Sulphur isotope equilibrium during sulphur hydrolysis at high temperatures

Studies of the sulphur hydrolysis reaction, 4S + 4H₂O /ag 3H₂S + HSO₄^? + H⁺, were conducted between 200 and 320°C in sealed silica glass tubes. The isotope exchange reaction: H₂¹⁸O + HS¹⁶O₄^? /ag H₂¹⁶O + HS¹⁸O₄^? is so rapid at the low pH (1.5–3) as to be unquenchable. However, the sulphur isotope exchange reaction: H₂³⁴S+ H³²SO₄^? /ag H₂³²S + H³⁴SO₄^? gave t12 values of 0.1, 0.3 and 1.7 days at 320, 260 and 200°C respectively and equilibrium H₂S - HSO₄^? sulphur isotope fractionation values of 20.9, 22.4, 24.8, 26.7 and 29.3‰ at 320, 290, 260, 230 and 200°C respectively. This latter data is represented by: 1000lnα_(HSO4^??H2S) = 5.07 (10⁶T^?2) + 6.33, and has valuable applications in geothermal and ore deposit studies.     

Between beds and sequences: stratigraphic organization at intermediate scales in the Quaternary of the Virginia coast, USA

ABSTRACT Stratigraphy presupposes a hierarchy of scales of spatial organization supplemented at the small‐scale end by sedimentological concepts (beds, bed sets and bed cosets) and, at larger spatial scales, by sequence‐stratigraphic concepts (systems tracts, parasequences, sequences). Between these two end‐members are intermediate‐scale bodies described as ‘lithofacies’, or simply ‘facies’. A more restricted concept, granulometric facies, can be described in terms of horizontal grain‐size gradients (‘facies change’) and cyclic vertical grain‐size gradients (‘stratification’). Assemblages of facies so defined (also called depositional systems) are not random, but occur in a limited suite of patterns. Such assemblages may be linked to two classes of bounding surfaces, a source diastem (the immediate source of the sediment) and a surface of closure (if preserved), between which is sandwiched a transgressive or regressive, basinward‐fining facies succession. Systems‐bounding surfaces are notably more continuous than internal (gradational) facies boundaries. By thus restricting the definition of a facies assemblage (depositional system), it is possible to describe the Quaternary of the Virginia coast with as few as 12 systems. Depositional systems in the Quaternary of the Virginia coast are allometric, in that any system can be derived from any other by plastic expansion of one or more facies relative to another, or by simple symmetry operations. Self‐similarity prevails across this intermediate scale of stratigraphic organization. Facies assemblages (depositional systems) consist of event beds, which themselves have erosional basal boundaries and internal successions of microfacies. At larger spatial scales, depositional systems are repeated, either autocyclic repetitions forced by processes within the basin of deposition or allocyclic repetitions, as ‘parasequences’ and high‐frequency sequences. In the Virginia Quaternary, systems architecture is compatible with sequence architecture and nests conformably within its framework, but analysis of systems architecture reveals rules beyond those governing sequence architecture.     

Sedimentary architecture and genesis of Holocene shallow-water mud-mounds, northern Belize

Abstract Cangrejo and Bulkhead Shoals are areally extensive, Holocene biodetrital mud‐mounds in northern Belize. They encompass areas of 20 km² and 35 km² in distal and proximal positions, respectively, on a wide and shallow‐water, microtidal carbonate shelf where storms are the major process affecting sediment dynamics. Sediments at each mound are primarily biodetrital and comprise part of a eustatically forced, dominantly subtidal cycle with a recognizable deepening‐upward transgressive systems tract, condensed section and shallowing‐upward highstand systems tract. Antecedent topographic relief on Pleistocene limestone bedrock also provided marine accommodation space for deposition of sediments that are a maximum of 7·6 m thick at Cangrejo and 4·5 m thick at Bulkhead. Despite differences in energy levels and location, facies and internal sedimentological architectures of the mud‐mounds are similar. On top of Pleistocene limestone or buried soil developed on it are mangrove peat and overlying to laterally correlative shelly gravels. Deposition of these basal transgressive, premound facies tracked the rapid rate of sea‐level rise from about 6400–6500 years BP to 4500 years BP, and the thin basal sedimentation unit of the overlying mound‐core appears to be a condensed section. Following this, the thick and complex facies mosaic comprising mound‐cores represents highstand systems tract sediments deposited in the last ≈ 4500 years during slow and decelerating sea‐level rise. Within these sections, there is an early phase of progradationally offlapping catch‐up deposition and a later (and current) phase of aggradational keep‐up deposition. The mound‐cores comprise stacked storm‐deposited autogenic sedimentation units, the upper bounding surfaces of which are mostly eroded former sediment–water interfaces below which depositional textures have largely been overprinted by biogenic processes associated with Thalassia‐colonized surfaces. Vertical stacking of these units imparts a quasi‐cyclic architecture to the section that superficially mimics metre‐scale parasequences in ancient rocks. The locations of the mud‐mounds and the tidal channels transecting them have apparently been stable over the last 50 years. Characteristics that might distinguish these mud‐mounds and those mudbanks deposited in more restricted settings such as Florida Bay are their broad areal extent, high proportion of sand‐size sediment fractions and relatively abundant biotic particles derived from adjoining open shelf areas.     

