Ar-Ar dating of plagioclase grain size separates from silicate inclusions in IAB iron meteorites and implications for the thermochronological evolution of the IAB parent body     

Nadia Vogel  Paul R. Renne 《Geochimica et cosmochimica acta》2008,72(4):1231-1255

In order to better constrain the thermochronological evolution of the IAB parent body we performed a ⁴⁰Ar/³⁹Ar age study on individual silicate inclusions of the IAB irons Caddo County, Campo del Cielo, Landes, and Ocotillo. In contrast to earlier studies, several plagioclase separates of different grain sizes and quality grades were extracted from each inclusion to reduce the complexity of the age spectra and study the influence of these parameters on the Ar-Ar ages. In nearly all inclusions we found significantly different Ar-Ar ages among the separates (Caddo County: 4.472 ± 0.02-4.562 ± 0.02 Ga; Campo del Cielo 2: 4.362 ± 0.04-4.442 ± 0.03 Ga; Landes 2: 4.412 ± 0.05-4.522 ± 0.04 Ga; Ocotillo: 4.382 ± 0.04-4.462 ± 0.03 Ga). These ages were calculated using the new ⁴⁰K decay constant published by [Mundil R., Renne P. R., Min K. and Ludwig K. R. (2006) Resolvable miscalibration of the ⁴⁰Ar/³⁹Ar geochronometer. Eos Trans. AGU 87, Fall Meet. Suppl., Abstract V21A-0543]. The ages did not systematically correlate with the respective grain size of the separate as expected, i.e., smaller grains did not necessarily show younger ages due to later closure to Ar diffusion or easier re-opening of the system in the course of a reheating event compared to larger grains. Based on the large range of Ar-Ar ages we suggest that the individual inclusions are composed of silicate grains from different locations within the IAB parent body. While some grains remained in a hot (deep) environment that allowed Ar diffusion over an extended time period—in some cases combined with grain coarsening—, others cooled significantly earlier (near surface) through the K/Ar blocking temperature. These different grains where brought together during an impact followed by mixing and reassembly of the debris as proposed by Benedix et al. [Benedix G. K., McCoy T. J., Keil K. and Love S. G. (2000) A petrologic study of the IAB iron meteorites: constraints on the formation of the IAB-Winonaite parent body. Meteorit. Planet. Sci.35, 1127-1141]. Due to rapid cooling after the impact some of the age differences among the grains could be preserved. Based mainly on our Caddo County Ar-Ar age information, the IAB parent body was destroyed by impact and reassembled between ∼4.5 and 4.47 Ga. However, IAB silicate Ar-Ar ages should depend much more on the pre- and post-impact cooling rate and burial depth than on the time of the actual impact. This is supported by a compilation of our and literature IAB and winonaite Ar-Ar ages ranging smoothly from the time of accretion of the chondritic IAB parent body down to the time of its final cooling through the K-Ar blocking temperature after impact and reassembly, instead of showing a peak in Ar-Ar ages at the time of the destructive impact.  相似文献   

Monitoring of a methane-seeping pockmark by cabled benthic observatory (Patras Gulf, Greece)     

Giuditta Marinaro  Giuseppe Etiope  Nadia Lo Bue  Paolo Favali  George Papatheodorou  Dimitris Christodoulou  Flavio Furlan  Francesco Gasparoni  George Ferentinos  Michel Masson  Jean-François Rolin 《Geo-Marine Letters》2006,26(5):297-302

A new seafloor observatory, the gas monitoring module (GMM), has been developed for continuous and long-term measurements of methane and hydrogen sulphide concentrations in seawater, integrated with temperature (T), pressure (P) and conductivity data at the seafloor. GMM was deployed in April 2004 within an active gas-bearing pockmark in the Gulf of Patras (Greece), at a water depth of 42 m. Through a submarine cable linked to an onshore station, it was possible to remotely check, via direct phone connection, GMM functioning and to receive data in near-real time. Recordings were carried out in two consecutive campaigns over the periods April–July 2004, and September 2004–January 2005, amounting to a combined dataset of ca. 6.5 months. This represents the first long-term monitoring ever done on gas leakage from pockmarks by means of CH₄+H₂S+T+P sensors. The results show frequent T and P drops associated with gas peaks, more than 60 events in 6.5 months, likely due to intermittent, pulsation-like seepage. Decreases in temperature in the order of 0.1–1°C (up to 1.7°C) below an ambient T of ca. 17°C (annual average) were associated with short-lived pulses (10–60 min) of increased CH₄+H₂S concentrations. This seepage “pulsation” can either be an active process driven by pressure build-up in the pockmark sediments, or a passive fluid release due to hydrostatic pressure drops induced by bottom currents cascading into the pockmark depression. Redundancy and comparison of data from different sensors were fundamental to interpret subtle proxy signals of temperature and pressure which would not be understood using only one sensor.  相似文献   

