Linked turbidite–debrite resulting from recent Sahara Slide headwall reactivation

The northwest African margin has been affected by numerous large-scale landslides during the late Quaternary. This study focuses on a recent collapse of the Sahara Slide headwall and characterises the resulting flow deposit. Core and seismic data from the base of the upper headwall reveal the presence of blocky slide debris, comprising heavily deformed hemipelagic slope sediments. The blocky slide debris spilled over a lower headwall 60 km downslope and formed a thick transparent debris flow unit. Cores recovered 200–250 km farther downslope contain a surficial turbidite that is interpreted to be linked to the headwall collapse event based on timing and composition. One core located approximately 200 km from the headwall scar (C13) contains debrite encased in turbidite. The debrite comprises sheared and contorted hemipelagic mudstone clasts similar as those seen in the vicinity of the Sahara Slide headwall, and lacks matrix. This debrite pinches out laterally within 25 km of C13, whereas the accompanying turbidite can be correlated across 700 km of the northwest African margin. The linked turbidite–debrite bed is interpreted to have formed through recent failure of the steep Sahara Slide headwall that either 1) generated both a debris flow and a turbidity current almost simultaneously, or 2) generated a debris flow which with entrainment of water and progressive dilution led to formation of an accompanying turbidity current.     

The role of frontal currents in larval fish transport on Georges Bank

The hydrography and distributions of cod larvae on Georges Bank were surveyed during two research cruises in April and May 1993 in order to relate larval drift between cruises to the vernal intensification of the frontal component of the residual circulation. We observed the transport of two patches of cod larvae. One patch, which had maximum larval cod densities of 45 larvae 100 m⁻³ in April, appeared to have been advected south about 75 km between surveys, while the other, which had maximum larval cod densities of 20 larvae 100 m⁻³ in April, appeared to have been advected north-northeast about 25 km. Maximum larval densities in each patch sampled during the second cruise in May were 15 and 18 larvae 100 m⁻³, respectively, and mean growth in total length for larvae in the two patches was approximately 5.5 mm and 4.5 mm, respectively, between the two cruises. During the April cruise there was a large volume of anomalous cold, fresh water, of Scotian Shelf origin, which occupied much of the eastern third of Georges Bank. During May, relatively cold, fresh water appeared in a band from the Northeast Peak along the Southern Flank, between Georges Bank water on the top of the Bank, and upper Slope Water offshore. The distribution of cold, fresh water suggests its participation in the general clockwise circulation around the Bank. The transport of cod larvae comprising the first patch appeared to become organized within, and move along, the frontal boundary established by the Scotian Shelf-like water mass, while larvae in the second patch, which we assumed to have moved to the north, may have been transported northward in an on-Bank flow of warmer and saltier upper Slope Water, which may have originated from a Gulf Stream Ring. Based upon observed transport of the first patch of larvae in relation to the frontal boundary, we present a conceptual model of frontal mixing currents on Georges Bank, where current velocities may reach 5 cm s⁻¹ at the depth of the pycnocline. We suggest that this frontal component of the residual circulation, which is in addition to that resulting from tidal rectification, may be important in the transport of fish larvae, and that interannual variability in the degree of intrusion of extrinsic water masses may contribute to variable larval cod drift patterns, to variable larval cod retention on the Bank, and ultimately, to variable larval fish recruitment to the early juvenile stage.     

Seasonal patterns in zooplankton community structure on the Newfoundland and Labrador Shelf

We report on the patterns in zooplankton community structure on the Newfoundland and Labrador Shelf from seasonal collections along oceanographic sections during 2000–2007. We use a combination of constrained and unconstrained multivariate methods to assess latitudinal and cross shelf patterns in community structure. Both physical and biological features of the region are dominated by the cross-shelf gradient in water mass characteristics although there is evidence of a latitudinal gradient that may be a reflection of the influence of freshwater outflow from the Arctic. All analyses reveal a strong and consistent pattern in species composition among water masses that extends across spring, summer and autumn, although there are some variations that occur among seasons that reflect differences in development state of certain taxa. The strong association between community structure and water mass characteristics in the region may be the result of the Newfoundland and Labrador Shelf being at the intersection of several major oceanographic domains, bounded by strong Labrador and Gulf Stream currents, that allows the formation and persistence of well defined zooplankton communities. Our findings have implications for the region’s potential as a monitoring location for long term changes in ocean state.     

