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21.
MARTINA HÄTTESTRAND ANN‐MARIE ROBERTSSON 《Boreas: An International Journal of Quaternary Research》2010,39(2):296-311
Hättestrand, M. & Robertsson, A.‐M. 2010: Weichselian interstadials at Riipiharju, northern Sweden – interpretation of vegetation and climate from fossil and modern pollen records. Boreas, 10.1111/j.1502‐3885.2009.00129.x. ISSN 0300‐9483. The most complete records of Weichselian ice‐free conditions in northern Sweden have been retrieved from kettleholes in the Riipiharju esker. In an earlier study, the Riipiharju I core was described as containing two Weichselian interstadials and Riipiharju was chosen as type site for the second Weichselian interstadial in northern Sweden. Here, we present a palynological investigation of two new sediment cores (Riipiharju II and III) retrieved from Riipiharju. Together, the new cores comprise a late cold part of the first Weichselian interstadial recorded in northeastern Sweden (Tärendö I, earlier correlated with Peräpohjola in Finland) as well as a long sequence of the second Weichselian interstadial (Tärendö II, earlier named Tärendö). The results indicate that the climate during deposition of the Tärendö II sequence was more variable than earlier suggested. According to the present interpretation it was relatively warm in the early part of Tärendö II; thereafter a long cold phase persisted, and finally the climate was warmer again in the late part of Tärendö II. The warm phases are characterized by Betula‐dominant pollen assemblages, while the cold phase is characterized by high percentages of Artemisia and Gramineae pollen. Since there is still no firm chronology established of the interstadials in northeastern Sweden, two possible correlations are discussed; either Tärendö I and II are correlated with Brörup (MIS 5c) and Odderade (MIS 5a), or, perhaps more likely, they are correlated with Odderade and early Middle Weichselian (MIS 3) time. 相似文献
22.
Nicholas P. Dunning Carmen McCane Tyler Swinney Matthew Purtill Jani Sparks Ashley Mann Jon‐Paul McCool Chantal Ivenso 《Geoarchaeology》2015,30(3):167-199
Over the past thirty years, geoarchaeology has moved from the fringe to mainstream status within Mesoamerican archaeological investigations. This review focuses on works published since the year 2000. Five themes are identified as central to recent studies: (1) the correlation of environmental change and cultural history; (2) anthropogenic environmental impacts; (3) ancient land cover, land use, and diet; (4) archaeological prospection; and (5) provenance studies. These themes are often interwoven in the application of complex systems approaches that allow scientists to more accurately model the intricacies of ancient human–environment interactions. 相似文献
23.
24.
Yun Jiang Piers Koefoed Olga Pravdivtseva Heng Chen Chun‐Hui Li Fang Huang Li‐Ping Qin Jia Liu Kun Wang 《Meteoritics & planetary science》2021,56(1):61-76
The alkali element K is moderately volatile and fluid mobile; thus, it can be influenced by both primary processes (evaporation and recondensation) in the solar nebula and secondary processes (thermal and aqueous alteration) in the parent body. Since these primary and secondary processes would induce different isotopic fractionations, K isotopes could become a potential tracer to distinguish them. Using recently developed methods with improved precision (0.05‰, 95% confidence interval), we systematically measured the K isotopic compositions and major/trace elemental compositions of chondritic components (18 chondrules, 3 CAIs, 2 matrices, and 5 bulks) in the carbonaceous chondrite fall Allende. Among all the components analyzed in this study, CAIs, which formed initially under high‐temperature conditions in the solar nebula and were dominated by nominally K‐free refractory minerals, have the highest K2O content (average 0.53 wt%) and have K isotope compositions most enriched in heavy isotopes (δ41K: ?0.30 to ?0.25‰). Such an observation is consistent with previous petrologic studies that show CAIs in Allende have undergone alkali enrichment during metasomatism. In contrast, chondrules contain lower K2O content (0.003–0.17 wt%) and generally lighter K isotope compositions (δ41K: ?0.87‰ to ?0.24‰). The matrix and bulks are nearly identical in K2O content and K isotope compositions (0.02–0.05 wt%; δ41K: ?0.62 to ? 0.46‰), which are, as expected, right in the middle of CAIs and chondrules. This strongly indicates that most of the chondritic components of Allende suffered aqueous alteration and their K isotopic compositions are the ramification of Allende parent‐body processing instead of primary nebular signatures. Nevertheless, we propose the small K isotope fractionations observed (< 1‰) among Allende components are likely similar to the overall range of K isotopic fractionation that occurred in nebular environment. Furthermore, the K isotope compositions seen in the components of Allende in this study are consistent with MC‐ICP‐MS analyses of the components in ordinary chondrites, which also show an absence of large (10‰) isotope fractionations. This is not expected as evaporation experiments in nebular conditions suggest there should be large K isotopic fractionations. Nevertheless, possible nebular processes such as chondrules back exchanging with ambient gas when they formed could explain this lack of large K isotopic variation. 相似文献
25.
