The fusion crust of the Winchcombe meteorite: A preserved record of atmospheric entry processes     

Matthew J. Genge  Luke Alesbrook  Natasha V. Almeida  Helena C. Bates  Phil A. Bland  Mark R. Boyd  Mark J. Burchell  Gareth S. Collins  Luke T. Cornwell  Luke Daly  Hadrien A. R. Devillepoix  Matthias van Ginneken  Ansgar Greshake  Daniel Hallatt  Christopher Hamann  Lutz Hecht  Laura E. Jenkins  Diane Johnson  Rosie Jones  Ashley J. King  Haithem Mansour  Sarah McMullan  Jennifer T. Mitchell  Gavyn Rollinson  Sara S. Russell  Christian Schröder  Natasha R. Stephen  Martin D. Suttle  Jon D. Tandy  Patrick Trimby  Eleanor K. Sansom  Vassilia Spathis  Francesca M. Willcocks  Penelope J. Wozniakiewicz 《Meteoritics & planetary science》2024,59(5):948-972

Fusion crusts form during the atmospheric entry heating of meteorites and preserve a record of the conditions that occurred during deceleration in the atmosphere. The fusion crust of the Winchcombe meteorite closely resembles that of other stony meteorites, and in particular CM2 chondrites, since it is dominated by olivine phenocrysts set in a glassy mesostasis with magnetite, and is highly vesicular. Dehydration cracks are unusually abundant in Winchcombe. Failure of this weak layer is an additional ablation mechanism to produce large numbers of particles during deceleration, consistent with the observation of pulses of plasma in videos of the Winchcombe fireball. Calving events might provide an observable phenomenon related to meteorites that are particularly susceptible to dehydration. Oscillatory zoning is observed within olivine phenocrysts in the fusion crust, in contrast to other meteorites, perhaps owing to temperature fluctuations resulting from calving events. Magnetite monolayers are found in the crust, and have also not been previously reported, and form discontinuous strata. These features grade into magnetite rims formed on the external surface of the crust and suggest the trapping of surface magnetite by collapse of melt. Magnetite monolayers may be a feature of meteorites that undergo significant degassing. Silicate warts with dendritic textures were observed and are suggested to be droplets ablated from another stone in the shower. They, therefore, represent the first evidence for intershower transfer of ablation materials and are consistent with the other evidence in the Winchcombe meteorite for unusually intense gas loss and ablation, despite its low entry velocity.  相似文献   

Palaeogeographical changes in response to glacial–interglacial cycles,as recorded in Middle and Late Pleistocene seismic stratigraphy,southern North Sea     

Stephen J. Eaton  David M. Hodgson  Natasha L. M. Barlow  Estelle E. J. Mortimer  Claire L. Mellett 《第四纪科学杂志》2020,35(6):760-775

Offshore stratigraphic records from the North Sea contain information to reconstruct palaeo-ice-sheet extent and understand sedimentary processes and landscape response to Pleistocene glacial–interglacial cycles. We document three major Middle to Late Pleistocene stratigraphic packages over a 401-km² area (Norfolk Vanguard/Boreas Offshore Wind Farm), offshore East Anglia, UK, through the integration of 2D seismic, borehole and cone penetration test data. The lowermost unit is predominantly fluviatile [Yarmouth Roads Formation, Marine Isotope Stage (MIS) 19–13], including three northward-draining valleys. The middle unit (Swarte Bank Formation) records the southernmost extent of tunnel valley-fills in this area of the North Sea, providing evidence for subglacial conditions most likely during the Anglian stage (MIS 12) glaciation. The Yarmouth Roads and Swarte Bank deposits are truncated and overlain by low-energy estuarine silts and clays (Brown Bank Formation; MIS 5d–4). Smaller scale features, including dune-scale bedforms, and abrupt changes in cone penetration test parameters, provide evidence for episodic changes in relative sea level within MIS 5. The landscape evolution recorded in deposits of ~MIS 19–5 are strongly related to glacial–interglacial cycles, although a distinctive aspect of this low-relief ice-marginal setting are opposing sediment transport directions under contrasting sedimentary process regimes.  相似文献   

游客原真性感知与世界遗产资源原真性内涵集合式关系研究(英文)     

