An unusual feature of the saline stratified lakes that were formed due to ongoing postglacial uplift on the White Sea coast is the presence of several differently colored thin layers in the zone with sharp gradients. Colored layers in five lakes at various stages of separation from the sea were investigated using optical microscopy, spectrophotometry, spectrofluorimetry, and photobiology. The upper greenish colored layer located in the aerobic strata of all lakes near the compensation depth of 1% light penetration contains green algae. In the chemocline, another layer, brightly green, red or pink, is dominated by mixotrophic flagellates. Despite the very low light intensities and the presence of H 2 S, active photosynthesis by these algae appears to be occurring, as indicated by high values of the maximum quantum yield of primary photochemistry, electron transport activity, photosynthetic activity of photosystem II, the fraction of active centers, and low values of heat dissipation. In the reduced zone of the chemocline, a dense green or brown suspension of anoxygenic phototrophs(green sulfur bacteria) is located. 相似文献
From 2002 through 2004, time-series sediment trap samples were collected from a depth of 410 m in Cuenca Alfonso, Bahía de La Paz, on the SW coast of the Gulf of California. The instrument recorded the impact of the local passage of hurricanes “Ignacio” (24–26 August) and “Marty” (21–23 September) in 2003. These two events accounted for 82% of the total rainfall measured in 2003, equivalent to the annual average precipitation in years without hurricanes. Mean total mass fluxes (TMFs) of 2.88 and 3.58 g m−2 d−1 were measured during the week of each hurricane as well as the following week. This may have been enough to produce a lamina in the underlying sediment with characteristics peculiar to such events. The terrigenous component was particularly abundant, with notably higher concentrations of Fe, Sc, Co and Cs and REEs. In contrast, TMFs throughout 2002–2004 (excluding the hurricane periods) averaged only 0.73 g m−2 d−1 and had a larger marine biogenic component. The extraordinary elemental fluxes during the 29 days of hurricane-influenced sedimentation represented a great proportion of the totals over an entire “normal” year: Co (67.8%) >Sc (62.6) >Fe (59.6) >Cs (53.4)>Lu (51.5)>La (51.3)>Yb (51.0)>Ce (49.5) >Tb (48.4) >Sm (44.7)>Cr (36.5) >Ca (31.0)>Eu (25.4%). The terrigenous fraction was calculated using (a) TMF minus the sum of CaCO3, biogenic silica and organic matter and (b) the ratio of Sc in the trap samples to the average in the Earth's crust. The latter was consistently smaller, but the two methods offered similar results following hurricanes (78% vs. 63%, respectively). For normal sedimentation, however, the difference method yielded values twice as large as the Sc method (58% vs. 30%) This suggests that the mineralogy of the terrigenous fraction may also vary, with unsorted dessert soil being carried to sea by the powerful flash floods associated with hurricanes. Eolian supply of particles, particularly Sc-free quartz grains, possibly from beyond the limited fluvial drainage basin, apparently dominates normal sedimentation. 相似文献
A coupled ocean and boundary layer flux numerical modeling system is used to study the upper ocean response to surface heat
and momentum fluxes associated with a major hurricane, namely, Hurricane Dennis (July 2005) in the Gulf of Mexico. A suite
of experiments is run using this modeling system, constructed by coupling a Navy Coastal Ocean Model simulation of the Gulf
of Mexico to an atmospheric flux model. The modeling system is forced by wind fields produced from satellite scatterometer
and atmospheric model wind data, and by numerical weather prediction air temperature data. The experiments are initialized
from a data assimilative hindcast model run and then forced by surface fluxes with no assimilation for the time during which
Hurricane Dennis impacted the region. Four experiments are run to aid in the analysis: one is forced by heat and momentum
fluxes, one by only momentum fluxes, one by only heat fluxes, and one with no surface forcing. An equation describing the
change in the upper ocean hurricane heat potential due to the storm is developed. Analysis of the model results show that
surface heat fluxes are primarily responsible for widespread reduction (0.5°–1.5°C) of sea surface temperature over the inner
West Florida Shelf 100–300 km away from the storm center. Momentum fluxes are responsible for stronger surface cooling (2°C)
near the center of the storm. The upper ocean heat loss near the storm center of more than 200 MJ/m2 is primarily due to the vertical flux of thermal energy between the surface layer and deep ocean. Heat loss to the atmosphere
during the storm’s passage is approximately 100–150 MJ/m2. The upper ocean cooling is enhanced where the preexisting mixed layer is shallow, e.g., within a cyclonic circulation feature,
although the heat flux to the atmosphere in these locations is markedly reduced. 相似文献
H2 photoproduction and nitrogenase activities in two strains ofAnabaena variabilis marked wild type ATCC 29413 and mutant PK84 exposed to thermal stress (temperature higher than the normal incubation temperature of 30°C) were studied. Cultures of both strains collected from any interval of logarithmic growth phase exhibited high H2 photoproduction and nitrogenase activities when exposed to limited time heat shock during the assay process. In contrast, the algal H2 photoproduction rate of both strains fluctuated with long term thermal stress caused by increasing the growth temperature from 30°C to 36°C.
