Sea level and diagenesis: a case study on Pleistocene beaches,Whalebone Bay,Bermuda     

R. Vollbrecht  D. Meischner 《International Journal of Earth Sciences》1993,82(2):248-262

Pleistocene fluctuations of sea level have left marine and aeolian limestones intercalated with glacial red soils on the Bermuda Carbonate Platform (Atlantic, 64°50W, 32°20N). Successive eustatic highstands of similar amplitude drowned the tectonically stable platform and piled up similar sets of sediments. Up to three Pleistocene beaches are stacked in shorelines sections. Post-depositional diagenetic histories of these beaches can be linked to repeated changes in sea level and pore waters.This paper presents field evidence and petrographic results (microscope, X-ray, cathodoluminescence, SEMEDAX) for the diagenetic histories of two superimposed Pleistocene beaches in Whalebone Bay, Bermuda North Shore. The younger beach was deposited during isotopic stage 5e, about 120 ka ago. The age of the older beach may be isotopic stage 9 or older.Diagenesis drastically altered the older beach before the stage 5e transgression. Primary high-Mg calcite (HMC) and aragonite were no longer present. Marine skeletal grains were instead leached or recrystallized to low-Mg calcite (LMC). Primary and secondary pore space were largely reduced by LMC cement. Lines of needle relics reminiscent of marine aragonite cement occur as inclusions within syntaxial rim cements around echinoderm grains, indicating that a marine influence had at least once interrupted this period of freshwater alteration. Finally, before the rocks became buried by the sediments of the younger beach, a crust of marine, bladed HMC cement was precipitated throughout the pore space.The younger beach consists of skeletal grains that are, apart from the effects of non-selective dissolution, essentially unaltered. The sediments are only weakly lithified by cryptocrystalline LMC showing an alveolar texture, tangential fibres and other features characteristic of calichification. A younger post-depositional marine influence is not recorded.These results suggest that, under favourable conditions, diagenetic processes can document sea-level fluctuations. The recorded fluctuations, however, are difficult to assess because even major sea-level highstands may not produce a diagenetic imprint. Correspondence to: R. Vollbrecht  相似文献   

Stable isotope study of carbonate-cemented rocks from the Pobitite Kamani area,north-eastern Bulgaria     

R. W. Botz  V. Georgiev  P. Stoffers  Kh. Khrischev  V. Kostadinov 《International Journal of Earth Sciences》1993,82(4):663-666

The Precipitation of carbonate cements in the Pobitite Kamani area (Lower Eocene) began during early diagenesis of sediments. There is evidence, however, that calcite is still forming today.The negative ¹³C values to –29.2 suggest that the carbonate formed during degradation of ¹²C-enriched organic matter (perhaps partly from oxidation of methane). The ¹⁸O values of –0.9 to –1.6 reflect the marine origin of the early diagenetic carbonate cements. Most of the carbonates, however, formed during late diagenesis (at approximately 1300 m burial depth) and/or recently (after uplift) from percolating groundwaters. These carbonates have an isotopic composition characteristic of carbonates which precipitated from meteoric waters under normal sedimentary temperatures in isotopic equilibrium with ¹²C-enriched soil carbon dioxide.  相似文献   

Late Quaternary palaeolimnology of a tropical marl lake: Wallywash Great Pond,Jamaica   总被引：1，自引：0，他引：1  

F. A. Street-Perrott  P. E. Hales  R. A. Perrott  J. C. Fontes  V. R. Switsur  A. Pearson 《Journal of Paleolimnology》1993,9(1):3-22

Wallywash Great Pond (17° 57 N, 77° 48 W, 7 m a.s.l.) is the largest perennial lake in Jamaica. It occupies a fault trough within the karstic White Limestone. The Great Pond is a hardwater lake with a pH of 8.2–8.6 and an alkalinity of 3.6–3.9 meq 1^–1. Its chemistry is strongly influenced by the spring discharge from the limestone. The lake water is subject to degassing, evaporation and bicarbonate assimilation by submerged plants and algae, resulting in marl precipitation. A 9.23 m core (WGP2), taken from a water depth of 2.8 m, was analysed for magnetic susceptibility, loss-on-ignition, carbonate content, mole % MgCO₃ in calcite, and stable isotopes in the fine carbonate fraction. The chronology is based on ten¹⁴C and four U/Th dates. Four main sediment types alternate in the core: marl; organic, calcareous mud; organic mud or peat; and earthy, brown, calcareous mud. The marls represent periods of wet/warm climate during sea-level highstands and the organic deposits, shallower, swampy conditions. In contrast, the brown, calcareous muds were laid down when the lake was dry or ephemeral. The last interglacial (120 000- 106 000 yr BP) is represented by three distinct marl units. After a dry interval, stable, wet/warm conditions set in from 106 000 to 93 000 yr BP. A dry/cool climate prevailed between 93 000 and at least 9500 yr BP. Three subsequent cycles of alternating wet and dry conditions culminated in flooding of the basin by the Black River during the late Holocene. These recent events cannot be accurately dated by¹⁴C due to significant and temporally-variable inputs of dead carbon from the springs.  相似文献   

