Investigations of the East Greenland continental margin between 70° and 72° N by deep seismic sounding and gravity studies

Results are presented from a deep seismic sounding experiment with the research vessel POLARSTERN in the Scoresby Sund area, East Greenland. For this continental margin study 9 seismic recording landstations were placed in Scoresby Sund and at the southeast end of Kong Oscars Fjord, and ocean bottom seismographs (OBS) were deployed at 26 positions in and out of Scoresby Sund offshore East Greenland between 70° and 72° N and on the west flank of the Kolbeinsey Ridge. The landstations were established using helicopters from RV POLARSTERN. Explosives, a 321 airgun and 81 airguns were used as seismic sources in the open sea. Gravity data were recorded in addition to the seismic measurements. A free-air gravity map is presented. The sea operations — shooting and OBS recording — were strongly influenced by varying ice conditions. Crustal structure 2-D models have been calculated from the deep seismic sounding results. Free-air gravity anomalies have been calculated from these models and compared to the observed gravity. In the inner Scoresby Sund — the Caledonian fold belt region — the crustal thickness is about 35 km, and thins seaward to 10 km. Sediments more than 10 km thick on Jameson Land are of mainly Mesozoic age. In the outer shelf region and deep sea a ‘Moho’ cannot clearly be identified by our data. There are only weak indications for the existence of a ‘Moho’ west of the Kolbeinsey Ridge. Inside and offshore Scoresby Sund there is clear evidence for a lower crust refractor characterised byp-velocities of 6.8–7.3 km s^?1 at depths between 6 and 10 km. We believe these velocities are related to magmatic processes of rifting and first drifting controlled by different scale mantle updoming during Paleocene to Eocene and Late Oligocene to Miocene times: the separation of Greenland/Norway and the separation of the Jan Mayen Ridge/Greenland, respectively. A thin igneous upper crust, interpreted to be of oceanic origin, begins about 50 km seaward of the Liverpool Land Escarpment and thickens oceanward. In the escarpment zone the crustal composition is not clear. Probably it is stretched and attenuated continental crust interspersed with basaltic intrusions. The great depth of the basement (about 5000 m) points to a high subsidence rate of about 0.25 mm yr^?1 due to sediment loading and cooling of the crust and upper mantle, mainly since Miocene time. The igneous upper crust thickens eastward under the Kolbeinsey Ridge to about 2.5 km; the thickening is likely caused by higher production of extrusives. The basementp-velocity of 5.8–6.0 km s^?1 is rather high. Such velocities are associated with young basalts and may also be caused by a higher percentage of dykes. Tertiary to recent sediments, about 5000 m thick, form most of the shelf east of Scoresby Sund, Liverpool Land and Kong Oscars Fjord. This points to a high sedimentation rate mainly since the Miocene. The deeper sediments have a rather high meanp-velocity of 4.5 km s^?1, perhaps due to pre-Cambrian to Caledonian deposits of continental origin. The upper sediments offshore Scoresby Sund are thick and have a rather low velocity. They are interpreted as eroded material transported from inside the Sund into the shelf region. Offshore Kong Oscars Fjord the upper sediments, likely Jurassic to Devonian deposits, are thin in the shelf region but thicken to more than 3000 m in the slope area. The crust and upper mantle structure in the ocean-continent transition zone is interpreted to be the result of the superposition of the activities of three rifting phases related to mantle plumes of different dimensions:

1. the ‘Greenland/Norway separation phase’ of high volcanic activity,
2. the ‘Jan Mayen Ridge/Greenland separation phase’ and
3. the ‘Kolbeinsey Ridge phase’ of ‘normal’ volcanic activity related to a more or less normal mantle temperature.

During period 2 and 3 only a few masses of extrusives were produced, but large volumes of intrusives were emplaced. So the margin between Scoresby Sund and Jan Mayen Fracture Zone is interpreted to be a stretched margin with low volcanic activity.     