The changes of nitrogenase (the key H2 photobiosynthetic enzyme) activities in the mutant PK84 showed variation tendency similar to that of H2 photoproduction during exposure to thermal stress, indicating that fluctuation of H2 photoproduction in the mutant was mainly due to the variation of nitrogenase activities. A temporary maximal H2 photoproduction in the mutant PK84 (wild type ATCC29413) was observed when cells grew at 36°C for 14 (6) days. However, the responses of nitrogenase activities in the wild type to thermal stress were not completely similar to those in the mutant in spite of similar variations of H2 photoproduction in both strains. The data obtained in these studies suggested that the activities of other enzymes (in the wild strain) involved in H2 photoproduction were affected by thermal stress since H2 photoproduction maximized or dropped to 0 without variation tendency similar to that of nitrogenase activities.
Furthermore, an enhancement of H2 photoproduction speed of the mutant strain cultured in a 4.4 L laboratory photobioreactor was also observed when it was subjected to short time continuous charge of argon, and temperature rise.
All these results indicated that high temperature plays an important role in the photo-autotrophic H2 photoproduction, and that long term thermal stress is unfavourable for net H2 photoproduction in both strains ofA. variabilis though short-time heat shock is conducive to H2 photoproduction.
Strong (B?109 G) and superstrong (B?1014 G) magnetic fields profoundly affect many thermodynamic and kinetic characteristics of dense plasmas in neutron star envelopes. In particular, they produce strongly anisotropic thermal conductivity in the neutron star crust and modify the equation of state and radiative opacities in the atmosphere, which are major ingredients of the cooling theory and spectral atmosphere models. As a result, both the radiation spectrum and the thermal luminosity of a neutron star can be affected by the magnetic field. We briefly review these effects and demonstrate the influence of magnetic field strength on the thermal structure of an isolated neutron star, putting emphasis on the differences brought about by the superstrong fields and high temperatures of magnetars. For the latter objects, it is important to take proper account of a combined effect of the magnetic field on thermal conduction and neutrino emission at densities ρ?1010 g?cm?3. We show that the neutrino emission puts a B-dependent upper limit on the effective surface temperature of a cooling neutron star. 相似文献
Palaeomagnetic and geochronological studies on mafic rocks in the Lake Ladoga region in South Russian Karelia provide a new, reliably dated Mesoproterozoic key paleopole for the East European Craton (Baltica). U–Pb dating on baddeleyite gives a crystallisation age of 1452 ± 12 Ma for one of the studied dolerite dykes. A mean palaeomagnetic pole for the Mesoproterozoic dolerite dykes, Valaam sill and Salmi basalts yields a paleopole at 15.2°N, 177.1°E, A95 = 5.5°. Positive baked contact test for the dolerite dykes and positive reversal test for the Salmi basalts and for the dykes confirm the primary nature of the magnetisation. Comparison of this Baltica palaeopole with coeval paleomagnetic data for Laurentia and Siberia provides a revised palaeoposition of these cratons. The results verify that the East European Craton, Laurentia and Siberia were part of the supercontinent Columbia from the Late Palaeoproterozoic to the Middle Neoproterozoic. 相似文献
Abstract We discuss the steady states of the αω-dynamo in a thin disc which arise due to α-quenching. Two asymptotic regimes are considered, one for the dynamo numberD near the generation thresholdD0, and the other for |D| ? 1. Asymptotic solutions for |D—D0| ? |D0| have a rather universal character provided only that the bifurcation is supercritical. For |D| ? 1 the asymptotic solution crucially depends on whether or not the mean helicity α, as a function ofB, has a positive root (hereB is the mean magnetic field). When such a root exists, the field value in the major portion of the disc is O(l), while near the disc surface thin boundary layers appear where the field rapidly decreases to zero (if the disc is surrounded by vacuum). Otherwise, when α = O(|B|?s) for |B| → ∞, we demonstrate that |B| = O(|D|1/s) and the solution is free of boundary layers. The results obtained here admit direct comparison with observations of magnetic fields in spiral galaxies, so that an appropriate model of nonlinear galactic dynamos hopefully could be specified. 相似文献
Here we characterize the magnetic properties of the Chelyabinsk chondrite (LL5, S4, W0) and constrain the composition, concentration, grain size distribution, and mineral fabric of the meteorite's magnetic mineral assemblage. Data were collected from 10 to 1073 K and include measurements of low‐field magnetic susceptibility (χ0), the anisotropy of χ0, hysteresis loops, first‐order reversal curves, Mössbauer spectroscopy, and X‐ray microtomography. The REM and REM′ paleointensity protocols suggest that the only magnetizations recorded by the chondrite are components of the Earth's magnetic field acquired during entry into our planet's atmosphere. The Chelyabinsk chondrite consists of light and dark lithologies. Fragments of the light lithology show logχ0 = 4.57 ± 0.09 (s.d.) (n =135), while the dark lithology shows 4.65 ± 0.09 (n =39) (where χ0 is in 10?9 m3 kg?1). Thus, Chelyabinsk is three times more magnetic than the average LL5 fall, but is similar to a subgroup of metal‐rich LL5 chondrites (Paragould, Aldsworth, Bawku, Richmond) and L/LL5 chondrites (Glanerbrug, Knyahinya). The meteorite's room‐temperature magnetization is dominated by multidomain FeNi alloys taenite and kamacite (no tetrataenite is present). However, below approximately 75 K remanence is dominated by chromite. The metal contents of the light and dark lithologies are 3.7 and 4.1 wt%, respectively, and are based on values of saturation magnetization. 相似文献