bluuebunny/crossref_metadata_2025 huggingface dataset:
Temperature and abundances in the Jovian auroral stratosphere: 2. Ethylene as a probe of the microbar region
Authors: Kostiuk Theodor , Romani Paul , Espenak Fred , Livengood Timothy A. , Goldstein Jeffrey J.
Published: February 2008

Individual emission lines of ethylene (C2H4) near 10.5 μm were measured from the equatorial and north polar regions of Jupiter. Observations were made at a spectral resolution of 0.00083 cm−1 using infrared heterodyne spectroscopy at the NASA Infrared Telescope Facility on Mauna Kea, Hawaii. The line shape information possible with this resolving power permitted the retrieval of quiescent ethylene abundances and the investigation of abundance and thermal structure variability in the polar auroral region. A rough distribution of the north polar emission as a function of longitude was obtained with an instantaneous field of view (full width at half maximum) of ∼1 arc sec on the planet. The greatest C2H4 emission was observed near the nominal north polar methane hot spot (60°N, 180° longitude, System III, 1965). It was found to be confined to <10° longitude on the planet. Using a Voyager‐derived thermal profile, retrieved ethylene mole fractions for equatorial and north polar quiescent (non‐hot spot) regions were consistent with results from existing photochemical models. At the hot spot an 18‐fold increase in abundance was required near the 10‐μbar level to reproduce the data. Alternatively varying the stratospheric thermal profile, a 67–137 K increase in temperature was required at the ∼10‐μbar level to satisfy the observed emission, if the C2H4 mole fraction is fixed to the quiescent value. These results provide the first direct probe of the upper stratosphere of Jupiter and give upper limits to the temperature increase near the source of the north polar thermal infrared aurora. Combined with results from similar measurements of auroral ethane emission (Livengood et al., this issue) probing the 1‐mbar region, altitude information on the thermal structure can be obtained for the first time. The ethylene line emission region extends to the few microbar pressure level and may overlap the region where the H2 ultraviolet aurora is formed; thus it can be used as a probe of the coupling between the ultraviolet and thermal infrared phenomena.
An episodic hypothesis for Venusian tectonics
Authors: Turcotte Donald L.
Published: February 2008

It is suggested that episodic plate tectonics occurs on Venus; episodes of rapid plate tectonics are separated by periods of surface quiescence. For the last 500 ± 200 m.y. it is postulated that the surface of Venus has been a single rigid plate that has been thickening due to conductive cooling. A near‐uniform surface age is consistent with observed crater densities and the relatively small number of craters modified by surface tectonics or embayed by lava flows. A lithosphere that has conductively thickened for some 500 m.y. has a thickness of about 300 km, nearly an order of magnitude greater than the thickness associated with steady state conductive heat loss. Such a thick lithosphere can support the high topography and associated gravity anomalies on Venus as well as the unrelaxed craters; studies of lithospheric flexure at coronae are also consistent with a thick elastic lithosphere. Incipient subduction associated with large coronae may represent the onset of a new episode of rapid plate tectonics. On the Earth, 75–90% of mantle heat transport is attributed to the creation of new oceanic lithosphere at ocean ridges. This process is not operative on Venus. This paper suggests that episodic plate tectonics on Venus constitutes the primary mechanism for mantle heat transport on that planet.
The nanophase iron mineral(s) in Mars soil
Authors: Banin A. , Ben‐Shlomo T. , Margulies L. , Blake D. F. , Mancinelli R. L. , Gehring A. U.
Published: February 2008

A series of surface‐modified clays containing nanophase (np) iron oxide/oxyhydroxides of extremely small particle sizes, with total iron contents as high as found in Mars soil, were prepared by iron deposition on the clay surface from ferrous chloride solution. Comprehensive studies of the iron mineralogy in these “Mars‐soil analogs” were conducted using chemical extractions, solubility analyses, pH and redox, x ray and electron diffractometry, electron microscopic imaging, specific surface area and particle size determinations, differential thermal analyses, magnetic properties characterization, spectral reflectance, and Viking biology simulation experiments. The clay matrix and the procedure used for synthesis produced nanophase iron oxides containing a certain proportion of divalent iron, which slowly converts to more stable, fully oxidized iron minerals. The clay acted as an effective matrix, both chemically and sterically, preventing the major part of the synthesized iron oxides from ripening, i.e., growing and developing larger crystals. The precipitated iron oxides appear as isodiametric or slightly elongated particles in the size range 1–10 nm, having large specific surface area. The noncrystalline nature of the iron compounds precipitated on the surface of the clay was verified by their complete extractability in oxalate. Lepidocrocite (γ‐FeOOH) was detected by selected area electron diffraction. It is formed from a double iron Fe(II)/Fe(III) hydroxy mineral such as “green rust,” or ferrosic hydroxide. Magnetic measurements suggested that lepidocrocite converted to the more stable maghemite (γ‐Fe2O3) by mild heat treatment and then to nanophase hematite (α‐Fe2O3) by extensive heat treatment. After mild heating, the iron‐enriched clay became slightly magnetic, to the extent that it adheres to a hand‐held magnet, as was observed with Mars soil. The chemical reactivity of the iron‐enriched clays strongly resembles, and offers a plausible mechanism for, the somewhat puzzling observations of the Viking biology experiments. Their unique chemical reactivities are attributed to the combined catalytic effects of the iron oxide/oxyhydroxide and silicate phase surfaces. The reflectance spectrum of the clay‐iron preparations in the visible range is generally similar to the reflectance curves of bright regions on Mars. This strengthens the evidence for the predominance of nanophase iron oxides/oxyhydroxides in Mars soil. The mode of formation of these nanophase iron oxides on Mars is still unknown. It is puzzling that despite the long period of time since aqueous weathering took place on Mars, they have not developed from their transitory stage to well‐crystallized end‐members. The possibility is suggested that these phases represent a continuously on‐going, extremely slow weathering process.
Topography of Apollinaris Patera and Ma'adim Vallis: Automated extraction of digital elevation models
Authors: Thornhill G. D. , Rothery D. A. , Murray J. B. , Cook A. C. , Day T. , Muller J. P. , Iliffe J. C.
Published: February 2008

