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31.
The influence of an internal boundary layer and a roughness sublayer on flux–profile relationships for momentum and sensible heat have been investigated for a closed beech forest canopy with limited fetch conditions. The influence was quantified by derivation of local scaling functions for sensible heat flux and momentum (h and m) and analysed as a function of atmospheric stability and fetch. For heat, the influences of the roughness sublayer and the internal boundary layer were in agreement with previous studies. For momentum, the strong vertical gradient of the flow just above the canopy top for some wind sectors led to an increase in m, a feature that has not previously been observed. For a fetch of 500 m over the beech forest during neutral atmospheric conditions, there is no height range at the site where profiles can be expected to be logarithmic with respect to the local surface. The different influence of the roughness sublayer on h and m is reflected in the aerodynamic resistance for the site. The aerodynamic resistance for sensible heat is considerably smaller than the corresponding value for momentum. 相似文献
32.
The concentration of selected trace metals (Cu, Pb and Zn) in salt-marsh sediments from within the Erme Estuary have been measured in order to assess possible historical sources of pollution. The Erme Estuary, south Devon, UK is an Area of Outstanding Natural Beauty and has remained largely unaffected by industrialisation, although a number of small silver–lead mines were in operation in the 1800s. Five cores reveal comparable geochemical profiles. An increase of lead at 40 cm depth is observed, reaching maximum values of 427 ppm. Less distinct trends are revealed by zinc and copper, probably reflecting the lack of widespread mining for ores of these elements within the catchment and possible post-depositional mobility rendering the metal concentrations non-contemporaneous with the chemostratigraphy of lead. The geochemical analysis of the salt-marsh sediments provides a fairly robust chemostratigraphic scheme and the likely sources of mine waste can be pinpointed within the catchment. Based upon reference to the historical mining record of these mines chemostratigraphic dating of the sediments can be achieved in order to provide an estimate of salt-marsh accretion rates and sea-level rise. 相似文献
33.
Niels Jöns Wolfgang Bach Timothy Schroeder 《Contributions to Mineralogy and Petrology》2009,157(5):625-639
We examined small-scale shear zones in drillcore samples of abyssal peridotites from the Mid-Atlantic Ridge. These shear zones
are associated with veins consisting of chlorite + actinolite/tremolite assemblages, with accessory phases zircon and apatite,
and they are interpreted as altered plagiogranite melt impregnations, which originate from hydrous partial melting of gabbroic
intrusion in an oceanic detachment fault. Ti-in-zircon thermometry yields temperatures around 820°C for the crystallization
of the evolved melt. Reaction path modeling indicates that the alteration assemblage includes serpentine of the adjacent altered
peridotites. Based on the model results, we propose that formation of chlorite occurred at higher temperatures than serpentinization,
thus leading to strain localization around former plagiogranites during alteration. The detachment fault represents a major
pathway for fluids through the oceanic crust, as evidenced by extremely low δ18O of altered plagiogranite veins (+3.0–4.2‰) and adjacent serpentinites (+ 2.6–3.7‰). The uniform oxygen isotope data indicate
that fluid flow in the detachment fault system affected veins and adjacent host serpentinites likewise.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
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36.
The ECMWF has been assimilating Feng-Yun-3B(FY-3B) satellite microwave humidity sounder(MWHS) data over ocean in an operational forecasting system since 24 September 2014. It is more difficult, however, to assimilate microwave observations over land and sea ice than over the open ocean due to higher uncertainties in land surface temperature, surface emissivity and less effective cloud screening. We compare approaches in which the emissivity is retrieved dynamically from MWHS channel 1 [150 GHz(vertical polarization)] with the use of an evolving emissivity atlas from 89 GHz observations from the MWHS onboard NOAA and EUMETSAT satellites. The assimilation of the additional data over land improves the fit of short-range forecasts to other observations, notably ATMS(Advanced Technology Microwave Sounder) humidity channels, and the forecast impacts are mainly neutral to slightly positive over the first five days. The forecast impacts are better in boreal summer and the Southern Hemisphere. These results suggest that the techniques tested allow for effective assimilation of MWHS/FY-3B data over land. 相似文献
37.