陈享尔  蔡建明  杨振山  Natasha WEBSTER 《资源与生态学报(英文版)》2014,5(1):20-31

游客原真性感知对于世界遗产资源的未来发展具有重要价值。本文基于原真性学说针对游客感知与世界遗产资源旅游发展内涵之间的关系进行了探讨。通过对世界遗产故宫博物院实证研究的结果表明:数学集合理论模型能够很好地模拟游客原真性感知与遗产资源发展内涵之间的内在联系,继而5种集合式关系被建立起来,即"相离关系"、"相交关系"、"包含关系I"、"包含关系II"和"重合关系"。根据调查样本的分配规律5种集合式关系可以被进一步分为主要集合式关系和次要集合式关系。使用双因素方差分析法从被调研游客的人口统计学特征(性别、年龄、受教育程度、游览频率和已有感知程度)出发,针对集合式关系进行研究的结果进一步显示:游客对于5种集合式关系类别的感知具有差异性。本论文研究成果对于基于原真性视角下的世界遗产资源发展成效评判以及从游客原真性感知出发提出世界遗产资源发展未来策略均具有实际意义。  相似文献   

Submarine Groundwater Discharge-Derived Nutrient Loads to San Francisco Bay: Implications to Future Ecosystem Changes     

Kimberly A. Null  Natasha T. Dimova  Karen L. Knee  Bradley K. Esser  Peter W. Swarzenski  Michael J. Singleton  Mark Stacey  Adina Paytan 《Estuaries and Coasts》2012,35(5):1299-1315

Submarine groundwater discharge (SGD) was quantified at select sites in San Francisco Bay (SFB) from radium (²²³Ra and ²²⁴Ra) and radon (²²²Rn) activities measured in groundwater and surface water using simple mass balance box models. Based on these models, discharge rates in South and Central Bays were 0.3?C7.4?m³?day^?1?m^?1. Although SGD fluxes at the two regions (Central and South Bays) of SFB were of the same order of magnitude, the dissolved inorganic nitrogen (DIN) species associated with SGD were different. In the South Bay, ammonium (NH ₄ ⁺ ) concentrations in groundwater were three-fold higher than in open bay waters, and NH ₄ ⁺ was the primary DIN form discharged by SGD. At the Central Bay site, the primary DIN form in groundwater and associated discharge was nitrate (NO ₃ ^? ). The stable isotope signatures (??¹⁵N_NO3 and ??¹⁸O_NO3) of NO ₃ ^? in the South Bay groundwater and surface waters were both consistent with NO ₃ ^? derived from NH ₄ ⁺ that was isotopically enriched in ¹⁵N by NH ₄ ⁺ volatilization. Based on the calculated SGD fluxes and groundwater nutrient concentrations, nutrient fluxes associated with SGD can account for up to 16?% of DIN and 22?% of DIP in South and Central Bays. The form of DIN contributed to surface waters from SGD may impact the ratio of NO ₃ ^? to NH ₄ ⁺ available to phytoplankton with implications to bay productivity, phytoplankton species distribution, and nutrient uptake rates. This assessment of nutrient delivery via groundwater discharge in SFB may provide vital information for future bay ecological wellbeing and sensitivity to future environmental stressors.  相似文献   

A natural tracer investigation of the hydrological regime of Spring Creek Springs, the largest submarine spring system in Florida     

Natasha T. Dimova  William C. BurnettKevin Speer 《Continental Shelf Research》2011,31(6):731-738

This work presents results from a nearly two-year monitoring of the hydrologic dynamics of the largest submarine spring system in Florida, Spring Creek Springs. During the summer of 2007 this spring system was observed to have significantly reduced flow due to persistent drought conditions. Our examination of the springs revealed that the salinity of the springs' waters had increased significantly, from 4 in 2004 to 33 in July 2007 with anomalous high radon (²²²Rn, t_1/2=3.8 days) in surface water concentrations indicating substantial saltwater intrusion into the local aquifer. During our investigation from August 2007 to May 2009 we deployed on an almost monthly basis a continuous radon-in-water measurement system and monitored the salinity fluctuations in the discharge area. To evaluate the springs' freshwater flux we developed three different models: two of them are based on water velocity measurements and either salinity or ²²²Rn in the associated surface waters as groundwater tracers. The third approach used only salinity changes within the spring area. The three models showed good agreement and the results confirmed that the hydrologic regime of the system is strongly correlated to local precipitation and water table fluctuations with higher discharges after major rain events and very low, even reverse flow during prolong droughts. High flow spring conditions were observed twice during our study, in the early spring and mid-late summer of 2008. However the freshwater spring flux during our observation period never reached that reported from a 1970s value of 4.9×10⁶ m³/day. The maximum spring flow was estimated at about 3.0×10⁶ m³/day after heavy precipitation in February-March 2008. As a result of this storm (total of 173 mm) the salinity in the spring area dropped from about 27 to 2 in only two days. The radon-in-water concentrations dramatically increased in parallel, from about 330 Bq/m³ to about 6600 Bq/m³. Such a rapid response suggests a direct connection between the deep and the surficial aquifers.  相似文献   