Impact of small-scale biogenic sediment structures on bacterial distribution and activity in Arctic deep-sea sediments   总被引：1，自引：0，他引：1  

Nadia V. Quéric  & Thomas Soltwedel 《Marine Ecology》2007,28(S1):66-74

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Baroclinic wind adjustment processes in the Mediterranean Sea     

Nadia Pinardi  Antonio Navarra 《Deep Sea Research Part II: Topical Studies in Oceanography》1993,40(6)

The wind-driven general circulation of the Mediterranean Sea is studied using a primitive equation model. The model uses a 0.25° horizontal resolution and eight or 16 levels in the vertical. The model uses the Mediterranean basin geometry, and the Strait of Gibraltar is closed. The vertical density structure is initialized with annual average data, and the temperature and salinity values are fixed at the surface to simulate perpetual annual mean conditions. The wind forcing consists of monthly mean climatological stresses.The results show that the general circulation of the Mediterranean Sea has a multiple time-scale character (seasonal excursions and steady state amplitudes are comparable) and it is composed by sub-basin scale gyres corresponding to the scale of the wind stress curl centers. The steady state circulation (annual mean average) is determined by a Sverdrup balacne modified by viscous effects.The unsteady vertically integrated transport circulation consists of sub-basin scale gyres similar to the steady state transport components, which amplify seasonally and the partial or total reversal of the currents in many subportions of the basin. The gyres can be stationary in position or propagating. This seasonal ocean response is partly constituted by Rossby modes due to the wind stress curl annual harmonic. The baroclinic circulation shows the seasonal shift of the North African Current from a position along the African coasts during winter to the center of the Balearic and Ionian basin during summer.  相似文献   

Estimation of boundary values in a North Atlantic circulation model using an adjoint method   总被引：2，自引：0，他引：2  

Nadia Ayoub   《Ocean Modelling》2006,12(3-4):319-347

A 1° × 1° resolution version of the MIT-GCM in the North Atlantic is used to test whether open-boundary conditions can be constrained by observations inside the domain using an adjoint method. In this preliminary feasibility study, the model is run during 1993 with a simplified vertical mixing physics. It is constrained by monthly SST fields, monthly climatological θ, S fields and TOPEX/POSEIDON altimetry. The adjoint model is built using automatic differentiation software. The method aims at bringing the model’s trajectory to consistency with data, by adjusting the initial θ, S fields, the time-varying atmospheric forcing fields and the time-varying open-boundary values. An originality of the work is the ‘nested approach’, which uses optimized fields from a global, coarser resolution model for the open-boundary conditions and for the prior estimates of the surface conditions adjustments. A solution is obtained after 75 iterations. This study shows that significant changes can be obtained on the open-boundary values, and that a general improvement in the circulation is achieved in the constrained solution, mainly in the Gulf Stream and equatorial regions. Changes at the open boundaries are characterized by a large temporal variability and small spatial scales. Large local adjustments are found close to the bottom and are likely unrealistic. There, the method tends to compensate for some model’s deficiencies by computing large corrections on the open-boundary values. The analysis of the cost function gradients with respect to the controls allows us to explore the local consistency between the constraints from the different data sets. This study suggests that no fundamental difficulty emerges when constraining open-boundary values. Its extension to a longer run with complete mixing physics can be envisaged.  相似文献   

Meiofauna of the deep Eastern Mediterranean Sea: distribution and abundance in relation to bacterial biomass, organic matter composition and other environmental factors     

Roberto Danovaro  Norberto Della Croce  Anastasios Eleftheriou  Mauro Fabiano  Nadia Papadopoulou  Chris Smith  Anastasios Tselepides 《Progress in Oceanography》1995,36(4)

