摘要:

Convective removal of lower lithosphere beneath the Tibetan Plateau can account for a rapid increase in the mean elevation of the Tibetan Plateau of 1000 m or more in a few million years. Such uplift seems to be required by abrupt tectonic and environmental changes in Asia and the Indian Ocean in late Cenozoic time. The composition of basaltic volcanism in northern Tibet, which apparently began at about 13 Ma, implies melting of lithosphere, not asthenosphere. The most plausible mechanism for rapid heat transfer to the midlithosphere is by convective removal of deeper lithosphere and its replacement by hotter asthenosphere. The initiation of normal faulting in Tibet at about 8 (± 3) Ma suggests that the plateau underwent an appreciable increase in elevation at that time. An increase due solely to the isostatic response to crustal thickening caused by India's penetration into Eurasia should have been slow and could not have triggered normal faulting. Another process, such as removal of relatively cold, dense lower lithosphere, must have caused a supplemental uplift of the surface. Folding and faulting of the Indo‐Australian plate south of India, the most prominent oceanic intraplate deformation on Earth, began between about 7.5 and 8 Ma and indicates an increased north‐south compressional stress within the Indo‐Australian plate. A Tibetan uplift of only 1000 m, if the result of removal of lower lithosphere, should have increased the compressional stress that the plateau applies to India and that resists India's northward movement, from an amount too small to fold oceanic lithosphere, to one sufficient to do so. The climate of the equatorial Indian Ocean and southern Asia changed at about 6–9 Ma: monsoonal winds apparently strengthened, northern Pakistan became more arid, but weathering of rock in the eastern Himalaya apparently increased. Because of its high altitude and lateral extent, the Tibetan Plateau provides a heat source at midlatitudes that should oppose classical (symmetric) Hadley circulation between the equator and temperate latitudes and that should help to drive an essentially opposite circulation characteristic of summer monsoons. For the simple case of axisymmetric heating (no dependence on longitude) of an atmosphere without dissipation, theoretical analyses by Hou, Lindzen, and Plumb show that an axisymmetric heat source displaced from the equator can drive a much stronger meridianal (monsoonlike) circulation than such a source centered on the equator, but only if heating exceeds a threshold whose level increases with the latitude of the heat source. Because heating of the atmosphere over Tibet should increase monotonically with elevation of the plateau, a modest uplift (1000–2500 m) of Tibet, already of substantial extent and height, might have been sufficient to exceed a threshold necessary for a strong monsoon. The virtual simultaneity of these phenomena suggests that uplift was rapid: approximately 1000 m to 2500 m in a few million years. Moreover, nearly simultaneously with the late Miocene strengthening of the monsoon, the calcite compensation depth in the oceans dropped, plants using the relatively efficient C4 pathway for photosynthesis evolved rapidly, and atmospheric CO2seems to have decreased, suggesting causal relationships and positive feedbacks among these phenomena. Both a supplemental uplift of the Himalaya, the southern edge of Tibet, and a strengthened monsoon may have accelerated erosion and weathering of silicate rock in the Himalaya that, in turn, enhanced extraction of CO2from the atmosphere. Thus these correlations offer some support for links between plateau uplift, a downdrawing of CO2from the atmosphere, and global climate change, as proposed by Raymo, Ruddiman, and Froehlich. Mantle dynamics beneath mountain belts not only may profoundly affect tectonic processes near and far from the belts, but might also play an important role in altering regional and globa

ISSN:8755-1209    

DOI:10.1029/93RG02030

年代:1993

数据来源: WILEY

摘要:

