Axial convergence in a well-mixed estuary     

R.A. Nunes  J.H. Simpson 《Estuarine, Coastal and Shelf Science》1985,20(5):637-649

Transverse secondary circulations involving surface convergence, observed in a well-mixed estuary in North Wales, are made visible by the collection of surface material along an axial line which extends continuously for many kilometres through the estuary. The circulation and axial convergence, however, are seen only during the flood phase of the tide and no similar behaviour has been observed during the ebb phase.Convergent circulations in the estuary are associated with small but steady transverse density gradients in the cross-section, produced by non-uniform advection of the longitudinal gradient through the channel. A diagnostic model, using measured mean distributions of cross-sectional density, indicates surface transverse velocities (～0.1 ms^?1) similar to those observed in the estuary. The model further predicts appreciable transverse divergent currents at a fractional depth of 0.75: a prediction which has been tested in the estuary using a vertical array of accurately resolving current direction indicators.  相似文献   

Trimpi perturbations from large ionisation enhancement patches     

《Journal of Atmospheric and Solar》2000,62(3):189-206

A number of increasingly sophisticated and realistic models have been developed in order to investigate the interaction between sub-ionospherically propagating VLF waves and regions of ionisation enhancement (LIE¹) in the D-region caused by lightning-induced electron precipitation enhancements (LEP). This LEP-produced LIE can result in phase and amplitude perturbations on received VLF radio signals that are referred to as Trimpis or more precisely, classic Trimpis, to distinguish them from “early/fast Trimpis” or “VLF sprites” which are not caused by LEP and are not considered here. It is important, for comparison with experimentally observed Trimpi effects, that the spatial extent of the D-region electron density (N_e) perturbation is modeled accurately. Here, it is argued that most previous modeling has used patch (LIE) sizes that are typically up to 100 km in both latitudinal and longitudinal extent, which are generally smaller than those that actually occur for real lightning induced electron precipitation events. It would also appear that maximum ΔN_e values assumed have often been too large, and the patches (LIEs) have been incorrectly modelled as circular rather than elliptical in horizontal extent. Consequently, in the present work, Trimpi perturbations are determined for LIEs with smaller maximum ΔN_e, larger spatial extent and elliptical shape. Calculations of VLF Trimpis have been made as a function of the horizontal coordinates of the LIE centre, over the whole rectangular corridor linking transmitter and receiver. The Trimpi modelling program is fully 3D, and takes account of modal mixing at the LIE. The underlying theory assumes weak Born scattering, but the code calculates a non-Born skin depth attenuation function for the LIE in question. The LIE is modelled as an electron density enhancement with a Gaussian profile in all coordinates. Results for a large elliptical LIE ∼ 200 × 600 km show that significant Trimpis, ∼−0.4 dB in amplitude and ∼+4° in phase are predicted, using modest maximum ΔN_e values ∼ 1.5 el/cc. Such an electron density enhancement is well within the range that would be expected to result from experimentally observed fluxes of electron precipitation following wave particle interactions with whistler-mode waves.  相似文献   

    

《International Journal of Coal Geology》1988,9(4):399-400

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《Journal of Structural Geology》1987,9(2):221-232

Pressure solution has caused substantial volume redistribution within the Purgatory Conglomerate from Rhode Island. Material has been removed from quartzite cobble surfaces parallel to the fold axes and mostly redeposited as fibrous pressure shadows at the long axis terminations of the cobbles. In the hinges of folds, 23% of the mean cobble volume has been removed, and in highly deformed and overturned fold limbs up to 55% volume reduction has occurred. The initial cobble shape and orientation can be measured at an undeformed locality and the deformation path can be easily deduced; thus these are real cobble volume reductions. Apparent volume losses (initial shapes not removed) range from 70% to 89%. Real strains for cobbles (axial ratios ranging from 1:0.65:0.38 to 1:0.47:0.15) have values of e_x, e_y, e_z which range from 0%, −20%, −11% to 0%, −37%, −42%, respectively, depending on structural position. The conglomerate itself has been extended parallel to the fold axes (e_r), but the extension is not recorded by the cobble shapes.  相似文献   

    

《Quaternary Science Reviews》1987,6(2):115-128

Human disturbances of ecosystems last a long time and have quantifiable influences on the structure and function of the systems. If long records (e.g. paleoecological) of both disturbances and the responses are available, the array of disturbances provides quasi-experimental treatments useful for the study of factors which govern ecosystems. This paper examines the paleoecology of a series of lake-drainage basin ecosystems that have been subject to disturbances which vary through time and space. In all cases studied, it has been demonstrated that human activities have increased the movement of materials from the catchment to the lake. Examples in Guatemala, Haiti and Florida demonstrate that the flow of macronutrients (nitrogen and phosphorus) is proportional to human population sizes, and that the flow of inorganic particulates is related to the nature of both the disturbances and the catchment. Lake eutrophication is driven by growing human populations, but the rate of increase can be slowed by activities such as urbanization, which increases siltation. Several tropical ecosystems have recovered from severe disturbances, but the rate of recovery was related to the severity and temporal extent of the disturbances.  相似文献   

    

