North Atlantic Water in the Barents Sea Opening, 1997 to 1999

North Atlantic Water (NAW) is an important source of heat and salt to the Nordic seas and the Arctic Ocean. To measure the transport and variability of one branch of NAW entering the Arctic, a transect across the entrance to the Barents Sea was occupied 13 times between July 1997 and November 1999, and hydrography and currents were measured. There is large variability between the cruises, but the mean currents and the hydrography show that the main inflow takes place in Bjørnøyrenna, with a transport of 1.6 Sv of NAW into the Barents Sea. Combining the flow field with measurements of temperature and salinity, this results in mean heat and salt transports by NAW into the Barents Sea of 3.9×10¹³ W and 5.7×10⁷ kg s⁻¹, respectively. The NAW core increased in temperature and salinity by 0.7 °C yr⁻¹ and 0.04 yr⁻¹, respectively, over the observation period. Variations in the transports of heat and salt are, however, dominated by the flow field, which did not exhibit a significant change.     

Axial Intrusion Zone beneath the Median Valley of the Mid-Atlantic Ridge at 37°N Detected by Explosion Seismology

Summary. Four seismic refraction lines, three of which had shots every 250 m, were shot across, along and parallel to the median valley of the Mid-Atlantic Ridge at 37° N. A method has been developed for calculating the effect on the travel times of the rough sea-floor relief beneath the profiles and has been used to correct all the travel times for this effect. Most arrivals were from a main refractor of apparent velocity 5·4 to 6·3 km s⁻¹; only beyond 35 km were faster arrivals observed from an 8·09 ± 36 km s⁻¹ refractor. The main refractor corresponds in depth, at least approximately, to the top of Layer 3 of the ocean basins but its velocity is significantly less than normal for Layer 3, perhaps due to dip. A study of time residuals along two profiles across the median valley indicates the presence of a 2 to 3 km wide low velocity zone (about 3·2 km s⁻¹) beneath the median valley floor. This zone extends over the upper 2·5 km of the crust and is believed to represent a zone of intrusion through which magma passes on its way to the sea floor.     

The Residual Water Flow through the Solent, South England

Summary. A reduced equation of motion is used to compute the residual velocity and the residual transport through the West Solent from the water levels recorded over an eight-month period at tide gauges at either end of the channel. A coefficient of bottom friction of 5·0 × 10⁻³ is assumed. There was a spring-neap variation and a significant correlation of fluctuations in the residual velocity with meteorological conditions. Westward residual velocities occurred at spring tides with low barometric pressure and south-westerly winds. Eastward residual velocities occurred at neap tides with high pressure and north-easterly winds. Because of the progressive nature of the tidal wave the long term residual transport appeared to be towards the west and the flushing time for the Solent system was long for considerable periods. The maximum velocities experienced during a tidal cycle half way along the channel are towards the west with a probability of values exceeding 160 cm s⁻¹ for 10 min in 5·4 days.     

Crustal and upper mantle structure of the northwestern North Island, New Zealand, from seismic refraction data

The crustal and upper mantle structure of the northwestern North Island of New Zealand is derived from the results of a seismic refraction experiment; shots were fired at the ends and middle of a 575 km-long line extending from Lake Taupo to Cape Reinga. The principal finding from the experiment is that the crust is 25 ± 2 km thick, and is underlain by what is interpreted to be an upper mantle of seismic velocity 7.6 ± 0.1 km s⁻¹, that increases to 7.9 km s⁻¹ at a depth of about 45 km. Crustal seismic velocities vary between 5.3 and 6.36 km s⁻¹ with an average value of 6.04 km s⁻¹. There are close geophysical and geological similarities between the north-western North Island of New Zealand and the Basin and Range province of the western United States. In particular, the conditions of low upper-mantle seismic velocities, thin crust with respect to surface elevation, and high heat-flow (70–100 mW m⁻²) observed in these two areas can be ascribed to their respective positions behind an active convergent margin for about the past 20 Myr.     