A mesoscale eddy driving spatial and temporal heterogeneity in the productivity of the euphotic zone of the northeast Atlantic

In this paper we show how different water masses from a similar geographic region provide an explanation for perturbations in the signal of declining productivity at the Porcupine Abyssal Plain (PAP) study site in the Northeast Atlantic. Furthermore we show that the passage of these different water masses is affected by the filamentary instabilities of a cyclonic eddy just southwest of the PAP site. We describe a high-resolution spatial hydrographic survey conducted with a towed instrument package, complemented by biogeochemical sampling. Maximum rates of primary production of 110 mmol C m^-2 d^-1 seen at the centre of the survey area were associated with the passage of an eddy filament and were enhanced 3 fold relative to far-field conditions (∼36 mmol C m^-2 d^-1). The rotation and stirring influence of the eddy resulted in the sequential passage of 3 distinct water masses past the observation point. This understanding of the lateral stirring around the site enabled us to explain the sharp changes observed in daily primary production rates and other biogeochemical parameters. The spatial survey also revealed a fluorescence maxima associated with the cyclonic eddy that was laterally displaced northwards away from the core, an observation supportive of recent modelling studies.     

Oxygen and carbon stable isotope tracers of Weddell Sea water masses: new data and some paleoceanographic implications

Stable oxygen isotopic composition of sea water and stable carbon isotopes of dissolved inorganic carbon (DIC) on the continental shelf in the southern Weddell Sea are presented. Using the stations sampled during the summer 1995 two sections can be constructed, one closely parallel to the ice shelf edge and the other perpendicular to the upper continental slope. Generally, δ¹⁸O values clearly separate between different shelf water masses depending on the content of meteoric meltwater added during melting of glacial ice. Extrapolation of the mixing line between the cores of High Salinity Shelf Water (HSSW) and supercooled Ice Shelf Water (ISW) reveals δ¹⁸O values of the glacial ice of −27‰, whereas extrapolation of the mixing line between the δ¹⁸O values of the most-saline HSSW and lowest temperature ISW results in δ¹⁸O values of −34‰ for glacial ice. These values point to an origin of meltwater from below the ice shelf, where ice is less depleted in ¹⁸O, since deep beneath the ice shelf close to the grounding line, values may reach −40‰. If values between −34 and −27‰ are used as δ¹⁸O end member values for glacial ice, the amount of meltwater from the ice shelf that adds to the formation of ISW off the Filchner–Ronne Ice Shelf ranges from 0.2 to 0.8%, in agreement with previous studies based on δ¹⁸O and ⁴He. Carbon isotopic fractionation due to gas exchange between the atmosphere and the ocean at cold temperatures results in Δδ¹³C_DIC values of 0.20±0.17‰ for Weddell Sea Deep Water, the water mass that ventilates the global abyssal ocean, typically defined as Antarctic Bottom Water (AABW). This confirms the low end of the range estimated previously (0.2–0.4‰), and thus corroborates the dominance of biology in shaping the deep and bottom water δ¹³C signal. It has been hypothesized that different modes of glacial/interglacial Antarctic bottom water formation may be separated by different stable isotopic compositions of deep-sea foraminiferal calcite. Here I show that differences between Δδ¹³C and δ¹⁸O values of HSSW and ISW, both of which contribute to bottom water formation today, are too small to be resolved in deep and bottom water masses. Therefore, glacial/interglacial changes in relative proportions of these water masses in Antarctic deep and bottom water cannot be separated by stable isotopes of fossil benthic foraminiferal calcite.     

