A Mesocyclone over the Northern Sea of Japan in February 2013

Oceanology - The article considers a particular event in the influence of a mesoscale cyclone (MC) on the northern Sea of Japan on February 10–11, 2013. The passage of the MC was accompanied...     

The Structure of the Bazhenov Horizon in the Northeast Part of West Siberia

A new variant of the structural-facies zonation of the Bazhenov horizon (Tithonian–Lower Berriasian) in the northeastern part of West Siberia was considered using lithofacies and structural analysis. The low content of terrigenous sedimentary material in the high-carbon facies of the Bazhenov Formation is explained by the capturing of the material by the relatively deep Pur–Taz Paleobasin, where the fan of the Yenisei–Khatanga strait was formed. The Pur–Taz Paleobasin was filled by the sediments of the Yanovstanov Formation. In the northeast the basin borders the vast and relatively shallow Ob Paleobasin, where the black shales of the oil-source Bazhenov Formation were accumulated.     

Dating of mine waste in lacustrine sediments using cesium-137

For over a century Medicine Lake in northern Idaho has received heavy-metal-laden tailings from the Coeur d'Alene mining district. Establishing the depositional chronology of the lake bottom sediments provides information on the source and rate of deposition of the tailings. Cesium-137, an isotope produced in the atmosphere by nuclear bomb tests, was virtually absent in the environment prior to 1951, but reached its apex in 1964. Our analysis of cesium-137 in the sediments of Medicine Lake revealed that 14 cm of fine-grained tailings were deposited in the lake from 1951 to 1964 and tailing deposition downstream was greatly reduced by the installation of tailings dams in the district in 1968. Cesium-137 analysis is accomplished by a fairly simple gamma-ray counting technique and should be a valuable tool for analyzing sedimentation in any lacustrine environment that was active during the 1950s and 1960s.     

Evolution of fluids associated with metasedimentary sequences from Chaves (North Portugal)

In order to identify and characterise fluids associated with metamorphic rocks from the Chaves region (North Portugal), fluid inclusions were studied in quartz veinlets, concordant with the main foliation, in graphitic-rich and nongraphitic-rich lithologies from areas with distinct metamorphic grade. The study indicates multiple fluid circulation events with a variety of compositions, broadly within the C–H–O–N–salt system. Primary fluid inclusions in quartz contain low salinity aqueous–carbonic, H₂O–CH₄–N₂–NaCl fluids that were trapped near the peak of regional metamorphism, which occurred during or immediately after D2. The calculated P–T conditions for the western area of Chaves (CW) is P=300–350 MPa and T500 °C, and for the eastern area (CE), P=200–250 MPa and T=400–450 °C. A first generation of secondary fluid inclusions is restricted to discrete cracks at the grain boundaries of quartz and consists of low salinity aqueous–carbonic, H₂O–CO₂–CH₄–N₂–NaCl fluids. P–T conditions from the fluid inclusions indicate that they were trapped during a thermal event, probably related with the emplacement of the two-mica granites.

A second generation of secondary inclusions occurs in intergranular fractures and is characterised by two types of aqueous inclusions. One type is a low salinity, H₂O–NaCl fluid and the second consists of a high salinity, H₂O–NaCl–CaCl₂ fluid. These fluid inclusions are not related to the metamorphic process and have been trapped after D3 at relatively low P (hydrostatic)–T conditions (P<100 MPa and T<300 °C).

Both the early H₂O–CH₄–N₂–NaCl fluids in quartz from the graphitic-rich lithologies and the later H₂O–CO₂–CH₄–N₂–NaCl carbonic fluid in quartz from graphitic-rich and nongraphitic-rich lithologies seem to have a common origin and evolution. They have low salinity, probably resulting from connate waters that were diluted by the water released from mineral dehydration during metamorphism. Their main component is water, but the early H₂O–CH₄–N₂–NaCl fluids are enriched in CH₄ due to interaction with the C-rich host rocks.

From the early H₂O–CH₄–N₂–NaCl to the later aqueous–carbonic H₂O–CO₂–CH₄–N₂–NaCl fluids, there is an enrichment in CO₂ that is more significant for the fluids associated with nongraphitic-rich lithologies.

The aqueous–carbonic fluids, enriched in H₂O and CH₄, are primarily associated with graphitic-rich lithologies. However, the aqueous–carbonic CO₂-rich fluids were found in both graphitic and nongraphitic-rich units from both the CW and CE studied areas, which are of medium and low metamorphic grade, respectively.     

