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排序方式: 共有182条查询结果,搜索用时 93 毫秒
1.
Richard Owen Hilary KennedyChristopher Richardson 《Geochimica et cosmochimica acta》2002,66(10):1727-1737
The relationship between molluscan shell growth rate and skeletal δ18O and δ13C was investigated in a detailed field study for the scallop, Pecten maximus. Seasonal variation in shell growth rate was found to be a governing factor influencing shell δ18O and δ13C. At low shell growth rates, shell δ18O were more positive (of the order +0.4‰) and δ13C more negative (up to −2‰) as compared with predicted values for precipitation of inorganic calcite in isotopic equilibrium with seawater. The deviations in δ18O were hypothesized as reflecting possible differences in solution carbonate chemistry at the site of mineralization in the extrapallial fluid as compared with that of the external seawater medium. The deviations in shell δ13C were consistent with incorporation of isotopically depleted respiratory 13C (i.e., a metabolic effect). A trend toward more depleted shell δ18O and δ13C values occurred at higher shell growth rates, with negative δ18O values as compared with predicted equilibrium at shell growth rates above 0.13 mm per day. These simultaneous negative deviations in skeletal δ18O and δ13C were interpreted as resulting from a kinetic effect. The implications for environmental reconstruction from molluscan isotopic records are discussed in light of a model of isotopic behavior based on the findings of the study. 相似文献
2.
Joseph J. Luczkovich Garcy P. Ward Jeffrey C. Johnson Robert R. Christian Daniel Baird Hilary Neckles William M. Rizzo 《Estuaries and Coasts》2002,25(6):1143-1163
We established trophic guilds of macroinvertebrate and fish taxa using correspondence analysis and a hierarchical clustering strategy for a seagrass food web in winter in the northeastern Gulf of Mexico. To create the diet matrix, we characterized the trophic linkages of macroinvertebrate and fish taxa present inHalodule wrightii seagrass habitat areas within the St. Marks National Wildlife Refuge (Florida) using binary data, combining dietary links obtained from relevant literature for macroinvertebrates with stomach analysis of common fishes collected during January and February of 1994. Heirarchical average-linkage cluster analysis of the 73 taxa of fishes and macroinvertebrates in the diet matrix yielded 14 clusters with diet similarity ≥ 0.60. We then used correspondence analysis with three factors to jointly plot the coordinates of the consumers (identified by cluster membership) and of the 33 food sources. Correspondence analysis served as a visualization tool for assigning each taxon to one of eight trophic guilds: herbivores, detritivores, suspension feeders, omnivores, molluscivores, meiobenthos consumers, macrobenthos consumers and piscivores. These trophic groups, corss-classified with major taxonomic groups, were further used to develop consumer compartments in a network analysis model of carbon flow in this seagrass ecosystem. The method presented here should greatly improve the development of future network models of food webs by providing an objective procedure for aggregating trophic groups. 相似文献
3.
Gábor Dobosi Pamela D. Kempton Hilary Downes Antal Embey-Isztin Matthew Thirlwall Peter Greenwood 《Contributions to Mineralogy and Petrology》2003,144(6):671-683
Mafic granulite xenoliths from the lower crust of the Pannonian Basin are dominated by LREE-depleted bulk-rock compositions. Many of these have MORB-like 143Nd/144Nd but 87Sr/86Sr is elevated relative to most MORBs. Their '18O values cover a wide range from +3.8 to +9.5. A group of LREE-enriched mafic granulites have higher 87Sr/86Sr (0.704-0.708) and lower 143Nd/144Nd (0.5128-0.5124), with higher '18O values on average (+7.8 to +10.6) than the LREE-depleted granulites. The LREE-enriched granulites are, however, isotopically similar to newly discovered metasedimentary granulite xenoliths. A sublinear correlation in )Hf-)Nd isotope space has a shallower slope than the crust-mantle array, with the metasedimentary rocks forming the low )Hf end member; the radiogenic end is restricted to the LREE-depleted granulites and these overlap the field of MORB. Pb isotopes for the LREE-depleted samples are less radiogenic on average than those of the LREE-enriched and metasedimentary xenoliths, and metasedimentary granulites have consistently higher 208Pb/204Pb. The wide range in '18O over a restricted range in Nd and Sr isotope values, in combination with the predominance of LREE-depleted trace-element compositions, is consistent with an origin as a package of hydrothermally altered oceanic crust. The existence of '18O values lower than average MORB and/or mantle peridotite requires that at least some of these rocks were hydrothermally altered at high temperature, presumably in the oceanic lower crust. The low 143Nd/144Nd of the LREE-enriched mafic granulites cannot be explained by simple mixing between a LREE-depleted melt and an enriched component, represented by the recovered metasediments. Instead, we interpret these rocks as the metamorphic equivalent of the shallowest levels of the ocean crust where pillow basalts are intimately intercalated with oceanic sediments. A possible model is accretion of oceanic crustal slices during subduction and convergence, followed by high-grade metamorphism during the Alpine orogeny. 相似文献
4.
