Gravity and S-wave modelling across the Jan Mayen Ridge,North Atlantic; implications for crustal lithology

The horizontal components from fourteen Ocean Bottom Seismometers deployed along four profiles focused along the western margin of the Jan Mayen microcontinent, North Atlantic, have been modelled with regard to S-waves, based on P-wave models obtained earlier. The seismic models have furthermore been constrained by 2D gravity modelling. High V _p/V _s-ratios (2.3–7.9) within the Cenozoic sedimentary section are attributed to significant porosities, whereas V _p/V _s-ratios in the order of 1.9–2.2 for the Mesozoic and Paleozoic sedimentary rocks indicate shale-dominated lithology throughout the area. The eastern side of the Jan Mayen Ridge is interpreted as a passive, volcanic margin, based on relatively high crustal V _p/V _s-ratios (1.9), whereas lower V _p/V _s-ratios (1.75–1.8) suggest the presence of intermediate composition crust and non-volcanic margin on the western side of the ridge. In the westernmost part of the Jan Mayen Basin, slightly increased upper mantle V _p/V _s-ratios may indicate some degree of serpentization of upper mantle peridotites.     

Crustal transect across the North Atlantic

Two dimensional crustal models derived from four different ocean bottom seismographic (OBS) surveys have been compiled into a 1,580 km long transect across the North Atlantic, from the Norwegian Møre coast, across the extinct Aegir Ridge, the continental Jan Mayen Ridge, the presently active Kolbeinsey Ridge north of Iceland, into Scoresby Sund in East Greenland. Backstripping of the transect suggests that the continental break-up at ca. 55 Ma occurred along a west-dipping detachment localized near the western end of a ca. 300 km wide basin thinned to less than 20 km crustal thickness. It is likely that an east-dipping detachment near the present day Liverpool Land Escarpment was active during the late stages of continental rifting. A lower crustal high-velocity layer (7.2–7.4 km/s) interpreted as mafic intrusions/underplating, was present beneath the entire basin. The observations are consistent with the plume hypothesis, involving the Early Tertiary arrival of a mantle plume beneath central Greenland and focused decompression melting beneath the thinnest portions of the lithosphere. The mid-Eocene to Oligocene continental extension in East Greenland is interpreted as fairly symmetric and strongly concentrated in the lower crustal layer. Continental break-up which rifted off the Jan Mayen Ridge, occurred at ca. 25 Ma, when the Aegir Ridge became extinct. The first ca. 2 m.y. of oceanic accretion along the Kolbeinsey Ridge was characterized by thin magmatic crust (ca. 5.5 km), whereas the oceanic crustal formation since ca. 23 Ma documents ca. 8 km thick crust and high magma budget.     

Lower crustal seismic velocity-anomalies; magmatic underplating or serpentinized peridotite? Evidence from the Vøring Margin,NE Atlantic

On the Vøring volcanic passive margin offshore mid-Norway, NE Atlantic, a lower crustal body with P-wave velocities in the range of 7.1–7.7 km/s has been mapped by twenty two-dimensional Ocean Bottom Seismograph (OBS) profiles. The main aim of the present paper is to evaluate to what extent the lower crust is consistent with magmatic intrusions or serpentinized peridotite. The relatively low V _p/V _s ratios of 1.75–1.78 modelled for the lower crust under the continental part of the Vøring Plateau are consistent with mafic intrusions mixed with blocks of stretched continental crust, but not with the presence of partially serpentinized peridotites. The lower crustal high-velocity body is restricted to the area of the Late Cretaceous/Early Tertiary rift that lead to continental break-up in Early Eocene. The same model can explain the observations in the northern Vøring Basin, but in the central and southern Vøring Basin the seismic velocities do not preclude a model involving serpentinized peridotite in addition to intrusions and continental remnants. On the west Iberia non-volcanic margin a similar layer is interpreted as serpentinized peridotite. The existence of Moho reflections, the observation of S-wave anisotropy but absence of P-wave anisotropy, uncertainties regarding supply of water to allow for significant serpentinization and very low stretching factors compared with the west Iberia Margin, are among factors that argue against the presence of serpentinized peridotite in the Vøring Basin.     