Zonal variability of plankton and particle export flux in the equatorial Pacific upwelling between 165° E and 150° W

Observations made during a “La Niña” situation (April–May 1996) in the equatorial Pacific upwelling, between 165° E and 150° W, show the classic deepening of hydrological isolines from east to west, resulting in zonal gradients for surface temperature and macronutrients. However, contrasting with such a gradient, no clear zonal variation could be seen for integrated planktonic biomasses and carbon fluxes, namely: chlorophyll a, bacterial abundances, particulate organic phosphorus, mesozooplankton ash-free dry weight, primary production, and the sinking flux of particulate organic carbon (POC). Moreover, mean values of these parameters along the zonal equatorial transect, are not significantly different from those of a 7-day-long time series station made at 0°, 150° W in October 1994 during an El Niño period. Such a steady zonal distribution of planktonic parameters seems to be characteristic of equatorial Pacific upwelling west of the Galapagos Islands so that the spatial distributions of nutrient concentrations and planktonic biomass appear to be uncoupled. This is consistent with the High Nutrient-Low Chlorophyll (HNLC) concept, in which primary production is not controlled directly by macronutrient concentrations. The lack of zonal gradient also suggests that carbon budget of the equatorial Pacific is primarily controlled by oscillations in the zonal and meridian extension of the HNLC area, rather than by values of planktonic biomasses and carbon fluxes within the upwelled water, which are quite constant.     

Climatic variability in the vicinity of Wallis,Futuna, and Samoa islands (13°–15° S, 180°–170° W)

Mean conditions, seasonal, and ENSO-related (El Niño Southern Oscillation) variability in the vicinity of Wallis, Futuna, and Samoa islands (13°–15° S, 180°–170° W) over the 1973–1995 period are analysed for wind pseudo-stress, satellite-derived and in situ precipitation, sea surface temperature (SST) and salinity (SSS), sea level, and 0–450 m temperature and geostrophic current. The mean local conditions reflect the presence of the large scale features such as the western Pacific warm pool, the South Pacific Convergence Zone (SPCZ), and the South Pacific anticyclonic gyre. The seasonal changes are closely related to the meridional migrations of the SPCZ, which passes twice a year over the region of study. During the warm phase of ENSO (El Niño), we generally observe saltier-than-average SSS (of the order of 0.4), consistent with a rainfall deficit (0.4 m yr⁻¹), a hint of colder-than-average surface temperature is also identified in subsurface (0.3°C), a weak tendency for westward geostrophic current anomalies (2 cm s⁻¹ at the surface), a sea level decrease (5–10 cm), together with easterly (5 m²s⁻²) and well marked southerly (10 m²s⁻²) wind pseudo-stress anomalies. Anomalies of similar magnitude, but of opposite sign, are detected during the cold phase of ENSO (La Niña). While these ENSO-related changes apply prior to the 1990s, they were not observed during the 1991–1994 period, which appears atypical.     

Spreading rate dependence of the axial topography and gravity of the Mid‐Atlantic ridge between 22°N and 38°N

A method is developed to quantify the relationship between the ridge axial topography and gravity and the spreading rate along the Mid‐Atlantic Ridge between 22 and 38°N. This relationship reflects the variations of slope of the best‐fit line of topography and gravity spectra with the spreading rate of the ridge segments. The slope of the best‐fit line of topography spectrum becomes smaller as the spreading rate increases, indicating that with increasing spreading rate more energy of the ridge axial topography shifts into high‐frequency bands. The spreading rate dependence of the ridge axial topography may be explained by an anomalous thermal structure beneath the ridge. No significant correlation was found between the slope of the best‐fit line of gravity spectrum and the spreading rate in this region. The lack of spreading rate dependence of the ridge axial gravity may be attributable to the isostatic compensation of the spreading center.     

