GSA home

Log In | GSA Community | GSA Store | Join GSA | Donate | Contact Us

GSA home

| GSA Community | GSA Store | Donate | Contact Us

About GSA

Connected Community

Divisions &
Associated Societies

Education & Outreach

GSA Foundation

Meetings

Membership

Newsroom

Public Policy

Publications

Resources & Jobs

Sections

Find Your Science at GSA
05 February 2010
GSA Release No. 10-03
Contact:
Christa Stratton
Director - GSA Communications & Marketing
+1-303-357-1093
Bookmark and Share

GEOLOGY AND GSA TODAY

FebrUARY 2010 MEDIA HIGHLIGHTS

Boulder, CO, USA GEOLOGY presents studies of mineralized microfossils in the Yukon; what the Great Barrier Reef tells us about sea level; a new find in Japan of fossilized iridescent leaf-beetle wings; the puzzle of Ediacara biota; mammalian fossils in Mongolia; a dust bowl long before the Dust Bowl in the North American Great Plains; fish-eating semi-aquatic spinosaurs; evidence against the Lilliput Effect; and geochemical mapping of Mars. GSA TODAY explains how rock is converted to soil.

Highlights are provided below. Representatives of the media may obtain complementary copies of GEOLOGY articles by contacting Christa Stratton at the address above. View abstracts for this issue of GEOLOGY at http://geology.gsapubs.org/.

GSA TODAY articles are open access. 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, .

Geology

Neoproterozoic orogeny along the margin of Rodinia: Valhalla orogen, North Atlantic
Peter A. Cawood et al., School of Earth and Environment, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia. Pages 99-102.

This paper by Cawood et al. studies the latest Mesoproterozoic to mid-Neoproterozoic (1030-710 million years ago) sedimentation and orogenic activity that developed on the northeast Laurentian substrate around the North Atlantic borderlands and is currently exposed in Scotland, Shetland, East Greenland, Svalbard, and Norway, defined as the Valhalla orogen. The site for the orogen was initiated by ~95 degrees of clockwise rotation of Baltica with respect to Laurentia at the end of the Mesoproterozoic. This created a triangular ocean basin, the Asgard Sea, which received orogenic detritus from the Grenville-Sveconorwegian-Sunsas orogen. Sedimentary successions within the orogen accumulated during two cycles at 1030-980 million years ago and 910-870 million years ago, with each cycle terminated and the successions stabilized during tectonothermal episodes involving crustal thickening and igneous activity, some of calc-alkaline affinity, associated with the Renlandian (980-910 million years ago) and Knoydartian (830-710 million years ago) orogenic events. The Valhalla orogen represents an exterior accretionary orogen that developed along the margin of Laurentia and the Asgard Sea. The early stages of the Valhalla orogen are coeval with the final stages of the Grenville-Sveconorwegian-Sunsas orogen to the south, but are tectonically discrete; they constitute part of an exterior orogen that is entirely distinct from the interior orogen formed between collision of Laurentia, Baltica, and Amazonia.

* * * * * * * * * * *
Testing planktic foraminiferal shell weight as a surface water [CO32−] proxy using plankton net samples
Christopher J. Beer et al., Dept. of Geology and Geophysics, Yale University, New Haven, Connecticut 06511, USA. Pages 103-106.

Atmospheric CO2 concentrations influence ocean chemistry such that, at elevated concentrations, ocean acidification occurs. Acidification hinders the ability of some marine animals, such as foraminifera (pinhead-sized animals that live in surface waters of the open ocean), to produce shells. The shell weight of foraminifera has therefore been proposed as a means by which to reconstruct past CO2 levels, and get a better understanding of its effect on climate beyond the range of records that are based on air bubbles trapped in polar ice cores. However, the relationship between CO2 and shell weight is poorly understood. As part of their research, Beer et al. weighed foraminifera shells collected from the Arabian Sea where the surface water chemistry varies in such a way that it provides situations analogous to those expected globally in a human-induced high-CO2 world. They find that different species respond very differently to changes in water chemistry, indicating that other environmental factors influence shell growth at a species-specific level. Hence, while they cannot use foraminiferal weight to reconstruct past CO2 concentrations, the future response of marine animals to human-induced environmental changes is likely to be highly complex, and additional research is therefore required if we are to mitigate against future climate changes appropriately.

