||23 April 2007
GSA Release No. 07-16
May Media Highlights: Geology
Boulder, CO, USA -- Topics include: evidence that Devonian-Silurian Prototaxites, Earth's largest land organisms up to that time, were actually giant fungi; discovery of a spectacular 300-million-year-old fossil forest in an Illinois coal mine; migration of sharks into freshwater systems during the Miocene; Hawaiian hotspot track preserved in the Bering Sea; and first evidence of tidal-channel sponge biostromes in southeastern Florida.
Highlights are provided below. Representatives of the media may obtain complimentary copies of articles by contacting Ann Cairns at . 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.
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- Impacts of the North Atlantic gyre circulation on Holocene climate off northwest Africa
Jung-Hyun Kim et al., Royal Netherlands Institute for Sea Research (NIOZ), Department of Marine Biogeochemistry and Toxicology, Den Burg, Texel 1790, Netherlands. Pages 387-390.
- To properly assess anthropogenic impacts on the post-industrial climate, it is essential to identify coherent natural climate variations in a long-term perspective, and their underlying mechanisms. Kim et al. present well-dated Holocene high-resolution records of sea-surface temperature and upwelling intensity off northwest Africa. On millennial time scales, sea-surface temperatures varied with the North Atlantic subtropical gyre circulation, while the upwelling intensity was linked to the Artic-North Atlantic Oscillation-like atmospheric circulation. This suggests that, as a mediator of solar forcing, the oceanic circulation linked to the North Atlantic thermohaline circulation had a stronger impact on Holocene millennial climate cycles than was previously considered.
- Global warming of the mantle at the origin of flood basalts over supercontinents
N. Coltice et al., Universite Lyon 1, Laboratoire de Sciences de la Terre, Villeurbanne, na 69100, France. Pages 391-394.
- Continental flood basalts, like the Deccan Traps, could potentially be at the origin of mass extinctions and global change. The Central Atlantic Magmatic Province, which extends over four continents and was emplaced over Pangea, constituted the largest continental flood basalt in terms of area. The plume head hypothesis is often advocated to explain enormous magmatism, but in this case many observations tend to exclude this classical model. Coltice et al. propose an alternative mechanism for the generation of continental flood basalts over supercontinents like Pangea. Numerical simulations of convection in the mantle, which only consider the radiogenic heat of rocks, show that the aggregation of continents modify the pattern of mantle flow, such that even in the absence of plumes, the temperature at the base of continents increases globally by ~100 °C. Coltice et al. suggest that the warming after supercontinental assembly could explain many features of gigantic continental flood basalts for which the plume head hypothesis fails.
- Extreme sediment and ice discharge from marine-based ice streams: New evidence from the North Sea
A. Nygård et al., University of Bergen, Department of Earth Science, Bergen, 5007, Norway. Pages 395-398.
- Some of Earth's most outstanding manifestations of ice age action are hidden beneath the sea. Along the edge of continents at high latitudes, large localized sediment deposits owe their existence to ice streams, or rivers of ice draining ice sheets during glacial times. One of the largest ice streams of the northeast Atlantic coastline drained ice from southern Scandinavia, and transported roughly 1 billion tons of sediment per year, a rate comparable to what the world's largest rivers transport today.