Morphotectonics, Volcanism and Hydrothermal Activity on the East Pacific Rise between 21°12′ S and 22°40′ S

The morphotectonic setting of the East Pacific Rise (EPR) between21°12 and 22°40 S and its recent and past hydrothermalactivity were the focus of the Russian R/V Geolog Fersmans expeditionin 1987–1988.The EPR axial zone in the study area is comprised of three segmentsseparated by overlapping spreading centers (OSCs) near 21°44 and22°08 S. The northern segment is the shallowest of three and hasa distinct massive axial ridge, trapeziodal in cross-section, toppedby a very wide flat summit surface and cut by a well-developedcentral graben. These features testify to intense magmatism and to avoluminous crustal magmatic chamber underlying the whole segment.Fine-scale segmentation is most clearly revealed in the structure ofthe central graben within which several 4th-order segments can bedistinguished. This scale of segmentation is also reflected on flanks of theaxis by variations in the character and intensity of faulting.According to structural and petrologic data, the magmatism is mostintense in the central part of the segment which is probably locateddirectly over a magmatic diapir supplying the melt to the whole segment.Magma migration at the subcrustal level from the center towards the ends ofthe segment with discrete injection into the crustal magmatic chamber ispresumed.The central segment is broken into two morphologically distinct partsseparated by a deval. In the subsided northern part, the wide summit of theaxial ridge is cut by a well-developed, intensely fractured axialgraben. In the southern part, the axial ridge is relatively elevated, butnarrow with an ephemeral graben along its crest. The character and intensityof faulting on the axial flanks are also considerably different in thenorthern and southern parts of the segment. Thus, the magmatic supply tothese two parts is thought to originate from two different sources. If so,then at present the magma chamber underlying the southern part of thesegment is probably at the stage of replenishment, while in the north it isat the stage of deep cooling.The southern segment is structurally similar to the central one. Howeverthere is considerably less intensive magmatic activity in this region,especially south of 22°30 S where the axial ridge is narrow, andtriangular in cross-section.Both OSCs studied are marked by abrupt narrowing and sharp subsidence ofthe tips of axial ridges within the northern limbs. The southern OSC limbsare morphologically similar to normal sections of axial ridges. In bothcases the flanks are structurally and morphologically disrupted adjacent tothe OSCs and oblique structures can be traced far southward of the OSCflanks. Due to the spatial position of oblique structures on the the flanksit is presumed that the OSC near 22°07 S is migrating northward.The 21°44 S OSC zone has apparently undergone small spatialoscillations. In spite of the small amplitude of lateral displacement, thiszone is marked by prominent bathymetric anomalies.Numerous massive sulfide deposits were discovered atop the axial ridgealong the entire length of the uplifted and hydrothermally active northernsegment. Ore metal concentrations in near-bottom waters are maximumover the southern part of the northern segment, while maximum concentrationsof the same metals in surficial sediments are confined to the central partof the same segment. We surmise that there has been a recentalong-axis shift of the zone of maximum hydrothermal activity fromthe middle of the segment to its present position in the southern part ofthe segment. Considering sedimentation rates, the age of this shift can beapproximately estimated to be 5 to 10 thousand years before the present.The relatively Mg-enriched basalts of the middle part of thenorthern segment represent a tike of a more primitive pattern, while therelatively Fe-rich rocks of its southern part probably reflect alarge degree of fractionation at shallow crustal levels. Considering thistrend, in addition to morphotectonic data we presume that subaxial magmaflow from the middle to the southern part of the segment is responsible forthe along-axis shift of hydrothermal activity.In the central segment of the study area, massive sulfides have only beendiscovered south of the 21°55 S deval, where the axial ridgeshoals and where the existence of a subjacent magma chamber is presumed.The very weak manifestations of recent volcanism within the southernsegment explain the absence of hydrothermal activity and sulfide depositswithin this segment.     

