In-situ Hf isotope analyses and U–Pb dates were obtained by laser ablation-MC-ICP-MS for a zircon-bearing mantle eclogite xenolith from the diamondiferous Jericho kimberlite located within the Archean Slave Province (Nunavut), Canada. The U–Pb zircon results yield a wide range of ages (2.0 to 0.8 Ga) indicating a complex geological history. Of importance, one zircon yields a U–Pb upper intercept date of 1989 ± 67 Ma, providing a new minimum age constraint for zircon crystallization and eclogite formation. In contrast, Hf isotope systematics for the same zircons display an intriguing uniformity, and corresponding Hf depleted mantle model ages range between 2.1 ± 0.1 and 2.3 ± 0.1 Ga; the youngest Hf model age is within error to the oldest U–Pb date.
The Jericho eclogites have previously been interpreted as representing remnants of metamorphosed oceanic crust, and their formation related to Paleoproterozoic subduction regimes along the western margin of the Archean Slave craton during the Wopmay orogeny. Hf isotope compositions and U–Pb results for the Jericho zircons reported here are in good agreement with a Paleoproterozoic subduction model, suggesting that generation of oceanic crust and eclogite formation occurred between 2.0 and 2.1 Ga. The slightly older Hf depleted mantle model ages (2.1 to 2.3 Ga) may be reconciled with this model by invoking mixing between ‘crustal’-derived Hf from sediments and more radiogenic Hf associated with the oceanic crust during the 2 Ga subduction event. This results in intermediate Hf isotope compositions for the Jericho zircons that yield ‘fictitiously’ older Hf model ages. 相似文献
Whalebacks are convex landforms created by the smoothing of bedrock by glacial processes. Their formation is attributed to glacial abrasion either by bodies of subglacial sediment sliding over bedrock or by individual clasts contained within ice. This paper reports field measurements of sediment depth around two whaleback landforms in order to investigate the relationship between glacigenic deposits and whaleback formation. The study site, at Lago Tranquilo in Chilean Patagonia, is situated within the Last Glacial Maximum (LGM) ice limits. The two whalebacks are separated by intervening depressions in which sediment depths are generally 0.2 to 0.3 m. Two facies occur on and around the whalebacks. These facies are: (1) angular gravel found only on the surface of the whalebacks, interpreted as bedrock fracturing in response to unloading of the rock following pressure release after ice recession, and (2) sandy boulder‐gravel in the sediment‐filled depressions between the two whalebacks, interpreted as an ice‐marginal deposit, with a mixture of sediment types including basal glacial and glaciofluvial sediment. Since the whalebacks have heavily abraded and striated surfaces but are surrounded by only a patchy and discontinuous layer of sediment, the implication is that surface abrasion of the whalebacks was achieved primarily by clasts entrained in basal ice, not by subglacial till sliding. 相似文献
A roving creel survey of the recreational shore fishery along the 16.4-km coastline in the Goukamma Marine Protected Area on the south coast of South Africa was conducted from 2009 to 2011. Some 838 patrols were stratified equally among months, areas and years, but intentionally biased towards weekends. Angler densities at Buffalo Bay and Groenvlei were 0.59 and 0.28 anglers km?1, respectively. Weekend densities were double to quadruple weekday densities and fishing during winter was more popular than during summer. Area, habitat and distance to access points explained variation in angler densities. Shannon–Wiener diversity in catches declined from 2.18 in an earlier (1993–2002) survey to 1.79. Although the order of species abundance in the catches remained largely unchanged, blacktail Diplodus capensis dominance increased to 57.3% by number, at the expense of galjoen Dichistius capensis. Habitat explained 27% of the variance in catch composition. The catch per unit effort (CPUE) for the top nine species ranged from 0.19 to 6.35 fish 100-h?1. The CPUE of all species, except spotted grunter Pomadasys commersonnii, declined. Blacktail and galjoen CPUE declined by 17% and 77%, respectively. The total catch estimate was 2 986 fish y?1. Transgressions of size limits were common. The results suggest that the fishery is overexploited and that catch rates are declining. 相似文献
A series of enclosed ecosystem experiments were conducted in a land-based tank near the seaside of West Xiamen Harbor. The
results of experiments conducted in different seasons and years showed a repeatable phytoplankton succession. In this relatively
stable ecosystem with added nutrients and trace metals, diatoms dominated initially, dinoflagellates dominated in the later
stage, and dinoflagellate red tides eventually occurred. Vitamin B12 enrichment may speed up this succession process. Stirring the water column could stop this process. Soluble Mn at a level
of 3–4 μg/L in seawater, which also is the existing concentration of soluble Mn in Xiamen Harbor seawater, is sufficient for
the multiplication of algae and occurrence of red tide. The present study showed that excessive soluble Mn in Xiamen Harbor
cannot cause red tide, and that Fe was one of the important factors causing diatiom red tide in this present study.
