||27 September 2006
GSA Release No. 06-41
Boulder, Colo. — Topics include: possibility that ancient Delphic oracle Pythia were breathing something other than ethylene; magma chamber recharge at Vesuvius prior to the A.D. 79 eruption; new insights into dynamics of sea-level change; the northern Adriatic Sea as modern laboratory for studying 450 million years of marine ecosystem evolution; and discovery of a southwestern Pacific region where no sediment has accumulated. The GSA TODAY science article addresses bioalteration of volcanic glass.
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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- Propagation of surface uplift, lower crustal flow, and Cenozoic tectonics of the southeast margin of the Tibetan Plateau
Lindsay M. Schoenbohm, Ohio State University, Department of Geological Sciences, Columbus, Ohio 43210, USA; et al. Pages 813-816.
- Schoenbohm et al. add to the compelling evidence for growth of the southeast margin of the Tibetan Plateau through lower crustal flow. They employ new and existing data for the timing of river incision to demonstrate progressively younger uplift away from the main plateau. In light of this new data, Schoenbohm et al. suggest a way to reconcile opposing regional tectonic models, in which intrusion of lower crust into the region triggered a gradual shift from extrusion tectonics to distributed deformation and rotation around the eastern Himalayan syntaxis.
- Sea-level reversal during Termination II
Mark Siddall, Physics Institute, University of Bern, Climate and Environmental Physics, Bern CH3012, Switzerland; et al. Pages 817-820.
- Earth's climate constantly shifts between states in which its continents are covered in glacial ice at high latitude (glacial periods) and states in which glacial ice is much reduced (interglacial periods). The storage of water (in the form of ice) on land during glacial periods reduces sea level by around 120 meters. At the end of the last glacial period, the ice disappeared in a relatively straightforward fashion — a mixture of rapid and gradual melting made up the transition between glacial and interglacial states. Taking the previous transition into consideration, however, makes analysis less clear because it has been suggested that a large amount of glacial ice grew back halfway through the transition. Siddall et al. provide new evidence that supports this theory of a Termination II sea-level reversal by considering the conditions in the Red Sea at that time. The Red Sea is sensitive to changes in sea level, and therefore the amount of ice stored in glaciers on land. Unlike previous work, Siddall et al.'s approach provides a continuous record of the rapid increase in ice cover during the second-to-last glacial-to-interglacial transition.
- The geological links of the ancient Delphic Oracle (Greece): A reappraisal of natural gas occurrence and origin
Guiseppe Etiope, Istituto Nazionale di Geofisica e Vulcanologia, Section Rome 2, Rome, Italy 00143; et al. Pages 825-828.
- The prophetic powers of Pythia, the woman of the ancient Delphic Oracle, at the Temple of Apollo in Greece, are said to have been induced by hydrocarbon vapors, specifically ethylene, rising from rock and producing neurotoxic effects, including trance and delirium. This study by Etiope et al. completes a trilogy of papers on the link between geology and archeology in Delphi. Gas occurrence, though not ethylene, was confirmed on the basis of detailed surveys of gas flux from soil, gas in groundwater, and isotopic analyses of spring scales. Etiope et al. provide evidence that little methane, ethane, and carbon dioxide are released from a thermogenic (catagenetic) hydrocarbon-prone environment. However, the possibility of significant ethylene emissions is not obvious. In neither the present nor the past could the deep carbonatic rocks of Delphi produce sufficient amounts of ethylene (hundreds of ppmv) to produce smelling vapors or generate neurotoxic effects on humans. The Temple of Apollo may have been the site of increased degassing of methane in the past. If gas-linked neurotoxic effects upon Pythia need to be invoked, they should be sought in the possibility of oxygen depletion due to CO2-CH4 exhalation in the indoor temple. Alternatively, a possible geological explanation behind the natural presence of sweet scents at the temple could be aromatic hydrocarbons, such as benzene, dissolved in the groundwater spring, the production of which in the Delphi rocks is theoretically possible, but remains to be experimentally proved.