Heat fluxes across 7°30′N and 4°30′S in the Atlantic Ocean

Heat fluxes are estimated across transatlantic sections made at 4°30′S and 7°30′N in January–March 1993, following Hall and Bryden (1982. Deep-Sea Research 29, 339–359). Particular care is given to the computation of Ekman volume and heat fluxes, which are assessed both (a) from the windstress data for the period of the cruise and (b) from the comparison between geostrophic and Vessel Mounted Acoustic Doppler Current Profiler (VM-ADCP) velocities. In contrast with previous studies, the two estimates for Ekman fluxes do not converge for either section: (a) (11.5±0.5 Sv; 1.01±0.05 PW) across 7°30′N and (−9.3±1.2 Sv; −0.85±0.12 PW) across 4°30′S when windstress data at the date of the hydrographic stations are used; (b) (6.3±1.1 Sv; 0.56±0.09 PW) across 7°30′N and (−3.4±3.0 Sv; −0.35±0.24 PW) across 4°30′N when the ageostrophic transport above the thermocline is used. The divergence would have been even greater at 4°30′S if the strong ageostrophic signal beneath the thermocline, which brings a transport of (8.4 Sv; 0.82 PW), had been considered. The corresponding total meridional heat fluxes are: (a) 1.40±0.16 PW and (b) 0.95±0.20 PW across 7°30′N, (a) 1.05±0.12 PW and (b) 1.67±0.14 PW (2.39±0.14 PW when the subthermocline ageostrophic transport is taken into account) across 4°30′S.The estimates based on windstress data are compared with the results from an inverse model (Lux and Mercier, 1999) to show the importance of the heat flux due to the deviation of the local depth-averaged potential temperature from its average over the section, which is neglected in the Hall and Bryden (1982. Deep-Sea Research 29, 339–359) method but is not negligible in our computation in which we do not isolate the transport of the western boundary current east of the 200 m isobath; this corrective flux amounts here to −0.19 PW across 7°30′N and 0.33 PW across 4°30′S.The seasonal variability of the meridional heat flux across 7°30′N is studied through the hydrographic data collected during the ETAMBOT 1–2 cruises, which repeated the 7°30′N section west of 35°W in September 1995 and April 1996. When the section is completed east of 35°W with CITHER 1 data and when windstress data are used for the computation of the Ekman transport, the estimates for the meridional heat fluxes are 0.20±0.14 PW in September 1995 and 1.69±0.27 PW in April 1996. The estimates fit well with results from numerical models.     

Bathymetric and magnetic survey of California Seamount (17°40′N × 124°W)

A bathymetric and magnetic survey of the California Seamount region (17°40′N × 124°00′W) shows that existing charts are in error. California Seamount is a peak extending to within 454 m (248 fathoms) of the surface. Its true location is 17°41′N × 124°01′W, 25 km southwest of the charted position. Near the old charted position there is an elongated feature which extends to within 1818 m (994 fathoms) of the surface. Both features are located on the Clarion Fracture Zone.     