* * * * * * * * * * *
Late Pliocene-Pleistocene development of the Barents Sea Ice Sheet
Jan Sverre Laberg et al., Dept. of Geology, University of Tromso, N-9037 Tromso, Norway. Pages 107-110.

Using three-dimensional seismic data, Laberg et al. studied the glacial sediments of the south-western Barents Sea. Their aim is to present an improved paleo-environmental reconstruction for the late Pliocene-Pleistocene period, and discuss implications for the properties and development of the Barents Sea Ice Sheet which repeatedly has covered this area. They infer the following evolution: (1) a temperate Barents Sea Ice Sheet with channelized melt-water flow developed during the late Pliocene-early Pleistocene, (2) alternating glacial periods of ice with channelized melt-water flow and the first periods of ice including paleo-ice streams characterized the early and middle Pleistocene, and (3) more polar ice conditions and a Barents Sea Ice Sheet that mainly included large paleo-ice streams with little or no channelized melt-water flow in the middle and late Pleistocene.
 
***************
Eocene-Oligocene transition in Central Asia and its effects on mammalian evolution
Brian P. Kraatz and Jonathan H. Geisler, Dept. of Anatomy, Western University of Health Sciences, Pomona, California 91766, USA. Pages 111-114.

This study uses geologic evidence to date sediments in Mongolia that hold significant fossil mammalian remains. These fossils are important because they represent a dramatic change in biological communities that has been previously recognized in Europe and North America. Kraatz and Geisler used both radioisotopic dating, which dates cooled volcano rocks directly, and paleomagnetic techniques, which date sediments by determining changes in the orientation of Earth's magnetic pole, to show that mammalian community change corresponded with climatic change in Asia. Their study also allows for the precise correlation of Asian fossils to similarly aged fossils in Europe, North America, and Africa, for the first time.

* * * * * * * * * * *
Influences of alkalinity and pCO2 on CaCO3 nucleation from estimated Cretaceous composition seawater representative of “calcite seas”
Janie Lee and John W. Morse, Dept. of Oceanography, Texas A&M University, College Station, Texas 77843, USA. Pages 115-118.

Lee and Morse studied CaCO3 nucleation in seawater of an estimated Cretaceous Period “calcite sea” (i.e., low-magnesium calcite being the primary inorganic marine calcium carbonate precipitate) to determine the influences of alkalinity and pCO2 on calcite versus aragonite formation. The results of detailed chemical analysis indicate that it may have been possible for CaCO3 to commonly nucleate directly from Cretaceous seawater due to elevated calcium and alkalinity concentrations, even though atmospheric pCO2 was higher.

* * * * * * * * * * *
Evidence for the episodic “turn on” and “turn off” of turbid-zone coral reefs during the late Holocene sea-level highstand
C.T. Perry and S.G. Smithers, Dept. of Environmental and Geographical Sciences, Manchester Metropolitan University, Chester Street, Manchester M1 5GD, UK. Pages 119-122.

Increased inputs of terrestrially-derived sediments are regarded as a major disturbance to nearshore coral reef growth. Such perceptions are, however, often based on an incomplete understanding of the time scales and modes of reef initiation and growth. Here, Perry and Smithers present evidence to support recent suggestions that, once established, rapid reef accretion and growth is possible in these environments and that the limited depths of coral reef initiation (as controlled by water turbidity), and thus accretion potential relative to sea level, act as far more important constraining factors to the longevity and extent of reef development. Specifically, this paper describes two discrete periods of reef growth that occurred within one small (about 600 m wide) coastal embayment around a high island on the terrigenous sediment-dominated inner shelf of the central Great Barrier Reef, Australia. These reef-building phases occurred at either end of the Holocene sea-level highstand, the first during the late stages of the post-glacial sea-level rise (between about 6,900 and 4,500 years ago), the second following the late Holocene sea-level regression (about 1,600 years ago). Importantly, an ~3,000 year hiatus occurred between these reef-building events — a hiatus probably caused by subtle changes in sea-level position and associated sea-level-controlled shoreline morphodynamics. The study demonstrates the potential for short-lived (ephemeral) phases of rapid reef growth and accretion to sea level - whereafter reefs naturally pass into phases of “turn-off” characterized by low carbonate production rates and low coral cover. Most significantly, these changes occur independently of any anthropogenic forcing.