We have used automatic extraction of digital elevation models (DEMs) from stereo imagery of Mars to produce DEMs of parts of the Aeolis quadrangle of Mars. The method uses automated stereo matching based on an adaptive least squares correlation technique, which can be applied to Viking Orbiter imagery. A demonstration of the technique is presented, which focuses on the volcano Apollinaris Patera (186°W, 9°S), and the channel Ma'adim Vallis (183°W, 20°S). The topographic information derived is compared with previous determinations of topography, and a brief discussion of the relationship of the topography to the processes that have acted in the area is given. DEMs derived from stereomatching are an invaluable resource for terrain interpretation, and will provide a complementary technique to direct topographic measurements such as future missions to Mars will provide.
A 300 KYR Record of Upwelling Off Oman during the Late Quaternary: Evidence of the Asian Southwest Monsoon
Authors: Anderson David M. , Prell Warren L.
Published: February 2008

In the northwest Arabian Sea upwelling occurs each summer, driven by the strong SW monsoon winds. Upwelling results in high biological productivity and a distinctive assemblage of plankton species in the surface waters off Oman that are preserved in the sediments along the Oman continental margin, creating a geologic record of monsoon‐driven upwelling. Sediments recovered from the Oman continental margin during Ocean Drilling Program leg 117 provide an opportunity to examine how upwelling has varied during the late Quaternary, spanning a longer interval than piston cores recovered prior to the ODP cruise. Variations in foraminifer shell accumulation and in the relative abundance of Globigerina bulloides indicate dominant cycles of variation at 1/100 kyr, the dominant frequency of glacial‐interglacial variations, and at 1/23 kyr, the frequency of precessionally driven cycles in seasonal insolation. The strongest monsoon winds (indicated by increased upwelling) occurred during interglacial times when perihelion was aligned with the summer solstice, an orbital change that increased the insolation received during summer in the northern hemisphere. During glacial times upwelling was reduced, and although the precessional cycles were still present their amplitude was smaller. At both frequencies the upwelling cycles are in phase with minimum ice volume, evidence that glacial‐interglacial climate changes also include changes to the climate system that influence the low‐latitude monsoon. We attribute the decrease in the monsoon winds observed during glacial times to changes in bare land albedo over Asia and/or to changes in the areal extent and seasonal cycle in Asian snow cover that decrease the summer land‐sea temperature contrast. Other mechanisms may also be involved. These new upwelling time series differ substantially from previous results, however the previous work relied on cores located farther offshore where upwelling is less intense and other physical mechanisms become important. Our results support the observations derived from atmospheric general circulation models of the atmosphere that indicate that both glacial boundary conditions, and the strength of summer insolation are important variables contributing to cycles in the monsoon winds during the late Quaternary.
Oxygen Isotope Records and Salinity Changes in the Northeastern Atlantic Ocean during the Last 18,000 Years
Authors: Duplessy J. C. , Bard E. , Labeyrie L. , Duprat J. , Moyes J.
Published: February 2008

A comparison of the deglaciation δ18O record of northeastern Atlantic core SU 81‐18 with that calculated by adding the δ18O changes due to sea surface temperature and ice volume variations shows that planktonic foraminifera have also recorded seawater δ18O changes due to salinity (local advection and evaporation‐precipitation (E‐P)) changes. Using a simple ocean box model of meltwater dilution during the deglaciation, we show that sea surface salinity covaried with temperature. Despite the injection of meltwater associated with the climate warming, sea surface salinity was high when sea surface water was warm and low during cold events. This implies that the combined effect of local E‐P changes and northward advection of saline subtropical water were the main factor responsible for the surface water salinity variations in the northeastern Atlantic Ocean during the last 18,000 years.
Holocene sediment fluxes in the northeast Atlantic from 230Thexcess and radiocarbon measurements
Authors: Thomson J. , Colley S. , Anderson R. , Cook G. T. , MacKenzie A. B. , Harkness D. D.
Published: February 2008