A detailed magnetic study of the Reykjanes Ridge between 63°00′N and 63°40′N

Immediately southwest of Iceland, the Reykjanes Ridge consists of a series ofen échelon, elongate ridges superposed on an elevated, smooth plateau. We have interpreted a detailed magnetic study of the portion of the Reykjanes Ridge between 63°00N and 63°40N on the Icelandic insular shelf. Because the seafloor is very shallow in our survey area (100–500 m), the surface magnetic survey is equivalent to a high-sensitivity, nearbottom experiment using a deep-towed magnetometer. We have performed two-dimensional inversions of the magnetic data along profiles perpendicular to the volcanic ridges. The inversions, which yield the magnetization distribution responsible for the observed magnetic field, allow us to locate the zones of most recent volcanism and to measure spreading rates accurately. We estimate the average half spreading rate over the last 0.72 m.y. to have been 10 mm/yr within the survey area. The two-dimensional inversions allow us also to measure polarity transition widths, which provide an indirect measure of the width of the zone of crustal accretion. We find a mean transition width on the order of 4.5±1.6 km. The observed range of transition widths (2 to 8.4 km) and their mean value are characteristic of slow-spreading centers, where the locus of crustal accretion may be prone to lateral shifts depending on the availability of magmatic sources. These results suggest that, despite the unique volcanotectonic setting of the Reykjanes Ridge, the scale at which crustal accretion occurs along it may be similar to that at which it occurs along other slow-spreading centers. The polarity transition width measurements suggest a zone of crustal accretion 4–9 km wide. This value is consistent with the observed width of volcanic systems of the Reykjanes Peninsula. The magnetization amplitudes inferred from our inversions are in general agreement with NRM intensity values of dredge samples measured by De Boer (1975) and ourselves. Our thermomagnetic measurements do not support the hypothesis that the low amplitude of magnetic anomalies near Iceland is the result of a high oxidation state of the basalts. We suggest that the observed reduction in magnetic anomaly amplitude toward Iceland may be the result of an increase in the size of pillows and other igneous units.     

东南亚10°—25°N,105°—130°E海区热带风暴异常路径的分析

本文４１ａ（１９４９－１９８９年）的资料，对东南亚１０°－２５°Ｎ，１０５°－１３０°Ｅ范围７－９月出现的热带风暴异常路长进行了普查统计和气候分析。结果表明：产生异常路径热风暴的机率约占区域内热带风暴总数的２０％；异常路径的产生与热带风暴所处的地理位置，季节，环下等因素有关。正确地考虑气候规律和具体的天气条件相结合是预报带式风暴异常路径成败的关键。     

Geophysical studies of the Atlantic sea floor near 40°N, 24°W,and its relation to King's Trough and the Azores

A detailed survey of a 1°-square of sea floor 100 miles northeast of the Azores has revealed the presence of scarps over 30 km long striking generally 120°, transverse to the regional topographic and magnetic lineations. They are not associated with a major fracture zone. Sedimentation in the area appears to have been by a mixture of pelagic and turbidity-current processes, with a Miocene increase in sedimentation rate probably indicating the subaerial emergence of the Azores Archipelago. Magnetic anomalies were identified along a number of profiles between the survey area and the Mid-Atlantic Ridge crest, and indicate a short period of increased spreading rate from about 12-3 m.y. ago.     

The responses of CO_2 to El Nino and La Nina in different seasons along 22°-18°N line

1 INTRoDUcTIONThe study results from Nov. 1986 to Aug. 1990 in sea area (ll5"E - 165"E, 20"N -l0"S) during TOGA experiments, indicated that there were outstanding responses of differenceof partial pressure of CO, between sea and air (APCO,) and total dissolved concentrationof CO, (TCO,) to El Nino and La Nina in NW region of the Western Tropical Pacific,including the cruise lines from ll4ofy 22"N to l30"E, l8"N and that from l8"N to 8"N,along l30"E. The characteristics of dis…     

Morphology,sedimentary infill and depositional environments of the Early Quaternary North Sea Basin (56°–62°N)

