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571.
Mollusc and brachiopod skeletal hard parts: Intricate archives of their marine environment
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The biogenic carbonate hard parts of fossil bivalves, cephalopods and brachiopods are among the most widely exploited marine archives of Phanerozoic environmental and climate dynamics research. The advent of novel analytical tools has led many workers to explore non‐traditional geochemical and petrographic proxies, and work performed in neighbouring disciplines sheds light on the complex biomineralization strategies applied by these organisms. These considerations form a strong motivation to review the potential and problems related to the compilation and interpretation of proxy data from bivalve, cephalopod and brachiopod hard parts from the viewpoint of the sedimentologist and palaeoceanographer. Specific focus is on the complex biomineralization pathways of a given dissolved ion or food particle from its aquatic environment via the digestion and biomineralization apparatus in molluscs and brachiopods and its incorporation into a biomineral. Given that molluscs and brachiopods do not secrete their hard parts from seawater but rather from their mantle and periostracum, this paper evaluates differences and similarities of seawater versus that of body fluids. Cephalopods, bivalves and brachiopods exert a strong biological control on biomineralization that, to some degree, may buffer their shell geochemistry against secular changes in seawater chemistry. Disordered (amorphous) calcium carbonate precursor phases, later transformed to crystalline biominerals, may be significant in carbonate archive research due to expected geochemical offset relative to the direct precipitation of stable phases. A reasonable level of understanding of the related mechanisms is thus crucial for those who use these skeletal hard parts as archives of the palaeo‐environment. The impact of what is commonly referred to as ‘biological factors’ on the geochemistry of mollusc and brachiopod hard parts is explored for conventional isotope systems such as carbon, oxygen, strontium and traditionally used element to calcium ratios. In particular, the often used δ13Ccarb or the Mg/Ca and Sr/Ca elemental proxies are fraught with problems. An interesting new research field represents the analysis, calibration and application of non‐traditional proxies to mollusc and brachiopod hard parts. Examples include the carbonate clumped isotope (Δ47) approach and the analysis of the isotopes of Ca, Mg, N, Li, S or element to Ca ratios such as Li/Ca or B/Ca and rare earth elements. Based on considerations discussed here, a series of “do's and don'ts” in mollusc and brachiopod archive research are proposed and suggestions for future work are presented. In essence, the suggestions proposed here include experimental work (also field experiments) making use of recent archive organisms or, where possible, a reasonable recent analogue in the case of extinct groups. Moreover, the detailed understanding of the architecture of mollusc and brachiopod hard parts and their ultra‐structures must guide sampling strategies for geochemical analyses. Where feasible, a detailed understanding of the diagenetic pathways and the application of multi‐proxy and multi‐archive approaches should form the foundation of fossil carbonate archive research. The uncritical compilation of large data sets from various carbonate‐shelled organisms collected at different locations is not encouraged. 相似文献
572.
Alexander?BorisovEmail authorView authors OrcID profile Harald?Behrens Francois?Holtz 《Contributions to Mineralogy and Petrology》2018,173(12):98
The effect of MgO and total FeO on ferric/ferrous ratio in model multicomponent silicate melts was investigated experimentally in the temperature range 1300–1500 °C at 1 atm total pressure in air. We demonstrate that the addition of these weak network modifier cations results in an increase of Fe3+/Fe2+ ratio in both mafic and silicic melts. Based on present and published experimental data, a new empirical equation is proposed to predict the ferric/ferrous ratio as a function of oxygen fugacity, temperature and melt composition. In contrast to previous equations, the compositional effect of melts on the Fe3+/Fe2+ ratio is not only modeled by the sum of the molar fraction of the individual oxide components. Additional interactions terms have also been incorporated. The main advantage of the proposed model is its applicability for a wide compositional range. However, its application to felsic melts (>?68 wt% SiO2) is not recommended. Other advantages of this equation and differences when compared with previous models are discussed. 相似文献
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575.
