Chlorine partitioning in the stratosphere based on in situ measurements

Balloon‐borne cryogenic air sampler experiments have been conducted from a tropical (Hyderabad, 17.5°N) and a midlatitude (GAP, southern France, 44°N) station since 1987 in the altitude range of about 8‐35 km. Air samples are analysed at the Max Planck Institute for Aeronomy (MPAE) and Physical Research Laboratory (PRL) using various gas chromatographic techniques to obtain the vertical distributions of several halogenated source gases and long‐lived dynamical tracers. These observations are used to determine the stratospheric partitioning of the chlorine species into their organic and inorganic forms for the altitude range of 20 to 35 km. Distributions of inorganic chlorine indicate increase in their abundances with altitude, latitude and time. Mixing ratio correlations of organic and inorganic chlorine with N₂O have been obtained which also suggest that the rate of increase in inorganic halogens component in the stratosphere is larger than the increase rate of total organic halogen. This supports previous assessments that halogen‐induced ozone depletion will continue to be observed for a few more years, despite the decrease of halocarbon loading into the stratosphere since 1994.     

The Late Quaternary sedimentary record in Scoresby Sund, East Greenland

Until recently, little was known about the Quaternary marine sedimentary record in East Greenland. Geophysical and geological investigations in Scoresby Sund were undertaken to characterize the nature and chronology of this record. Seismic records show that almost 70% of the outer fjord system is covered by about 10 m of unlithified sediments, making direct correlation with the Quaternary records on land and the adjacent continental margin difficult. These acoustically unstratified sediments are scoured by icebergs above 550 m water depth. Almost 90% of core material is massive diamicton of Holocene age, deposited mainly from iceberg rafting and turbid meltwater. Sedimentation rates are 0.1 -0.3 m 1000 yr-¹. Thicker accumulations of unlithified Quaternary sediments in Scoresby Sund occur as sediment ridges and in two other major depocentres. A low sediment ridge runs across the mouth of Scoresby Sund, and is interpreted as an end moraine of Late Weichselian Flakkerhuk stadial age. The very restricted sediment thickness suggests that grounded ice filled the fjord during the Flakkerhuk and an ice shelf was not present. High inputs of ice rafted debris to the continental margin at about 18 000 BP indicate this as a probable age for the moraine. During the Allerød Interstadial, ice probably retreated from the outer fjord system, since massive diamictons similar to those of Holocene age are present at the base of most cores. A major depocentre of acoustically stratified sediments at the head of Hall Bredning is interpreted to represent ice proximal deposits from a glacier margin extending across the fjord. It is adjacent to dated moraines on land and is inferred to be of Milne Land stadial age (about 10 000 BP). A similar age is interpreted for acoustically laminated sediments and a moraine at the entrance of Vikingebugt, on the south side of Scoresby Sund. Dated kame terraces in the inner fjord system indicate that ice retreated to its present position 6–7000 years ago.     

Anomalous arboreal pollen assemblages in Late Devensian and Early Flandrian deposits at Creich Castle, Fife, Scotland

Pollen analysis of a radiocarbon-dated core from a shallow basin in volcanic hills in eastern Scotland revealed the existence of Late Devensian and Flandrian deposits. Much of the vegetation history of the Flandrian proved to be missing due to a hiatus in the profile. The Late Devensian pollen record included anomalous arboreal taxa of three possible origins: contamination, long-distance wind transport and reworking of interstadial material. Because of lacking information on Late Devensian climates and the location of thermophilous arboreal refugia in Europe a re-examination of supposedly contaminated British Late Devensian pollen records might well be rewarding.     

THREE-DIMENSIONAL OPTIMUM MULTICHANNEL VELOCITY FILTERS*

Design procedures and characteristics of three stacking filters are discussed which may find application in various three-dimensional velocity filtering problems. These filters are derived in the time-domain as optimum multichannel Wiener filters. Random stationary functions are assumed as stochastic models for the seismic traces. All power and crosspower spectra which are the basic elements of the multichannel normal equations are statistically averaged according to specific three-dimensional considerations. Various properties of the input traces may be incorporated in the design of the optimum filters. With fairly general assumptions about the input these filters are deterministic in the sense that they are applicable to a broad class of input traces with similar statistics in amplitudes and arrival times of signals.     