The distribution of tritium and CFCs in the Weddell Sea during the mid-1980s

Transient tracer data (tritium, CFC11 and CFC12) from the southern, central and northwestern Weddell Sea collected during Polarstern cruises ANT III-3, ANT V-2/3/4 and during Andenes cruise NARE 85 are presented and discussed in the context of hydrographic observations. A kinematic, time-dependent, multi-box model is used to estimate mean residence times and formation rates of several water masses observed in the Weddell Sea.Ice Shelf Water is marked by higher tritium and lower CFC concentrations compared to surface waters. The tracer signature of Ice Shelf Water can only be explained by assuming that its source water mass, Western Shelf Water, has characteristics different from those of surface waters. Using the transient nature of tritium and the CFCs, the mean residence time of Western Shelf Water on the shelf is estimated to be approximately 5 years. Ice Shelf Water is renewed on a time scale of about 14 years from Western Shelf Water by interaction of this water mass with glacial ice underneath the Filchner-Ronne Ice shelf. The Ice Shelf Water signature can be traced across the sill of the Filchner Depression and down the continental slope of the southern Weddell Sea. On the continental slope, new Weddell Sea Bottom Water is formed by entrainment of Weddell Deep Water and Weddell Sea Deep Water into the Ice Shelf Water plume. In the northwestern Weddell Sea, new Weddell Sea Bottom Water is observed in two narrow, deep boundary currents flowing along the base of the continental slope. Classically defined Weddell Sea Bottom Water (θ ≤ −0.7°C) and Weddell Sea Deep Water (−0.7°C ≤ θ ≤ 0°C) are ventilated from the deeper of these boundary currents by lateral spreading and mixing. Model-based estimates yield a total formation rate of 3.5Sv for new Weddell Sea Bottom Water (θ = −1.0°C) and a formation rate of at least 11Sv for Antarctic Bottom Water (θ = −0.5°C).     

The Brunei slide: A giant submarine landslide on the North West Borneo Margin revealed by 3D seismic data

Three dimensional seismic data, offshore Brunei, provide evidence for a giant landslide with a volume of 1200 km³, an area of ∼ 5300 km² and an average thickness of ∼ 240 m. It extends for over 120 km from the Baram Canyon in ∼ 200 m water depth to the deep basin floor of the North West Borneo Trough. The landslide is a unique example of a major submarine landslide located on a steep, tectonically active margin adjacent to a large river and canyon system. The landslide is mappable using 3D seismic data, which allow detailed imaging of internal flow structures, erosional headwall and the basal sliding surface. The landslide is a complex deposit, involving a chaotic debris flow matrix, with flow structures and blocks 500 to 1000 m wide and up to 250 m thick. Imaging of the basal sliding surface reveals large striations ∼ 30-120 km long, ∼ 100-600 m wide, and ∼ 10-30 m deep that show significant amounts of basal erosion. In the landslide source area we describe fluid escape structures, gas buildups and bottom simulating reflectors, which may provide a mechanism for weakening and triggering slope failure. We also report older landslides, buried several hundred meters beneath the basin floor that indicate giant landsliding is a recurrent process in the NW Borneo Trough.     

The Rockall Bank Mass Flow: Collapse of a moated contourite drift onlapping the eastern flank of Rockall Bank,west of Ireland

The Rockall Bank Mass Flow (RBMF) is a large, multi-phase submarine slope failure and mass flow complex. It is located in an area where the Feni Drift impinges upon the eastern flank of the Rockall Bank in the NE Atlantic. A 6100 km² region of slope failure scarps, extending over a wide water depth range and with individual scarps reaching up to 22 km long and 150 m high, lies upslope of a series of mass flow lobes that cover at least 18,000 km² of the base of slope and floor of the Rockall Trough. The downslope lobe complex has a negative topographic relief along much of its northern boundary, being inset below the level of the undisplaced contourite drift at the base of slope. The southern margin is topographically more subtle but is marked by the sharp termination of sediment waves outside the lobe. Within the lobe complex the southern margin of the largest lobe shows a positive relief along its southern margin. The initial failure is suggested to have occurred along coherent layer-parallel detachment surfaces at depths of up to 100 m and this promoted initial downslope block sliding which in turn transformed into debris flows which moved out into the basin. The remains of a deep erosional moat linked to the onlapping contourite complex bisects the region of failed slope, and post-failure thermohaline currents have continued to modify the mass flow in this area. Differential sedimentation and erosion associated with the moat may have promoted slope instability. Following the major failure phase, continuous readjustments of the slope occurred and resulted in small-volume turbidites found in shallow gravity cores collected on the lobes. The short term trigger for the failure remains uncertain but earthquake events associated with a deep-seated tectonic lineament to the north of the mass flow may have been important. A Late Pleistocene age for the slope failure is likely. The RBMF is unusual in that it records large-scale collapse of a contourite body that impinged on a sediment-undersupplied slope system. Unlike many other large slope failure complexes along the NE Atlantic margin, the RBMF occurs in a region where there was little overloading by glacial sediment.     

