth of Kohala volcano and formation of long Hawaiian rift zones

Peter W. Lipman and Andrew T. Calvert et al., U.S. Geological Survey, Menlo Park, California 94025, USA; doi: 10.1130/G31929.1.

Samples of submarine-erupted basalt lava from the toe of the Hilo Ridge at water depths of about 3800 m below sea level have yielded the oldest ages known from the Island of Hawaii: 1138 plus or minus 34 to 1159 plus or minus 33 thousand years old. These submarine rift-zone ages constrain the inception of large-volume eruptions at Kohala volcano, provide the first measured duration of shield-building (870 thousand years) for any Hawaiian volcano, and show that this 125-km rift zone developed to near-total length during early growth of Kohala. Long easterly-trending rift zones of Hawaiian volcanoes may follow fractures in oceanic crust activated by arching of the Hawaiian Swell in front of the propagating hotspot. The new samples were collected by deep-diving submersibles operated by the Japan Marine Science and Technology Center (JAMSTEC) during the collaborative Japan-USA research project Deep Underwater Hawaiian Volcanism.


Dunites from Isua, Greenland: A ca. 3720 Ma window into subcrustal metasomatism of depleted mantle

C.R.L. Friend and A.P. Nutman, 45 Stanway Road, Headington, Oxford OX3 8HU, UK; doi: 10.1130/G31904.1.

Understanding how early crust formed, and its structure, is hampered because all of the rocks older than 3600 million years old occur in gneiss terrains that have suffered high-grade metamorphism. Similarly, attempts to understand mantle evolution are hampered because identifying such rocks is very difficult. The chemistry of any surviving pieces of Eoarchaean mantle together with related crust, therefore, is extremely important in determining early crust-forming mechanisms. Within the 3720-million-year-old part of the Isua supracrustal belt (southern West Greenland) lenses of high-magnesium, low-aluminum dunite belt occur that are interpreted as relicts of Eoarchaean mantle with minimal crustal disturbance. The lenses occur within altered, higher aluminum, calcium ultramafic schists that are intercalated with pillow basalts and gabbros with island arc chemical signatures, all intruded by 3710-million-year-old tonalites. One variety of dunite is dominated by olivine (Fo90-92), which does not show high field strength element (HFSE) anomalies. Another variety contains Fo96-98 olivine intergrown with titanium-humite group minerals that have strong positive HFSE anomalies that are complementary to the negative HFSE anomalies of the adjacent amphibolites. C.R.L. Friend of the Beijing SHRIMP Center and A.P. Nutman of the University of Wollongong, Australia, propose that the dunites are tectonic slivers of 3720-million-year-old subarc mantle, preserving evidence for varying interaction with mafic magmas in subcrustal environment. These are by far the oldest direct geochemical link between coeval mantle and crustal rocks, and are new evidence for subduction zone-like environments on early Earth.


Diamond and coesite discovered in Saxony-type granulite: Solution to the Variscan garnet peridotite enigma

Jana Kotková et al., Czech Geological Survey, Klárov 3, 118 21 Praha 1, Czech Republic; doi: 10.1130/G31971.1.

Microdiamond and coesite were newly discovered in situ in minerals of high-pressure granulites of the northern Bohemian Massif. The discovery confirms the long-disputed provenance of Europe's first reliable diamond, the "Bohemian diamond" found in 1870, which is on display in the National Museum in Prague. The diamond and coesite represent the first robust evidence for ultrahigh-pressure (>3 GPa) metamorphism of a major rather than exotic rock type of the European Variscides. This discovery by Jana Kotkova of the Czech Geological Survey and colleagues of diamond and coesite as 5-30 micrometer inclusions in the major minerals of the peak granulite assemblage garnet and kyanite means that previously determined equilibrium pressure-temperature conditions for these rocks are significantly underestimated, and that the granulites are in fact eclogite facies rocks. The ultra high-pressure conditions imply widespread, deep (100-150 km) subduction of upper continental crust in Central Europe, which provides an explanation for the common, but until now not fully understood, association of crustal high-pressure granulites and mantle garnet peridotites in the European Variscan belt. Existing tectonometamorphic models for these rocks involving homogeneous crustal thickening, heating, and extrusion of the lower crust now need significant revision.


40Ar/39Ar age of the Lonar crater and consequence for the geochronology of planetary impacts

F. Jourdan et al., Western Australian Argon Isotope Facility, JdL Centre & Applied Geology, Curtin University, GPO Box U1987, Perth WA 6845, Australia; doi: 10.1130/G31888.1.