《Applied Geochemistry》2006,21(1):166-183

The composition of organic matter (OM) in pine vegetation and soil samples from a pine forest which was charred by a wildfire was analyzed using solid-state nuclear magnetic resonance (¹³C NMR) and gas chromatography–mass spectrometry (GC–MS) of solvent extracts to study the effects of thermal alteration on soil organic matter (SOM). The NMR data revealed the presence of unaltered biomolecules (cellulose, proteins) and low contents of aromatic C (15%) in the charred pine wood and cones while the charred soil samples exhibited higher contents of aromatic C (39–56%). The solvent extraction of charred and uncharred plant and soil samples yielded diterpenoids, triterpenoids, steroids, a series of aliphatic lipids, phenols and carbohydrates indicating the predominant input of higher plant OM and minor contributions from microorganisms and/or fauna. The lower yield of solvent extractable aliphatic lipids in the charred samples versus the uncharred samples suggests that these compounds are thermally degraded during a wildfire. Molecular markers for the burning of cellulose (levoglucosan, mannosan, galactosan) were detected in all charred samples. The comparison of charred and uncharred samples allowed the identification of unaltered pine derived biomolecules and their thermal alteration products in the charred samples. Terpenoid and steroid biomolecules were in part altered during incomplete combustion to aromatic, unsaturated and polar derivatives (“pyromolecules”) that still retained the characteristic skeleton of their precursors. Since some of the polar degradation products found in the charred soils can be generated either from thermal or microbial degradation, the aromatic and unsaturated hydrocarbon products are preferred as molecular markers for SOM burning. Ratios of biological precursors to aromatic (diterpenoids) or unsaturated products (steroids) indicate that the cyclic lipids in the pine wood and the soil surface horizon were highly altered. In conclusion, the solvent extractable lipids and carbohydrates in charred SOM are valuable, source-specific molecular markers for the burning of plant biomass and for tracing the biogeochemistry of charred residues in soils.  相似文献   

    

《Quaternary Science Reviews》2005,24(14-15):1655-1671

During the glacial–interglacial transition that began subsequent to the Last Glacial Maximum approximately 21,000 calendar years ago, globally averaged (eustatic) sea-level rose by approximately 120 m as climate warmed to its current (Holocene) state. This rise of relative sea-level (RSL) did not occur smoothly, however, but was characterized by the occurrence of one or more episodes of extremely rapid increase. The most extreme of these events has come to be referred to as meltwater pulse 1a, and was initially identified in the coral based record of RSL history from the island of Barbados in the Caribbean Sea. Although it has usually been assumed that this episode of rapid RSL rise was derivative of a partial collapse of the northern hemisphere ice sheets, it has recently been suggested that this pulse could have originated in a dramatic melt-back of the Antarctic Ice Sheet. In this paper the arguments presented in favour of the southern hemisphere source are revisited in order to assess the plausibility of this alternative scenario.Based upon the analyses presented, it is concluded that the evidence previously provided in support of the southern hemisphere scenario is in fact unable to rule out an entirely northern hemisphere source for the meltwater pulse 1a. Since explicit evidence does exist that both the Laurentide and Fennoscandian ice sheets contributed to this event and that Antarctic ice sheet melting occurred significantly later, the southern hemisphere appears not to have been a prime mover of northern hemisphere events.  相似文献   

Impact of multiple satellite ocean color samplings in a day on assessing phytoplankton dynamics     

ZhongPing Lee  Mingshun Jiang  Curtiss Davis  Nima Pahlevan  Yu-Hwan Ahn  Ronghua Ma 《Ocean Science Journal》2012,47(3):323-329

Ocean-color imagers on conventional polar-orbiting satellites have a revisit time of ??2 days for most regions, which is further reduced if the area is frequently cloudy. The Geostationary Ocean Color Imager (GOCI), the first ocean-color imager on a geostationary satellite, provides measurements 8 times a day, thus significantly improving the frequency of measurements for studies of ocean environments. Here, we use results derived from GOCI measurements over Taihu Lake to demonstrate that the extra sampling can be used to improve the accuracy of statistically averaged longer-term (daily) measurements. Additionally, using numerical simulations, we demonstrate that the coupling of diurnal variations of both biomass and photosynthetic available radiation can improve the accuracy of daily primary production estimates. These results echo that higher sampling frequency can improve our estimates of longer-term dynamics of biogeochemical processes and highlights the value of ocean color measurements from geostationary satellites.  相似文献   

    

《Organic Geochemistry》1986,9(5):245-253

The effects of mineral matter on the pyrolysis behavior of an Australian aliphatic oil shale (Rundle, aromaticity = 0.21) have been studied. Besides confirming previous reports that mineral matter in highly aliphatic shales reduces the yield of oil from pyrolysis, the results clearly show that some mineral matter components such as montmorillonites and poorly ordered aluminosilicates can reduce the ratio of 1-alkenes to internal alkenes in oils. Isomerization is dependent on the type of aluminium coordination in the mineral matter and the tetrahedral aluminium is most efficient at altering alkene ratios. Although there is no evidence that mineral matter operates through a carbocationic mechanism to reduce the oil yield, the results strongly suggest that such a mechanism operates during alkene isomerization.  相似文献   

    

《Cretaceous Research》1986,7(3):227-252

Angiosperms appeared in the coastal plains of eastern North America during the latest Barremian or early Aptian, reaching California by the Albian, or perhaps as early as Aptian. In Alaska, however, they are found in sediments only as old as latest Albian. From their earliest Cretaceous appearance, and well into the Late Cretaceous, angiosperms were most abundant and diverse in coastal and fluvial depositional environments, conifers remaining dominant in well drained and inland soils. Extensive dispersal of pioneering, coastal angiosperms occurred at a time of extensive sea-level fluctuation and went hand in hand with their mid-Cretaceous diversification. The early successes of invading angiosperms can most easily be attributed to their ability to colonise disturbed sedimentary surfaces. It is unlikely that they were initially successful in high salinity or other unfavourable habitats, or that they could compete directly with pre-existing plants for nutrients, pollinators or other resources.  相似文献