- Devonian landscape heterogeneity recorded by a giant fungus
C. Kevin Boyce et al., University of Chicago, Geophysical Sciences, Chicago, IL 60637, USA. Pages 399-402.
- Prototaxites fossils are large trunks up to a meter wide and eight meters tall, yet are constructed only of minute interwoven tubes 10 to 50 microns in diameter. The subject of debate for over 150 years, these terrestrial fossils of the Devonian and Silurian (approximately 420 to 350 million years ago) have been considered conifers, lichen, fungi, or different types of algae. Because their form and anatomy may always be controversial—a 20-ft-tall lichen doesn't make much more sense than a 20-ft-tall terrestrial alga—Boyce et al. have used the isotopic geochemistry of the fossils to address their identity. A primary producer will be relatively stable in the extent to which its metabolism discriminates between different carbon isotopes, while a consumer will take on the isotope ratios of whatever it eats and thereby can end up with widely differing isotope ratios by eating producers with different metabolisms. Our geochemical analyses have found too much isotopic variance between Prototaxites fossils for them to be photosynthetic primary producers. Instead, Prototaxites was a consumer, in this context indicating that these enormous fossils—the largest land organisms to have lived up to that point in time—were actually giant fungi. Furthermore, the isotopes of Prototaxites fossils can also help constrain the types of primary producers available for it to consume, and a number of the Prototaxites individuals appear to have come from environments that were devoid of the vascular plants that dominate the modern world. This discovery was unexpected because the conquest of the land by vascular plants had already begun about 40 million years earlier. Instead, large portions of the environment appear to have still consisted only of the alternative producers, such as soil-based microbial algae, that dominated terrestrial environments before the vascular plants had evolved.
- Highly metalliferous carbonaceous shale and Early Cambrian seawater
Bernd Lehmann et al., Technical University of Clausthal, Institute of Mineralogy and Mineral Resources, Clausthal-Zellerfeld, n/a 38678, Germany. Pages 403-406.
- Can a large ore deposit of nickel-, molybdenum-, gold-, and platinum-group elements form from average seawater? Lehmann et al. document a rare case from South China, where the environmental conditions in the Early Cambrian (about 540 million years ago) were such that upwelling ocean currents on the passive continental margin of the Yangtze Platform precipitated the minute amounts of redox-sensitive trace metals dissolved in seawater. This was possible only in extremely sediment-starved euxinic (sulfidic) basins—where very little clastic input is derived from the continent—for which there is no modern analog. The isotope composition of molybdenum, which was quantitatively scavenged from the seawater, has preserved the signature of the Early Cambrian oceans, and reflects a much less oxidized global marine environment than today's environment.
- Plate-tectonic reconstructions predict part of the Hawaiian hotspot track to be preserved in the Bering Sea
Bernhard Steinberger (corresponding author) and Carmen Gaina, Norges Geologiske Undersøkelse, Center for Geodynamics, Sør-Trøndelag 7491, Norway. Pages 407-410.
- The Hawaiian-Emperor chain is the textbook example for a hotspot track: volcanoes erupt at the "hotspot" in Hawaii. As the Pacific plate moves over the hotspot, volcanic edifices, formed at the hotspot, are carried away with the moving plate. Thus a chain of extinct volcanoes, getting older with increasing distance from Hawaii, forms. The chain ends near Kamchatka, where the Pacific plate is subducted. However, it could be possible to find older parts of the hotspot track, because sometimes parts of a subducted plate are attached to the overriding plate. The ocean floor in the Bering Sea is believed to be formed by such a process—as a remnant of the Izanagi plate, which has otherwise been completely subducted. Steinberger and Gaina's plate reconstructions indicate that the Hawaiian hotspot was beneath the Izanagi plate before about 83 million years ago, and its track could be preserved in the Bering Sea. They speculate that Bowers and Shirshov Ridge may be its present-day expression.
- Two-stage rifting of Zealandia-Australia-Antarctica: Evidence from 40Ar/39Ar thermochronometry of the Sisters shear zone, Stewart Island, New Zealand
Joseph Kula et al., University of Nevada at Las Vegas, Geoscience, Las Vegas, NV 89154-4010, USA. Pages 411-414.
- The Sisters shear zone, an extensional shear zone and associated detachment fault, has been recognized along the southeast coast of Stewart Island, southernmost New Zealand. Structural data from the shear zone indicates extensional motion along the shear zone was consistent with the direction of spreading along the Pacific-Antarctic oceanic ridge. Argon thermochronometry data constrains activity along the Sisters shear zone from roughly 89 to 82 million years ago. Motion ceased along the shear zone at the same time seafloor spreading began, indicating the Sisters shear zone accommodated continental extension leading to formation of the Pacific-Antarctic spreading ridge, and separation of eastern New Zealand from West Antarctica. Combining this new structural and thermochronometry data with that from elsewhere in New Zealand and Antarctica reveals a two-stage rift evolution for the breakup of this region of the Gondwana supercontinent. The first stage was extension between Australia-East Antarctica and New Zealand-West Antarctica, resulting in formation of the Tasman ridge. The second stage, as recorded by the Sisters shear zone, was extension between New Zealand and West Antarctica leading to formation of the Pacific-Antarctic ridge.