Asteroid impacts play an important role in the evolution of planetary surfaces. A great majority of impacts in the solar system occurred on bodies covered by basaltic rocks, as illustrated by the surfaces of Venus, Mars, the moon, and some large asteroids. F. Jourdan of Curtin University, Australia, and colleagues have dated the Lonar crater, which is located on the Deccan traps in India and is one of the rare craters on Earth emplaced on basaltic lava flows. Their new argon-argon age shows that the crater is ten times older than previously thought and occurred 570,000 years ago. By measuring the composition of the trapped argon from Lonar impact rocks, Jourdan and colleagues also provide the first evidence that asteroid impacts are extremely efficient at resetting the atomic clock of basaltic rocks, and thus, that basaltic rocks are excellent candidates for recording impact events of any size. They also suggest that basaltic rocks should be a priority target of sample recovery by future space missions interested in the impact history of the solar system.


Major oceanic redox condition change correlated with the rebound of marine animal diversity during the Late Ordovician

Tonggang Zhang et al., Chinese Academy of Sciences, Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China; doi: 10.1130/G32020.1.

The Middle-Late Ordovician was a time of dramatic change in the biosphere and environment. The Great Ordovician Biodiversification Event (GOBE) that possibly began ~470 million years ago, yielded arguably the most important increase of marine animal diversity during Earth's history. At the end of the Ordovician (~445-443 million years ago), a large mass extinction occurred, which has been causally linked to the Late Ordovician glaciations. Moreover, global fossil records indicate that the diversity of marine animals decreased following the GOBE and rebounded into the late Katian before the end-Ordovician mass extinction. No study has been carried out, however, to explain this decrease and rebound of marine animal diversity. To understand probable influences of environmental changes on the decrease and rebound of biodiversity during the Late Ordovician, Tonggang Zhang of the Chinese Academy of Sciences and colleagues performed detailed geochemical analyses on Late Ordovician sediments from Poland. They used multiple proxies to reconstruct the chemical evolution of the Ordovician oceans, and the new data reveal a major oceanic redox condition change during the late Katian. The results from the Baltica area suggest that changes of the redox condition may have contributed to the decrease and rebound of marine animal diversity during the Late Ordovician.


The Rhone Glacier was smaller than today for most of the Holocene

Brent M. Goehring et al., Dept. of Earth and Environmental Science and Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York 10964-1000, USA; doi: 10.1130/G32145.1.

The past 11,500 years, also known as the Holocene, represents a period of relative climate quiescence compared to the Last Ice Age. Even though the climate has been relatively stable, natural variability of Holocene climate and, in turn, the response of glaciers to those changes, remains underconstrained, there is consensus that the changes impacted humans and society. Brent M. Goehring of Columbia University and colleagues use new means to measure periods during which glaciers were larger and smaller than today and apply this new technique to the Rhone Glacier, the dominant glacier in the Alps during the Last Ice Age. Rare nuclides of carbon and beryllium, produced in the rock surfaces underlying the glacier by cosmic rays, provide a new chronometer to measure the integrated period that the rock surfaces were ice-free. The new chronometer also allows Goehring and colleagues to determine the rate of abrasive action by the sliding glacier across the rock surfaces. Their results indicate that for much of the Holocene (6500 years), the Rhone Glacier was smaller than it is today. Therefore the Rhone Glacier, and likely many glaciers in the Alps, responded sensitively to the modest climate fluctuations of the Holocene. Finally, their results show that abrasion rates increase non-linearly with ice velocity, as has been previously suggested.


Seawater calcium isotope ratios across the Eocene-Oligocene transition

Elizabeth M. Griffith et al., Dept. of Geology, 221 McGilvrey Hall, Kent State University, Kent, Ohio 44242, USA; doi: 10.1130/G31872.1.

During the Eocene-Oligocene Transition (~34 million years ago), Earth's climate cooled from a "greenhouse" to an "icehouse" state. Antarctic ice sheets reached sea level, ocean chemistry changed in concert, and calcite sedimentation in open ocean, one of the controlling factors of Earth's climate on long time scales, changed dramatically-increasing the amount of carbonate deposited on the Pacific Ocean floor. Elizabeth M. Griffith of Kent State University and colleagues used stable calcium isotope ratios in two different minerals precipitated in the ocean to document that the isotopic composition of dissolved calcium in the oceans was stable throughout this period of global cooling, whereas at other times of global cooling (e.g., the middle Miocene, ~13-15 million years ago), the calcium isotopic chemistry of the ocean changed considerably. The response of ocean chemistry to climate change thus depends upon the composition of seawater and the rate of change. Griffith and colleagues also show that calcite precipitated by marine micro-organisms records not only changes in the seawater isotope ratio of calcium, but also changes in temperature and/or the assemblages of such organisms, which are sensitive to climate change.


