近百年里海海平面的升降与伏尔加三角洲的发育

位于里海北部的伏尔加三角洲属河控型三角洲，其水下坡度极度平缓。近百年来里海海平面升降幅度达3．46m，其变化可分为3个阶段：1900－1929年缓慢下降期，平均下降速率为1.1cm/a；1930－1977年快速下降期，平均速率为6.3cm/a；1978－1999年为快速上升期，平均上升速率达到8.8cm/a。在海平面下降阶段，伏尔加三角洲增长幅度达到每年2－180km^2，这是由于河流泥沙的沉积和浅水区出露水面而造成。海平面上升之时，较浅水下三角洲部分成为“缓冲带”，降低了海平面上升对三角洲的影响。尽管近20多年来海平面迅速上升，伏尔加三角洲并未发现海岸侵蚀、湿地损失、盐水入侵、海岸洪涝灾害等一般大河三角洲常见的现象。对比其它三角洲可以发现，各个大河三角洲各以不同的方式响应海平面的上升。     

Changes in plankton community structure and function in response to variable freshwater flow in two tributaries of the Chesapeake Bay

The biomass of phytoplankton, microzooplankton, copepods, and gelatinous zooplankton were measured in two tributaries of the Chesapeake Bay during the springs of consecutive dry (below average freshwater flow), wet (above average freshwater flow), and average freshwater flow years. The potential for copepod control of microzooplankton biomass in the dry and wet years was evaluated by comparing the estimated grazing rates of microzooplankton by the dominant copepod species (Acartia spp. andEurytemora affinis) to microzooplankton growth rates and by calculating the percent of daily microzooplanton standing stock removed through copepod grazing. There were significant increases in phytoplankton and copepod biomass, but not for microzooplankton biomass in the wet year as compared to the dry year. The ctenophoreMnemiopsis leidyi was present during the dry year but was absent during the sampling period of the wet and average freshwater flow years. Grazing pressure on microzooplankton was greatest in the wet year, withAcartia spp. andE. affinis ingesting 0.21–2.64 μg of microzooplankton C copepod⁻¹ d⁻¹ and removing up to 60% of the microzooplankton standing stock per day. In the dry year, these copepod species ingested 0.10–0.73 μg of microzooplankton C copepod⁻¹ d⁻¹ with a maximum daily removal of approximately 3% of the microzooplankton standing stock. Potential copepod grazing pressure was significantly less than microzooplankton growth in the dry year, but was equivalent to microzooplankton growth in the wet year, implying strong top-down control of the microzooplankton community in the wet year. These results suggest that increased grazing control of microzooplankton populations by more copepods in the wet year released top-down control of phytoplankton. Reduced microzooplankton grazing, in conjunction with increased nutrient availability, resulted in large increases in phytoplankton biomass in the wet year. Increased freshwater flow has the potential to influence trophic cascades and the partitioning of plankton production in estuarine systems.     

Extreme rainfalls in Eastern Himalaya and southern slope of Meghalaya Plateau and their geomorphologic impacts

This paper presents the detailed rainfall characteristics of 3 key areas located in the eastern monsoon India: the margin of Darjeeling Himalaya, the margin of Bhutanese Himalaya and the Cherrapunji region at the southern slope of Meghalaya Upland. All these areas are sensitive to changes but differ in annual rainfall totals (2000–4000 mm, 4000–6000 m and 6000–23,000 mm respectively) and in the frequency of extreme rainfalls. Therefore the response of geomorphic processes is different, also due to various human impact. In the Darjeeling Himalaya the thresholds may be passed 2–3 times in one century and the system may return to the former equilibrium. At the margin of western Bhutanese Himalaya in 1990s, the clustering of three events caused an acceleration in the transformation and formation of a new trend of evolution, especially in the piedmont zone. In the Cherrapunji of Meghalaya region in the natural conditions the effects of dozens of extreme rainfalls every year were checked by the dense vegetation cover. After deforestation and extensive land use the fertile soil was removed and either the exposed bedrock or armoured debris top layer protect the surface against degradation and facilitate only rapid overland flow. A new “sterile” system has been formed.     

On the linkages among density, flow, and bathymetry gradients at the entrance to the Chesapeake Bay

Linkages among density, flow, and bathymetry gradients were explored at the entrance to the Chesapeake Bay with underway measurements of density and flow profiles. Four tidal cycles were sampled along a transect that crossed the bay entrance during cruises in April–May of 1997 and in July of 1997. The April–May cruise coincided with neap tides, while the July cruise occurred during spring tides. The bathymetry of the bay entrance transect featured a broad Chesapeake Channel, 8 km wide and 17 m deep, and a narrow North Channel, 2 km wide and 14 m deep. The two channels were separated by an area with typical depths of 7 m. Linkages among flows, bathymetry, and water density were best established over the North Channel during both cruises. Over this channel, greatest convergence rates alternated from the left (looking into the estuary) slope of the channel during ebb to the right slope during flood as a result of the coupling between bathymetry and tidal flow through bottom friction. These convergences were linked to the strongest transverse shears in the along-estuary tidal flow and to the appearance of salinity fronts, most markedly during ebb periods. In the wide channel, the Chesapeake Channel, frontogenesis mechanisms over the northern slope of the channel were similar to those in the North Channel only in July, when buoyancy was relatively weak and tidal forcing was relatively strong. In April–May, when buoyancy was relatively large and tidal forcing was relatively weak, the recurrence of fronts over the same northern slope of the Chesapeake Channel was independent of the tidal phase. The distinct frontogenesis in the Chesapeake Channel during the increased buoyancy period was attributed to a strong pycnocline that insulated the surface tidal flow from the effects of bottom friction, which tends to decrease the strength of the tidal flow over relatively shallow areas.     

