Hydrography and phytoplankton of waters off south-west Ireland

Distributions of physical, chemical and biological characteristics were recorded for coastal waters off south-west Ireland during the summers of 1985–1987. A number of thermal fronts were identified from both shipboard measurements and satellite imagery. Upwelling was shown to be an important process in the area and enhanced phytoplankton biomass and productivity were associated with areas of upwelling and the fronts. Phytoplankton species composition, although varying between cruises, was associated with differences in hydrographic features.     

Experimental analysis of arsenic precipitation during microbial sulfate and iron reduction in model aquifer sediment reactors

Microbial SO₄²⁻ reduction limits accumulation of aqueous As in reducing aquifers where the sulfide that is produced forms minerals that sequester As. We examined the potential for As partitioning into As- and Fe-sulfide minerals in anaerobic, semi-continuous flow bioreactors inoculated with 0.5% (g mL⁻¹) fine-grained alluvial aquifer sediment. A fluid residence time of three weeks was maintained over a ca. 300-d incubation period by replacing one-third of the aqueous phase volume of the reactors with fresh medium every seven days. The medium had a composition comparable to natural As-contaminated groundwater with slightly basic pH (7.3) and 7.5 μM aqueous As(V) and also contained 0.8 mM acetate to stimulate microbial activity. Medium was delivered to a reactor system with and without 10 mmol L⁻¹ synthetic goethite (α-FeOOH). In both reactors, influent As(V) was almost completely reduced to As(III). Pure As-sulfide minerals did not form in the Fe-limited reactor. Realgar (As₄S₄) and As₂S₃(am) were undersaturated throughout the experiment. Orpiment (As₂S₃) was saturated while sulfide content was low (∼50 to 150 μM), but precipitation was likely limited by slow kinetics. Reaction-path modeling suggests that, even if these minerals had formed, the dissolved As content of the reactor would have remained at hazardous levels. Mackinawite (Fe_1 + xS; x ? 0.07) formed readily in the Fe-bearing reactor and held dissolved sulfide at levels below saturation for orpiment and realgar. The mackinawite sequestered little As (<0.1 wt.%), however, and aqueous As accumulated to levels above the influent concentration as microbial Fe(III) reduction consumed goethite and mobilized adsorbed As. A relatively small amount of pyrite (FeS₂) and greigite (Fe₃S₄) formed in the Fe-bearing reactor when we injected a polysulfide solution (Na₂S₄) to a final concentration of 0.5 mM after 216, 230, 279, and 286 days. The pyrite, and to a lesser extent the greigite, that formed did sequester As from solution, containing 0.84 and 0.23 wt.% As on average, respectively. Our results suggest that As precipitation during Fe-sulfide formation in nature occurs mainly in conjunction with pyrite formation. Our findings imply that the effectiveness of stimulating microbial SO₄²⁻ reduction to remediate As contamination may be limited by the rate and extent of pyrite formation and the solubility of As-sulfides.     

The Nippon/Norway Svalbard Meteor Radar: First results of small-scale structure observations

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Oxidation and metal-insertion in molybdenite surfaces: evaluation of charge-transfer mechanisms and dynamics

Molybdenum disulfide (MoS₂), a layered transition-metal dichalcogenide, has been of special importance to the research community of geochemistry, materials and environmental chemistry, and geotechnical engineering. Understanding the oxidation behavior and charge-transfer mechanisms in MoS₂ is important to gain better insight into the degradation of this mineral in the environment. In addition, understanding the insertion of metals into molybdenite and evaluation of charge-transfer mechanism and dynamics is important to utilize these minerals in technological applications. Furthermore, a detailed investigation of thermal oxidation behavior and metal-insertion will provide a basis to further explore and model the mechanism of adsorption of metal ions onto geomedia.     

白云鄂博碳酸岩的方解石-白云石地质温度计

利用方解石-白云石地质温度计对白云鄂博地区碳酸岩的平衡温度进行了测定。出露于东矿下盘的白云岩质火山岩和出露于尖山的方解石-白云石型火山岩获得了较高的温度,分别为681℃和648℃。这些样品中的方解石呈二十微米左右晶形较完整的小片,被稍大粒度的白云石颗粒包裹,未受交代作用影响,推测这种碳酸岩在快速冷却的情况下保存下了其岩浆侵位时的成分特点,从而指示出接近碳酸岩浆侵位时的温度。但本区多数碳酸岩的平衡温度在400～500℃之间,有下列三种情况:(1)具有自形-半自形中粗粒粒状变晶结构的碳酸岩最后的平衡温度为415～496℃;(2)产自东矿的其余样品(火山岩),所测最后平衡温度为431～485℃,在测温的微区范围内可见极细粒白云石方解石与稀土等矿物共生的现象;(3)为交代重结晶结构的碳酸岩明显受到后期热液流体的交代,在流体的作用下共生方解石和白云石在成分上达到新的平衡,平衡温度为432～507℃。本文所分析的样品多数结果(371～507℃)与用白云石(方解石)和磁铁矿氧同位素温度计对白云鄂博碳酸岩的计算结果(360～546℃)十分一致。虽然有研究者对方解石-白云石温度计用于火成碳酸岩表示过质疑,但本文资料表明火成碳酸岩最后的平衡温度是可以运用方解石-白云石温度计法来计算的。     

Stable Fe isotope fractionations produced by aqueous Fe(II)-hematite surface interactions

