Close interval sampling of interstitial silicate and porosity in marine sediments     

Daniel Andrews  Barry T. Hargrave 《Geochimica et cosmochimica acta》1984,48(4):711-722

A method was developed to measure porosity and dissolved interstitial silicate at millimeter intervals or less in a sediment core. In cores from Emerald Basin (Scotian Shelf), interstitial concentrations near the sediment surface did not drop rapidly to bottom-water concentrations as measured in bottle casts (28 μM) but remained as high as 166 μM in the upper 0.5 mm of sediment High rates of benthic silicate release were measured which could not be accounted for by interstitial concentration gradients or by ventilation of macro-invertebrate burrows. The silicate discontinuity observed between the sediments and water column suggests that a diffusive sublayer exists in a zone of viscous flow above the sediment surface. This is possible only if a surface reaction is primarily responsible for silicate release. By assuming a linear concentration gradient across this diffusive sublayer, the silicate release rates were used to estimate the thickness of the sublayer to be about 2 mm.  相似文献   

Reply to A. S. Dyke's discussion     

J.T. Andrews  W.W. Shilts  G.H. Miller 《Quaternary Research》1984,22(2):253-258

In our opinion the amino acid data are consistent with the till/nonglacial stratigraphy. We reject Dyke's proposal that the plotting of data in 0.04 increments is appropriate as an unwarranted interpretation that errors are cumulative. We also see no grounds for accepting his alternative interpretation that the groups of amino acid ratios reflect various transport (read temperature) histories of a single population of Bell Sea shells. It is our opinion that the relative sequence of marine incursions in Hudson Bay is reliable and we repeat that the evidence favors one or more deglacial events. We stress that the ages of the units between the Tyrrell Sea and Bell sea end membrers are interpolated and that the chronology of events is currently based on the assumption that the Bell Sea represents marine incursion at the onset of marine isotope stage 5.Dyke has raised a number of points which have concerned us since we started our joint research on the aminostratigraphy of the Hudson Bay Lowlands. The answer to many questions will come, not from the amino acid results per se, but from detailed litho-, and biostratigraphic logging of the thousands of kilometers exposed along the large rivers that drain into James Bay and southwestern Hudson Bay. This work is presently going on. Let us say in conclusion that analysis of a further 63 shells and shell fragments resulted in a virtually identical frequency distribution to that discussed in our paper. We are currently evaluating the stratigraphic integrity of these results. Field expeditions by the Geological Survey of Canada in 1982 and 1983 into the Hudson Bay Lowlands were specifically designed to log new sections and make additional shell collections. We hope to report on these new data in due course.  相似文献   

Decadal to millennial-scale periodicities in North Iceland shelf sediments over the last 12 000 cal yr: long-term North Atlantic oceanographic variability and solar forcing     

John T. Andrews  Jorunn Hardadottir  Joseph S. Stoner  Michael E. Mann  Greta B. Kristjansdottir  Nalan Koc 《Earth and Planetary Science Letters》2003,210(3-4):453-465

Giant piston core MD99-2269 recovered 25 m of sediment in Hunáfloáall, a deep trough on the North Iceland margin fronting the Iceland Sea, and the site of a shelf sediment drift. The rate of sediment accumulation is 2 m/kyr (5 yr/cm); the core terminated in the Vedde tephra (12 cal ka). The sediment was sampled at between 5 and 50 yr/sample, including rock magnetic, grain-size, and sediment properties. Data reduction was carried out using principal component analysis. Two PC axes for the 5-yr/sample magnetic data are strongly correlated with measures of coercivity (ARM_{20 mT}/ARM) and magnetic concentrations (kARM). In turn ARM_{20 mT}/ARM is highly correlated (negatively) with grain-size and the mean size of the sortable silt fraction. Analyses of the two PC axes with MTM spectral methods indicate a series of significant (>99%) periodicities at millennial to multidecadal scales, including those at 200, 125, and 88 yr which are associated with solar variability. We also document a strong correlation between the sediment magnetic properties and the ∂¹⁸O on benthic foraminifera on the North Iceland inner shelf. We hypothesize that the links between variations in grain-size, magnetic concentrations, and solar forcing are controlled by atmospheric and oceanographic changes linked to changes in the relative advection of Atlantic and polar waters along the North Iceland margin. Today these changes are associated with variations in the deep convection in the Greenland and Iceland Seas. The precise linkages are, however, presently elusive although a combination of coarser sediments and low ∂¹⁸O values define a Holocene thermal maximum between 8 and 6 cal ka.  相似文献   

促进地震研究成果的应用     

安德鲁斯 《国际地震动态》1999,(5):23-26

加利福尼亚人生活在一个多姿多彩的世界。我们的家园面临经常的变动和永恒的挑战。宏伟的逆冲山脉、深蓝色的河谷、起伏的沙漠、湖泊与河流以及太平洋海岸为背景不同、兴趣各异的人们提供了天然的栖息之地。对于那些厌倦可预报的天气,偶尔被绵延起伏的草原隔断的平原,看...  相似文献   

Containment of Sources of Groundwater Contamination: Analysis of Mass Fluxes     

Christopher J. Neville  Charles B. Andrews 《Ground water》2020,58(2):183-188

Cutoff walls and liners are used frequently as barriers to isolate contaminants at both controlled and uncontrolled hazardous waste sites. Neville and Andrews (2006) presented a containment criterion for contaminant isolation by a barrier. The analysis of Neville and Andrews (2006) yields the inward Darcy flux that balances the diffusive mass flux from the source so that the net mass flux is zero. A requirement of zero net mass flux may not be achievable in all situations. The analysis developed by Neville and Andrews (2006) is extended to develop straightforward expressions for the long-term mass fluxes across a barrier for any conditions. In cases where it may not be possible to satisfy a criterion of zero net mass flux, the results from an exact solution for transient solute transport are used to show how the mass fluxes evolve to their long-term values.  相似文献   

