Numerical forward modelling of peritidal carbonate parasequence development: implications for outcrop interpretation

The mechanisms responsible for formation of peritidal parasequences have been a focus of debate between proponents of contrasting autocyclic and allocyclic models. To contribute to this debate a three‐dimensional numerical forward model of carbonate production, transport and deposition has been developed. Shallowing‐upward parasequences are produced in the model via carbonate island formation and progradation, with an element of self‐organization, and no external forcing. These autocyclic parasequences have characteristics comparable with peritidal parasequences observed in outcrop. Modelled parasequence thickness and duration depend primarily on subsidence rate and sediment transport rate, illustrating the significance of sediment flux in formation of peritidal parasequences. Adding an element of stochastic variation of sediment transport rate and transport path leads to formation of nonuniform‐thickness parasequences that generate Fischer plots showing apparent hierarchies similar to those often interpreted as evidence of eustatic forcing. The model results do not rule out allocylic mechanisms, but suggest that shoreline and island progradation are also plausible mechanisms to create variable‐thickness, shallowing‐upward peritidal parasequences and should be considered in interpretations of such strata.     

Rates of rock surface lowering,Princess Elizabeth land,Eastern Antarctica

A series of micro-erosion-meter sites on different rock types and in differing wind regimes was established and re-read after four years on two sites in the Larsemann and Vestfold Hills. These two oases in Eastern Antarctica are subjected to both wind abrasion and salt wedging. The measurements displayed bimodal distributions, indicating that both abrasion and single-grain detachment could be observed. Surface lowering rates of 0·015 and 0·022 mm a^?1 were demonstrated for the Larsemann and Vestfold Hills, respectively.     

Noble gas and halogen evidence for the origin of Scandinavian sandstone-hosted Pb-Zn deposits

Fluid origins in the sandstone-hosted Pb-Zn class of ore deposit have been investigated in three deposits from Scandinavia; Laisvall, Vassbo and Osen. The deposits studied are hosted by autochthonous Cambrian sandstones that preserve a near original structural relationship to the underlying Precambrian basement, enabling the role of basement interaction to be assessed.Mineral samples have been collected from across the paragenetic sequence: sphalerite, galena, pyrite, fluorite and barite, of impregnation and related joint-hosted mineralization. Fluid-inclusion halogen (Cl, Br and I) and noble gas isotope (⁴⁰Ar, ³⁶Ar, ⁸⁴Kr) compositions were determined simultaneously by noble gas mass spectrometry of irradiated sample splits. Complementary He isotope analyses are obtained from nonirradiated splits of the same samples.³He/⁴He values at Laisvall and Osen are highly radiogenic, 0.02 Ra, and the ⁴He/⁴⁰Ar* ratio extends to values greater than the crustal production value of 5, characteristic of low-temperature crustal fluids. At Vassbo, a slightly elevated ³He/⁴He ratio of 0.1-0.3 Ra is compatible with a very minor mantle component (1%-4%) suggesting a distal source for the basinal brine-dominated fluid.Br/Cl molar ratios 3.2-8.2 × 10⁻³ are greater than the present seawater value of 1.54 × 10⁻³ and correspond with I/Cl molar ratios in the range 64-1600 × 10⁻⁶. The upper limits of both the I/Cl and Br/Cl values are amongst the highest measured in crustal fluids. Together, the data indicate acquisition of salinity by the evaporation of seawater beyond the point of halite saturation and subsequent fluid interaction with I-rich organic matter in the subsurface. The data are compatible with the independent transport of sulfate and sulfide and indicate that fluids responsible for joint-hosted mineralization were distinct to those responsible for impregnation mineralization.All three deposits preserve fluids with ⁴⁰Ar/³⁶Ar in the range of 6,000-10,000 and fluid inclusion ⁴⁰Ar* concentrations of >0.02-0.05 cm³cm⁻³. Fluid-inclusion ⁴He concentrations are also extremely elevated with maximum values of ∼0.1 cm³cm⁻³ in Laisvall fluorite and sphalerite. The high ⁴⁰Ar/³⁶Ar values, together with the high ⁴He and ⁴⁰Ar* concentrations, result from a very long premineralization crustal residence time on the order of 100-200 Ma.Together, the noble gas and halogen data are compatible with a Caledonian mineralization event (∼425 Ma) caused by mixing of two or more, long-lived, hydrothermal basinal brines and pore fluids at the sites of mineralization. The data suggest negligible recharge of the basinal brines by meteoric water and indicate extensive fluid-basement interaction before mineralization. The similar noble gas composition of each deposit, suggests that similar processes operated at all three deposits and favors a single-pass fluid-flow model for mineralization.     