Paleohydrology and paleoclimate records in a saline prairie lake core: mineral,isotope and organic indicators     

D. R. Van Stempvoort  T. W. D. Edwards  M. S. Evans  W. M. Last 《Journal of Paleolimnology》1993,8(2):135-147

This pilot study examines the potential of obtaining a sedimentary record of paleoenvironmental/climatic/hydrologic conditions for saline Redberry Lake in southern Saskatchewan, Canada. The tools are mineralogy, stable isotopes and pigments. The upper meter of an offshore sediment core contains 10 to 20% by weight aragonite (CaCO₃), which apparently precipitated in the water column. The ¹⁸O and ¹³C of the bulk aragonite (corrected for content of detrital calcite) vary by 4 to 5. Enrichment in ¹⁸O in aragonite is significantly correlated with pigment concentrations (chlorophyll a, phaeophytin). The ¹⁸O and pigment data provide evidence for relatively dry and/or warm conditions and high limnetic productivity for the period 2500 to 1500 yrs B.P. After 1500 B.P., the climate was apparently similar to the present, with two episodes of relatively enhanced productivity, dryness and/or warmth, at around 1000 to 900 and 500 to 200 B.P. During the past century, Redberry Lake has decreased approximately 8 m in depth and its salinity has doubled. No clear sedimentary signal was observed in response to these recent hydrologic trends. These changes have not been associated with a significant climate trend in the region, but may have been induced by land use changes in the catchment.This publication is the third of a series of papers presented at the Conference on Sedimentary and Paleolimnological Records of Saline Lakes. This Conference was held August 13–16, 1991 at the University of Saskatchewan, Saskatoon, Canada. Dr. Evans is serving as Guest Editor for this series.  相似文献   

Generalized Lyapunov exponents indicators in Hamiltonian dynamics: An application to a double star system     

E. Lohinger  C. Froeschlé  R. Dvorak 《Celestial Mechanics and Dynamical Astronomy》1993,56(1-2):315-322

The Lyapunov characteristic numbers (LCNs) which are defined as the mean value of the distribution of the local variations of the tangent vectors to the flow (=ln _k ⁱ ) (see Froeschlé, 1984) have been found to be sensitive indicators of stochasticity. So we computed the distribution of these local variations and determined the moments of higher order for the integrable and stochastic regions in a binary star system with =0.5.  相似文献   

The starburst in the Wolf-Rayet nucleus of the Liner NGC 6764     

A. Eckart  M. Cameron  R. Genzel  N. Nakai  S. Wagner 《Astrophysics and Space Science》1993,205(1):29-35

We present high resolution millimeter, near-infrared, and optical data on the Wolf-Rayet nucleus of the Liner NGC 6764. The millimeter¹²CO(1-0) maps were obtained using the Nobeyama Millimeter Interferometer. Near-infrared images in the K-band continuum and the 2.12µm H₂, 2.06µm He I, 2.17µm Br, and 1.64µm [Fe II] lines were taken with the MPE imaging spectrometer FAST at the William Herschel Telescope on La Palma, Spain. The optical data were obtained at the 3.5m telescope on Calar Alto, Spain. The measurements indicate a strong concentration of molecular gas and a massive starburst at the nucleus of NGC 6764. The interferometric position velocity map of the nucleus shows the presence of distinct molecular cloud complexes with an apparently asymmetric velocity field shifted towards the blue with respect to the systemic velocity of 2420 km s^–1. The distribution of the 2.12µm H₂ line flux exhibits extensions approximately perpendicular to the bar which are in agreement with structural features in VLA radio maps and IRAM 30m maps of the¹²CO(2–1) line emission. This may represent evidence for combination of a nuclear outflow and a central oval distortion of gas predicted by gas dynamical calculations as a response to a bar like potential. A detailed investigation of the Wolf-Rayet-feature at 466 nm indicates that it is spatially extended on a scale of a few arcseconds.  相似文献   

