The timing and extent of the eruption of the Siberian Traps large igneous province: Implications for the end-Permian environmental crisis

We present new high-precision ⁴⁰Ar/³⁹Ar ages on feldspar and biotite separates to establish the age, duration and extent of the larger Siberian Traps volcanic province. Samples include basalts and gabbros from Noril'sk, the Lower Tunguska area on the Siberian craton, the Taimyr Peninsula, the Kuznetsk Basin, Vorkuta in the Polar Urals, and from Chelyabinsk in the southern Urals. Most of the ages, except for those from Chelyabinsk, are indistinguishable from those found at Noril'sk. Cessation of activity at Noril'sk is constrained by a ⁴⁰Ar/³⁹Ar age of 250.3 ± 1.1 Ma for the uppermost Kumginsky Suite.The new ⁴⁰Ar/³⁹Ar data confirm that the bulk of Siberian volcanism occurred at 250 Ma during a period of less than 2 Ma, extending over an area of up to 5 million km². The resolution of the data allows us to confidently conclude that the main stage of volcanism either immediately predates, or is synchronous with, the end-Permian mass extinction, further strengthening an association between volcanism and the end-Permian crisis. A sanidine age of 249.25 ± 0.14 Ma from Bed 28 tuff at the global section and stratotype at Meishan, China, allows us to bracket the P–Tr boundary to 0.58 ± 0.21 myr, and enables a direct comparison between the ⁴⁰Ar/³⁹Ar age of the Traps and the Permo–Triassic boundary section.Younger ages (243 Ma) obtained for basalts from Chelyabinsk indicate that volcanism in at least the southern part of the province continued into the Triassic.     

A hypothesis on the locations of lunar gas venting

Both lunar transient phenomena and ²²²Rn/²¹⁰Po anomalies observed by Apollo-15 and -16 orbital alpha spectrometers display preferences for certain kinds of locations: rims of circular maria and craters with central peaks and/or dark floors. If these classes of observations are due to lunar gas venting, why are these types of locations preferred? The hypothesis offered is that these are locations at which cracks or channels exist extending deep enough into the moon to tap lunar volatile reservoirs. Possible channels include circumferential cracks around circular maria, old lava tubes for dark-floor and volcanic central peak craters, and shattered subsurface rock structure for impact central peak craters.     

Numerical simulation of gas venting for NAPL site remediation

A control volume, finite element method is used to discretize the three phase, three component equations for simulation of gas venting. The discrete equations are solved using full Newton iteration. Any combinations of phases can exist, and variable substitution is used to take into account phase appearance and disappearance. Some example computations are presented for two dimensional axisymmetric geometry. Several different scenarios for gas venting are examined. High rate air injection can be effective at removing NAPL both in the unsaturated and saturated zones. The numerical techniques can handle problems having node pore volume gas throughputs (in a timestep) of the order of 10⁶, which greatly exceeds the maximum stable explicit timestep size.     

水合物层下伏游离气渗漏过程的数值模拟及实例分析

海洋环境中天然气水合物层是理想的毛细管封闭层,游离气被抑制在水合物层下,游离气层的气体压力随气体聚集和气层厚度的增加而升高,当气压超过封闭层的毛细管力时,游离气会克服毛细管进入压力、刺入上伏封闭层孔隙空间,毛细管封闭作用随之消失,从而形成水合物下伏游离气向海底的渗漏.通过对该过程进行的数值模拟计算表明：渗漏气体是以活塞式驱动上伏沉积层中的孔隙水向海底排出,水合物稳定带内流体渗漏速度随水流柱高度的减小而增加,当水流阻抗大于相应沉积层段的静岩压力时,沉积层将转变为流沙,流沙沉积被海流移除后便在海底留下凹陷麻坑.麻坑形成后流体运移通道演化为气体通道,气体快速排放.麻坑深度主要取决于游离气层的厚度和水合物封闭层（底界）的深度,而与沉积层的渗透率无关.麻坑深度一定程度上指示了渗漏前水合物层下伏游离气层的资源量.对布莱克海台海底麻坑深度的数值模拟计算表明,形成4 m深的海底麻坑需要至少22 m厚的游离气层.     