Quantitative information on the abundance and biomass of metazoan meiofauna was obtained from samples collected at 15 deep-sea stations in the Eastern Mediterranean Sea (533–2400m). Meiofaunal abundance was compared to bacterial biomass and other environmental factors such as the total sedimentary organic matter content, the concentrations of the main biochemical classes of organic compounds (i.e. proteins, carbohydrates and lipids) and to ATP. To estimate the sedimentation potential of primary organic matter, sediment bound chloroplastic pigment equivalents (CPE) were assayed. Meiofaunal density was very low ranging from 4 ind.10cm⁻² (Station A4, 1658m depth) to 290 ind.10cm⁻² (Station A12, 636m depth). Nematodes were the numerically dominant taxon (68% of total meiofauna) and were usually confined to the top 6cm of the sediments. Total meiofaunal biomass ranged from 2.78μgC 10cm⁻² (Station A4) to 598.34μgC 10cm⁻² (Station 15A). There was a significant decrease in the density of metazoan meiofauna with water depth. Bacterial biomass largely dominated the total biomass (as the sum of bacterial and meiofaunal biomass) with an average of 73.2% and accounted for 35.8% of the living biomass (as ATP carbon) whereas meiofaunal biomass accounted only for 6.56%. Bacterial biomass was significantly related to the DNA concentrations of the sediment. A significant correlation between ATP concentration and CPE content was also found. No correlations were found between meiofauna, ATP and CPE, or between meiofauna and bacterial parameters. The significant relationship between meiofaunal density and the ratio of labile organic matter/total organic matter indicates that deep-sea meiofauna inhabiting an extremely oligotrophic environment (such as the Eastern Mediterranean) may be more nutritionally dependent upon the quality than on the quantity of sedimentary organic matter.  相似文献   

Constrained Kriging: An Alternative to Predict Global Recoverable Resources     

Mery  Nadia  Marcotte  Denis  Dutaut  Raphael 《Natural Resources Research》2020,29(4):2275-2289

Natural Resources Research - In most NI-43-101 resource assessment reports the prediction of global in situ resources is performed by either inverse distance weighting, ordinary kriging (OK) or...  相似文献   

Impact of data assimilation of glider observations in the Ionian Sea (Eastern Mediterranean)   总被引：1，自引：0，他引：1  

Srdjan Dobricic  Nadia Pinardi  Pierre Testor  Uwe Send 《Dynamics of Atmospheres and Oceans》2010

Glider observations of temperature, salinity and vertically averaged velocity in the Ionian Sea (Eastern Mediterranean Sea), made in the period October 2004–December 2004, were assimilated into an operational forecasting model together with other in situ and satellite observations. The study area has a high spatial and temporal variability of near surface dynamics, characterized by the entrance of the Atlantic Ionian Stream (AIS) into the Northern Ionian Sea. The impact of glider observations on the estimation of the circulation is studied, and it is found that their assimilation locally improves the prediction of temperature, salinity, velocity and surface elevation fields. However, only the assimilation of temperature and salinity together with the vertically averaged velocity improves the forecast of all observed parameters. It is also found that glider observations rapidly impact the analyses even remotely, and the remote impacts on the analyses remain several months after the presence of the glider. The study emphasizes the importance of assimilating as much as possible all available information from gliders, especially in dynamically complex areas.  相似文献   

Influence of geomagnetic field on the motion of low satellites     

Nadia A. Saad  M. N. Ismail 《Astrophysics and Space Science》2010,325(2):177-184

The first aim of the present work is to compute a more accurate and recent model for the Earth’s magnetic field. The second aim is to determine the effects of the Earth’s magnetic field on the motion of a charged artificial satellite to evaluate the variations of the orbital elements of the satellite due to these effects. The magnetic field and its variation with time have been studied at different heights, longitudes and latitudes. The geomagnetic field is considered as a multiple potential field and the electrical charge of the satellite is assumed to be constant. A new computer code has been constructed to follow the components of the magnetic field in spherical harmonic models. The Gauss equations are solved numerically. The results concentrate on the computation of the numerical values of orbital perturbation for the case of a low Earth satellite. RS-1 satellite and space craft gravity probe B (GPB) are chosen as cases of studies for a detailed numerical analysis.  相似文献   

Stability of banded vegetation patterns under seasonal rainfall and limited soil moisture storage capacity   总被引：1，自引：0，他引：1  

Nadia Ursino  Samuel Contarini 《Advances in water resources》2006

The delicate equilibrium of soil moisture and biomass may become unstable under water scarcity conditions causing banded vegetation patterns to form on hillsides of semi-arid catchments. Soil related processes that induce instability (namely: soil moisture advection and diffusion), have been evaluated numerically for different rainfall regimes. This study addresses the combined influence of some relevant soil characteristics, and the effect of seasonal precipitation on vegetation patterns, advancing the comprehension of those mechanisms that cause shifts toward banded vegetation patterns or bare states.  相似文献