April GEOLOGY and GSA TODAY media highlights

Surface erosion control on the evolution of the deep lithosphere
Russell N. Pysklywec, University of Toronto, Geology, Toronto, Ontario M5S 3B1, Canada. Pages 225-228.

Over the last decade there has been growing appreciation that surface weathering and erosion exerts an important control on the deformation and evolution of Earth's crust. This paper reports numerical experiments that demonstrate that this control extends well into Earth's mantle. Specifically, the experiments show that erosion of tectonically uplifted surfaces at modest rates can dramatically alter the general style of plate boundary evolution. Erosion, for example, can trigger a transition from stable subduction to delamination/retreat of the over-riding plate. This intriguing interaction between short-timescale climate-driven processes and large- scale mantle evolution, which has not previously been recognized, has fundamental implications for the fields of earth system sciences and comparative planetology.

Cracks and fins in sulfate sand: Evidence for recent mineral-atmospheric water cycling in Meridiani Planum outcrops?
Gregory V. Chavdarian and Dawn Y. Summer (corresponding author), University of California-Davis, Department of Geology, Davis, CA 95616-8605, USA. Pages 229-232.

Features similar to those on Mars can be seen here on Earth. Images from the Mars Exploration Rover Opportunity show numerous sulfate-rich outcrops that contain polygonal cracks that crosscut bedding. Some cracks are associated with millimeter-thick, platy fins that protrude a few centimeters above the outcrops. Cracks that crosscut bedding also form in the sulfate-rich interdune areas and dune slopes at White Sands National Monument, New Mexico. Some of these cracks are also associated with fins that protrude up a few centimeters from the crack edges. Cracks at White Sands form from cementation of damp gypsum sand followed by contraction due to dehydration. Fins form from cementation along crack edges by evaporating water, followed by adhesion of fine-grained particles to the fin surfaces. Similar processes may be important for crack and fin formation in Meridiani Planum outcrops on Mars, implying recent water cycling between the sulfate outcrops and the Martian atmosphere.

Shelf and open-ocean calcareous phytoplankton assemblages across the Paleocene-Eocene Thermal Maximum: Implications for global productivity gradients
Samantha J. Gibbs, Department of Geosciences, Pennsylvania State University, University Park, PA 16802, USA; et al. Pages 233-236.

The Paleocene-Eocene Thermal Maximum (PETM) was a transient interval of global warming and ocean acidification that resulted from the rapid release and oxidation of fossil fuel from ocean sediments. Gibbs et al. focus on the hypothesis that increased phytoplankton productivity could have contributed to increased CO2 drawdown returning CO2 and temperatures to background levels. They address discrepancies between previous conclusions of when and where this increased productivity occurred. Nannofossil records, i.e., records of fossil calcareous algae, provide a powerful tool to quantify primary plankton production, allowing scientists to reconstruct past marine productivity variations. The nannofossil data of Gibbs et al. combined with published biotic records indicate that across the onset of the PETM gradients, in-shelf–off-shelf productivity steepened, with a decrease in open-ocean productivity coeval with increased productivity in shelf areas. Productivity levels recovered in the open ocean during the later stages of the event, which may have contributed to CO2 drawdown.

Completeness of the fossil record: Estimating losses due to small body size
Roger A. Cooper, Geological and Nuclear Sciences, CRI, Gracefield, Wellington, Wellington, New Zealand; et al. Pages 241-244.

The rock record of fossils is increasingly being used to interpret patterns of macroevolution, past biodiversity, and environmental change. However, museum collections of fossil populations are known to be biased by the depletion of small species because of their fragility and difficulty of collection ('size culling'). This limits the value of museum collections for paleobiological interpretation. In a novel approach, the exceptionally full and well-documented record of New Zealand fossil mollusks through the last 60 million years has been used to quantify the extent of this size bias. No fewer than 36% of all species have been removed by size culling, 27% from the smallest size fraction (less than 5 millimeters) alone. To get a true picture of the size structure and diversity of past communities, geological scientists must increase the number of small species accordingly. If this result is duplicated elsewhere, it will open the way to mitigation of the size bias in estimates of past biodiversity based on marine mollusks and improve scientific understanding of the history of life and life communities.

