All ICDP Publications with Abstracts
From parent-sysfolder "Publications" + 2 folder-levels deep
694.
An 85-ka record of climate change in lowland Central America
Quaternary Science Reviews,
27
(11-12)
1152 – 1165
2008
ISSN: 02773791Keywords:▾
Arctic; Atlantic Ocean; Atlantic Ocean (North); Cariaco Basin; Caribbean Sea; Central America; Greenland; Guatemala [Central America]; Lake Peten Itza; Peten; Drilling; Glaciers; Gypsum; Isotopes; Sediments; Soil moisture; climate change; climate conditions; deglaciation; gypsum; Holocene; hydrodynamics; ice core; interstadial; intertropical convergence zone; Last Glacial Maximum; last glaciation; lowland environment; Neotropical region; oceanic circulation; precipitation (chemistry); precipitation intensity; proxy climate record; reconstruction; sediment core; stratigraphy; Younger Dryas; Central America; Drill cores; Terrestrial climate; Climate change
Abstract: ▾ Drill cores obtained from Lake Petén Itzá, Petén, Guatemala, contain a ∼85-kyr record of terrestrial climate from lowland Central America that was used to reconstruct hydrologic changes in the northern Neotropics during the last glaciation. Sediments are composed of alternating clay and gypsum reflecting relatively wet and dry climate conditions, respectively. From ∼85 to 48 ka, sediments were dominated by carbonate clay indicating moist conditions during Marine Isotope Stages (MIS) 5a, 4, and early 3. The first gypsum layer was deposited at ∼48 ka, signifying a shift toward drier hydrologic conditions and the onset of wet-dry oscillations. During the latter part of MIS 3, Petén climate varied between wetter conditions during interstadials and drier states during stadials. The pattern of clay-gypsum (wet-dry) oscillations during the latter part of MIS 3 (∼48-23 ka) closely resembles the temperature records from Greenland ice cores and North Atlantic marine sediment cores and precipitation proxies from the Cariaco Basin. The most arid periods coincided with Heinrich Events when cold sea surface temperatures prevailed in the North Atlantic, meridional overturning circulation was reduced, and the Intertropical Convergence Zone (ITCZ) was displaced southward. A thick clay unit was deposited from 23 to 18 ka suggesting deposition in a deep lake, and pollen accumulated during the same period indicates vegetation consisted of a temperate pine-oak forest. This finding contradicts previous inferences that climate was arid during the Last Glacial Maximum (LGM) chronozone (21±2 ka). At ∼18 ka, Petén climate switched from moist to arid conditions and remained dry from 18 to 14.7 ka during the early deglaciation. Moister conditions prevailed during the warmer Bolling-Allerod (14.7-12.8 ka) with the exception of a brief return to dry conditions at ∼13.8 ka that coincides with the Older Dryas and meltwater pulse 1A. The onset of the Younger Dryas at 12.8 ka marked the return of gypsum and hence dry conditions. The lake continued to precipitate gypsum until ∼10.3 ka when rainfall increased markedly in the early Holocene. © 2008 Elsevier Ltd. All rights reserved.
693.
A layered model of the upper crust in the Aigion region of Greece, inferred from arrival times of the 2001 earthquake sequence
Studia Geophysica et Geodaetica,
52
(1)
123 – 131
2008
ISSN: 00393169
Publisher: Springer New York
Keywords:▾
Eurasia; Europe; Greece; Gulf of Corinth; Ionian Sea; Mediterranean Sea; Southern Europe; active fault; algorithm; arrival time; crustal structure; earthquake event; gradient analysis; layered medium; modeling; seismic velocity; travel time; upper crust; urban area
Abstract: ▾ The western part of the Gulf of Corinth attracts attention due to its seismically active fault system and considerable seismic hazard. Detailed studies of the seismic activity of the region have been carried out especially as part of the so-called Corinth Rift Laboratory (CRL) Project. For standard earthquake locations, the CRL uses the HYPO algorithm and a special structural model that is composed of homogeneous layers (Rigo et al., 1996). This model was derived from a passive seismic experiment in a broader area around the western part of the Gulf. A significant part of the seismic activity is concentrated close to the town of Aigion, which was damaged by a strong earthquake in 1995. A sequence of smaller earthquakes occurred to the south of this town in the year 2001. In the present paper, we have used this sequence to derive an improved structural model for the region in the vicinity of the town of Aigion. This new model is based on the minimization of travel-time residuals. In particular, we used arrival times from a subset of 88 events recorded by at least 5 stations of the southern part of the CRL network, had magnitudes of over 2.3, and were recorded at the nearest station (station code AIO). A variant of the method of conjugate gradients has been used for this purpose. In comparison with the model derived by Rigo et al. (1996), the new model is characterized by a higher νP/νS velocity ratio and by higher velocities to a depth of about 7 km. The new model was derived with the aim to get more accurate locations of future events in the vicinity of the town of Aigion. © Institute of Geophysics of the ASCR, v.v.i 2008.
692.
