|3 February 2009
GSA Release No. 09-02
Director - GSA Communications & Marketing
February MEDIA Highlights
Boulder, CO, USA - GEOLOGY includes three papers about Mars; continuation of the "jelly sandwich" versus "crème brûlée" debate; support for the Snowball Earth hypothesis; what nine-million-year-old tooth enamel says about vegetation in an ancient sub-Himalayan ecosystem; anthropogenic lead in the Tyrrhenian Sea; evidence for a prehistoric South Pacific tsunami; a multicentennial megadrought in medieval Europe; and a newly discovered fossil turtle in the Canadian Arctic. GSA Today’s science article proposes a new method for classifying Quaternary glacial deposits.
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Tropical sea temperatures in the high-latitude South Pacific during the Eocene
Christopher J. Hollis et al., GNS Science, P.O. Box 30368, Lower Hutt 5040, New Zealand. Pages 99-102.
Hollis et al. provide evidence for a tropical climate in the high-latitude southwest Pacific in Eocene time. The early Eocene, around 50 million years ago, has been long recognized as a time of pronounced global warming. In New Zealand, warm climate conditions were inferred for this time period from a wide range of fossil evidence but, until now, the degree of warmth was uncertain. Hollis et al. used three independent geochemical methods for extracting sea temperatures from the sedimentary rock record and determined that surface waters exceeded 30 degrees Celsius, and water at the sea floor hovered around 20 degrees Celsius, during what is inferred to have been a 2-3-million-year episode of greenhouse gas-induced global warming. These temperatures are at the extreme end of modern tropical water masses (annual sea surface temperatures of 25-30 degrees Celsius). Tropical temperatures at this high-latitude location (55 degrees south) present a huge challenge for climate modelers. Even under extreme greenhouse conditions of greater than 2000 parts per million of carbon dioxide, modeled temperatures for Eocene era New Zealand do not exceed 20 degrees Celsius. Anomalously warm conditions have also been reported for early Eocene records from high-latitude regions in the Northern Hemisphere. It now seems likely that some as yet unknown heat transport mechanism comes into play during times of extreme global warmth.
Lateral trends in carbon isotope ratios reveal a Miocene vegetation gradient in the Siwaliks of Pakistan
Michele E. Morgan et al., Peabody Museum, Harvard University, Cambridge, Massachusetts 02138, USA. Pages 103-106.
It is unusual to be able to look back in time at ancient ecosystems, but through analysis of carbon atoms preserved in fossils, paleontologists and chemists are opening new windows onto Earth’s ecological past. Morgan et al. use stable carbon isotope ratios measured in 9-million-year-old mammalian tooth enamel from northern Pakistan to reveal how the landscape and vegetation varied across two different paleo-river systems on the ancient sub-Himalayan plain. The teeth were sampled along a 32-km transect within a time slice of about 150,000 years. Fossil teeth from the southwestern end have more of the heavier 13-C isotope compared with fossil teeth from the northeastern end, evidence that there was increased evaporation of moisture from the soil and plants in that direction. The carbon isotopic data indicate that animals living on the river plain to the southwest were foraging in more open vegetation compared with the river system to the northeast, which had moister soil conditions with more closed-canopy woodland and forest. The data also imply that the resident mammals, including giraffes, antelopes, pigs, the hippo-like anthracotheres, and primates, stayed in a limited area and did not move from one river system to the other on a yearly or seasonal basis. This approach offers new insights into kilometer-scale habitat variation and mammalian landscape-use patterns in an ancient sub-Himalayan ecosystem.
Mafic magma recharge supplies high CO2 and SO2 gas fluxes from Popocatépetl volcano, Mexico
Julie Roberge et al., Departamento de Geoquímica, Instituto de Geología, Universidad Nacional Autónoma de Mexico, Coyoacan, Mexico D.F. 04510, Mexico. Pages 107-110.