The North Sea Basin has been subsiding during the Quaternary and contains hundreds of metres of fill. Seismic surveys (170 000 km²) provide new evidence on Early Quaternary sedimentation, from about 2.75 Ma to around the Brunhes-Matuyama boundary (0.78 Ma). We present an informal seismic stratigraphy for the Early Quaternary of the North Sea, and calculate sediment volumes for major units. Early Quaternary sediment thickness is > 1000 m in the northern basin and >700 m in the central basin (total about 40 000 km³). Northern North Sea basin-fill comprises several clinoform units, prograding westward over 60 000 km². Architecture of the central basin also comprises clinoforms, building from the southeast. To the west, an acoustically layered and mounded unit (Unit Z) was deposited. Remaining accommodation space was filled with fine-grained sediments of two Central Basin units. Above these units, an Upper Regional Unconformity-equivalent (URU) records a conformable surface with flat-lying units that indicate stronger direct glacial influence than on the sediments below. On the North Sea Plateau north of 59°N, the Upper Regional Unconformity (URU) is defined by a shift from westward to eastward dipping seismic reflectors, recording a major change in sedimentation, with the Shetland Platform becoming a significant source. A model of Early Quaternary sediment delivery to the North Sea shows sources from the Scandinavian ice sheet and major European rivers. Clinoforms prograding west in the northern North Sea Basin, representing glacigenic debris flows, indicate an ice sheet on the western Scandinavian margin. In the central basin, sediments are generally fine-grained, suggesting a distal fluvial or glacifluvial origin from European rivers. Ploughmarks also demonstrate that icebergs, derived from an ice sheet to the north, drifted into the central North Sea Basin. By contrast, sediments and glacial landforms above the URU provide evidence for the later presence of a grounded ice sheet.     

Segmentation and Morphotectonic Variations Along a Super Slow-Spreading Center: The Southwest Indian Ridge (57° E-70° E)

Bathymetric data along the Southwest Indian Ridge (SWIR) between 57°E and 70° E have been used to analyze the characteristics of thesegmentation and the morphotectonic variations along this ridge. Higheraxial volcanic ridges on the SWIR than on the central Mid-Atlantic Ridge(MAR) indicate that the lithosphere beneath the SWIR axis that supportsthese volcanic ridges, is thicker than the lithosphere beneath the MAR. Astronger/thicker lithosphere allows less along-axis melt flow andenhances the large crustal thickness variations due to 3D mantle upwellings.Magmatic processes beneath the SWIR are more focused, producing segmentsthat are shorter (30 km mean length) with higher along-axis relief (1200 mmean amplitude) than on the MAR. The dramatic variations in the length andamplitude of the swells (8–50 km and 500–2300 m respectively),the height of axial volcanic ridges (200–1400 m) and the number ofvolcanoes (5–58) between the different types of segments identifiedon the SWIR presumably reflect large differences in the volume, focusing andtemporal continuity of magmatic upwelling beneath the axis. To the east ofMelville fracture zone (60°42 E), the spreading center isdeeper, the bathymetric undulation of the axial-valley floor is less regularand the number of volcanoes is much lower than to the west. The spreadingsegments are also shorter and have higher along-axis amplitudes than to thewest of Melville fracture zone where segments are morphologically similar tothose observed on the central MAR. The lower magmatic activity together withshorter and higher segments suggest colder mantle temperatures withgenerally reduced and more focused magma supply in the deepest part of thesurvey area between 60°42 E and 70° E. The non-transformdiscontinuities show offsets as large as 70 km and orientations up toN36° E as compared to the N0° E spreading direction. We suggest thatin regions of low or sporadic melt generation, the lithosphere neardiscontinuities is laterally heterogeneous and mechanically unable tosustain focused strike-slip deformation.     

Crucial physical characteristics of sea ice in the Arctic section of 143°–180°W during August and early September 2008

Sea-ice physical characteristics were investigated in the Arctic section of 143°-180°W during August and early September 2008. Ship-based observations show that both the sea-ice thickness and concentration recorded during southward navigation from 30 August to 6 September were remarkably less than those recorded during northward navigation from 3 to 30 August, especially at low latitudes. Accordingly, the marginal ice zone moved from about 74.0°N to about 79.5°N from mid-August to early September. Melt-pond coverage increased with increasing latitude, peaking at 84.4°N, where about 27% of ice was covered by melt ponds. Above this latitude, melt-pond coverage decreased evidently as the ice at high latitudes experienced a relatively short melt season and commenced its growth stage by the end of August. Regional mean ice thickness increased from 0.8 (±0.5) m at 75.0°N to 1.5 (±0.4) m at 85.0°N along the northward navigation while it decreased rapidly to 0.6 (±0.3) m at 78.0°N along the southward navigation. Because of relatively low ice concentration and thin ice in the investigated Arctic sector, both the short-term ice stations and ice camp could only be set up over multiyear sea ice. Observations of ice properties based on ice cores collected at the short-term ice stations and the ice camp show that all investigated floes were essentially isothermal with high temperature and porosity, and low density and salinity. Most ices had salinity below 2 and mean density of 800-860 kg/m~3 . Significant ice loss in the investigated Arctic sector during the last 15 a can be identified by comparison with the previous observations.     