A deep crustal fluid channel into the San Andreas Fault system near Parkfield, California
Geophysical Journal International,
173
(2)
718-732
2008
ISSN: 0956540XKeywords:▾
aeromagnetic survey; crustal structure; data interpretation; data inversion; electrical conductivity; electrical resistivity; lower crust; magnetotelluric method; San Andreas Fault; seismic reflection; transform fault; two-dimensional modeling, California; North America; Parkfield; United States
Abstract: ▾ Magnetotelluric (MT) data from 66 sites along a 45-km-long profile across the San Andreas Fault (SAF) were inverted to obtain the 2-D electrical resistivity structure of the crust near the San Andreas Fault Observatory at Depth (SAFOD). The most intriguing feature of the resistivity model is a steeply dipping upper crustal high-conductivity zone flanking the seismically defined SAF to the NE, that widens into the lower crust and appears to be connected to a broad conductivity anomaly in the upper mantle. Hypothesis tests of the inversion model suggest that upper and lower crustal and upper-mantle anomalies may be interconnected. We speculate that the high conductivities are caused by fluids and may represent a deep-rooted channel for crustal and/or mantle fluid ascent. Based on the chemical analysis of well waters, it was previously suggested that fluids can enter the brittle regime of the SAF system from the lower crust and mantle. At high pressures, these fluids can contribute to fault-weakening at seismogenic depths. These geochemical studies predicted the existence of a deep fluid source and a permeable pathway through the crust. Our resistivity model images a conductive pathway, which penetrates the entire crust, in agreement with the geochemical interpretation. However, the resistivity model also shows that the upper crustal branch of the high-conductivity zone is located NE of the seismically defined SAF, suggesting that the SAF does not itself act as a major fluid pathway. This interpretation is supported by both, the location of the upper crustal high-conductivity zone and recent studies within the SAFOD main hole, which indicate that pore pressures within the core of the SAF zone are not anomalously high, that mantle-derived fluids are minor constituents to the fault-zone fluid composition and that both the volume of mantle fluids and the fluid pressure increase to the NE of the SAF. We further infer from the MT model that the resistive Salinian block basement to the SW of the SAFOD represents an isolated body, being 5-8km wide and reaching to depths >7km, in agreement with aeromagnetic data. This body is separated from a massive block of Salinian crust farther to the SW. The NE terminus of resistive Salinian crust has a spatial relationship with a near-vertical zone of increased seismic reflectivity ∼15km SW of the SAF and likely represents a deep-reaching fault zone. © 2008 The Authors Journal compilation © 2008 RAS.
691.
Origin and spatial distribution of gas at seismogenic depths of the San Andreas Fault from drill-mud gas analysis
Applied Geochemistry,
23
(6)
1675-1690
2008
ISSN: 08832927Keywords:▾
Boreholes; Drilling; Gas fuel analysis; Hydrocarbons; Sedimentation, Gas compositions; Seismogenic zones; Spatial distribution, Seismographs, analytical method; chemical composition; concentration (composition); gas; hydrocarbon; isotopic composition; measurement method; permeability; sandstone; seismic zone; spatial distribution, California; North America; San Andreas Fault Zone; United States
Abstract: ▾ Data are presented on the molecular composition of drill-mud gas from the lower sedimentary section (1800-3987 m) of the SAFOD (San Andreas Fault Observatory at Depth) Main Hole measured on-line during drilling, as well as C and H isotope data from off-line mud gas samples. Hydrocarbons, H2 and CO2 are the most abundant non-atmospheric gases in drill-mud when drilling seismogenic zones. Gas influx into the well at depth is related to the lithology and permeability of the drilled strata: larger formation gas influx was detected when drilling through organic-rich shales and permeable sandstones. The SAF (San Andreas Fault), encountered between approximately 3100 m and 3450 m borehole depth, is generally low in gas, but is encompassed by two gas-rich zones (2700-2900 m and below 3550 m) at the fault margins with enhanced 222Rn activities and distinct gas compositions. Within the fault, two interstratified gas-rich lenses (3150-3200 m and 3310-3340 m) consist of CO2 and hydrocarbons (upper zone), but almost exclusively of hydrocarbons (lower zone). The isotopic composition indicates an organic source of hydrocarbons and CO2 in the entire sedimentary section of the well. Hydrocarbons in sedimentary strata are partly of microbial origin down to ∼2500 m borehole depth. The contribution of thermogenic gas increases between ∼2500 m and 3200 m. Below ∼3200 m, hydrocarbons fully derive from thermal degradation of organic matter. The lack of H2 in the center of the fault and the high concentration of H2 in the fractured zones at the fault margins are consistent with H2 formation by interaction of water with fresh silica mineral surfaces generated by tectonic activities, however, this needs to be verified by laboratory experiments. Based on these studies, it is concluded that the fault zone margins consist of strata with enhanced permeability, separated by a low-permeability fault center. © 2008 Elsevier Ltd. All rights reserved.
690.
Drilling the North Anatolian Fault
Scientific Drilling,
6
(1)
58-59
2008
ISSN: 18168957
Publisher: ICDP-IODP Göttingen, Germany
689.
Character and origin of lithofacies in the conduit of Unzen volcano, Japan
Journal of Volcanology and Geothermal Research,
175
(1-2)
45 – 59
2008
688.