Project 39570145 supported by NSFC. 相似文献
1 Introduction The Gaojiacun intrusive complex is one of the numerous ultramafic-mafic intrusions in Sichuan Province of China. It was mapped during the 1970s and studied mainly by Chinese scientists (e.g. Geological Team 106, 1975; Shen et al., 1986, 1989; CGGJC, 1986; Yang et al., 1993; Li et al., 1995; Shen et al., 2003; Zhu et al., 2004a). Since the year 2000, China has become one of the largest PGE consumers. While the country can produce only less than 1 ton PGE/year, the Chin… 相似文献
The importance of the nitrogen source for phytoplankton growth in a highly eutrophic embayment, Dokai Bay, was investigated.
The DIN concentration often exceeded 100 μM of which 40–70% was NH4+. During two incubation experiments, the natural assemblage of mainly diatoms took up NH4+ instead of NO3−. The growth of two Skeletonema species isolated in Dokai Bay were significantly faster on NH4+ (1.86 and 1.27 div. d−1 respectively) than on NO3− (1.55 and 1.04 div. d−1 respectively). Our results indicated that these diatoms could grow faster by using NH4+ compared to NO3− in this eutrophic bay. 相似文献
Changes in lake levels during the last 12000 years in eastern North America show spatially coherent patterns, implying climatic control. Conditions were generally wetter than today during the late glacial, becoming more arid towards 6000 years BP when most lakes were low. Lakes rose after 6000 years BP, reaching modern levels by about 3000 years BP. These palaeohydrological changes broadly agree with simulated changes in moisture balance derived from experiments with the NCAR Community Climate Model (Kutzbach and Guetter 1986) with changing orbital parameters and lower boundary conditions (sea-surface temperature and ice extent). However, the model simulates maximum aridity at 9000 years BP. Data and model show broadly similar spatial patterns, implying that the lake-level changes can be explained by the changing boundary conditions and their effects on atmospheric circulation. At 12000 years BP most lakes were high because of increased precipitation along the jet-stream storm-track south of the ice sheet. By 9000 years BP, with the much reduced ice sheet, many lakes along the eastern seaboard and in the southeast were lower than present because of greater evaporation due to high summer insolation. The warming of the continental interior generated an enhanced monsoon low in the southwest, causing increased southerly flow which helped to maintain higher lakes in the Midwest. Dry conditions spread eastwards across the Midwest between 9000 and 6000 years BP. This effect is not shown by the model, which continues to bring monsoonal precipitation into the Midwest while simulating enhanced westerly flow and drier conditions further to the west. Such displacements of circulation features are unimportant at the continental scale, but could be significant if general circulation models are used for regionalscale predictions of changes in the moisture balance. 相似文献
Viewing K-feldspars as containing a discrete distribution of diffusion domain sizes reconciles otherwise disconsonant features common in their 40Ar/39Ar age spectra and Arrhenius plots but raises a fundamental question. What are the features in K-feldspar that endow it with this behavior? We report here the results of two different kinds of experiments that help isolate the nature of the responsible diffusion properties. To assess the thermal stability of the diffusion domains during laboratory treatment, MH-10 K-feldspar was step-heated to 850°C, removed from the furnace and split. One split was reirradiated and the other returned to the furnace and completely degassed. Following re-irradiation, the original heating schedule was used to degas the second aliquot. Apart from the first 5% of gas released, the diffusion properties show little change relative to the original result but, it appears, the physical character of a portion of the smallest domain has been altered. Results of duplicate step-heating experiments of samples treated at 750°C, 950°C and 1100°C prior to irradiation are consistent with the conclusions of the double irradiation experiment. In a second series of experiments, sized aggregates of MH-10 K-feldspar were analyzed by the 40Ar/39Ar step-heating method. The resultant log(r/ro) plots reveal that the largest domain is annihilated when the particle size is reduced to about 50 μm. From this result we infer that the largest diffusion domain size is between 60 and about 130 μm in diameter. This estimate, together with knowledge of the relative domain size distribution obtained from modeling the log(r/ro) plot, sets the size of the smallest domain to be less than about 1 μm. Microstructural examination of MH-10 K-feldspar identifies sub-grain features that correspond in size to our independent estimates for the largest and smallest diffusion domains. These results strongly support the view that low-temperature K-feldspars contain a distribution of diffusion length scales that are well approximated as discrete domain sizes and that laboratory heating below the onset of melting does not significatly affect the ability to obtain thermal reconstructions from the 40Ar/39Ar systematics. 相似文献