Radiocarbon and 230Thexcess data from six NE Atlantic box cores are considered. The cores form a transect from the Porcupine Abyssal Plain over the East Thulean Rise to the southern end of Feni Drift. The chronology for the cores is established from bulk sediment carbonate radiocarbon data and reveals that sections exhibiting constant accumulation rates can be identified in all the cores, with rates of 3.0‐3.5 cm kyr−1 on the plain through the Holocene and late Holocene rates of 4.3–6.6 cm kyr−1 elsewhere. Five out of the six cores show accumulations of more 230Thexcess than is produced in the overlying water column, with the greatest inventories (up to 225% of production) in the cores from the rise and drift. A size fraction comparison between two cores from the plain and rise reveals that the higher overall accumulation rates and 230Thexcess inventories in the off‐plain cores are due to an increased fine (<5 µm) component fraction, whereas the flux of coarser material is similar to that received on the plain. This suggests that the higher fluxes of materials observed are physically (rather than biogeochemically) driven and also that drift formation has been continuously active in the late Holocene. Sections of all the cores where regular accumulation is defined by the radiocarbon data are modeled first by a linear radiocarbon age/depth model and second by a constant rain (230Thexcess)0 model prorated for the observed core inventories. These modeling approaches yield historical mass accumulation rate estimates which are generally in reasonable agreement (±30%), but the differences observed appear to be well organized in time rather than random.
Late Cretaceous upwelling system along the Southern Tethys Margin (Israel): Interrelationship between productivity, bottom water environments, and organic matter preservation
Authors: Almogi‐Labin Ahuva , Bein Amos , Sass Eytan
Published: February 2008

Organic‐rich Upper Cretaceous sequences in Israel were deposited in an extensive, highly productive upwelling‐linked system which prevailed along the southern Tethys margin, and lasted for ∼19 m.y. (Santonian to late Maastrichtian). An understanding of the spatial and temporal characteristics of this system was gained through detailed paleontological and geochemical analyses of subsurface sequences in two basins in Israel, representing an outer (Shefela) and an inner (Zin) facies belt. The nature of the upwelling system, and its effect on the sedimentary record, is related to two basic environmental parameters, namely paleoproductivity intensity and oxygen levels at the bottom. The assessment of these parameters and their interrelationship has been performed through the development of paleontological (foraminiferal) criteria, which are independent of the organic matter content. Following the establishment of these criteria, it is concluded that the productivity reached its maximum intensity during the late Campanian, which was also the time of most notable differentiation between the center of the upwelling system in the inner belt and the less intense conditions in the outer basin. This distribution is expressed in varied lithology (organic‐rich carbonates, phosphorites, and siliceous rocks) at the core of upwelling and a uniform lithology (organic‐rich carbonates) at the margin of this system. The uniform lithology of the Maastrichtian in both basins, composed of organic‐rich carbonates, is ascribed to a gradual weakening of productivity. The bottom conditions in the inner belt during the late Campanian (the time of maximum surface productivity) were near anoxic, changing to more aerated (dysaerobic) conditions during the early Maastrichtian. In the outer belt a more aerated bottom (dysaerobic) prevailed throughout the late Campanian to late Maastrichtian. The elevated organic matter content in both basins reflects the overall environment of high productivity; its actual variations, however, seem to be unrelated to changes in surface productivity and bottom oxygen levels.
Binge/purge oscillations of the Laurentide Ice Sheet as a cause of the North Atlantic's Heinrich events
Authors: MacAyeal D. R.
Published: February 2008

Ice‐rafted debris in sediment cores from the North Atlantic suggests that the Laurentide ice sheet (LIS) periodically disgorged icebergs in brief but violent episodes which occurred approximately every 7,000 years. Here, I propose that Heinrich events (i.e., what these episodes are called) were caused by free oscillations in the flow of the Laurentide ice sheet which arose because the floor of Hudson Bay and Hudson Strait is covered with soft, unconsolidated sediment that forms a slippery lubricant when thawed. The proposed Heinrich event cycle has two phases. The growth phase occurs when the sediment is frozen and the LIS is stranded (immobile) on a rigid bed. The volume of the LIS slowly grows during this phase at a rate dictated by snow accumulation. The purge phase occurs when the basal sediment thaws and a basally lubricated discharge pathway (i.e., an ice stream such as those which occur in West Antarctica today) developes through Hudson Strait. The volume of the LIS rapidly equilibrates to the reduced basal friction during this phase by dumping icebergs into the Labrador Sea. The periodicity
years of the proposed Heinrich event cycle is a function of the thermal conductivity and diffusivity of ice, k and κ, respectively, the atmospheric sea level temperature θsl (in degrees Celsius), and the excess geothermal heat flux defined by
where Γ is the atmospheric lapse rate, and G is the geothermal heat flux. Agreement between the predicted T and the apparent periodicity implied by the marine record is the main virtue of the free oscillation mechanism I propose. An alternative mechanism in which Heinrich events are forced by periodic variations in external climate is implausible, because periodic atmospheric temperature perturbations are strongly attenuated with depth in an ice sheet.
Improved formulas for the diffraction by a wedge
Authors: Liu Y. , Ciric I. R.
Published: February 2008

New analytical expressions for the diffraction integral in the case of a perfectly conducting wedge of an arbitrary angle illuminated by a plane wave or by a line source field are presented. They are independent of the poles in the original integrals and avoid the process of selection of the integer parameter in previous formulations, thus giving a continuous total field at the geometrical optics boundaries. The uniform solution formula in the case of a line source field illumination is obtained without using the asymptotic expansion of the Hankel functions in the cylindrical waves, which extends its validity in the near zone, when the electrical distance from the edge is less than unity. The proposed formulas allow a clearer physical interpretation of the diffraction phenomena and a more convenient and efficient calculation of the diffracted fields even in the region about the geometrical optics boundaries.
Kinematics of the central Hellenides
Authors: Doutsos T. , Pe‐Piper G. , Boronkay K. , Koukouvelas I.
Published: February 2008