- Ecological gradients within a Pennsylvanian mire forest
William A. DiMichele, Howard J. Falcon-Lang (corresponding author), et al., University of Bristol, Earth Sciences, Bristol, Avon BS8 1RJ, UK. Pages 415-418.
- Discovery of a spectacular fossil forest is transforming geologists' understanding of the Earth's first rainforests. The 300-million-year-old fossils were found in underground coal mines in Illinois, United States, and preserve intact many square miles of ancient rainforest. The fossils show that the earliest rainforests were highly diverse, and that the species make-up changed across the landscape. The fossil forest was preserved when a fault in Earth's crust abruptly dropped the region below sea level. It preserves a rare snapshot of what life was like 300 million years ago when Illinois lay over the equator.
- 1400 yr multiproxy record of climate variability from the northern Gulf of Mexico
J.N. Richey et al., University of South Florida, College of Marine Science, St. Petersburg, FL 33701, USA. Pages 423-426.
- A continuous record of climate variability over the past 1400 years in the northern Gulf of Mexico was constructed from a sediment core recovered in the Pigmy Basin. Analysis of magnesium/calcium (Mg/Ca) and oxygen isotopes in the surface-dwelling foraminifer Globigerinoides ruber indicates large changes in sea-surface temperature (SST) and salinity over the past millennium. Two intervals of sustained high Mg/Ca indicate Gulf of Mexico SSTs were as warm, or warmer than, near-modern conditions between 1000 and 1400 years before present (yr B.P.). Foraminiferal Mg/Ca during the coolest interval of the Little Ice Age (ca. 250 yr B.P.) indicates that SST was 2-2.5 °C below modern SST. Four minima in the Mg/Ca record between 900 and 250 yr B.P. correspond with the Maunder, Spörer, Wolf, and Oort sunspot minima, suggesting a link between changes in solar insolation and SST variability in the Gulf of Mexico.
- Morphotectonic evolution of the central Kenya rift flanks: Implications for late Cenozoic environmental change in East Africa
Cornelia Spiegel et al., Universität Tübingen, Institut für Geowissenschaften, BW 72076, Germany. Pages 427-430.
- The East African rift system is a globally significant geological feature, hosting some of the most important sites for hominin remnants. Environmental changes in East Africa, toward drier conditions and an open grassland-dominated vegetation during the last ~8 million years, have been related to the appearance and evolution of hominins and Homo. The aridification is widely viewed as a result of world climate changes, and more specifically related to the closing of the Panama Isthmus and/or the closing of the Indonesian seaway. In this paper, potential temporal interrelations between rift formation, rift morphotectonic evolution, and environmental changes for the central part of the Kenya rift were investigated using the apatite (Uranium-Thorium)/Helium dating technique. The results suggest that the present high-mountainous relief bordering the Rift Valley (up to 3400 meters in central Kenya) formed between roughly 8 million years ago and today. Relief formation was therefore temporally coeval with the change towards drier conditions and grassland-dominated vegetation in East Africa, suggesting that the rising mountains bordering the Rift Valley formed orographic barriers with high-precipitation areas to the west and rain shadows to the east. The geomorphic evolution of East Africa modified regional climate and precipitation patterns, thereby superimposing a pronounced local aridification effect on global climate change at that time.
- Detrital mineral chronology of the Uinta Mountain Group: Implications for the Grenville flood in southwestern Laurentia
Paul A. Mueller et al., University of Florida, Department of Geological Sciences, Gainesville, FL 32611-2120, USA. Pages 431-434.
