Strontium isotope zoning in garnet: implications for metamorphic matrix equilibration,geochronology and phase equilibrium modelling     

J. Sousa  M. J. Kohn  M. D. Schmitz  C. J. Northrup  F. S. Spear 《Journal of Metamorphic Geology》2013,31(4):437-452

In principle, garnet growth rates may be calculated from ⁸⁷Rb/⁸⁶Sr and ⁸⁷Sr/⁸⁶Sr measurements in garnet subsamples and the surrounding rock matrix. Because of low Rb/Sr, garnet should passively record the matrix decay of ⁸⁷Rb to ⁸⁷Sr as a progressive increase in ⁸⁷Sr/⁸⁶Sr from core to rim. This concept was tested by collecting Rb‐Sr data for five garnet grains from four major orogenic belts: eastern Vermont (c. 380 Ma), western New Hampshire (c. 320 Ma), southern Chile (c. 75 Ma) and northwestern Italy (c. 35 Ma). Both normal Sr isotope zoning (increasing ⁸⁷Sr/⁸⁶Sr from core to rim) and inverse Sr zoning (decreasing ⁸⁷Sr/⁸⁶Sr from core to rim) were observed. Garnet and matrix isotope data commonly yielded grossly inaccurate model ages. Incomplete Rb and Sr equilibration among matrix minerals is invoked to explain the deviations between theoretical v. measured zoning patterns and the age disparities. Initially, the reactive matrix is dominated by rapidly equilibrating Rb‐rich mica, which imparts high ⁸⁷Sr/⁸⁶Sr values in garnet cores. Progressive participation of slower equilibrating Sr‐rich plagioclase buffers or even reduces ⁸⁷Sr/⁸⁶Sr, possibly leading to flat or decreasing ⁸⁷Sr/⁸⁶Sr from garnet cores to rims. Unusually high ⁸⁷Sr/⁸⁶Sr in garnet in combination with bulk matrix compositions causes erroneously young apparent ages, so metamorphic ages, growth rates, and associated heating and loading rates are likely suspect. Although Rb‐Sr may be the most susceptible because of the profound disparities between mica and feldspar, zircon reactivity might influence the Lu‐Hf system by up to a few per cent. The Sm‐Nd system seems generally immune to these effects. Pseudosection analysis and conventional garnet geochronology, which presume complete matrix equilibration during metamorphism, may require modification to account for differences between whole‐rock v. reactive matrix compositions.  相似文献   

Limitations of chemical dating of monazite     

Frank S. Spear  Joseph M. Pyle  Daniele Cherniak 《Chemical Geology》2009,266(3-4):227-239

Instrumental and spectral characteristics germane to chemical dating of monazite have been tested using the Cameca SX-100 at Rensselaer Polytechnic Institute. Statistical analysis demonstrates that, for trace element analysis, equal counting time on peak and background is required for optimal statistical precision, thus rendering impractical the procedure of fitting the entire spectrum to obtain background values. Energy shifts require shifting the detector voltage window between peak and background positions, and it is concluded that the differential auto PHA mode works optimally for this.Analyses of Pb-free phosphates, silicates, and oxides are used to measure spectral interferences with the PbM_α peak and background positions. Backgrounds were modeled using both linear and exponential fits. It was found that the difference in background counts using the two fits varies with each of the five spectrometers examined, and that the high-pressure (3 bar) detectors show larger differences in exponential vs. linear peak-minus-background (P-B) values than the low-pressure (1 bar) detectors. In addition, every spectrometer requires a unique correction for every major element in monazite. An analytical protocol is presented that incorporates these results. This protocol was applied to several monazite standards to determine inter-spectrometer variability, and spectrometer reproducibility from session to session. It was found that the difference in composition (and age) between spectrometers on identical spots exceeds the 2 sigma standard error of the mean of composition (or age) on either spectrometer. This means that (a) additional sources of error beyond the counting statistics exist between spectrometers; (b) the precision of microprobe ages cannot be continuously improved by additional counting; and (c) the minimum realistic precision is on the order of ± 2–3% for monazites with around 1500–2000 ppm total Pb, or an additional absolute uncertainty of 20–50 ppm Pb.  相似文献   

Investigation of calcareous beach sands in the Akumal and Tulum areas for use in constructed wetlands,Eastern Yucatan Peninsula     

Mark P. S. Krekeler  Erin P. Argyilan  Jill Lepp  Lance E. Kearns 《Environmental Earth Sciences》2009,59(2):411-420