天然和人为地震活动性的对比研究

为了充分发挥现有的区域性、地方性数字地震台站在防震减灾中的作用，中国地震局地球物理研究所与美国达拉斯南卫理大学地球科学系合作开展“中美合作延庆一怀来盆地和海城地区天然和人为地震活动性的对比研究”。该合作项目以北京西北的延庆一怀来盆地与东北辽宁的海城地区作为流动地震观测试验场，计划在现有台网的基础上，在近震源与区域性的距离上布设15套由STS—2型宽频带数字地震仪组成的流动地震台网加密观测，以改善在区域性距离上台距过长、不利于分析、解释、研究所观测到地震记录的状况。2002年秋，双方在河北怀来一带首先架设了5个宽频带数字地震台并随即投入观测。到2003年3月上旬，中方又在预定地点架设了另5个宽频带数字地震台。目前，已建成的10个宽频带数字地震台仪器运行正常，运用已获得的研究区内天然和人为地震的高质量记录，合作双方按照计划开展了相关的研究工作。     

Triple-Hot-Film Anemometer Performance in Cases-99 and A Comparison With Sonic Anemometer Measurements

Two levels of triple-hot-film and sonic anemometers were deployed on a 5.5-m towerduring the Cooperative Atmospheric Surface Exchange Study (CASES-99) in October1999. Each triple-hot-film probe was collocated 50 mm from the sonic sensing path ona common boom. Various problems with using triple-hot-films in the atmosphere toresolve wind components are addressed including the derivation of a yaw angle correction using the collocated sensors. It was found that output voltage drift due to changes in environmental temperature could be monitored and corrected using an automated system. Non-unique solutions to heat transfer equations can be resolved using a collocated sonic anemometer. Multi-resolution decomposition of the hot-film data was used to estimate appropriate day and night averaging periods for turbulent flux measurements in and near the roughness sub-layer. Finally, triple-hot-film measurements of mean wind magnitude (M), turbulent kinetic energy (TKE), sensible heat flux (H), and local friction velocity (u_*) are compared to those of the collocated CSAT3 sonic anemometers. Overall, the mean wind magnitudes measured by the triple-hot-film and the collocated sonic sensorswere close, consistent and independent of stability or proximity to the ground. The turbulent statistics, TKE, u_*, and H, measured by the two sensor systems were reasonably close together at z = 5 m. However, the ratio of sonic measurement/hot-film measurement decreased toward the ground surface, especially during stable conditions.     

Lacaille 250 years on

Brian Warner , Department of Astronomy, University of Cape Town, tells the story of Nicolas Louis de Lacaille's sojourn in South Africa.
Two hundred and fifty years ago French astronomer Nicolas Louis de la Caille spent two years at the Cape of Good Hope, carrying out astronomical and geodetic observations that had long-lasting repercussions. His measurements of the positions of nearly 10 000 southern stars led to the definition of the southern constellations which are used to this day; his discrepant measurement of the radius of the Earth provided a challenge that took much 19th-century labour to rectify, but led to an accurate trigonometric survey of South Africa that would otherwise not have occurred until much later.     