Significant advances are being made in our understanding of the Arctic sea ice‐climate system. The mean circulation of the Arctic sea ice cover is now well defined through analysis of data from drifting stations and buoys. Analysis of nearly 20 years of daily satellite data from optical, infrared, and passive microwave sensors has documented the regional variability in monthly ice extent, concentration, and surface albedo. Advances in modeling include better treatments of sea ice dynamics and thermodynamics, improved atmosphere‐ice‐ocean coupling, and the development of high resolution regional models. Diagnostic studies of monthly and interannual sea ice variability have benefited from better sea ice data and geostrophic wind analyses that incorporate drifting buoy data. Some evidence exists for a small retreat of Arctic sea ice over the last 2 decades, but there are large decadal fluctuations in regional ice extent. Antiphase relationships between ice anomalies in different sectors can be related to changes in atmospheric circulation. Evidence suggests that episodes of significant salinity reduction in the North Atlantic, associated with extensive sea ice in the Greenland Sea, may be a manifestation of a decadal oscillation in the Arctic climate system. Aspects of the Arctic system in need of further attention include the surface energy budget and its variability, particularly with respect to the roles of cloud cover and surface types in summer. Sea ice thickness distribution data remain meager, and there are many unknowns regarding the circulation and hydrologic cycle of the Arctic Ocean and its links to the world ocean. Planned measurements from a new generation of satellites, supported by field programs, will provide much needed data to address these i

ISSN:8755-1209    

DOI:10.1029/93RG01998

年代:1993

数据来源: WILEY

摘要:

Implantation of solar corpuscular radiation into the lunar surface generates a population of solar atoms in the rims of lunar regolith grains. Laboratory analysis of those atoms can yield a measure of solar composition. Nitrogen trapped in the lunar regolith consists of at least two components, putatively originating in the Sun, differing in release temperature and therefore probably in implantation energy. The higher‐energy component is depleted in15N relative to the lower‐energy component by amounts that range up to at least 20%. These components superficially resemble those identified previously in the solar‐derived light noble gases, though with several marked differences. Thus the higher‐energy noble gas components are depleted in the lighter isotope. Unlike the noble gas case, the15N/14N ratios of both N components vary with antiquity in a complex fashion; the lower‐energy component echoes the variations in the higher‐energy component which dominate the isotopic evolution of the bulk samples. The magnitude of the bulk sample variation exceeds 30%; the higher‐energy component varies by at least 25%. The bulk long‐term trend in15N/14N does not result from variations in mixing ratio of the two components. Both the compositional difference between the components and the long‐term variations within them apparently originate in the Sun, though this conclusion is inconsistent with current understanding of solar structure and evolution. The nitrogen isotopic record therefore appears to represent a major challenge

ISSN:8755-1209    

DOI:10.1029/93RG01953

年代:1993

数据来源: WILEY

摘要:

Seismic studies have established that large‐offset transforms along the slow spreading Mid‐Atlantic Ridge exhibit anomalous crustal structures that fall well outside the range typically associated with oceanic crust. Seismically, fracture zone crust in the North Atlantic is extremely heterogeneous in both thickness and internal structure. It is frequently quite thin (<1–2 km thick) and is characterized by low compressional wave velocities and the absence of a normal seismic layer 3. A more gradual crustal thinning can extend up to several tens of kilometers from these fracture zones. Anomalously thin crust has also been inferred from both seismic and gravity studies at smaller ridge axis discontinuities along the Mid‐Atlantic Ridge. The geological nature of the seismically anomalous crust found within Atlantic fracture zones, and how this crust forms, are still controversial. One interpretation consistent with available seismic observations is that the crust within North Atlantic fracture zones consists of a thin, intensely fractured, and hydrothermally altered basaltic section overlying ultramafics that are extensively serpentinized in places. Variations in apparent seismic crustal thickness along fracture zones may reflect different degrees of serpentinization of the upper mantle section or changes in the thickness of the igneous crust. The existence of a thinner crustal section in fracture zones can be explained by a reduced magma supply within a broad region near ridge offsets due to the three‐dimensional nature of upwelling beneath a segmented spreading center and by tectonic dismemberment of the crust by large‐scale detachment faults that form preferentially in the cold, brittle lithosphere near the ends

ISSN:8755-1209    

DOI:10.1029/93RG01952

年代:1993

数据来源: WILEY