- This Uinta Mountains of Utah are distinctive for the ancient (roughly 700-million-year-old) sedimentary rocks that are exposed throughout the range. As such, they are part of a larger system preserved in small areas throughout the southwestern United States that contain vast quantities of sediment eroded from roughly one-billion-year-old rocks. Because very few rocks of this age are found in the Southwest, it has been unclear where this vast quantity of sediment originated. Mueller et al. conducted U-Pb age determinations on the mineral zircon using an ion microprobe, hafnium isotopic analyses using a laser ablation technique (also on zircons), and potassium-argon analyses on individual mica crystals to determine the source of the sediment. They determined that there were two primary sources. One was local and derived from the adjoining ancient rocks of Wyoming (2.5 billion years old), and the other was apparently from the area under the present Appalachian Mountains of eastern North America. This implies an extensive river system was present 700 million years ago that carried sediment from a Himalayan-type mountain range in the region of the current Appalachian Mountain system thousands of kilometers to the modern American Southwest. This river system persisted for at least 200 million years and was eventually disrupted during the separation of nascent North America from the supercontinent Rodinia.
- Soft-sediment deformation produced by tides in a meizoseismic area, Turnagain Arm, Alaska
Stephen F. Greb (corresponding author) and Allen Archer, Kentucky Geological Survey, Energy and Minerals, University of Kentucky, Lexington, KY 40506-0107, USA. Pages 435-439.
- Sediments were studied on tidal flats in the Turnagain Arm estuary, Alaska, where the twice-daily tidal range may exceed 9 meters. During their study, Greb and Archer encountered deformed beds that had formed during or after the previous day's tidal cycle. This type of bedding can form by many mechanisms, including earthquakes. In this case, however, the deformation was caused by over-pressurization of the tidal flats during rapid tidal drawdown, or by the shear stress created by the passage of a 1.8-meter-high tidal bore; a true "tidal wave" that forms on the flood tide. This finding sheds light on mechanisms that deform fine sediment in tidal settings. It also shows that caution is needed when using horizons of flow rolls in ancient sediments or rocks to infer ancient earthquakes in estuarine deposits, because some of the mechanisms that can cause deformation in rapidly deposited, unconsolidated silts and sands in estuarine settings are orders of magnitude more common than great earthquakes.
- Cantor set model of eolian dust deposits on desert alluvial fan terraces
Jon D. Pelletier, University of Arizona, Department of Geosciences, Tucson, AZ 85721, USA. Pages 439-442.
- Windblown dust plays an important role in the geology of deserts. Pelletier explores how dust accumulates in soils of the Mojave Desert (southwest United States) over different time scales and shows that the vast majority of dust that lands on surfaces is reworked, leading to highly incomplete sequences of dust accumulation over time. These results point to the dynamic nature of landform evolution in deserts.
- Migration of sharks into freshwater systems during the Miocene and implications for Alpine paleoelevation
László Kocsis et al., University of Lausanne, Mineralogy and Geochemistry, Lausanne, CH-1015, Switzerland. Pages 451-454.
- Trace-element and isotopic compositions of fossilized shark teeth sampled from Miocene marine sediments of the north Alpine Molasse Basin, the Vienna Basin, and the Pannonian Basin generally show evidence of formation in a marine environment under conditions geochemically equivalent to the open ocean. In contrast, two of eight shark teeth from the Swiss Upper Marine Molasse locality of La Molière have extremely low d18O values and low 87Sr/86Sr ratios compared to other teeth from this locality. The compositions of these two teeth suggest that they were formed while the sharks frequented a freshwater environment with very low 18O content and Sr isotopic composition controlled by Mesozoic calcareous rocks. Such an environment is consistent with the paleogeography of high-elevation (~2300 meters) Miocene Alps adjacent to a marginal sea. The discovery is intriguing because today only some bull sharks are known to be able to live in both freshwater and marine habitats, which requires an unusual adaptation to osmoregulation. The existence of these ancient sharks in freshwater supports such adaptation, which had already appeared during the Miocene.
- Geochemical evidence of secondary microbial methane from very slight biodegradation of undersaturated oils in a deep hot reservoir
Alexei V. Milkov and Leon Dzou, BP America, EPTG, Houston, TX 77079, USA. Pages 455-458.