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.     

The northern boundary of the Jan Mayen microcontinent, North Atlantic determined from ocean bottom seismic, multichannel seismic, and gravity data

The Jan Mayen microcontinent was as a result of two major North Atlantic evolutionary cornerstones—the separation of Greenland from Norway (~54 Ma), accompanied by voluminous volcanic activity, and the jump of spreading from the Aegir to the Kolbeinsey ridge (~33 Ma), which resulted in the separation of the microcontinent itself from Eastern Greenland (~24 Ma). The resulting eastern and western sides of the Jan Mayen microcontinent are respectively volcanic and non-volcanic rifted margins. Until now the northern boundary of the microcontinent was not precisely known. In order to locate this boundary, two combined refraction and reflection seismic profiles were acquired in 2006: one trending S–N and consisting of two separate segments south and north of the island of Jan Mayen respectively, and the second one trending SW–NE east of the island. Crustal P-wave velocity models were derived and constrained using gravity data collected during the same expedition. North of the West Jan Mayen Fracture Zone (WJMFZ) the models show oceanic crust that thickens from west to east. This thickening is explained by an increase in volcanic activity expressed as a bathymetric high and most likely related to the proximity of the Mohn ridge. East of the island and south of the WJMFZ, oceanic Layers 2 and 3 have normal seismic velocities but above normal average crustal thickness (~11 km). The similarity of the crustal thickness and seismic velocities to those observed on the conjugate M?re margin confirm the volcanic origin of the eastern side of the microcontinent. Thick continental crust is observed in the southern parts of both profiles. The northern boundary of the microcontinent is a continuation of the northern lineament of the East Jan Mayen Fracture Zone. It is thus located farther north than previously assumed. The crust in the middle parts of both models, around Jan Mayen island, is more enigmatic as the data suggest two possible interpretations—Icelandic type of oceanic crust or thinned and heavily intruded continental crust. We prefer the first interpretation but the latter cannot be completely ruled out. We infer that the volcanism on Jan Mayen is related to the Icelandic plume.     

Seismic refraction observations in the Transkei Basin and adjacent areas

Some seismic refraction observations undertaken during the IGY are reported here together with a summary of other refraction studies carried out within the Transkei Basin, the Mozambique Ridge and the South African continental shelf area.A 2.5 km section of Cretaceous and younger rocks is associated with profiles observed on the continental shelf; directly below this group are rocks with velocities in the range 4.0–5.5 km s^-1, probably representatives of the Karroo and Cape supergroups. The basement material velocity variations were from 5.3 to 6.5 with an average of 5.9 km s^-1, and is correlated with granite or Malmesbury Formation plus granite. This crustal structure is similar to that found on the eastern continental shelf of southern South America.The profiles in the Transkei Basin show a thick layer of sediment with velocity range 1.50 to 3.50 km s^-1, underlain by a refracting layer in which the average velocity is 4.5 km s^-1. The velocity of 6.6 km s^-1 obtained for the oceanic layer is similar to the velocities of the crustal layer measured in the Argentine Basin. The mantle velocity (8.1 km s^-1) is consistent with the average mantle velocity for the Indian Ocean but significantly lower than the Pacific Ocean average of 8.20 km s^-1. The depth to Moho is about 12.0 km and the crustal section is typical oceanic. A plate tectonic model of the early opening of the South Atlantic is used to describe the evolution of the Transkei Basin.On the Mozambique Ridge the thin sediments (0.7 km) are underlain by rocks with velocities averaging 5.6 km s^-1. This is more than 1.0 km s^-1 faster than the velocity for layer 2 from the Transkei Basin and the Agulhas Plateau, indicating rocks of a younger age or of a different type. Moreover the crustal section of the Ridge has a thickness in excess of 22 km and is in isostatic equilibrium when compared with the adjacent Transkei Basin and Agulhas Plateau. DSDP site 249, situated on the Ridge, penetrated basalt at a depth of 0.4 km. Whether this is continental or oceanic basalt is not known; when this site 249 basalt was compared to the cored basalts of the adjacent Mozambique Basin, inconclusive results were obtained. The essential constitution of the Mozambique Ridge remains an enigma, but solution of this problem is vital for the proper understanding of the Mesozoic history of this oceanic region.     