A Search for CMEs Associated with Big Flares     

Andrews  M.D. 《Solar physics》2003,218(1-2):261-279

Solar Physics - The relationship between flares and coronal mass ejections (CMEs) remains a topic of active research. This paper considers a complete set of 311 M- and X-class GOES soft X-ray...  相似文献   

Night-time radial plasma drifts and coupling fluxes at L = 2.3 from whistler mode measurements     

M.K. Andrews 《Planetary and Space Science》1980,28(4):407-417

A method recently reported for measuring radial drifts in the equatorial plane and ionosphere-magnetosphere coupling fluxes from the Doppler shifts and group delays on whistler mode signals is applied to VLF transmissions from station NLK on 18.6 kHz. Data from 22 nights, primarily during the months November to February, are analysed. When averaged over a time of about 90 min, drifts found are accurate to ±20 ms^?1, corresponding to an equatorial electric field accuracy of ±0.05 mVm^?1 and fluxes, to ±1.5 × 10¹² el m^?2 s^?1 (two hemisphere total). Given currently accepted values of coupling fluxes, the flux accuracy is of marginal value on individual nights, but useful information on average behaviour may be obtained.We find fluxes generally contribute less than 20% to the measured Doppler shift, most of which is therefore produced by cross-L drifts. To an accuracy of about 20% then, Doppler data alone may give information on these drifts. Doppler shift data previously accumulated over a number of years and relating to signals in ducts near L = 2.3 are re-examined. Dominating the nightly behaviour is an inward drift which reaches a maximum of ～ 100 ms^?1 as the duct ends cross the dusk terminator and an outward drift at dawn of the same magnitude which is initiated when the duct end crosses the terminator in the E or lower F-region. In some months, separate effects can be seen corresponding to sunrise at each end of the duct.During the night, there are clear differences between December and June solstice drift behaviour which are as yet unexplained.Two-hemisphere-total coupling fluxes found from the 22 nights of data are ～ 1?2 × 10¹² elm^?2s^?1. At the time of the evening maximum of inward drift the flow is to the ionosphere, but later in the night upfluxes were measured.  相似文献   

Wavelike irregularities in the mid-latitude 6300 Å airglow     

M.K. Andrews 《Planetary and Space Science》1976,24(6):521-527

The operation of a sky-scanning photometer at mid-latitudes has revealed the presence of elongated irregularities in the 6300 Å airglow, which can have a wavelike nature. Wavefronts are aligned near the magnetic meridian, and the motion is to the east with speeds ～100 m/sec. One airglow event in which the ionosphere appeared to corrugate produced a period of moderate spread-F on a nearby ionosonde.It is not clear from comparison with radio experiments that the airglow disturbances are due to the passage in the ionosphere of gravity waves; some characteristics suggest the irregularities could be due to an instability of the ionospheric plasma.  相似文献   

Numerical Modelling of Tectonic Flow behind Island Arcs   总被引：2，自引：0，他引：2  

D. J. Andrews  N. H. Sleep 《Geophysical Journal International》1974,38(2):237-251

  相似文献   

Palaeoenvironmental records in Holocene Spanish tufas: a stable isotope approach in search of reliable climatic archives   总被引：3，自引：0，他引：3  

Julian E. Andrews  Martyn Pedley  & Paul F. Dennis 《Sedimentology》2000,47(5):961-978

Tufa deposits are potential terrestrial archives of palaeoenvironmental and palaeoclimatic information. This study assesses the potential of stable isotopic archives from two closely juxtaposed Holocene tufa sites in SE Spain. The Ruidera site contains deep‐water lacustrine micrites and tufas, whereas the nearby Alcaraz site represents a shallow barrage tufa. Understanding site characteristics is critical to interpreting the stable isotopic variations. These Holocene lacustrine micrites have isotopic compositions consistent with modern European lake shore microbial carbonates, where the isotopic chemistry is strongly influenced by hydrological and residence time effects. All the lacustrine micrite δ¹³C values were influenced by microenvironmental microbial effects to some degree. Because of these effects, stable isotope data from lacustrine microbial micrites and tufas will not normally yield precise information on the isotopic composition of palaeoprecipitation, temperature or vegetation composition of an area. In contrast, Holocene tufas that formed in shallow, fast‐flowing riverine settings may contain valuable palaeoclimatic archives. The tufa deposits must be largely autochthonous, as at Alcaraz, where in situ reed stem encrustations are present. Records of relative change in air temperature and changes in the source of airmasses are potentially resolvable in the δ¹⁸O data. These interpretations can be verified by other independent climatic data where chronology is constrained. Variations in riverine tufa δ¹³C values probably record changes in local vegetation and/or soil respiration. Covariation between δ¹⁸O and δ¹³C values may be intrinsically linked to climatic factors such as aridity. Tentative palaeoclimatic interpretations for the middle Holocene at Alcaraz based on the isotope data suggest warming (or increasing influence of Mediterranean‐sourced precipitation) between approximately 5000–3000 radiocarbon years BP, accompanied by increased aridity. These interpretations are consistent with the sparse independent palaeoclimatic data and climate modelling results for the Holocene of SE Spain. This study supports the growing evidence that well‐chosen tufa sites could yield valuable palaeoclimatic information.  相似文献