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961.
Michael M. McGlue Kiram E. Lezzar Andrew S. Cohen James M. Russell Jean-Jacques Tiercelin Anna A. Felton Evelyne Mbede Hudson H. Nkotagu 《Journal of Paleolimnology》2008,40(2):635-653
New intermediate-resolution, normal-incidence seismic reflection profiles from Lake Tanganyika’s central basin capture dramatic
evidence of base-level change during two intervals of the late Pleistocene. Four seismically-defined stratigraphic sequences
(A–D) tied to radiocarbon-dated sediment cores provide a chronology for fluctuating environmental conditions along the Kalya
Platform. Stacked, oblique clinoforms in Sequence C are interpreted as prograding siliciclastic deltas deposited during a
major regression that shifted the paleo-lake shore ∼21 km towards the west prior to ∼106 ka. The topset-to-foreset transitions
in these deltas suggest lake level was reduced by ∼435 m during the period of deposition. Mounded reflections in the overlying
sequence are interpreted as the backstepping remnants of the delta system, deposited during the termination of the lowstand
and the onset of transgressive conditions in the basin. The youngest depositional sequence reflects the onset of profundal
sedimentation during the lake level highstand. High amplitude reflections and deeply incised channels suggest a short-lived
desiccation event that reduced lake level by ∼260 m, interpreted as a product of Last Glacial Maximum (32–14 ka) aridity.
Paleobathymetric maps constructed for the two interpreted regressions reveal that despite the positive lake-floor topography
created by the Kavala Island Ridge Accommodation Zone, Lake Tanganyika remained a large, mostly connected water body throughout
the late Pleistocene. The results of this analysis further imply that Lake Tanganyika is the most drought resistant water
body in the East African tropics, and may have acted as a refuge for local and migrating fauna during periods of prolonged
aridity. 相似文献
962.
Abstract— The characterization of comet and asteroid interiors will eventually require in situ exploration with drills, penetrators/penetrometers, hypervelocity impactors, excavators or other devices. Because they offer desirable scientific capabilities and relative mechanical simplicity, penetrators and penetrometers, which use only axial force to push beneath the surface, are a good choice for near‐term missions. Penetrometers are instruments, generally deployed from a larger vehicle, that measure subsurface mechanical properties and may also contain additional scientific instruments. There are three basic types: “fast” penetrometers are released from above and plunge into the surface. Static and dynamic (collectively referred to as “slow”) penetrometers use, respectively, a constant slow penetration speed and periodic hammering impulses. The low gravity environment of asteroids and comets presents a key challenge to instrument deployment and also greatly affects the mechanical properties of surface materials, and in turn penetrometer performance. The Rosetta mission, currently en route to comet 67P/Churyumov‐Gerasimenko, will be the next mission to try both fast and slow, dynamic penetrometry, when it arrives in 2014. We present some new concepts of static penetrometers for small body exploration that are adapted to the low gravity environment. The low gravity environment also presents challenges for the testing of penetrometers on Earth and a number of previous solutions are described and new methods suggested. In the next generation of missions to study comets and asteroids, penetrometers could provide important data on their mechanical, seismic, thermal, electromagnetic, and chemical characteristics, as well as sample collection. 相似文献
963.
I briefly review the physics of accretion at rates well above the Eddington value. In stellar-mass systems, the result is generally an ultraluminous X-ray binary, and I show how SS433 fits into this picture. Conditions are relatively less extreme in active galactic nuclei, as the greatest likely accretion rates are not far above the electron-scattering Eddington value. However, even quite modest mass inflow rates exceed the corresponding value for dust opacity. I show that the obscuring torus invoked in unified AGN schemes is the probable result, and note that the matter in the torus probably does not supply the matter accreting on to the central black hole and thus powering the AGN. 相似文献
964.
965.