A new window into Early Archean life: Microbial mats in Earth's oldest siliciclastic tidal deposits (3.2 Ga Moodies Group, South Africa)
Nora Noffke, Old Dominion University, OEAS, Norfolk, VA 23529, USA; et al. Pages 253-256.

Microbial effects on marine sediments are generally understood to result from biogeochemical processes that generate biogene structures such as stromatolites. In contrast, the interaction of microbial mats with physical dynamic parameters has been long underestimated, although this interaction plays an important role in the formation of sedimentary structures as well. Siliciclastic tidal flats are ruled predominantly by the physical sediment dynamics of tidal currents or waves, whereas chemical sedimentary processes play a minor role. In such settings, microbial mats constructed by cyanobacteria and/or other microorganisms do not produce stromatolites, but microbially induced sedimentary structures (MISS). MISS were defined first in modern tidal flats, but have since been detected in sandstones of Phanerozoic, Proterozoic, and Archean ages. Noffke et al. describe MISS from Earth's oldest siliciclastic tidal flat: the Early Archean Moodies Group, South Africa. Independent of silicified microfossils and stromatolites, the newly detected MISS support the presence of bacterial life in the Early Archean.

Dynamics of the Salton block: Absolute fault strength and crust-mantle coupling in Southern California
Noah Fay (corresponding author) and Eugene Humphreys, University of Oregon, Geological Sciences, Eugene, OR 97403, USA. Pages 261-264.

Fay and Humphreys analyze the forces causing mountain building and earthquakes in Southern California. They apply basic physical principles in new ways to study the deformation of the crust, and their results suggest that faults in Southern California, such as the San Andreas, are "weak," i.e., they experience less stress than one might expect. Despite this weakness, stress in the crust is still sufficient to cause earthquakes and mountain building, as evidenced by the impressive topography of the Transverse Ranges (e.g., San Gabriel Mountains, San Bernardino Mountains).

Typical Triassic Gondwanan floral elements in the Upper Permian of the paleotropics
Hans Kerp, Forschungsstelle für Paläobotanik, Dept. of Geology, Universität Münster, Münster, Germany; et al. Pages 265-268.

The Permian-Triassic boundary marks the largest extinction event in Earth's history. Plants belonging to a group that is traditionally considered as typical for the Triassic of the Southern Hemisphere have now for the first time been found in Permian sediments deposited in the equatorial region. This discovery indicates that the tropics were apparently a region and that these plants were present before the extinction event and survived it. Moreover, these findings provide evidence for the hypothesis that the tropics have been a radiation center where new gymnosperm groups have evolved. After the Permian-Triassic boundary event, which is marked by a global warming, this group migrated southward, and in the Triassic it had colonized the entire Southern Hemisphere, where it often was the dominant floral element.

Constraints on Pennsylvanian glacioeustatic sea-level changes using oxygen isotopes of conodont apatite
Michael M. Joachimski, University of Erlangen-Nuernberg, Institute of Geology and Mineralogy, Erlangen, Bavaria D-91054, Germany; et al. Pages 277-280.

Conodonts from cyclothems in the mid-continental United States were studied for oxygen isotopes in order to constrain sea-level fluctuations during the late Carboniferous glaciation. The late Carboniferous deposits of this region are composed of cyclic alternations of thin transgressive limestones, offshore gray to black phosphatic shales, and thick regressive limestones, a sequence that is under- and overlain by nearshore to terrestrial shales with paleosols and coal beds. Glacioeustatic sea-level fluctuations are considered the primary cause for the formation of these cyclothems. The oxygen isotope data of conodonts (extinct early jawless vertebrates that possessed a complex feeding apparatus of elements composed of calcium phosphate) indicate that these Carboniferous glacioeustatic sea-level changes were probably larger than the 120-meter fluctuations recorded for the glaciations in the Pleistocene.

Modern-day tectonic subsidence in coastal Louisiana
Roy K. Dokka, Louisiana State University, Center for GeoInformatics, Department of Civil & Environmental Engineering, Baton Rouge, LA 70803, USA. Pages 281-284.