Dynamic modeling suggests terrace zone asymmetry in the Chicxulub crater is caused by target heterogeneity
Earth and Planetary Science Letters,
270
(3-4)
221-230
2008
Abstract: ▾ We investigate the cause of terrace zone asymmetry in the Chicxulub impact crater using dynamic models of crater formation. Marine seismic data acquired across the crater show that the geometry of the crater's terrace zone, a series of sedimentary megablocks that slumped into the crater from the crater rim, varies significantly around the offshore half of the crater. The seismic data also reveal that, at the time of impact, both the water depth and sediment thickness varied with azimuth around the impact site. To test whether the observed heterogeneity in the pre-impact target might have affected terrace zone geometry we constructed two end-member models of upper-target structure at Chicxulub, based on the seismic data at different azimuths. One model, representing the northwest sector, had no water layer and a 3-km thick sediment layer; the other model, representing the northeast sector, had a 2-km water layer above a 4-km sediment layer. Numerical models of vertical impacts into these two targets produced final craters that differ substantially in terrace zone geometry, suggesting that the initial water depth and sediment thickness variations affected the structure of the terrace zone at Chicxulub. Moreover, the differences in terrace zone geometry between the two numerical models are consistent with the observed differences in the geometry of the terrace zone at different azimuths around the Chicxulub crater. We conclude that asymmetry in the pre-impact target rocks at Chicxulub is likely to be the primary cause of asymmetry in the terrace zone. © 2008 Elsevier B.V. All rights reserved.
687.
Interferometry by deconvolution: Part 2 - Theory for elastic waves and application to drill-bit seismic imaging
Geophysics,
73
(3)
S129-S141
2008
ISSN: 00168033
Publisher: Society of Exploration Geophysicists
Keywords:▾
Autocorrelation; Deconvolution; Drilling; Elastic waves; Imaging techniques; Impulse response; Interferometry; Seismic waves, Drill-bit seismic imaging; Elastic deconvolution interferometry; Multicomponent data; Seismic-while-drilling (SWD), Seismology, autocorrelation; boundary condition; deconvolution; drill bit; elastic wave; imaging method; interferometry; San Andreas Fault; seismic data; wave propagation; wave scattering
Abstract: ▾ Deconvolution interferometry successfully recovers the impulse response between two receivers without the need for an independent estimate of the source function. Here we extend the method of interferometry by deconvolution to multicomponent data in elastic media. As in the acoustic case, elastic deconvolution interferometry retrieves only causal scattered waves that propagate between two receivers as if one acts as a pseudosource of the point-force type. Interferometry by deconvolution in elastic media also generates artifacts because of a clamped-point boundary condition imposed by the deconvolution process. In seismic-while-drilling (SWD) practice, the goal is to determine the subsurface impulse response from drill-bit noise records. Most SWD technologies rely on pilot sensors and/or models to predict the drill-bit source function, whose imprint is then removed from the data. Interferometry by deconvolution is of most use to SWD applications in which pilot records are absent or provide unreliable estimates of bit excitation. With a numerical SWD subsalt example, we show that deconvolution interferometry provides an image of the subsurface that cannot be obtained by correlations without an estimate of the source autocorrelation. Finally, we test the use of deconvolution interferometry in processing SWD field data acquired at the San Andreas Fault Observatory at Depth (SAFOD). Because no pilot records were available for these data, deconvolution outperforms correlation in obtaining an interferometric image of the San Andreas fault zone at depth. © 2008 Society of Exploration Geophysicists. All rights reserved.
686.
Early response of continental Asia to Pleistocene climate cycles (retuned orbital chronologies for Baikal, marine, and ice core records)
Russian Geology and Geophysics,
49
(1)
40 – 45
2008
ISSN: 10687971Keywords:▾
chronology; climate cycle; eccentricity; geochemistry; ice core; methane; paleoclimate; Pleistocene
Abstract: ▾ Assuming orbital modulation of Pleistocene climate cycles, we have generated a new time scale for the Asian geochemical limnic record in the BDP-96-2 Baikal and the KDP-01 Hovsgol cores and updated the chronologies for the global marine δ18O and Vostok ice-gas records. The time scales were obtained by orbital tuning with the assumption of arbitrary but time invariable amplitudes and phase lags of the orbital parameters and responses. The retuned chronologies highlighted the cycles of eccentricity (100 kyr), obliquity (41 kyr), and precession (23 and 19 kyr), but the combined 70- and 30-kyr cycles became less prominent in the continental (Baikal) record though persisted in the global data (Vostok δD). The residual 70- and 30-kyr harmonics more likely result from errors in the untuned chronology for the Baikal record but are rather due to nonlinearity in the climate response. We investigated the leads and lags of orbital climate signals with a special focus on the 100-kyr cycle. The phases of precession, obliquity, and eccentricity cycles, compared among the records, showed the lead of the continent. The Baikal geochemical signal at the 100-kyr band led the global glacial and greenhouse CH4 responses and was almost synchronous with the earliest responding polar temperature signal. The reported results characterize the continent as a system highly responsive to eccentricity variations but do not contradict alternative hypotheses for the origin of the 100-kyr cycle in the Earth's climate history. © 2007.
685.
Observations and interpretations at Vredefort, Sudbury, and Chicxulub: Towards an empirical model of terrestrial impact basin formation
Meteoritics and Planetary Science,
43
(5)
855-882
2008
Abstract: ▾ The structural, topographic and other characteristics of the Vredefort, Sudbury, and Chicxulub impact structures are described. Assuming that the structures originally had the same morphology, the observations/ interpretations for each structure are compared and extended to the other structures. This does not result in any major inconsistencies but requires that the observations be scaled spatially. In the case of Vredefort and Sudbury, this is accomplished by scaling the outer limit of particular shock metamorphic features. In the case of Chicxulub, scaling requires a reasoned assumption as to the formation mechanism of an interior peak ring. The observations/interpretations are then used to construct an integrated, empirical kinematic model for a terrestrial peak-ring basin. The major attributes of the model include: a set of outward-directed thrusts in the parautochthonous rocks of the outermost environs of the crater floor, some of which are pre-existing structures that have been reactivated during transient cavity formation; inward-directed motions along the same outermost structures and along a set of structures, at intermediate radial distances, during transient cavity collapse; structural uplift in the center followed by a final set of radially outward-directed thrusts at the outer edges of the structural uplift, during uplift collapse. The rock displacements on the intermediate, inward and innermost, outward sets of structures are consistent with the assumption that a peak ring will result from the convergence of the collapse of the transient cavity rim area and the collapse of the structural uplift. © The Meteoritical Society, 2008.