Since late 1994, open-vent eruptive activity and degassing at Popocatépetl volcano, Mexico, have released large masses of carbon dioxide and sulfur dioxide. Tephra and lava produced by these eruptions show evidence for mixing of mafic and silicic magmas shortly before eruption. Roberge et al. present the first measurements of dissolved carbon dioxide in the mafic magma endmember based on analyses of olivine-hosted melt inclusions that were trapped at pressures as high as about 400 megapascals (MPa) (about 17 km) beneath the volcano. They combine their data with thermodynamic models to show that degassing of mafic magma at about 150–350 MPa pressure can explain the carbon-dioxide/sulfur-dioxide mass ratios (1:8) of volcanic gases released from the volcano during 1995-1997. Their results demonstrate that mafic magma recharge was responsible for the high measured fluxes of carbon dioxide and sulfur dioxide from 1995-1997. Their results suggest that the ongoing eruption of Popocatépetl is essentially an intrusive event. More generally, they suggest that intrusion and deep degassing may explain the high gas fluxes at some other open-vent volcanoes, rather than convection of magma in the uppermost parts of subvolcanic conduits.
Anthropogenic Pb in recent hydrothermal sediments from the Tyrrhenian Sea: Implications for seawater Pb control on low-temperature hydrothermal systems
George D. Kamenov et al., Department of Geological Sciences, University of Florida, 241 Williamson Hall, Gainesville, Florida 32611, USA. Pages 111-114.
Hydrothermal sediments from a submarine mineralization site on the Eolo Seamount (Tyrrhenian Sea), exhibit unusual lead (Pb) that cannot be solely derived from natural sources in the region. Lead isotopic abundances indicate a significant part of the lead in these sediments is anthropogenic in origin. This finding by Kamenov et al. suggests that modern low-temperature hydrothermal processes do not contribute a significant amount of lead to seawater, and implies that anthropogenic activities can influence the chemistry of natural hydrothermal systems.
The impact of lithification on the diversity, size distribution, and recovery dynamics of marine invertebrate assemblages
Jocelyn A. Sessa et al., Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802-2714, USA. Pages 115-118.
The process of turning sediments into stone has recently been recognized as having the potential to significantly alter fossil collections. Sessa et al. find that the diversity of fossil assemblages in sediments is about twice as high as the diversity found in coeval lithified rocks, due to a loss of taxa smaller than 5 millimeters. In some of the world's best studied and stratigraphically expanded Cretaceous-Paleogene mass extinction boundary sections, this bias prolongs the recovery period by about seven million years, the duration that lithified rocks persist after unlithified diversities recover to pre-extinction values.
Stratigraphic and tectonic implications of a newly discovered glacial diamictite-cap carbonate couplet in southwestern Mongolia
Francis A. Macdonald et al., Dept. of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138, USA. Pages 123-126.
Macdonald et al. report stratigraphic studies on a newly discovered Neoproterozoic glacial deposit and bounding carbonate rocks in western Mongolia. The juxtaposition of cold-water glacial deposits and warm-water carbonate rocks is consistent with the Snowball Earth hypothesis. Integration of this stratum into the Neoproterozoic record of climate change also places constraints on the timing of basin formation and early accretion in Central Asia.
Tails of two plume types in one mantle
A. Lenardic and A.M. Jellinek, Department of Earth Science, MS 126, P.O. Box 1892, Rice University, Houston, Texas 77251-1892, USA. Pages 127-130.