Morphology of a ‘superfast’ Mid-Ocean Ridge crest and flanks: The East Pacific Rise, 7°–9° S

Detailed bathymetric data from a Hydrosweep multibeam sonar survey of a 250 km-long portion of the superfast-spreading southern East Pacific Rise crest and flanks show that the along-axis variation in morphology and axial depth differs significantly from that observed at the fast-spreading northern East Pacific Rise. While the deep mantle upwelling pattern is similar under the northern and southern East Pacific Rise, our observations require that the connectivity of the shallow, subcrestal plumbing system be more efficient beneath the super-fast spreading southern East Pacific Rise than beneath the slower spreading northern East Pacific Rise.     

坎昆56°蓝

正位处墨西哥东南角的坎昆是一座纯粹的旅游度假城市,这里的海有着56不同的蓝。从地图上看,向左是平静无波的墨西哥湾,向右是神秘瑰丽的加勒比海。海水像孩子般纯洁清透,将海底的变幻毫无保留地呈现。40年前,坎昆只是个居住着十几户渔民的小渔村。墨西哥政府看着北边美国、加拿大两个富有的邻居,冥思苦想怎么把他们的钱揣进自己的口袋。美国人爱海,加拿大人爱     

Morphology and Evolution of the Remnant Colville and Active Kermadec Arc Ridges South of 33°30′ S

Swath MR1 data from the remnant Colville and active Kermadec arc margins, south of 33°30 S (SW Pacific), record the structural morphology and evolution of the rifted, and now separate portions, of the proto-Colville–Kermadec arc flanking the actively widening southern Havre Trough back-arc basin associated with Pacific-Australian plate convergence. Both the remnant Colville and active Kermadec arc margins comprise opposing, asymmetric, partially basement exposed, segmented ridges. Differences in morphology between the two ridges are, however, observed. The single, near linear, border fault system, with relief of 1000 m, along the western edge of the Kermadec margin is interpreted to be the exposed fault escarpment of a major, west-dipping, detachment fault. In contrast, two major zig-zag border fault systems along the eastern edge of the Colville Ridge, bounding a back-tilted ridge flank terrace, are interpreted as major antithetic faults between the remnant arc and back-arc region. This contrast in structural morphology coincides with, respectively, lesser and greater degrees of both active tectonism and channel-canyon erosion, on the remnant Colville and active Kermadec margins. These differences are interpreted to reflect the progressive trenchward collapse and associated greater rift flank uplift and incisive erosion of the Kermadec foot-wall contrasting with the non-collapse and relatively lesser rift flank uplift and ridge erosion of the Colville hanging-wall. The data provide further constraints on the early evolution of the Havre Trough in particular, and back-arc basins in general.     

Phytoplankton community structures in shelf and oceanic waters off southeast Brazil (20°–25°S), as determined by pigment signatures