Trophic functioning of the St. Lucia estuarine lake during a drought phase assessed using stable isotopes     

Natasha Govender  Albertus J. SmitRenzo Perissinotto 《Estuarine, Coastal and Shelf Science》2011

The St. Lucia Estuary is Africa’s largest estuarine system and is currently experiencing the stress of prolonged freshwater deprivation, manifested by extremely low water levels and hypersalinity. These unprecedented conditions have raised questions regarding the trophic functioning of the ecosystem. Despite the substantial amount of research previously undertaken within this system, no studies of food web structure and function have yet been documented. This study therefore aimed to examine the food web structure of the St. Lucia estuary system through the use of carbon and nitrogen stable isotope analysis. Analysis of carbon isotope ratios indicates that benthic carbon sources are most utilised at sites with low water levels and generally higher salinity (Catalina Bay, Charter’s Creek). Conversely, the estuarine region of the mouth and Narrows, with its elevated water levels and lower salinity, still sustains a viable pelagic food web. Analysis of δ¹⁵N ratios indicates that the number of trophic transfers (food chain length) might be related to water levels. Overall, the study provides a greater understanding of the ecological processes of this complex estuarine lake, which may allow for future comparisons of trophic functioning under drought and normal/wet conditions to be made.  相似文献   

The gravitational field and the Geoid of Australia     

N. P. Grushinsky  Natasha B. Sazhina  D. A. Brown 《Australian Journal of Earth Sciences》2013,60(3):183-199

A map of the geoid in Australia and surrounding oceanic areas has been produced on a scale of 1:5M. There are two variants, both of which employ Bouguer anomalies for the land areas. For the ocean areas in the first variant, anomalies have been obtained as the difference between the observed and theoretical values of gravity at sea level. In the second variant, a correction has been introduced to reduce the density of the water layer to the average density of the Earth's crust. The height of the geoid in Australia is calculated in the same way. The effect of distant zones has been allowed for in accordance with the data of Kozai (1967) and Köhnlein (1967), and a high precision for the geoid height for Australia has been obtained. Discrepancies between the new data and the observations of Mather (1969) amount to + 2 m for the regions investigated and ± 4 m for the entire continent. Coordinates have also been obtained for the centre of the Australian ellipsoid relative to the centre of the Earth's mass.  相似文献   

Constructing motion primitive sets to summarize periodic orbit families and hyperbolic invariant manifolds in a multi-body system     

Smith  Thomas R.  Bosanac  Natasha 《Celestial Mechanics and Dynamical Astronomy》2022,134(1):1-19

Celestial Mechanics and Dynamical Astronomy - The equilibrium figure of an inviscid tidally deformed body is the starting point for the construction of many tidal theories such as Darwinian tidal...  相似文献   

Enhanced application of root‐reinforcement algorithms for bank‐stability modeling     