* * * * * * * * * * *
First evidence for locomotion in the Ediacara biota from the 565 Ma Mistaken Point Formation, Newfoundland
Alexander G. Liu et al., Dept. of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK. Pages 123-126.

The rapid appearance of animal life — the so-called Cambrian explosion — greatly troubled Darwin and has long remained a puzzle. The Ediacara biota, a strange assemblage of large and soft-bodied fossils some 30 million years older than the Cambrian period, has been the subject of considerable debate since the first discoveries around 60 years ago. Many paleontologists consider that these frondose organisms do not closely resemble modern animal groups, while others suggest they may even have been closer to fungi. Liu et al. show that evidence for complex movement can be found preserved along the wild Atlantic shores of Newfoundland, Canada. These 565 million-year-old trails form long narrow furrows along the ancient seabed. Structural features within the trails suggest production by an organism moving in a similar way to a modern sea anemone. Such markings clearly indicate a degree of muscular control during locomotion. That is important because it provides evidence that animals not only with collagen, but with muscles, were likely to have been present within this ancient assemblage. At present it is not possible to say which Ediacaran organism created the traces, but evidence for musculature in Ediacaran organisms is now on the cards.

* * * * * * * * * * *
Original structural color preserved in an ancient leaf beetle
Gengo Tanaka et al., Gunma Museum of Natural History, 1674-1, Kamikuroiwa, Tomioka, Gunma 370-2345, Japan. Pages 127-130.

In a study by Tanaka et al., greenish-blue metallic colored fossil elytra of the leaf beetle (Plateumaris sp.) have been discovered in the peat sediment of the middle Pleistocene (about 600,000 years ago) Hirabaru Formation, Kyushu, Japan. Scanning and transmission electron microscopy revealed the smooth outer surface of the fossil elytra and five alternating electron-dense and electron-lucent layers in the epicuticle. By applying the matrix method to the epicuticle, three reflective peaks of wavelengths 388 nanometers, 544 nanometers, and 656 nanometers were computed. These peaks correspond to the reflective color observed under white light. Thus, the coloration of the fossils was caused by multilayer reflectors of the epicuticle of the elytra. Pyrolysis-gas chromatography-mass spectrometric analysis revealed that the fossil elytra have preserved some of their original macromolecules (chitin, protein, and amino acids), which are similar to those of a related Holocene species. The high-porosity matrix of the peat contains many fragments of diatoms, indicating the high productivity of the water column but a reducing bottom environment. Slight acidity of the interstitial water also seems to contribute to the preservation of some original macromolecules of fossil insects through geologic time.

* * * * * * * * * * *
Formation of accretionary prisms influenced by sediment subduction and supplied by sediments from adjacent continents
Guy D.H. Simpson, Dept. of Geology and Paleontology, University of Geneva, Boulevard du Pont-d’Arve 40, CH-1211 Geneva, Switzerland. Pages 131-134.

Accretionary prisms associated with convergent plate boundaries are often thought of as being analogous to the wedges of snow or sand that form in front of moving bulldozers. However this view ignores two important aspects of real accretionary prisms: the fact that most sediments fed into subduction zones are transported over the top of the deforming wedge from the adjacent hinterland, and that typically more than half of the sediment entering subduction zones is subducted into the mantle. Simpson carried out two-dimensional numerical experiments with a mechanical viscoelastic-plastic model to show how sediment subduction and a hinterland sediment supply may influence the geometry and dynamics of accretionary wedges, relative to classic accretionary wedges formed by offscraping oceanic sediments from the downgoing slab.

* * * * * * * * * * *
Rapid, time-transgressive, and variable responses to early Holocene midcontinental drying in North America
John W. Williams et al., Dept. of Geography, Center for Climatic Research, University of Wisconsin, Madison, Wisconsin 53706, USA. Pages 135-138.

Because water is scarce in the Great Plains of North America, and may be scarcer in the future, paleoclimatologists for decades have sought to understand the patterns and drivers of past hydrological changes. Much of this work has focused on the early Holocene, roughly 11,000 to 7,000 years ago, when the Great Plains experienced a prolonged dry period more severe than the Dust Bowl or any of the megadroughts of the last millennium. During this time lakes shrank and became saltier, dunes activated, the prairie expanded, and warm-season grasses became more abundant. Now, Williams et al. have compiled the most comprehensive synthesis yet of paleoclimatic records from the Great Plains, in which they document the temporal and spatial patterns of drying. They show that the drying signal progressed from west to east, with sites in the Great Plains interior drying out before peripheral sites. They also show that sites varied in the rapidity of response, with a cluster of sites abruptly drying out around 8,000 years before present. The authors use these patterns to argue that the Great Plains drying was driven by a combination of enhanced summer insolation and the collapse of the Laurentide Ice Sheet 8,400 years ago.
 