Evolution of the spin of Venus   总被引：1，自引：0，他引：1  

J. McCue  J. R. Dormand 《Earth, Moon, and Planets》1993,63(3):209-225

Evolutionary sequences of the Venusian sidereal rotation period, and of its obliquity, under the action of solar gravitational torques on the body of the planet and on its atmosphere are here presented. They show that Venus initial configuration in the early history of the solar system may well have been of rotation period between 33 and 48 days with an angle of obliquity of about 170°.  相似文献   

Preservation of biological information in thermal spring deposits: developing a strategy for the search for fossil life on Mars   总被引：1，自引：0，他引：1  

Walter MR  Des Marais DJ 《Icarus》1993,101(1):129-143

Current interpretations of the early history of Mars suggest many similarities with the early Earth and therefore raise the possibility that the Archean and Proterozoic history of life on Earth could have a counterpart on Mars. Terrestrial experience suggests that, with techniques that can be employed remotely, ancient springs, including thermal springs, could well yield important information. By delivering water and various dissolved species to the sunlit surface of Mars, springs very likely created an environment suitable for life, which could have been difficult, if not impossible, to attain elsewhere. The chemical and temperature gradients associated with thermal springs sort organisms into sharply delineated, distinctive and different communities, and so diverse organisms are concentrated into relatively small areas in a predictable and informative fashion. A wide range of metabolic strategies are concentrated into small areas, thus furnishing a useful and representative sampling of the existing biota. Mineral-charged springwaters frequently deposit chemical precipitates of silica and/or carbonate which incorporate microorganisms and preserve them as fossils. The juxtaposition of stream valley headwaters with volcanoes and impact craters on Mars strongly implies that subsurface heating of groundwater created thermal springs. On Earth, thermal springs create distinctive geomorphic features and chemical signatures which can be detected by remote sensing. Spring deposits can be quite different chemically from adjacent rocks. Individual springs can be hundreds of meters wide, and complexes of springs occupy areas up to several kilometers wide. Benthic microbial mats and the resultant stromatolites occupy a large fraction of the available area. The relatively high densities of fossils and microbial mat fabrics within these deposits make them highly prospective in any search for morphological evidence of life, and there are examples of microbial fossils in spring deposits as old as 300 Myr.  相似文献   

Coupled atmosphere-ocean models of Titan's past     

McKay CP  Pollack JB  Lunine JI  Courtin R 《Icarus》1993,102(1):88-98

We have developed a coupled atmosphere and ocean model of Titan's surface. The atmospheric model is a 1-D spectrally-resolved radiative-convective model. The ocean thermodynamics are based upon solution theory. The ocean, initially composed of CH4, becomes progressively enriched in ethane over time. The partial pressures of N2 and CH4 in the atmosphere are dependent on the ocean temperature and composition. We find that the resulting system is stable against a runaway greenhouse. Accounting for the decreased solar luminosity, we find that Titan's surface temperature was about 20 K colder 4 Gyr ago. Without an ocean, but only small CH4 lakes, the temperature change is 12 K. In both cases we find that the surface of Titan may have been ice covered about 3 Gyr ago. In the lakes case condensation of N2 provides the ice, whereas in the ocean case the ocean freezes. The dominant factor influencing the evolution of Titan's surface temperature is the change in the solar constant--amplified, if an ocean is present, by the temperature dependence of the solubility of N2. Accretional heating can dramatically alter the surface temperature; a surface thermal flux of 500 erg cm-2 sec-1, representative of small levels of accretional heating, results in a approximately 20 K change in surface temperatures.  相似文献   

Discrete emissions and whistler precursors recorded at low latitude ground station Gulmarg     

R. P. Singh  Lalmani  U. P. Singh 《Earth, Moon, and Planets》1993,60(3):225-232

Discrete chorus-type emission and whistler precursors recorded in March 1972 during day time hours at our ground based station Gulmarg are presented. It is shown that discrete chorus type emissions are generated in the equatorial region (L 1.2) during cyclotron resonance interaction between the propagating whistler wave and the gyrating electrons. The whistler precursors are explained in terms of the mechanism suggested by Dowden (1972).  相似文献