Natasha Pollen‐Bankhead  Andrew Simon 《地球表面变化过程与地形》2009,34(4):471-480

Riparian vegetation is known to exert a number of mechanical and hydrologic controls on bank stability. In particular, plant roots provide mechanical reinforcement to a soil matrix due to the different responses of soils and roots to stress. Root reinforcement is largely a function of the strength of the roots crossing potential shear planes, and the number and diameter of such roots. However, previous bank stability models have been constrained by limited field data pertaining to the spatial and temporal variability of root networks within stream banks. In this paper, a method is developed to use root‐architecture data to derive parameters required for modeling temporal and spatial changes in root reinforcement. Changes in root numbers over time were assumed to follow a sigmoidal curve, which commonly represents the growth rates of organisms. Regressions for numbers of roots crossing potential shear planes over time showed small variations between species during the juvenile growth phase, but extrapolation led to large variations in root numbers by the time the senescent phase of the sigmoidal growth curve had been reached. In light of potential variability in the field data, the mean number of roots crossing a potential shear plane at each year of tree growth was also calculated using data from all species and an additional sigmoidal regression was run. After 30 years the mean number of roots predicted to cross a 1 m shear plane was 484, compared with species‐specific curves whose values ranged from 240 roots for black willow trees to 890 roots for western cottonwood trees. In addition, the effect of spatial variations in rooting density with depth on stream‐bank stability was modeled using the bank stability and toe erosion model (BSTEM). Three root distributions, all approximating the same average root reinforcement (5 kPa) over the top 1 m of the bank profile, were modeled, but with differing vertical distributions (concentrated near surface, non‐linear decline with depth, uniform over top meter). It was found that stream‐bank F_S varied the most when the proportion of the failure plane length to the depth of the rooting zone was greatest. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

Paleogene evolution and demise of the proto‐Paratethys Sea in Central Asia (Tarim and Tajik basins): Role of intensified tectonic activity at ca. 41 Ma     

Mustafa Y. Kaya  Guillaume Dupont‐Nivet  Jean‐Noël Proust  Pierrick Roperch  Laurie Bougeois  Niels Meijer  Joost Frieling  Chiara Fioroni  Sevin zkan Alt&#x;ner  Ezgi Vardar  Natasha Barbolini  Marius Stoica  Jovid Aminov  Mehmut Mamtimin  Guo Zhaojie 《Basin Research》2019,31(3):461-486

The proto‐Paratethys Sea covered a vast area extending from the Mediterranean Tethys to the Tarim Basin in western China during Cretaceous and early Paleogene. Climate modelling and proxy studies suggest that Asian aridification has been governed by westerly moisture modulated by fluctuations of the proto‐Paratethys Sea. Transgressive and regressive episodes of the proto‐Paratethys Sea have been previously recognized but their timing, extent and depositional environments remain poorly constrained. This hampers understanding of their driving mechanisms (tectonic and/or eustatic) and their contribution to Asian aridification. Here, we present a new chronostratigraphic framework based on biostratigraphy and magnetostratigraphy as well as a detailed palaeoenvironmental analysis for the Paleogene proto‐Paratethys Sea incursions in the Tajik and Tarim basins. This enables us to identify the major drivers of marine fluctuations and their potential consequences on Asian aridification. A major regional restriction event, marked by the exceptionally thick (≤ 400 m) shelf evaporites is assigned a Danian‐Selandian age (ca. 63–59 Ma) in the Aertashi Formation. This is followed by the largest recorded proto‐Paratethys Sea incursion with a transgression estimated as early Thanetian (ca. 59–57 Ma) and a regression within the Ypresian (ca. 53–52 Ma), both within the Qimugen Formation. The transgression of the next incursion in the Kalatar and Wulagen formations is now constrained as early Lutetian (ca. 47–46 Ma), whereas its regression in the Bashibulake Formation is constrained as late Lutetian (ca. 41 Ma) and is associated with a drastic increase in both tectonic subsidence and basin infilling. The age of the final and least pronounced sea incursion restricted to the westernmost margin of the Tarim Basin is assigned as Bartonian–Priabonian (ca. 39.7–36.7 Ma). We interpret the long‐term westward retreat of the proto‐Paratethys Sea starting at ca. 41 Ma to be associated with far‐field tectonic effects of the Indo‐Asia collision and Pamir/Tibetan plateau uplift. Short‐term eustatic sea level transgressions are superimposed on this long‐term regression and seem coeval with the transgression events in the other northern Peri‐Tethyan sedimentary provinces for the 1st and 2nd sea incursions. However, the 3rd sea incursion is interpreted as related to tectonism. The transgressive and regressive intervals of the proto‐Paratethys Sea correlate well with the reported humid and arid phases, respectively in the Qaidam and Xining basins, thus demonstrating the role of the proto‐Paratethys Sea as an important moisture source for the Asian interior and its regression as a contributor to Asian aridification.  相似文献