Quartz Cement in Middle Jurassic Reservoir Sandstones in North Sea A Review. Part II： Duration and Silica Sources

The timing and duration of quartz cementation in sandstones have been mainly inferred from diagenetic texture, relationship between pore filling minerals, fluid inclusions and isotopic data. Fluid inclusion temperatures from North Sea reservoir sandstones indicate that most of the quartz cement forms at temperature exceeding 90℃ and is continually proceeding after oil emplacement, based on the fluid inclusion temperatures in quartz overgrowth which is approaching the bottom-hole temperatures. The duration of quartz cement after oil emplacement depends upon the saturation of porewater and the distribution of pore water film and the property of water-wet or oil-wet of the reactants. The leaching of K-feldspar by meteoric water requires pore water flow to move the released potassium and sodium and silica out the solution, which suggests the mechanism does not appear to be a major source of silica for quartz cementation. The quartz cementation coincidence with the compaction and pressure solution suggests the major source of silica. The alteration of feldspar by illitization of kaolinite may serve as another important source of silica at deep burial depth. External sources are not need to call on for illustrating the quartz cementation, because there is no evidences for large scale convection of pore water flow occurred in the burial history of reservoir sandstones of middle Jurassic in the North Sea.     

Characteristics of Danish estuaries

We review various aspects of the structure and functioning of Danish estuaries from data collected by the National Monitoring Program and from information in published sources. We present data on the physical, chemical, and biological characteristics of estuaries in Denmark, we evaluate the functioning of these systems as filters and transformers of nutrients and we evaluate the outlook for Danish estuaries in the future. Danish estuarine systems are for the most part shallow (<3 m deep), have short residence times, and tend to be heavily loaded with nutrients primarily from agricultural sources. Total average loads from land per unit watershed area are 112 kg P km^?2 yr^?1 and 2,400 kg N km^?2 yr^?1 during the period 1989–1995. The total phosphorus (TP) load in estuaries has been significantly reduced over the last decade, following implementation of the 1987 Action Plan for the Aquatic Environment (Vandmiljøplan in Danish) that prescribed that nitrogen loads to the total aquatic environment should be reduced by 50% and phosphorus loads by 80%. Reductions in the total nitrogen (TN) load have been more modest. Nutrient loading is one of the primary determinants of estuarine nutrient concentration with 70% of the annual variation in TN concentration and 55% of the annual variation in TP concentration explained by variation in the load. Many Danish estuaries have rich communities of macrophytes and benthic filter feeders, such asMytlis edulis andCiona intestinalis, that can control water column chlorophyll concentrations by their filter feeding activities. Many of the estuaries experience hypoxia and anoxia, especially during warm and calm summer months. Further reductions in nutrient loading are expected following implementation of the Action Plan for the Aquatic Environment II, with predicted improvements in oxygen concentrations and in the functioning of these shallow, dynamic estuarine systems.     

Sensitivity Kernels for Time-Distance Inversion

Inversion of local-area helioseismic time-distance data has so far only been done in the ray approximation (Kosovichev, 1996). Since this is a high-frequency approximation its applicability can be questioned for the solar case. Bogdan (1997) showed that for a simple solar model the localized wave packets do follow, but are not confined to, the ray path. We use an approximation based on the first Fresnel zone that has been developed in geophysics by Snieder and Lomax (1996) to go beyond the ray approximation in the inversions. We have calculated sensitivity kernels using both approximations. To test them we use a finite-difference forward modeling of the whole wave field in an acoustic medium reminiscent of the Sun. We use the finite-difference modeling to calculate sensitivity kernels for the full wave field and compare this with the other kernels. The results show that the Fresnel-zone-based kernels are in good agreement with the sensitivity obtained from the modeling. Thus these new kernels represent a significant step forward in the inversion of time-distance data.     

Raised beach deposits and new 14C ages from Picepynten, Prins Karls Forland, Western Svalbard

This note reports the occurrence of two generations of 14C dated raised beach deposits at Pricepynten. southern Prins Karls Forland, western Svalbard, dated to < 12 Kya and >30 Kya. In addition two new 14C ages of ca. 9.2 and 1.8 Ky are presented from Myrilus eclulis and a buried peat deposit, respectively.     

Facies distribution and dissolution depths of surface sediment components from the Vema channel and the Rio Grande rise (southwest Atlantic Ocean)

Surface sediments from the Vema channel and from the Rio Grande rise (western extremity) contain less calcareous fossils with increasing water depth. The pteropodal (aragonite) lysocline corresponds to the 3200-m isobath. The pteropodal compensation depth is found above the lower boundary of the North Atlantic Deep Water: 3500 m. The planktonic foraminiferal lysocline (4050 m) seems to be very close to the abyssal thermocline and is therefore here the upper limit of the Antarctic Bottom Water. The foraminiferal and the coccolith compensation depths seem to coincide: 4500 m. Distribution and dissolution of calcareous sediment components are the same all around the western extremity of the Rio Grande rise (southern, western and northern flanks).     