Structural analysis along 24 cross sections crosscutting several windows in the central Hellenides provides the sense of nappe movements as well as the location of destroyed oceans lying between the Apulian and Eurasian continents from the Mesozoic. Orogeny took place in two phases: The first phase, “the Eo‐Hellenic” phase, was initiated by convergence of the Apulian and Pelagonian plates with west directed subduction and closure of the Pindos Ocean. Late Jurassic obduction of oceanic lithosphere over the western margin of the Pelagonian plate was followed by footwall imbrication, mylonites and sheath folds. During the late Cretaceous, uplift was associated with ductile normal faulting at depth and tectonic unroofing at shallow crustal levels. The second phase, “the Meso‐Hellenic” phase, comprised the closure of the Ambelakia Ocean at the eastern margin of the Pelagonian plate and continental subduction along the eastern margin of the Apulian plate. West directed subduction of the Ambelakia Ocean was associated with eastward directed ductile thrusting, folding and blueschist metamorphism. Blueschist formed within a simple duplex structure at depth and was subsequently overthrusted in the late Eocene onto the Olympos microcontinent, which acted as a major obstacle to the eastward directed nappe movements. Up to 150‐m‐thick cataclasites, kink folds and a spaced cleavage were formed during the late stage of the continental collision. “A subduction” along the eastern margin of the Apulian plate caused kink folding and reimbrication of the western parts of the Pelagonian basement. Since the Oligocene, the overthickened crust collapsed by means of low‐angle normal faults.
Contrasting styles of rifting: Models and examples from the Eastern Canadian Margin
Authors: Bassi Gianna , Keen Charlotte E. , Potter Patrick
Published: February 2008

We present the results of a dynamical model of lithospheric rifting and rupture which show that a wide range of crustal thinning patterns across rifted passive margins can be produced by varying the steady state geotherm, lithospheric composition (dry versus wet materials), and strain rate. The basic mechanism of continental rupture is assumed to be passive rifting and necking. We use a numerical thermomechanical model of lithosphere extension based on a finite element approach. When plasticity is significant (i.e., at lower temperatures or for “harder” materials) deformation is unstable and thinning takes place abruptly, over a narrow area. Conversely, a progressive thinning across the margin is observed when creep is dominant (i.e., in warm or ductile conditions). Cooling and associated hardening of the thinned area can occur during extension and cause the locus of extension to migrate laterally. In these circumstances, rupture is likely to take place asymmetrically along one edge of the thinned area, producing a narrow margin and a very wide conjugate. The eastern margins of Canada and their conjugates across the North Atlantic provide examples which cover this range of theoretical profiles. The crustal thinning patterns, inferred from deep seismic data, and the duration of rifting compare well with model results. We discuss also the constraints that these geodynamical models provide on such current issues as the seismic reflectivity of the lower crust, or the location of the ocean‐continent boundary in wide areas supposedly underlain by 5‐km thin continental crust.
Cretaceous to Miocene thrusting and wrenching along the central south Carpathians due to a corner effect during collision and orocline formation
Authors: Ratschbacher Lothar , Linzer Hans‐Gert , Moser Franz , Strusievicz Robert‐Octavian , Bedelean Horia , Har Nicolae , Mogoş Petru‐Adrian
Published: February 2008

Field studies in the Romanian South Carpathians (longitude 22.5° to 24.2°E and latitude 45.2° to 45.6°N) demonstrate (1) Cretaceous top‐to‐NE shearing parallel to the present strike of the thrust system connected with coaxial flattening within the generally northwest dipping foliation, (2) Paleogene ductile‐brittle dextral wrenching, E‐W compression (σ1: 87±15°), and basin formation (Petroşani basin) along the Cerna‐Jiu fault system, (3) large‐scale Miocene dextral wrenching along the northern margin of Moesia (σ1: 143±16°), and (4) probably Pliocene–early Pleistocene N‐S compression (σ1: 205±25°). We discuss the tectonics of the South Carpathians stressing the corner effect of the Moesian foreland promontory during convergence and formation of the Carpathian orocline. Up to the late Early Cretaceous, subduction of oceanic crust was active between Europe‐Moesia on one side and East Carpathia‐Rhodopia on the other side. Collision and intracontinental deformation occurred during the late Early and Late Cretaceous. The pinning of the thrust front at the western tip of Moesia and the foreland recess north of it caused superposition of thrusting and wrenching during collision and lateral translation, tangential stretching during orocline formation, and spreading into the recess. Further convergence during the early Tertiary resulted in dislocation of the previously welded East Carpathian‐Rhodopian and Moesian fragments along the Cerna‐Jiu fault system and the further northeast translation of the western segment. The intramontane Petroşani basin opened as a northeasterly propagating, transient pull‐apart structure along the Cerna‐Jiu fault system, which acquired a curved, northwesterly convex, transtensional trace due to the shape of the Moesian promontory. Tightening of the Carpathian orocline and/or rearrangement of the microplate geometry during the formation of the Pannonian basin system led to large‐scale dextral wrenching along the northern margin of Moesia. Pliocene N‐S compression reflects final shortening in the Carpathian system before ongoing convergence between Europe and Africa was transferred to the Mediterranean. Rotation of material lines around the Moesian corner is corroborated by paleomagnetic studies.
Large‐scale crustal structure of the Northwestern Yilgarn Craton, western Australia: Evidence from Nd isotopic data and zircon geochronology
Authors: Nutman Allen P. , Bennett Victoria C. , Kinny Peter D. , Price Richard
Published: February 2008

