Effects of hypoxia on benthic macrofauna and bioturbation in the Estuary and Gulf of St. Lawrence,Canada

The bottom water in the >300 m deep Lower St. Lawrence Estuary (LSLE) is persistently hypoxic in contrast to the normoxic bottom waters in the Gulf of St. Lawrence (GSL). We photographed the seabed at 11 stations in the Estuary and Gulf of St. Lawrence (EGSL) during the summers 2006 and 2007 and analysed the images to identify bioturbation traces (lebensspuren) and benthic macrofauna. The objective was to identify the environmental variables that influence the density and diversity of benthic macrofauna and bioturbation traces, and the differences that exist among regions with high, medium and low oxygen levels in the bottom water. The bottom water oxygen concentration is the variable that best explains the densities of total-traces as well as surface-traces. However, the density of these traces was higher in hypoxic regions than in well-oxygenated regions. The higher density of traces in the hypoxic region of the LSLE is mainly due to the activities of the surface deposit feeder Ophiura sp., which occurs in large numbers in this region. Possible explanations explored are stress behaviour of the organisms in response to hypoxia and different benthic macrofauna community structures between the hypoxic regions of the LSLE and the normoxic regions of the GSL. In the former, surface deposit feeders and low-oxygen tolerant species dominate over suspension feeders and low-oxygen intolerant species.     

Modeling the timing of spring phytoplankton bloom and biological production of the Gulf of St. Lawrence (Canada): Effects of colored dissolved organic matter and temperature

The effects of colored dissolved organic matter (CDOM) from freshwater runoff and seasonal cycle of temperature on the dynamic of phytoplankton and zooplankton biomass and production in the Gulf of St. Lawrence (GSL) are studied using a 3-D coupled physical-plankton ecosystem model. Three simulations are conducted: (1) the reference simulation based on Le Fouest et al. (2005), in which light attenuation by CDOM is not considered and maximum growth rate (_μmax${\mu }_{\max }$) of phytoplankton and zooplankton are not temperature-dependent (REF simulation); (2) light attenuation by CDOM is added to REF simulation (CDOM simulation); and (3) in addition to CDOM, the _μmax${\mu }_{\max }$ of phytoplankton and zooplankton are regulated by temperature (CDOM+TEMP simulation). CDOM simulation shows that CDOM substantially reduces phytoplankton biomass and production in the Lower St. Lawrence Estuary (LSLE), but slightly reduces overall primary production in the GSL. In the LSLE, the spring phytoplankton bloom is delayed from mid-March to mid-April, resulted from light attenuation by CDOM. The CDOM+TEMP simulation shows that the spring phytoplankton bloom in the LSLE is further delayed to July, which is more consistent with observations. Annual primary production is reduced by 33% in CDOM+TEMP simulation from REF and CDOM simulations. Zooplankton production is the same in all three simulations, and export of organic matter to depth is reduced in CDOM+TEMP simulation, suggesting that temperature controlled growth of phytoplankton and zooplankton enhances the coupling between primary production and zooplankton production under the seasonal temperature cycle of the GSL.     

Rock fracture as a precursor to lava dome eruptions at Mount St Helens from June 1980 to October 1986

Following its plinian eruption on 18 May 1980, Mount St Helens (Washington State, USA) entered a period of intermittent lava-dome extrusion until 1986. Renewed extrusion was frequently preceded by accelerating rates of seismicity, with more precursory seismicity observed prior to eruptions later in the sequence. Here the failure forecasting method (FFM) is used to investigate changes in the observed rate of volcano–tectonic (VT) seismicity. The analysis indicates that: (1) all VT crises resulted in an eruption within 3 weeks (usually less than 10 days), (2) the majority of eruptions had VT precursors, and (3) patterns of precursory seismicity showed fluctuations about the ideal model trend. Thus, although these seismic events could be used to warn of an impending eruption, specific forecasts were subject to an uncertainty of weeks or more. It is proposed that: (1) increased seismicity prior to later eruptions is a result of a larger and more solidified dome acting as a greater impediment to magma ascent; (2) the consistency of seismic swarms resulting in an eruption indicates that stresses high enough to initiate fracturing in the country rock and lava dome carapace were only achieved once the approach to an eruption had already begun; and (3) discrepancies between models of accelerating rock fracture and the observed seismicity may arise due to a significant amount of the rocks deforming through ductile mechanisms rather than seismogenic fracture.     

