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141.
We used statistical analyses to determine which subset of 36 environmental variables best explained variations in surface
sediment δ13C and δ15N from 50 lakes in western Ireland that span a human-impact gradient. The factors controlling lake sediment δ13C and δ15N depended on whether organics in the lake sediment were mostly derived from the lake catchment (allochthonous) or from productivity
within the lake (autochthonous). Lake sediments with a dominantly allochthonous organic source (high C:N ratio sediments)
produced δ13C and δ15N measurements similar to values from catchment vegetation. δ13C and δ15N measurements from lake sediments with a dominantly autochthonous organic source (low C:N ratio sediments) were influenced
by fractionation in the lake and catchment leading up to assimilation of carbon and nitrogen by lacustrine biota. δ13C values from lake sediment samples in agricultural catchments were more negative than δ13C values from lake sediment samples in non-impacted, bogland catchments. Hypolimnetic oxygen concentrations and methane production
had a greater influence on δ13C values than fractionation due to algal productivity. δ15N from lake sediment samples in agricultural catchments were more positive than δ15N in non-impacted bogland catchments. Lower δ15N values from non-impacted lake catchments reflected δ15N values of catchment vegetation, while higher δ15N values in agricultural catchments reflected the high δ15N values of cattle manure and inorganic fertilisers. The influence of changing nitrogen sources and lake/catchment fractionation
processes were more important than early diagenesis for lake sediment δ15N values in this dataset. The results from this study suggest a possible influence of bound inorganic nitrogen on the bulk
sediment δ15N values. We recommend using a suitable method to control for bound inorganic nitrogen in lake sediments, especially when
working with clay-rich sediments. This study confirms the usefulness of δ13C and δ15N from bulk lake sediments, as long as we are mindful of the multiple factors that can influence these values. This study
also highlights how stable isotope datasets from lake surface sediments can complement site-specific isotope source/process
studies and help identify key processes controlling lake sediment δ13C and δ15N in a study area. 相似文献
142.
Andrei M. Sarna-Wojcicki Charles E. Meyer Harry R. Bowman N. Timothy Hall Paul C. Russell Marta J. Woodward Janet L. Slate 《Quaternary Research》1985,23(2):236-257
Outcrops of an ash bed at several localities in northern California and western Nevada belong to a single air-fall ash layer, the informally named Rockland ash bed, dated at about 400,000 yr B.P. The informal Rockland pumice tuff breccia, a thick, coarse, compound tephra deposit southwest of Lassen Peak in northeastern California, is the near-source equivalent of the Rockland ash bed. Relations between initial thickness of the Rockland ash bed and distances to eruptive source suggest that the eruption was at least as great as that of the Mazama ash from Crater Lake, Oregon. Identification of the Rockland tephra allows temporal correlation of associated middle Pleistocene strata of diverse facies in separate depositional basins. Specifically, marine, littoral, estuarine, and fluvial strata of the Hookton and type Merced formations correlate with fluvial strata of the Santa Clara Formation and unnamed alluvium of Willits Valley and the Hollister area, in northwestern and west-central California, and with lacustrine beds of Mohawk Valley, fluvial deposits of the Red Bluff Formation of the eastern Sacramento Valley, and fluvial and glaciofluvial deposits of Fales Hot Spring, Carson City, and Washoe Valley areas in northeastern California and western Nevada. Stratigraphic relations of the Rockland ash bed and older tephra layers in the Great Valley and near San Francisco suggest that the southern Great Valley emerged above sea level about 2 my ago, that its southerly outlet to the ocean was closed sometime after about 2 my ago, and that drainage from the Great Valley to the ocean was established near the present, northerly outlet in the vicinity of San Francisco Bay about 0.6 my ago. 相似文献
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149.
D. J. Woodward 《Geophysical Journal International》1977,50(2):459-472
Summary. An empirical equation of state, assuming mineralogical equilibrium, is developed for the top 700 km of the mantle. Assuming a uniform viscosity, this equation of state is used to show that the stresses due to the changes in phase induced in a descending lithospheric plate in a subduction zone are an order of magnitude larger than those due to the negative buoyancy of the slab in the asthenosphere. The stresses predicted are well within the power law creep region for likely mantle materials and so the effective viscosity will vary within the slab. Consequently the stresses will be smaller than those of 7.0 × 108 N/m2 obtained here using uniform viscosities. These stresses are relatively compressional near the sides of the slab and tensional in the centre. 相似文献
150.
A coupled neutral-ionic photochemical model has been used to interpret the ionic composition of the Venusian dayside ionosphere measured by the orbiter retarding potential analyser (ORPA) experiment on board the NASA Pioneer-Venus orbiter spacecraft. The electron and ion temperatures also measured by the ORPA are used for calculating the plasma-diffusion coefficients and scale heights for ions. The neutral temperature profiles and the densities of neutral constituents, particularly CO2 and O, play key roles in the determination of the height profiles of the ionic constituents. All these quantities vary substantially in the Venusian thermosphere near the terminator; the models presented are representatives of the solar zenith angle ~65°. The predicted O2+ densities below ~200km agree particularly well with observations by the ORPA, but the model values are significantly less than those measured by the orbiter ion mass spectrometer (OIMS). Models predict much smaller densities than observed values for all molecular ions above ~200km. The reason for the turn-up trend of the concentration gradient of molecular ions observed at these heights by both ORPA and OIMS is unknown. One of the models can predict O+ ion densities above ~200km compatible with observations, if an effective vertical escape flux (φ108cm?2sec?1) is assumed at the ionopause. The neutral air density required to explain the observed ion composition is about 1.4 times larger than the values measured by the orbiter neutral mass spectrometer (ONMS). 相似文献