Sea-ice retreat processes are examined in the Sea of Okhotsk. A heat budget analysis in the sea-ice zone shows that net heat
flux from the atmosphere at the water surface is about 77 W m−2 on average in the active ice melt season (April) due to large solar heating, while that at the ice surface is about 12 W m−2 because of the difference in surface albedo. The temporal variation of the heat input into the upper ocean through the open
water fraction corresponds well to that of the latent heat required for ice retreat. These results suggest that heat input
into the ice–upper ocean system from the atmosphere mainly occurs at the open water fraction, and this heat input into the
upper ocean is an important heat source for ice melting. The decrease in ice area in the active melt season (April) and the
geostrophic wind just before the melt season (March) show a correlation: the decrease is large when the offshoreward wind
is strong. This relationship can be explained by the following process. Once ice concentration is decreased (increased) by
the offshoreward (onshoreward) wind just before the melt season, solar heating of the upper ocean through the increased (decreased)
open water fraction is enhanced (reduced), leading to (suppressing) a further decrease in ice concentration. This positive
feedback is regarded as the ice–ocean albedo feedback, and explains in part the large interannual variability of the ice cover
in the ice melt season. 相似文献
As part of a collaborative study between the 3rd Institute of Oceanography, Xiamen, and the U. K. Universities of East Anglia
and St. Andrews, a 4-day experiment was conducted on a ridge- and- runnel beach of north Norfolk on the North Sea coast. Detailed
surveys were made of every low water of an area 10m by 30m and electromagnetic current meters were used to measure wave-induced
currents over the ridge and in the runnel. The locations of bedform fields were noted, fluorescent sand was used to follow
the sand movement and, at the end of the experiment, lacquer peels were taken of the top 0.2m of a vertical section through
the ridge- and- runnel.
During the 4 days 0.75m3 of sand per metre width of beach accreted in the runnel and a similar volume was eroded from the upper foreshore. The dominant
shoreward transport, identified qualitatively by the movement of the fluorescent sand, suggests the ridge- and- runnel system
migrated shorewards up the general beach slope of 1o by 2–3m. The wave orbital currents were used to predict the bedforms to be expected over the tidal cycle: an upper phase
plane- bed was predicted for most of the period but vortex and rolling- grain ripples were predicted and observed when the
water level over the ridge was low. As the tide dropped ripples on the ridge were eliminated by swash action but the ripples
in the runnel were protected by the ridge and remained on the beach after the tide dropped.
The observed accretion of the beach in the runnel and on the lee slope of the ridge was used to calculate that a net average
shoreward transport of 0.11g/cm·s−1 occurred over the ridge crest during the period it was underwater. The current meter measurements of the wave orbital currents
and the mean currents over the ridge crest were used with the wave- current interaction model of Grant and Madsen (1982),
ripple dimensions defined by Nielsen (1981), and resuspension coefficient of Vincent and Green (1990), to compute bedload
sand transport rates from the equations of Madsen and Grant (1976), Sleath (1978), and Vincent et al. (1981), and also the
suspended sand transport rates. The results from one of these bedload equations (Madsen and Grant, 1976) compared well with
the observed net transport. The calculated suspended load transport rates (due to steady currents alone) were a factor of
5 too great, and were also several orders of magnitude greater than suspended transport rates measured directly under similar
or more energetic wave and current conditions. 相似文献
The Oligo-Miocene Somuncura province is the largest (55 000km2) back-arc mafic volcanic field in Patagonia, and one ofEarth's largest with no clear link to a hotspot or major extension.Major and trace element and SrNdPb isotopic datasuggest involvement of a plume-like component in the mantlemagma source mixed with hydrous, but not high field strengthelement (HFSE)-depleted components, from a disintegrating subductingplate. Magmatism is attributed to mantle upwelling related todisturbances during plate reorganization, possibly at a timewhen the South America plate was nearly stationary over theunderlying mantle. Melting was enhanced by hydration of themantle during Paleogene subduction. Crustal contamination wasminimal in a refractory crust that had been extensively meltedin the Jurassic. Eruption began with low-volume intraplate alkalinemafic flows with depleted NdSr isotopic signatures. Thesewere followed by voluminous 2925 Ma tholeiitic maficflows with flat light and steep heavy rare earth element (REE)patterns, intraplate-like La/Ta ratios, arc-like Ba/La ratiosand enriched SrNd isotopic signatures. Their source canbe explained by mixing EM1Tristan da Cunha-like and depletedmantle components with subduction-related components. Post-plateau2417 Ma alkaline flows with steep REE patterns, highincompatible element abundances, and depleted SrNd isotopicsignatures mark the ebbing of the mantle upwelling. KEY WORDS: Somuncura plateau; slab interaction; Patagonia; large igneous province (LIP); plume-like upwelling相似文献