Constructed wetlands can be a low-cost, ecologically based technology to locally treat domestic wastewater prior to discharge. Wetland systems implemented in the resort village of Akumal in the state of Quintana Roo, in the Yucatan Peninsula of Mexico have been considered moderately successful over approximately 10 years of operation. Yet the wide-scale implementation of constructed wetlands has not occurred in the region nor throughout Mexico due largely to the cost of the crushed aggregate used in these systems. Variation in the geotechnical and mineralogical properties of 29 samples of local calcareous beach sands collected from the tourist areas of Akumal and Tulum in the Yucatan, Mexico were analyzed and compared to those of crushed aggregate currently used in constructed wetland systems in Akumal. Powder X-ray diffraction on beach sand samples from Akumal and Tulum indicates that the proportion of aragonite varies from 24 to 84%, calcite varies from 7 to 38% and Mg-calcite varied from 8 to 57% compared to crushed aggregate which consists of 0–25% aragonite and 75–100% calcite. Compared to crushed aggregate beach sands are unimodal and have better uniformity coefficients, making them a better geotechnical choice than crushed sands with respect to these properties. Grain-size of the beach sands is much smaller compared to that of the crushed aggregate currently used in the constructed wetland systems in Akumal which should enhance the efficiency of the filtration of organic matter and bacteria. The average hydraulic conductivity measurements of 0.010 and 0.016 cm/s for beach sands from Akumal and Tulum, respectively are comparable or less than the average of 0.016 cm/s for crushed aggregate currently used in engineered wetland systems. Yet a combination of high bulk porosity, intragranular pores, and the presence of Mg-calcite observed in beach sands may likely result in higher dissolution rates versus the crushed aggregate. Preliminary assessment suggests that the use of local sand may be a technical, economic, and perceptual enhancement to the systems. Still a careful and thorough analysis of the potential environmental and aesthetic impacts of mining beach sands is necessary.  相似文献   

Magnetic susceptibility as a proxy for coal ash pollution within riverbed sediments in a watershed with complex geology (southeastern USA)     

Ellen A. Cowan  Erin E. Epperson  Keith C. Seramur  Stefanie A. Brachfeld  Steven J. Hageman 《Environmental Earth Sciences》2017,76(19):657

A study of near surface sediments from the Dan River (southeastern USA) was conducted to assess the use of magnetic properties as proxies of coal ash after a recent spill. The watershed geology is diverse and potentially contributes magnetic minerals to riverbed sediment from diabase dikes in the Dan River Triassic Basin and from granitic gneiss outside the basin. Coal ash is heterogeneous, including aluminosilicate spheres, amorphous particles and carbonaceous rods and lacy particles. The magnetic fraction of ash from the failed storage pond is up to 17 wt% and is mostly composed of black spheres with maghemite and magnetite. Ash was detected in riverbed sediment from quiet water settings such as inside of meander bends, the confluence of tributary streams and near islands between the spill site and 20 miles downstream in the Schoolfield Reservoir, Danville, VA. The strong magnetic signal is detected above background in riverbed samples and is strongly positively correlated with total ash; elevated low field magnetic susceptibility (χ _LF) is evident in samples with ≥ 12% ash content. Anhysteretic remanent magnetization and hysteresis parameters delineate native sediment, ash-bearing sediment, and diabase dikes. Between 20 and 70 miles downstream of the spill site, ash concentrations were either buried or too low due to dilution with native sediment to be detected with χ _LF in riverbed samples.  相似文献   

Spatial and temporal variation of diverse inter‐tidal algal assemblages in Southwest O‘ahu          下载免费PDF全文

Traci Erin Cox  Heather L. Spalding  Michael S. Foster 《Marine Ecology》2017,38(3)

Spatial and temporal variation in tropical inter‐tidal communities is poorly known, making predictions about the effects of climate change and other anthropogenic disturbances difficult. Along Southwest O‘ahu, Hawai'i, local residents are concerned about the environmental effects of coastal development and the perceived loss of targeted algal species, which are collected for human consumption. To describe the coastal benthic community and better understand the processes that form and maintain it, the abundance and composition of macroalgae were sampled in the region's inter‐tidal zone from 2006 to 2015. Sixty‐six macroalgal species and two broad algal assemblages were identified that corresponded to substrate topography and sand influence at a similar tidal elevation. Along flat carbonate benches with a sand beach, Phaeophyceae and Rhodophytes occurred in almost equal proportions, while shores with slightly more topographic relief and angular substrate were dominated by Rhodophytes. Foliose or turf algal forms were most common. Surveys captured the local invasion of an alga, Avrainvillea sp. and significant declines in abundant macroalgae in 2015 after a period of unseasonably warm, calm water. Temporal changes in algal assemblages were related to maximum water temperature and wave height but not precipitation. Thus, algal assemblages appear to be structured by local beach morphology as they interact with sand and wave activity and episodically by unusual weather events. However, manipulation and continuous monitoring of the algal assemblages coupled to sensing of the localized environment is necessary to confirm factors related to assembly maintenance and recent species shifts.  相似文献   