Surface boundary-layer variability off Northern California, USA, during upwelling

A five-element mooring array is used to study surface boundary-layer transport over the Northern California shelf from May to August 2001. In this region, upwelling favorable winds increase in strength offshore, leading to a strong positive wind stress curl. We examine the cross-shelf variation in surface Ekman transport calculated from the wind stress and the actual surface boundary-layer transport estimated from oceanic observations. The two quantities are highly correlated with a regression slope near one. Both the Ekman transport and surface boundary layer transport imply curl-driven upwelling rates of about 3×10⁻⁴ m s⁻¹ between the 40 and 90 m isobaths (1.5 and 11.0 km from the coast, respectively) and curl-driven upwelling rates about 1.5×10⁻⁴m s⁻¹ between the 90 and 130 m isobaths (11.0 and 28.4 km from the coast, respectively). Thus curl-driven upwelling extends to at least 25 km from the coast. In contrast, upwelling driven by the adjustment to the coastal boundary condition occurs primarily inshore of the 40-m isobath. The upwelling rates implied by the differentiating the 40-m transport observations with the coastal boundary condition are up to 8×10⁻⁴ m s⁻¹. The estimated upwelling rates and the temperature–nitrate relationship imply curl-driven vertical nitrate flux divergences are about half of those driven by coastal boundary upwelling.     

Seismic geomorphology of cretaceous megaslides offshore Namibia (Orange Basin): Insights into segmentation and degradation of gravity-driven linked systems

This study applies modern seismic geomorphology techniques to deep-water collapse features in the Orange Basin (Namibian margin, Southwest Africa) in order to provide unprecedented insights into the segmentation and degradation processes of gravity-driven linked systems. The seismic analysis was carried out using a high-quality, depth-migrated 3D volume that images the Upper Cretaceous post-rift succession of the basin, where two buried collapse features with strongly contrasting seismic expression are observed. The lower Megaslide Complex is a typical margin-scale, extensional-contractional gravity-driven linked system that deformed at least 2 km of post-rift section. The complex is laterally segmented into scoop-shaped megaslides up to 20 km wide that extend downdip for distances in excess of 30 km. The megaslides comprise extensional headwall fault systems with associated 3D rollover structures and thrust imbricates at their toes. Lateral segmentation occurs along sidewall fault systems which, in the proximal part of the megaslides, exhibit oblique extensional motion and define horst structures up to 6 km wide between individual megaslides. In the toe areas, reverse slip along these same sidewall faults, creates lateral ramps with hanging wall thrust-related folds up to 2 km wide. Headwall rollover anticlines, sidewall horsts and ramp anticlines may represent novel traps for hydrocarbon exploration on the Namibian margin.The Megaslide Complex is unconformably overlain by few hundreds of metres of highly contorted strata which define an upper Slump Complex. Combined seismic attributes and detailed seismic facies analysis allowed mapping of headscarps, thrust imbrications and longitudinal shear zones within the Slump Complex that indicate a dominantly downslope movement of a number of coalesced collapse systems. Spatial and stratal relationships between these shallow failures and the underlying megaslides suggest that the Slump Complex was likely triggered by the development of topography created by the activation of the main structural elements of the lower Megaslide Complex.This study reveals that gravity-driven linked systems undergo lateral segmentation during their evolution, and that their upper section can become unstable, favouring the initiation of a number of shallow failures that produce widespread degradation of the underlying megaslide structures. Gravity-driven linked systems along other margins are likely to share similar processes of segmentation and degradation, implying that the megaslide-related, hydrocarbon trapping structures discovered in the Namibian margin may be common elsewhere, making megaslides an attractive element of deep-water exploration along other gravitationally unstable margins.     

