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481.
482.
Edward J. Olsen T.E. Bunch Eugene Jarosewich Albert F. Noonan Glenn I Huss 《Meteoritics & planetary science》1977,12(2):109-124
Happy Canyon [found: 1971, 34° 46.5′N, 101° 33.6′W, Texas] consists of about 85 vol. % enstatite (Fs 0.4%), 5 to 10 vol % plagioclase (An 26%), and 5 vol % diopside (Fs 0.9%). In addition, there are minor remnants of metal (Ni 6.35 wt %, Si-free) and troilite (with 5.10 wt % Cr and 1.15 wt % Ti) that have survived extensive terrestrial weathering. The meteorite has a cumulate texture, uniform-size euhedral, prismatic crystals of enstatite (0.3 to 0.4 mm long) with interstitial plagioclase, diopside, troilite, and metal. The enstatite crystals are dominantly disordered and occur in alignments that suggest flow. There are no chondrules or remnants of chondrules. The enstatite crystals contain internal negative crystal voids, which are charactieristic of enstatite achondrites, as well as internal branching submicron rivulet dislocations. The bulk composition is that of an E6 enstatite chondrite, however, it has the texture of a crystal cumulate; achondritic, but unlike that of enstatite achondrites. Glass of a granitic composition occurs mainly in the mesostasis and is compositionally like the glass found inside pyroxene crystals in the Cumberland Falls enstatite achondrite. Happy Canyon is most simply explained as an E6 composition that has melted and reprecipitated at a slightly higher oxidation state, at some depth (> 7 km), possibly in the core volume of a small, asteroidal-size parent body. In terms of classification, it occupies the gap between the recrystallized enstatite chondrites and the igneous, crystalline, unbrecciated enstatite achondrites like Shallowater. Happy Canyon is a new type of enstatite achondrite 相似文献
483.
Data from a two-year period of monthly slackwater surveys reveal that in addition to the classical estuary turbidity maximum (ETM), another peak of bottom total suspended sediment (TSS) concentration, or a so-called secondary turbidity maximum (STM), often exists in the middle part of the York River estuary, Virginia. This STM, observed in most (but not all) of the slackwater surveys, moves back and forth in the region of about 20 to 40 km from the York River mouth where the mud percentage of bottom sediment is very high. The distribution of the potential energy anomaly, which was calculated using salinity data, indicates that the STM usually resides in the transition zone between the upstream well mixed and the downstream more stratified water columns. An analysis using the conservation equation of suspended sediment concentration in the water column reveals that four processes may contribute to the formation of the STM: convergence of bottom residual flow, tidal asymmetry, inhibition of turbulent diffusion by stratification, and bottom resuspension. The along-channel variations of the strength of bottom residual flow, the effect of tidal asymmetry, and the stratification patterns are probably due to the geometric features of the York River estuary. 相似文献
484.
The mass-dependent fractionation laws that describe the partitioning of isotopes are different for kinetic and equilibrium reactions. These laws are characterized by the exponent relating the fractionation factors for two isotope ratios such that α2/1 = α3/1β. The exponent β for equilibrium exchange is (1/m1 − 1/m2)/(1/m1 − 1/m3), where mi are the atomic masses and m1 < m2 < m3. For kinetic fractionation, the masses used to evaluate β depend upon the isotopic species in motion. Reduced masses apply for breaking bonds whereas molecular or atomic masses apply for transport processes. In each case the functional form of the kinetic β is ln(M1/M2)/ln(M1/M3), where Mi are the reduced, molecular, or atomic masses. New high-precision Mg isotope ratio data confirm that the distinct equilibrium and kinetic fractionation laws can be resolved for changes in isotope ratios of only 3‰ per amu. The variability in mass-dependent fractionation laws is sufficient to explain the negative Δ17O of tropospheric O2 relative to rocks and differences in Δ17O between carbonate, hydroxyl, and anhydrous silicate in Martian meteorites. (For simplicity, we use integer amu values for masses when evaluating β throughout this paper.) 相似文献
485.
486.