The effects of heating on low-temperature susceptibility and hysteresis properties of basalts

The variation of the low-field susceptibility of basalts down to liquid-nitrogen temperature always falls into one of three types that depend on the composition and grain size of the titanomagnetite grains present. Group 1 basalts contain predominantly unoxidised, multidomain homogeneous titanomagnetites having x 0.3. Group 2 basalts contain predominantly titanomagnetite grains with many exsolved ilmenite lamallae that subdivide the grains so that they act similarly to single domains. Group 3 basalts contain predominantly multidomain magnetite or magnetite-rich titanomagnetite having x 0.15. After repeated heating to 615°C, the group 1 basalts gradually oxidise above 300°C to produce the characteristics of group 2 basalts, owing to the exsolution of ilmenite. On the other hand, both group 2 and 3 basalts are stable to oxidation until at least 500°C. They are therefore the most useful material for palaeointensity studies.     

Fluids in the lithosphere, 2. Experimental constraints on CO2 transport in dunite and quartzite at elevated P-T conditions with implications for mantle and crustal decarbonation processes

In a series of experiments at 0.5–1.3 GPa and 1050–1200°C we have monitored the transport, via crack propagation, of CO₂ into well-annealed olivine and quartz aggregates. The objectives were to determine (1) the extent and rate of fluid penetration; (2) the effect of varying both P-T conditions and microstructure; and (3) the fluid penetration pathways. Experiments on CO₂ penetration into dunite annealed in the absence of MgO indicate rapid and pervasive fluid transport on a grain-dimension scale, but a limited penetration distance ( 1 mm). Additional experiments on dunite annealed in the presence of MgO (either dispersed or present at both ends), however, resulted in CO₂ penetration that was both pervasive on the scale of individual grains and almost always completely through the 5 mm long samples. The abundance of fine (10 μm) grains in the MgO-free dunite, in contrast to the much larger grain sizes of the samples annealed with MgO present, suggests the difference in fluid penetration behavior may arise because the strength variation in dunite scales with the grain size. Effects arising from changes in olivine point defect chemistry, however, are an additional possibility. The response of synthetic quartzite to CO₂ overpressure is distinct from that of dunite: Quartzite experiences rapid and complete penetration of CO₂, via a macroscopically visible system of transgranular fractures, over the range of P-T conditions investigated.The small amount of porosity ( 2–3%) present in most rock samples fabricated for this study, lacks three-dimensional connectivity, thus precluding any enhanced fluid penetration via porous flow. Pores could possibly enhance fluid penetration as the result of a small reduction in resistance to fracture, but the probable abundance of strength-controlling flaws in natural rocks is likely to produce similar behavior.The results of our experiments on olivine and olivine + MgO suggest that the transport of pressurized CO₂ in very olivine-rich mantle environments will be pervasive on the scale of individual grains and its extent may be dependent on rock microstructure and/or crystal chemical effects. Such pervasive fluid transport, perhaps associated with magma decarbonation, may have interesting implications for both magma transport and local LREE enrichment of adjacent mantle wall-rock. The ease with which quartzite is penetrated by CO₂ at the conditions of our experiments underscores the possible role of decarbonation reactions in crustal permeability-enhancement processes.     

Residual curvature migration velocity analysis for angle domain common imaging gathers

Pre-stack depth migration velocity analysis is one of the keys to influencing the imaging quality of pre-stack migration. In this paper we cover a residual curvature velocity analysis method on angle-domain common image gathers (ADCIGs) which can depict the relationship between incident angle and migration depth at imaging points and update the migration velocity. Differing from offset-domain common image gathers (ODCIGs), ADCIGs are not disturbed by the multi-path problem which contributes to imaging artifacts, thus influencing the velocity analysis. On the basis of horizontal layers, we derive the residual depth equation and also propose a velocity analysis workflow for velocity scanning. The tests to synthetic and field data prove the velocity analysis methods adopted in this paper are robust and valid.     