Magnetostratigraphy of Palaeocene basalts from the Vaigat Formation of West Greenland 总被引:1,自引:0,他引:1
A palaeomagnetic study comprising the directional results from 289 individual lava flows, sampled along eight sections in the Palaeocene basalts of West Greenland, is reported. The eight individual sections are correlated using lithostratigraphical marker horizons to form a single composite profile. Generally, the lithological correlation is in good agreement with the record of geomagnetic secular variation.
The total composite palaeomagnetic profile represents a stratigraphic thickness of 1.6 km through the Vaigat Formation, which is the lowermost of the two volcanic formations formed during the main stage of plateau volcanism. Only two polarity zones are found in the composite profile, suggesting a very short duration for the West Greenland main plateau-building volcanism.40 Ar/39 Ar dates support a high extrusion rate and also indicate that the lower normal polarity zone is Chron C27n and that the upper reverse polarity zone is Chron C26r.
The C27n–C26r transition is fully recorded along one of the sections (Nuusap Qaqqarsua), with intermediate directions covering a 200 m thick succession of lavas. A combined palaeomagnetic, field and geochemical study along this profile showed good agreement; that is, geochemically and geologically derived single magmatic events show groupings of the palaeomagnetic directions. Supposing a duration for the geomagnetic transition of 5000 years, the eruption frequency during this period was as high as one flow every 80 years. 相似文献
The total composite palaeomagnetic profile represents a stratigraphic thickness of 1.6 km through the Vaigat Formation, which is the lowermost of the two volcanic formations formed during the main stage of plateau volcanism. Only two polarity zones are found in the composite profile, suggesting a very short duration for the West Greenland main plateau-building volcanism.
The C27n–C26r transition is fully recorded along one of the sections (Nuusap Qaqqarsua), with intermediate directions covering a 200 m thick succession of lavas. A combined palaeomagnetic, field and geochemical study along this profile showed good agreement; that is, geochemically and geologically derived single magmatic events show groupings of the palaeomagnetic directions. Supposing a duration for the geomagnetic transition of 5000 years, the eruption frequency during this period was as high as one flow every 80 years. 相似文献
38.
Effects of salinity on NH4
+ adsorption capacity, nitrification, and denitrification in Danish estuarine sediments 总被引:1,自引:0,他引:1
Søren Rysgaard Peter Thastum Tage Dalsgaard Peter Bondo Christensen Niels P. Sloth 《Estuaries and Coasts》1999,22(1):21-30
The regulatory effect of salinity on nitrogen dynamics in estuarine sediments was investigated in the Randers Fjord estuary, Denmark, using sediment slurries and intact sediment cores and applying 15N-isotope techniques. Sediment was sampled at three representative stations varying in salinity, and all experiments were run at 0‰, 10‰, 20‰, and 30‰. The sediment NH4 + adsorption capacity decreased markedly at all stations when salinity was increased from 0‰ to 10‰; further increase showed little effect. In situ nitrification and denitrification also decreased with increasing salinities, with the most pronounced reduction of approximately 50% being observed when the salinity was raised from 0‰ to 10‰. The salinity-induced reduction in NH4 + adsorption capacity and stimulation of NH4 + efflux has previously been argued to cause a reduction in nitrification activity since the nitrifying bacteria become limited by NH4 + availability at higher salinities. However, using a potential nitrification assay where NH4 + was added in excess, it was demonstrated that potential nitrification activity also decreased with increasing salinity, indicating that the inhibitory salinity effect may also be a physiological effect on the microorganisms. This hypothesis was supported by the finding that denitrification based on NO3 − from the overlying water (Dw), which is independent of the nitrification process, and hence NH4 + availability, also decreased with increasing salinity. We conclude that changes in salinity have a significant effect on nitrogen dynamics in estuarine sediments, which must be considered when nitrogen transformations are measured and evaluated. 相似文献
39.