The crater lake and hydrothermal system of Mount Pinatubo,Philippines: evolution in the decade after eruption

The June 1991 eruption of Mount Pinatubo, Philippines breached a significant, pre-eruptive magmatic-hydrothermal system consisting of a hot (>300 °C) core at two-phase conditions and surrounding, cooler (<260 °C) liquid outflows to the N and S. The eruption created a large, closed crater that accumulated hydrothermal upwellings, near-surface aquifer and meteoric inflows. A shallow lake formed by early September 1991, and showed a long-term increase in level of ~1 m/month until an artificial drainage was created in September 2001. Comparison of the temporal trends in lake chemistry to pre- and post-eruptive springs distinguishes processes important in lake evolution. The lake was initially near-neutral pH and dominated by meteoric influx and Cl–SO₄ and Cl–HCO₃ hydrothermal waters, with peaks in SO₄ and Ca concentrations resulting from leaching of anhydrite and aerosol-laden tephra. Magmatic discharge, acidity (pH~2) and rock dissolution peaked in late 1992, during and immediately after eruption of a lava dome on the crater floor. Since cessation of dome growth, trends in lake pH (increase from 3 to 5.5), temperature (decline from 40 to 26 °C), and chemical and isotopic composition indicate that magmatic degassing and rock dissolution have declined significantly relative to the input of meteoric water and immature hydrothermal brine. Higher concentrations of Cl, Na, K, Li and B, and lower concentrations of Mg, Ca, Fe, SO₄ and F up to 1999 highlight the importance of a dilute hydrothermal contribution, as do stable-isotope and tritium compositions of the various fluids. However, samples taken since that time indicate further dilution and steeper trends of increasing pH and declining temperature. Present gas and brine compositions from crater fumaroles and hot springs indicate boiling of an immature Cl–SO₄ geothermal fluid of near-neutral pH at approximately 200 °C, rather than direct discharge from magma. It appears that remnants of the pre-eruptive hydrothermal system invaded the magma conduit shortly after the end of dome emplacement, blocking the direct degassing path. This, along with the large catchment area (~5 km²) and the high precipitation rate of the area, led to a rapid transition from a small and hot acid lake to a large lake with near-ambient temperature and pH. This behavior contrasts with that of peak-activity lakes that have more sustained volcanic gas influx (e.g., Kawah Ijen, Indonesia; Poas and Rincón de la Vieja, Costa Rica).Editorial responsibility: H. Shinohara     

ASSESSMENT OF MACCORMACK SCHEME IN UNSTEADY SEDIMENT LADEN FLOW COMPUTATION

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Internal stratigraphic architecture of the komatiitic dunite-hosted MKD5 disseminated nickel sulfide deposit,Mount Keith Domain,Agnew-Wiluna Greenstone Belt,Western Australia

The MKD5 nickel deposit is hosted by the Mount Keith Ultramafic Complex (MKUC), a dunite body of komatiitic affinity located in the Agnew-Wiluna Greenstone Belt, Western Australia. The internal architecture of the MKUC comprises seven distinct internal units that range from extreme adcumulate dunite to relatively fractionated pyroxenitic and gabbroic horizons. The MKUC is divided into three packages of units. The main adcumulate domain (MAD), which is situated in the lower portion of the complex, contains the bulk of disseminated nickel sulfide and is dominated by coarse adcumulate olivine textures. Overlying the MAD is an upper fractionated zone, which is dominated by mesocumulate-to-orthocumulate peridotite with domains containing oikocrystic pyroxenite and gabbroic lenses. An aerially restricted unit comprising texturally and chemically distinct olivine cumulate rocks is known as the western mineralized zone (WMZ). The fractionation trend between the MAD and UPZ indicates a westerly facing for the MKUC, conformable with the bounding stratigraphy. In the MKUC, truncation of some of the uppermost internal stratigraphic units by the hangingwall contact indicates that the unit has undergone structural modification since its emplacement and suggests that a proportion of the upper section has been removed by faulting. Furthermore, vertical and lateral textural transitions within the internal stratigraphy suggest that the MKUC (excluding the WMZ) was emplaced from an essentially continuous magma flow, with the MAD representing the period of highest magma flux and the major constructional period of emplacement. Conversely, the WMZ is interpreted to represent a later pulse of ultramafic magma, emplaced stratigraphically above the main MKUC. Comparison of the MKUC with other komatiitic dunites from both within the Agnew-Wiluna Greenstone Belt and worldwide indicates that despite apparent geometrical differences between lens and sheet komatiitic dunites, a broad facies architecture can be defined. We suggest that the differences in geometry are related to differing degrees of flow localization within dunitic units and are a function of both differences in the thermal characteristics and the degree of litho-facies heterogeneity of the enclosing lithologies.     