Shallow velocity structure along the Hirapur–Mandla profile using traveltime inversion of wide-angle seismic data, and its tectonic implications

In order to investigate the velocity structure, and hence shed light on the related tectonics, across the Narmada–Son lineament, traveltimes of wide-angle seismic data along the 240 km long Hirapur–Mandla profile in central India have been inverted. A blocky, laterally heterogeneous, three-layer velocity model down to a depth of 10 km has been derived. The first layer shows a maximum thickness of the upper Vindhyans (4.5 km s⁻¹ ) of about 1.35 km and rests on top of normal crystalline basement, represented by the 5.9 km s⁻¹ velocity layer. The anomalous feature of the study is the absence of normal granitic basement in the great Vindhyan Graben, where lower Vindhyan sediments (5.3 km s⁻¹ ) were deposited during the Precambrian on high-velocity (6.3 km s⁻¹ ) metamorphic rock. The block beneath the Narmada–Son lineament represents a horst feature in which high-velocity (6.5 km s⁻¹ ) lower crustal material has risen to a depth of less than 2 km. South of the lineament, the Deccan Traps were deposited on normal basement during the upper Cretaceous period and attained a maximum thickness of about 800 m.     

Crust and upper mantle structure of the central Iberian Meseta (Spain)

Summary. Quarry blasts recorded along three lines on the central Iberian Meseta are used in an attempt to interpret the crustal structure. The results of the interpretation of the data, together with published surface wave and earthquake data, suggest a layered structure of the crust having the following features: the basement, in some areas covered by up to 4 km of sediments, has a P -velocity of 6.1 km s⁻¹; a low-velocity layer, between 7 and 11 km depth, seems to exist on the basis of both P and S interpretation of seismic data; a thick middle crust of 12 km has a P -velocity of 6.4 km s⁻¹ and overlies a lower crust with a mean P -velocity of 6.9 km s⁻¹ and a possible slight negative gradient; the mean v _p/ v _s ratio for the crust is about 1.75; the Moho is reached at about 31 km depth and consists of a transition zone at least 1.5 km thick. The P -velocity of the upper mantle is close to 8.1 km s⁻¹ and the S -velocity about 4.5 km s⁻¹, which gives a v _p /v _s ratio of 1.8 for the uppermost mantle. A tentative petrological interpretation of the velocities and composition of the layers is given.     

Crustal structure of the Mid-Atlantic ridge crest at 37° N

Summary. A structural model of the Mid-Atlantic Ridge at 37° N is proposed on the basis of travel-time data and synthetic seismograms. At the ridge axis the crust is only 3 km thick and overlies material with an anomalously low'upper mantle'velocity of 7.2 km s⁻¹. Crustal thickening and the formation of layer 3 and a layer with velocity 7.2–7.3 km s⁻¹ takes place within a few kilometres of the axis, producing a 6–7 km thick crust by less than 10 km from the axis. A normal upper mantle velocity of 8.1 km s⁻¹ exists within 10 km of the axis. Shear waves propagate across the axis, thus precluding the existence of any sizeable magma chamber at shallow depth.     

南北极海冰变化及其影响因素的对比分析

海冰是海洋-大气交互系统的重要组成部分,与全球气候系统间存在灵敏的响应和反馈机制。本文选用欧洲空间局发布的1992—2008年海冰密集度数据分析了南北极海冰在时间和空间上的变化规律与趋势,并结合由美国环境预报中心(National Centers for Environmental Prediction,NCEP)和美国大气研究中心(National Center for Atmospheric Research, NCAR)联合制作的NCEP/NCAR气温数据和ENSO指数探讨了南北极海冰变化的影响因素。结果表明,北极海冰面积呈明显的减少趋势,其中夏季海冰最小月的减少更快。北冰洋中央海盆区、巴伦支海、喀拉海、巴芬湾和拉布拉多海的减少最明显。南极海冰面积呈微弱增加趋势,罗斯海、太平洋扇区和大西洋扇区的海冰增加。北极海冰面积与气温有显著的滞后1个月的负相关关系(P0.01)。北极升温显著,北冰洋中央海盆区、喀拉海、巴伦支海、巴芬湾和楚科奇海升温趋势最大,海冰减少很明显。南极在南大西洋、南太平洋呈降温趋势,海冰增加。北极海冰减少与39个月之后ONI的下降、40个月之后SOI的上升密切相关;南极海冰增加与7个月之后ONI的下降、6个月之后SOI的上升存在很好的响应关系。南北极海冰变化与三次ENSO的强暖与强冷事件有很好的对应关系。     

Velocity structure of upper-mantle transition zones beneath central eurasia from seismic inversion using genetic algorithms