Explosion craters associated with shallow submarine gas venting off Panarea island, Italy

Explosions of hot water, steam, and gas are common periodic events of subaerial geothermal systems. These highly destructive events may cause loss of life and substantial damage to infrastructure, especially in densely populated areas and where geothermal systems are actively exploited for energy. We report on the occurrence of a large number of explosion craters associated with the offshore venting of gas and thermal waters at the volcanic island of Panarea, Italy, demonstrating that violent explosions similar to those observed on land also are common in the shallow submarine environment. With diameters ranging from 5 to over 100?m, the observed circular seafloor depressions record a history of major gas explosions caused by frequent perturbation of the submarine geothermal system over the past 10,000?years. Estimates of the total gas flux indicate that the Panarea geothermal system released over 70?Mt of CO₂ over this period of time, suggesting that CO₂ venting at submerged arc volcanoes contributes significantly to the global atmospheric budget of this greenhouse gas. The findings at Panarea highlight that shallow submarine gas explosions represent a previously unrecognized volcanic hazard around populated volcanic islands that needs to be taken into account in the development of risk management strategies.     

Correlations between fatty acid composition of seston and zooplankton and effects of environmental parameters in a eutrophic Siberian reservoir

During two sampling seasons we analyzed on weekly basis fatty acid (FA) composition of seston fraction <130 μm and zooplankton fraction >130 μm, and compared them using a multivariate canonical correlation analysis (CCA). Besides, we evaluated a possible impact of water temperature and inorganic nutrients on FA composition of the seston and the zooplankton.In spite of significant differences in percentages of several individual FAs, we found very strong canonical correlation (cross-correlation, 1-week lag) between FA composition of the seston and the zooplankton. The most important factor, providing the overall canonical cross-correlation between FA profiles of the seston and the zooplankton fractions was eicosapentaenoic acid (20:5ω3, EPA). FA composition of the zooplankton fraction had comparatively poor correlations with taxonomic composition of the zooplankton. Thus, seasonal variations of FA composition of the zooplankton were determined primarily by seasonal changes in FA composition of the seston, rather than by taxonomic differences of FA profiles between rotifers, cyclopoids and cladocerans. FA composition of the seston was strongly affected by its taxonomic composition, namely by that of phytoplankton. According to CCA, the highest factor loadings pertained to diatoms interacting with their marker acids, including EPA, and cyanobacteria and greens, interacting with their marker acids. Ciliates and small rotifers composed considerable and sometimes major part of the seston biomass, but according to CCA their contributions to seasonal variations of the total FA profile of the seston were insignificant. This finding indirectly support the conclusion of the other authors, that the main source of FAs presented in ciliates and rotifers must be sought in algae and that they do not modify FA composition of food consumed, apart from repackaging it.Water temperature was the principal environmental parameter which drove the overall variations of FA composition. Factor loadings for the inorganic nutrients were comparatively negligible. The main contribution in the seasonal variation of FA composition of the seston was given by negative interaction between water temperature and percentage of EPA in the seston.     

Paleomagnetism, microplates and Siberian plate consolidation

Paleomagnetic research in the Siberian area provides new data to derive models of relative horizontal movements and the process of amalgamation or consolidation of the Siberian plate. Comparison of the Apparent Polar Wander (APW) curves for different regions of Siberia show that there were several stages of plate consolidation, the most important of which belonged to Baikalian time (c. 520–800 Ma). The consolidation was complicated by rifting processes. There were six megaterrains in pre-Vendian times, which underwent differential horizontal displacements. The joining of the Altai-Sayan fold area and the North-East microplates and fold belts to the Siberian plate, and the separation of the Taimyr peninsula, were the main post-Baikalian events. The most complete geologic sections were studied in conjunction with radiometric age dating and related to the geochronological Precambrian time scale of the USSR. This has led to the discovery of a stratigraphic polarity bias in the early Riphean-Vendian interval. There were five polarity stages in this interval 200–240 Ma each.     