Combining fractal analysis of mineral deposit clustering with weights of evidence to evaluate patterns of mineralization: Application to copper deposits of the Mount Isa Inlier, NW Queensland, Australia

The clustering of mineral occurrences and their spatial associations with particular geological features are critical aspects of mineral distributions for exploration and understanding ore genesis. Variations in the degree of clustering of mineral occurrences or geological features can be measured by fractal dimensions, obtained from a shifting box counting method. Spatial associations between mineral occurrences and geological features can be quantified by the weights of evidence (WofE) method using the contrast value, which increases with the strength of the spatial relationship. A new method is proposed to evaluate mineral occurrence distributions by combining the power of fractal analysis of clustering with the WofE approach. The method compares the correlation between the variation in degree of clustering of mineral occurrences and a geological feature in a study area, with the contrast value of the same feature. The possible outcomes can be simplified into four scenarios, depending on whether the correlation in variation of clustering and the contrast are high or low, respectively. Each outcome has specific exploration implications. If either a high correlation in variation of clustering or a high contrast value is obtained, the geological feature can be used for exploration targeting.The integrated fractal and WofE approach is applied to copper occurrences in the Proterozoic Mount Isa Inlier, NW Queensland, Australia, which hosts large numbers of copper deposits (1,869 occurrences), including the world class Mount Isa copper deposit. Variation in clustering of copper occurrences has a positive correlation with variation in clustering of fault bends (R = 0.823), fault intersections (R = 0.862) and mafic rocks (R = 0.885). WofE results indicate that the copper occurrences are spatially associated with fault intersections and bends and with mafic rocks. Analyses were carried out separately for the two major lithostratigraphic sequences in the Inlier, the Eastern and Western Successions. The Western Succession copper occurrences are apparently more clustered than those of the Eastern Succession, which may reflect a lower degree of exploration and/or geological factors. The association of copper occurrences with mafic rocks compared with fault bends and intersections is greater in the Eastern Succession, which may reflect genetic factors. Correlations in the variation of clustering of mineral occurrences and geological features have a linear relationship with the contrast values, and the spatial association between all geological features and copper occurrences constitute high correlation/high contrast cases. The linear relationship suggests that the geological features that control the clustering of the copper occurrences could be the same features that control their localization.     

Stable Sulfur Isotopic Evidence for Historical Changes of Sulfur Cycling in Estuarine Sediments from Northern Florida

Data on abundance and isotopic composition of porewater and sedimentary sulfur species are reported for relatively uncontaminated and highly contaminated fine-grained anoxic sediments of St. Andrew Bay, Florida. A strong contrast in amount and composition of sedimentary organic matter at the two sites allows a comparative study of the historical effects of increased organic loading on sulfur cycling and sulfur isotopic fractionation. In the contaminated sediments, an increase in organic loading caused increased sedimentary carbon/sulfur ratios and resulted in higher rates of bacterial sulfate reduction, but a lower efficiency of sulfide oxidation. These differences are well reflected in the isotopic composition of dissolved sulfate, sulfide, and sedimentary pyrite. Concentration and isotopic profiles of dissolved sulfate, organic carbon, and total sulfur suggest that the anaerobic decomposition of organic matter is most active in the upper 8cm but proceeds at very slow rates below this depth. The rapid formation of more than 90% of pyrite in the uppermost 2 cm which corresponds to about 3 years of sediment deposition allows the use of pyrite isotopic composition for tracing changing diagenetic conditions. Sediment profiles of the sulfur isotopic composition of pyrite reflect present-day higher rates of bacterial sulfate reduction and lower rates of sulfide oxidation, and record a profound change in the diagenetic cycling of sulfur in the contaminated sediments coincident with urban and industrial development of the St. Andrew Bay area.     

Switching bulk horizontal shortening directions and regional-scale partitioning of deformation during the Isan Orogeny in the Eastern Fold Belt,Mount Isa Inlier,Australia

Prolonged deformation for ca 150 Ma along the Eastern Fold Belt, Mount Isa Inlier, differentially partitioned into three distinct Mesoproterozoic tectonic domains. NW–SE-trending structures dominate the northern domain, whereas E–W- and N–S-trending structures dominate the central and southern domains, respectively.

Changing the direction of bulk horizontal shortening from NE–SW to N–S to E–W shifted the locus of maximum tectono-metamorphic effect. This accounts for the different generations of structures preserved in these three domains. Overprinting relationships and geochronological data reveal a component of deformation partitioning in time as well as space.

Rheological contrasts in the Soldiers Cap Group between a thick interlayered pelitic, psammitic and volcanic units on the one hand, and ca 1686 Ma, competent mafic intrusives and genetically related metasomatic albitite bodies present in its lower part, on the other, enhanced strain localisation during the long-lived Isan Orogeny (ca 1670–1500 Ma).     

浅水流动的可压缩流数学模拟

利用浅水流动的可压缩流比拟来认识浅水方程组的特性及其有关物理现象,过去的讨论多限于矩形断面棱柱形明渠的最简单情况。本文首先将该模拟推广到任意断面非棱柱形明渠的一般情况,给出了等价绝热指数公式γ=H/h.(H为水力水深,h_c为形心水深),并指出γ=2为矩形断面的特例.然后着重阐明这一数学模拟对计算水力学具有普遍实用价值,并将计算气体动力学中的两种通量向量分裂(FVS)算法通过比拟应用于浅水流动计算。最后,本文讨论了浅水流动可压缩流模拟与一般完全气体可压缩流物理特性的异同点,据此提出正确应用这一模拟所应遵循的原则。