Observations related to flood basalts suggest the existence of mantle plumes with large heads and thin trailing tails (cavity plumes). Seismic data suggest the existence of mantle plumes with thick tails (diapir plumes). The conditions required for diapir versus cavity plume generation are different, and in a chemically homogeneous mantle both types are not predicted to coexist. Lenardic and Jellinek show, however, that if a variable-thickness chemical layer exists at the base of the mantle, consistent with seismic observations, then the coexistence of morphologically distinct plumes types is expected. The chemical layer governs temperature and, thus, viscosity variations in the thermal boundary layer from which mantle plumes rise. A locally thick layer leads to small viscosity variation instabilities and hence to diapir plumes. A locally thin chemical layer allows for large viscosity variations across the active portion of the lower mantle thermal boundary layer and, hence, for cavity plume formation. A chemical layer that can move in response to changing flow patterns allows for the potential that plumes can morphologically transition over their lifetimes. An expectation that the morphology and thermal structure of mantle plumes should vary according to the thickness of a chemical layer is consistent with correlations between seismic observations of chemically distinct material at the core-mantle boundary, the varied morphology of mantle thermal anomalies, and the inferred diversity in hotspot buoyancy fluxes and excess temperatures.
Huge erratic boulders in Tonga deposited by a prehistoric tsunami
Cliff Frohlich et al., Institute for Geophysics, John A. and Katherine G. Jackson School of Geosciences, University of Texas at Austin, Austin, Texas 78758-4445, USA. Pages 131-134.
Seven huge and hugely puzzling coral limestone boulders sit 100–200 meters from the shoreline on the island of Tongatapu in the southwest Pacific. In 2007, Frohlich et al. studied these boulders, which have dimensions as large as 9 meters and weigh up to 1600 tons, and concluded that the boulders originated at the shoreline about 120,000 years ago and have since been displaced by a prehistoric tsunami. Frohlich et al. analyzed undersea volcanic calderas in the Tofua arc west of Tongatapu and local slump features just offshore from the boulders and concluded that a caldera collapse was the likely cause of a tsunami large enough to move the boulders. A systematic census and analysis of erratic boulders and other tsunamigenic features along shorelines elsewhere in the world may provide a means for extending the historic record and thus more accurately assessing tsunami hazard.
Volcanic spreading and lateral variations in the structure of Olympus Mons, Mars
Patrick J. McGovern and Julia K. Morgan, Lunar and Planetary Institute, Universities Space Research Association, 3600 Bay Area Boulevard, Houston, Texas 77058, USA. Pages 139-142.
The immense Olympus Mons volcano on Mars (about 23 km tall and 600 km wide) exhibits a somewhat lopsided structure: elongated to the northwest, shortened to the southeast, with corresponding types of faulting (extensional and compressional, respectively) prevalent in each sector. However, the overall shape of the edifice is characterized by a decrease in slope with increasing distance from the center, somewhat like the surface of a tent that is supported by a single central pole. In order to uncover the mechanism that generates the distinctive features of the shape of Olympus Mons, McGovern and Morgan constructed models of Olympus Mons as collections of particles that slip and slide past each other, controlled by the weight of other particles and the friction value assigned to each. They find that a low-friction zone at the base of the volcanic pile is required, and lateral variations in basal friction are critical. Friction decreasing outward from the center of the edifice can explain the overall "circus tent" shape, whereas friction decreasing from southeast to northwest accounts for the shortening and elongation of the respective quadrants. The most likely origin for the basal low-friction zone is in a layer of clay sediment, overpressured by pore water. This layer would be thicker in the "downhill" (northwest) direction, accounting for the asymmetry. Such a layer likely corresponds to widespread and ancient clay deposits discovered by the Mars Express mission, and may constitute a favored environment for extant thermophilic ("heat-loving") organisms on Mars.
Mantle weakening and strain localization: Implications for the long-term strength of the continental lithosphere
Jacques Précigout and Frédéric Gueydan, Géosciences Rennes, Université de Rennes 1, CNRS UMR 6118, 35042 Rennes, France. Pages 147-150.
The long-term strength of the continental lithosphere remains a matter of debate, implying two opposite models with either a stiff or a weak lithosphere mantle, e.g. “jelly sandwich” versus "crème brûlée," respectively. In the study by Précigout and Gueydan, numerical results show that the dominance of grain boundary sliding during dynamic grain size reduction can trigger intense strain localization. This process of ductile strain localization could drastically reduce the strength of the subcontinental mantle. Therefore, in response to the grain boundary sliding–induced weakening in mantle shear zones, the long-term deformation of such a ductile localizing mantle could change the lithosphere strength layering from "jelly sandwich" to "crème brûlée."