Chia-Yu Chen Sean D. Willett A. Joshua West Simon Dadson Niels Hovius Marcus Christl J. Bruce H. Shyu 《地球表面变化过程与地形》2020,45(3):548-564
Extreme erosion events can produce large short-term sediment fluxes. Such events complicate erosion rates estimated from cosmogenic nuclide concentrations in river sediment by providing sediment with a concentration different from the long-term basin average. We present a detrital 10Be study in southern Taiwan, with multiple samples obtained in a time sequence bracketing the 2009 Typhoon Morakot, to assess the impact of landslide sediment on 10Be concentrations (N10Be) in river sediment. Sediment samples were collected from 13 major basins, two or three times over the last decade, to observe the temporal variation of N10Be. Landslide inventories with time intervals of 5–6 years were used to quantify sediment flux changes. A negative correlation between N10Be and landslide areal density indicates dilution of N10Be by landslide sediment. Denudation rates estimated from the diluted N10Be can be up to three times higher than the lowest rate derived from the same basins. Observed increases imply that, 3 years after the passage of Typhoon Morakot, fluvial channels still contain a considerable amount of sediment provided by hillslope landslides during the event. However, higher N10Be in 2016 samples indicate that the contribution from landslide sediment at the sampled grain size has decreased with time. The correlation between changes in N10Be and landslide area and volume is not strong, likely resulting from the stochastic and complex nature of sediment transport. To simultaneously evaluate the volume of landslide-derived sediment and estimate the background denudation rate, associated with less impulsive sediment supply, we constructed a sediment-mixing model with the time series of N10Be and landslide inventories. The spatial pattern of background erosion rate in southern Taiwan is consistent with the regional tectonic framework, indicating that the landscape is evolving mainly in response to the tectonic forcing, and this signal is modified, but not obscured by impulsive sediment supply. © 2019 John Wiley & Sons, Ltd. 相似文献
40.
Josh A. Calkins George Zandt James Girardi Ken Dueker George E. Gehrels Mihai N. Ducea 《Earth and Planetary Science Letters》2010,289(1-2):145-155
The late Triassic to early Tertiary Coast Mountains Batholith (CMB) of British Columbia provides an ideal locale to study the processes whereby accreted terranes and subduction-related melts interact to form stable continental crust of intermediate to felsic composition and complementary ultramafic residuals. Seismic measurements, combined with calculated elastic properties of various CMB rock compositions, provide a window into the deep-crustal lithologies that are key to understanding the processes of continental growth and evolution. We use a combination of seismic observations and petrologic modeling to construct hypothetical crustal sections at representative locations across the CMB, then test the viability of these sections via forward modeling with synthetic seismic data. The compositions that make up our petrologic forward models are based on calculations using the free energy minimization program Perple_X to predict mineral assemblages at depth for the bulk compositions of exposed plutonic rocks collected in the study area. Seismic data were collected along two transects in west-central British Columbia: a southern line that crossed the CMB near the town of Bella Coola (near 52° N), and a northern line centered on the towns of Terrace and Kitimat (near 54° N). Along both transects, seismic receiver functions reveal high Vp/Vs ratios near the Insular/Intermontane terrane boundary and crustal thickness increasing from 26 ± 3 km to 34 ± 3 km (at the 1 sigma certainty level) from west to east across the Coast Shear Zone (CSZ). On the southern line, we observe an anomalous region of complex receiver functions and diminished Moho signals beneath the central portion of the CMB. Our petrologic and seismic profiles show that observed seismic data from much of the CMB can be well-matched in terms of crustal thickness and structure, average Vp/Vs, and amplitude of the Moho converted phase, without including ultramafic residual material in the lower crust. 相似文献