Ether, Luminosity and Galactic Source Counts

An interpretation of the cosmological redshift in terms of a cosmic ether is given. We study a Robertson-Walker cosmology in which the ether is phenomenologically defined by a homogeneous and isotropic permeability tensor. The speed of light becomes so a function of cosmic time like in a dielectric medium. However, the cosmic ether is dispersion free, it does not lead to a broadening of spectral lines. Locally, in Euclidean frames, the scale factors of the permeability tensor get absorbed in the fundamental constants. Mass and charge scale with cosmic time, and so do atomic energy levels. This substantially changes the interpretation of the cosmological redshift as a Doppler shift. Photon frequencies are independent of the expansion factor; their time scaling is determined by the permeability tensor. The impact of the ether on the luminosity-distance, on the distance-redshift relation, and on galactic number counts is discussed. The Hubble constant is related to the scale factors of the metric and the permeability tensor. We study the effects of the ether at first in comoving Robertson-Walker coordinates, and then, in the context of a flat but expanding space- time, in the globally geodesic rest frames of galactic observers. This revised version was published online in July 2006 with corrections to the Cover Date.     

Elemental abundances in chondrules from unequilibrated chondrites: Evidence for chondrule origin by melting of pre-existing materials

Bulk abundances of Na, Mg, Al, Ca, Sc, V, Cr, Mn, Fe, Co, Ni, Zn, La, Sm, Eu, Yb, Lu, Ir, and Au were determined by neutron activation analysis of chondrules separated from unequilibrated H-, L-, and LL-chondrites (Tieschitz, Hallingeberg, Chainpur, Semarkona) and correlated with chondrule petrographic properties. Despite wellknown compositional differences among the whole-rock chondrites, the geometric mean compositions of their respective chondrule suites are nearly indistinguishable from each other for many elements. Relative to the condensible bulk solar system (approximated by the Cl chondrite Orgueil), chondrules are enriched in lithophile and depleted in siderophile elements in a pattern consistent with chondrule formation by melting of pre-existing materials, preceded or attended by silicate/metal fractionation. Relative to nonporphyritic chondrules, porphyritic chondrules are enriched in refractory and siderophile elements, suggesting that these two chondrule groups may have formed from different precursor materials.     

Ukinrek Maars, Alaska, I. April 1977 eruption sequence, petrology and tectonic setting

During ten days of phreatomagmatic activity in early April 1977, two maars formed 13 km behind the Aleutian arc near Peulik volcano on the Alaska Peninsula. They have been named “Ukinrek Maars”, meaning “two holes in the ground” in Yupik Eskimo. The western maar formed at the northwestern end of a low ridge within the first three days and is up to 170 m in diameter and 35 m in depth. The eastern maar formed during the next seven days 600 m east of West Maar at a lower elevation in a shallow saddle on the same ridge and is more circular, up to 300 m in diameter and 70 m in depth. The maars formed in terrain that was heavily glaciated in Pleistocene times. The groundwater contained in the underlying till and silicic volcanics from nearby Peulik volcano controlled the dominantly phreatomagmatic course of the eruption.During the eruptions, steam and ash clouds reached maximum heights of about 6 km and a thin blanket of fine ash was deposited north and east of the vents up to a distance of at least 160 km. Magma started to pool on the floor of East Maar after four days of intense phreatomagmatic activity.The new melt is a weakly undersaturated alkali olivine basalt (Ne = 1.2%) showing some transitional character toward high-alumina basalts. The chemistry, an anomaly in the tholeitic basalt-andesite-dominated Aleutian arc, suggests that the new melt is primitive, generated at a depth of 80 km or greater by a low degree of partial melting of garnet peridotite mantle with little subsequent fractionization during transport.The Pacific plate subduction zone lies at a depth of 150 km beneath the maars. Their position appears to be tectonically controlled by a major regional fault, the Bruin Bay fault, and its intersection with cross-arc structural features. We favor a model for the emplacement of the Ukinrek Maars that does not link the Ukinrek conduit to the plumbing system of nearby Peulik volcano. The Ukinrek eruptions probably represent a genetically distinct magma pulse originating at asthenospheric depths beneath the continental lithosphere.     

Resonances of low orders in the planetary system of HD37124

The full set of published radial velocity data (52 measurements from Keck + 58 ones from ELODIE + 17 ones from CORALIE) for the star HD37124 is analysed. Two families of dynamically stable high-eccentricity orbital solutions for the planetary system are found. In the first one, the outer planets c and d are trapped in the 2/1 mean-motion resonance. The second family of solutions corresponds to the 5/2 mean-motion resonance between these planets. In both families, the planets are locked in (or close to) an apsidal corotation resonance. In the case of the 2/1 MMR, it is an asymmetric apsidal corotation (with the difference between the longitudes of periastra Δω ~ 60°), whereas in the case of the 5/2 MMR it is a symmetric antialigned one (Δω = 180°). It remains also possible that the two outer planets are not trapped in an orbital resonance. Then their orbital eccentricities should be relatively small (less than, say, 0.15) and the ratio of their orbital periods is unlikely to exceed 2.3 − 2.5.