U‐Pb zircon geochronology and Nd isotope geochemistry have been used in the northwestern Yilgarn Craton region of Western Australia to map gneiss units of different ages and to provide a model for late Archean crustal evolution, despite the inherent difficulties of minimal exposure and variations in the appearance of units brought about by heterogeneous strain. The 3300–3730 Ma gneisses and intercalated metasedimentary units crop out in a ≤100 km wide tract, bounded on both sides by areas containing 2920–3000 Ma gneisses. Between 2750 and 2620 Ma, several generations of granitoids were emplaced throughout the region. Regardless of whether the granitoids intruded the 3300–3730 Ma or 2920–3000 Ma gneisses, modeling of their initial Nd isotopic compositions shows that they were most likely formed by partial melting of predominantly 2920–3000 Ma gneisses; only a minor input from early Archean sources is indicated for granitoids cutting the 3300–3730 Ma gneisses and intercalated metasediments. Some of the granitoids that intrude the 3300–3730 Ma gneisses contain 2920 Ma inherited zircons, supporting the Nd evidence that their source is dominated by 2920–3000 Ma gneisses. The 3300–3730 Ma gneisses are interpreted as belonging to an allochthonous terrain emplaced over a younger terrain comprising the 2920–3000 Ma gneisses. Subsequently, partial melting concentrated in the underlying 2920–3000 Ma terrain gave rise to the late Archean granitoids.
Ophiolitic and metamorphic assemblages of southeast Anatolia and their significance in the geological evolution of the orogenic belt
Authors: Yilmaz Yücel , Yiğitbaş Erdinç , Genç Ş. Can
Published: February 2008

The southeast Anatolian orogen may be divided into three roughly east‐west trending structural zones formed as a result of continental collision between the Taurus platform and the Arabian continent. Along the orogenic belt, metamorphic and ophiolitic rocks occur widely. The ophiolites represent remnants of the ocean or oceans which were totally consumed between these converging continental blocks during Late Cretaceous to Miocene period. Metamorphic rocks formed from the oceanic as well as the continental rocks which were incorporated into a nappe stack during the consumption of the oceanic lithosphere and the progressive southward advance of the nappes toward the Arabian continent. The metamorphic units, together with the ophiolite associations, provide stratigraphic and petrologic evidence indicating time, place, and environment of formation of these units; the metamorphic units also provide evidence of nappe transportation stages which are complementary to the data derived from the sedimentary successions in the evaluation of the orogenic evolution of southeast Anatolian orogen.
Stretching and subsidence: Rifting of conjugate margins in the North Atlantic Region
Authors: Keen Charlotte E. , Dehler Sonya A.
Published: February 2008

Subsidence on rifted conjugate continental margins around the North Atlantic is analyzed to derive the amount and areal distribution of stretching in the crust and in the lower lithosphere during continental rifting. Study areas are the Grand Banks and Orphan Basin regions of the eastern Canadian continental margin and the Goban Spur and Galicia Bank regions off western Europe. In all areas, maps of synrift and postrift sediment thickness and bathymetry were used to derive maps of post‐ and synrift subsidence. A two‐layer lithospheric stretching model with independent amounts of stretching in the crust and in the lower lithosphere was assumed to be applicable, with the rifting history approximated by several instantaneous episodes of extension. This model was used to derive estimates of stretching at all points on a 0.05° geographical grid, where subsidence values were available within the study regions. The models are constrained with seismic measurements of crustal thickness. The results imply that pure shear stretching predominates at a lithospheric scale, while simple shear is more localized laterally and confined to the crust. In places there is significant decoupling between crustal and mantle stretching. Near the continent‐ocean boundary, final continental breakup may be localized on one side of the rift between conjugate margin pairs, rather than symmetrically located. Total extension of the margins is compatible with that estimated from normal fault geometries and indicates that the continent‐ocean boundary has been extended up to 350 km seaward of its original position, which should be considered in plate kinematic reconstructions.
Anatomy and evolution of the Triassic‐Jurassic Continental Rift System, eastern North America
Authors: Schlische Roy W.
Published: February 2008