Zoltn Vci Carl B. Agee Munir Humayun Karen Ziegler Yemane Asmerom Victor Polyak Henner Busemann Daniela Krietsch Matthew Heizler Matthew E. Sanborn Qing‐Zhu Yin 《Meteoritics & planetary science》2020,55(3):622-648
Northwest Africa (NWA) 11042 is a heavily shocked achondrite with medium‐grained cumulate textures. Its olivine and pyroxene compositions, oxygen isotopic composition, and chromium isotopic composition are consistent with L chondrites. Sm‐Nd dating of its primary phases shows a crystallization age of 4100 ± 160 Ma. Ar‐Ar dating of its shocked mineral maskelynite reveals an age of 484.0 ± 1.5 Ma. This age coincides roughly with the breakup event of the L chondrite parent body evident in the shock ages of many L chondrites and the terrestrial record of fossil L chondritic chromite. NWA 11042 shows large depletions in siderophile elements (<0.01×CI) suggestive of a complex igneous history involving extraction of a Fe‐Ni‐S liquid on the L chondrite parent body. Due to its relatively young crystallization age, the heat source for such an igneous process is most likely impact. Because its mineralogy, petrology, and O isotopes are similar to the ungrouped achondrite NWA 4284 (this work), the two meteorites are likely paired and derived from the same parent body. 相似文献
26.
Michael J. Steventon Christopher A.‐L. Jackson David M. Hodgson Howard D. Johnson 《Basin Research》2019,31(3):600-620
Strain style, magnitude and distribution within mass‐transport complexes (MTCs) are important for understanding the process evolution of submarine mass flows and for estimating their runout distances. Structural restoration and quantification of strain in gravitationally driven passive margins have been shown to approximately balance between updip extensional and downdip contractional domains; such an exercise has not yet been attempted for MTCs. We here interpret and structurally restore a shallowly buried (c. 1,500 mbsf) and well‐imaged MTC, offshore Uruguay using a high‐resolution (12.5 m vertical and 15 × 12.5 m horizontal resolution) three‐dimensional seismic‐reflection survey. This allows us to characterise and quantify vertical and lateral strain distribution within the deposit. Detailed seismic mapping and attribute analysis shows that the MTC is characterised by a complicated array of kinematic indicators, which vary spatially in style and concentration. Seismic‐attribute extractions reveal several previously undocumented fabrics preserved in the MTC, including internal shearing in the form of sub‐orthogonal shear zones, and fold‐thrust systems within the basal shear zone beneath rafted‐blocks. These features suggest multiple transport directions and phases of flow during emplacement. The MTC is characterised by a broadly tripartite strain distribution, with extensional (e.g. normal faults), translational and contractional (e.g. folds and thrusts) domains, along with a radial frontally emergent zone. We also show how strain is preferentially concentrated around intra‐MTC rafted‐blocks due to their kinematic interactions with the underlying basal shear zone. Overall, and even when volume loss within the frontally emergent zone is included, a strain difference between extension (1.6–1.9 km) and contraction (6.7–7.3 km) is calculated. We attribute this to a combination of distributed, sub‐seismic, ‘cryptic’ strain, likely related to de‐watering, grain‐scale deformation and related changes in bulk sediment volume. This work has implications for assessing MTCs strain distribution and provides a practical approach for evaluating structural interpretations within such deposits. 相似文献
27.
Samuel Ebert Addi Bischoff Dennis Harries Sarah Lentfort Jean‐Alix Barrat Andreas Pack Jrme Gattacceca Robbin Visser Peter Schmid‐Beurmann Stephan Kimpel 《Meteoritics & planetary science》2019,54(2):328-356
Based on the high abundance of fine‐grained material and its dark appearance, NWA 11024 was recognized as a CM chondrite, which is also confirmed by oxygen isotope measurements. But contrary to known CM chondrites, the typical phases indicating aqueous alteration (e.g., phyllosilicates, carbonates) are missing. Using multiple analytical techniques, this study reveals the differences and similarities to known CM chondrites and will discuss the possibility that NWA 11024 is the first type 3 CM chondrite. During the investigation, two texturally apparent tochilinite–cronstedtite intergrowths were identified within two thin sections. However, the former phyllosilicates were recrystallized to Fe‐rich olivine during a heating event without changing the textural appearance. A peak temperature of 400–600 °C is estimated, which is not high enough to destroy or recrystallize calcite grains. Thus, calcites were never constituents of the mineral paragenesis. Another remarkable feature of NWA 11024 is the occurrence of unknown clot‐like inclusions (UCLIs) within fine‐grained rims, which are unique in this clarity. Their density and S concentration are significantly higher than of the surrounding fine‐grained rim and UCLIs can be seen as primary objects that were not formed by secondary alteration processes inside the rims. Similarities to chondritic and cometary interplanetary dust particles suggest an ice‐rich first‐generation planetesimal for their origin. In the earliest evolution, NWA 11024 experienced the lowest degree of aqueous alteration of all known CM chondrites and subsequently, a heating event dehydrated the sample. We suggest to classify the meteorite NWA 11024 as the first type 3 CM chondrite similar to the classification of CV3 chondrites (like Allende) that could also have lost their matrix phyllosilicates by thermal dehydration. 相似文献
28.
