To evaluate the potential of using surficial shell accumulations for paleoenvironmental studies, an extensive time series of individually dated specimens of the marine infaunal bivalve mollusk Semele casali was assembled using amino acid racemization (AAR) ratios (n = 270) calibrated against radiocarbon ages (n = 32). The shells were collected from surface sediments at multiple sites across a sediment-starved shelf in the shallow sub-tropical São Paulo Bight (São Paulo State, Brazil). The resulting 14C-calibrated AAR time series, one of the largest AAR datasets compiled to date, ranges from modern to 10,307 cal yr BP, is right skewed, and represents a remarkably complete time series: the completeness of the Holocene record is 66% at 250-yr binning resolution and 81% at 500-yr binning resolution. Extensive time-averaging is observed for all sites across the sampled bathymetric range indicating long water depth-invariant survival of carbonate shells at the sediment surface with low net sedimentation rates. Benthic organisms collected from active depositional surfaces can provide multi-millennial time series of biomineral records and serve as a source of geochemical proxy data for reconstructing environmental and climatic trends throughout the Holocene at centennial resolution. Surface sediments can contain time-rich shell accumulations that record the entire Holocene, not just the present. 相似文献
Isotopic and cladoceran investigations of Eemian (MIS 5e) lake sediments from Imbramowice, SW Poland, allow us to reconstruct the environmental conditions, especially changes of water level and trophic status, during the early and middle Eemian Interglaciation. We analyzed the sediments from 6.5 to 11.0 m depth in a core provided by Mamakowa (1989). The upper 6.5 m had insufficient carbonates and cladoceran contents for analyses. The analyzed section consists of sandy and organic silts at the bottom, followed by gyttja characterized by increasing CaCO3 content. Measured δ18O values oscillate from ca. - 9 to - 4‰ and δ13C from - 3.5 to above + 6‰. Based on stable isotope analyses of carbonates, we define and characterize eight isotopic horizons (Is). We identify 26 taxa of subfossil Cladocera and seven zones (CLZ) of faunal development. Probably the greatest depth of the lake occurred with pollen zone E2; shallowing then took place. During pollen zones E2-E3, gradual warming is observed and expressed through a positive trend in both δ18O and δ13C values. Pollen zone E4 is characterized by frequent changes of water level. During the Eemian Interglaciation, excluding the initial phase of lake evolution, the lake was meso-eutrophic and eutrophic with high phytoplankton productivity. 相似文献
The term ‘hiatus concretions’ was introduced for the first time by the late Ehrhard Voigt, a well‐known German bryozoan specialist, in 1968 (originally as Hiatus‐Konkretionen). Hiatus concretions are early diagenetic bodies that formed within the host sediment. In this respect, they are similar in composition to other concretions that are very common in siliciclastic deposits of different ages, some of which are known to contain fossils or minerals. Hiatus concretions, however, differ from conventional concretions in their complex post‐diagenetic history, including exhumation on the sea‐floor, colonization by various encrusting and/or boring organisms during a break in sedimentation, and final burial. Thus, the name ‘hiatus concretions’ refers to the fact that they indicate hiatal surfaces in sedimentary sequences ( Fig. 1 ). It is known that hiatus concretions may have developed during very different time‐spans: for example, within a part of one ammonite subzone (i.e. tens of thousands to a few hundreds of thousands of years) or much longer, during a time embracing more than a stage (i.e. several millions of years). The majority of the hiatus concretions known from the fossil record have carbonate cements, usually calcite, and these are the main focus of this article. Some hiatus concretions, however, are cemented by phosphate minerals. Figure 1 Open in figure viewer PowerPoint Hiatus concretions in the field. A. A horizon of hiatus concretions weathered out from the Middle Jurassic (Bathonian) clay sediments at Krzyworzeka, Polish Jura, Poland. B. Close‐up of the hiatus concretions. 相似文献
The article presents results of CCD photometry in R-band of a dynamically new Comet C/2001 K5 (LINEAR), obtained at a heliocentric distance of about 5.6 AU, after the perihelion passage. Being so distant from the Sun, this comet was extremely active (Afρ close to 2000 cm), exhibiting quite well developed dust coma and tail. During the observations, general photometric behavior of the comet with heliocentric distance r was well described by the 2.5nlog(r) function with coefficient n=5. The radial profiles of the coma were found to be undulated, with mean slope of the dependence between cometary magnitude and 2.5log of aperture radius (at comet distance) equal to . The light curve of Comet LINEAR exhibited short-term variability which we attributed to cyclic changes of dust emission, induced by nucleus rotation. Model computations by some authors have revealed that active comets can change their spin status quite substantially even during a single orbital revolution. Thus, attempting to search for a rotation frequency, we have modified the classical PDM approach by including the spin acceleration term. Such DynamicalPDM (DPDM) method revealed the most reliable solution for the frequency f0=0.019048±0.000013 h−1 and its first time-derivative (index “zero” denotes reference to the mid time of the whole observing run), indicating a rapid spin-down of the nucleus. These parameters are equivalent to the rotation period of 52.499±0.036 h and its relative increment of 0.02729±0.00013. We present the most probable evolution of the rotation frequency of Comet LINEAR, based on the results of periodicity analysis and a simple, almost parameter independent, dynamical model of nucleus rotation. It is also shown that the DPDM may be an effective tool for determination of a nucleus radius, which provided us with the value of 1.53±0.25 km for Comet LINEAR. 相似文献
In this paper, I present a general discussion of several astrophysical processes likely to play a role in the production of
non-thermal emission in massive stars, with emphasis on massive binaries. Even though the discussion will start in the radio
domain where the non-thermal emission was first detected, the census of physical processes involved in the non-thermal emission
from massive stars shows that many spectral domains are concerned, from the radio to the very high energies. First, the theoretical
aspects of the non-thermal emission from early-type stars will be addressed. The main topics that will be discussed are respectively
the physics of individual stellar winds and their interaction in binary systems, the acceleration of relativistic electrons,
the magnetic field of massive stars, and finally the non-thermal emission processes relevant to the case of massive stars.