- Mapping stress and structurally controlled crustal shear velocity anisotropy in California
Naomi L. Boness (corresponding author), Chevron, ETC, San Ramon, California 94583, USA; and Mark D. Zoback, Stanford University, Geophysics, Stanford University, Stanford, California 94305-2215, USA. Pages 825-828.
- Quantifying the forces that are active within tectonic plates is critical to furthering our understanding of fault motion, earthquakes, and other geologic phenomena. The techniques typically used to measure stress in Earth's crust are limited because they require either boreholes, which are not ubiquitous, or significant numbers of earthquakes with well-constrained focal mechanisms at a single location. In this paper, Boness and Zoback describe a method for determining the direction of maximum stress in Earth's crust using shear velocity anisotropy from earthquakes beneath a single three-component seismic station. The technique does not require focal mechanisms and would work, in theory, with only a single, high-quality earthquake record. Boness and Zoback applied the method in the vicinity of the San Andreas fault (where hundreds of earthquake seismograms are available for many of the stations) and where they could compare their observations to direct measurements of the maximum stress orientation using traditional methods. At seismic stations located directly on the San Andreas fault, the direction of anisotropy is controlled by the structural fabric of the fault system and not the regional stress field.
- Salt-marsh erosion associated with a hurricane landfall in southern New England in the fifteenth and seventeenth centuries
Orson van de Plassche, Faculty of Earth and Life Sciences, Vrije Universiteit, Paleoclimatology and Geomorphology, Amsterdam, Noord-Holland 1081 HV, Netherlands; et al. Pages 829-832.
- The stratigraphy of salt marshes in southern New England is characterized by beds of salt-marsh peat alternating with beds (with an erosive lower boundary) of mineral sediment (tidal mud). Each bed of tidal mud represents a major environmental change from high intertidal salt marsh to shallow marine and/or low intertidal mud flat and, therefore, could be deposited only after erosion had created the necessary space. Each bed of salt-marsh peat overlying a bed of tidal mud demonstrates that sooner or later the sediment surface accreted to a level sufficiently shallow for the salt-marsh environment to re-establish. What caused this repeated erosion and recovery of salt marshes? van de Plassche et al. investigated this question in the sheltered inland part of Pattagansett River Marsh, near Niantic, Connecticut, which has a record of at least six cycles of marsh erosion, tidal sedimentation, and renewed marsh-peat accumulation. They hypothesized that the erosion was caused by wave and current action under storm-surge conditions created by a hurricane, and tested this hypothesis for the shallowest two erosion-deposition cycles. van de Plassche et al. show and argue that the configuration of the erosion cannot be attributed to gradual migration of tidal creeks, sudden increases in tidal prism, regular marsh-cliff retreat, or pond-hole formation. In contrast, the erosion patterns, the approximate time of erosion compared to known hurricane landfalls, and the rapid infilling of eroded space with sediment all support an interpretation of the data in terms of hurricane-related erosion and rapid post-hurricane recovery of the marsh environment. This result expands the frame for interpreting salt-marsh stratigraphy and is relevant for paleo-storm research as well as the study of salt marsh vulnerability to and recovery from storm erosion.
- Instability in tropical Pacific sea-surface temperatures during the early Aptian
Mirela Dumitrescu and Simon C. Brassell (corresponding author), Indiana University-Bloomington, Geological Sciences, Biogeochemical Laboratories, Bloomington, Indiana 47405, USA. Pages 833-836.
- The rock record preserves different lines of evidence about ancient climates. These include the chemical remnants from microbes that biochemically regulate their composition in response to fluctuations in the temperature of their immediate environment. Thus, molecular records can provide a proxy for ocean surface temperatures in the past, and their temporal variations attest to the rate and magnitude of climate change. Dumitrescu and Brassell et al. focus on a sequence of 120 million-year-old sediments that were deposited in the central Pacific. Their investigation of the sediments' temperature profile reveals two sustained intervals of time when ocean temperatures cooled by approximately ~4 °C. Such instability in tropical temperatures was likely triggered by changes in carbon cycling, induced by enhanced burial of organic matter, which is associated with a so-called "Oceanic Anoxic Event" and widespread deposition of petroleum source rocks. Thus, Earth's climate appears to be coupled to the global carbon cycle, changing in response to its perturbations, even at times of extreme warmth caused by elevated atmospheric CO2.