Absolutely referenced geostrophic velocity and transport on a section across the North Atlantic Current

A transect of CTD profiles crossing the North Atlantic Current (NAC) along WOCE line ACM6 near 42.5°N during August 1–7, 1993, provides geostrophic shear velocity profiles, which were absolutely referenced using simultaneous POGO transport float measurements and velocity measurements from a ship-mounted acoustic doppler current profiler (ADCP). The NAC absolute transport was 112±23×10⁶ m³ s⁻¹, which includes a portion of the transport of the Mann Eddy, a large permanent anticyclonic eddy commonly adjacent to the NAC. The NAC transport estimated relative to a level of no motion at the bottom would have underestimated the true total absolute transport by 20%. A surprisingly large 58×10⁶ m³ s⁻¹ flowed southward just inshore of the NAC. This flow, centered near 1500 dbars about 200 km offshore of the shelf-break, was fairly barotropic with a peak velocity of greater than 20 cm s⁻¹, and the water mass characteristics were of Labrador Sea Water. These absolute transport observations suggest southward recirculation inshore of the NAC at 42.5°N and a stronger NAC than has previously been observed.     

Pockmarks in the inner Oslofjord,Norway

Multibeam bathymetric surveys of the Inner Oslofjord, Norway have revealed a high density of pockmarks in the 179-km² inner fjord area, which contains over 500 pockmarks of varying size, typically 20–50 m in diameter and 2–10 m deep. These pockmarks have been investigated with a variety of techniques, including acoustic subbottom profiling, sedimentological and geochemical analyses of cores, remotely operated vehicle observation, and morphometry. Both the distribution and shapes of the pockmarks suggest that they are related to structures in the bedrock underlying relatively thin (<50 m) unconsolidated glacial and postglacial sediments. The data provide no direct indication of a particular mode of pockmark formation, but release of large amounts of biogenic, shallow methane seems unlikely. Several lines of evidence point to a continuous process of pockmark formation followed by inactivity, with some pockmarks recently active whereas others have been inactive for a considerable time. Some pockmarks are characterised by coarse sediment in their centres. The density, variety and easy access make this pockmark field an ideal model area for pockmark research. John S. Gray is deceased.     

Turbidity events observed in situ along the Congo submarine channel

As part of the multidisciplinary programme BIOZAIRE devoted to studying deep-sea benthic ecosystems in the Gulf of Guinea, particulate input and its relationship with near-bottom hydrodynamics were monitored using long-term moorings from 2000 to early 2005. Particular attention was given to material input through the Congo (ex-Zaïre) submarine channel that extends 760 km from the Congo River mouth to the abyssal plain (>5100 m) near 6°S. Due to its direct connection to the Congo River, the Congo canyon and channel system are characterised by particularly active recent sediment transport. During this first in situ long-term monitoring along the channel, an energetic turbidity event was observed in January 2004 at three locations along the channel from 3420 to 4790 m in depth. This event tilted and displaced the moorings installed at 3420 m (site ZR′) and 4070 m (site ZD′), and resulted in high sediment deposition at all three mooring sites. The event moved at an average velocity of 3.5 m s⁻¹ along the numerous channel meanders between 3420 and 4070 m, then at 0.7 m s⁻¹ between 4070 m and the end of the channel at 4790 m. The particle cloud rose above the top of the valley at 4070 m (site ZD′), but not at 3420 m (site ZR′) where the channel was too deep. Lastly, the mooring line broke at site ZD′ in October 2004 probably due to a strong event like that of 2001 previously described by Khripounoff et al. [Khripounoff, A., Vangriesheim, A., Babonneau, N., Crassous, P., Denniellou, B., Savoye, B., 2003. Direct observation of intense turbidity activity in the Zaire submarine valley at 4000 m water depth. Marine Geology (194), 151–158]. Between these strong events, several peaks of high turbidity and particle flux occurred, but without noticeable current increases. These events were probably due to local sliding of sediment accumulated on the walls or terraces on the side of the channel. The area near 4000 m depth and the lobe appear to be the main depocentres of particulate input rich in organic matter derived from the Congo River.     

