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1.
Nature ; 518(7537): 85-8, 2015 Feb 05.
Artigo em Inglês | MEDLINE | ID: mdl-25653000

RESUMO

Plate tectonics successfully describes the surface of Earth as a mosaic of moving lithospheric plates. But it is not clear what happens at the base of the plates, the lithosphere-asthenosphere boundary (LAB). The LAB has been well imaged with converted teleseismic waves, whose 10-40-kilometre wavelength controls the structural resolution. Here we use explosion-generated seismic waves (of about 0.5-kilometre wavelength) to form a high-resolution image for the base of an oceanic plate that is subducting beneath North Island, New Zealand. Our 80-kilometre-wide image is based on P-wave reflections and shows an approximately 15° dipping, abrupt, seismic wave-speed transition (less than 1 kilometre thick) at a depth of about 100 kilometres. The boundary is parallel to the top of the plate and seismic attributes indicate a P-wave speed decrease of at least 8 ± 3 per cent across it. A parallel reflection event approximately 10 kilometres deeper shows that the decrease in P-wave speed is confined to a channel at the base of the plate, which we interpret as a sheared zone of ponded partial melts or volatiles. This is independent, high-resolution evidence for a low-viscosity channel at the LAB that decouples plates from mantle flow beneath, and allows plate tectonics to work.

2.
Nature ; 458(7236): 322-8, 2009 Mar 19.
Artigo em Inglês | MEDLINE | ID: mdl-19295607

RESUMO

Thirty years after oxygen isotope records from microfossils deposited in ocean sediments confirmed the hypothesis that variations in the Earth's orbital geometry control the ice ages, fundamental questions remain over the response of the Antarctic ice sheets to orbital cycles. Furthermore, an understanding of the behaviour of the marine-based West Antarctic ice sheet (WAIS) during the 'warmer-than-present' early-Pliocene epoch ( approximately 5-3 Myr ago) is needed to better constrain the possible range of ice-sheet behaviour in the context of future global warming. Here we present a marine glacial record from the upper 600 m of the AND-1B sediment core recovered from beneath the northwest part of the Ross ice shelf by the ANDRILL programme and demonstrate well-dated, approximately 40-kyr cyclic variations in ice-sheet extent linked to cycles in insolation influenced by changes in the Earth's axial tilt (obliquity) during the Pliocene. Our data provide direct evidence for orbitally induced oscillations in the WAIS, which periodically collapsed, resulting in a switch from grounded ice, or ice shelves, to open waters in the Ross embayment when planetary temperatures were up to approximately 3 degrees C warmer than today and atmospheric CO(2) concentration was as high as approximately 400 p.p.m.v. (refs 5, 6). The evidence is consistent with a new ice-sheet/ice-shelf model that simulates fluctuations in Antarctic ice volume of up to +7 m in equivalent sea level associated with the loss of the WAIS and up to +3 m in equivalent sea level from the East Antarctic ice sheet, in response to ocean-induced melting paced by obliquity. During interglacial times, diatomaceous sediments indicate high surface-water productivity, minimal summer sea ice and air temperatures above freezing, suggesting an additional influence of surface melt under conditions of elevated CO(2).


Assuntos
Camada de Gelo , Regiões Antárticas , Atmosfera/análise , Atmosfera/química , Calibragem , Dióxido de Carbono/análise , Diatomáceas/química , Diatomáceas/isolamento & purificação , Fósseis , História Antiga , Isótopos de Oxigênio , Temperatura
3.
Nature ; 413(6857): 719-23, 2001 Oct 18.
Artigo em Inglês | MEDLINE | ID: mdl-11607028

RESUMO

Between 34 and 15 million years (Myr) ago, when planetary temperatures were 3-4 degrees C warmer than at present and atmospheric CO2 concentrations were twice as high as today, the Antarctic ice sheets may have been unstable. Oxygen isotope records from deep-sea sediment cores suggest that during this time fluctuations in global temperatures and high-latitude continental ice volumes were influenced by orbital cycles. But it has hitherto not been possible to calibrate the inferred changes in ice volume with direct evidence for oscillations of the Antarctic ice sheets. Here we present sediment data from shallow marine cores in the western Ross Sea that exhibit well dated cyclic variations, and which link the extent of the East Antarctic ice sheet directly to orbital cycles during the Oligocene/Miocene transition (24.1-23.7 Myr ago). Three rapidly deposited glacimarine sequences are constrained to a period of less than 450 kyr by our age model, suggesting that orbital influences at the frequencies of obliquity (40 kyr) and eccentricity (125 kyr) controlled the oscillations of the ice margin at that time. An erosional hiatus covering 250 kyr provides direct evidence for a major episode of global cooling and ice-sheet expansion about 23.7 Myr ago, which had previously been inferred from oxygen isotope data (Mi1 event).

4.
Yonsei Medical Journal ; : 105-115, 1984.
Artigo em Inglês | WPRIM | ID: wpr-82276

RESUMO

Activities of hepatic cytosol enzymes involved in UDP-g1ucuronic acid synthesis as well as in glutathione reduction and conjugation systems were determined following administrations of butylated hydroxyanisole (approximately 5 mmol/kg body weight/day) and of equimolar intake doses of its structural anglogs. These compounds included the multi-functional group side chain compounds (t-butyl hydroquinone, 4-hydroxy- anisole, hydroquinone, benzoquinone) and the mono-functional side chain compounds (t-butyl benzene, anisole, phenol). They were administered to mice for 10 days either by mixing them in the diet or by oral intubations. Results showed that glutathione Stransferase activities were markedly increased by all tested compounds except for the t-butyl benzene. Activities of glutathione reductase and glucose 6-phosphate dehydrogenase were increased together on1y by BHA and t-butyl hydroguinone. UDP-glucose dehydrogenase and NADH:quinone reductase activities were significantly elevated by the multi-functional side chain compounds, but not by the mono-functional analogs. The relations between chemical structures of tested BHA analogs and elevations of the measured hepatic cytosol conjugation (detoxification) system enzyme activities for the metabolism and excretion of BHA analogs are discussed.


Assuntos
Camundongos , Animais , Anisóis/metabolismo , Hidroxianisol Butilado/análogos & derivados , Hidroxianisol Butilado/metabolismo , Citosol/enzimologia , Glutationa/metabolismo , Uridina Difosfato Ácido Glucurônico/biossíntese , Açúcares de Uridina Difosfato/biossíntese
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