We present velocity constraints for the upper-mantle transition zones beneath Central Siberia based on observations of the 1982 RIFT Deep Seismic Sounding (DSS) profile. The data consist of seismic recordings of a nuclear explosion in north-western Siberia along a 2600 km long seismic profile extending from the Yamal Peninsula to Lake Baikal. We invert seismic data from the mantle transition zones using a non-linear inversion scheme using a genetic algorithm for optimization and the WKBJ method to compute the synthetic seismograms. A statistical error analysis using a graph-binning technique was performed to provide uncertainty values in the velocity models.
Our best model for the upper-mantle velocity discontinuity near 410 km depth has a two-stage velocity-gradient structure, with velocities increasing from 8.70–9.25 km s⁻¹ over a depth range of 400–415 km, a gradient of 0.0433 s⁻¹, and from 9.25–9.60 km s⁻¹ over a depth range of 415–435 km, a gradient of 0.0175 s⁻¹. This derived model is consistent with other seismological observations and mineral-physics models. The model for the velocity discontinuity near 660 km depth is simple, sharp and includes velocities increasing from 10.15 km s⁻¹ at 655 km depth to 10.70 km s⁻¹ at 660 km depth, a gradient of 0.055 s⁻¹.     

Crustal structure of the Whipple Mountains, southeastern California: a refraction study across a region of large continental extension

Summary. Closely spaced refraction profiling across the Whipple Mountains metamorphic core complex in southeastern California yields a complex picture of crustal structure in this region of large continental extension. A NE-directed profile, parallel to the extension direction, reveals a high-velocity mid-crustal layer (6.6–6.8 km s⁻¹) at 16-18 km depth, bounded above and below by laterally discontinuous low-velocity zones (<6.0 km s⁻¹). In marked contrast, a NW-directed profile shows a more uniform 6.0 km s⁻¹ crust down to the crust-mantle boundary. The apparent contrast between these two perpendicular profiles may be related not only to a more complex geologic structure in the NW-SE direction, but also to velocity anisotropy associated with mid-crustal mylonites. Despite the differences between the two refraction profiles, both define a flat Moho at 26-27 km depth with an associated upper mantle-velocity of 7.8 km s⁻¹. This observation is significant as it suggests that, although the amount of extension has been highly variable regionally, the crust is no thinner beneath the Whipple Mountains (where extension has been extreme) than the surrounding mountain ranges. Such an observation requires either that the crust was considerably thicker prior to extension, or that lateral flow in the lower crust and/or inflation of the crust via magmatism occurred contemporaneous with extension.     

The seismic structure of thinned continental crust in the northern Bay of Biscay

Summary. The stretching and thinning of the continental crust, which occurs during the formation of passive continental margins, may cause important changes in the velocity structure of such crust. Further, crust attenuated to a few kilometres' thickness, can be found underlying 'oceanic' water depths. This paper poses the question of whether thinned continental crust can be distinguished seismically from normal oceanic crust of about the same thickness. A single seismic refraction line shot over thinned continental crust as part of the North Biscay margin transect in 1979 was studied in detail. Tau— p inversion suggested that there are differences between oceanic and continental crust in the lower crustal structure. This was confirmed when synthetic seismograms were calculated. The thinned continental crust (β± 7.0) exhibits a two-gradient structure in the non-sedimentary crust with velocities between 5.9 and 7.4 km s⁻¹; an upper 0.8 s⁻¹ layer overlies a 0.4 s⁻¹ layer. No layer comparable to oceanic layer 3 was detected. The uppermost mantle also contains a low-velocity zone.     

The Storfjorden polynya: ERS-2 SAR observations and overview

Persistent polynyas have been observed over several winters in Storfjorden, situated between Spitsbergen and Barentsøya/Edgeøya in the south of the Svalbard archipelago. Polynyas are in general active regions with respect to ocean-atmosphere heat exchange, presenting strong convection phenomena and as such being involved in important water mass formation and having an impact on the marine ecosystem. Hydrographic observations have revealed very dense (cold and saline) brine-enriched bottom waters leaving the continental shelf as gravity driven plumes into the deep sea west of Spitsbergen. Satellite observations, using ERS-2 SAR imagery, reveal the evolution of the Storfjorden polynya during winter 1997/98. After forming a complete ice cover until mid-January, Storfjorden responds dynamically to northerly winds by opening a large latent heat polynya. It occupies at its largest extent a region of up to 6000 km² of open water, thin ice and brash ice. Comparable in size to other large Arctic polynyas, the Storfjorden polynya might have the same or even greater importance in the thermohaline circulation and bottom water mass formation. Ice production is estimated at 30 km³ in Storfjorden, rejecting around 700 Mt (Megatons) of salt that can raise the salinity in Storfjorden by 0.9-1.0 PSU. First studies and the winter 1997/98 evolution of this polynya are presented in this paper.     