Experiencing agricultural failure: Internal migration, tourism and local perceptions of regional change in the Yucatan     

Lindsey Carte  Erin Daley 《Geoforum》2010,41(5):700-710

In response to a growing body of literature encouraging geographers to present textured, regional case studies that demonstrate how neoliberalism functions at diverse scales, this article presents a nuanced account of neoliberalism in Mexico’s Yucatan peninsula. The study seeks to illustrate how neoliberal reforms to agriculture and ongoing neoliberal tourism development in the Yucatecan state of Quintana Roo intersect to create a unique regional context for migration between the state’s rural areas and Mexico’s largest planned tourism development zone in Cancun. The research presented is based on a series of in-depth qualitative interviews that took place in 2003 with rural residents and regional migrants working in Cancun. Through the employment of an experiential definition of neoliberalism, grounded in the lived experiences and perceptions of study participants, the article suggests that rural-urban tourist pole migration is the result of a complex negotiation of the region’s neoliberal landscape. The article argues that rural residents’ and migrants’ perceptions of the failure of agriculture as a livelihood is shaped relative to the perceived success of the tourism industry and better livelihood opportunities in Cancun. These widespread perceptions of agricultural failure in the Yucatan peninsula are constructed not solely in response to local conditions but rather in response to the overall regional outcome of neoliberal economic restructuring initiatives which have negatively affected traditional agriculture in the rural sphere while privileging tourism development in the urban sphere centered in Cancun.  相似文献   

A shallow subsurface controlled release facility in Bozeman, Montana, USA, for testing near surface CO2 detection techniques and transport models   总被引：1，自引：0，他引：1  

Lee H. Spangler  Laura M. Dobeck  Kevin S. Repasky  Amin R. Nehrir  Seth D. Humphries  Jamie L. Barr  Charlie J. Keith  Joseph A. Shaw  Joshua H. Rouse  Alfred B. Cunningham  Sally M. Benson  Curtis M. Oldenburg  Jennifer L. Lewicki  Arthur W. Wells  J. Rodney Diehl  Brian R. Strazisar  Julianna E. Fessenden  Thom A. Rahn  James E. Amonette  Jon L. Barr  William L. Pickles  James D. Jacobson  Eli A. Silver  Erin J. Male  Henry W. Rauch  Kadie S. Gullickson  Robert Trautz  Yousif Kharaka  Jens Birkholzer  Lucien Wielopolski 《Environmental Earth Sciences》2010,60(2):227-239

A controlled field pilot has been developed in Bozeman, Montana, USA, to study near surface CO₂ transport and detection technologies. A slotted horizontal well divided into six zones was installed in the shallow subsurface. The scale and CO₂ release rates were chosen to be relevant to developing monitoring strategies for geological carbon storage. The field site was characterized before injection, and CO₂ transport and concentrations in saturated soil and the vadose zone were modeled. Controlled releases of CO₂ from the horizontal well were performed in the summers of 2007 and 2008, and collaborators from six national labs, three universities, and the U.S. Geological Survey investigated movement of CO₂ through the soil, water, plants, and air with a wide range of near surface detection techniques. An overview of these results will be presented.  相似文献   

Can Water Vapour Raman Lidar Resolve Profiles of Turbulent Variables in the Convective Boundary Layer?     

Volker Wulfmeyer  Sandip Pal  David D. Turner  Erin Wagner 《Boundary-Layer Meteorology》2010,136(2):253-284

