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271.
Geochemical estimates of N2 fixation in the North Atlantic often serve as a foundation for estimating global marine diazotrophy. Yet despite being well-studied, estimations of nitrogen fixation rates in this basin vary widely. Here we investigate the variability in published estimates of excess nitrogen accumulation rates in the main thermocline of the subtropical North Atlantic, testing the assumptions and choices made in the analyses. Employing one of these previously described methods, modified here with improved estimates of excess N spatial gradients and ventilation rates of the main thermocline, we determine a total excess N accumulation rate of 7.8 ± 1.7 × 1011 mol N yr− 1. Contributions to excess N development include atmospheric deposition of high N:P nutrients (adding excess N at a rate of 3.0 ± 0.9 × 1011 mol N yr− 1 for 38% of the total), high N:P dissolved organic matter advected into and mineralized in the main thermocline (adding excess N at 2.2 ± 1.1 × 1011 mol N yr− 1 for 28% of the total), and, calculated by mass balance of the excess N field, N2 fixation (adding excess N at 2.6 ± 2.2 × 1011 mol N yr− 1 for 33% of the total). Assuming an N:P of 40 and this rate of excess N accumulation due to the process, N2 fixation in the North Atlantic subtropical gyre is estimated at 4 × 1011 mol N yr− 1. This relatively low rate of N2 fixation suggests that i) the rate of N2 fixation in the North Atlantic is greatly overestimated in some previous analyses, ii) the main thermocline is not the primary repository of N fixed by diazotrophs, and/or iii) the N:P ratio of exported diazotrophic organic matter is much lower than generally assumed. It is this last possibility, and our uncertainty in the N:P ratios of exported material supporting excess N development, that greatly lessens our confidence in geochemical measures of N2 fixation. 相似文献
272.
273.
Irene Ylla Isis Sanpera-Calbet Isabel Muñoz Anna M. Romaní Sergi Sabater 《Aquatic Sciences - Research Across Boundaries》2011,73(4):523-535
Amino acid composition (quality) and abundance (quantity) of organic matter (OM) in an intermittent Mediterranean stream were
followed during transitions from wet to dry and dry to wet conditions. Amino acids were analyzed in benthic material (epilithic
biofilms, sand sediments, leaf material) as well as in the flowing water (dissolved organic matter, DOM). A principal component
analysis and the estimation of the amino acid degradation index (DI) elucidated differences in amino acid composition and
quality among the wet–drought–wet cycle. Amino acid content and composition were dependent on the source of OM as well as
on its diagenetic state. The highest-quality OM (high DI and high N content) occurred on epilithic biofilms and the most degraded
and lowest-quality OM occurred in sandy sediments. Differences between the pre- and post-drought periods were evident in DOM
quality; autochthonous-derived material was predominant during the pre-drought (wet period preceding drying), while allochthonous
inputs dominated during the post-drought period (wet period following drying). In contrast, benthic OM compartments were more
stable, but benthic OM quality decreased continuously throughout the drought period. This study revealed that wet–drought–wet
cycles resulted in subtle changes in benthic OM quality, and degradation of DOM was related to flow intermittency. 相似文献
274.
A significant number of volcano-tectonic (VT) earthquake swarms, some of which are accompanied by ground deformation and/or
volcanic gas emissions, do not culminate in an eruption. These swarms are often thought to represent stalled intrusions of
magma into the mid- or shallow-level crust. Real-time assessment of the likelihood that a VT swarm will culminate in an eruption
is one of the key challenges of volcano monitoring, and retrospective analysis of non-eruptive swarms provides an important
framework for future assessments. Here we explore models for a non-eruptive VT earthquake swarm located beneath Iliamna Volcano,
Alaska, in May 1996–June 1997 through calculation and inversion of fault-plane solutions for swarm and background periods,
and through Coulomb stress modeling of faulting types and hypocenter locations observed during the swarm. Through a comparison
of models of deep and shallow intrusions to swarm observations, we aim to test the hypothesis that the 1996–97 swarm represented
a shallow intrusion, or “failed” eruption. Observations of the 1996–97 swarm are found to be consistent with several scenarios
including both shallow and deep intrusion, most likely involving a relatively small volume of intruded magma and/or a low
degree of magma pressurization corresponding to a relatively low likelihood of eruption. 相似文献
275.
Cases of modern and prehistoric liquefaction illustrate that sand deposits can be liquefied again (or “reliquefied”) by a subsequent earthquake after initially liquefying during seismic shaking. In order to test the validity of two postulates regarding reliquefaction mechanisms and to examine the role of gradational characteristics on reliquefaction resistance, 1 g shaking table tests were performed using five sands with differing gradation characteristics. The test results demonstrate that the number of cycles required to reliquefy each sand decreased significantly following the 1st liquefaction event as a result of destroying the “aged” sand fabric developed prior to the 1st shaking event via secondary compression of the initially loose sands. Reliquefaction resistance correlated reasonably well with a proxy for cv (∝D102Dr2.8), illustrating that both the permeability and compressibility of the sand play significant roles in the post-liquefaction fabric (and hence reliquefaction resistance) formed by a sand. While the initial decrease in reliquefaction resistance supports both the Oda et al. [8] and the Olson et al. [5] reliquefaction postulates, only the Olson et al. [5] postulate reasonably explains the subsequent, large increase in reliquefaction resistance observed during the 3rd through 5th shaking events. These tests suggest that the coefficient of consolidation, cv=kv/γwmv (or proxy values such as D102Dr2.8 or D10/CU) may be a useful tool for evaluating reliquefaction potential in forward and inverse (i.e., paleoliquefaction) analysis. 相似文献
276.
