Numerical models for diagenesis and the Neogene Sr isotopic evolution of seawater from DSDP Site 590B

A numerical model for the diagenetic exchange of Sr between carbonates and their pore fluids during sedimentation and compaction has been developed. The model has been applied to data from DSDP Site 590B in order to assess the accuracy with which the Sr isotope record in the carbonate sediment reflects that of seawater. The most important process affecting the Sr in the solid carbonate is exchange with the pore fluid due to solution-reprecipitation, but the concentration or isotopic composition of Sr in the solid itself gives little or no information as to the magnitude of this exchange. The key to determining the rate of exchange is the pore fluid, where the variations of Sr²⁺ and⁸⁷Sr/⁸⁶Sr with depth are very sensitive indicators. The logical structure of applying the model to data from DSDP 590B is to find by successive iteration an ocean history (i.e., the initial⁸⁷Sr/⁸⁶Sr and Sr concentration of each increment of carbonate deposited) and a rate of Sr exchange between pore water and solid carbonate such that the model matches the present Sr concentration and⁸⁷Sr/⁸⁶Sr of both pore water and solid carbonate.Once all the data are matched, the model provides an estimate of the rate of Sr exchange due to solution-reprecipitation and the evolution of⁸⁷Sr/⁸⁶Sr in seawater over the past 20 million years. For DSDP 590B we find that solution-reprecipitation decreases rapidly with depth, from a near surface value of about 10% per million years to about 1% per million years below 200 m. This rate of exchange of Sr results in the carbonates of DSDP 590B preserving an accurate record of the Sr isotopic evolution of the ocean over the past 5 million years, but for ages greater than 5 million years the⁸⁷Sr/⁸⁶Sr ratio of the carbonate is systematically displaced from that of the seawater in which it was deposited. The maximum difference is of order 5 × 10⁻⁵.     

Approaches to dynamic equilibrium in theoretical simulations of slope development

The model SLOP3D is used to identify characteristic aspects of slope development under different conditions. A series of slopes along the Kall valley in the northern Eifel is reproduced by a developmental sequence of model slopes. The spatial and temporal variations of the components of slope development systems (relief, slope form, weathering rates, denudation rates) are discussed by means of mass movement and wash denudation models. The negative feedbacks between system components create a tendency towards the establishment of a dynamic equilibrium which is reached, however, only if the endogenic and exogenic conditions remain constant for the required relaxation time. The latter is shown to be a function of slope length, of the rate of uplift, of the denudational process type, and of the intensity of exogenic inputs (e.g. precipitation). Varying rock resistance leads to the development of shield inselbergs only if the rate of denudation becomes higher than the maximum possible weathering rate of the resistant bedrock.     

Simple models for trace element fractionation during melt segregation

A dynamical calculation for melt segregation from a deformable matrix is used as a framework for studying chemical fractionation during melting and melt migration. The chemical aspects of the problem are modelled in terms of modal melting and local diffusive equilibrium, allowing for the use of unchanging distribution coefficients. The chemical properties of the melt segregated is a synthetic data set that is used to test how well simple equations for perfect equilibrium partial melt and perfect fractional partial melt recover the actual degree of melting and original source composition. The general conclusion is that either equation does quite well (to within a factor generally less than 2) even when the physical situation is quite different from that required for deriving these simple relations between degree of melting, distribution coefficient and magma composition. In terms of the apparent source composition as inferred from the properties of the segregated melt, the typical result is to find a source enriched in the more incompatible elements, which could easily be misinterpreted as requiring metasomatic alteration.     

The Grüneisen parameter at high pressure: a molecular dynamical study

The method of molecular dynamics is used to calculate values for the thermal Grüneisen parameter, γ, for a face-centred cubic crystal with several simple central force atomic potential functions at three compressions for comparison with free volume and acoustic γ formulations. Neither is found to agree with the computer experiments. A defect in the free-volume formula for γ is shown to arise from the assumption that motions of neighbouring atoms in a crystal at high temperature are uncorrelated whereas the computer models demonstrate a 25–35% correlation of in-line motions and 5% correlation of transverse motions. The formula can be modified to allow for the empirically observed correlations, but it is concluded that there are still difficulties in analytical approaches to equation-of-state studies of the Earth's deep interior and that the computer modelling method of molecular dynamics has important advantages.     

Unmelted meteoritic debris in the Late Pliocene iridium anomaly: Evidence for the ocean impact of a nonchondritic asteroid

Ir-bearing particles have been recovered from 2 piston cores in the Antarctic Basin in the southeastern Pacific. In core E13-3 the particles closely correspond to the Late Pliocene Ir anomaly and have a fluence of ~100 mg cm^?2. In core E13-4, 120 km to the southwest, the particle fluence is ~4 mg/cm^?2. Particles with diameters from 0.5 to 4 mm contain at least 35% of the Ir in this horizon. Three types of particles have been identified: 1) vesicular, 2) basaltic, and 3) metal. The vesicular particles appear to be shock-melted debris derived from the oceanic impact of a howarditic asteroid containing a minor metal component. These particles have recrystallized from a melt and impact into the ocean has resulted in the incorporation of Na, K, Cl, and radiogenic Sr from the ocean water target. The basaltic clasts appear to be unmelted fragments of the original asteroid which may have separated from the main body prior to impact. Combined vesicular and basaltic particles are believed to have formed by collisions in the debris cloud. Estimates of the diameter of the projectile range from 100 to 500 m. By many orders of magnitude this is the most massive achondrite sampled by a single meteorite fall.     

