June Geology media highlights

May 24, 2007

Boulder, CO, USA -- Topics include: how oceans reacted to greenhouse conditions of the Cretaceous; first direct age determination of an Archean microfossil; evidence of a swimming dinosaur; understanding volcanoes that don't eject lava or debris; and new insights into the relationship of megathrust earthquakes and volcanic eruptions.

Highlights are provided below. Representatives of the media may obtain complimentary copies of articles by contacting Ann Cairns. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOLOGY in articles published. Contact Ann Cairns for additional information or other assistance.

Non-media requests for articles may be directed to GSA Sales and Service, gsaservice@geosociety.org.

Phosphorus and the roles of productivity and nutrient recycling during oceanic anoxic event 2

Haydon P. Mort, University of Neuchâtel, Institute of Geology, Neuchâtel 2009, Switzerland; et al. Pages 483-486.

Sediments deposited on the ocean floor around 100 million years ago--during the Cretaceous period--provide a glimpse of how oceans react under greenhouse conditions. This period is important in predicting future changes because its high temperatures and carbon dioxide (CO2) concentrations are similar to potential future conditions on Earth. Mort et al. find that dramatic changes in the ocean and atmosphere millions of years ago were exacerbated by a decrease in oxygen on the sea-floor, which is also being seen in many places today. This oxygen decrease appears to have caused changes in the oceans that lasted about 500,000 years, when they would otherwise have been much shorter. Today's decrease in seafloor oxygen is a result of CO2 emissions and an increased amount of chemicals used in agriculture making their way out to sea. The resulting increase in productivity causes the oxygen at the bottom of the ocean to be consumed by the bacteria that feed off organic material. Disturbing changes have been discovered in several coastal areas, including coastal regions of the Gulf of Mexico and the Baltic Sea, where 'dead zones' (areas devoid of all life) have formed. The 100-million-year-old sediments in Mort et al.'s study suggest that these zones may become a permanent feature of our oceans, even if humans stop polluting coastal waters.

Direct dating of Archean microbial ichnofossils

Neil R. Banerjee, University of Western Ontario, Department of Earth Sciences, London, Ontario N6A 5B7, Canada; et al. Pages 487-490.

Well-preserved, roughly 3.35-billion-year-old pillow lavas from Western Australia contain sub-millimeter tubular microfossils that represent some of the earliest vestiges of life on Earth. The microfossils are produced by the boring action of tiny microbes as they search for nutrients in the glassy margins of pillow lavas. The tubular microfossils discovered in Australia are identical to microbial ichnofossils in modern basalts, where DNA and organic remains have shown a direct link between the microbes and the tunnels they excavate. Direct radiometric dating of the minerals in the tubular structures suggests microbial alteration of the volcanic rocks occurred immediately after eruption, but may have continued for a protracted length of geological time. Banerjee et al.'s study marks the first direct age determination of an Archean microfossil. Microbial colonization of basaltic glass thus appears to have been part of a deep subsurface biosphere that was well established and possibly widespread within the first billion years of Earth's history. Pillow lavas are the most common rock type in Archean greenstone belts, so volcanic environments may not only have been one of the earliest places where life began; they may have also been where life first flourished, warmed by geothermal energy and protected from harmful radiation. Archean microbial biosignatures preserved in pillow basalts from greenstone belts may therefore help elucidate not only the presence of early life on Earth but also illuminate the conditions under which life began.

Were non-avian theropod dinosaurs able to swim" Supportive evidence from an Early Cretaceous trackway, Cameros Basin (La Rioja, Spain)

Loic Costeur (corresponding author), Université de Nantes, UMR CNRS 6112 LPGN, Nantes, France; et al. Pages 507-510.

Ezquerra et al. describe the discovery of an exceptional trackway that was made on a lakeshore, underwater, by a swimming bipedal dinosaur about 125 million years ago. The animal scratched the bottom, leaving long and slender sets of grooves on the sediment. Ripple marks on the surface of the site indicate that the dinosaur was swimming against a current but struggled to maintain a straight path and balance through accentuated movements of its right hind limb. This discovery is the first definitive evidence of an active swimming behavior in dinosaurs, and it bears strong implications on potential new ecological niches to be investigated by paleontologists.