***************
Oxygen isotope evidence for semi-aquatic habits among spinosaurid theropods
Romain Amiot et al., Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, #142 XiZhiMenWai DaJie, Beijing 100044, China. Pages 139-142.

Spinosaurs were large theropod dinosaurs showing peculiar specializations, including somewhat crocodile-like elongate jaws and conical teeth. Their biology has been much discussed, and a piscivorous diet has been suggested on the basis of jaw as well as tooth morphology and stomach contents. Although fish eating has been considered plausible, an aquatic or semi-aquatic lifestyle has seldom been suggested because of the apparent lack of corresponding adaptations in the postcranial skeleton of spinosaurs, which on the whole is reminiscent of that of other, terrestrial, large theropods. On the basis of the oxygen isotopic composition of their phosphatic remains compared with those of coexisting terrestrial theropod dinosaurs and semi-aquatic crocodilians and turtles, Amiot et al. conclude that spinosaurs had semi-aquatic lifestyles, i.e., they spent a large part of their daily time in water, like extant crocodilians or hippopotamuses. Their results shed light on niche partitioning between large predatory dinosaurs, since spinosaurs coexisted with other large theropods such as carcharodontosaurids or tyrannosaurids. The likely ichthyophagy and aquatic habits of spinosaurids may have allowed them to coexist with other large theropods by reducing competition for food and territory.

* * * * * * * * * * *
Early Neoproterozoic scale microfossils in the Lower Tindir Group of Alaska and the Yukon Territory
Francis A. Macdonald et al., Dept. of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138, USA. Pages 143-146.

Unique mineralized scale microfossils have been recovered from stratigraphic sections in the Yukon Territory and previously correlated with the Upper Tindir Group. These strata have been interpreted variously as late Neoproterozoic to early Cambrian in age. Macdonald et al.’s remapping of the area indicates that these sections are stratigraphically below an early Cryogenian glacial diamictite, unit 2 of the Upper Tindir Group, and are actually part of the Lower Tindir Group. That is, the Tinidr microfossils are about 200 million years older than previously thought. This new age model is consistent with carbon and strontium isotope geochemistry and indicates that these fossils can be added to the early Neoproterozoic record of eukaryotic evolution.

* * * * * * * * * * *
Gastropod evidence against the Early Triassic Lilliput effect
Arnaud Brayard et al., UMR 5561 CNRS Biogeosciences, Universite de Bourgogne, 21000 Dijon, France. Pages 147-150.

The Permian-Triassic (PT) mass extinction removed more than 80% of marine genera. Size reduction in its aftermath has repeatedly been described for various marine organisms, including gastropods. This phenomenon has been termed the Lilliput Effect and interpreted as a consequence of potential recurrent or prolonged harsh environmental conditions including, for instance, anoxia, productivity decline, or collapse of food webs during the mass extinction and its recovery. Few doubts were raised whether the Early Triassic Lilliput Effect is really such a significant phenomenon. Indeed, small gastropods are common in many fossil and extant assemblages. Brayard et al., present a new Smithian gastropod assemblage from Utah revealing numerous large-sized specimens, as large as 70 mm, the largest ever reported from the Early Triassic. Size frequency distributions of the studied assemblage also indicate that they were not unusually small when compared with later Mesozoic and modern faunas. This new occurrence of large-sized gastropods refutes the Lilliput hypothesis in this clade, at least for about the last two-thirds of the Early Triassic period. The simultaneous explosive diversification of many marine groups and re-appearance of large gastropods suggest that recovery in the marine realm was well under way, only less than 2 million years after the PT mass extinction.

* * * * * * * * * * *
Enriched Grenvillian lithospheric mantle as a consequence of long-lived subduction beneath Laurentia
Jeff Chiarenzelli et al., Dept. of Geology, St. Lawrence University, Canton, New York 13617, USA. Pages 151-154.

