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421.
William O. Hobbs Stefan V. Lalonde Rolf D. Vinebrooke Kurt O. Konhauser R. Paul Weidman Mark D. Graham Alexander P. Wolfe 《Journal of Paleolimnology》2010,44(2):613-628
The quality and interpretability of the paleobiological record depends on the preservation of morphological and geochemical
fossils. Siliceous microfossils and sedimentary pigments are often cornerstones in paleoecology, although the microbial and
geochemical processes conducive to their preservation remain poorly constrained. We examined sediments from an alpine lake
in Banff National Park (Alberta, Canada) where diatom frustules are completely dissolved within 50 years of deposition. Diatom
dissolution, silica recycling, and diagenetic alteration of algal pigments were investigated, in conjunction with porewater
geochemistry and microelectrode profiling of the sediment–water interface. Analysis of sediment trap material showed ~90%
of biogenic silica (BSi) production is lost prior to burial. Silica flux calculations, based on dissolved silica (as H4SiO4) in pore-waters, show a further ~6% of total BSi is returned to the water column from the upper 4 cm of sediments, implying
that only ~4% of total BSi is permanently archived in sediments. In situ sediment pH and O2 profiles reveal that aerobic respiration by bacteria fully consumes oxygen by a depth of 4 mm into the sediment, with associated
strong pH and redox gradients. During sedimentation and early diagenesis, diatoms undergo loss of extracellular polymeric
substances that coat their frustules, promoting silica dissolution and leading to the loss of the microfossil record by a
depth of 3.25 cm. Sedimentary pigments similarly undergo rapid degradation, but diatom-related carotenoids persist below the
depth of silica dissolution. This work provides new insights on diagenetic processes in lakes, with broad implications for
the interpretation of sedimentary proxies for algal production. 相似文献
422.
The freshwater budget of a tidal flat area is evaluated from long-term hydrographic time series from an observation pole positioned in a tidal channel in the Hörnum Basin (Germany). For each tidal cycle, the freshwater budget is calculated from the total imported and exported water volumes and the corresponding mean densities. The variability of the budget on a tidal scale is characterised by a period of twice the tidal period, exhibiting a minimum when the tidal flats are dry around daylight hours during the foregoing low tide, and a maximum when low tide occurs at night; enhanced evaporation on the flats at daylight hours is identified as the driving process. On the average over one year, while winter observations are missing, the freshwater budget is negative for the years 2002–2005 and positive only for 2006. The interannual mean is negative and amounts to a freshwater loss of about 2 mm day−1, although the large-scale climate in this region is humid. The results demonstrate that the bulk parametrisations for the latent and sensible heat flux between the ocean and the atmosphere must not be applied for the tidelands. 相似文献
423.
John W. Morse Dwight K. Gledhill Karen S. Sell Rolf S. Arvidson 《Aquatic Geochemistry》2002,8(1):3-13
In sediments from the continental slope of the Northern Gulf of Mexico, generally,the degree of iron pyritization (DOP) is low (<0.1) and dissolved sulfide is belowdetection limits (5 M), whereas dissolved Fe is typically about 50 to100 M. Therefore, the dissolution of kinetically reactive iron minerals generallydominates over the rate of sulfide production in sediments throughout this region.However, in sediments where hydrocarbons have been added via seepage from thesubsurface, dissolved-Fe is undetectable, DOP can approach 1, and high concentrationsof dissolved sulfide (up to 11 mM) are commonly present. Even though thesesediments have high total reduced sulfide (TRS) concentrations (typically 150 to370 mol gdw-1), their average C/S ratio is about 4 times that of normal marine sediments reflecting the major input of hydrocarbons. DOP is significantly (20%) higher when calculated using reactive-Fe extracted by citrate dithionite than by cold 1N HCl. This difference is primarily due to the greater extraction efficiency of the cold HCl method for silicate-Fe. TRS tends to rise to a maximum, and remains close to constant even at high (mM) dissolved sulfide concentrations. These TRS concentrations, therefore, represent the size of the ``kinetically' reactive-Fe pool during early diagenesis. 相似文献
424.