Orthopyroxene-rich olivine websterite xenoliths (OWB2) in Palaeogene basanites in East Serbia are mostly composed of tabular low-Al2O3 orthopyroxene (> 70 vol.%, Mg# 85–87) containing tiny Cr spinel inclusions. Orthopyroxene shows a slightly U-shaped primitive mantle-normalized trace element pattern with strong peaks at U and Pb, similar to that of orthopyroxene from normal regional peridotitic mantle. In between the orthopyroxenes are interstitial spaces composed of partially altered olivine (Mg# 85–87), clinopyroxene, Ti-rich spinel, Mg-bearing calcite, K-feldspar, apatite, ilmenite and relicts of a hydrous mineral. Clinopyroxene appears as selvages around orthopyroxene and as coarser euhedral crystals. Trace element patterns of the clinopyroxene selvages resemble those of adjacent orthopyroxene, whereas the coarser ones have flatter and more LREE- and LILE-enriched patterns, similar to that of metasomatic clinopyroxene. The OWB2 xenoliths are interpreted as having formed in two stages. During Stage I orthopyroxene crystallized, along with some spinel, olivine and probably hydrous phase(s). This original OWB2 lithology was a hydrous olivine-bearing orthopyroxenite that crystallised from subduction-related SiO2-saturated, boninite-like magmas. During Stage II the interstitial minerals formed due to infiltration of a low-SiO2, high-CaO and CO2-rich external melt, accompanied by decomposition of original H2O-bearing minerals. The calculated composition of the infiltrating liquid corresponds to a mafic alkaline melt similar to the basanitic host but more enriched in CO2, LREE and LILE. Metasomatism is interpreted in terms of small degree melts related to the Palaeogene mafic alkaline magmatism. 相似文献
5.
Summary For two pairs of arctic and antarctic stations, one coastal and one mountainous, an intercomparison between the summer radiation and surface energy budgets was carried out. The station pairs were similar in both latitude and altitude. It was found that the global radiation was larger for both antarctic stations. This is the result of a smaller Earth-Sun distance and cleaner atmosphere in Antarctica. Cloudiness, and for the arctic mountainous station substantial screening of the sun, also contributed. Further, large differences were found in the albedo. In the Arctic, the summer surfaces considered were bare tundra and melting snow, with respective albedos of 20 and 59%, while in Antarctica the surfaces considered were melting and dry snow, with albedos of 67 and 83% respectively.This results in a less positive radiation balance at both antarctic stations, despite the higher incoming global radiation. In turn, less sensible heat transfer from the surface to air results in lower temperatures in the Antarctic. The reduced rate of evaporation in Antarctica results in a drier atmosphere and less cloudy conditions.