Cumulative probability functions (CPFs) for large numbers of radiocarbon age determinations are increasingly being used by scientists as a methodology to discern environmental histories. While the recent compilation of regional databases of the radiocarbon dating control for fluvial sediment sequences has been beneficial for identifying gaps in knowledge and stimulating new research, there are a number of problems that critically undermine the use of these CPFs as sensitive hydroclimatic proxies. (i) The CPF method is underpinned by the assumption that each radiocarbon measurement is a true age estimate for a point in time, whereas each measured age in fact forms a scatter around the true age of the sample; (ii) calibration of radiocarbon ages is responsible for much of the structure in CPFs and compounds the problem of scatter and smears the chronological control; (iii) the databases incorporate multiple types of environmental changes differing chronological relationships between the 14C measurements and the dated events, with pre‐dating, dating or post‐dating chronological control each displaying variable length temporal lags all mixed together in the same analysis; and (iv) the radiocarbon ages from individual case studies need to be more robustly tested before being incorporated into regional databases. All these factors negate the value of CPFs as sensitive proxies of environmental change, because peaks in probability for individual radiocarbon measurements are likely to be an incorrect estimate for the age of a geomorphological event and this problem is compounded by combining probabilities for multiple unrelated events. In this paper we present a critical analysis of CPFs and their interpretation before suggesting alternative approaches to analysing radiocarbon geochronologies of geomorphic events, which include: (i) Bayesian age modelling of river terrace development; (ii) developing regional databases that test specific geomorphic hypotheses; (iii) Bayesian age modelling of palaeoflood records; and (iv) analysis of sedimentation rates. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
576.
Jérôme Carrayrou Joachim Hoffmann Peter Knabner Serge Kräutle Caroline de Dieuleveult Jocelyne Erhel Jan Van der Lee V. Lagneau K. Ulrich Mayer Kerry T. B. MacQuarrie 《Computational Geosciences》2010,14(3):483-502
Although multicomponent reactive transport modeling is gaining wider application in various geoscience fields, it continues
to present significant mathematical and computational challenges. There is a need to solve and compare the solutions to complex
benchmark problems, using a variety of codes, because such intercomparisons can reveal promising numerical solution approaches
and increase confidence in the application of reactive transport codes. In this contribution, the results and performance
of five current reactive transport codes are compared for the 1D and 2D subproblems of the so-called easy test case of the
MoMaS benchmark (Carrayrou et al., Comput Geosci, 2009, this issue). This benchmark presents a simple fictitious reactive transport problem that highlights the main numerical difficulties
encountered in real reactive transport problems. As a group, the codes include iterative and noniterative operator splitting
and global implicit solution approaches. The 1D easy advective and 1D easy diffusive scenarios were solved using all codes,
and, in general, there was a good agreement, with solution discrepancies limited to regions with rapid concentration changes.
Computational demands were typically consistent with what was expected for the various solution approaches. The differences
between solutions given by the three codes solving the 2D problem are more important. The very high computing effort required
by the 2D problem illustrates the importance of parallel computations. The most important outcome of the benchmark exercise
is that all codes are able to generate comparable results for problems of significant complexity and computational difficulty. 相似文献
577.
Alberto Flandes Harald Krüger Douglas P. Hamilton J. Francisco Valdés-Galicia Linda Spilker Rogelio Caballero 《Planetary and Space Science》2011,59(13):1455-1471
High speed dust streams emanating from near Jupiter were first discovered by the Ulysses spacecraft in 1992. Since then the phenomenon has been re-observed by Galileo in 1995, Cassini in 2000, and Ulysses in 2004. The dust grains are expected to be charged to a potential of , which is sufficient to allow the planet's magnetic field to accelerate them away from the planet, where they are subsequently influenced by the interplanetary magnetic field (IMF). A similar phenomenon was observed near Saturn by Cassini. Here, we report and analyze simultaneous dust, IMF and solar wind data for all dust streams from the two Ulysses Jupiter flybys. We find that compression regions (CRs) in the IMF – regions of enhanced magnetic field – precede most dust streams. Furthermore, the duration of a dust stream is roughly comparable with that of the precedent CR, and the occurrence of a dust stream and the occurrence of the previous CR are separated by a time interval that depends on the distance to Jupiter. The intensity of the dust streams and their precedent CRs are also correlated, but this correlation is only evident at distances from the planet no greater than 2 AU. Combining these observations, we argue that CRs strongly affect dust streams, probably by deflecting dust grain trajectories, so that they can reach the spacecraft and be detected by its dust sensor. 相似文献
578.