Volume, heat and salt transport by the West Spitsbergen Current

During the summer of 2000 (June-July) 14 CTD and ADCP transects perpendicular to the West Spitsbergen Current and along the western border of the Barents Sea were made. The measurements covered the area between 69° 43'and 80° N and 01° and 20° E. The main purpose was to follow changes in volume, heat and salt content of Atlantic Water (AW) on its way north. The strongest and most stable flow of AW was located along the continental slope where northward flowing currents exceeding 40 cm/sec were measured. A few weaker northward branches were also found to the west of the slope. South-directed currents were recorded between them and eddy-like mesoscale structures were commonly observed. Measured by vessel-mounted acoustic Doppler current profiler (VM-ADCP), the net northward transport of AW volume in the upper 136 m layer decreased from nearly 6 Sv at the southernmost transect to below 1 Sv at a latitude of 78° 50'N. Similarly, heat transport drops from about 173 TW to about 9 TW and relative salt transport (over 34.92 psu) from 980 × 10³ kg/sec to 14 × 10³ kg/sec. Transport in the southern direction prevails at the transect located between 79° 07'and 79° 30'N. The calculated baroclinic geostrophic transport of AW volume, heat and salt in the upper 1000 m layer behaves similarly. East-directed transport dominates at the Barents Sea boundary while westward flow prevails on the western side of the West Spitsbergen Current.     

A seismic study of deep geological structure in the Bristol Channel area, SW Britain

Summary. Results from eight seismic refraction lines, 35–90 km long, in the Bristol Channel area are presented. The data, mostly land recordings of marine shots, have been interpreted mainly by ray-tracing and time-term modelling. Upper layer velocities through Palaeozoic rocks usually fall within the range 4.8–5.2 km s⁻¹. Below the Carboniferous Limestone with a normal velocity of 5.1–5.2 kms⁻¹, the Old Red Sandstone with a velocity of 4.7–4.8 kms⁻¹ acts as a low velocity layer, as do parts of the underlying Lower Palaeozoic succession. In the central South Wales/Bristol Channel area and the Mendips, a 5.4–5.5 km s⁻¹ refractor is correlated with a horizon at or near the top of the Lower Palaeozoic succession. Under the whole area, except for north Devon, a 6.0–6.2 km s⁻¹ basal refractor has been located and is correlated with Precambrian crystalline basement rocks. In general, this refractor deepens southwards from a series of basement highs, which existed before the major movements of the Variscan orogeny in South Wales, resulting in a southerly thickening of the pre Upper Carboniferous supra-basement sequence. In north Devon, a 6.2 km s⁻¹ refractor at shallow depth, interpreted as a horizon in the Devonian or Lower Palaeozoic succession, overlies a deep reflector that may represent the Precambrian crystalline basement.     

Ivrea seismic array: a study of continental crust and upper mantle

A seismic-array study of the continental crust and upper mantle in the Ivrea-Yerbano and Strona-Ceneri zones (northwestern Italy) is presented. A short-period network is used to define crustal P - and S -wave velocity models from earthquakes. The analysis of the seismic-refraction profile LOND of the CROP-ECORS project provided independent information and control on the array-data interpretation.
Apparent-velocity measurements from both local and regional earthquakes, and time-term analysis are used to estimate the velocity in the lower crust and in the upper mantle. The geometry of the upper-lower crust and Moho boundaries is determined from the station delay times.
We have obtained a three-layer crustal seismic model. The P -wave velocity in the upper crust, lower crust and upper mantle is 6.1±0.2 km s⁻¹, 6.5±0.3 km s⁻¹ and 7.8±0.3 km s⁻¹ respectively. Pronounced low-velocity zones in the upper and lower crust are not observed. A clear change in the velocity structure between the upper and lower crust is documented, constraining the petrological interpretation of the Ivrea-type reflective lower continental crust derived from small-scale petrophysical data. Moreover, we found a V _P/ V _S ratio of 1.69±0.04 for the upper crust and 1.82±0.08 for the lower crust and upper mantle. This is consistent with the structural and petrophysical differences between a compositionally uniform and seismically transparent upper crust and a layered and reflective lower crust. The thickness of the lower crust ranges from about 8 km in front of the Ivrea body (ARVO, Arvonio station) in the northern part of the array to a maximum of about 15 km in the southern part of the array. The lower crust reaches a minimum depth of 5 km below the PROV (Provola) station.     