The influences of the hydrological features and environmental conditions in the phytoplankton community found in the Campos Basin area in the Atlantic Ocean (20° to 25°S; 42° to 38°W) were studied using HPLC/CHEMTAX pigment analysis. Samples were collected at 72 stations distributed along the 25–3000 m isobaths at two depths during two seasonal periods (rainy and dry). Seven taxonomic groups of phytoplankton were detected (diatoms, dinoflagellates, prasinophytes, cryptophytes, haptophytes, pelagophytes and cyanobacteria). Redundancy analysis showed that the spatial and temporal patterns observed in the distribution of the phytoplanktonic groups were primarily related to variations in the availability of light and nutrients. Nutrient variations were caused by South Atlantic Central Water seasonal intrusions over the continental shelf region. Cyanobacteria predominated in the rainy season, while diatoms, Haptophyceae and Prasinophyceae, were associated with higher nutrient availability in the dry season. In the inner shelf region, diatoms dominated and were associated with increased conditions of turbulence and nutrient availability. Haptophytes and prasinophytes were predominant on the outer shelf and shelf-break regions associated with high nutrient concentrations and availability of light. Prochlorococcus was related to oceanic waters (in both dry and rainy periods) or to low nutrient/strongly stratified shelf waters (rainy period). In contrast, Synechococcus was widely distributed in both the shelf and oceanic regions. Variation in the quality of light between coastal and oceanic waters was probably responsible for the distributions observed. Through HPLC/CHEMTAX pigment analysis we have developed a detailed picture of the influence of hydrological regime on the dynamics of the phytoplankton community in an under-studied shelf/ocean system in the tropical southern Atlantic Ocean.     

Peculiarities of the tectonic and magma evolution of the southwestern Indian middle-ocean crust within the range of 51°–67° eastern longitude

The comparative estimation of the parameters of the lithosphere of the Mid-Ocean Southwestern Indian range in the areas westwards and eastwards of the Atlantis II transform fault zone shows that, within this zone, an alteration in the basalt composition occurred. Eastwards of this zone, a decrease of the anomaly of the magnetic field occurred and increased average depths of the axial part (4.7 km) and thinning (up to 4–5 km) of the ocean crust with increased rates of seismic waves in the upper mantle were observed. This, first of all, indicates an anomalously cold mantle below the oceanic crust. The changes that occurred in the location of the Euler pole within the last millions of years resulted in slanting spreading in the area of the investigation with rates of opening lower than 1.8 cm/year probably accompanied by the phenomena of transtension in the active parts of the transform faults. The interaction between the Landly and Somali lithosphere plates occurred along the diffusion boundary and was accompanied by problems with tracing the chrones between the neighboring profiles of geomagnetic observations. Consequently, the more detailed investigation of the configuration of the diffusion boundary will contribute to the more accurate reconstruction of the paleogeodynamics of the central part of the Indian Ocean.     

The western Svalbard margin (74°–80°N)

A regional study of the continental margin between the Senja and Molloy-Spitsbergen fracture zones reveals that the transition from continental to oceanic crust occurs in a narrow zone beneath the outer shelf and uppermost slope. The postulated continent-ocean boundary appears to be fault-related consisting of sheared and rifted segments. The marginal structures are compatible with a plate tectonic model in which the southern Greenland Sea opened along a northeasterly propagating plate boundary in the Eocene, whereas the northern Greenland Sea started opening in the early Oligocene. The main structure at the margin is the Hornsund Fault Zone which probably reflects an old zone of weakness rejuvenated in the Tertiary, first by shear and later by extensional movements. In the early Tertiary local transpressional and transtensional components along the plate boundary are associated with the Spitsbergen Orogeny, emplacement of belts of high-density oceanic crust and tectonism in the western Barents Sea. A complex volcanic rifted margin characterized by the Bjørnøya Marginal High links the predominantly sheared margin segments on either side. The main ridge-like segment of the Hovgaard Fracture Zone was originally part of the Spitsbergen margin. In a regional sense, the Hornsund Fault Zone demarcates the eastern boundary of the Tertiary sedimentary wedge which reaches a total thickness of more than 7 km. There appears to have been a considerable increase in deposition of sediments the last 5–6 my. Depocentres located seaward of the east-west fjord systems and submarine depressions indicate a relationship between late Cenozoic glaciations and high sedimentation rates.