- Fault dating in the Canadian Rocky Mountains: Evidence for Late Cretaceous and early Eocene orogenic pulses
Ben A. van der Pluijm, University of Michigan, Department of Geological Sciences, Ann Arbor, Michigan 48109-1005, USA; et al. Pages 837-840.
- University of Michigan scientists have found a way to use gouge — a soft, often clayey material created in the contact zone between moving fault blocks — to date near-surface fault activity. Although geologists have been able to determine the age of deep faults for more than a decade, establishing the absolute age of near-surface faults has been difficult. Fault activity tends to occur in cycles of a few million years of movement followed by long quiet periods. van der Pluijm et al. present data from the analysis of gouge in major thrust faults in the southern Canadian Rockies near Banff. Until now, geologists could only estimate (based on location and on sedimentary deposits) that the date of activity of these faults occurred somewhere between the early Eocene and Late Cretaceous (50 million to 75 million years ago). New analysis, however, suggests that activity occurred in two tectonic pulses around 72 and 52 million years ago. Most geologists consider gouge to be grinding scum created in a mechanical process when rocks move past each other; however, van der Pluijm et al. show that most gouge material is actually created by chemical reactions, rather than from rock grinding. The change involves a dehydration process and the transformation of original clays called smectite into other forms of clay called illite. In addition to their potential for radiometric dating, these changes also have significant mechanical and hydrologic consequences for shallow faults. Clays are inherently weak, making it possible for a fault to move more easily. This is not necessarily a bad thing, if you live near one. Weak faults generate a lot of small slippage motion, instead of one huge slip that could result in a catastrophic earthquake. In subduction zones and other major faults, gouge may work like a lubicant between bricks, greasing the fault and making it easier for blocks to slide. Because heat and pressure increase with depth beneath Earth's surface, the chemistry involved when deep faults slip produces a completely different type of material than gouge. To unlock the secrets stored in gouge, van der Pluijm et al. used X-ray equipment and radiogenic dating technology. They used argon isotopic dating technology to date grains of newly grown clays in gouge deposits, and date fault activity in the Canadian Rockies.
- Magma chamber recharge at Vesuvius in the century prior to the eruption of A.D. 79
D.J. Morgan, Université Joseph Fourier, LGCA, Domaine Universitaire, Saint Martin D'Hères, Isère 38400, France; et al. Pages 845-848.
- The events that led up to the historically important eruption of Vesuvius volcano in A.D. 79 have been investigated and may assist in understanding the volcano in its current dormant state. By analyzing chemical variations within crystals found in A.D. 79 pumice, Morgan et al. have discovered that fresh hot magma rose into the Vesuvius system in at least four pulses before the eruption. These happened about 80 years, 40 years, and then twice around 20 years before the eruption. Morgan et al.'s method is likely to be applicable at other volcanoes and has the potential to reveal the timing of events that can potentially trigger volcanic eruptions.
- Sedimentation and diagenesis of Chinese loess: Implications for the preservation of continuous, high-resolution climate records
Thomas Stevens, University of Oxford, Centre for the Environment, Oxford, Oxfordshire OX1 3QY, UK; et al. Pages 849-852.