684.
Numerical model of the hydrothermal system beneath Unzen Volcano, Japan
Journal of Volcanology and Geothermal Research,
175
(1-2)
35-44
2008
ISSN: 03770273Keywords:▾
Computer simulation; Cooling; Drilling; Estimation; Geothermal fields; Geothermal prospecting; Geothermal springs; Geothermal wells; Hot springs; Landforms; Numerical methods; Oceanography; Offshore oil well production; Offshore oil wells; Oil well logging; Sea level; Springs (components); Temperature; Volcanic rocks; Volcanoes; Water levels; Well logging, Bottom temperature; Fumarolic field; Geodetic data; geothermal fluid; Geothermal systems; heat source; Heat sourcing; High temperature; Hydro-thermal systems; hydrothermal system; Initial temperature; Large-scale structures; Logging data; numerical model; Numerical modelling; Numerical simulations; Recharge area; Shimabara Peninsula; Unzen Volcano; Volcanic conduits, Well drilling, heat source; hydrothermal fluid; hydrothermal system; magma; numerical model; thermal spring; volcanic eruption, Asia; Eurasia; Far East; Japan; Kyushu; Nagasaki [Kyushu]; Shimabara Peninsula; Unzen Volcano
Abstract: ▾ Drilling of the volcanic conduit in the Unzen Scientific Drilling Project (USDP) was completed in 2004. Some conduit materials of the 1990-95 eruption were encountered at the bottom of Well USDP-4 (150 m below sea level), and a bottom temperature of about 200 °C was estimated using logging data, although before drilling a temperature of about 700 °C had been inferred. Accordingly, conduit cooling from the initial temperature (850 °C) to the estimated temperature (about 200 °C) was evaluated by numerical simulation. The drilling provides constraints for the numerical model. The drilling indicates that the N-S width of the conduit of the latest eruption is 20 to 30 m and that it occupies a zone of about 300 m, which includes conduits of past eruptions. The process of cooling in the conduit, from an initial temperature of 850 °C in 1995 (the end of the eruption) to 200 °C in 2004 (completion of the conduit drilling), was replicated in models with permeabilities of 1 and 10 mdarcys. This result demonstrates that a highly permeable volcanic body surrounding a small conduit is required to explain the estimated bottom temperature. Our study also aimed to use a numerical simulation to construct a comprehensive hydrothermal model beneath Unzen Volcano. There are four large geothermal systems in the Shimabara Peninsula (Obama hot springs, Unzen fumarolic field, Shimabara hot springs and the West Unzen High Temperature Body [WUHTB]). Three pressure sources ("Sources A", "B" and "C" from shallow to the deep) were determined by geodetic data during the 1990-95 eruption. Source C is located at about 8 km deep at WUHTB, and is considered to be a magma body. We attempted to explain the existence of the four geothermal systems from the large-scale structures (the topography of the Shimabara Peninsula and Unzen Graben) and the various heat sources. We first set a heat source around Source C and changed its position and size. This numerical model produced the upflow zones at the Obama and Shimabara hot springs and WUHTB; however, the Unzen fumarolic field became a recharge area. This result indicates that another heat source is required to explain the Unzen fumarolic field and that two heat sources beneath WUHTB and the Unzen fumarolic field are involved in the formation of the four hydrothermal systems in the Shimabara Peninsula. © 2008 Elsevier B.V. All rights reserved.
683.
Neural network analysis of crosshole tomographic images: The seismic signature of gas hydrate bearing sediments in the Mackenzie Delta (NW Canada)
Geophysical Research Letters,
35
(19)
2008
ISSN: 00948276Keywords:▾
Anisotropy; Bearings (structural); Conformal mapping; Diagnostic radiography; Earthquakes; Electric network analysis; Feature extraction; Graph theory; Hydration; Image enhancement; Lithology; Medical imaging; Neural networks; Petroleum engineering; Petroleum prospecting; Sedimentology; Seismic prospecting; Seismology; Self organizing maps; Strength of materials; Tomography; Vectors, Attribute datums; Crosshole; Data vectors; Exploration strategies; High velocities; In-situ; Interlayering; Kohonen layers; Model vectors; Neural network analyses; Neural Network techniques; P velocities; P waves; Seismic signatures; Seismic tomographies; Strong anisotropies; Tomographic images; Watershed segmentation algorithms, Gas hydrates, algorithm; artificial neural network; crosshole seismic method; data set; gas hydrate; lithology; lithotype; P-wave; seismic anisotropy; seismic attenuation; seismic tomography; seismic velocity, Canada; Mackenzie Delta; North America; Northwest Territories
Abstract: ▾ Crosshole seismic experiments were conducted to study the in-situ properties of gas hydrate bearing sediments (GHBS) in the Mackenzie Delta (NW Canada). Seismic tomography provided images of P velocity, anisotropy, and attenuation. Self-organizing maps (SOM) are powerful neural network techniques to classify and interpret multi-attribute data sets. The coincident tomographic images are translated to a set of data vectors in order to train a Kohonen layer. The total gradient of the model vectors is determined for the trained SOM and a watershed segmentation algorithm is used to visualize and map the lithological clusters with well-defined seismic signatures. Application to the Mallik data reveals four major litho-types: (1) GHBS, (2) sands, (3) shale/coal interlayering, and (4) silt. The signature of seismic P wave characteristics distinguished for the GHBS (high velocities, strong anisotropy and attenuation) is new and can be used for new exploration strategies to map and quantify gas hydrates. Copyright 2008 by the American Geophysical Union.