Crustal growth by magmatic overplating in the Galápagos

Dennis Geist (corresponding author), University of Idaho, Department of Geological Sciences, Moscow, ID 83844-3022, USA; et al. Pages 511-514.

The Galápagos Islands are best known for their biota and formative influence on Charles Darwin's concepts of evolution by natural selection. But the islands also serve as one of the planet's premier natural laboratories for the study of the relationship between magmatism and tectonics. Lavas from Floreana Island bear accidental inclusions of rocks derived from the deep oceanic crust and mantle, which provide samples of otherwise inaccessible parts of the volcano. Some of these inclusions have chemical attributes unlike those of the island itself, suggesting that the volcanoes evolve as they are carried eastward by plate tectonics. The inclusions also contain mineralogical evidence that indicates they have undergone compression. This suggests that the crust in the Galápagos grows mostly from above, by lavas piling up on Earth's surface.

Large-scale hydrological change drove the late Miocene C4 plant expansion in the Himalayan foreland and Arabian Peninsula

Yongsong Huang et al., Brown University, Geological Sciences, Providence, Rhode Island 02912, USA. Pages 531-534.

Carbon isotope changes in paleosols from Siwalik, Pakistan, and marine sediments from the Bengal Fan indicate a major C4 plant expansion in the Himalayan foreland during the late Miocene. However, the timing and mechanisms behind the C4 plant expansion remain enigmatic. Huang et al. present high-resolution (~60 k.y.) biomarker and compound-specific isotope data spanning the past 11 million years from Ocean Drilling Program Site 722 in the Arabian Sea. An ~5‰-6‰ increase in leaf wax d13C values indicates a marked rise of C4 plants from 10 to 5.5 Ma, with accelerated expansion from 7.9 to 5.5 Ma. A concurrent ~50‰ rise in leaf wax dD values is attributed to a combined effect of changes in precipitation amount and evaporation, indicating that source regions for the plant waxes became progressively drier from 10 to 5.5 Ma. In contrast to earlier reports, our isotope records, biomarker abundances, alkenone UK'37, and Globigerina bulloides abundance data do not suggest enhanced summer monsoon circulation during this time interval. Rather, Huang et al. suggest that large-scale hydrological changes drove the late Miocene expansion of C4 plants in the Himalayan foreland and Arabian Peninsula.

Stability and instability of quiescently active volcanoes: The case of Masaya, Nicaragua

John Stix, McGill University, Earth and Planetary Sciences, Montréal, PQ H3A 2A7, Canada. Pages 535-538.

When most volcanoes erupt, they eject lava or fragmental debris. But some volcanoes do neither. They are clearly restless, as manifested by large gas emissions, significant seismicity, and other signs of unrest. Yet little solid material is erupted. What factors cause this situation" Stix presents a model of a plumbing system beneath such a volcano, Masaya in Nicaragua. He proposes that gas is supplied to the volcano through a coupled conduit and shallow reservoir system. The conduit supplies gas-rich magma to the reservoir. In turn, bubbles accumulate in a foam layer at the top of the reservoir. The system is steady-state since bubbles supplied to the foam layer from below are balanced by gas release from the reservoir to the vent at the surface. Volcanoes such as Masaya can be destabilized, for example by a large input of gas-rich magma, which could result in eruption.

Volcanic eruptions following M ≥ 9 megathrust earthquakes: Implications for the Sumatra-Andaman volcanoes

Thomas R. Walter, GeoForschungsZentrum, Physics of the Earth, Potsdam, GER 14473, Germany; and Falk Amelung, Department of Marine Geology and Geophysics, Rosenstiel School of Marine and Atmospheric Sciences, University of Miami, Miami, FL, USA. Pages 539-542.

The study is motivated by two volcanic eruptions in the Sumatra-Andaman arc that followed the disastrous 9.3 magnitude earthquake of 26 December 2004 and its aftershocks, reviving a century-long dispute about volcanism related to large earthquakes. Walter and Amelung analyze the four largest instrumentally recorded earthquakes and calculate the earthquake-induced strain and eruption rate changes along the nearby volcanic arcs. They show that eruptions occurred at volcanoes that experienced volumetric expansion during the preceding megathrust earthquake, suggesting that evaluation of the coseismic deformation provides an estimate of whether an earthquake increased or decreased the eruption potential of a volcano.