Mesozoic rift basins in eastern North America formed during continental extension associated with the separation of North America and Africa. These basins locally overprint the Appalachian orogen and involve the extensional reactivation of Paleozoic faults. Half graben are thought to have formed where Mesozoic extension was subperpendicular to orogenic strike. Transtensional basins formed where the extension was more oblique. Segmented border fault systems and predominantly synthetic intrabasinal faults characterize the half graben. These basins resemble elongate synclines in longitudinal section; this geometry resulted from border‐fault displacement that was greatest near the center of the fault and decreased toward both ends. Large‐scale segmentation of some border fault systems resulted in the formation of multiple synclinal subbasins separated by transverse anticlines at segment boundaries, where fault displacement was less. As displacement increased on individual fault segments, the faults and associated basins grew in length, perhaps linking originally isolated basins. Smaller‐scale fault segmentation resulted in the formation of relay ramps, rider blocks, and transverse folds. Some transverse synclines are located near the centers of fault segments, and related anticlines are located at segment boundaries. Adjacent half graben units within larger rift zones do not alternate polarity along strike and are generally not linked via accommodation zones, as in the East African rift system. Strike‐slip‐dominated basins are characterized by a network of strike‐slip and normal faults, and are shallower and narrower than dip‐slip‐dominated basins.
Structure and kinematics of Upper Cenozoic extensional detachment on Naxos and Paros (Cyclades Islands, Greece)
Authors: Gautier Pierre , Brun Jean‐Pierre , Jolivet Laurent
Published: February 2008

This paper presents a study of the ductile and brittle deformation on Naxos and Paros islands (Cyclades, Greece). Previous maps and studies of the two islands have shown that a major low‐angle fault zone separates surface rocks above the contact from an initially deep‐seated unit below, showing a metamorphic evolution from high to low pressures. Structural analysis, as well as available stratigraphical, metamorphic, and geochronological data taken together demonstrate that this fault zone is a major normal‐sense detachment zone dipping to the north. Rapid denudation of footwall rocks subsequent to high temperature metamorphism, at an estimated rate of 1.8–9.5 mm/yr, attests for tectonic unroofing during regional‐scale top‐to‐the north ductile shearing. The change from ductile to brittle behavior of the footwall rocks together with a progressive localization of high strain intensity deformations just below the hangingwall is explained by the progressive cooling of the uprisen footwall of the detachment. Mio‐Pliocene clastic sediments in the hangingwall represent the infilling of half grabens opened in between major normal faults that are synthetic to the underlying ductile shear zone. These sediments are as old as (Aquitanian‐25 Ma), or younger than the earliest recognized evidence of ductile extension in the footwall. This provides a minimum age for the onset of extension in the Cyclades, which appears significantly older than maximum ages reported up to now (13–5 Ma). Structural data strongly suggest that the detachment fault was initially rather low dipping (≈35°). An evolutionary model is proposed, in which migmatite domes in the footwall correspond to the uprise of the lower ductile crust between two separating upper crustal blocks, during a process of asymmetric boudinage of the crust. This detachment model applies to a previously thickened continental lithosphere, which then suffers thermal relaxation and weakening, allowing extensional deformation to reach a climax during and subsequent to high temperature metamorphism. In the Cyclades, crustal‐scale extension started after Early Cenozoic thrusting, while the crust was still thick, or less likely, before late underthrusting below the present surface.
Age, height, and deformation of Holocene marine terraces at Mahia Peninsula, Hikurangi Subduction Margin, New Zealand
Authors: Berryman Kelvin
Published: February 2008

Mahia Peninsula is a prominent coastal landmark in eastern North Island and is the closest point of land in the North Island to the Hikurangi Trough, where the Pacific plate plunges beneath the subduction complex at the eastern margin of the Australian plate. Uplifted Holocene marine deposits of both estuarine and open beach affinities are found in many parts of the peninsula and provide the basis for Holocene tectonic characterization. Estuarine deposits record the later part of the postglacial transgression that culminated about 6500 years B.P. in New Zealand. The deposits have been differentially uplifted since that time at a rate of 2.5 ± 0.3 mm/yr in the central north coast area, decreasing to 0.7 ± 0.2 mm/yr about 6 km to the west. The coastal plain is characterised in many places by a stepped sequence of emergent shore platforms overlain by fossiliferous beach deposits. Extensive radiocarbon dating of samples from beach deposits shows that terraces in widespread parts of the peninsula are of five distinct ages. Each of the terraces is inferred to have formed in conjunction with a large prehistoric earthquake because of the stepped terrace morphology, clustering of ages on each terrace, differential uplift of terraces across the peninsula, and historic coseismic uplift events in this tectonic setting. Paleoseismic events of Mw 7.5–8.0 are estimated to have occurred approximately 250, ∼1600, ∼1900, ∼3500, and ∼4500 years B.P. Uplift distribution of the Holocene and late Pleistocene marine terraces shows that the peninsula lies on the west (gentle) flank of the active Lachlan anticline, which is cored by a major west dipping reverse fault (the Lachlan fault). Holocene active deformation at Mahia Peninsula and other coastal areas of eastern North Island is a continuation of structures developed in Pleistocene time in the landward part of the subduction complex adjacent to the Hikurangi subduction zone.
Tectonics of an ultrahigh‐pressure metamorphic terrane: The Dabie Shan/Tongbai Shan Orogen, China
Authors: Okay Aral I. , Şengör A. M. Celal , Satir Muharrem
Published: February 2008