Lu–Hf and U–Pb isotope systematics of zircons from the Storgangen intrusion, Rogaland Intrusive Complex, SW Norway: implications for the composition and evolution of Precambrian lower crust in the Baltic Shield

The Storgangen orebody is a concordantly layered, sill-like body of ilmenite-rich norite, intruding anorthosites of the Rogaland Intrusive Complex (RIC), SW Norway. 17 zircon grains were separated from ca. 5 kg of sand-size flotation waste collected from the on-site repository from ilmenite mining. These zircons were analysed for major and trace elements by electron microprobe, and for U–Pb and Lu–Hf isotopes by laser ablation microprobe plasma source mass spectrometry. Eight of the zircons define a well-constrained (MSWD=0.37) concordant population with an age of 949±7 Ma, which is significantly older than the 920–930 Ma ages previously reported for zircon inclusions in orthopyroxene megacrysts from the RIC. The remaining zircons, interpreted as inherited grains, show a range of ²⁰⁷Pb/²⁰⁶Pb ages up to 1407±14 Ma, with an upper intercept age at ca. 1520 Ma. The concordant zircons have similar trace element patterns, and a mean initial Hf isotope composition of ¹⁷⁶Hf/¹⁷⁷Hf_{949 Ma}=0.28223±5 (_Hf=+2±2). This is similar to the Hf-isotope composition of zircons in a range of post-tectonic Sveconorwegian granites from South Norway, and slightly more radiogenic than expected for mid-Proterozoic juvenile crust. The older, inherited zircons show Lu–Hf crustal residence ages in the range 1.85–2.04 Ga. One (undated) zircon plots well within the field of Hf isotope evolution of Paleoproterozoic rocks of the Baltic Shield. These findings indicate the presence of Paleoproterozoic components in the deep crust of the Rogaland area, but do not demonstrate that such rocks, or a Sveconorwegian mantle-derived component, contributed significantly to the petrogenesis of the RIC. If the parent magma was derived from a homogeneous, lower crustal mafic granulite source, the lower crustal protolith must be at least 1.5 Ga old, and it must have an elevated Rb/Sr ratio. This component would be indistinguishable in Sr, Nd and Hf isotopes from some intermediate mixtures between Sveconorwegian mantle and Paleoprotoerzoic felsic crust, but it cannot account for the initial ¹⁴³Nd/¹⁴⁴Nd of the most primitive, late Sveconorwegian granite in the region, without the addition of mantle-derived material.     

Comparison of isotope pairing and N2:Ar methods for measuring sediment denitrification—Assumption, modifications, and implications

Denitrification has been measured during the last few years using two different methods in particular: isotope pairing measured on a triple-collector isotopic ratio mass spectrometer and N₂:Ar ratios measured on a membrane inlet mass spectrometer (MIMS). This study compares these two techniques in short-term batch experiments. Rates obtained using the original N₂∶Ar method were up to 3 to 4 times higher than rates obtained using the isotope pairing technique due to O₂ reacting with the N₂ during MIMS analysis. Oxygen combines with N₂ within the mass spectrometer ion source forming NO⁺ which reduces the N₂ concentration. The decrease in N₂ is least at lower O₂ concentrations and since oxygen is typically consumed during incubations of sediment cores, the result is often a pseudo-increase in N₂ concentration being interpreted as denitrification activity. The magnitude of this ocygen effect may be instrument specific. The reaction of O₂ with N₂ and the subsequent decrease in N₂ was only partly correctly using an O₂ correction curve for the relationship between N₂ and O₂ concentrations. The O₂ corrected N₂∶Ar denitrification rates were lower, but still did not match the isotope pairing rates and the variability between replicates was much higher. Using a copper reduction column heated to 600°C to remove all of the O₂ from the sample before MIMS analysis resulted in comparable rates (slightly lower), and comparable variability between replicates, to the isotope pairing technique. The N₂:Ar technique determines the net N₂ production as the difference between N₂ production by denitrification and N₂ consumption by N-fixation, while N-fixation has little effect on the isotope pairing technique which determines a rate very close to the gross N₂ production. When the two different techniques were applied on the same sediment, the small difference in rates obtained by the two methods seemed to reflect N-fixation as also supported from measurements of ethylene production in acetylene enriched sediment cores. The N₂:Ar and isotope pairing techniques may be combined to provide simultaneous measurements of denitrification and N-fixation. Both techniques have several assumptions that must be met to achieve accurate rates; a number of tests are outlined that can be applied to demonstrate that these assumptions are being meet.     

