Young asteroid mixing revealed in ordinary chondrites: The case of NWA 5764, a polymict LL breccia with L clasts          下载免费PDF全文

Jérome Gattacceca  Agata M. Krzesińska  Yves Marrocchi  Matthias M. M. Meier  Michèle Bourot‐Denise  Rob Lenssen 《Meteoritics & planetary science》2017,52(11):2289-2304

Polymict chondritic breccias—rocks composed of fragments originating from different chondritic parent bodies—are of particular interest because they give insights into the mixing of asteroids in the main asteroid belt (occurrence, encounter velocity, transfer time). We describe Northwest Africa (NWA) 5764, a brecciated LL6 chondrite that contains a >16 cm³ L4 clast. The L clast was incorporated in the breccia through a nondestructive, low‐velocity impact. Identical cosmic‐ray exposure ages of the L clast and the LL host (36.6 ± 5.8 Myr), suggest a short transfer time of the L meteoroid to the LL parent body of 0.1 ± 8.1 Myr, if that meteoroid was no larger than a few meters. NWA 5764 (together with St. Mesmin, Dimmitt, and Glanerbrug) shows that effective mixing is possible between ordinary chondrite parent bodies. In NWA 5764 this mixing occurred after the peak of thermal metamorphism on the LL parent body, i.e., at least several tens of Myr after the formation of the solar system. The U,Th‐He ages of the L clast and LL host, identical at about 2.9 Ga, might date the final assembly of the breccia, indicating relatively young mixing in the main asteroid belt as previously evidenced in St. Mesmin.  相似文献   

广东徐闻县西部沿岸海域的珊瑚虫纲物种   总被引：4，自引：1，他引：3  

赵焕庭  王丽荣  宋朝景  方静威  Denise McCorry  黄志俊 《台湾海峡》2009,28(3):405-409

广东省徐闻县西部沿岸海域分布着珊瑚礁，现生珊瑚虫纲物种较丰富，本文将2004年调查研究的结果，结合文献记录的资料，综合整理，统计有石珊瑚目12科29属56种和5未定种，其它珊瑚虫纲物种有6目15科31种和12未定种．如进一步深入调查研究，还会有本区新的记录．本区石珊瑚目比处于更低纬度的海南岛种属少，但比处于更高纬度的涠洲岛种属多，符合喜热的物种依自然带分布的规律．  相似文献   

Geochronology,Nd isotopes and reconnaissance geochemistry of volcanic and metavolcanic rocks of the São Luís Craton,northern Brazil: Implications for tectonic setting and crustal evolution     

Evandro L. Klein  Renê Luzardo  Candido A.V. Moura  Denise C. Lobato  Reinaldo S.C. Brito  Richard Armstrong 《Journal of South American Earth Sciences》2009,27(2-3):129-145

New field work, in addition to zircon geochronology, Nd isotopes and reconnaissance geochemical data allow the recognition of Paleoproterozoic volcanic and metavolcanic sequences in the São Luís Craton of northern Brazil. These sequences record at least five volcanic pulses occurring probably in three distinct epochs and in different tectonic settings. (1) The Pirocaua Formation of the Aurizona Group comprises early arc-related calc-alkaline metapyroclastic rocks of 2240 ± 5 Ma formed from juvenile protoliths in addition to minor older crustal components. (2) The Matará Formation of the Aurizona Group holds mafic tholeiitic and ultramafic metavolcanic rocks of back arc and/or island arc setting, which are likely coeval to the Pirocaua Formation. (3) The Serra do Jacaré volcanic unit is composed of tholeiitic basalts and predominantly metaluminous, normal- to high-K calc-alkaline andesites of 2164 ± 3 Ma formed in mature arc or active continental margin from juvenile protoliths along with subordinate older (Paleoproterozoic) materials and associated to the main calc-alkaline orogenic stage. (4) The Rio Diamante Formation consists of late-orogenic metaluminous, medium-K, calc-alkaline rhyolite to dacite and tuffs of 2160 ± 8 Ma formed in continental margin setting from reworked Paleoproterozoic crust (island arc) with incipient Archean contribution. (5) The Rosilha volcanic unit is composed of weakly peraluminous, medium-K, calc-alkaline dacite and tuff formed probably at about 2068 Ma from reworked crustal protoliths. As a whole the volcanic and metavolcanic rocks record and characterized better the previously proposed orogenic evolution of the São Luís Craton.  相似文献   

Seismic architecture and sedimentology of a major grounding zone system deposited by the Bjørnøyrenna Ice Stream during Late Weichselian deglaciation     

Denise Christina Rüther  Rune Mattingsdal  Karin Andreassen  Matthias Forwick  Katrine Husum 《Quaternary Science Reviews》2011,30(19-20):2776-2792