Attenuation of seismic waves in the Jan Mayen island area

An analysis of the attenuation of seismic waves as measured by the quality factorQc (for coda waves) has been performed for the volcanic Jan Mayen island in the Norwegian Sea, using earthquakes near the Jan Mayen Fracture Zone and local seismic stations on the Jan Mayen island.Qc values of the order of 100 at a frequency of 1 Hz are found, increasing to about 300 at 10 Hz. These values are typical of what usually is observed in tectonically influenced areas near oceanic/continental plate boundaries. It is considered likely that these results are influenced by the fact that the Jan Mayen island, in spite of its proximity to a fracture zone, is located in the northern end of the Jan Mayen Ridge, which now is accepted as being a micro-continent. The presence of the active Beerenberg volcano on the Jan Mayen island does give rise to a somewhat stronger attenuation for waves traversing that area, but this effect is weak and quite limited in spatial extent. There is also a slight increase in attenuation as a function of depth, but less than what is observed in terms of lateral variations. This is reasonable in view of the very strong lateral variations in lithospheric structure exhibited in this area.     

The Agulhas Ridge, South Atlantic: The Peculiar Structure of a Fracture Zone

The Agulhas Ridge is a prominent topographic feature that parallels the Agulhas-Falkland Fracture Zone (AFFZ). Seismic reflection and wide angle/refraction data have led to the classification of this feature as a transverse ridge. Changes in spreading rate and direction associated with ridge jumps, combined with asymmetric spreading within the Agulhas Basin, modified the stress field across the fracture zone. Moreover, passing the Agulhas Ridge’s location between 80 and 69 Ma, the Bouvet and Shona Hotspots may have supplied excess material to this part of the AFFZ thus altering the ridge’s structure. The low crustal velocities and overthickened crust of the northern Agulhas Ridge segment indicate a possible continental affinity that suggests it may be formed by a small continental sliver, which was severed off the Maurice Ewing Bank during the opening of the South Atlantic. In early Oligocene times the Agulhas Ridge was tectono-magmatically reactivated, as documented by the presence of basement highs disturbing and disrupting the sedimentary column in the Cape Basin. We consider the Discovery Hotspot, which distributes plume material southwards across the AAFZ, as a source for the magmatic material.     

Geological history and magmatism of the Jan Mayen microcontinent (Polar Atlantic)

Two main events determined the formation, geological history, magmatism, and geodynamics of the Jan Mayen microcontinent: (1) drift of this segment of the Laurasian plate over the Iceland plume in the Early Paleogene; (2) propagation of the rift zone of the mid-Atlantic Ridge into this region and separation of the Jan Mayen lithospheric block from northeastern Greenland. The lithosphere was reduced at the block boundary when it was separated. This process was accompanied by the formation of depressions intruded by magma of the Iceland plume, which resulted in the appearance of a new volcanic center with active volcanoes of the central type. They supplied pyroclastic material to the sedimentary cover of the expanding Norwegian?Greenland Basin in the Eocene and Oligocene. The wedging of the Jan Mayen plate (microcontinent) into the triple junction of the plates (Greenland, Eurasian, Jan Mayen) promoted intense volcanism and the formation of two large volcanic complexes: (1) the Greenland?Faroes and the (2) Trail?Vøring. Recent volcanoes of the Jan Mayen hot spot are fed by magma from the Iceland plume as well as from relict and newly formed cambers in a zone of deep-seated Jan Mayen transform faults.     