石羊河下游民勤绿洲变化的人文机制研究

本文根据社会调查资料分析了甘肃省河西地区石羊河下游民勤绿洲变化的趋势及其影响 ,进而从用水行为和水量均衡角度讨论了绿洲变化的人文作用机制 ,认为石羊河流域下游绿洲变化与传统农业外延式扩张机制密切相关 ,而传统农业扩张机制由于各种因素的制约在短时期内难以打破。而不打破导致绿洲环境恶化的人文机制 ,任何试图扭转此类地区绿洲环境恶化趋势的措施都可能是无效的。     

Icehouse world sea-level behaviour and resulting stratal patterns in late Visean (Mississippian) carbonate platforms: integration of numerical forward modelling and outcrop studies

Late Visean (Asbian–Brigantian) platform carbonates in the British Isles show a pronounced cyclicity marked by the alternation of mainly subtidal carbonates and subaerial exposure surfaces. Whereas the cyclicity of shallow‐water limestones of this age has been well‐documented, there has been little attempt to understand the controls on larger‐scale patterns such as those recognized in Pennsylvanian successions in the U.S.A. The principal aim of this study is to test two contrasting theories of cycle stacking via numerical forward modelling. Earlier studies of Pennsylvanian‐early Permian platform carbonates in south‐west U.S.A. suggest that cycle stacking patterns were controlled by the interaction of third‐ and fourth‐order sea‐level oscillations, with relatively uniform fourth‐order oscillations altered mainly by the harmonic effect of lower‐order sea‐level changes. An alternative model is based on an insolation curve for the Carboniferous calculated using Milankovitch parameters. This model predicts a considerable variability in levels of solar insolation that would have affected the amplitude of fourth‐order sea‐level changes and cycle composition. Both of these ideas were examined via numerous model runs using CARBONATE 6.0 and a new program, CARBOSMUT. Model results were evaluated through the use of 3 key criteria derived from well‐documented outcropping stratigraphies in the U.K.: (1) cycle stacking patterns and the stratigraphic position of major transgressions, (2) stratigraphic position of major faunal changes, (3) degree of development of subaerial exposure surfaces. Computer simulations and comparison with outcrop data suggest that a model invoking the interaction of relatively uniform fourth‐ (c.100 Kyr) and third‐order sea‐level oscillations is most appropriate for much of the late Visean, with major lowstands occurring at the mid‐Asbian and Asbian–Brigantian boundary. Late Visean cycles are important exploration targets in the Pri‐Caspian Basin, Kazakhstan and understanding the controls on stratal patterns is important as a potential exploration tool.     

Field studies of velocity,salinity and suspended solids concentration in a shallow tidal channel near tidal flap gates

The design and operation of mathematical models of solute mixing and sediment transport in estuaries rely heavily on the provision of good-quality field data. We present some observations of salinity, suspended sediment concentration and velocity at one of the tidal limits of a semi-enclosed tidal lagoon in Southern England (Pagham Harbour, West Sussex, UK) where the natural processes of tidal incursion and solute mixing have been heavily modified as a result of the construction of sea walls dating back to the 18th Century. These observations, made immediately downstream of two parallel tidal flap gates by conductivity-temperature-depth (CTD) profiler, and also using bed-mounted sensor frames to measure velocity at 2 fixed depths, have yielded a set of results covering 11 tidal cycles over the period 2002–04. It is clear from the results obtained that over a typical tidal cycle, the greatest vertical salinity gradients occur in the 1–2 h immediately after the onset of the flood tide, and that subsequently, energetic mixing acts to rapidly break down this stratification. Under moderate-to-high fresh water flows (>0.5 m³/s), the break-down in vertical salinity gradient is more gradual, while under low fresh water flows (<0.2 m³/s), the vertical salinity gradient is generally less pronounced. Estimates of Richardson number during the early flood-tide period reveal values that vary rapidly between <1 and about 20, with lower values occurring after around 1.5–2 h after low water. Observations of suspended sediment concentration vary widely even for similar tidal and fresh water flow conditions, revealing the possible influence of wind speed, the storage effects of the water in the lagoon downstream of the observation site, and the complexity of the hydrodynamics downstream of tidal flap gates. The data also show that most of the sediment transport is landward, and occurs during flood tides, with estimated total tidal landward flood tide flux of fine sediment of the order of 50–120 kg under low fresh water flow conditions. These observations, which reinforce the results presented in Warner et al. (2004) and elsewhere, can help to provide information about the appropriate techniques for managing sediments and pollutants, including nutrients from sewage effluent waters, in estuaries where hydraulic flap gates are used to control the entry of fresh water over the tidal cycle.     

The Pioneer 9 electric field experiment

We present a detailed analysis of the Pioneer 9 VLF electric field observations for 20 selected storm periods covering a heliocentric range extending from 0.754 AU to 0.99 AU. Although data from only two low frequency channels are available, the results of the present study tend to confirm the preliminary speculation by Scarf and Siscoe (1971) that the turbulentE-field spectrum in the disturbed solar wind has a significant radial gradient.     

Concentration and distribution of hydrophobic organic contaminants and metals in the estuaries of Ukraine

In this study of Ukrainian estuaries, sediments and tissues from the Dnieper and Boh estuaries and Danube Delta on the mainland, Sevastopol and Balaklava Bays on the Crimean Peninsula, and coastal Black Sea along the Crimean Peninsula were collected in 2006. Contaminant analyses included several metals, the hydrophobic organic chemicals (HOCs) polychlorinated biphenyls, several chlorinated pesticides, and polycyclic aromatic hydrocarbons. When compared to estuarine sediments globally, the Ukrainian sediments were found to be moderately contaminated. However, several metals, especially mercury, were often shown to be elevated in the tissues of the Ukrainian organisms in comparison to organisms from other estuarine locations. Sediment quality guidelines indicate some of the estuarine sediments could be sufficiently contaminated to cause adverse toxicological effects. This investigation represents the first extensive study of HOC and metal baseline concentrations and distributions in Ukrainian estuaries and seeks to characterize exposures to aquatic organisms living in these systems.