M. B. Biren J.‐A. Wartho M. C. VAN Soest K. V. Hodges H. Cathey B. P. Glass C. Koeberl J. W. Horton W. Hale 《Meteoritics & planetary science》2019,54(8):1840-1852
Single crystal (U‐Th)/He dating has been undertaken on 21 detrital zircon grains extracted from a core sample from Ocean Drilling Project (ODP) site 1073, which is located ~390 km northeast of the center of the Chesapeake Bay impact structure. Optical and electron imaging in combination with energy dispersive X‐ray microanalysis (EDS) of zircon grains from this late Eocene sediment shows clear evidence of shock metamorphism in some zircon grains, which suggests that these shocked zircon crystals are distal ejecta from the formation of the ~40 km diameter Chesapeake Bay impact structure. (U‐Th/He) dates for zircon crystals from this sediment range from 33.49 ± 0.94 to 305.1 ± 8.6 Ma (2σ), implying crystal‐to‐crystal variability in the degree of impact‐related resetting of (U‐Th)/He systematics and a range of different possible sources. The two youngest zircon grains yield an inverse‐variance weighted mean (U‐Th)/He age of 33.99 ± 0.71 Ma (2σ uncertainties n = 2; mean square weighted deviation = 2.6; probability [P] = 11%), which is interpreted to be the (U‐Th)/He age of formation of the Chesapeake Bay impact structure. This age is in agreement with K/Ar, 40Ar/39Ar, and fission track dates for tektites from the North American strewn field, which have been interpreted as associated with the Chesapeake Bay impact event. 相似文献
29.
Marina Martinez Adrian J. Brearley Josep M. Trigo‐Rodríguez Jordi Llorca 《Meteoritics & planetary science》2019,54(11):2845-2863
The petrology and mineralogy of shock melt veins in the L6 ordinary chondrite host of Villalbeto de la Peña, a highly shocked, L chondrite polymict breccia, have been investigated in detail using scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, and electron probe microanalysis. Entrained olivine, enstatite, diopside, and plagioclase are transformed into ringwoodite, low‐Ca majorite, high‐Ca majorite, and an assemblage of jadeite‐lingunite, respectively, in several shock melt veins and pockets. We have focused on the shock behavior of diopside in a particularly large shock melt vein (10 mm long and up to 4 mm wide) in order to provide additional insights into its high‐pressure polymorphic phase transformation mechanisms. We report the first evidence of diopside undergoing shock‐induced melting, and the occurrence of natural Ca‐majorite formed by solid‐state transformation from diopside. Magnesiowüstite has also been found as veins injected into diopside in the form of nanocrystalline grains that crystallized from a melt and also occurs interstitially between majorite‐pyrope grains in the melt‐vein matrix. In addition, we have observed compositional zoning in majorite‐pyrope grains in the matrix of the shock‐melt vein, which has not been described previously in any shocked meteorite. Collectively, all these different lines of evidence are suggestive of a major shock event with high cooling rates. The minimum peak shock conditions are difficult to constrain, because of the uncertainties in applying experimentally determined high‐pressure phase equilibria to complex natural systems. However, our results suggest that conditions between 16 and 28 GPa and 2000–2200 °C were reached. 相似文献
30.
Massimo Ponti Andrea PasterisRoberta Guerra Marco Abbiati 《Estuarine, Coastal and Shelf Science》2009
Coastal lagoons are ephemeral habitats whose conservation requires human intervention, such as maintenance dredging of inner channels. Dredging can reduce the abundance of benthic species due to the removal of individuals with the sediment, modify sediment properties, and resuspend fine sediment, nutrients and pollutants, which can lead to eutrophication, hypoxic events and increasing toxicity. Both direct effects in the dredged channel and possible indirect effects in surrounding shallow areas could be expected. This study assesses the effects of the channel maintenance dredging, performed between October 2004 and August 2005, on the invertebrate assemblages both in channels and adjacent ponds in the northern Adriatic coastal lagoon of Pialassa Baiona. The lagoon is affected by eutrophication, chemical and thermal pollution from wastewater treatment and power plants. Three impacted sites were located in the dredged channel and three in the adjacent interconnected shallow water ponds, while three non-impacted sites were located in a channel and in a pond far from the dredged area. Replicate samples were collected from each site one time before and one time after the dredging operations. Despite the extent of the intervention, effects of the dredging on macrobenthic assemblages were detected only within the dredged channel, while in the surrounding ponds no clear and unequivocal effects were found. In particular the dredging could have promoted the increase of the abundance of the polychaete Streblospio shrubsolii in the southern and central parts of the dredged channel and the increase in abundance of the amphipod Corophium insidiosum in the northern side, compared to the controls. Instead, species diversity was reduced in the central and northern parts of the dredged channel. These effects on the macrobenthic invertebrate assemblages could be related to the observed changes of sediment characteristics, contamination and toxicity. Overall, direct effects on benthic assemblages in the dredged channels were more detectable than the possible secondary effects in the surrounding shallow ponds, where the higher spatial heterogeneity can mask any relevant effects. 相似文献