Oceanic crustal basement: A comparison of seismic properties of D.S.D.P. basalts and consolidated sediments

Compressional (V_P) and shear (V_s) wave velocities have been measured to 1.0 kbar for 14 cores of well-consolidated sedimentary rock from Atlantic and Pacific sites of the Deep Sea Drilling Project. The range of V_P (2.05–5.38 km/sec at 0.5 kbar) shows significant overlap with the range of oceanic layer-2 seismic velocities determined by marine refraction surveys, suggesting that sedimentary rocks may, in some regions, constitute the upper portion of layer 2. Differing linear relationships between V_P and V_s for basalts and sedimentary rocks, however, may provide a method of resolving layer-2 composition. This is illustra ted for a refraction survey site on the flank of the Mid-Atlantic Ridge where layer-2 velocities agree with basalt, and two sites on the Saya de Malha Bank in the Indian Ocean where layer-2 velocities appear to represent sedimentary rock.     

Flow of Canadian basin deep water in the Western Eurasian Basin of the Arctic Ocean

The LOMROG 2007 expedition targeted the previously unexplored southern part of the Lomonosov Ridge north of Greenland together with a section from the Morris Jesup Rise to Gakkel Ridge. The oceanographic data show that Canadian Basin Deep Water (CBDW) passes the Lomonosov Ridge in the area of the Intra Basin close to the North Pole and then continues along the ridge towards Greenland and further along its northernmost continental slope. The CBDW is clearly evident as a salinity maximum and oxygen minimum at a depth of about 2000 m. The cross-slope sections at the Amundsen Basin side of the Lomonosov Ridge and further south at the Morris Jesup Rise show a sharp frontal structure higher up in the water column between Makarov Basin water and Amundsen Basin water. The frontal structure continues upward into the Atlantic Water up to a depth of about 300 m. The observed water mass division at levels well above the ridge crest indicates a strong topographic steering of the flow and that different water masses tend to pass the ridge guided by ridge-crossing isobaths at local topographic heights and depressions. A rough scaling analysis shows that the extremely steep and sharply turning bathymetry of the Morris Jesup Rise may force the boundary current to separate and generate deep eddies.     

Analysis of multi-channel seismic reflection and magnetic data along 13° N latitude across the Bay of Bengal

Analysis of the multi-channel seismic reflection, magnetic and bathymetric data collected along a transect, 1110 km long parallel to 13° N latitude across the Bay of Bengal was made. The transect is from the continental shelf off Madras to the continental slope off Andaman Island in water depths of 525 m to 3350 m and across the Western Basin (bounded by foot of the continental slope of Madras and 85° E Ridge), the 85° E Ridge, the Central Basin (between the 85° E Ridge and the Ninetyeast Ridge), the Ninetyeast Ridge and the Sunda Arc. The study revealed eight seismic sequences, H1 to H8 of parallel continuous to discontinuous reflectors. Considering especially depth to the horizons, nature of reflection and on comparison with the published seismic reflection results of Currayet al. (1982), the early Eocene (P) and Miocene (M) unconformities and the base of the Quaternary sediments (Q) are identified on the seismic section. Marked changes in velocities also occur at their boundaries.In the Western Basin the acoustic basement deepening landward is inferred as a crystalline basement overlain by about 6.7 km of sediment. In the Central Basin possibly thicker sediments than in the Western Basin are estimated. The sediments in the Sunda Arc area are relatively thick and appears to have no distinct horizons. But the entire sedimentary section appears to be consisting of folded and possibly faulted layers.The comparatively broader wavelength magnetic anomalies of the Central Basin also indicate deeper depth of their origin. Very prominent double humped feature of the 85° E Ridge and broad basement swell of the Ninetyeast Ridge are buried under about 2.8 km thick sediments except over the prominent basement high near 92° E longitude. The positive structural relief of the buried 85° E Ridge in the area is reflected in magnetic signature of about 450 nT amplitude. Flexural bulge of the 85° E Ridge and subsidence of the Ninetyeast Ridge about 24 cm my^–1 rate since early Eocene period have been inferred from the seismic sequence analysis.     