Michael?BauEmail author Rolf?L.?Romer Volker?Lüders Peter?Dulski 《Mineralium Deposita》2003,38(8):992-1008
Fluorite from Mississippi Valley Type (MVT) deposits in the South Pennine Orefield, England, displays significantly different distributions of rare earths and yttrium (REY) compared to fluorite from similar MVT deposits in the North Pennine Orefield. Samples from the South Pennine Orefield display negative Ce and positive Gd and Y anomalies but lack any Eu anomaly, indicating that the REY were mobilized from relatively pure marine sedimentary carbonates. In marked contrast, fluorite from the North Pennine Orefield lacks any Ce and Gd anomalies but shows a pronounced positive Eu anomaly, suggesting that the REY were provided by different source rock(s), that the mineralizing hydrothermal fluid had experienced higher temperatures prior to fluorite precipitation, and that it was derived from deeper crustal levels in the north compared to the south. The isotopic composition of Sr in Blue John fluorite from the South Pennine Orefield suggests that Sr was mobilized from Lower Carboniferous (Tournaisian) limestones, whereas Pb isotopes suggest that in contrast to REY and Sr, Pb was derived from aluminosilicate rocks. Neither Nd nor Sr or Pb isotopes can be used to radiometrically date the formation of Blue John fluorite. All isotope systems studied indicate that the limestone host rock of this fluorite mineralization did not contribute to the trace element budget of the hydrothermal fluid. Our results show that different solutes in a natural water (hydrothermal fluid, groundwater, etc.) may be derived from different sources, and that the study of a small set of elements or isotope ratios may not provide full insight into the genesis or history of a mineralization or a hydrothermal fluid. Our data provide evidence for the uncoupling of Sr, Nd and Pb during fluid-rock interaction and fluid migration, and show that the use of plots such as 87Sr/86Sr vs. Nd. to learn about mixing relationships (as is commonly done in igneous geochemistry) is unreliable when applied to natural waters and their precipitates.Editorial handling: B. Lehmann 相似文献
425.
We analyzed 17 fragments from a zoned allanite–epidote crystal (ca 2.2 mm × 4.0 mm), which had formed during different prograde and retrograde stages of ultra high pressure (UHP) and amphibolite facies metamorphism (240–230 Ma, Sulu Belt, E China), for the isotopic composition of Pb, Nd, and Sr and contents of Pb, U, and Th, Sr and Rb, and Nd and Sm. Since most fragments had 238U/204Pb and 232Th/204Pb values less than 1, corrections for in situ Pb growth are small and uncertainties in the recalculation of the Pb isotopic compositions to 240 Ma are insignificant. The recalculated Pb falls on a linear trend in the 206Pb/204Pb vs 207Pb/204Pb diagram with the allanite defining the low–206Pb/204Pb end (17.07) of this trend and the epidote defining its high–206Pb/204Pb end (17.56). The recalculated data scatter in the 206Pb/204Pb vs 208Pb/204Pb diagram, which implies that the initial Pb isotopic variation reflects the involvement of at least three different Pb sources. The low 87Rb/86Sr values account for a change in 87Sr/86Sr by in situ 87Sr growth of less than 0.0007, which implies that the isotopic heterogeneity of 87Sr/86Sr (0.70601–0.7200) is a primary feature. The Pb and Sr isotope data unequivocally demonstrate that contributions from different precursor minerals result in initial isotopic heterogeneity in the metamorphic reaction product. It is likely that such an initial isotopic heterogeneity also exists for Nd, but it could not be resolved in the present study. Initially heterogeneous Pb and Sr isotope compositions imply that age differences between core and rim of large crystals may result in the determination of highly arbitrary geological rates, especially for minerals with relatively low parent-to-daughter ratios. 相似文献
426.
Eddy Covariance Measurements On Mountain Slopes: The Advantage Of Surface-Normal Sensor Orientation Over A Vertical Set-Up 总被引:1,自引:1,他引:0
The measurement of scalar fluxes employing the eddy covariance method is a widely used experimental approach,for which the flow distortion due to obstacles (e.g., sensor mounts and mast)is a well-known but not fully solved problem. In order to reduce flow distortion we installed a sonic anemometer in a surface-normal orientationrelative to the terrain slope, and a second instrument in a verticalposition at a horizontal distance of 1.54 m from the first instrumentWe found a significant reduction in the rotation angle necessary for the coordinaterotation procedure in the x-z plane whencomputing 30-minute flux averages with the surface-normal orientation. In 91% of all cases this rotation angleremained within the angle of incidence of ±10° recommended bythe manufacturer. In contrast, only 24% of the measurements taken with the vertically mounted anemometer were obtained at an angle of incidencewithin ±10°, and 3% were outside the ±30° range specified for an acceptable operation.A data quality test based on the variance of vertical windspeed normalized with friction velocity (w/u*) revealed problems for application under stable conditions due to large uncertainties in the determination of the Monin–Obukhov stability parameter z/L. An alternative test using the bulk drag coefficient CD revealed other problems related to the dependence of CD on z/z0, the measuring height normalized by the roughness length, which do not appear to be constantin complex terrain. With both tests, a tendency for a slightly improved dataquality was found for the surface normal set-up, which, however, proved statistically insignificant.It is concluded that the surface-normal set-up of a sonic anemometer significantly reduces flow distortion by thesensor head. Although the surface-normal mounting position therefore appears to be the preferred one, with decreased flow distortion and a slightly improved data quality, no significant differences in turbulent quantities were found between the two set-uppositions. Hence, the consequences for short-term measurements of massand energy fluxes with a surface-normal set-up in complex terrain appearto be relevant only if single flux events are to be inspected, while for long-term measurements of integrated fluxes both the surface-normaland vertical installation of the sonic anemometer are adequate,indicating that eddy covariance measurements in complex terrain are lessdelicate than expected. 相似文献
427.