Kevin P. Norton Friedhelm von Blanckenburg Roman DiBiase Fritz Schlunegger Peter W. Kubik 《International Journal of Earth Sciences》2011,100(5):1163-1179
Denudation rates from cosmogenic 10Be measured in quartz from recent river sediment have previously been used in the Central Alps to argue that rock uplift occurs
through isostatic response to erosion in the absence of ongoing convergence. We present new basin-averaged denudation rates
from large rivers in the Eastern and Southern European Alps together with a detailed topographic analysis in order to infer
the forces driving erosion. Denudation rates in the Eastern and Southern Alps of 170–1,400 mm ky−1 are within a similar range to those in the Central Alps for similar lithologies. However, these denudation rates vary considerably
with lithology, and their variability generally increases with steeper landscapes, where correlations with topographic metrics
also become poorer. Tertiary igneous rocks are associated with steep hillslopes and channels and low denudation rates, whereas
pre-Alpine gneisses usually exhibit steep hillslopes and higher denudation rates. Molasse, flysch, and schists display lower
mean basin slopes and channel gradients, and, despite their high erodibility, low erosion rates. Exceptionally low denudation
rates are also measured in Permian rhyolite, which has high mean basin slopes. We invoke geomorphic inheritance as a major
factor controlling erosion, such that large erosive glaciers in the late Quaternary cold periods were more effective in priming
landscapes in the Central Alps for erosion than in the interior Eastern Alps. However, the difference in tectonic evolution
of the Eastern and Central Alps potentially adds to differences in their geomorphic response; their deep structures differ
significantly and, unlike the Central Alps, the Eastern Alps are affected by ongoing tectonic influx due to the slow motion
and rotation of Adria. The result is a complex pattern of high mountain erosion in the Eastern Alps, which has evolved from
one confined to the narrow belt of the Tauern Window in late Tertiary time to one affecting the entire underthrust basement,
orogenic lid, and parts of the Southern Alps today. 相似文献
277.
Experiments simulating flow in the Earth's liquid core induced by luni-solar precession of the solid mantle indicate, to a first approximation, that the core behaves like a rigidized fluid sphere spinning slower than the mantle and with its spin axis lagging the mantle spin axis in precession. Secondary flow patterns are always present. At low precession rates the fluid sphere is subdivided into a set of cylinders coaxial with the fluid spin axis, the cylinders rotating alternately at slightly faster and slower rates relative to the net retrograde motion of the fluid as a whole. Slow non-axisymmetric columnar wave patterns develop between the differentially rotating cylinders. Axial flows between the spheroidal cavity boundary and the interior are observed. Fluid motion becomes turbulent only at precession rates large enough to cause the fluid spin axis to align nearly with the precession axis. There is no evidence that the Earth's liquid spin axis direction departs more than a fraction of a degree from geographic north. Our observations suggest precession induces a complex variety of laminar flows, including slowly varying and/or periodic patterns, in the Earth's liquid core. 相似文献
278.
Alan J. Lewitus Roman V. Jesien Todd M. Kana JoAnn M. Burkholder Howard B. Glasgow Eric May 《Estuaries and Coasts》1995,18(2):373-378
Since its discovery in natural estuarine habitat of North Carolina in 1991, the widespread impact of the toxic dinoflagellate, Pfiesteria piscicida (gen. et sp. nov.), popularly called the “phantom” dinoflagellate, on North Carolina fish stocks has been established, yet little is known about its influence outside of North Carolina estuaries. Here, we document the presence of P. piscicida in Chesapeake Bay. A fish kill was observed after inoculating an aquarium containing mummichogs with sediment samples from Jenkins Creek, a brackish creek (salinity 11‰) of the Chesapeake Bay system. P. piscicida was the cause of the kill, as supported by morphological, physiological, and histological evidence. The appearance and behavior of the algae and symptoms associated with fish mortality were consistent with those previously observed in P. piscicida-associated aquaria fish kills in North Carolina. The discovery of P. piscicida in Chesapeake Bay supports the speculation that these toxic dinoflagellates have a dramatic and far-reaching impact on fish stocks in shallow, eutrophic estuaries along the eastern United States. 相似文献
279.
280.
Viatcheslav S. Solomatov Peter Olson David J. Stevenson 《Earth and Planetary Science Letters》1993,120(3-4):387-393
Differentiation in magma chambers, in the Earth's core and in the partially molten early Earth is a competitive process between sedimentation and re-entrainment of crystals in the presence of convection. Previous studies show that the particles suspended in convective layers eventually settle and do so almost as fast as in the absence of convection. However, the nature and magnitude of the competing entrainment has remained unclear. Here we provide a simple theory and experimental evidence showing that entrainment occurs at the crests of dunes created in the particle bed at the base of a convecting fluid. In both laminar and turbulent regimes, the dune formation and entrainment are driven by viscous stresses produced by thermal plumes. At sufficiently high Rayleigh numbers the particles are probably entrained by Reynolds stresses. Entrainment in the Earth's core is hardly possible because it requires unreasonably small crystals. Entrainment of 10−2–10−1 cm diameter crystals is very likely in magma oceans. For magma chambers entrainment requires large viscosities (> 106 P) and even when it occurs, the total amount of the suspended solid fraction is very small. 相似文献