Hydrothermal alteration and zeolitization of the Fohberg phonolite,Kaiserstuhl Volcanic Complex,Germany

The subvolcanic Fohberg phonolite (Kaiserstuhl Volcanic Complex, Germany) is an economic zeolite deposit, formed by hydrothermal alteration of primary magmatic minerals. It is mined due to the high (>40 wt%) zeolite content, which accounts for the remarkable zeolitic physicochemical properties of the ground rock. New mineralogical and geochemical studies are carried out (a) to evaluate the manifestation of hydrothermal alteration, and (b) to constrain the physical and chemical properties of the fluids, which promoted hydrothermal replacement. The alkaline intrusion is characterized by the primary mineralogy: feldspathoid minerals, K-feldspar, aegirine–augite, wollastonite, and andradite. The rare-earth elements-phase götzenite is formed during the late-stage magmatic crystallization. Fluid-induced re-equilibration of feldspathoid minerals and wollastonite caused breakdown to a set of secondary phases. Feldspathoid minerals are totally replaced by various zeolite species, calcite, and barite. Wollastonite breakdown results in the formation of various zeolites, calcite, pectolite, sepiolite, and quartz. Zeolites are formed during subsolidus hydrothermal alteration (<150 °C) under alkaline conditions. A sequence of Ca–Na-dominated zeolite species (gonnardite, thomsonite, mesolite) is followed by natrolite. The sequence reflects an increase in \(\log [(a_{{{\text{Na}}^{ + } }} )/(a_{{{\text{H}}^{ + } }} )]\) and decrease in \(\log [(a_{{{\text{Ca}}^{2 + } }} )/(a_{{{\text{H}}^{ + } }}^{2} )]\) of the precipitating fluid. Low radiogenic ⁸⁷Sr/⁸⁶Sr values indicate a local origin of the elements necessary for secondary mineral formation from primary igneous phases. In addition, fractures cut the intrusive body, which contain zeolites, followed by calcite and a variety of other silicates, carbonates, and sulfates as younger generations. Stable isotope analysis of late-fracture calcite indicates very late circulation of meteoric fluids and mobilization of organic matter from surrounding sedimentary units.     

Using sewage sludge as a sealing layer to remediate sulphidic mine tailings: a pilot-scale experiment, northern Sweden

At the Kristineberg mine, northern Sweden, sulphidic mine tailings were remediated in an 8-year pilot-scale experiment using sewage sludge to evaluate its applicability as a sealing layer in a composite dry cover. Sediment, leachate water, and pore gas geochemistry were collected in the aim of determining if the sludge was an effective barrier material to mitigate acid rock drainage (ARD) formation. The sludge was an effective barrier to oxygen influx as it formed both a physical obstruction and functioned as an organic reactive barrier to prevent oxygen to the underlying tailings. Sulphide oxidation and consequential ARD formation did not occur. Sludge-borne trace elements accumulated in a reductive, alkaline environment in the underlying tailings, resulting in an effluent drainage geochemistry of Cd, Cu, Pb and Zn below 10 μg/L, high alkalinity (810 mg/L) and low sulphate (38 mg/L). In contrast, the uncovered reference tailings received a 0.35-m deep oxidation front and typical ARD, with dissolved concentrations of Cd, Zn and sulphate, 20.8 μg/L, 16,100 μg/L and 1,390 mg/L, respectively. Organic matter degradation in the sludge may be a limiting factor to the function of the sealing layer over time as 85 % loss of the organic fraction occurred over the 8-year experimental period due to aerobic and anaerobic degradation. Though the cover may function in the short to medium term (100 years), it is unlikely to meet the demands of a long-term remedial solution.     

Oxygen and nutrient dynamics within mats of the filamentous macroalga Chaetomorpha linum

Concentration profiles of O₂, NH₄ ⁺, NO₃ ⁻, and PO₄ ³⁻ were measured at high spatial resolution in a 12-cm thick benthic mat of the filamentous macroalga Chaetomorpha linum. Oxygen and nutrient concentration profiles varied depending on algal activity and water turbulence. High surface irradiance stimulated O₂ production in the surface layers and introduced O₂ to deeper parts of the mat while the bottom layers of the mat and the underlying sediment were anoxic. Nutrient concentrations were highest in the bottom layers of the mat directly above the sediment nutrient source and decreased towards the surface layers due to algal assimilation and enhanced mixing with the overlying water column. Increased turbulence during windy periods resulted in more homogeneous oxygen and nutrient concentration profiles and shifted the oxic-anoxic interface downward. Denitrification within the mat, as measured by the isotope pairing technique on addition of ¹⁵NO₃ ⁻, was found to take place directly below the oxic-anoxic interface. Denitrification activity was always due to coupled nitrification-denitrification, whereby nitrifiers in the mat utilize NH₄ ⁺ diffusing from below and O₂ diffusing from above. The denitrification rate in the mat ranged from 22 μmol m⁻² h⁻¹ to 28 μmol m⁻² h⁻¹, approximately equivalent to that measured in the surrounding nonvegetated sediment. Although sediment denitrification is suppressed when the sediment surface is covered by a dense macroalgal mat, the denitrification zone may migrate up into the mat. In eutrophic estuaries with a large area of macroalgal cover, the physical structure and growth stage of algal mats may thus play an important role in the regulation of nitrogen removal by denitrification.