Joan E. Cartes Vanesa Papiol Albert Palanques Jorge Guilln Montserrat Demestre 《Estuarine, Coastal and Shelf Science》2007,75(4):501-515
Dynamics of suprabenthos (hyperbenthos) composition and biomass have been simultaneously analyzed at two sites (S1, S2) off the Ebro River Delta (western Mediterranean). The stations, separated by ca. 5 km, differed in terms of depth (S1, 47 m; S2, 61 m), distance to the river mouth (S2 south of S1 and farther from the mouth) and fishing activity (S1 is a fishing ground; S2 is in an area closed to fishing). Peracarids (gammaridean amphipods, mysids, and cumaceans) were dominant among suprabenthic taxa. Seasonality was the main explanation for changes in taxonomic composition, with two seasonal groups indicated by MDS analyses (late summer–autumn, August–September, and November 2003; early summer, June and July samples). Peracarids at both S1 and S2 showed a peak of abundance in early July, with the highest densities reaching 5400 individuals (100 m)−2 at S2. There was a sharp decrease of density in late July (S1) and August (S2), then an increase in August (S1) and in September (S2), respectively. A secondary peak of abundance occurred in November (S1) and December (S2). There was, therefore, a similar picture in the dynamics of suprabenthic peracarids at both sites, though with a delay of 1 month at the deeper S2. This pattern coincided with changes in river discharge (specifically, a decrease of suprabenthos when influx was below 200 m3 s−1 in the period June–September 2003), and with the formation of a thermal gradient (also in June–September 2003) between S1 and S2 associated with the 15 °C isotherm. In addition, the decrease of suprabenthos in late July–August also coincided with the massive occurrence of mucilaginous aggregates close to the bottom (at between 0 and 4 m above the bottom), in the usual habitat of suprabenthos. C/N ratios in sediments (an indicator of the degree of degradation of organic matter (OM)) increased during this peak abundance of mucilaginous aggregates. The impoverishment of sediments in total organic carbon (TOC) was parallel to the decline of suprabenthos in late July–August, which is consistent with negative or non-significant correlations (evidenced both by Spearman's r and multi-regression models) between suprabenthos densities and TOC. Zooplankton taxa (e.g., copepods, chaetognaths, and fish/crustacean larvae living in the entire water column) showed significant correlations with a number of environmental factors (basically temperature), in contrast to suprabenthos. In conclusion, suprabenthos abundance was influenced by a number of natural factors, both intra-annual (e.g., OM quality, river discharges) and inter-annual (e.g., accumulation of mucilaginous remains). Probably due to their swimming capability, suprabenthos were not influenced by trawling activity. Considering the high P/B exhibited by suprabenthic crustaceans in comparison to infauna, this compartment likely has an important role in regulating food webs in those communities submitted to high fishing impact. 相似文献
487.
Hermann Albert J.; Hinckley Sarah; Megrey Bernard A.; Napp Jeffrey M. 《ICES Journal of Marine Science》2001,58(5):1030-1041
488.
Gregg R. Brooks Larry J. Doyle Beau C. Suthard Stan D. Locker Albert C. Hine 《Marine Geology》2003,200(1-4):325-349
Sediment vibracores and surface samples were collected from the mixed carbonate/siliciclastic inner shelf of west–central Florida in an effort to determine the three-dimensional facies architecture and Holocene geologic development of the coastal barrier-island and adjacent shallow marine environments. The unconsolidated sediment veneer is thin (generally <3 m), with a patchy distribution. Nine facies are identified representing Miocene platform deposits (limestone gravel and blue–green clay facies), Pleistocene restricted marine deposits (lime mud facies), and Holocene back-barrier (organic muddy sand, olive-gray mud, and muddy sand facies) and open marine (well-sorted quartz sand, shelly sand, and black sand facies) deposits. Holocene back-barrier facies are separated from overlying open marine facies by a ravinement surface formed during the late Holocene rise in sea level. Facies associations are naturally divided into four discrete types. The pattern of distribution and ages of facies suggest that barrier islands developed approximately 8200 yr BP and in excess of 20 km seaward of the present coastline in the north, and more recently and nearer to their present position in the south. No barrier-island development prior to approximately 8200 yr BP is indicated. Initiation of barrier-island development is most likely due to a slowing in the Holocene sea-level rise ca. 8000 yr BP, coupled with the intersection of the coast with quartz sand deposits formed during Pleistocene sea-level highstands. This study is an example of a mixed carbonate/siliciclastic shallow marine depositional system that is tightly constrained in both time and sea-level position. It provides a useful analog for the study of other, similar depositional systems in both the modern and ancient rock record. 相似文献
489.
Using dual-isotope data to trace the origin and processes of dissolved sulphate: a case study in Calders stream (Llobregat basin,Spain) 总被引:1,自引:0,他引:1
Whereas most of the reported δ34S values of dissolved sulphate are positive in the Llobregat basin, Calders stream, which is a tributary of the Llobregat
River, is characterised by negative values. Stream waters, sampled monthly between 1997 and 1998, and quarterly in 1999, show
an overall increase in δ34S from −10‰ to 0‰, coupled with an increase in Na and Cl concentration. On the other hand, the oxygen isotopic composition
of dissolved sulphate, δ18O, displayed an opposite trend with a slight decrease, from +9‰ to +6‰. Detailed sampling up stream in November 2000 indicated
that, contrary to most of the surficial waters of the Llobregat basin with a δ34SSO4 mainly controlled by evaporites, in Calders stream, sulphate is derived from pyrite oxidation. The dual-isotope approach,
coupled with chemical data, allowed us to identify the contribution of 34S-rich sulphate effluents from anthropogenic sources, while mixing models, calculated between natural and anthropogenic sources,
enabled us to estimate their contribution. Sudden increases of δ34S and δ18O of dissolved sulphate in stream waters are believed to be caused by a sulphate reduction process related to oil spillage.
The long-term enrichment in δ34S, coupled with a decrease in δ18OSO4, from Jan-97 to Aug-99, is interpreted as a progressive increase in the contribution of pig manure. 相似文献
490.