- Sequences of eolian loess sediments in China have been widely acclaimed as the most detailed and continuous terrestrial archives of late Cenozoic paleoclimate. Stevens et al. apply luminescence dating at very high resolution to suggest that, contrary to the current belief, loess does not contain continuous records of past climate. In particular, strata exhibit unconformities and highly variable rates of sedimentation, while post-depositional processes can obscure large portions of past climate records and invalidate stratigraphic correlations with more complete climatic archives. Consequently, previous work based on stratigraphic correlations should be regarded with caution. However, despite holding a fragmentary past climate record, loess still exhibits records of unparalleled detail in certain strata. As such, extremely valuable information concerning past climate change is obtainable using radiometric dating methods. The initial results of Stevens et al. support the assertion that the East Asian monsoon exhibits abrupt shifts in intensity.
- Complex Holocene lunette dune development, South Africa: Implications for palaeoclimate and models of pan development in arid regions
M.W. Telfer (corresponding author) and D.S.G. Thomas, University of Oxford, OUCE, Oxford, Oxon OX1 3QY, UK. Pages 853-856.
- On the downwind side of many arid-zone lake basins (playas), there is a crescent-shaped dune around the margin of the basin. Often referred to as lunettes, these dunes have been used to infer past environmental conditions, as they are believed to form under certain diagnostic conditions (wetter or drier). Telfer and Thomas have established the timing of the deposition of a South African lunette in a very detailed manner, using the relatively new technique of Optically Stimulated Luminescence (OSL) dating. Past models of lunette development have usually assumed a simple model for dune development (it is either active or it is not), yet Telfer and Thomas's data suggest that the dunes respond in a complex manner (different parts of a dune may be active at different times). This is apparently due to the varying sediment supplies from the surrounding area, rather than any simple link to climatic forcing. If such dunes are to be used for improving our understanding of past dry land environments, Telfer and Thomas argue that an equally thorough strategy would be necessary for each example.
- Acatlán Complex, southern Mexico: Record spanning the assembly and breakup of Pangea
R. Damian Nance, Ohio University, Geological Sciences, Athens, Ohio 45701, USA; et al. Pages 857-860.
- At the largest of scales, the geologic history of the past half-billion years can be said to reflect the assembly and breakup of the supercontinent Pangea. In North America, the assembly of this supercontinent culminated in the closure of the Rheic Ocean and the formation of the Appalachians and Ouachitas. Pangea's life span is marked by subduction and collision in the Cordillera, and its breakup is recorded in the opening of the Atlantic Ocean and the Gulf of Mexico. The complete history of Pangea is consequently manifested in areas a continent apart. In southern Mexico, however, a unique record of Pangea assembly to breakup is preserved within a single package of rocks known as the Acatlán Complex. Nance et al. document the history of this complex and show it to be the best preserved vestige of the Rheic Ocean in North America
- Rapid sea-level rise and Holocene climate in the Chukchi Sea
Lloyd D. Keigwin, Woods Hole Oceanographic Institution, Geology and Geophysics, Woods Hole, Massachusetts 02543-1541, USA; et al. Pages 861-864.
- For decades geologists have cored the Arctic Ocean seafloor to understand the Arctic's role in climate change. However, knowledge of oceanic and climate change in the Arctic has lagged behind knowledge of other parts of the ocean because most Artic core locations accumulated only about 1 centimeter of sediment every 1000 years. At such low rates, it is impossible to distinguish one millennium from the next. Keigwin et al. report three new core sites north and west of Alaska in the Chukchi Sea. At these locations, accumulation of sediment is more than 100 times greater than at previous sites, allowing identification of climate changes that were previously unseen. Cores from these sites reveal that rising sea level flooded the Bering Strait at about 12,000 years before present, that since 7000 years ago there has been very little sediment eroded from Alaska compared to before that time, and that beginning about 4000 years ago, there was a decline in biological productivity that may have resulted from increased sea ice or decreased nutrient supply from the Bering Strait.
- Broad region of no sediment in the southwest Pacific Basin
David K. Rea, University of Michigan, Department of Geological Sciences, Ann Arbor, Michigan 48109-1005, USA; et al. Pages 873-876.