The Geology of the Great 'Dyke', Zimbabwe: The Ultramafic Rocks

Textural and mineral chemistry data for the ultramafic sequenceof the Hartley Complex are presented with the object of evaluatingemplacement mechanisms, crystallization history and sub-solidusre-equilibration processes for the Great ‘Dyke’.Mineral chemistry indicates in situ crystallizaration for theultramafic sequence, whereas textural evidence suggests thatlimited crystal settling of chromite took place. It is concludedthat crystallization of cumulus phases occurred at or near thefloor of the magma chamber. The mineral chemistry indicates that the volume of magma fromwhich each unit crystallized was significantly smaller thanthat represented by the stratigraphic succession of the HartleyComplex. The magma chamber may effectively have been part ofan open system during the crystallization of the ultramaficsequence. The results are consistent with the concept of a stratifiedmagma chamber and the process of double-diffusion convection. Modelling of the liquid line of descent and crystallizationsequences indicate that none of the previously proposed initialliquid compositions are likely to have constituted the parentalmagma of the Great ‘Dyke’. Rather than komatüticor exceptionally high magnesium liquids, as previously suggested,a parental magma with about 15 per cent MgO, similar to thecomposition of the chill phase of a dyke parallel to and inclose proximity to the East Dyke is in closest agreement withthe observed and modelled results. Chromite compositions are strongly related to textural and mineralogicalenvironments. Seam chromitites are higher in Cr, Mg and Fe³⁺than chromites enclosed in silicates. Chromite enclosed in cumulusolivine is higher in Fe²⁺ than that in coexisting pyroxenesbut there is little difference in the proportions of the trivalentcaptions. Seam chromitites are considered to have precipitatedin response to increases in foi associated with periodic influxesof magma into the magma chamber. The higher ferric iron contentof the seam chromitites compared with the chromite enclosedin the silicates is consistent with such a mechanism. Compositional zoning in olivine and pyroxene adjacent to enclosedchromite grains is interpreted as reflecting subsolidus re-equilibrationwith cooling. Zoning profiles exhibit strong crystallographiccontrol. Computer modelling using finite difference approximationshas allowed controlling factors to be assessed by optimizationof the modelled parameters to give closest agreement to themeasured results. Interdiffusion coefficients and distributioncoefficients for Fe²⁺ and Mg for olivine and pyroxene with chromiteare modelled and compared with published data. Indicated blockingtemperatures for olivine are of the order of 600 °C to 700°C and 750 °C to 850 °C for orthopyroxene. Thuschromites enclosed in orthopyroxene are more Mg-rich than thoseenclosed in olivine. Coarse-grained seam chromitites have beenlittle modified subsequent to crystallization but the compositionsof the associated silicates have been influenced by the modalabundance of the chromite. Geothermometers based on chromite-silicate equilibria are probablynot applicable to layered intrusions, but information on thermalhistories may be provided by evaluation of the diffusion profiles.     

Thermobarometry of phengite-bearing eclogites in the Dabie Mountains of central China

Pressure–temperature conditions for formation of the peak metamorphic mineral assemblages in phengite-bearing eclogites from Dabieshan have been assessed through a consideration of Fe²⁺–Mg²⁺ partitioning between garnet–omphacite and garnet–phengite pairs and of the reaction equilibrium celadonite+pyrope+grossular=muscovite+diopside, which incorporates an evaluation of the extent of the strongly pressure-dependent inverse Tschermak's molecule substitution in the phengites. For the latter equilibrium, the calibration and recommended activity–composition models indicated by 45 ) have been employed and importantly yield results consistent with petrographic evidence for the stability at peak conditions of coesite in certain samples and quartz in others. Confirmation that in some phengite-eclogite samples peak silicate mineral assemblages have equilibrated at confining pressures sufficient for the stability of coesite (and in some cases even diamond) rather negates previous suggestions that coesite may have been stabilized in only very localized, possibly just intracrystalline, domains. Inherent difficulties in the evaluation of peak metamorphic temperatures from Fe²⁺–Mg²⁺ partitioning between mineral phases, due to uncertainties over Fe³⁺/Fe²⁺ ratios in the minerals (especially omphacites), and to re-equilibration during extensive retrograde overprinting in some samples, are also assessed and discussed. Our results indicate the existence in south-central Dabieshan of phengite eclogites with markedly different equilibration conditions within two structurally distinct tectonometamorphic terranes. Thus our data do not support earlier contentions that south-central Dabieshan comprises a structurally coherent continental-crust terrane with a regional P–T gradient signalling previous deepest-level subduction in the north. Instead, we recognize the Central Dabie ultra-high-pressure (coesite eclogite-bearing) terrane to be structurally overlain by a Southern Dabie high-pressure (quartz eclogite-bearing) terrane at a major southerly dipping shear zone along which late orogenic extensional collapse appears to have eliminated at least 20 km of crustal section.