Recent tectonics of the Blanco Ridge,eastern blanco transform fault zone

Bathymetric, hydro-acoustic, seismic, submersible, and gravity data are used to investigate the active tectonics of the eastern Blanco Transform Fault Zone (BTFZ). The eastern BTFZ is dominated by the 150 km long transform-parallel Blanco Ridge (BR) which is a right-lateral strike-slip fault bordered to the east and west by the Gorda and Cascadia Depressions. Acoustic locations, fault-parameter information, and slip vector estimates of 43 earthquakes (M _w3.8) that occurred along the eastern BTFZ over the last 5 years reveal that the Blanco Ridge is a high-angle right-lateral strike-slip fault, with a small component of dip-slip motion, where the Juan de Fuca plate is the hanging wall relative to the Pacific plate. Furthermore, the Cascadia and Gorda basins are undergoing normal faulting with extension predominantly oblique to the transform trend. Seafloor submersible observations agree with previous hypotheses that the active transform fault trace is the elongate basin that runs the length of the BR summit. Brecciated and undeformed basalt, diabase, and gabbro samples were collected at the four submersible survey sites along the Blanco Ridge. These petrologic samples indicate the Blanco Ridge is composed of an ocean crustal sequence that has been uplifted and highly fractured. The petrologic samples also appear to show an increase in elevation of the crustal section from east to west along the Blanco Ridge, with gabbros exposed at a shallower depth farther west along the southern (Pacific plate side) BR ridge flank. Further supporting evidence for BR uplift exists in the seismic reflection profiles across the BR showing uplift of turbidite sequences along the north and south ridge base, and gravity and magnetics profiles that indicate possible basement uplift and a low-density zone centered on the ridge's Pacific plate side. The BR formation mechanism preferred here is first, uplift achieved partially through strike-slip motion (with a small dip-slip component). Second, seawater penetration along the fault into the lower crust upper mantle, which then enhanced formation and intrusion of a mantle-derived serpentinized-peridotite diapir into the shallow ocean crust, causing further uplift along the fault.     

First results from the North Iceland experiment

Between June 2004 and September 2004 a temporary seismic network was installed on the northern insular shelf of Iceland and onshore in north Iceland. The seismic setup aimed at resolving the subsurface structure and, thus, the geodynamical transition from Icelandic crust to typical oceanic crust along Kolbeinsey Ridge. The experiment recorded about 1,000 earthquakes. The region encloses the Tjörnes Fracture Zone containing the Husavik–Flatey strike-slip fault and the extensional seismic Grimsey Lineament. Most of the seismicity occurs in swarms offshore. Preliminary results reveal typical mid-ocean crust north of Grimsey and a heterogeneous structure with major velocity anomalies along the seismic lineaments and north–south trending subsurface features. Complementary bathymetric mapping highlight numerous extrusion features along the Grimsey Lineament and Kolbeinsey Ridge. The seismic dataset promises to deliver new insights into the tectonic framework for earthquakes in an extensional transform zone along the global mid-ocean ridge system.     

Early Eocene sea floor spreading and continent-ocean boundary between Jan Mayen and Senja fracture zones in the Norwegian-Greenland Sea

Sea floor spreading anomalies in the Lofoten-Greenland basins reveal an unstable plate boundary characterized by several small-offset transforms for a period of 4 m.y. after opening. North of the Jan Mayen Fracture Zone, integrated analysis of magnetic and seismic data also document a distinct, persistent magnetic anomaly associated with the continent-ocean boundary and a locally, robust anomaly along the inner boundary of the break-up lavas. These results provide improved constraints on early opening plate reconstructions, which include a new anomaly 23-to-opening pole of rotation yielding more northerly relative motion vectors than previously recognized; and a solution of the enigmatic, azimuthal difference between the conjugate Eocene parts of the Greenland-Senja Fracture Zone if the Greenland Ridge is considered a continental sliver. The results confirm high, 2.36–2.40 cm yr^–1, early opening spreading rates, and are consistent with the start of sea floor spreading during Chron 24r. The potential field data along the landward prolongations of the Bivrost Fracture Zone suggest that its location is determined by a Mesozoic transfer system which has acted as a first-order, across-margin tectono-magmatic boundary between the regional Jan Mayen and Greenland-Senja Fracture Zone systems, greatly influencing the pre-, syn- and post-breakup margin development.     