Siberian traps: Hypotheses and seismology data

Siberian traps are the result of huge basalt eruptions which took place about 250 Ma ago over a vast territory of Siberia. The genesis of Siberian traps is attributed to a mantle plume with a center in the region of Iceland or beneath the central Urals in terms of their present coordinates. The eruption mechanism is associated with delamination—replacement of the mantle lithosphere by the deep magma material. The receiver function analysis of the records from the Norilsk seismic station (NRIL) allows comparing these hypotheses with the factual data on the depth structure of the region of Siberian traps. The S-wave velocity section place the seismic lithosphere/asthenosphere boundary (LAB) at a depth of 155–190 km, commensurate with the data for the other cratons. The mantle lithosphere has a high S-wave velocity characteristic of cratons (4.6–4.8 km/s instead of the typical value 4.5 km/s). The seismic boundary, which is located at a depth around 410 km beneath the continents is depressed by ~10 km in the region of the NRIL station. The phase diagram of olivine/wadsleyite transformation accounts for this depression by a 50–100°С increase in temperature. At the depths of 350–400 km, the S-wave velocity drops due to partial melting. A new reduction in the S-wave velocities is observed at a depth of 460 km. The similar anomalies (deepening of the 410-km seismic boundary and low shear wave velocity at depths of 350–400 and 460–500 km, respectively) were previously revealed in the other regions of the Meso-Cenozoic volcanism. In the case of a differently directed drift of the Siberian lithosphere and underlying mantle at depths down to 500 km, these anomalies are barely accountable. In particular, if the mantle at a depth ranging from 200 to 500 km is fixed, the anomalies should be observed at the original locations where they emerged 250 Ma ago, i.e. thousands of km from the Siberian traps. Our seismic data suggest that despite the low viscosity of the asthenosphere, the mantle drift at depths ranging from 200 to 500 km is correlated with the drift of the Siberian lithospheric plate. Furthermore, the position of the mantle plume beneath the Urals is easier to reconcile with the seismic data than its position beneath Iceland because of the Siberian traps being less remote from the Urals.     

Seismicity of the southern Siberian platform: Spatiotemporal characteristics and genesis

Analysis of instrumental data on earthquakes of 1960–2005 in the Irkutsk amphitheater shows that the majority of the earthquakes form a wide (150–300 km) band of diffuse seismicity along the marginal suture of the Siberian platform. In accordance with established regular spatiotemporal patterns of the distribution of earthquakes, this band belongs to the Sayan-Baikal seismic belt, associated with the destruction process at the boundary of large lithospheric blocks. The band is located on the northern periphery of the belt and, the deformable substrate being highly monolithic, this sharply weakens the seismicity within the band. Because of the tectonic origin of earthquakes in such a vast platform territory, undoubted evidence for induced seismicity around the Angara cascade reservoirs, and the intense economic development of the region, the problem of seismic hazard in the southern Siberian platform should be regarded as one of the most significant objects of geodynamic research.     

Factors responsible for the high position of the Siberian platform

A new model accounting for the origin of anomalously high elevations of the Siberian platform (SP) topography is presented. It is shown that the traditional interpretation of these topographic anomalies is at variance with the available evidence for the geological history of the SP development. The ideas elaborated in the paper are based on the concept of the formation of a mantle plume that has led to the supply of large volumes of molten material into the upper crust and surface basalt eruptions. A new approach is proposed for the construction of a density model of the Siberian upper mantle. A density model of the crust based on the available seismic and petrological data is constructed at the first stage. The calculated anomalous gravity field produced by this model is then subtracted from the observed field. The resulting residual mantle anomalies are used, together with seismological data, for the construction of an upper mantle density model. The formation of the present SP topography is shown to have been controlled by the thickening of the crust due to underplating caused by the development of a giant mantle plume at 251 Ma.     

Infrasonic tracking of large bubble bursts and ash venting at Erebus Volcano,Antarctica