- Rea et al. have discovered a broad region in the southwestern Pacific Ocean, the most remote part of the global ocean, where no sediment has ever accumulated. This region, the South Pacific Bare Zone, is about 2 million square kilometers in area — roughly the size of Western Europe or the Mediterranean Sea. Marine geologists have always understood that there are places where deposition rates are slow and resulting deposits are thin, such as in the central North Pacific, but this is the first documentation of a region where nothing has happened for the last 80 million years. Such an extended period of nondeposition requires 1) a long history of very low sea-surface biological productivity, 2) that all biogenic siliceous and calcareous material dissolve before it can accumulate, 3) no input of dust or other land-derived material from continents, and 4) no deposition of chemically formed oxide and hydroxide sediment. Nowhere else in today's ocean have these four conditions been known to co-exist for such a long period of time.
- Paleozoic to modern marine ecological shift displayed in the northern Adriatic Sea
Frank K. McKinney (corresponding author) and Steven J. Hageman, Appalachian State University, Department of Geology, Boone, North Carolina 28608 USA. Pages 881-884.
- Life on the sea floor has changed markedly over the past 450 million years. During the Paleozoic era, most bottom-dwelling organisms lived on the sea floor, or developed above it, resulting in widespread "gardens" of invertebrate animals, with many stuck to the sea floor. Between about 250 to 150 million years ago, during the Mesozoic era, something caused a major ecological change in the modern benthic ecosystem. Today, most of the bottom-dwelling life of the sea lives below the sea floor, burrowing through soft sediment. McKinney and Hagemann report that ecological conditions in the northern part of the Adriatic Sea today provide a modern laboratory for testing ideas about the evolution of marine ecosystem during the past 450 million years. This region of the Adriatic, between Italy and Croatia, is special because it has a flat, shallow sea floor and is dominated by a system of currents that slowly flow north along the eastern side (Croatia) and then turn and flow south along the western side (Italy). The two scientists characterized the bottom-dwelling communities of the Adriatic and found that they occur in two well-defined ecological groups. Those in the east, off the Croatian coast, resemble and function like Paleozoic communities. In contrast, the animals off the Italian coast are organized into "modern" communities. Comparison of the distribution with several important ecological conditions showed that by far the strongest correlation was with abundance of available food in the water. The water in the eastern part of the northern Adriatic Sea flows in from the nutrient-poor Aegean part of the Mediterranean Sea. Then as it flows along the Italian coast, it rapidly gains nutrients from the many Italian rivers that flow into it, causing an abundant growth of microscopic floating algae in the western region. These observations support the hypothesis that the change from the Paleozoic to the more modern marine ecosystem was driven by an increase in nutrients. An increase in nutrients along the margin of continents is thought to be the result of the evolution of ever more complex, widespread, and abundant plants on land, which help release nutrients from rocks and bind nitrogen from the air, making them available to be flushed down river systems and into the sea.
- Microbes and volcanoes: A tale from the oceans, ophiolites, and greenstone belts
Hubert Staudigel, Scripps Institution of Oceanography, University of California, La Jolla, California 92093-0225, USA; et al.
- Microbes Kick Glass: Ever since 1922, when Mellor reported microbial etching of glass in stained glass windows, geologists have wondered if such a process could happen to volcanic glass formed by rapid cooling and quenching of lava. Hubert Staudigel, a professor at Scripps Institution, and colleagues have provided an answer, and it is a definitive "yes." Reported in the October issue of GSA Today, Staudigel and colleagues offer a comprehensive description of evidence for microbial attack and alteration of basalt glass on the modern sea floor. They also report microscopic textures in rocks from around the world, including the 3.5 Ga Pilbara Greenstone belt, suggesting that this process of bioalteration of volcanic glass has been going on for most of Earth's history, from the beginning of the earliest life. The alteration of glass by microbial activity is a previously unappreciated means of liberating chemical elements from rocks and hence affecting the composition of seawater. This work casts new light on the extreme environments that microbes call home and puts a biologic spin on the alteration of volcanic glass and the evolution of seawater chemistry.
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