A 280 km wide sediment wedge in outer Bjørnøyrenna (Bear Island Trough), south-western Barents Sea, has been investigated using 2D and 3D seismic data, sediment gravity cores, as well as regional swath and large scale bathymetry data. The bathymetry data indicate a division into an up to 35 m high frontal wedge with large depressions, and an upstream part characterized by mega scale glacial lineations (MSGL). From seismic sections increasing erosion is demonstrated for the upstream part, coinciding with the location of MSGL. Whether the latter are depositional features postdating an extensive erosional event or formed by erosion remains inconclusive. Based on the distinct morphology and internal structures, we infer that the system was deposited during a rapid readvance whereby the ice front pushed and bulldozed predominantly soft, diluted proglacial sediments. Analyses in the eastern part of the sediment system reveal the existence of imbricated thrust sheets in the frontal part of the wedge. This is suggested to imply upstream erosion of sedimentary rock and incorporation of thrusted blocks into the moraine, forming a composite ridge locally. We argue that observed large scale depressions are dead-ice features in the marine environment. It is envisioned that intense englacial thrusting may have developed into a decollement as the cold glacier snout got overrun by ice masses from the interior, thereby enabling the inclusion of slabs of ice in the push moraine mass. Radiocarbon dates indicate that the sediment wedge was deposited around 17,090 cal yrs BP (14,530 ¹⁴C yrs BP) and that the ice front probably remained stable until 16,580 cal yrs BP (13,835 ¹⁴C yrs BP).  相似文献   

Metamorphic grade of organic matter in six unequilibrated ordinary chondrites     

Eric Quirico  Pierre‐Ivan Raynal  Michle Bourot‐Denise 《Meteoritics & planetary science》2003,38(5):795-811

Abstract— The thermal metamorphism grade of organic matter (OM) trapped in 6 unequilibrated ordinary chondrites (UOCs) (Semarkona [LL 3.0], Bishunpur [L/LL 3.1], Krymka [LL 3.1], Chainpur [LL 3.4], Inman [L/LL 3.4], and Tieschitz [H/L 3.6]) has been investigated with Raman spectroscopy in the region of the first‐order carbon bands. The carbonaceous chondrite Renazzo (CR2) was also investigated and used as a reference object for comparison, owing to the fact that previous studies pointed to the OM in this meteorite as being the most pristine among all chondrites. The results show that the OM thermal metamorphic grade: 1) follows the hierarchy Renazzo << Semarkona << other UOCs; 2) is well correlated to the petrographic type of the studied objects; and 3) is also well correlated with the isotopic enrichment δ¹⁵N. These results are strikingly consistent with earlier cosmochemical studies, in particular, the scenario proposed by Alexander et al. (1998). Thermal metamorphism in the parent body appears as the main evolution process of OM in UOCs, demonstrating that nebular heating was extremely weak and that OM burial results in the destabilization of an initial isotopic composition with high δD and δ¹⁵N. Furthermore, the clear discrimination between Renazzo, Semarkona, and other UOCs shows: 1) Semarkona is a very peculiar UOC—by far the most pristine; and 2) Raman spectroscopy is a valid and valuable tool for deriving petrographic sub‐types (especially the low ones) that should be used in the future to complement current techniques. We compare our results with other current techniques, namely, induced thermo‐luminescence and opaques petrography. Other results have been obtained. First, humic coals are not strictly valid standard materials for meteoritic OM but are helpful in the study of evolutionary trends due to thermal metamorphism. Second, terrestrial weathering has a huge effect on OM structure, particularly in Inman, which is a find. Finally, the earlier statement that fine‐grained chondrule rims and matrix in Semarkona could be the source of smectite‐rich IDPs is not valid, given the different degree of structural order of their OM.  相似文献   

A numerical model for simulation of fluid mud with different rheological behaviors   总被引：1，自引：1，他引：0  

Denise Knoch  Andreas Malcherek 《Ocean Dynamics》2011,61(2-3):245-256

In this paper, a three-dimensional isopycnal approach is presented to simulate the dynamics of fluid mud covering the formation, development, transport, and disappearance of fluid mud. The basic assumption is the assignment of the fluid’s density as the indicating parameter for the rheological behavior. Considering stable stratification, as is usually the case for fluid mud, layers of constant density discretize the vertical domain. The non-Newtonian dynamics of fluid mud is simulated by solving the Cauchy equations for general continuum dynamics. Instead of using a turbulent viscosity approach, the viscosity is allowed to vary according to the rheological behavior of mud suspensions. This apparent viscosity can be determined for different rheological formulations in dependence of the volume solid fraction and the shear rate. An existing three-dimensional isopycnal hydrodynamic model was extended for vertical mass transport processes and was applied on a schematic system with hindered settling. For including the rheological behavior of fluid mud, the Worrall–Tuliani approach was parameterized and implemented. The resulting flow behavior is shown on a model application of fluid mud layers moving down an inclined plane. With these changes, it is demonstrated that the isopycnal model is capable of simulating fluid mud dynamics.  相似文献   