Structure of the Northern Symmetrical Segment of the Juan de Fuca Ridge

A seismic refraction profile was shot along the axis of the Northern Symmetrical Segment of the Juan de Fuca Ridge system. Three models of the along-axis crustal structure fit the observed data equally well. One model includes a low-velocity zone, the top of which is at a depth below the seafloor of approximately 3 km, that is continuous along-axis for at least 30 km. A second model includes a low-Q layer, the top of which is also at a depth of approximately 3 km below the seafloor and is continuous along-axis for at least 30 km. Both the low-Q layer and low-velocity zone can be explained geologically by a region of elevated temperatures. The third model is characterized by a homogeneous seismic layer 3. All models contain an ~1 km s^–1 discontinuity at the seismic layer 2/3 boundary; a wide-angle reflection from this boundary is seen on all record sections. Kappel and Ryan (1986) had previously proposed that the Northern Symmetrical Segment was in a stage of volcanic inactivity, and this theory is supported by the seismic observations. Two-dimensional modelling of travel times to ocean bottom hydrophone instruments shows that the amplitude variations in the along-axis depth to intracrustal seismic layers (a few hundred meters) is on the order of the lateral changes in topographic relief. It is suggested that the crustal emplacement processes reflect the deeper style of 3-D mantle upwelling beneath the ridge.     

The return of Pacific waters to the upper layers of the central Arctic Ocean

Temperature, salinity, and chemical measurements, including the nutrients silicic acid (Si), nitrate (NO₃), nitrite (NO₂), ammonium (NH₄), and phosphate (PO₄ or P), the oxygen isotopic composition of seawater (δ¹⁸O), and barium (Ba) concentrations were obtained from the central Arctic Ocean along transects radiating from the North Pole in early spring, 2000–2006. Stations that were reoccupied over this time period were grouped into five regions: from Ellesmere Island, (1) north along 70°W and (2) northwest along 90°W; near the North Pole, (3) on the Amundsen Basin flank and (4) directly over the Lomonosov Ridge; (5) through the Makarov Basin along 170–180°W. These regions had been shown by others to have undergone marked changes in water-mass assemblies in the early 1990s, but our time series tracer hydrographic data indicate a partial return of Pacific origin water within the mixed layer and the upper halocline layers beginning in 2003–2004. Back-trajectories derived from satellite-tracked ice buoys for these stations indicate that the upper levels of Pacific water in the central Arctic in 2004–2006 transited westward from the Bering Strait along the Siberian continental slope into the East Siberian Sea before entering the Transpolar Drift Stream (TPD). By 2004, the TPD shifted back from an alignment over the Alpha-Mendeleev Ridge toward the Lomonosov Ridge, as was characteristic prior to the early 1990s. At most stations occupied in 2006, a decrease in the Pacific influence was observed, both in the mixed layer and in the upper halocline, which suggests the Canadian branch of the TPD was shifting back toward North America. Clearly the system is more variable than has been previously appreciated.     

On the accuracy of upper ocean particulate organic carbon export fluxes estimated from 234Th/238U disequilibrium