Subsidence, or the sinking of the land, has long been suspected to play a role in the ongoing inundation of Louisiana's coastal lands and marshes. Sinking has lowered the land and associated levee defenses, and thus created much of the flooding vulnerability that was recently and tragically exploited by hurricanes Katrina and Rita. Dokka suggests that subsidence of the Michoud area of New Orleans has a large, but previously unreported tectonic component. This tectonic component is associated with the Mississippi River delta, a large, unstable volume of sediment that has accumulated in south Louisiana since the last ice age. Previously, most scientists considered subsidence to be the result of young sediment compaction/consolidation and human activities. Using official NOAA surveying measurements collected between 1955 and 2005 to determine vertical movements of the land, the new study reports that tectonic fault movements may account for 73% and 50% of the total subsidence during the intervals 1969–1971 and 1971–1977, respectively. These results are timely because post-Katrina and Rita rebuilding of levees, infrastructure, and neighborhoods will require accurate understanding of the current and future behavior of the land.

The Huygens-Hellas giant dike system on Mars: Implications for Late Noachian-Early Hesperian volcanic resurfacing and climatic evolution
J.W. Head, Brown University, Department of Geological Sciences, Providence, RI 02912-1846, USA; et al. Pages 285-288.

The cold, very dry conditions on Mars mean that erosion is generally much less significant than on Earth. Head et al. report on the discovery of narrow ridges that extend across the surface of Mars for over 600 kilometers north of the Hellas basin, interpreting these to be the tops of basaltic dikes that were intruded into the shallow crust and then were eroded and exhumed. The stratigraphic position of these ridges suggest that they represent some of the long sought after feeder dikes for the widespread ancient volcanic plains thought to have been emplaced in a flood-basalt mode.

Segmentation of transform systems on the East Pacific Rise: Implications for earthquake processes at fast-slipping oceanic transform faults
Patricia M. Gregg, MIT/WHOI Joint Program in Oceanography, Geology and Geophysics, Woods Hole, MA 02543, USA; et al. Pages 289-292.

Scientists have long been puzzled by the question of what factors limit the size of earthquakes at a transform fault system, where two tectonic plates slide past each other. Traditionally, studies of transform faults are carried out on land, such as along the San Andreas system in California. A group of researchers from the Woods Hole Oceanographic Institution (WHOI) and Massachusetts Institute of Technology (MIT) have gained a new understanding of this question by investigating a group of oceanic transform faults in the Pacific Ocean near the equator. They found that the commonly occurring volcanic centers within oceanic transform faults, called "intra-transform spreading centers" are capable of preventing static earthquake stress interactions between adjacent sub-segments of strike-slip faulting, thus prohibiting larger magnitude earthquakes on major oceanic transform systems. This finding provides a fresh angle in understanding the fundamental earthquake behaviors of transform faults.

Transition from seamount chain to intraplate volcanic ridge at the East Pacific Rise
Scott M. White, University of South Carolina, Geological Sciences, Columbia, SC 29208, USA; et al. Pages 293-296.

Several seamount chains and ridges stretch across the Pacific Ocean basin, but their origins are highly controversial. Recent mapping of the flanks of the East Pacific Rise spreading center has revealed the origin of one such feature to be a single line of individual seamounts that coalesce into a continuous linear ridge named Sojourn Ridge. The development of the Sojourn Ridge from a seamount chain in such close proximity to the East Pacific Rise indicates that intense, focused upwelling of magma is a likely cause for these features, rather than the previous models, which advanced large-scale cracking of the Pacific Plate. This finding highlights the importance of direct magma supply in creating the numerous seamounts dotting the Pacific seafloor.

Pelagic evolution and environmental recovery after the Cretaceous-Paleogene mass extinction
Helen K. Coxall, University of Rhode Island, Graduate School of Oceanography, Narragansett, RI 02882, USA; et al. Pages 297-300.