- Milkov and Dzou present some of the first evidence of abundant methane originating from microbial degradation of petroleum in hot deep reservoirs. Although potentially common around the world, this secondary microbial methane is usually difficult to recognize because its isotopic composition overlaps with the composition of other sources. The authors investigate an early mature, slightly biodegraded oil with low gas/oil ratio, which allowed them to recognize the isotopic signature of secondary microbial methane using both carbon and hydrogen stable isotopes. The results open new research directions for petroleum geologists (e.g., to investigate the resources of secondary microbial gas worldwide), geochemists (Milkov and Dzou report isotopically light thermogenic gases, which should result in a significant paradigm shift), and microbiologists (regarding biodegradation in the deep hot biosphere).
- Conditions for branching in depositional rivers
Douglas J. Jerolmack and David Mohrig, University of Pennsylvania, Department of Earth and Environmental Science, Philadelphia, PA 19104, USA. Pages 463-466.
- Rivers usually act to focus water from the surrounding landscape into a single channel. Where sediment is being deposited, however, some rivers disperse water into numerous channels. This large-scale branching pattern is commonly observed on deltas and in some inland rivers, but is poorly understood. Jerolmack and Mohrig find that branching rivers also tend to experience frequent avulsions—an instability where water leaves the channel to find a new path at lower elevation. By compiling data from rivers all over the world, the authors determine the conditions that make rivers susceptible to avulsion and that lead to branching. These results may be used to predict how human disturbance or changes in sea level may induce changes in river pattern.
- Oligocene development of the West Antarctic Ice Sheet recorded in eastern Ross Sea strata
Christopher C. Sorlien et al., University of California, Santa Barbara, Institute for Crustal Studies, Santa Barbara, CA 93106-1100, USA. Pages 467-470.
- Sorlien et al. use sub-bottom imaging of the eastern Ross Sea, Antarctica, to provide evidence that large glaciers extended offshore into the Ross Sea from Marie Byrd Land prior to 25 million years ago. These seismic reflection profiles image U-shaped erosional troughs that are interpreted as having been carved by glaciers, and flat-topped ridges that are interpreted as giant moraines. The orientation of the troughs, and other spatial relations, indicate that glaciers were sourced from highlands in Marie Byrd Land, part of West Antarctica. Analysis by others of oxygen and carbon isotopes in cores recovered from the world's oceans indicates the growth of ice sheets after 34 million years ago. However, there has been little evidence that West Antarctica was included in that glaciation. Global climate models succeed in reproducing ice sheet growth for a high-standing East Antarctica at that time, but not for a low-elevation archipelago of West Antarctica. Other recent publications and Sorlien et al. suggest the presence of an ice cap in Marie Byrd Land before 25 million years ago. For given atmospheric conditions, it is more likely that ice sheets could grow if adjacent parts of Marie Byrd Land were elevated. This supports a hypothesis that this area has been subsiding since the end of major crustal extension about 80 million years ago.
- First documentation of tidal-channel sponge biostromes (upper Pleistocene, southeastern Florida)
Kevin J. Cunningham et al., U.S. Geological Survey, FISC, Fort Lauderdale, FL 33315, USA. Pages 475-478.
- Reef-building corals and coralline algae construct the most widespread and volumetrically important reefs in the world's modern, tropical oceans. Although sponges regularly are faunal elements of coral-algal reefs, they characteristically are accessory. Further, though sponges inhabit effectively all marine environments, they are generally dominant only in deep-water or cold-water settings, or both. Cunningham et al. report on their recent discovery of several large sponge aggregations called biostromes in southeastern Florida that formed within tidal channels between ooid shoals on a tropical, shallow-water, marine, limestone-producing platform during the last interglacial period about 125,000 years ago. Biostromes are a widely extensive blanket-like mass of rock built by and composed of mainly sedentary organisms. The Floridian sponge biostromes are constructed of densely distributed demosponges that range from barrel to vase shape and are up to 2 meters in height. The well-preserved sponge biostromes are important because, to our knowledge, they are 1) the only sponge biostromes that occur in tidal channels cutting through a complex of ooid shoals, and 2) outstandingly well-preserved examples of large tropical, shallow-marine sponge aggregations that contrast with much more common deep- or cold-water (or both) Cenozoic sponge-reef examples.
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