682.
Natural laboratory NW Bohemia: Comprehensive fluid studies between 1992 and 2005 used to trace geodynamic processes
Geochemistry, Geophysics, Geosystems,
9
(4)
2008
ISSN: 15252027Abstract: ▾ Comprehensive studies of CO2-rich fluids close to the swarm earthquake region Nový Kostel at the Czech-German border have been started 15 years ago and have in particular included two extended chemical and isotope monitoring studies lasting for several years each. The regional surface distribution patterns of the fluid signatures including the identification of the origin of fluid components are the focus of the detailed studies. Three degassing centers (Cheb basin, Mariánské Lázně, and Karlovy Vary) with high CO2 flux and the same level of δ13C values, but different levels of 3He/ 4He ratios, have been identified. The studies have located the CO2 source and have investigated seismically induced changes in fluid characteristics on the basis of unique weekly sampling campaigns at selected locations. A seismically triggered release of crustal helium was confirmed by both monitoring campaigns. Finally, indications for a presently active magmatic process beneath the Cheb basin have been found. In contrast to volcanically active regions, magma accumulation in the study area takes place at the crust-mantle boundary and is not yet accompanied by heat transfer to the surface. Likewise, reactive magma-derived components are absent in the degassing fluids. The area of investigation has the potential to be a natural laboratory for fundamental studies of active geodynamic processes. The results of our fluid monitoring, including the stunning observation of mantle-derived free fluids marked by 3He/4He ratios within the subcontinental mantle range, are supported by geophysical findings from seismic studies and geologic indications. Copyright 2008 by the American Geophysical Union.
681.
Monte-Carlo simulation for estimating topographic disturbance to heat flow data
Russian Geology and Geophysics,
49
(4)
291 – 296
2008
ISSN: 10687971Keywords:▾
Eurasia; Lake Baikal; Russian Federation; Calluna vulgaris; algorithm; computer simulation; correction; digital elevation model; estimation method; heat flow; lacustrine deposit; Monte Carlo analysis; performance assessment; temperature gradient; topographic effect
Abstract: ▾ The Earth's surface topography disturbs the thermal field making measured superficial thermal gradients (heat flow) higher or lower than the deep value. Topographic disturbance can be removed from data by applying the respective correction. We estimated the effect of the rugged bottom of Lake Baikal on thermal gradients from the vicinity of boreholes BDP-93 and BDP-96 drilled in the lake sediments and deep borehole L-2 on the western shore of the South Baikal basin near Listvyanka Village. The corrections were computed using a Monte-Carlo simulation algorithm specially designed to quantify the effect of surface topography on the thermal field. The performance of the algorithm was checked by a test example. The corrections turned out to be vanishing in the area of underwater drilling but significant at L-2 where the thermal gradient in the upper 1 km of rocks was about 20% underestimated. The corrected heat flow in L-2 data increased from 50 to 60 mW/m2 and approached the values measured in the central South Baikal basin. © 2008.
680.
Microtubules in basalt glass from Hawaii Scientific Driling Project #2 phase 1 core and Hilina slope, Hawaii: Evidence of the occurrence and behavior of endolithic microorganisms
Geobiology,
6
(4)
351-364
2008
ISSN: 14724677Keywords:▾
silicate; silicon dioxide, basalt; endolithon; microorganism; olivine; palagonite; smectite; species occurrence, article; basalt; chemistry; microbiology; microscopy; microtubule; United States, Hawaii; Microscopy; Microtubules; Silicates; Soil Microbiology
Abstract: ▾ Elongate, fine tubes, ∼1 μm wide and up to 200 μm long, extend from fractured surfaces, vesicle walls, and internal fractures into fragments of basalt glass in samples from the Hawaii Scientific Drilling Project #2 phase 1 (HSDP #21) core and the Hilina slope, Hawaii. Several features indicate that these tubes are microbial endolithic microborings: the tubes resemble many described microborings from oceanic basalt glass, their formation is postdepositional but restricted to certain but different ranges of time in the two sets of samples, and they are not uniformly distributed throughout glass fragments. Microtubules record several characteristic behaviors including boring into glass, mining, seeking olivine, and avoiding plagioclase. They also are highly associated with a particular form of glass-replacing smectite. Evidence of behavior should join morphological and geochemical criteria in indicating microbial alteration of basalt glass. In some samples, steeply conical tubes, ∼10-20 μm in diameter tapering to 1 μm and commonly filled with smectite, appear to be modifications or elaborations of the microtubules. These also curve toward olivine and are associated with replacement smectite. In HSDP #21 samples, microtubules initiated at margins of shards before palagonite replaced those margins and are preserved during palagonitization. In fact, microtubules appear to have provided routes that enhanced the efficiency of water's reaching of unaltered glass. In Hilina Slope samples, the microtubules appear to postdate palagonitization because they initiate at the boundary between palagonite and unaltered sideromelane. Preservation of microtubules during palagonitization in samples together with recognition of other associated characteristics representing behavior suggests that such features may be recognizable in more heavily altered ancient rocks. © 2008 The Author.
