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81.
Reconstruction of early Cenozoic deep‐water circulation is one of the keys to modelling Earth's greenhouse‐to‐icehouse surface evolution, but it has long been hampered by the paucity of information from the central South Pacific. To help overcome this knowledge gap, we present new micropalaeontological data from dredged carbonates (R/V Sonne Expedition SO193) at several eastern volcanic salients of the Manihiki Plateau. Interestingly, despite appreciable longitudinal separations among the dredged sites, ages indicated by the foraminiferal assemblages are consistently around the Middle Eocene (including mixed Turonian [Late Cretaceous]/Eocene at a single site), suggesting widespread post‐Eocene cessation of the pelagic sedimentation. By integrating with independent seismic and chronostratigraphic data (Deep Sea Drilling Project Leg 33) for large‐scale erosion of top‐Eocene–Oligocene sedimentary units on the eastern Manihiki Plateau, our results can be viewed as novel physical evidence for the intensification of central South Pacific deep‐water circulation since the Eocene/Oligocene climatic transition.  相似文献   
82.
The Cenozoic sedimentary succession of Bangladesh provides an archive of Himalayan erosion. However, its potential as an archive is currently hampered by a poor lithostratigaphic framework with limited age control. We focus on the Hatia Trough of the Bengal Basin and the adjacent fold belt of the Chittagong Hill Tracts which forms the outermost part of the west‐propagating Indo‐Burmese wedge. We present a basin‐wide seismic stratigraphic framework for the Neogene rocks, calibrated by biostratigraphy, which divides the succession into three seismically distinct and regionally correlatable Megasequences (MS). MS1 extends to NN15‐NN16 (ca. 2.5–3.9 Ma), MS2 to NN19‐NN20 (ca. 0.4–1.9 Ma) and MS3 to present day. Our seismic mapping, thermochronological analyses of detrital mineral grains, isotopic analyses of bulk rock, heavy mineral and petrographic data, show that the Neogene rocks of the Hatia Trough and Chittagong Hill Tracts are predominantly Himalayan‐derived, with a subordinate arc‐derived input possibly from the Paleogene IndoBurman Ranges as well as the Trans‐Himalaya. Our seismic data allow us to concur with previous work that suggests folding of the outer part of the west‐propagating wedge only commenced recently, within the last few million years. We suggest that it could have been the westward encroachment and final abutment of the Chittagong Hill Tracts fold belt onto the already‐uplifted Shillong Plateau that caused diversion of the palaeo‐Brahmaputra to the west of the plateau as the north‐east drainage route closed.  相似文献   
83.
We estimate the flux of the gamma-ray burst (GRB) neutrino background and compute the event rate at SK and TITAND in the collapsar model, assuming that GRB formation rate is proportional to the star formation rate. We find that the predicted background neutrino flux is highly sensitive to unknown model parameters, mainly to the mass–accretion rate, to the fraction of disk energy emitted in thermal neutrinos (as opposed to emission through electromagnetic processes), and to the fraction of collapsar events leading to GRBs. The predicted neutrino flux varies over many orders of magnitude as the values of unknown model parameters are varied. We investigate the detection possibility of thermal neutrinos from collapsars which lead to GRBs by TITAND. We find that the GRB neutrino background might be detected by TITAND within 10 yrs only for the optimistic cases in which the average mass–accretion rate is high ( a few M s−1), and the probability that one collapsar generates a GRB is high (f=0.5–1.0).  相似文献   
84.
Abstract

The static equilibrium of a thin vertical magnetic tube embedded in the solar atmosphere is shown to be dynamically unstable against the fundamental mode of perturbation having no nodes in the vertical displacement. The instability has its origin in the convection zone, and the eigenfunction is extended further up in the stable upper layers by the magnetic field which guides the displacement mainly in the longitudinal direction. It is suggested that the downdraft observed in the solar network structure is a finite amplitude consequence of this instability. The overtone modes are found to be stable.  相似文献   
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To clarify the global deep-water circulation in the northwest Pacific, we conducted current observations with seven moorings at 40°N east of Japan from May 2007 to October 2008, together with hydrographic observations. By analyzing the data, while taking into consideration that the deep circulation has a northward component in this region and carries low-silica, high-dissolved-oxygen water, we clarified that the deep circulation flows within the region between 144°30′ and 146°10′E at 40°N on and east of the eastern slope of the Japan Trench with marked variability; the deep circulation flows partly on the eastern slope of the trench and mainly to the east during P1 (10 May–24 November 2007), is confined to the eastern slope of the trench during P2 (25 November 2007–20 May 2008), and flows on and to the immediate east of the eastern slope of the trench during P3 (21 May–15 October 2008). Previous studies have identified two branches of the deep circulation at lower latitudes in the western North Pacific; one flows off the western trenches and the other detours near the Shatsky Rise. It was thus concluded that the eastern branch flows westward at 38°N and then northward to the east of the trench, finally joining the western branch around 40°N during P1 and P3, whereas the eastern branch passes westward south of 38°N, joins the western branch around 38°N, and flows northward on the eastern slope of the trench during P2.  相似文献   
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Plotosaurus is a highly aquatically adapted mosasaur, which is supposed to inhabit the deep ocean basin. The geographic occurrence of this genus has been limited only to the west coast of North America. In this study, two Plotosaurus-type mosasaur caudal vertebrae derived from the Upper Cretaceous Nakaminato Group in Ibaraki Prefecture, Japan, are described with discussion on the paleobiogeographic significance of the Late Cretaceous mosasaur fauna in the Northwestern Pacific region. The two specimens are an intermediate caudal vertebra found in a beach cobble, which presumably originated from the Hiraiso Formation (upper Campanian), and a terminal caudal vertebra found in situ in the lower Isoai Formation (lower Maastrichtian). Because their relative centrum lengths (ratio of centrum length/centrum height, ~0.7) are very close to that of Plotosaurus, the specimens are referred to cf. Plotosaurus sp. The two specimens provide the first evidence that highly specialized Plotosaurus-type mosasaurs inhabited the Northwestern Pacific Ocean, suggesting that such forms had a wider distribution than previously recognized and might have existed since the late Campanian in the Northwestern Pacific Ocean.  相似文献   
90.
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