Rolf Amundsen 《Planetary and Space Science》1973,21(11):1863-1874
The pitch-angle distributions in and near the loss cone, of ~ (100–200) and ~ (200–350) keV protons observed by the ESRO IB satellite during the period 7–15 October 1969 are presented. The data include periods of relative quiet as well as more disturbed geomagnetic conditions. Spatial characteristics and dynamics of the protons, both on the night-and dayside of the Earth are described. The actual pitch-angle distribution is interpreted as produced by wave-particle interactions, and the diffusion coefficient and lifetime against pitch angle scattering have been estimated from existing theories. During slightly disturbed conditions, the observations suggest an average random walk in pitch angle made by a particle during a crossing of the diffusion region of about one half of the loss cone half angle for 4 ? L ? 6. The lifetime against pitch angle scattering into the loss cone is found to be somewhat less than the charge exchange lifetime for these (100–350) keV protons. The spectral density of interacting waves is tentatively estimated to about 0·1 , and compares with estimates arrived at from completely different approaches. 相似文献
428.
Chironomids typical of cold, well-oxygenated, oligotrophic environments are common in late-Pleistocene deposits, but these taxa are rare in Holocene sediments of most small temperate lakes. Hypotheses to explain the demise of these taxa include variations in climate, lake trophic state, lake levels, terrestrial vegetation, and/or sediment composition. In southwestern British Columbia, this demise correlates with palynological evidence for a lodgepole pine decline, and for rapid climatic amelioration, at about 10 000 yr B.P. Faunal changes are poorly correlated with lithological boundaries. The similar timing of the declines among lakes suggests that a regional influence, climate, has possibly been the principal determinant of early chironomid faunal succession.This is the eleventh of a series of papers to be published by this journal that was presented in the paleolimnology sessions organized by R.B. Davis for the XIIth Congress of the International Union for Quaternary Research (INQUA), which took place in Ottawa, Canada in August 1987. Dr. Davis is serving as guest editor of this series. 相似文献
429.
Juliane Ramelow Ulrich Riller Rolf L. Romer Onno Oncken 《International Journal of Earth Sciences》2006,95(3):529-541
A combined geochronological and structural analysis of the Miocene Negra Muerta Caldera was designed to better understand caldera formation associated with prominent faults on the central Andean plateau. Rb–Sr ages of the caldera outflow facies indicate that caldera formation occurred in two volcano-tectonic episodes. The first episode commenced with explosive eruption of the 9.0±0.1 Ma andesitic Acay Ignimbrite followed by a period of volcanic quiescence and moderate tectonic activity. Dominant volcanic and tectonic activity occurred during the second episode, which is bracketed by eruption of the 7.6±0.1 Ma rhyolitic Toba 1 Ignimbrite and effusive discharge of the 7.3±0.1 Ma rhyodacitic to andesitic lava flows. Structural relationships between rocks of the Negra Muerta Volcanic Complex and collapse-induced normal faults, notably NE-striking normal faults, agree with simultaneous volcanic activity and floor subsidence of the caldera during the second episode. Floor subsidence was achieved by tilting on an outward dipping reverse fault to the northwest of the caldera floor around a hinge zone located south of the caldera floor. This induced horizontal extension of the caldera floor and was accomplished by fragmentation of, and intrusion of dikes into, the floor. Collapse-induced and post-collapse fault populations of the caldera do not differ significantly in the directions of their axes of maximum extension and are in this respect kinematically compatible with left-lateral slip on the nearby Olacapato-El Toro Fault Zone. This furnishes evidence for a kinematic control by prominent faults on the formation of collapse calderas in the central Andes. The structural analysis of the Negra Muerta Caldera shows that collapse calderas can serve as deformation markers that contribute in elucidating the regional kinematic regime and the time of activity of prominent dislocations genetically related to collapse calderas. 相似文献
430.
Magnesium inhibition of calcite dissolution kinetics 总被引:1,自引:0,他引:1
Rolf S. Arvidson Martin Collier Michael D. Vinson Andreas Luttge 《Geochimica et cosmochimica acta》2006,70(3):583-594
We present evidence of inhibition of calcite dissolution by dissolved magnesium through direct observations of the (104) surface using atomic force microscopy (AFM) and vertical scanning interferometry (VSI). Far from equilibrium, the pattern of magnesium inhibition is dependent on solution composition and specific to surface step geometry. In CO2-free solutions (pH 8.8), dissolved magnesium brings about little inhibition even at concentrations of 0.8 × 10−3 molal. At the same pH, magnesium concentrations of less than 0.05 × 10−3 molal in carbonate-buffered solutions generate significant inhibition, although no changes in surface and etch pit morphology are observed. As concentrations exceed magnesite saturation, the dissolution rate shows little additional decrease; however, selective pinning of step edges results in unique etch pit profiles, seen in both AFM and VSI datasets. Despite the decreases in step velocity, magnesium addition in carbonated solutions also appears to activate the surface by increasing the nucleation rate of new defects. These relationships suggest that the modest depression of the bulk rate measured by VSI reflects a balance between competing reaction mechanisms that simultaneously depress the rate through selective inhibition of step movement, but also enhance reactivity on terraces by lowering the energy barrier to new etch pit formation. 相似文献