A 1500 yr record of North Atlantic storm activity based on optically dated relict beach scarps

Ilya V. Buynevich, et al., Woods Hole Oceanographic Institution, Geology & Geophysics, Woods Hole, MA 02543, USA. Pages 543-546.

Buynevich et al. reconstruct a history of intense storms in the Gulf of Maine based on erosional features buried under a sandy beach. These relict beach scarps are defined by their steep slopes and enrichment in heavy minerals. Imaged with ground-penetrating radar and dated using optically stimulated luminescence technique, these scarps indicate a massive erosional event circa 1,600 years ago, followed by a series of storms within the past 400 years. The paper demonstrates that prograded coastal sequences contain long-term records of erosion with widespread application to other sandy coasts around the world.

Exposure ages from mountain dipsticks in Mac. Robertson Land, East Antarctica, indicate little change in ice-sheet thickness since the Last Glacial Maximum

Andrew Mackintosh, Victoria University of Wellington School of Geography, Environment and Earth Sciences, Wellington 6001, New Zealand; et al. Pages 551-554.

Very little is known about how Antarctica has changed since the last ice age. Understanding ice sheet behavior since this time is important for assessing the present-day stability of Antarctic ice sheets and predicting future behavior. Mackintosh et al. use a novel 'mountain dipstick' method to show that the East Antarctic Ice Sheet has remained fairly stable during the last 20,000 years. The small changes in the East Antarctic Ice Sheet volume that did occur were probably due to changing global sea levels driven by the waxing and waning of Northern Hemisphere ice sheets. Consequently, the East Antarctic Ice Sheet reached its present volume about 6000 years ago when global sea levels stabilized. This is in contrast to the dynamic West Antarctic Ice Sheet which continues to retreat today.

Age and dynamics of linear dunes in the Namib Desert

C.S. Bristow, N. Lancaster (corresponding author), Desert Research Institute, Earth and Ecosystem Sciences, 2215 Raggio Parkway, Reno, NV 89512, USA; et al. Pages 555-558.

Bristow et al. report the results of combined ground penetrating radar (GPR) and optically stimulated luminescence (OSL) dating studies of a large, complex, linear dune in the northern part of the Namib Sand Sea in southwestern Africa. They provide new information on the age and internal sedimentary structures of these dunes. The unique combination of GPR imaging of the internal sedimentary structures of these dunes with high-precision OSL dating of the sediments at multiple locations provides a clear picture of how a large sand dune may develop over a period of thousands of years, and help to resolve many of the questions raised by earlier investigators with regard to the processes of formation and development of linear dunes, which are the most widespread of all sand dune types. Bristow et al.'s results provide firm evidence for lateral migration of linear dunes and indicate that the deposits of many dunes preserved in the rock record previously interpreted to be transverse to the mean transport direction may in fact be those of dunes of linear form, which combine the deposits of flow-parallel and flow-transverse elements. This has important implications for interpretation of ancient aeolian sandstones, past wind regimes, and resulting paleoclimatic and paleogeographic reconstructions.

Geodynamics of the southeastern Tibetan plateau from seismic anisotropy and geodesy

S. Sol, Lehigh University, Earth and Sciences, Bethlehem, PA 18015, USA; et al. Pages 563-566.

The ongoing continental collision between India and Eurasia has given rise to two of the most spectacular topographic features on Earth: the Himalayas and the Tibetan plateau. Outstanding questions remain about how the plateau is deforming and how Earth's surface and mantle deform in response to the collision. Sol et al. address these questions using both seismic and Global Positioning System data in the southeastern corner of the Tibetan plateau and off the plateau proper in Yunnan province, China. Sol et al. show that lithospheric deformation in the southeastern corner of the plateau appears to be coherent and continuous throughout the lithosphere. The comparison between the surface and the mantle strain fields suggests mechanical coupling between the crust and the mantle. The new data indicate a more intricate relationship in Yunnan where the role of the subducting Burma slab appears important. These new data reveal the importance of lateral variations in lithospheric structure and boundary conditions in the development of the Tibetan plateau.
To review the abstracts for these articles, go to www.gsajournals.org. To view the complete table of contents for the current issue of GEOLOGY, go to www.gsajournals.org/gsaonline/"request=get-current-toc&issn=0091-7613.


Geological Society of America

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