Coxall et al. report the results of an investigation into the evolutionary recovery of the marine ecosystem after the mass extinction at the end of the Cretaceous period, when dinosaurs and many other groups of organisms suddenly disappeared from Earth. The study combines fossil records of marine plankton (planktic foraminifera) with geochemical records of carbon cycling (carbon stable isotope surface to deep sea gradients) through the Cretaceous-Paleogene extinction and into the subsequent Paleocene epoch. The results demonstrate that full recovery of marine plankton communities took almost 4 million years and directly followed the full recovery of the marine carbon system that is responsible for cycling life-dependent carbon between the atmosphere and deep sea sediments through marine plants and animals. The important factor in the plankton fossil records is that the final stage of evolutionary recovery involved diversification of photo symbiotic taxa that rely on algal 'partners' for nutrition when there is not much food available. Coxall et al. hypothesize that this development signals the reappearance of low-nutrient conditions in the central ocean basins (apparent today) as larger organisms and complex food chains recovered from the mass extinction and restarted the transfer of organic material from surface waters to the deep sea. Coxall et al. conclude that long delays such as this in evolutionary recovery after mass extinctions are dependant on the time required to rebuild integrated ecosystems, which is in turn closely linked to reestablishment of marine biogeochemical cycles.

Margin architecture reveals the transition to the modern Antarctic ice sheet ca. 3 Ma
Michele Rebesco, Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS), Dip. RIMA, Trieste 34010, Italy; et al. Pages 301-304.

The study of marine sediments around Antarctica reveals that the cooling of the planet 3 million years ago, which led to the onset of the major Northern Hemisphere glaciations, also coincided with a major change in the geometry of Antarctic sedimentary strata that Rebesco et al. interpret to result from the transition to the modern, cold, dry-based, stable Antarctic Ice Sheet apparent today.

Ancient diets indicate significant uplift of southern Tibet after ca. 7 Ma
Yang Wang, Florida State University, Department of Geological Sciences, Tallahassee, FL 32306-4100, USA; et al. Pages 309-312.

The uplift of the Himalayan–Tibetan Plateau plays a critical role in controlling global climate. Yet, the timing of the uplift remains unresolved. Here, Wang et al. present carbon isotopic evidence, preserved in teeth from 7 million-year-old horses and rhinos from the high Himalayas, which indicates that, unlike modern herbivores in the area, these ancient herbivores ate substantial amounts of warm-climate grasses. The presence of significant amounts of warm-climate grasses in the diets of these ancient mammals indicates that the climate was much warmer and the elevation was much lower in the area about 7 million years ago than it is today. This implies that the present high elevation in that part of the Tibetan Plateau was attained after 7 million years ago, much later than is generally believed.

Flow switching and large-scale deposition by ice streams draining former ice sheets
Julian A. Dowdeswell, University of Cambridge, Scott Polar Reseach Institute, Cambridge, CB2 1ER, UK; et al. Pages 313-316.

Fast-flowing ice streams are responsible for the bulk of mass transfer through large ice sheets. Dowdeswell et al. use extensive three-dimensional seismic data from the western Norwegian margin to explain how a 400-kilometer-long ice stream has undergone major switching in flow direction from one glaciation to the next. The direction of ice flow is inferred from the pattern of build-up of tens of thousands of cubic kilometers of glacier-derived debris and observations of large-scale streamlined landforms on former subglacial beds. Dowdeswell et al. demonstrate that ice streams can undergo major changes in flow direction through modification of their large-scale topographic setting. Whereas ice-stream switching in modern ice sheets has been regarded mainly as a reflection of internal changes in ice-sheet dynamics, switching over successive 100,000-year glacial cycles is in this case a response to the effects of continuing sediment deposition and the large-scale development of ice-influenced continental margins.

GSA TODAY Science Article
The 1906 earthquake and a century of progress in understanding earthquakes and their hazards
Mary Lou Zoback, U.S. Geological Survey, Menlo Park, California 94025, USA.

Centennial of the 1906 San Francisco earthquake: 18 April 2006 marks the centennial of the San Francisco earthquake, one of the most devastating natural disasters to strike the United States. In this month's issue of GSA Today, Mary Lou Zoback, a seismologist with the U.S. Geological Survey, presents an historical view of the earthquake and focuses on the major discoveries that gave birth to modern earthquake science and seismic hazard assessment. A commission established by the governor of California and led by Andrew Lawson, a professor at the University of California at Berkeley at that time, systematically mapped many of the features related to the earthquake and associated geology. Their report included many "firsts," such as the extent of the San Andreas fault through most of California, the important relationship of shaking intensity to geologic substrate (bedrock vs. filled ground), and the establishment of earthquake periodicity from historical records. Zoback concludes her paper with a look at modern seismic risk assessment, highlighting the high probability of one or more damaging earthquakes in the San Francisco Bay area in the next 30 years.

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