Inter-mineral Fe isotope variations in mantle-derived rocks and implications for the Fe geochemical cycle

Iron isotope and major- and minor-element compositions of coexisting olivine, clinopyroxene, and orthopyroxene from eight spinel peridotite mantle xenoliths; olivine, magnetite, amphibole, and biotite from four andesitic volcanic rocks; and garnet and clinopyroxene from seven garnet peridotite and eclogites have been measured to evaluate if inter-mineral Fe isotope fractionation occurs in high-temperature igneous and metamorphic minerals and if isotopic fractionation is related to equilibrium Fe isotope partitioning or a result of open-system behavior. There is no measurable fractionation between silicate minerals and magnetite in andesitic volcanic rocks, nor between olivine and orthopyroxene in spinel peridotite mantle xenoliths. There are some inter-mineral differences (up to 0.2‰ in ⁵⁶Fe/⁵⁴Fe) in the Fe isotope composition of coexisting olivine and clinopyroxene in spinel peridotites. The Fe isotope fractionation observed between clinopyroxene and olivine appears to be a result of open-system behavior based on a positive correlation between the Δ⁵⁶Fe_{clinopyroxene-olivine} fractionation and the δ⁵⁶Fe value of clinopyroxene and olivine. There is also a significant difference in the isotopic compositions of garnet and clinopyroxene in garnet peridotites and eclogites, where the average Δ⁵⁶Fe_{clinopyroxene-garnet} fractionation is +0.32 ± 0.07‰ for six of the seven samples. The one sample that has a lower Δ⁵⁶Fe_{clinopyroxene-garnet} fractionation of 0.08‰ has a low Ca content in garnet, which may reflect some crystal chemical control on Fe isotope fractionation. The Fe isotope variability in mantle-derived minerals is interpreted to reflect subduction of isotopically variable oceanic crust, followed by transport through metasomatic fluids. Isotopic variability in the mantle might also occur during crystal fractionation of basaltic magmas within the mantle if garnet is a liquidus phase. The isotopic variations in the mantle are apparently homogenized during melting processes, producing homogenous Fe isotope compositions during crust formation.     

Experimental constraints on Fe isotope fractionation during magnetite and Fe carbonate formation coupled to dissimilatory hydrous ferric oxide reduction

Iron isotope fractionation between aqueous Fe(II) and biogenic magnetite and Fe carbonates produced during reduction of hydrous ferric oxide (HFO) by Shewanella putrefaciens, Shewanella algae, and Geobacter sulfurreducens in laboratory experiments is a function of Fe(III) reduction rates and pathways by which biogenic minerals are formed. High Fe(III) reduction rates produced ⁵⁶Fe/⁵⁴Fe ratios for Fe(II)_aq that are 2-3‰ lower than the HFO substrate, reflecting a kinetic isotope fractionation that was associated with rapid sorption of Fe(II) to HFO. In long-term experiments at low Fe(III) reduction rates, the Fe(II)_aq-magnetite fractionation is −1.3‰, and this is interpreted to be the equilibrium fractionation factor at 22°C in the biologic reduction systems studied here. In experiments where Fe carbonate was the major ferrous product of HFO reduction, the estimated equilibrium Fe(II)_aq-Fe carbonate fractionations were ca. 0.0‰ for siderite (FeCO₃) and ca. +0.9‰ for Ca-substituted siderite (Ca_0.15Fe_0.85CO₃) at 22°C. Formation of precursor phases such as amorphous nonmagnetic, noncarbonate Fe(II) solids are important in the pathways to formation of biogenic magnetite or siderite, particularly at high Fe(III) reduction rates, and these solids may have ⁵⁶Fe/⁵⁴Fe ratios that are up to 1‰ lower than Fe(II)_aq. Under low Fe(III) reduction rates, where equilibrium is likely to be attained, it appears that both sorbed Fe(II) and amorphous Fe(II)(s) components have isotopic compositions that are similar to those of Fe(II)_aq.The relative order of δ⁵⁶Fe values for these biogenic minerals and aqueous Fe(II) is: magnetite > siderite ≈ Fe(II)_aq > Ca-bearing Fe carbonate, and this is similar to that observed for minerals from natural samples such as Banded Iron Formations (BIFs). Where magnetite from BIFs has δ⁵⁶Fe >0‰, the calculated δ⁵⁶Fe value for aqueous Fe(II) suggests a source from midocean ridge (MOR) hydrothermal fluids. In contrast, magnetite from BIFs that has δ⁵⁶Fe ≤0‰ apparently requires formation from aqueous Fe(II) that had very low δ⁵⁶Fe values. Based on this experimental study, formation of low-δ⁵⁶Fe Fe(II)_aq in nonsulfidic systems seems most likely to have been produced by dissimilatory reduction of ferric oxides by Fe(III)-reducing bacteria.