Evolution of a Pharaonic harbor on the Red Sea: Implications for coastal response to changes in sea level and climate

Christopher J. Hein et al., Dept. of Earth Sciences, Boston University, 675 Commonwealth Avenue, Boston, Massachusetts 02215, USA; doi: 10.1130/G31928.1.

Archaeological excavations at the site of Mersa/Wadi Gawasis along the Egyptian Red Sea coast have uncovered the world's oldest archaeological evidence of long-distance seafaring. Finds have included coiled ropes, boxes used to ship precious goods, stelae inscribed with the details of numerous trading expeditions, and parts of the sailing vessels themselves. Christopher J. Hein of Boston University and colleagues investigate the existence of an ancient embayment at this site, and provide a detailed evolutionary model for the bay, from its opening 7,000 years ago, through the time of its use as a harbor by the ancient Egyptians 4,000 years ago when sea level in the Red Sea was more than one meter higher than at present, and through the closure of the bay ~1,000 years ago. Their primary discover was that, due to higher sediment loads in rivers during a period of wetter climate, the bay was shallowing and closing at a time when sea level was still rising, and was only half of its maximum size when it was occupied by the ancient Egyptians. Whereas it is common now to study the effects of accelerated sea-level rise in terms of coastal inundation, this study serves as an example of the need to account for possible changes in sediment supplies as precipitation patterns shift in response to climate change.


Geological record of ice shelf break-up and grounding line retreat, Pine Island Bay, West Antarctica

Martin Jakobsson et al., Dept. of Geological Sciences, Stockholm University, 106 91 Stockholm, Sweden; doi: 10.1130/G32153.1.

The catastrophic break-ups of the floating Larsen A and B ice shelves in 1995 and 2002, and associated acceleration of glaciers that flowed into these ice shelves, were among the most dramatic glaciological events observed in historical time. This raises a question about the larger West Antarctic ice shelves. Do these shelves, with their much greater glacial discharge, have a history of collapse? New high-resolution images by Martin Jakobsson Stockholm University and colleagues of the seafloor in Pine Island Bay, West Antarctica, reveal evidence of a massive ice shelf break-up and associated grounding line retreat that occurred about 12,000 years ago. The evidence from the glacial break-up exists in the form of huge iceberg furrows on the seafloor that are characterized by well-preserved small ridges. The ridges were produced daily as a consequence of the tidally influenced motion of mega icebergs, maintained upright in an iceberg armada, produced from the disintegrating ice shelf and retreating grounding line. These new data indicate that events such as the break-ups witnessed in 1995 and 2002 of Larsen A and B also happened in the past, but on an even larger scale.


Column collapse and generation of pyroclastic density currents during the A.D. 79 eruption of Vesuvius: The role of pyroclast density

Thomas Shea et al., Dept. of Geology and Geophysics, University of Hawaii, Honolulu, Hawaii 96822, USA; doi: 10.1130/G32092.1.

The renowned A.D. 79 eruption of Vesuvius produced an enormous ash column (~35 km high at its maximum) that alternated several times between fully stable, buoyantly rising phases, and unstable phases of partial or total collapse. Column collapses during the latter phases produced at least six pyroclastic density currents which caused the destruction of Roman towns around the volcano, such as Pompeii. The density of erupted juvenile clasts and the abundance in dense rock ripped from the conduit walls were measured within several deposits covering stable and unstable column regimes to establish whether they may have played a role in destabilizing the eruptive column. Data presented by Thomas Shea of the University of Hawaii and colleagues suggest that repeated changes in both juvenile pumice and wall-rock content combined to produce the multiple deadly pyroclastic currents during this eruption.


Deep-water erosional remnants in eastern offshore Trinidad as terrestrial analogs for teardrop-shaped islands on Mars: Implications for outflow channel formation

Lorena Moscardelli and Lesli Wood, Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas 78758, USA; doi: 10.1130/G31949.1.

Discovery of geomorphological elements such as valleys and channel-like features on the surface of Mars has prompted debate about alternative origins for these morphologies, including erosion by lava, liquid carbon dioxide, glaciations, and submarine landslides. Similarities between Martian geomorphological elements and those of certain terrestrial environments suggest that water processes were involved in the formation of some visible Martian landscapes. Lorena Moscardelli and Lesli Wood of The University of Texas at Austin explore the similarities between erosional remnants imaged within the deepwater regions of eastern offshore Trinidad and teardrop-shaped islands described within the downstream end of outflow channels on Mars. Moscardelli and Wood propose that teardrop-shaped islands on Mars formed as the result of catastrophic submarine landslide processes affecting the circum-Chryse Planitia region, and that these processes were similar to those documented within continental margins on Earth.

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