Stable Fe isotope fractionations were investigated during exposure of hematite to aqueous Fe(II) under conditions of variable Fe(II)/hematite ratios, the presence/absence of dissolved Si, and neutral versus alkaline pH. When Fe(II) undergoes electron transfer to hematite, Fe(II) is initially oxidized to Fe(III), and structural Fe(III) on the hematite surface is reduced to Fe(II). During this redox reaction, the newly formed reactive Fe(III) layer becomes enriched in heavy Fe isotopes and light Fe isotopes partition into aqueous and sorbed Fe(II). Our results indicate that in most cases the reactive Fe(III) that undergoes isotopic exchange accounts for less than one octahedral layer on the hematite surface. With higher Fe(II)/hematite molar ratios, and the presence of dissolved Si at alkaline pH, stable Fe isotope fractionations move away from those expected for equilibrium between aqueous Fe(II) and hematite, towards those expected for aqueous Fe(II) and goethite. These results point to formation of new phases on the hematite surface as a result of distortion of Fe-O bonds and Si polymerization at high pH. Our findings demonstrate how stable Fe isotope fractionations can be used to investigate changes in surface Fe phases during exposure of Fe(III) oxides to aqueous Fe(II) under different environmental conditions. These results confirm the coupled electron and atom exchange mechanism proposed to explain Fe isotope fractionation during dissimilatory iron reduction (DIR). Although abiologic Fe(II)_aq - oxide interaction will produce low δ⁵⁶Fe values for Fe(II)_aq, similar to that produced by Fe(II) oxidation, only small quantities of low-δ⁵⁶Fe Fe(II)_aq are formed by these processes. In contrast, DIR, which continually exposes new surface Fe(III) atoms during reduction, as well as production of Fe(II), remains the most efficient mechanism for generating large quantities of low-δ⁵⁶Fe aqueous Fe(II) in many natural systems.     

安徽巢湖凤凰山剖面石炭系微相和层序地层特征

应用露头层序地层学基本原理和方法 ,对巢湖凤凰山石炭系剖面进行重新研究。巢湖凤凰山石炭系总厚度为79.0 2 m ,在这个剖面中可以识别出 6种主要岩相、18种微相和 11个三级沉积层序 ,其中六个三级沉积层序属于 I型沉积层序 ,三个属于 II型沉积层序 ,缺失第 9和第 10沉积层序。三级沉积层序在不同的古地理单元和不同的沉积环境中可以进行对比。根据各微相特征和成因 ,揭示了沉积间断面上存在的多期古岩溶现象。阐述了三级层序及其对应的三级海平面升降旋回的特征     

Petrology and geochemistry of the Huerto Andesite,San Juan volcanic field,Colorado

The Huerto Andesite is the largest of several andesite sequences interlayered with the large-volume ash-flow tuffs of the San Juan volcanic field, Colorado. Stratigraphically this andesite is between the region's largest tuff (the 27.8 Ma, 3,000 km³ Fish Canyon Tuff) and the evolved product of the Fish Canyon Tuff (the 27.4 Ma, 1,000 km³ Carpenter Ridge Tuff), and eruption was from vents located approximately 20–30 km southwest and southeast of calderas associated with these ashflow tuffs. Olivine phenocrysts are present in the more mafic, SiO₂-poor samples of andesite, hence the parent magma was most likely a mantle-derived basaltic magma. The bulk compositions of the olivine-bearing andesites compared to those containing orthopyroxene phenocrysts suggest the phenocryst assemblage equilibrated at 2–5 kbar. Two-pyroxene geothermometry yields equilibrium temperatures consistent with near-peritectic magmas at 2–5 kbar. Fractionation of phenocryst phases (olivine or orthopyroxene + clinopyroxene + plagioclase + Ti-magnetite + apatite) can explain most major and trace element variations of the andesites, although assimilation of some crustal material may explain abundances of some highly incompatible trace elements (Rb, Ba, Nb, Ta, Zr, Hf) in the most evolved lavas. Despite the great distance of the San Juan volcanic field from the inferred Oligocene destructive margin, the Huerto Andesite is similar to typical plate-margin andesites: both have relatively low abundances of Nb and Ta and similar values for trace-element ratios such as La/Yb and La/Nb.Deriving the Fish Canyon and Carpenter Ridge Tuffs by crystal fractionation from the Huerto Andesite cannot be dismissed by major-element models, although limited trace-element data indicate the tuffs may not have been derived by such direct evolution. Alternatively, heat of crystallization released as basaltic magmas evolved to andesitic compositions may have caused melting of crust to produce the felsic-ash flows. Mafic magmas may have been gravitationally trapped below lighter felsic magmas; mafic magmas which ascended to the surface probably migrated upwards around the margins of silicic chambers, as suggested by the present-day outcrops of andesitic units around the margins of recognized ash-flow calderas.     

The 1980/1981 phytoplankton cycle in the coastal waters off Connemara,Ireland

The results of a phytoplankton survey conducted in coastal waters off western Ireland in 1980/1981 are reported. Surface values of temperature, salinity, NO₃N, PO₄P, Si, total N, total P and chlorophyll a (Chla) collected at 6 stations during 14 cruises are presented along with the species composition of the net phytoplankton.The spring bloom occurred in late April and was dominated by diatoms. Between April and July further diatom blooms occurred. In July and August dinoflagellates were dominant. This change was associated with the stratification of offshore water and low concentrations of Chla and nutrients. In Autumn large blooms of Ceratium tripos were found; it is suggested that the cause was heavy freshwater runoff. In autumn and winter some warm water oceanic species (e.g. Oxytoxum scolopax) occurred.