Is there an orbital signal in the polar layered deposits on Mars?
J. Taylor Perron and Peter Huybers, Dept. of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138, USA. Pages 155-158.
Climate records in ice cores and ocean sediments record temperature changes, including repeated ice ages that bear a clear imprint of long-term variations in the Earth's orbit. In their study, Perron and Huybers ask whether the polar ice caps on Mars contain a similar record of orbital changes. The martian polar caps contain hundreds of meters of layers composed of mixed ice and dust, and scientists have speculated that these layers formed in response to variations in orbital characteristics like the tilt of Mars' rotational axis, which controls the intensity of the seasons. Perron and Huybers used spacecraft images of exposed layers and elevation measurements to construct virtual "cores" through the north polar cap. Their analysis of these cores revealed repeating layers 1.6 meters (about 5 feet) thick in many areas of the polar cap. Because the age of the polar caps is unknown, it is not currently possible to determine whether the layering formed in response to orbital variations, contrary to the conclusions of several previous studies. To confirm an orbital cause, repeating layers with multiple thicknesses, corresponding to the multiple orbital cycles, would be required. The martian layers could be evidence of a shorter-term process that affects the deposition of ice and dust at the poles, perhaps similar to the way the El Niño Southern Oscillation produces intermittent changes in Pacific Ocean temperatures and rainfall. Perron and Huybers do not rule out the possibility that the layers they observe are part of an orbital signature, and they note that new data from current and future Mars missions could reveal whether the layers on Mars are, or are not, like the ones buried in Earth's oceans and Antarctic ice.
Choking the Mediterranean to dehydration: The Messinian salinity crisis
Rob Govers, Department of Earth Sciences, Utrecht University, P.O. Box 80.021, 3508 TA Utrecht, Netherlands. Pages 167-170.
Motions of the Earth's crust, not sea-level variations, caused the Mediterranean Sea to dry up and re-flood about six million years ago. Model results by Govers shed new light on the cause of this Messinian Salinity Crisis, which left kilometer-thick evaporite layers as massive footprints of this major climate event.
Discovery of columnar jointing on Mars
M.P. Milazzo et al., Astrogeology Team, U.S. Geological Survey, Flagstaff, Arizona 86001, USA. Pages 171-174.
Part of NASA's Mars Exploration Program is the search for liquid water (and potential life) on Mars. One of the most promising ways to locate water near the surface is to search for geological features in which water and lava have interacted. On Earth, columnar jointing and entablature are two such geologic features that indicate cooling lava and liquid water have interacted near the Earth's surface. Milazzo et al. show that the High Resolution Imaging Science Experiment onboard NASA's Mars Reconnaissance Orbiter has discovered columnar jointing and entablature near the martian surface, suggesting geologically recent interactions between cooling lava and liquid water.
Multicentennial megadrought in Northern Europe coincided with a global El Niño-Southern Oscillation drought pattern during the Medieval Climate Anomaly
Samuli Helama et al., Department of Geology, P.O. Box 64, University of Helsinki, 00014 Helsinki, Finland. Pages 175-178.
Climatologists from Finland report a severe multi-centennial drought in Northern Europe. This drought occurred during the late medieval times, approximately 1,000 years ago, between A.D. 1000 and 1200. During this interval, only a few moister periods (with duration of tens of years) seem to have ameliorated the climate. Hundreds of moisture-sensitive tree-ring records enabled a precise reconstruction of precipitation variability. The study by Helama et al. is the first European dendroclimatic precipitation reconstruction that runs through the alternating climate phases of the Medieval Climate Anomaly and the Little Ice Age. Northern Europe thus suffered from a severe precipitation deficit during the Medieval Climate Anomaly. Subsequent centuries during the Little Ice Age show a markedly wetter climate. The medieval drought was strikingly synchronous with droughts experienced in various regions sensitive to El Niño-Southern Oscillation, worldwide, including North America, eastern South America, and equatorial East Africa. Results of the study are important since the ongoing climate change may be causing current and future regional droughts, similar to droughts in medieval times. Results thus underscore the need for a higher number of longer high-precision precipitation reconstructions, to better understand the temporal and spatial variability of the worldwide late Holocene droughts.