In a study by Chiarenzelli et al., geochemical characteristics and the neodymium isotopic composition of mafic and, newly discovered ultramafic rocks in the Adirondack Lowlands of New York State suggest widespread enrichment of the mantle beneath the northeastern margin of North America approximately 1.3 billion years ago. This enrichment, found in subsequent igneous rocks suites in the Adirondacks over a period of more than 300 million years, was likely caused by subduction of oceanic crust beneath the leading edge of the ancestral North American continent of Laurentia. This is caused by the vertical migration of fluids and melts through the lithosphere above subduction zones derived from the devolatilization and partial melting of the subducted oceanic crust. This geochemical signature is similar to that found in igneous rocks of other continents and may be a predictable and important differentiation process on Earth.

* * * * * * * * * * *
Mantle upwelling after Gondwana subduction death explains anomalous topography and subsidence histories of eastern New Zealand and West Antarctica
Rupert Sutherland et al., GNS Science, PO Box 30368, Lower Hutt 5040, New Zealand. Pages 155-158.

A careful look at the topography and seabed of West Antarctica by Sutherland et al. shows that it is about a kilometer higher than would be expected from its known geological structure. This observation is explained by a model of mantle upwelling that is rooted at depths greater than 1000 km. Such a model is able to explain the subsidence histories of sedimentary basins in southern New Zealand, is consistent with images of the mantle constructed from seismic waves, and may also be the reason for some Antarctic volcanoes. It is suggested that a long history of subduction - the sinking of plates back into the mantle - at the margin of the supercontinent Gondwana resulted in chemical and physical processes that produced relatively low-density rock to depths of more than 1000 km. When subduction stopped about 100 million years ago, the drag from the sinking plates dissipated and the low-density material was free to rise. Sutherland et al. suggest that this type of upwelling is an inevitable consequence of subduction death and may represent a previously unrecognized mode of mantle convection.

* * * * * * * * * * *
Local and regional geochemical signatures of surface sediments from the Cariaco Basin and Orinoco Delta, Venezuela
N.C. Martinez et al., Dept. of Earth Sciences, Boston University, Boston, Massachusetts 02215, USA. Pages 159-162.

Martinez et al. have characterized sediment from the Cariaco shelf (Venezuela) and Orinoco delta to better discriminate the fluvial sources of material to the Cariaco Basin. The chemical data from these sediments is later compared to that from modern sediment traps and also from paleorecords from the Basin to provide insight into the climatic mechanisms regulating Cariaco sediment composition over multiple time scales. They observe distinctive patterns in composition, with shelf sediments found near the Tuy River, Manzanares River, the Araya-Margarita region, and in the northern portion of the Unare Platform recording compositional differences. However, statistical models showed that the variability observed in modern sediment trap samples from the Basin cannot be explain by only mixing material from the mentioned local areas. Average trap values are best approximated by mixing upper crust (30-50%), local river sources (50-70% of either the Tuy, Unare, or Neveri rivers), and eolian dust (<2%). When compared to the paleorecords, the chemical data suggest a glacial-interglacial switching of these local sources. Interestingly, high glacial titanium/aluminum values could be supplied by some of the local rivers and not exclusively by Saharan dust, as previously thought. The present work does not offer support for the presence of any significant Orinoco source.

* * * * * * * * * * *
Past ice-sheet flow east of Svalbard inferred from streamlined subglacial landforms
J.A. Dowdeswell et al., Scott Polar Research Institute, University of Cambridge, Cambridge CB2 1ER, UK. Pages 163-166.

Evidence from terrestrial sediments and landforms has long been used to reconstruct the configuration of past ice sheets. As more geophysical and geological data have become available from polar waters, imagery of well-preserved submarine landforms is adding important new insights on the flow patterns of former marine-based ice sheets. The pattern of ice flow in the northern Barents Sea during the last full-glacial period (about 20,000 years ago) is not well known, due mainly to a lack of marine data east of Svalbard. Several years with little summer sea ice have allowed ship access and the acquisition of multibeam echo-sounding imagery of well-preserved subglacial landforms characterizing full-glacial ice-flow directions over ~150,000 square kilometers of the northwest Barents Sea. Dowdeswell et al. show that a major ice dome was located on easternmost Spitsbergen, at least 500 km west of its previously inferred position in the northern Barents Sea. This dome controlled the regional flow pattern; ice flowed eastward around Kong Karls Land into Franz Victoria Trough and north through Hinlopen Strait. An ice dome west of Kong Karls Land is required to explain the observed ice-flow pattern, but does not preclude an additional ice dome to the southeast. Discrepancies with earlier ice-sheet reconstructions reflect the lack of previous sea-floor observations, with evidence limited mainly to past ice loading and post-glacial rebound. The new pattern of ice-flow directions shows predominantly eastward rather than northward flow, with Franz Victoria Trough a major drainage pathway with a full-glacial balance flux of more than 40 cubic kilometers per year.