An analysis of the ²³⁴Th method for determining the export flux of particulate organic carbon (P_POC) from the upper ocean using in situ pumps or water bottles shows that the accuracy of the method (the ratio of the experimental value of P_POC divided by the true POC flux, F_POC), defined as the p-ratio, is equal to the mean settling velocity of particulate ²³⁴Th divided by that of POC. Therefore, P_POC is equivalent to the true POC flux (F_POC) and the p-ratio is equal to unity if, and only if, POC and particulate ²³⁴Th have the same mean sinking velocities. A simple particle settling model is discussed that invokes Stokes’ Law settling velocities, volume:surface area (V:SA) fractionation of C:²³⁴Th and two assemblages of particles having different sizes and densities. The model is used to illustrate the ranges of parameter values that conform to values of the p-ratio sufficiently close to unity that the experimentally determined POC flux can be considered to be an accurate representation of the true POC flux.Despite the over-simplification of real systems implicit in the model, the results suggest that p-ratios<1, equivalent to an under-estimate of the POC flux, are representative of single particle settling regimes in which the larger particles dominate the vertical flux. This follows from the assumption that the ratio of C/²³⁴Th on particles is governed by the volume to surface area (V:SA) ratio of the particles. This results in a greater proportion of ²³⁴Th compared to C being associated with the smaller, more slowly settling particles and, as a result, normalization of the POC flux to the ²³⁴Th flux provides an under-estimate of the former quantity. However, when the smaller particle assemblage dominates the vertical flux, as could occur in open ocean regimes having high aeolian inputs of dense, rapidly settling, inorganic particles, then the p-ratio could exceed unity (p-ratio >1) resulting in an over-estimate of the POC flux using the ²³⁴Th method. High levels of flocculation associated with phytoplankton blooms in productive regions of the ocean are likely to produce p-ratios approaching unity, because flocs would tend to preserve the V:SA partitioning of the original particle size distribution and thereby minimize differences in the mean settling speeds of POC and particulate ²³⁴Th (Waite, A.W., Hill, P.S., 2006. Flocculation, phytoplankton and the accuracy of ²³⁴Th-based estimates of the vertical flux of particulate organic carbon in the sea. Marine Chemistry in press). Selective sampling of the large particle fraction using, for example, 53-μm screens can produce a more accurate estimate of the true POC flux, but may not entirely shift the p-ratio to a value of unity.     

Tectonic core of a sedimentary drift: a potential ridge propagation feature beneath the Blake Outer Ridge

The Blake Outer Ridge is a 480–kilometer long linear sedimentary drift ridge striking perpendicular to the North American coastline. By modeling free-air gravity anomalies we tested for the presence of a crustal feature that may control the location and orientation of the Blake Outer Ridge. Most of our crustal density models that match observed gravity anomalies require an increase in oceanic crustal thickness of 1–3 km on the southwest side of the Blake Outer Ridge relative to the northeast side. Most of these models also require 1–4 km of crustal thinning in zone 20–30 km southwest of the crest of the Blake Outer Ridge. Although these features are consistent with the structure of oceanic fracture zones, the Blake Outer Ridge is not parallel to adjacent known fracture zones. Magnetic anomalies suggest that the ocean crust beneath this feature formed during a period of mid-ocean ridge reorganization, and that the Blake Outer Ridge may be built upon the bathymetric expression of an oblique extensional feature associated with ridge propagation. It is likely that the orientation of this trough acted as a catalyst for sediment deposition with the start of the Western Boundary Undercurrent in the mid-Oligocene.     

Crustal structure of the ultra-slow spreading Knipovich Ridge,North Atlantic,along a presumed ridge segment center

A combined ocean bottom seismometer, multichannel seismic reflection and gravity study has been carried out along the spreading direction of the Knipovich Ridge over a topographic high that defines a segment center. The youngest parts of the crust in the immediate vicinity of the ridge reveal fractured Oceanic Layer 2 and thermally expanded and possibly serpentinized Oceanic Layer 3. The mature part of the crust has normal thickness and seismic velocities with no significant crustal thickness and seismic velocity variations. Mature Oceanic Layer 2 is in addition broken into several rotated fault blocks. Comparison with a profile acquired ~40 km north of the segment center reveals significant differences. Along this profile, reported earlier, periods of slower spreading led to generation of thin crust with a high P-wave velocity (Vp), composed of a mixture of gabbro and serpentinized mantle, while periods of faster spreading led to generation of more normal gabbroic crust. For the profile across the segment center no clear relation exists between spreading rate and crustal thickness and seismic velocity. In this study we have found that higher magmatism may lead to generation of oceanic crust with normal thickness even at ultra-slow spreading rates.