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71.
T.E. Johnson C.L. Kirkland D.R. Viete S. Fischer S.M. Reddy N.J. Evans B.J. McDonald 《地学前缘(英文版)》2017,8(6):1469-1478
The type locality for high-temperature,low-pressure regional metamorphism,the Buchan Block in NE Scotland,exhibits profound differences to the rest of the Grampian Terrane.These differences have led some to regard the Buchan Block as an exotic crustal fragment comprising Precambrian basement gneisses and cover rocks thrust into their current position during Grampian orogenesis.Although rocks of the Buchan Block are now generally correlated with Dalradian strata elsewhere,the origin of the gneisses and the cause of the high heat flow and associated magmatism is debated.We report SIMS U-Pb and LA-ICPMS Hf isotopic data in zircon from high-grade rocks from the northeast(Inzie Head Gneiss)and northwest(Portsoy)corners of the Buchan Block.Around Inzie Head,upper amphibolite to granulite facies metasedimentary gneisses coexist with diorite sheets that were emplaced contemporaneously with partial melting of their host rocks,at least locally.U-Pb geochronology indicates a crystallisation age for the diorite of 486±9 Ma.Highly-deformed diorites within the Portsoy Gabbro have a crystallisation age of 493±8 Ma.Ages of ca.490 Ma for magmatism and high-grade metamorphism,which are broadly contemporaneous with ophiolite obduction and the onset of orogenesis,are significantly older than the established peak of Grampian metamorphism(ca.470 Ma).We propose a new model for the Grampian Orogeny involving punctuated tectonothermal activity due to tectonic switching during accretionary orogenesis.Rollback of a NW-dipping subduction zone at ca.490 Ma produced a back-arc environment(the Buchan Block)with associated arc magmatism and high dT/dP metamorphism.Arrival of an outboard arc resulted in shortening(the initial phase of the Grampian Orogeny)at ca.488 Ma.Rollback of a NW-dipping subduction zone to the SE of the ca.488 Ma suture began at 473 Ma and led to lithospheric-scale extension,decompression melting and advective heating of the middle crust,producing the widespread ca.470 Ma Grampian(classic Barrovian and Buchan)regional metamorphism.Resumed hinge advance and the final phase of shortening cut off the heat supply at ca.465 Ma,marking the end of the Grampian Orogeny. 相似文献
72.
Understanding the influence of climatic and non-climatic factors on geochemical signals in corals is critical for assessing coral-derived records of tropical climate variability. Porites microatolls form large, disk-shaped colonies constrained in their upward growth by exposure at or close to mean spring low water level, and occur on Indo-Pacific reefs. Microatolls appear suitable for paleoclimate reconstruction, however the systematics of the microatoll chemistry-climate relationship are yet to be characterized. In this study, the δ18O signal in Porites microatolls from well-flushed reef flats on Kiritimati (Christmas) Island, central Pacific was investigated for intra-coral (growth aspect and extension rate effects) and between-coral effects, and to explore the climate signal contained within their skeletons. Samples for δ18O analysis were taken from six individual transects from different positions within Porites microatoll XM22. The results show that: (1) the average standard deviation for the mean δ18O values of transects that represent the same time periods is 0.03‰, and is within measurement error for a single analysis (0.04‰); (2) the average standard deviation for time-equivalent, near-monthly samples along the transects within the same microatoll is 0.07‰ and; (3) comparison of the average δ18O values of records for different microatolls from across Kiritimati Island show only a small between-coral differences of 0.04‰ and 0.11‰ for different time periods. These differences in mean δ18O are within the range for intra- and inter-colony differences in seasonal and interannual δ18O reported for dome-shaped Porites. Based on these results, a stacked microatoll δ18O record was constructed for the period 1978-2007 for comparison with published coral δ18O records for nearby dome-shaped Porites. There is a systematic offset between the two types of records, which is probably due to variations in δ18O seawater across Kiritimati Island. Despite the offset, all records show similar amplitudes for the seasonal-cycle of δ18O, and there is a strong correlation (r = −0.71) between microatoll δ18O and local sea surface temperature (SST). The δ18O-SST slope relationship for microatolls is −0.15‰/°C, very similar to that reported for fast-growing domed corals (−0.18‰ to −0.22‰/°C). Statistical analysis of the stacked microatoll δ18O record shows that it is correlated with both local and large-scale climate variables (primarily SST) at semiannual, annual and interannual timescales. Our results show that the signal reproducibility and fidelity of skeletal δ18O in coral microatolls is comparable to that observed for more conventional coral growth forms. Longer-lived, and fossil, Porites microatolls, where they have grown in suitably flushed environments, are likely to contain δ18O signals that can significantly extend instrumental records of tropical climate variability. 相似文献
73.