Eruptions in shallow water typically produce cones of volcaniclastic material. In order to identify any systematic effects of water depth and other environmental parameters on cone morphology, we have measured the heights and widths of cones in multibeam echo-sounder data from a submarine ridge extending southeast from Pico Island, Azores. XRF analyses of dredged samples show that lavas here vary compositionally from alkali basalt to trachybasalt and trachyandesite. Cones in deeper water are generally steep-sided with upper flanks close to 30°, the dip of talus at the angle of repose. However, height/width ratios of cones vary more in shallow water (200?C400-m summit depth) with extreme values below 0.1; while some shallow-water cones are steep-sided as in deep water, others are much flatter. Three such cones lie on a bench at 300-m depth immediately east of Pico Island and have flank slopes of only 10?C20°. We speculate that exceptionally shallow cone slopes here were produced by forced spreading of the erupting columns on reaching the water?Cair density barrier. 相似文献
The relativistic Sunyaev–Zel'dovich (SZ) effect offers a method, independent of X-ray, for measuring the temperature of the intracluster medium (ICM) in the hottest systems. Here, using N -body/hydrodynamic simulations of three galaxy clusters, we compare the two quantities for a non-radiative ICM, and for one that is subject both to radiative cooling and to strong energy feedback from galaxies. Our study has yielded two interesting results. First, in all cases, the SZ temperature is hotter than the X-ray temperature and is within 10 per cent of the virial temperature of the cluster. Secondly, the mean SZ temperature is less affected by cooling and feedback than the X-ray temperature. Both these results can be explained by the SZ temperature being less sensitive to the distribution of cool gas associated with cluster substructure. A comparison of the SZ and X-ray temperatures (measured for a sample of hot clusters) would therefore yield interesting constraints on the thermodynamic structure of the intracluster gas. 相似文献
The sequence of extreme September sea ice extent minima over the past decade suggests acceleration in the response of the Arctic sea ice cover to external forcing, hastening the ongoing transition towards a seasonally open Arctic Ocean. This reflects several mutually supporting processes. Because of the extensive open water in recent Septembers, ice cover in the following spring is increasingly dominated by thin, first-year ice (ice formed during the previous autumn and winter) that is vulnerable to melting out in summer. Thinner ice in spring in turn fosters a stronger summer ice-albedo feedback through earlier formation of open water areas. A thin ice cover is also more vulnerable to strong summer retreat under anomalous atmospheric forcing. Finally, general warming of the Arctic has reduced the likelihood of cold years that could bring about temporary recovery of the ice cover. Events leading to the September ice extent minima of recent years exemplify these processes. 相似文献
We investigated the distribution of naturally occurring geochemical tracers (222Rn, 223Ra, 224Ra, 226Ra, CH4, δ18O, and δ2H) in the water column and adjacent groundwater of Mangueira Lagoon as proxies of groundwater discharge. Mangueira Lagoon is a large (90 km long), shallow (4–5 m deep), fresh, and non-tidal coastal lagoon in southern Brazil surrounded by extensively irrigated rice plantations and numerous irrigation canals. We hypothesized that the annual, intense irrigation for rice agriculture creates extreme conditions that seasonally change groundwater discharge patterns in the adjacent lagoon. We further supposed that dredging of irrigation canals alters groundwater fluxes.
While the activities of 222Rn in shallow groundwater were 2–3 orders of magnitude higher than in surface water, CH4 and radium isotopes were only 1 order of magnitude higher. Therefore, 222Rn appears to be the preferred groundwater tracer in this system. Radon concentrations and conductivities were dramatically higher near the pump house of rice irrigation canals, consistent with a groundwater source. Modeling of radon inventories accounting for total inputs (groundwater advection, diffusion from sediments, and decay of 226Ra) and losses (atmospheric evasion, horizontal mixing and decay) indicated that groundwater advection rates in the irrigation canals (25 cm/d) are over 2 orders of magnitude higher than along the shoreline (0.1 cm/d). Nearly 75% of the total area of the canals is found in the southern half of the lagoon, where groundwater inputs seem to be higher as also indicated by methane and stable isotope trends. In spite of the relatively small area of the canals, we estimate that they contribute nearly 70% of the total (57,000 m3/d) groundwater input into the entire Mangueira Lagoon. We suggest that the dredging of these canals cut through aquitards which previously restricted upward advection from the underlying permeable strata. The irrigation channels may therefore represent an important but previously overlooked source of nutrients and other dissolved chemicals derived from agricultural practices into the lagoon. 相似文献