679.
Mechanical intrusion models and their implications for the possibility of magma-driven swarms in NW Bohemia Region
Studia Geophysica et Geodaetica,
52
(4)
529 – 548
2008
ISSN: 00393169Keywords:▾
Bohemia; Central Europe; Czech Republic; Eurasia; Europe; earthquake swarm; fracture; igneous intrusion; magmatism; modeling; seismicity; stress
Abstract: ▾ Earthquake swarms are often assumed to be caused by magmatic or fluid intrusions, where the stress changes in the vicinity of the intrusion control the position, strength and rate of seismicity. Fracture mechanical models of natural intrusions or man-made hydrofractures pose constraints on orientation, magnitude, shape and growing rate of fractures and can be used to estimate stress changes in the vicinity of the intrusions. Although the idea of intrusion-induced seismicity is widely accepted, specific comparisons of seismicity patterns with fracture models of stress changes are rarely done. The goal of the study is to review patterns of intrusion-induced earthquake swarms in comparison to the observations of the swarm in NW Bohemia in 2000. We analyse and discuss the theoretical 3D shape of intrusions under mixed mode loading and apparent buoyancy. The aspect ratio and form of the intrusion is used to constrain parameters of the fluid, the surrounding rock and stress. We conclude that the 2000 NW Bohemia swarm could have been driven by a magmatic intrusion. The intrusion was, however, inclined to the maximal principal stress and caused shear displacement additional to opening. We estimate that the density diference between magma and rock was small. The feeding reservoir was possibly much larger than the area affected from earthquakes and may be a vertical dike beneath the swarm region. © Institute of Geophysics of the ASCR, v.v.i 2008.
678.
Matching Technique for Drilling Engineering in Well-1of Songliao Scientific Drilling(in Chinese with English abstract); 松科一井(主井)钻探工程技术配套
Exploration Engineering (Rock &, Soil Drilling and Tunneling),
35
(12)
1-5
2008
677.
Lithospheric topography, tilted plumes, and the track of the Snake River-Yellowstone hot spot
Tectonics,
27
(5)
2008
ISSN: 02787407Keywords:▾
basalt; continental lithosphere; craton; extensional tectonics; hot spot; mantle plume; structural control; topography; volcanic eruption; volcanism, Idaho; Nevada; North America; Oregon; Snake River; United States; Yellowstone River
Abstract: ▾ The trace of the Snake River-Yellowstone hot spot is the world's best example of a mantle plume that has been overridden by continental lithosphere. The "standard model" calls for the plume head to rise under northern Nevada and be forced northward to form basalts of the Columbia Plateau; subsequent movement of North America to the southwest over the plume tail created a hot spot trace on the surface. We present a new conceptual model for the origin of this feature that resolves inconsistencies in the current standard model and explains the recent documentation of a thermal anomaly in the mantle below Yellowstone today that plunges ∼ 65° WNW. Our model implies that the plume tail was forced beneath thinned cratonic lithosphere to the SE along with part of the plume head and has remained in this orientation for the last 12 Ma. We infer that almost all of the voicanism in SE Oregon and SW Idaho prior to 12 Ma results from overriding the southern extension of the plume head, not the plume tail, and that a distinct plume tail hot spot track was not established until formation of the Bruneau-Jarbidge eruptive center around 12 Ma. The plume tail track may also be controlled by a preexisting structural boundary in lithosphere that is thinner than adjacent lithosphere. This model demonstrates the potential importance of lithospheric topography on controlling the surface manifestation of plume volcanism and the complexity that may arise when lithospheric thickness is nonuniform. Copyright 2008 by the American Geophysical Union.
676.
Lithospheric structure of the Bohemian Massif and adjacent Variscan belt in central Europe based on profile S01 from the SUDETES 2003 experiment
Journal of Geophysical Research: Solid Earth,
113
(10)
2008
ISSN: 21699313
Publisher: Blackwell Publishing Ltd
Keywords:▾
Bohemian Massif; Central Europe; Eurasia; Europe; Cenozoic; craton; Hercynian orogeny; lithospheric structure; low velocity zone; Moho; orogenic belt; P-wave; ray tracing; S-wave; suture zone
Abstract: ▾ The SUDETES 2003 seismic experiment investigated the lithospheric structure of the eastern part of the Variscan belt of central Europe. The key profile of this experiment (S01) was 630 km long and extended southwestward from the margin of the East European craton, across the Trans-European suture zone (TESZ) and Sudetes, and across the Bohemian Massif that contains the active Eger (Ohře) rift, which is an element of the European Cenozoic rift system. Good quality first arrivals and later phases of refracted/reflected P and S waves were interpreted using 2-D ray-tracing techniques. The derived seismic model shows large variations in the internal structure of the crust, while the depth to the Moho varies in the relatively narrow depth interval of 28-35 km. Except for the Polish basin on the northeast end of the profile, the sedimentary cover is thin. The crystalline upper and middle crust with velocities of 5.9-6.4 km S-1 is about 20 km thick, and the 7-10 km thick lower crust can be divided into three regions based on P wave velocities: a low-velocity region (6.5-6.6 km s-1 beneath Eger rift and Sudetes) that is bounded on the southwest and northeast by regions of significantly higher velocity (6.8-7.1 km s-1 beneath the Saxothuringian and Moldanubian in the southwest and Fore-Sudetic Monocline and Polish Basin in the northeast). High-velocity bodies (Vp > 6.5 km s-1) were delineated in the upper crust of the Eger rift region. The seismic structure along the S01 profile images a Variscan orogenic wedge resting on the down warped margin of the plate margin containing the TESZ. This situation implies the northerly directed subduction of the Rheic Ocean that existed between the southern margin of the Old Red Continent and the Armorican terranes presently accreted into the Variscan belt. Closure of this ocean produced the Rheic suture between low-velocity crust of the Variscan orogenic wedge and higher-velocity crust of the TESZ. Copyright 2008 by the American Geophysical Union.