74.
Julia L. Barringer Timothy P. Wilson Zoltan Szabo Jennifer L. Bonin Jeffrey M. Fischer Nicholas P. Smith 《Environmental Geology》2008,53(6):1183-1199
Diurnal variations in particulate and dissolved As and metal concentrations were observed in mildly alkaline water from a
wetlands site on the Wallkill River in northwestern New Jersey. The site, underlain by glacial sediments over dolomite bedrock,
is 10 km downstream from a mined area of the Franklin Marble, host to Zn ores, also As and Mn minerals. In mid-September 2005,
maxima and minima in dissolved-oxygen-concentration and pH, typically caused by photosynthesis and respiration, occurred at
2000 and 0800 hours. Concentrations of dissolved As (1.52–1.95 μg/L) peaked at dusk (2000 hours), whereas dissolved Mn and
Zn concentrations (76.5–96.9 and 8.55–12.8 μg/L, respectively) were lowest at dusk and peaked at 1000 hours. These opposing
cycles probably reflect sorption and desorption of As (an anion), and Mn and Zn (cations) as pH varied throughout the 24-h
period. Doubly-peaked cycles of B, Cl, SO4, and nutrients also were observed; these may result from upstream discharges of septic-system effluent. Both recoverable
amd particulate Al, Fe, Mn, and Zn concentrations peaked between 0200 and 0600 hours. The particulate metals cycle, with perturbations
at 0400 hours, may be influenced by biological activity. 相似文献
75.
Stefan Gödeke Helmut Geistlinger Anko Fischer Hans-Hermann Richnow Mario Schirmer 《Environmental Geology》2008,55(6):1255-1261
Reactive tracer tests are performed to derive flow, transport and in situ biodegradation parameters. This paper describes
the 3D simulation of a reactive tracer test using the transition probability geostatistical approach. Fifty different equally
probable aquifer realizations were generated based on the geological information of 107 boreholes in an area of 62,500 m2. One realization was chosen for the reactive transport simulation based on the results of groundwater flow modeling and on
particle tracking calculations for the site. Field velocities at the site vary between 0.4 and 3 m/d. The transport of the
reactive tracers deuterium ring labeled toluene-d5 and fully deuterated toluene-d8 was simulated and first-order biodegradation rates of 0.017 d−1 for toluene-d5 and 0.012 d−1 for toluene-d8 were determined. 相似文献
76.