Francesca Mietta Claire Chassagne Andrew J. Manning Johan C. Winterwerp 《Ocean Dynamics》2009,59(5):751-763
The purpose of this paper is to establish a relation between a few measurable quantities (the so-called ζ potential, organic matter content, and shear rate) and the flocculation behavior of mud. The results obtained with small-scale
flocculation experiments (mixing jar) are compared to results of large-scale experiments (settling column). The mud used for
all experiments has been collected in October 2007 in the lower Western Schelde, near Antwerp, Belgium. From this study, it
was found that the mean floc size and the Kolmogorov microscale vary in a similar way with the shear rate for suspensions
with different pH and salt concentrations. The size of flocs at a given shear rate depends on the properties of the suspension,
which affect the electrokinetic properties of the sediment; these can be described by means of the ζ potential. The main findings of this paper are: (1) In saline suspensions at pH = 8, the mean floc size increases when the
salt concentration and the ζ potential increase. (2) For a given ζ potential, the mean floc size at low pH is larger than observed at pH = 8 for any added salt. (3) The mean floc size increases
with increasing organic matter content. (4) Mud with no organic matter at pH = 8 and no added salt flocculates very little.
The response of mud suspensions to variations in salinity and pH is similar to that of kaolinite. This suggests that a general
trend can be established for different and complex types of clays and mud. This systematic study can therefore be used for
further development of flocculation models. 相似文献
966.
967.
Sediment source changes over the last 250 years in a dry-tropical catchment, central Queensland, Australia 总被引:2,自引:0,他引:2
Rivers draining to the Great Barrier Reef are receiving increased attention with the realisation that European land use changes over the last 150 years may have increased river sediment yields, and that these may have adversely affected the reef environment. Mitigation of the effects associated with such changes is only possible if information on the spatial provenance and dominant types of erosion is known. To date, very few field-based studies have attempted to provide this information. This study uses fallout radionuclide (137Cs and 210Pbex) and geochemical tracing of river bed and floodplain sediments to examine sources over the last 250 years for Theresa Creek, a subcatchment of the Fitzroy River basin, central Queensland, Australia. A Monte Carlo style mixing model is used to predict the relative contribution of both the spatial (geological) sources and erosion types. The results indicate that sheetwash and rill erosion from cultivated basaltic land and channel erosion from non-basaltic parts of the catchment are currently contributing most sediment to the river system. Evidence indicates that the dominant form of channel erosion is gully headcut and sidewall erosion. Sheetwash and rill erosion from uncultivated land (i.e., grazed pasture/woodland) is a comparatively minor contributor of sediment to the river network. Analysis of the spatial provenance of floodplain core sediments, in conjunction with optical dating and 137Cs depth profile data, suggests that a phase of channel erosion was initiated in the late nineteenth century. With the development of land underlain by basalt in the mid-twentieth century the dominant source of erosion shifted to cultivated land, although improvements in land management practices have probably resulted in a decrease in sediment yield from cultivated areas in the later half of the twentieth century. On a basin-wide scale, because of the limited spatial extent of cultivation, channel sources are likely to be the largest contributor of sediment to the Fitzroy River. Accordingly, catchment management measures focused on reducing sediment delivery to the Great Barrier Reef should focus primarily on decreasing erosion from channel sources. 相似文献
968.
Danny Redo J.O. Joby Bass Andrew C. Millington 《Applied geography (Sevenoaks, England)》2009,29(1):91-110
Analyses of landscape change using remotely sensed satellite imagery constitute a large component of forest transition research, allowing for assessments of large areas. In the western highlands of Honduras is an area of complex forest dynamics (~45,000 ha) that has seen significant forest regeneration in recent years. However, analysis of the larger region (~500,000 ha) shows net forest loss. The comparative aspects highlight the importance of site selection and scale in forest transition analysis, a process often ignored in the land-use and land-cover change (LULCC) and forest transition literature. Results also highlight the importance of analyzing human-induced fragmentation at a variety of selected sites and a range of spatial scales, and producing quality, accurate forest cover and change maps. 相似文献
969.
970.