Second, this general qualitative discussion will be followed by a more quantitative one, devoted to the most probable scenario
where non-thermal radio emitters are massive binaries. I will show how several stellar, wind and orbital parameters can be
combined in order to make some semi-quantitative predictions on the high-energy counterpart to the non-thermal emission detected
in the radio domain. These theoretical considerations will be followed by a census of results obtained so far, and related
to this topic. These results concern the radio, the visible, the X-ray and the γ-ray domains. Prospects for the very high energy γ-ray emission from massive stars will also be addressed. Two particularly interesting examples—one O-type and one Wolf-Rayet
binary—will be considered in details. Finally, strategies for future developments in this field will be discussed. 相似文献
COMPASS is an on-axis 2.6-m telescope coupled to a correlation polarimeter. The entire instrument was built specifically for CMB polarization studies. Careful attention was given to receiver and optics design, stability of the pointing platform, avoidance of systematic offsets, and development of data analysis techniques. Here we describe the experiment, its strengths and weaknesses, and the various things we have learned that may benefit future efforts to measure the polarization of the CMB. 相似文献
New fission track and Ar/Ar geochronological data provide time constraints on the exhumation history of the Himalayan nappes in the Mandi (Beas valley) – Tso Morari transect of the NW Indian Himalaya. Results from this and previous studies suggest that the SW-directed North Himalayan nappes were emplaced by detachment from the underthrusted upper Indian crust by 55 Ma and metamorphosed by ca. 48–40 Ma. The nappe stack was subsequently exhumed to shallow upper crustal depths (<10 km) by 40–30 Ma in the Tso Morari dome (northern section of the transect) and by 30–20 Ma close to frontal thrusts in the Baralacha La region. From the Oligocene to the present, exhumation continued slowly.Metamorphism started in the High Himalayan nappe prior to the Late Oligocene.High temperatures and anatexis of the subducting upper Indian crust engendered the buoyancy-driven ductile detachment and extrusion of the High Himalayan nappe in the zone of continental collision. Late extrusion of the High Himalayan nappe started about 26 Ma ago, accompanied by ductile extensional shearing in the Zanskar shear zone in its roof between 22 and 19 Ma concomitant with thrusting along the basal Main Central Thrust to the south. The northern part of the nappe was then rapidly exhumed to shallow depth (<10 km) between 20 and 6 Ma, while its southern front reached this depth at 10–5 Ma. 相似文献
The spatial transport of charged particles in the presence of pure slab Alfvén waves, pure isotropic magnetosonic waves and their mixture is considered using Monte Carlo particle simulations. We show that the mean free path of solar cosmic ray protons strongly depends on the assumed spectrum and amplitude of MHD turbulence but much less on the type of the considered waves. It is demonstrated that, for realistic solar wind parameters, the presented wide range wave spectrum models can reproduce the observed mean free path in a particular SEP event but not in a wide range of rigidity characterized by the Palmer's `consensus'. 相似文献
We have analyzed photometric lightcurves of 30 asteroids, and present here the obtained shapes, rotational periods and pole directions. We also present new photometric observations of five asteroids. The shape models indicate the existence of many features of varying degrees of irregularity. Even large main-belt asteroids display such features, so the resulting poles and periods are more consistent than those obtained by simple ellipsoid-like models. In some cases the new rotational parameters are rather different from those obtained previously, and in a few cases there were no proper previous estimates at all. 相似文献
Depth filtration is still one of the most important water treatment processes. It remains difficult to model mathematically because of the complexity of the process and the impact of numerous suspension parameters. The shape of the grains and suspension particles or flocs are among these parameters. The results of theoretical calculations suggest their visible impact on filtration. The shape of the media grain is relatively easy to measure. The shapes of the flocs or suspension particles, which are commonly not perfectly spherical, are much more difficult to predict. However, a few apparatuses developed lasting recent years make this possible. The FPIA 3000, manufactured by Malvern Instruments, uses flow particle image analysis method to measure 26 shape parameters of the suspension particles. Experimental measurement of suspension particle shape after flocculation and after depth filtration has allowed us to answer the question regarding the possible application of this measuring technique in water filtration technology. The experiments have also provided us with detailed information about the shape of flocs and suspension particles in filtered water. High number of particles characterized by circularity lower than 0.8 was detected in flocculated water inflowing to the filter. The particles remained in the outflow was characterized by visibly higher circularity than the particles inflowing to the filter. The results revealed much better removal of the particles characterized by lower circularity during filtration, whereas relation between convexity of the particles and their removal efficiency was not so evident. 相似文献