Ultrahigh‐pressure metamorphic rocks with coesite and diamond form a tectonic slice over 20 km thick, called the eclogite zone, within the Dabie Shan complex in the Qinling orogen in central China. The orogen separates the Sino‐Korean block in the north from the Yangtze block in the south. The Dabie Shan Complex is a composite terrane made up of eclogite facies and amphibolite facies gneiss slices and represents fragments of the lower continental crust of the Yangtze block. The Dabie Shan Complex is bounded in the south by a Triassic foreland fold‐thrust belt and in the north by a greenschist facies metaclastic unit, the Foziling Group, which probably represents the passive continental apron deposits of the Yangtze block. Farther north is a granulite facies gneiss complex, the Qinling Group, which has ultramafic slivers and includes the remnants of an island arc with two bounding suture zones. North of the Qinling Group are early Paleozoic active margin deposits of the Sino‐Korean block. The eclogite zone in the Dabie Shan Complex is sandwiched between amphibolite facies gneiss slices. Dating by Sm‐Nd, Rb‐Sr, and Ar‐Ar of two eclogite samples from the eclogite zone gives early to middle Triassic ages (236–246 Ma); the initial εNdvalues indicate reworking of a 2.11 and 1.55 Ga continental crust. A Himalayan‐type tectonic evolution is envisaged for the Qinling orogen with the creation of a 100‐km‐thick crustal thrust wedge through continuous underplating during the subduction of the Yangtze continental lithosphere. Exhumation of the ultrahigh‐pressure metamorphic rocks was chiefly achieved by the southward propagation of the thrust planes, thereby isostatically uplifting and eroding the earlier deeply subducted parts of the orogen. A total of 680 km of southward thrusting in front of Dabie Shan is inferred, based on the abrupt termination of the Tanlu fault. Normal faulting possibly caused by gravitational collapse probably also had a role in the exhumation process.
The growth of fault‐bounded tilt blocks
Authors: Anders Mark H. , Spiegelman Marc , Rodgers David W. , Hagstrum Jonathan T.
Published: February 2008

A series of uniformly tilted fault‐bounded blocks is a common feature in actively extending regions, such as the Basin and Range province. If the tilted blocks were produced by rigid “domino‐style” rotation, one would predict large voids at either end of a series of these blocks. Using tilt data and a simple flexural calculation, we suggest that much of the apparent rigid behavior could also be produced by internal block deformation. In our model of normal fault growth, isostatic/elastic uplift of the footwall is coupled with hanging wall downdrop within the region between faults, resulting in the appearance of a tilted rigid block. We present tilt data sampled at varying distances from several block‐defining faults within the northeast Basin and Range province. Tilt measurements between a series of 30‐km spaced block‐defining faults are found to be uniform, while tilts between more widely spaced faults exhibit a pattern of tilt that diminishes to zero in less than 30 km. Using a simple flexural calculation for internal block deformation, we show that for this region the patterns of tilt are consistent with a flexural length scale of ∼8–12 km and deflections of 2–4 km. These estimates are compatible with both the lower limit to seismicity and basin depth determined from earthquake and seismic reflection studies.
Use of Precipitation and Groundwater Isotopes to Interpret Regional Hydrology on a Tropical Volcanic Island: Kilauea Volcano Area, Hawaii
Authors: Scholl M. A. , Ingebritsen S. E. , Janik C. J. , Kauahikaua J. P.
Published: February 2008

Isotope tracer methods were used to determine flow paths, recharge areas, and relative age for groundwater in the Kilauea volcano area of the Island of Hawaii. A network of up to 66 precipitation collectors was emplaced in the study area and sampled twice yearly for a 3‐year period. Stable isotopes in rainfall show three distinct isotopic gradients with elevation, which are correlated with trade wind, rain shadow, and highelevation climatological patterns. Temporal variations in precipitation isotopes are controlled more by the frequency of storms than by seasonal temperature fluctuations. Results from this study suggest that (1) sampling network design must take into account areal variations in rainfall patterns on islands and in continental coastal areas and (2) isotope/elevation gradients on other tropical islands may be predictable on the basis of similar climatology. Groundwater was sampled yearly in coastal springs, wells, and a few high‐elevation springs. Areal contrasts in groundwater stable isotopes and tritium indicate that the volcanic rift zones compartmentalize the regional groundwater system, isolating the groundwater south of Kilauea's summit and rift zones. Part of the Southwest Rift Zone appears to act as a conduit for water from higher elevation, but there is no evidence for downrift flow in the springs and shallow wells sampled in the lower East Rift Zone.
The Use of Artificial Neural Networks for the Prediction of Water Quality Parameters
Authors: Maier Holger R. , Dandy Graeme C.
Published: February 2008

This paper presents the use of artificial neural networks (ANNs) as a viable means of forecasting water quality parameters. A review of ANNs is given, and a case study is presented in which ANN methods are used to forecast salinity in the River Murray at Murray Bridge (South Australia) 14 days in advance. It is estimated that high salinity levels in the Murray cause $US 22 million damage per year to water users in Adelaide. Previous studies have shown that the average salinity of the water supplied to Adelaide could be reduced by about 10% if pumping from the Murray were to be scheduled in an optimal manner. This requires forecasts of salinity several weeks in advance. The results obtained were most promising. The average absolute percentage errors of the independent 14‐day forecasts for four different years of data varied from 5.3% to 7.0%. The average absolute percentage error obtained as part of a real‐time forecasting simulation for 1991 was 6.5%.
Stable carbon isotope chronologies from trees in the southwestern United States
Authors: Leavitt Steven W. , Long Austin
Published: February 2008