Explosive degassing at Erebus Volcano produces infrasound that can be used to locate, characterize, and quantify eruptive activity from multiple vents. We use a three element distributed microphone network to pinpoint eruption sources and track the activity at the prominent vents through time. Eruptive mechanisms for both source types are analyzed in conjunction with the telemetered time-synced video imagery. We identify two commonly active vents corresponding to the large (often > 10-m diameter) bubble bursts at the free surface of a persistent phonolitic lava lake (‘Ray Lake’), and the less frequent ash-rich eruptions from a constricted vent (‘Active Vent’) located ∼ 80 m from the lava lake. During a 3-month study interval from 6 January to 13 April 2006 we identified and mapped more than 350 eruptive sources from the lava lake and 20 sources from the ash vent. Lava lake events are characterized by high-amplitude infrasonic transients that reflect rapid (less than a few s) acceleration and rupture of magma bubble films followed by an explosion of pressurized gases. Precise infrasonic localization of the lava lake events to accuracies of a few m indicates variable bubble source locations across a 40 by 50-m region spanning the lava lake. Spatial variability is corroborated by the video data. In contrast, degassing from the ash vent produces longer-duration (tens of s), lower amplitude transients that reflect diminished impulsivity and an extended degassing duration, features that are corroborated by video. Because infrasound networks can operate continuously in all weather conditions and during both diurnal and seasonal polar darkness, and are easily incorporated into automatic processing, they significantly contribute to the completeness and quantification of eruption catalogues for Erebus.     

New insights into the initiation and venting of the Bronze-Age eruption of Santorini (Greece), from component analysis

The late-seventeenth century BC Minoan eruption of Santorini discharged 30–60 km³ of magma, and caldera collapse deepened and widened the existing 22 ka caldera. A study of juvenile, cognate, and accidental components in the eruption products provides new constraints on vent development during the five eruptive phases, and on the processes that initiated the eruption. The eruption began with subplinian (phase 0) and plinian (phase 1) phases from a vent on a NE–SW fault line that bisects the volcanic field. During phase 1, the magma fragmentation level dropped from the surface to the level of subvolcanic basement and magmatic intrusions. The fragmentation level shallowed again, and the vent migrated northwards (during phase 2) into the flooded 22 ka caldera. The eruption then became strongly phreatomagmatic and discharged low-temperature ignimbrite containing abundant fragments of post-22 ka, pre-Minoan intracaldera lavas (phase 3). Phase 4 discharged hot, fluidized pyroclastic flows from subaerial vents and constructed three main ignimbrite fans (northwestern, eastern, and southern) around the volcano. The first phase-4 flows were discharged from a vent, or vents, in the northern half of the volcanic field, and laid down lithic-block-rich ignimbrite and lag breccias across much of the NW fan. About a tenth of the lithic debris in these flows was subvolcanic basement. New subaerial vents then opened up, probably across much of the volcanic field, and finer-grained ignimbrite was discharged to form the E and S fans. If major caldera collapse took place during the eruption, it probably occurred during phase 4. Three juvenile components were discharged during the eruption—a volumetrically dominant rhyodacitic pumice and two andesitic components: microphenocryst-rich andesitic pumices and quenched andesitic enclaves. The microphenocryst-rich pumices form a textural, mineralogical, chemical, and thermal continuum with co-erupted hornblende diorite nodules, and together they are interpreted as the contents of a small, variably crystallized intrusion that was fragmented and discharged during the eruption, mostly during phases 0 and 1. The microphenocryst-rich pumices, hornblende diorite, andesitic enclaves, and fragments of pre-Minoan intracaldera andesitic lava together form a chemically distinct suite of Ba-rich, Zr-poor andesites that is unique in the products of Santorini since 530 ka. Once the Minoan magma reservoir was primed for eruption by recharge-generated pressurization, the rhyodacite moved upwards by exploiting the plane of weakness offered by the pre-existing andesite–diorite intrusion, dragging some of the crystal-rich contents of the intrusion with it.     

Estimating the variations in paleointensity from the Siberian traps of Maymecha-Kotui and Norilsk regions

The results of determining the paleointensity of the Siberian traps sampled from the Kotui sequence, Truba ravine, Onkuchak Suite, and dated to Permian-Triassic are presented. Most of the H _anc values for separate lava flows are significantly lower than the values of the present geomagnetic field at the observation point (approximately 50 μT). This is consistent with the known conclusion that the intensity of the geomagnetic field at the Permian-Triassic boundary was lower by a factor of two to three lower than its present value. We carried out the comparative analysis of the behavior of H _an and VDM for two trap sequences (Kotui (the Onkuchak Suite) and Norilsk) from the standpoint of the eruptive pulse hypothesis. In both sections, the average VDM values and their dispersion are similar. For comparison, similar analysis of the VDM behavior is conducted for the Brunhes epoch and Miocene in the interval of 5–10 Ma. It is hypothesized that during the three considered periods, namely, the eruption of the Siberian traps, in the Brunhes epoch, and in Miocene (in the interval of 5–10 Ma), the time behavior of the geomagnetic field was close to a stationary stochastic process.     