* * * * * * * * * * *
Recovering tectonic events from the sedimentary record: Detrital monazite plays in high fidelity
Jack Hietpas et al., Syracuse University, Dept. of Earth Sciences, Syracuse, New York 13244, USA. Pages 167-170.

Deciphering ancient geologic events by examining the rock record is a daunting task. When erosion has removed much of the solid rock record the difficulty of the task increases enormously. Some minerals, e.g. zircon, survive erosion/transportation and can be used as a proxy record for eroded crust. The age of zircon, for example, is inferred to reflect the age of the eroded material. But studies of modern sediment show that the zircon age record is significantly biased. A study by Hietpas et al. of the age of monazite (a thorium-rich mineral) much more accurately reflects the age of eroded continents, providing “high-fidelity” geologic information for long-gone mountain belts.

* * * * * * * * * * *
Eastern North American freshwater discharge during the Bølling-Allerød warm periods
Elizabeth A. Obbink et al., Dept. of Geoscience, University of Wisconsin, Madison, Wisconsin 53706, USA. Pages 171-174.

Freshwater runoff from ice sheets during the last ice age (about 19,000-6,500 years ago) likely caused changes in northern Atlantic Ocean circulation and hence changes in Northern Hemisphere climate. However, locations and rates of freshwater runoff from the North American Laurentide Ice Sheet have, up until now, been poorly constrained. Obbink et al. reconstruct sea-surface temperatures and salinity adjacent to the eastern outlets of North America during the Bølling-Allerød warm periods (about 14,600-12,900 years ago) to more fully understand the timing and effects of ice sheet runoff on Northern Hemisphere climate. Their results indicate that periods of increased freshwater runoff (reduced salinity) to the North Atlantic are associated with reduced ocean circulation, cooler North Atlantic temperatures, reduced Southeast Asian Monsoon intensity, and a southward shift of the Intertropical Convergence Zone. This work provides a better understanding of the effects of increased freshwater runoff to the North Atlantic, which has important implications for further understanding how the climate system may respond in the future to increased runoff due to more rapid melting of the Greenland Ice Sheet.

* * * * * * * * * * *
Initiation of subduction in the Alps: Continent or ocean?
Kurt Stüwe and Ralf Schuster, Dept. of Earth Sciences, University of Graz, A-8010 Graz, Austria. Pages 175-178.

Subduction is the technical term describing when a continent or oceanic plate is pushed underneath another. Typically, subduction occurs only in oceans, because oceanic lithosphere is much denser and heavier than continental lithosphere. In the study of the evolution of the European Alps, there has been a long discussion about how the Tethys Ocean disappeared underneath the European plate to cause the beginning of the evolution of the Alps in the Jurassic period. Recent evidence suggests that the onset of this subduction did not occur in the Tethys ocean, but inside the plate that later formed the Alps. While field observations are consistent with this idea, it has not been generally accepted, as it is unclear how such subduction can begin inside a continent. Stüwe and Schuster present some mechanical calculations and paleogeographic reconstructions that illustrate that this is mechanically plausible. They show that subduction inside the continent may be the natural consequence of the earlier Triassic evolution which formed the northern calcareous Alps and the Dolomites.

* * * * * * * * * * *
Shakedown in Madagascar: Occurrence of lavakas (erosional gullies) associated with seismic activity
Rónadh Cox et al., Dept. of Geosciences, Williams College, Williamstown, Massachusetts 01276, USA. Pages 179-182.