C. Warneke R. Holzinger A. Hansel A. Jordan W. Lindinger U. Pöschl J. Williams P. Hoor H. Fischer P. J. Crutzen H. A. Scheeren J. Lelieveld 《Journal of Atmospheric Chemistry》2001,38(2):167-185
Airborne measurements of volatile organic compounds (VOC) were performed overthe tropical rainforest in Surinam (0–12 km altitude,2°–7° N, 54°–58° W) using the proton transferreaction mass spectrometry (PTR-MS) technique, which allows online monitoringof compounds like isoprene, its oxidation products methyl vinyl ketone,methacrolein, tentatively identified hydroxy-isoprene-hydroperoxides, andseveral other organic compounds. Isoprene volume mixing ratios (VMR) variedfrom below the detection limit at the highest altitudes to about 7 nmol/molin the planetary boundary layer shortly before sunset. Correlations betweenisoprene and its product compounds were made for different times of day andaltitudes, with the isoprene-hydroperoxides showing the highest correlation.Model calculated mixing ratios of the isoprene oxidation products using adetailed hydrocarbon oxidation mechanism, as well as the intercomparisonmeasurement with air samples collected during the flights in canisters andlater analysed with a GC-FID, showed good agreement with the PTR-MSmeasurements, in particular at the higher mixing ratios.Low OH concentrations in the range of 1–3 × 105molecules cm-3 averaged over 24 hours were calculated due to lossof OH and HO2 in the isoprene oxidation chain, thereby stronglyenhancing the lifetime of gases in the forest boundary layer. 相似文献
77.
Four cores of anoxic sediments were collected from the Seine estuary to assess the early diagenesis pathways leading to the
formation of previously reactive phase. Pore waters were analyzed for dissolved iron (Fe) and manganese (Mn) and different
ligands (e.g., sulfate, chloride, total inorganic carbon). The anoxic zone is present up to the first centimeter depth, in
these conditions the reduction of Mn and Fe oxides and SO4
2− was verified. The sulfate reduction was well established with a subsequent carbon mineralization in the NORMAI94 core. The
chemical speciation of Mn and Fe in the dissolved and solid phases was determined. For the dissolved phase, thermodynamic
calculations were used to characterize and illustrate the importance of carbonate and phosphate phases as sinks for Fe and
Mn. The ion activity product (IAP) of Fe and Mn species was compared to the solubility products (Ks) of these species. In
the solid phase, the presence of higher concentration of calcium carbonate in the Seine sediments is an important factor controlling
Mn cycle. The carbonate-bound Mn can reach more than 75% of the total concentration. This result is confirmed by the use of
electron spin resonance (ESR) spectroscopy. The reduction of Fe is closely coupled to the sulfate reduction by the formation
of new solid phases such as FeS and FeS2, which can be regarded as temporal sinks for sulfides. These forms were quantified in all cores as acid volatile sulfide
(AVS: FeS+ free sulfide) and chromium reducible sulfide (CRS: FeS2+elemental sulfur S0). 相似文献
78.
79.
Epiphytic orchids and their ecological niche under anthropogenic influence in central Himalayas,Nepal 总被引:1,自引:1,他引:0
The survival chance of epiphytic orchids today not only depends on the natural site conditions required by the orchids but also on anthropogenic changes in site conditions.This study answers two questions:(1) What is the ecological niche of the different epiphytic orchid species?(2) What are the ecological factors that threaten epiphytic orchid's population under anthropogenic disturbances? Our study area was the Kathmandu valley,Nepal,with its subtropical forest.We established 156 systematically selected sampling points in the Kathmandu area covering different types of ecosystems under human impacts such as densely populated area,agricultural land,mixed agricultural and settled area,old tree patches,and a natural forest in a national park.The ecological niche of the orchid species was analyzed with a principal component analysis(PCA).The correlations between the different site factors were statistically significant.Spearman's rank correlation matrices showed that the variables land-use intensities with altitude,and height with diameter in breast height(dbh) of host had the highest significant positive correlation coefficient(0.67 and 0.64 respectively).On the other hand,host bark p H and altitude as well as land use had a significantly strong negative correlation coefficient(-0.80 and-0.61,respectively).Different epiphytic orchid species interact differently with the given set ofenvironmental factors:for occurrence of Vanda cristata there is no single environmental factor of special influence,while for Rhynchostylis retusa high bark p H and high light availability are important.First two axis of the PCA explained more than 50% of the total variance.Most orchid species occupy a specific,narrow niche in this ecological space.The main causes of anthropogenic influence of orchid population in the Kathmandu Valley are loss of adequate host trees(species and size) and increasing air pollution,resulting in increasing host bark p H. 相似文献
80.