Texas environmental flow standards and the hydrology-based environmental flow regime methodology

Abstract

In 2007, the Texas legislature created a program to identify environmental flow standards statewide through the coordinated efforts of scientific and stakeholder groups and rulemaking by the Texas Commission on Environmental Quality. To aid in this task, a Hydrology-based Environmental Flow Regime (HEFR) method was developed that combines a suite of user-customizable hydrologic statistics with an implementation framework. Following the concepts of the Natural Flow Paradigm, the methodology includes the separation of a long-term hydrograph into key flow components (e.g. subsistence, base, high-flow pulse and overbank) defined by the Texas Instream Flow Program. Seasonal, annual and inter-annual flow component statistics were then coupled with biology, water quality and geomorphology overlays, where available, and with implementation rules applied to example large-scale water supply projects to support development of environmental flow standards for use in water rights permit conditions. The HEFR methodology and resulting flow recommendations are compared to two contemporary in-stream flow studies and adopted environmental flow standards. Subsistence flows were fairly similar. Baseflows were in a similar range, but fewer than three seasonal levels have sometimes been specified in in-stream flow studies. Episodic events are quite different in terms of magnitude, frequency, duration and applicable number.

Editor D. Koutsoyiannis; Guest editor M. Acreman

Citation Opdyke, D.R., Oborny, E.L., Vaugh, S.K., and Mayes, K.B., 2014. Texas environmental flow standards and the hydrology-based environmental flow regime methodology. Hydrological Sciences Journal, 59 (3–4), 820–830.     

Upper Crustal Velocity Model for the Southeastern Boundary of the Siberian Platform

Izvestiya, Physics of the Solid Earth - Abstract—Based on the common-mid-point (CMP) seismic data on the 3-DV reference profile, using dynamic conversion of refracted waves into time...     

Siberian paleomagnetic data and the problem of rigidity of the Northern Eurasian continent in the post-Paleozoic

The Meso-Cenozoic paleomagnetic poles from the Siberian platform and its folded margins, which comply with the modern technical and methodological standards, are analyzed. The analysis suggests the following conclusions. (1) The geometrical relationship between the Permo-Triassic poles of the Stable Europe and Siberian Platform prohibits the possibility of relative displacements of these platforms in the post-Paleozoic time. (2) The Mesozoic paleomagnetic poles of the Siberian Platform support the hypothesis of rigid Northern Eurasia. (3) The paleolatitudes of the Mesozoic sections located on the folded margins of the Siberian Platform closely agree with the Apparent Polar Wandering Path (APWP) for Europe. (4) The available data indicate that the vertical-axis rotation of separate local blocks within the folded margins of the Siberian Platform was a widespread phenomenon. Therefore, (1) the modern paleomagnetic data quite certainly show that consolidation of the Northern Eurasian continent was completed by the end of Permian, and, since the very beginning of the Mesozoic, the Siberian and East-European platforms have been parts of a single rigid megablock. (2) The Meso-Cenozoic segment of the APWP for Europe can be used as reference for the Siberian platform.     

Geochemical and biogeochemical observations on the biological communities associated with fluid venting in Nankai Trough and Japan Trench subduction zones

We have studied the compositions and structure of the lamellibranchiaCalyptogena sp associated with fluid venting in subduction zones off Japan. The gills of theCalyptogena are the habitat of symbiotic bacteria which develop owing to chemoautotrophy. They most probably assimilate hydrogen sulfide, methane and/or carbon dioxide and reduce nitrogen. The isotopic budget of methane uptake enables to establish the thermal diagenetic origin of the light hydrocarbons brought by the upwelling fluids. The microscopic examination of thin sections of the bacteria enables to establish that their membranes are compatible with uptake of inorganic nutrients and to establish relations with the metabolism of archeobacteria.