Plant fixation of CO2 and subsequent incorporation of this carbon into growth rings is the basis for attempts to use tree rings to reconstruct 13C/12C ratios (δ13C) of atmospheric carbon dioxide. Accurate δ13C chronologies of atmospheric CO2 are important to distinguishing biospheric and fossil fuel CO2 inputs to the atmosphere. We have sampled 14 sites in the American Southwest, pooling four cores from each of four trees at most of the sites. Cellulose from pooled 5‐year ring groups was analyzed to develop δ13C chronologies for each of the sites. The 10 pinyon (Pinus edulis, P. monophylla) chronologies, plus two additional pinyon chronologies formed from groups of sites where single trees had been analyzed, were examined in detail. Significant correlations of differences in successive values of ring width indices and δ13C (first differences) were used to “correct” each chronology for climate influences primarily responsible for high‐frequency fluctuation. The master corrected δ13C chronology reveals an overall decrease of 1.2–1.4‰ from 1700 to present, greater than the atmospheric change expected from fossil fuel CO2 inputs alone and comparable to that measured in ice cores. The biospheric CO2 input to the atmosphere implicated in this chronology is greatest is the nineteenth and early twentieth centuries. High‐frequency fluctuations in the chronologies appear dominantly related to climate (drought).
Testing models of past ocean chemistry using foraminifera 15N/14N
Authors: Altabet Mark A. , Curry William B.
Published: February 2008

Previous models have attempted to explain glacial to interglacial changes in atmospheric pCO2 by invoking changes in the ocean's nutrient concentration or regional changes in nutrient utilization. Nitrogen is limiting to primary production and has a residence time in the ocean compatible with glacial to interglacial variations. But up to now, there has been no geological indicator for glacial to interglacial changes in the ocean's nitrogen cycle. We propose that 15N/14N ratios for the organic matrix of preserved foraminifera yield an interpretable δ15N record. We have developed models to study the relationship between plausible changes in the ocean's nitrogen cycle and resulting changes in the 15N/14N ratio of dissolved NO3−. Our model results demonstrate that 15N/14N ratios exhibit excursions during periods in which the oceans are accumulating or losing nitrogen regardless of the means by which these changes occur. Foraminifera 15N/14N results for a Pacific and an Atlantic core provide evidence against wide spread glacial anoxia, a consequence of many models of ocean chemistry. We suggest that increasing shelf‐sediment denitrification upon deglaciation reduced nitrogen concentrations in the modern ocean. Ambiguities between the two foraminifera 15N/14N records indicate that local effects associated with changes in hydrography or ecology need to be studied further.
An ombrotrophic bog as a methane reservoir
Authors: Brown Ann , Mathur S. P. , Kushner D. J.
Published: February 2008

The distribution of methane in an ombrotrophic bog was examined by extracting the gas from different depths, using evacuated bottles and sampling tubes. Methane was extracted during 75 min, from 60 cm, 90 cm and 120 cm depths at 25 stations in a 24 meter‐square area. The amount of methane extracted varied widely between samples, and this variation was greater within each depth than between the different depths. Approximately two and a half times as much methane was found at 90 cm and 120 cm depths as at 60 cm. The amount of methane from sites more than 60 cm below the surface can be correlated with the methane microbially produced from peat from the same site during laboratory incubations. Much less methane could be extracted from near‐surface peat than from deeper levels, but laboratory incubations of peat from this level produced 10‐fold more methane. Our results show that there is a considerable amount of methane trapped below 50 cm depth within an ombrotrophic bog, which we calculate in the bog studied to be 1.7 Gg. We suggest this entrapped methane reduces the hydraulic conductivity in the lower layers of the peat by blocking the soil pore spaces and preventing fluid movement; this in turn generates an elevated water table allowing the formation of a raised bog. We further suggest that if this peat is disturbed by mining or farming, the entrapped methane could be released into the atmosphere and add to the atmospheric pollution by greenhouse gases.
Losses and transport of odd nitrogen species(NOy) over the western Atlantic Ocean during GCE/CASE/WATOX
Authors: Ray John D. , Luria Menachem , Hastie Donald R. , Malle Sue , Keene William C. , Sievering Herman
Published: February 2008

Aircraft and shipboard measurements of mixing ratios for odd nitrogen species (NOy) were made over the western Atlantic Ocean during the summer of 1988. NOy in boundary layer air ranged from 4.8 ppbv near the U.S. east coast to 1.7 ppbv near a research ship 160 km from the coast. A vertical NOy gradient was observed near shore; mixing ratios decreased with altitude from 3.0 ppbv at 150 m to 1.3 ppbv at 2600 m. A smaller NOy gradient was observed near the ship, with mixing ratios also decreasing with altitude. During the observation period of this experiment, a high‐pressure system over the mid‐Atlantic limited advection of polluted continental air to a band just along the coast, thus preserving less polluted conditions to the east of the band. Loss rates for NOy advected from the continent, based on 3 eastward flights from the U.S. east coast, were estimated to be 2.0 ± 1.5% hour−1. Measurements near Bermuda were in marine air with average mixing ratios near the surface of 0.8 ppbv for NOy and 0.3 ppbv for total NO3−. No significant vertical gradient was observed for NOy near Bermuda, where the high‐altitude NOy mixing ratio was 0.7 ppbv. Overall, NOy mixing ratios near Bermuda were higher than would be expected on the basis of either measurements or computer modeling for remote marine air, suggesting possible influence from local anthropogenic sources.