675.
Modeling of CO2-hydrate formation in geological reservoirs by injection of CO2 Gas
Journal of Energy Resources Technology, Transactions of the ASME,
130
(3)
0325021-03250211
2008
ISSN: 01950738Keywords:▾
Atmospheric chemistry; Capillarity; Carbon dioxide; Data reduction; Gas hydrates; Gas industry; Gases; Hydration; Liquids; Numerical analysis; Petroleum reservoirs; Porosity; Solids; Water injection, CH4 hydrate; CO2 hydrate; CO2 sequestration; Geological reservoir; Hydrate decomposition; Hydrate formation; Numerical simulation, Petroleum reservoir engineering
Abstract: ▾ Continuing concern about the impacts of atmospheric carbon dioxide (CO 2) on the global climate system provides an impetus for the development of methods for long-term disposal of CO2 produced by industrial and other activities. Investigations of the CO2-hydrate properties indicate the feasibility of geologic sequestration CO2 as gas hydrate and the possibility of coincident CO2 sequestration/CH4 production from natural gas hydrate reservoirs. Numerical studies can provide an integrated understanding of the process mechanisms in predicting the potential and economic viability of CO2 gas sequestration, especially when utilizing realistic geological reservoir characteristics in the models. This study numerically investigates possible sequestration of CO2 as a stable gas hydrate in various reservoir geological formations. As such, this paper extends the applicability of a previously developed model to more realistic and relevant reservoir scenarios. A unified gas hydrate model coupled with a thermal reservoir simulator (CMG STARS) was applied to simulate CO2-hydrate formation in four reservoir geological formations. These reservoirs can be described as follows. The first reservoir (Reservoir I) is similar to tight gas reservoir with mean porosity 0.25 and mean absolute permeability 10 mD. The second reservoir (Reservoir II) is similar to a conventional sandstone reservoir with mean porosity 0.25 and mean permeability 20 mD. The third reservoir (Reservoir III) is similar to hydrate-free Mallik silt with mean porosity 0.30 and mean permeability 100 mD. The fourth reservoir (Reservoir IV) is similar to hydrate-free Mallik sand with mean porosity 0.35 and mean permeability 1000 mD. The Mallik gas hydrate bearing formation itself can be described as several layers of variable thickness with permeability variations from 1 mD to 1000 mD, and is addressed as a separate part of this study. This paper describes numerical methodology, model input data selection, and reservoir simulation results, including an enhancement to model the effects of ice formation and decay. The numerical investigation shows that the gas hydrate model effectively captures the spatial and temporal dynamics of CO2-hydrate formation in geological reservoirs by injection of CO2 gas. Practical limitations to CO2-hydrate formation by gas injection are identified and potential improvements to the process are suggested. Copyright © 2008 by ASME.
674.
Integrated interpretation of physical properties of rocks of the borehole Yaxcopoil-1 (Chicxulub impact structure)
Journal of Geophysical Research: Solid Earth,
113
(7)
2008
Abstract: ▾ The borehole Yaxcopoil-1, drilled within the Chicxulub meteoritic impact structure (Mexico), was completely cored from 404 to 1511 in through postimpact Tertiary limestones underlain by impactites. The impactites comprise impact melt-rich, suevitic breccia followed by megablocks of Cretaceous limestones, calcarenites, dolomites, and anhydrites. Measurements of porosity, density, and thermal parameters on 450 samples (equidistant sampling, complete depth range) and of ultrasonic velocities and electric resistivity on 80 representative samples are used to investigate the physical properties of carbonate rocks and to study the influence of the impact. Experiments under elevated pressure, calculations using frequency-dependent Biot-Gassmann theory, and cross-checking with borehole logs, where available, show that ultrasonic laboratory and sonic in situ data correspond. Sonic and electric quasi-continuous logs are obtained from empirical correlations with thermal conductivity, density, and porosity and consideration of mineralogical composition and microstructure. These data give constraints on interpretation and geophysical modeling of, e.g., seismic and gravity data. In the Tertiary postimpact limestone section, the rock fabric (porosity) influences the physical properties. The upper boundary of the impactites is distinctly determined by the high inhomogeneity factor and anisotropy coefficient of thermal conductivity and by the temperature gradient from high-resolution borehole temperature measurements. All physical properties indicate that the upper part of the suevitic breccia can be distinguished from the lower suevite unit. In the Cretaceous megablocks, a high variability of all properties (particularly, thermal conductivity, density of solid material, and temperature gradient) due to the high variability in the mineral composition (calcite, dolomite, anhydrite) is observed. Copyright 2008 by the American Geophysical Union.
673.