With 5 Figures 相似文献
Zusammenfassung Für jeweils zwei arktische und antarktische Stationen, von denen jeweils eine an der Küste, eine in den Bergen liegt, wurden Vergleiche in bezug auf sommerliche Strahlungs- und Oberflächenenergiebilanz angestellt. Die Stationen liegen auf vergleichbarer Höhe und Breite. Es hat sich gezeigt, daß die Gesamt-Einstrahlung in den antarktischen Stationen größer war, aufgrund geringerer Erde-Sonnen-Distanz und der reineren Atmosphäre. Bewölkung und vor allem die für die arktischen Gebirgsstationen ausschlaggebende Sonnenabschirmung tragen auch zu dieser Differenz bei. Weiters wurden starke Albedounterschiede beobachtet. In der Arktis waren die Beobachtungsoberflächen im Sommer offene Tundra und schmelzender Schnee mit einer Albedo von 20 bzw. 59%, während die Antarktisoberflächen, nasser und trockener Schnee, eine Albedo von 67 bzw. 83% aufwiesen.Dies ergibt eine weniger positive Strahlungsbilanz für die beiden antarktischen Stationen trotz höherer Gesamteinstrahlung. Infolgedessen bewirkt die geringere Wärmeabgabe des Bodens an die Luft niedere Temperaturen in der Antarktis. Geringere Verdunstung ergibt somit geringere Bewölkung und eine trockenere Atmosphäre über der Antarktis.
With 5 Figures 相似文献
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9.
Marcello A. Mannino Kenneth D. Thomas Melanie J. Leng Hilary J. Sloane 《Geo-Marine Letters》2008,28(5-6):309-325
Shells of the rocky shore intertidal gastropod Osilinus turbinatus (von Born), often abundant in archaeological deposits in the Mediterranean region, are a potential source of data on palaeotemperature, palaeoseasonality and archaeological seasonality. To evaluate this species as a climate archive, investigations of annual patterns of shell growth and of monthly variations in oxygen isotopes in shell carbonates were made on different populations in NW Sicily. Mark-recapture experiments at San Vito lo Capo and Mazzaforno show that O. turbinatus grows almost continuously throughout the year but at different rates in different seasons. Around 75% of the yearly shell growth occurs in the autumn and winter. On average, larger/older shells produce narrower annual growth increments than smaller/younger ones. Conspicuous growth lines in larger/older shells show that growth stops during the hottest part of the summer. Oxygen isotope analyses on monthly collected shells of O. turbinatus from three shores (Cala Grande, Monte Cofano and Mazzaforno) show that the isotope values record temperature variations through the year. In all the datasets, surface seawater temperatures (SSTs) calculated from δ18OSHELL mostly underestimate measured SSTs, offsets being generally greater in summer. Minimum annual offsets range from 0.0°C to 0.7°C and maximum annual offsets from 3.1°C to 8.7°C. δ18OSHELL values fail to record temperatures higher than 25°C. Careful selection of shells to be analysed can reduce offsets between δ18OSHELL temperature estimates and measured SSTs for many parts of the year, except the hottest. Allowing for this, shells of O. turbinatus offer good potential as climate archives and for archaeological studies of seasonal patterns of human foraging for shellfish. 相似文献
10.
Hilary A. Neckles Eric T. Koepfler Leonard W. Haas Richard L. Wetzel Robert J. Orth 《Estuaries and Coasts》1994,17(3):597-605
The combined effects of nutrient enrichment and grazing by isopods and amphipods on abundances of seagrass epiphytes were tested inZostera marina L. (eelgrass) microcosms. Using epifluorescence microscopy, densities of epiphytic diatoms, cyanobacteria, heterotrophic flagellates, and heterotrophic bacteria were enumerated after 1 mo and 2 mo of treatment. In general, numbers of diatoms decreased, in the presence of grazers and showed little response to nutrient enrichment, whereas numbers of cyanobacteria increased with nutrient enrichment and showed little response to grazing. Thus, macrofaunal grazing maintained a photoautotrophic community domainated by cyanobacteria, particularly under nutrient enriched conditions. Following 2 mo of treatment, dense macroalgal growth under nutrient-enriched conditins with grazers absent appeared to limit populations of both epiphytic autotrophs. Patterns of abundance of heterotrophic bacteria suggested that the original bacteria population was nutrient limited. Bacteria populations may have been limited by organic carbon supplies at the end of the experiment. Abundances of heterotrophic flagellates and bacteria were strongly correlated on both sampling dates. Results suggest that heterotrophic flagellates might serve as a link between heterotrophic bacterial production and higher trophic levels in seagrass epiphyte food webs. 相似文献