ECONOMIC-LOGIC, CONCEPTS AND METHODS: ANALYTICAL AIDS IN SEDIMENT MANAGEMENT

I. INTRODUCTION AND PERSPECTIVEIn planning, implementing and monitoring water and related resources developmentS, the effectsof erosion and sedimentation on man,s economic activities, his health and welfare and on the environment, have to be taken into ac…     

Sediment accumulation in lake chicot,Arkansas

Lake Chicot is an oxbow lake located along the western side of the Mississippi River in southeastern Arkansas. A major flood in 1927, levee construction, land use changes from bottomland hardwood to agriculture, a large increase in drainage area, and stream channelization have altered the appearance of contributing watersheds and the lake. The lake often has high suspended sediment concentrations making it undesirable for recreation and aquatic production. As part of a coordinated study to determine changes in the lake, sediment accumulation patterns and rates were determined in Lake Chicot using the¹³⁷Cs technique. Major sediment accumulation is occurring near the major inlet and along the thalweg of the oxbow. Average sediment accumulation of 1.8 cm/yr since 1963 was measured for twelve samples. Calculations show sediment accumulation is decreasing. This study has shown that the¹³⁷Cs technique can provide useful planning information for the environmental geologist or reservoir manager. Results of the current study will be used to develop better sediment and water management strategies for Lake Chicot. Contribution of the U.S. Department of Agriculture, Agricultural Research Service, in cooperation with the Vicksburg District of the U.S. Army Corps of Engineers.     

Climate Variability-Observations, Reconstructions, and Model Simulations for the Atlantic-European and Alpine Region from 1500-2100 AD

A detailed analysis is undertaken of the Atlantic-European climate using data from 500-year-long proxy-based climate reconstructions, a long climate simulation with perpetual 1990 forcing, as well as two global and one regional climate change scenarios. The observed and simulated interannual variability and teleconnectivity are compared and interpreted in order to improve the understanding of natural climate variability on interannual to decadal time scales for the late Holocene. The focus is set on the Atlantic-European and Alpine regions during the winter and summer seasons, using temperature, precipitation, and 500 hPa geopotential height fields. The climate reconstruction shows pronounced interdecadal variations that appear to “lock” the atmospheric circulation in quasi-steady long-term patterns over multi-decadal periods controlling at least part of the temperature and precipitation variability. Different circulation patterns are persistent over several decades for the period 1500 to 1900. The 500-year-long simulation with perpetual 1990 forcing shows some substantial differences, with a more unsteady teleconnectivity behaviour. Two global scenario simulations indicate a transition towards more stable teleconnectivity for the next 100 years. Time series of reconstructed and simulated temperature and precipitation over the Alpine region show comparatively small changes in interannual variability within the time frame considered, with the exception of the summer season, where a substantial increase in interannual variability is simulated by regional climate models.     

Palynomorphe und organisches Material aus den Raibler Schichten einer oberostalpinen Schuppe der Iberger Klippen (Kanton Schwyz, Schweiz)

A palynology and organic matter study has been carried out on samples of the Upper Triassic Raibl beds from an Upper Austroalpine thrust sheet, preserved in the area of the Iberg Klippen (Laucherenst?ckli). The palynological assemblages indicate an early Carnian (Julian) age. Comparison with well calibrated successions from the Southern Alps suggests a correlation with the Late Julian ammonoid zone Austrotrachyceras austriacum. The lithofacies of the studied sections suggests a substantial similarity to Raibl beds of the Upper Austroalpine of the Northern Calcareous Alps and those of the Silvretta- and the S-charl nappe of the Grisons. The minute Upper Austroalpine thrust sheet represents the westward extension of series with similar facies, which are still preserved in the Northern Calcareous Alps of Liechtenstein and western Austria (Vorarlberg). According to the Thermal Alteration Scale (TAS) of Batten (1996) the observed medium brown colors of the pollen grains correspond to a value of 4/5, which is equivalent to vitrinite reflectance (VR) values of 0.7–0.9 %R_o. In comparison with other sites in Liechtenstein, Vorarlberg and the Upper Austroalpine of Central Grisons (Silvretta and S-charl nappe) with measured VR values of > 2%R_o the organic matter of the studied samples shows only minor thermal alteration, indicating that the Upper Austroalpine thrust sheets of Iberg represent the highest part of the nappe stack in Central Switzerland, formerly covered by a comparatively thin overburden.