There is no doubt that human activities can lead to exceptionally high rates of landscape erosion; the challenge for modern studies of landscape change is to understand and measure those effects relative to the background chemical and physical processes driven by climate and tectonics. Madagascar is a case in point. Conventional wisdom holds that Madagascar’s erosion rates are exceptionally high, and that the lavakas (gullies) that pepper the Malagasy landscape are the result of deforestation and local agricultural practices. Analysis of the literature, however, reveals that interpretation to be based on narrative evaluations and regional extrapolations from a few local datapoints. Mapping of gully distributions from satellite images, and analysis of the occurrence of seismic events, by Cox et al. shows a very strong spatial correlation between earthquake distribution and the locations of dense lavaka clusters, underscoring the importance of tectonics in generating this complicated landscape. This new understanding may provide a tool to help predict areas most at risk for lavaka erosion in areas of ongoing forest clearance. Quantifying cause and effect in erosion - rather than relying on received wisdom - is central to the development of effective erosion control strategies.

* * * * * * * * * * *
Mapping Mars geochemically
G. Jeffrey Taylor et al., Hawaii Institute of Geophysics and Planetology, University of Hawaii, Honolulu, Hawaii 96822, USA. Pages 183-186.

Using a statistical technique called cluster analysis, Taylor et al. identify six distinct geochemical provinces on Mars from the concentrations of K, Th, Fe, Si, Ca, Cl, and H2O determined by the Mars Odyssey gamma-ray spectrometer, which has been collecting data since June 2002. The results show that the Martian surface has an overall composition like basalts found on Earth. The geochemical provinces lie in large, contiguous regions that differ in their chemical compositions, suggesting that a range of basalt compositions erupted onto the Martian surface. The K/Th ratio is surprisingly uniform among the geochemical provinces in spite of their tendency to separate from each other when rocks are weathered by water. To prevent measurable fractionation of K from Th, aqueous events must have been brief and/or the total throughput of water small. The muted weathering effects led to deposition of younger sedimentary deposits with the same compositions as older igneous units, explaining why a geochemical province may contain a range of geologic units.

* * * * * * * * * * *
Changes in productivity and redox conditions in the Panthalassic Ocean during the latest Permian
Thomas J. Algeo et al., Dept. of Geology, University of Cincinnati, Cincinnati, Ohio 45221-0013, USA. Pages 187-190.

During the Permian-Triassic boundary mass extinction event, the largest die-off in Earth history, ~90% of all species of marine organisms disappeared. Widespread development of anoxia in the global ocean has been suggested as a cause of this crisis. Algeo et al. investigated a Permian-Triassic boundary section at Gujo-Hachiman, Japan, representing a rare window into environmental conditions within the contemporaneous Panthalassic Ocean, which covered more than half the Earth’s surface at 252 million years ago. They showed that the deep watermass and seafloor of the Panthalassic Ocean became only slightly less oxic during this event, but that a strong shift toward sulfidic conditions occurred higher in the water column, probably in the oxygen-minimum zone around 500 m water depth, as reflected in an intensified rainout of pyrite framboids. They also showed that changes in sediment lithology that had previously been cited as evidence of deep-ocean anoxia, namely a shift from chert to black shale, instead can be explained simply through loss of the sinking flux of siliceous radiolarian shells, which would have been a likely consequence of redox changes in the upper water column, and concomitant changes in the composition of the phytoplankton community.

GSA Today Science Article

Rock to regolith conversion: producing hospitable substrates for terrestrial ecosystems
Robert C. Graham, Ann M. Rossi, Soil & Water Science Program, Dept. of Environmental Sciences, University of California, Riverside, California 92521-0424, USA, and Kenneth R. Hubbert, USDA Forest Service, Pacific Southwest Research Station, 3644 Avtech Parkway, Redding, California 96002, USA. Pages 4-10.

The Earth System processes that attract the most attention and research are those that result in catastrophic events; earthquakes, tsunamis, volcanic eruptions and landslides all spring to mind. But terrestrial life as we know it is dependent upon a far less dramatic, but no less important, geological process: the conversion of rock into soil. And yet, as is demonstrated in a recent paper in GSA Today by Robert Graham of the University of California and his colleagues, the formation of soil is one of the least well understood of Earth’s surficial processes. Based on field studies and experimentation, Graham and his co-authors show that while soil is a necessary component of a healthy, vibrant eco-system, ironically it is the presence of an ecosystem that facilitates the production of soil. Microbes and rootlets maintain a constant physical and chemical attack on rocky substrates, breaking them down, and generating pore spaces in which water can reside. Water stored in weathered rock in turn facilitates the development of water-dependent ecosystems. The rate at which soil is produced by this cyclic feedback between rock and life is the ultimate measure of the sustainability of our terrestrial ecosystem.

www.geosociety.org

###