Frank Postberg Eberhard Grün Mihaly Horanyi Sascha Kempf Harald Krüger Jürgen Schmidt Frank Spahn Ralf Srama Zoltan Sternovsky Mario Trieloff 《Planetary and Space Science》2011,59(14):1815-1825
Classical methods to analyze the surface composition of atmosphereless planetary objects from an orbiter are IR and gamma ray spectroscopy and neutron backscatter measurements. The idea to analyze surface properties with an in-situ instrument has been proposed by Johnson et al. (1998). There, it was suggested to analyze Europa's thin atmosphere with an ion and neutral gas spectrometer. Since the atmospheric components are released by sputtering of the moon's surface, they provide a link to surface composition. Here we present an improved, complementary method to analyze rocky or icy dust particles as samples of planetary objects from which they were ejected. Such particles, generated by the ambient meteoroid bombardment that erodes the surface, are naturally present on all atmosphereless moons and planets. The planetary bodies are enshrouded in clouds of ballistic dust particles, which are characteristic samples of their surfaces. In situ mass spectroscopic analysis of these dust particles impacting onto a detector of an orbiting spacecraft reveals their composition. Recent instrumental developments and tests allow the chemical characterization of ice and dust particles encountered at speeds as low as 1 km/s and an accurate reconstruction of their trajectories. Depending on the sampling altitude, a dust trajectory sensor can trace back the origin of each analyzed grain with about 10 km accuracy at the surface. Since the detection rates are of the order of thousand per orbit, a spatially resolved mapping of the surface composition can be achieved. Certain bodies (e.g., Europa) with particularly dense dust clouds, could provide impact statistics that allow for compositional mapping even on single flybys. Dust impact velocities are in general sufficiently high at orbiters about planetary objects with a radius >1000 km and with only a thin or no atmosphere. In this work we focus on the scientific benefit of a dust spectrometer on a spacecraft orbiting Earth's Moon as well as Jupiter's Galilean satellites. This ‘dust spectrometer' approach provides key chemical and isotopic constraints for varying provinces or geological formations on the surfaces, leading to better understanding of the body's geological evolution. 相似文献
579.
A novel approach for upscaling land-surface parameters based on inverse stochastic surface-vegetation-atmosphere transfer
(SVAT) modelling is presented. It allows estimation of effective parameters that yield scale invariant outputs e.g. for sensible
and latent heat fluxes and evaporative fraction. The general methodology is used to estimate effective parameters for the
Oregon State University Land-Surface Model, including surface albedo, surface emissivity, roughness length, minimum stomatal
resistance, leaf area index, vapour pressure deficit factor, solar insolation factor and the Clapp–Hornberger soil parameter.
Upscaling laws were developed that map the mean and standard deviation of the distributed land-surface parameters at the subgrid
scale to their corresponding effective parameter at the grid scale. Both linear and bi-parabolic upscaling laws were obtained
for the roughness length. The bi-parabolic upscaling law fitted best for the remaining land-surface parameters, except surface
albedo and emissivity, which were best fitted with linear upscaling laws. 相似文献
580.
Ingrid H. Ellingsen Padmini Dalpadado Dag Slagstad Harald Loeng 《Climatic change》2008,87(1-2):155-175
The Barents Sea is a high latitude ecosystem and is an important nursery and feeding area for commercial fish stocks such
as cod, capelin and herring. There is a large inter-annual variability both in physical and biological conditions in the Barents
Sea. Understanding and predicting changes in the system requires insight into the coupled nature of the physical and biological
interactions. A coupled physical and biological ocean model is used to study the impact of postulated future atmospheric changes
on the physical and biological conditions in the Barents Sea. Results from this simulation not only show that there is a large
variability in the physical conditions on a wide range of time scales, but also that the temperature in the Barents Sea is
increasing. The corresponding ice cover decrease is most noticeable in the summer months. The changes in physical properties
will most likely have an impact on the biotope. On average, the primary production increases slightly over a 65 year long
period, about 8%, partly due to an increased production in the northern Barents Sea. The model further simulates that the
production of Atlantic zooplankton species increases approximately 20% and becomes more abundant in the east. The Arctic zooplankton
biomass decreases significantly (50%) causing the total simulated production to decrease. 相似文献