Exploration for gas hydrates in the deepwater, northern Gulf of Mexico: Part I. A seismic approach based on geologic model, inversion, and rock physics principles
Marine and Petroleum Geology,
25
(9)
830-844
2008
ISSN: 02648172Keywords:▾
Bottom simulating reflector (BSR); Gas hydrate; Gulf of Mexico; Rock physics models; Seismic inversion, Bearings (structural); Gases; Geologic models; Hydration; Modal analysis; Offshore oil well production; Offshore oil wells; Offshore petroleum prospecting; Parameter estimation; Petroleum engineering; Petroleum prospecting; Reflection; Sedimentation; Sedimentology; Seismic response; Seismic waves; Seismology; Site selection; Stratigraphy; Three dimensional, Gas hydrates, deep water; estimation method; gas hydrate; hydrocarbon exploration; hydrocarbon reservoir; inverse analysis; P-wave; S-wave; seismic data; seismic method; seismic reflection; seismic velocity; three-dimensional modeling, Atlantic Ocean; Gulf of Mexico
Abstract: ▾ This article presents the results of applying a five-step process for using high-quality seismic data to locate marine gas hydrates. The process involved (1) reprocessing of seismic data for higher resolution, (2) detailed stratigraphic evaluation and interpretation to locate possible hydrate-bearing zones, (3) seismic attribute analysis to further delineate these zones, (4) seismic inversion to obtain appropriate elastic parameters of these zones in 3D, and (5) quantitative estimation of gas hydrate saturation from seismic data using inversion and rock physics principles. We used seismic data from Keathley Canyon 151 and Atwater Valley 14 in the northern Gulf of Mexico. Although careful analysis did indicate the presence of a bottom simulating reflector (BSR), our study mainly relied on a host of other seismic attributes (e.g., gas chimneys, seismically transparent zones, other features associated with the petroleum system) to characterize the occurrence of gas hydrates in these areas. We tested and verified a viable rock model for hydrate-bearing sediments using data from the Mallik (McKenzie Delta, Canada) and Blake Ridge (ODP Leg 164, southeast U.S. Atlantic margin) wells and modified it for application in the current area. We then used this model to estimate gas hydrate saturation in the host sediments of the northern Gulf of Mexico focus areas using estimates of P-wave and S-wave velocities from inversion of reflection seismic data. In this paper, we present the gas hydrate saturations predicted from the seismic processing methodology prior to 2005 drilling in the focus areas. In frontier areas where no well data are available and lithologic heterogeneities are poorly understood, implementing a seismic-based technique like the one described here to identify potential gas hydrates can provide valuable pre-drill information for site selection and for planning future characterization studies of gas hydrate-bearing sediments.
672.
Environment: Carbon measures slow to emerge
Petroleum Economist,
75
(12)
2008
ISSN: 0306395X
Publisher: Petroleum Economist Ltd.
Abstract: ▾ The US Geological Survey (USGS) estimated that 85.4 tcf of methane could be trapped in these cage-like lattices of ice about 2000 ft below the permafrost. According to USGS Director Mark Myers, a growing body of evidence indicates that concentrated gas-hydrate accumulations in conventional hydrocarbons reservoirs, such as those in northern Alaska, can be produced with existing technology, particularly depressurization. This technique lowers the pressure in the well, which causes the hydrates to become unstable and dissociate into water and gas that can be pumped to the surface. The Mallik 2002 Gas Hydrate Production Research Well Program, conducted in the Mackenzie Delta, Northwest Territories, Canada, represented the first modern, fully integrated field study and production testing of a gas-hydrate accumulation. It demonstrated that methane could be produced from gas hydrates by using pressure stimulation and that combining depressurisation with heating increased gas production. Numerous technical problems must still be overcome before this potential resource can be considered economically producible. One, for example, is the risk of releasing methane into the atmosphere; this gas is about 20 times more potent than CO2 as a greenhouse gas, which contributes to global warming. However, the effort would be worthwhile. The North Slope's hydrate reserves could provide enough gas to heat 100 m homes for up to a decade. The opportunity to develop them would also provide another justification for the proposed $26 billion, 1700 mi pipeline to carry conventionally produced natural gas from the North Slope to markets in the lower 48 states.
671.
Impact ejecta and carbonate sequence in the eastern sector of the Chicxulub crater
Comptes Rendus - Geoscience,
340
(12)
801-810
2008
Abstract: ▾ The Chicxulub 200 km diameter crater located in the Yucatan platform of the Gulf of Mexico formed 65 Myr ago and has since been covered by Tertiary post-impact carbonates. The sediment cover and absence of significant volcanic and tectonic activity in the carbonate platform have protected the crater from erosion and deformation, making Chicxulub the only large multi-ring crater in which ejecta is well preserved. Ejecta deposits have been studied by drilling/coring in the southern crater sector and at outcrops in Belize, Quintana Roo and Campeche; little information is available from other sectors. Here, we report on the drilling/coring of a section of ∼34 m of carbonate breccias at 250 m depth in the Valladolid area (120 km away from crater center), which are interpreted as Chicxulub proximal ejecta deposits. The Valladolid breccias correlate with the carbonate breccias cored in the Peto and Tekax boreholes to the south and at similar radial distance. This constitutes the first report of breccias in the eastern sector close to the crater rim. Thickness of the Valladolid breccias is less than that at the other sites, which may indicate erosion of the ejecta deposits before reestablishment of carbonate deposition. The region east of the crater rim appears different from regions to the south and west, characterized by high density and scattered distribution of sinkholes. © 2008 Académie des sciences.
670.
Establishment of floating astronomical time scale for the terrestrial Late Cretaceous Qingshankou Formation in the Songliao Basin of Northeast China
Earth Science Frontiers,
15
(4)
159-169
2008