The eutrophication history of a tropical water supply reservoir in Brazil   总被引：1，自引：0，他引：1  

Luciane Fontana  Ana Luiza S. Albuquerque  Mark Brenner  Daniel M. Bonotto  Tatiani P. P. Sabaris  Maria A. F. Pires  Marycel E. B. Cotrim  Denise C. Bicudo 《Journal of Paleolimnology》2014,51(1):29-43

Guarapiranga Reservoir is the second most important public water supply in São Paulo, Brazil and has been eutrophic for several decades. We inferred the major ecological shifts for the period 1919–2010 related to multiple stressors (forest flooding, hydrological change, use of algicide and eutrophication), using geochemistry (TOC, TN, TP, C/N, δ¹⁵N, δ¹³C) and diatom assemblages in a short (75-cm) sediment core. Thirty-two diatom species were abundant in the core and stratigraphically constrained incremental sum of squares analysis enabled identification of three diatom zones and four subzones, i.e. depths at which marked changes in species composition occurred. Early diatom assemblages were dominated by benthic, oligotrophic taxa, mainly Eunotia, influenced by flooded vegetation after dam construction. A shift to dominance by a planktonic species (Eunotia tukanorum) occurred ca. 1932, during the period of initial physical disturbance and early use of the water body as a public water supply. Diatoms and geochemical variables show that the reservoir was oligotrophic from ~1919 to 1947. Eutrophication began ~1975 and by the early 1980s the reservoir had become eutrophic, in response to an explosive increase in human population in the watershed. Severe cultural eutrophication has persisted since ~1990. Higher concentrations of copper in the sediments, beginning in 1991, reflect the increased use of copper sulfate to control cyanobacteria blooms and provide a chronological marker. Higher δ¹⁵N values in recent sediments indicate greater sewage inputs and low C/N values reflect the predominant contribution of algae to sediment organic matter. Eutrophic taxa Cyclotella meneghiniana and Nitzschia sp. dominate recent diatom assemblages, along with Aulacoseira granulata, a species that is tolerant of copper sulfate. Diatom assemblages reflect multiple stressors, however, geochemical information provides a better understanding of the early phase of the reservoir. Paleolimnologically documented trophic state changes in this important drinking water supply are largely attributable to increased urbanization of the drainage basin and inputs of sewage. Management efforts should focus on mitigating this nutrient source.  相似文献   

Preface—From ecology to economics: Tracing human influence in the Patuxent River estuary and its watershed     

Denise L. Breitburg  Thomas E. Jordan  Douglas Lipton 《Estuaries and Coasts》2003,26(2):167-170

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Water for Texas by Jim Norwine,John R. Giardino,and Sushma Krishnamurthy,eds.     

R. Denise Blanchard‐Boehm 《The Professional geographer》2006,58(3):362-365

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Rhizome growth dynamics of native and exotic haplotypes ofPhragmites australis (Common reed)     

Michael T. League  Erin P. Colbert  Denise M. Seliskar  John L. Gallagher 《Estuaries and Coasts》2006,29(2):269-276

Phragmites australis (common reed), a clonal grass, has expanded from a minor component of the mid-Atlantic wetlands to a dominant species. It has been suggested that invasive populations ofPhragmites are an exotic haplotype responsible for the dramatic increase in the distribution of the species. We used field observations and measurements and a greenhouse assay to compare native (haplotype F) and exotic (haplotype M) populations, growing adjacent to one another in a brackish marsh near Odessa, Delaware. In the marsh, shoots of the exotic strain emerged from the rhizomes earlier than those of the native and by March there was an order of magnitude more new shoots of the exotic strain than the native. In August, the exotic strain was 30% taller than the native, had twice the leaf biomass, and twice the total biomass. Nine of ten morphological and biomass characteristics measured differed significantly between the native and exotic strains. A greenhouse assay was conducted by planting rhizomes collected in March in shallow trays and growing them for 70 d followed by shoot harvest (Harvest 1). Rhizomes were measured, replanted, and grown for 35 d after which time they were measured and shoots were harvested (Harvest 2). At Harvest 1, shoot height was approximately 80% greater in the exotic strain, shoot biomass was three times higher, aboveground to belowground biomass ratio was twice as high, and rhizome internode length was 50% greater in the exotic strain than the native. These traits, in addition to number of shoots, were also greater in the exotic strain at Harvest 2. The number of rhizome buds at Harvest 1 was three times greater in the native than in the exotic strain. The greater number of rhizome buds in the native would seem to be an advantage, but it did not result in more shoot production. Buds were maintained in an inactive state that does not allow this strain to compete well in a wetland environment inhabited by a more efficient spreader. The earlier emergence of new shoots from the rhizomes, the greater aboveground structure, the greater rhizome internode length, and the quick transition of rhizome buds to shoot or rhizome explain in part the exotic strain's advantage over the native and the mechanisms for its invasive nature.  相似文献