Modulation of tropical ocean surface chlorophyll by the Madden–Julian Oscillation

The MJO modulation of sea surface chlorophyll-a (Chl) examined initially by Waliser et al. in Geophys Res Lett, (2005) is revisited with a significantly longer time-series of observations and a more systematic approach to characterizing the possible mechanisms underlying the MJO-Chl relationships. The MJO composite analysis of Chl and lead-lag correlations between Chl and other physical variables reveal regional variability of Chl and corresponding indicative temporal relationships among variables. Along the path of the MJO convection, wind speed—a proxy for oceanic vertical turbulent mixing and corresponding entrainment—is most strongly correlated with Chl when wind leads Chl by a few days. Composite Chl also displays MJO influences away from the path of the MJO convection. The role of wind speed in those regions is generally the same for Chl variability as that along the path of the MJO convection, although Ekman pumping also plays a role in generating Chl variability in limited regions. However, the wind forcing away from the MJO convection path is less coherent, rendering the temporal link relatively weak. Lastly, the potential for bio-physical feedbacks at the MJO time-scale is examined. The correlation analysis provides tantalizing evidence for local bio-feedbacks to the physical MJO system. Plausible hypothesis for Chl to amplify the MJO phase transition is presented though it cannot be affirmed in this study and will be examined and reported in a future modeling study.     

Sensitivity study of the seasonal mean circulation in the northern South China Sea

A study of the circulation in the northern South China Sea （SCS） is carried out with the aid of a three-dimensional, high-resolution regional ocean model. One control and two sensitivity experiments are performed to qualitatively investigate the effects of surface wind forcing, Kuroshio intrusion, and bottom topographic influence on the circulation in the northern SCS. The model results show that a branch of the Kuroshio in the upper layer can intrude into the SCS and have direct influence on the circulation over the continental shelf break in the northern SCS. There are strong southward pressure gradients along a zonal belt largely seaward of the continental slope. The pressure gradients are opposite in the southern and northern parts of the Luzon Strait, indicating inflow and outflow through the strait, respectively. The sensitivity experiments suggest that the Kuroshio intrusion is responsible for generating the imposed pressure head along the shelf break and has no obvious seasonal variations. The lateral forcing through the Luzon Strait and Taiwan Strait can induce the southwestward slope current and the northeastward SCS Warm Current in the northern SCS. Without the lateral forcing, there is the continental slope. The wind forcing mainly causes the The wind-induced water pile-up results in the southward no high-pressure-gradient zonal belt seaward of seasonal variation of the circulation in the SCS. high pressure gradient along the northwestern boundary of the basin. Without the blocking of the plateau around Dongsha Islands, the intruded Kuroshio tends to extend northwest and the SCS branch of the Kuroshio becomes wider and stronger. The analyses presented here are qualitative in nature but should lead to a better understanding of the oceanic responses in the northern SCS to these external influence factors.     

Volcano-tectonic controls and emplacement kinematics of the Iriga debris avalanches (Philippines)

Mt Iriga in southeastern Luzon is known for its spectacular collapse scar that was possibly created in 1628 ad by a 1.5-km³ debris avalanche. The debris avalanche deposit (DAD) covered 70?km² and dammed the Barit River to form Lake Buhi. The collapse has been ascribed to a non-volcanic trigger related to a major strike-slip fault under the volcano. Using a combination of fieldwork and remote sensing, we have identified a similar size, older DAD to the southwest of the edifice that originated from a sector oblique to the underlying strike-slip fault. Both deposits cover wide areas of low, waterlogged plains, to a distance of about 16?km for the oldest and 12?km for the youngest. Hundreds of metre-wide and up to 50-m-high hummocks of intact conglomerate, sand and clay units derived from the base of the volcano show that the initial failure planes cut deep into the substrata. In addition, large proportions of both DAD consist of ring-plain sediments that were incorporated by soft-sediment bulking and extensive bulldozing. An ignimbrite unit incorporated into the younger DAD forms small (less than 5?m high) discrete hummocks between the larger ones. Both debris avalanches slid over water-saturated soft sediment or ignimbrite and spread out on a basal shear zone, accommodated by horst and graben formation and strike-slip faults in the main mass. The faults are listric and flatten into a well-developed basal shear zone. This shear zone contains components from the substrate and has a diffuse contact with the intact substrata. Long, transport-normal ridges in the distal parts are evidence of compression related to deceleration and bulldozing. The collapse orientation and structure on both sectors and DAD constituents are consistent with collapse being a response to combined transtensional faulting and gravity spreading. Iriga can serve as a model for other volcanoes, such as Mayon, that stand in sedimentary basins undergoing transtensional strike-slip faulting.