High-resolution water vapour measurements made by the Atmospheric Radiation Measurement (ARM) Raman lidar operated at the Southern Great Plains Climate Research Facility site near Lamont, Oklahoma, U.S.A. are presented. Using a 2-h measurement period for the convective boundary layer (CBL) on 13 September 2005, with temporal and spatial resolutions of 10 s and 75 m, respectively, spectral and autocovariance analyses of water vapour mixing ratio time series are performed. It is demonstrated that the major part of the inertial subrange was detected and that the integral scale was significantly larger than the time resolution. Consequently, the major part of the turbulent fluctuations was resolved. Different methods to retrieve noise error profiles yield consistent results and compare well with noise profiles estimated using Poisson statistics of the Raman lidar signals. Integral scale, mixing-ratio variance, skewness, and kurtosis profiles were determined including error bars with respect to statistical and sampling errors. The integral scale ranges between 70 and 130 s at the top of the CBL. Within the CBL, up to the third order, noise errors are significantly smaller than sampling errors and the absolute values of turbulent variables, respectively. The mixing-ratio variance profile rises monotonically from ≈0.07 to ≈3.7 g² kg⁻² in the entrainment zone. The skewness is nearly zero up to 0.6 z/z _i , becomes −1 around 0.7–0.8 z/z _i , crosses zero at about 0.95 z/z _i , and reaches about 1.7 at 1.1 z/z _i (here, z is the height and z _i is the CBL depth). The noise errors are too large to derive fourth-order moments with sufficient accuracy. Consequently, to the best of our knowledge, the ARM Raman lidar is the first water vapour Raman lidar with demonstrated capability to retrieve profiles of turbulent variables up to the third order during daytime throughout the atmospheric CBL.  相似文献   

Coastal seas as a context for science teaching: a lesson from Chesapeake Bay     

Bell WH  Fowler EM  Stein JA 《Marine pollution bulletin》2003,47(1-6):253-259

Lessons that employ authentic environmental data can enhance the ability of students to understand fundamental science concepts. This differs from traditional "environmental education" in that school curricula need not set aside time for educators to teach only environmental topics. Rather, the "environment" is used to advance student learning in science and technology. The success of this approach depends on programs that encourage scientists to communicate more effectively with teachers at all education levels. The expanding diversity of research and monitoring activities on the world's marine waters constitutes an outstanding potential education resource. Many of these projects involve remote sensing with sophisticated instrumentation and employ Internet technology to compile measurements, interpret data using graphs and satellite imagery, and share the results among scientific colleagues and the general public alike. Unfortunately, these resources, which constitute a much shortened path between research findings and textbook presentation, are seldom interpreted for use by K-12 educators. We have developed an example that uses the Chesapeake Bay as a paradigm to demonstrate how such interpretation can assist educators in teaching important principles in physical oceanography and marine ecology. We present this example using PowerPoint to conduct a virtual tour of selected Internet sources. Our example begins with the conceptual "salt wedge" circulation model of Chesapeake Bay as a partially mixed estuary. Teachers have the opportunity to explore this model using salinity, temperature, and dissolved oxygen data taken from a research vessel platform during summer professional development programs. This source of authentic data, originally obtained by teachers themselves, clearly demonstrates the presence of a picnocline and deep-water anoxia. Our lesson plan proceeds to interpret these data using additional Internet-based resources at increasing scales of time and space. The "salt wedge," picnocline, and anoxia are examined using graphics derived from data taken by researchers using "ScanFish," a towed instrument that samples temperature, salinity, and dissolved oxygen at a resolution of only a few meters vertically and horizontally. The seasonal dynamics of these parameters at a given location are interpreted using biweekly monitoring data obtained as part of the state-federal Chesapeake Bay Program. The influence of annual variations in freshwater input is examined using stream flow data from US. Geological Survey gauging stations. Satellite remote sensing images from the TOPEX/Poseidon project are used to show how El Ni?o and La Ni?a events in the mid-Pacific affect the Chesapeake Bay system via rainfall on its watershed. Finally, the life cycle of the blue crab (Callinectes sapidus) is presented to show how an estuarine organism has adapted to this truly unique and dynamic coastal environment.  相似文献   

Crystallization rates of shock melts in three martian basalts: Experimental simulation with implications for meteoroid dimensions     

Erin Walton  Cliff Shaw  John Spray 《Geochimica et cosmochimica acta》2006,70(4):1059-1075

Dynamic crystallization experiments have been performed on synthetic glasses representative of shock-generated melts observed in Los Angeles, Sayh al Uhaymir 150 and Dar al Gani 476 martian basalts. On the basis of qualitative (texture) and quantitative (fractal analysis) results, we show that melt pockets in Los Angeles cooled at a rate of 1040-1560 °C/h. Sayh al Uhaymir 150 and Dar al Gani 476 melt pockets cooled at 780 °C/h. Conductive cooling models, for a range of meteoroid diameters (10-50 cm), indicate that the minimum meteoroid diameter was small, on the order of 10-15 cm and that melt pockets cooled from post-shock temperatures within minutes. Our results also have bearing on shock implanted martian atmospheric components because it is during cooling that the melt pockets have the potential to lose gases. Modeling of argon diffusion in a spherical melt pocket indicates that during cooling and quench crystallization ∼4-60% of trapped martian atmospheric argon may be lost from the melt pocket through diffusive transport.  相似文献