Upper crustal shear velocity models from higher mode Rayleigh wave dispersion in Scotland

Summary. Group velocities for first and second higher mode Rayleigh waves, in the frequency range 0.8–4.8 Hz, generated from a local earthquake of magnitude 3.7 M _L in western Scotland, are measured at stations along the 1974 LISPB line. These provide detailed information about the crustal structure west of the line. The data divide the region into seven apparently homogeneous provinces. Averaged higher mode velocity dispersion curves for each province are analysed simultaneously using a linearized inversion technique, yielding regionalized shear velocity profiles down to a depth of 17 km into the upper crust. Shear wave velocity is between 3.0 and 3.4 km s⁻¹ in the upper 2 km, with a slow increase to around 3.8 km s⁻¹. P -wave models computed using these results agree with profiles from the LISPB and LUST refraction experiments.     

Comparison of the Upper Mantle Shear Wave Velocity Structure of the Indian Shield and the Tibetan Plateau and Tectonic Implications

Summary. Travel times and waveforms of long-period SH -waves recorded at distances of 10–30° and some SS waveforms are used to constrain the upper mantle velocities down to a depth of 400km beneath both the Indian Shield and the Tibetan Plateau. the shear velocity in the uppermost mantle beneath both the Indian Shield and the Tibetan Plateau is high and close to 4.7 km s⁻¹. the Indian Shield has a fairly thick high velocity lid, and the mean velocity between 40 and 250 km is between 4.58 and 4.68 km s⁻¹. In contrast, S -wave travel times and waveforms of S -waves, as well as a few for SS , show that the mean velocity between 70 and 250km beneath the central and northern part of the Tibetan Plateau is slower by 4 per cent or more than that beneath the Indian Shield and probably is between 4.4 and 4.5km s⁻¹. No large differences in the structure of the two areas below 250 km are required to explain both the arrival times and the waveforms of SH phases crossing Tibet or the Indian Shield. These results show that the structure of Tibet is not that of a shield and imply that the Indian plate is not underthrusting the whole of the Tibetan Plateau at the present time.     

One-year records of current and bottom pressure in the strait between Nordaustlandet and Kvitøya, Svalbard, 1980-81

we have obtained one year of measurements from a subsurface instrumented mooring carrying two current meters and one bottom pressure recorder in the strait between Nordaustlandet and Kvitøya in the northeastern Svalbard archipelago. The observations show a mixed tide with typical amplitudes 0.4 db and 10cm sec⁻¹. The semidiurnal tide is characterized by a progressive wave propagating toward the south. together with a cross-channel baroclinic mode. The annual average (non-tidal) current is less than 2cm sec⁻¹ toward the north-east, suggesting that the transport into the Arctic Ocean is approximately 0.2 × 10⁶m³s⁻¹.     

Terrestrial Heat Flow and Heat Generation in South-west England

Three heat flow values for south-west England are presented. Two of the sites, Geevor and South Crofty, are operating tin mines on the northern contacts of the Land's End and Carnmenellis Granites, respectively, while the third, Wilsey Down, is a stratigraphical borehole 5 km north of the Bodmin Moor Granite. After applying topographic corrections values of 128·6 mW m^-2 (3·07 μ cal cm^-2 s^-1) for Geevor, 128·9 mW m^-2 (3·08 (A cal cm^-2 s^-1) for South Crofty and 67·3 mW cm^-2 (1·61 cal cm^-2 s^-1) for Wilsey Down, were determined. The value at Wilsey Down is shown to be consistent with that for an environment in which the Hercynian orogeny was the last significant thermal event. An additional heat source term must clearly be involved at Geevor and South Crofty to account for the unusually high values at these sites. Radiogenic heat production has been determined on granites from these sites and in spite of the fact that it is high it does not fully account for the measured heat flow. A compilation of underground temperature measurements made in the nineteenth century suggests that high heat flow is a general feature of the mineralized belt. At least part of this can be explained in terms of hot spring activity recorded widely throughout the area but the ultimate cause remains to be evaluated.