A New Late Cretaceous macrobaenid turtle with Asian affinities from the High Canadian Arctic: Dispersal via ice-free polar routes
Deborah Vandermark et al., Department of Earth & Environmental Sciences, University of Rochester, Rochester, New York 14627, USA. Pages 183-186.
The waning volume of Arctic sea ice highlights the future potential impact of climate change. Such changes can be placed in a broader context by looking at times in the past when the Arctic had a much warmer climate. Aurorachelys, a newly discovered fossil turtle found in the High Canadian Arctic, lived in ice-free polar latitudes some 92-86 million years ago. Aurorachelys has affinities with turtles in Mongolia, and documents faunal interchange between Asia and North America during the Late Cretaceous. Vandermark et al. postulate that the extreme polar warmth of the time (similar to the climate of northern Florida today), may have been driven by volcanic carbon dioxide missions, allowing reptiles to expand their temperature-dependent ranges poleward. Other features of the ancient, volcanically active Arctic Ocean, such as islands and fresh surface waters, could have assisted the faunal interchange.
Fetch-limited self-organization of elongate water bodies
Andrew D. Ashton et al., Coastal Systems Group, Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA. Pages 187-190.
Series of rounded water bodies can be found in many environments worldwide. These enigmatic water bodies often appear to be the result of the segmentation of a larger water body. Ashton et al. present numerical modeling showing the first quantitative demonstration of how an elongate water body with sandy coastlines could self-organize and segment itself into equally sized rounded lakes. As a result of the long fetch along an elongate water body, waves approach most of the coast at large relative angles. These "high-angle" waves cause capes and spits to form and to continue to grow along these coasts. As these capes extend across the water body, they affect the fetch felt by capes on the opposing coast, attracting capes on the opposite shore, causing the elongate water body to segment into smaller, round ponds or lakes. These results can explain the morphology of many wave-formed features in many environments, potentially explaining the reworking of elongate back barriers, kettle ponds, and even Carolina bays into series of rounded ponds.
A shift from lithostratigraphic to allostratigraphic classification of Quaternary glacial deposits
Matti E. Räsänen et al., Dept. of Geology, University of Turku, 20014 Turku, Finland
Surficial, Quaternary glacial deposits historically have not received much attention from stratigraphers, and the economic importance of such deposits has, until recently, gone largely unappreciated. Moreover, the geologically unique characteristics of Quaternary glacial deposits have caused them to be regarded differently in geological research than older, generally more lithified strata. This has been exacerbated by the segregation of the Quaternary research community into different schools of geology and geography, producing redundant nomenclature, multiple classification approaches, and inconsistent stratigraphies that ultimately are not well suited for describing glaciated surficial deposits in a predictive fashion. Here, the authors propose a classification of Quaternary glacial deposits based on the combined use of conventional lithostratigraphic and sequence stratigraphic data. In this proposal, alloformations are treated as the fundamental stratigraphic unit, and lithostratigraphic units fill out the framework where appropriate. This classification provides a predictive, hierarchical framework for glaciogenic deposits that will potentially support stratigraphic information systems, databases, and digital spatial models more effectively than the traditional lithostratigraphic frameworks. The increased interest in and recognition of the importance of surficial Quaternary deposits makes this a timely and important contribution. The bedrock of the northern halves of North America and Europe is covered by Quaternary glacial deposits, forming a surficial overburden that is relatively thin, nonlithified, lithologically variable on a small scale (in terms of grain-size, mineralogy, texture, fabric, structure, and color), and often has a well-preserved depositional topography.
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