An inverse modeling study in Fram Strait. Part II: Water mass distribution and transports

A water-mass analysis is carried out in Fram Strait, between 77.15 and 81.15°N, based on three-dimensional large-scale potential temperature and salinity distributions reconstructed from the MIZEX 84 hydrographic data collected in summer 1984. Combining these distributions with the geostrophic flow field derived from the same data in a companion paper (Schlichtholz and Houssais, 1999), the heat, fresh water and volume transports are estimated for each of the water masses identified in the strait. Twelve water masses are selected based on their different origins. Among them, the Polar Water (PW) enters Fram Strait from the Arctic Ocean both over the Greenland Slope and over the western slope of the Yermak Plateau. In the Atlantic Water (AW) range, four modes with distinct geographical distributions are indentified. In the Deep Water range, the Eurasian Basin Deep Water (EBDW) is confined to the Lena Trough and to the Molloy Deep area where it is involved in a cyclonic circulation. The warm and shallower mode of the Norwegian Sea Deep Water (NSDW), concentrated to the west, is mainly seen as an outflow from the Arctic Ocean while the cold and deeper mode, essentially observed to the east, enters the strait from the Greenland Sea. Apart from the EBDW, there is a tendency for all water masses of polar origin to flow along the Greenland Slope. The two most abundant water masses, the AW and the NSDW, occupy as much as 67% of the total water volume. The southward net transport of PW through Fram Strait is about 1 Sv at 78.9°N. At the same latitude, the net transport of AW is southward and equal to about 1.7 Sv. Only the transport of the warm mode (AWw) is northward, amounting to 0.2 Sv. The overall net outflow of the Deep Waters to the Greenland Sea is about 2.6 Sv. Two upper water masses, the fresh (AWf) and the cold (AWc) mode of the AW, and one deep-water mass, the NSDW, appear to be produced in the strait, with production rates, between 77.6 and 79.9°N, of about 0.2, 1.0 and 1.7 Sv, respectively. A southward net fresh-water transport through the strait of about 2000 km³ yr⁻¹ (relative to a salinity of 34.93) is mainly due to the PW. The net heat transport relative to −0.1°C is northward, but undergoes a rapid northward decrease, suggesting an area-averaged surface heat loss of 50–100 W m⁻² in the strait.     

On the spatial scale needed for benthos community monitoring in the coastal North Sea

In community monitoring an attempt is made to identify long-term trends by regular sampling of selected sites. Since the benthos is reputed to be fairly sedentary, the spatial resolution is often reduced to single sites. However, members of many benthic invertebrate species have been found drifting across sedimentary seabeds in shallow waters. Transportation by currents may result in changes of their spatial pattern in the sediment, thereby changing local community composition. The quantitative importance of drifting was tested by repeated sampling of a 2-km² shallow (10 m) offshore area west of the island of Sylt (North Sea). Within the fortnight period between two samplings the benthic community composition had changed dramatically. Despite fairly calm weather, translocation of organisms by currents exceeded 1 km. In about half of the species, the spatial changes in abundance within these two weeks roughly equalled the average variation between consecutive years. This example suggests that community monitoring needs a wide spatial scale to discriminate long-term temporal changes from short-term variability. Extending the sampling area from 2 to 180 km² strongly reduced the variability of abundance estimates. However, only in a few species was the spatial distribution over the sampling sites found on one sampling date a suitable estimator for the spatial pattern found one or two months later, at the same sites. Instead the spatial patterns of the fauna changed strongly during a single month with a spatial scale of re-distribution exceeding several km in some species. At the same time the granulometric sediment composition changed, indicating changes of habitat quality. Hence, sampling of a large area, with random selection of the sampling sites on each sampling date, is suggested to yield the most reliable estimates of population development in the coastal North Sea. However, in view of the expected spatial scale of re-distribution during storm tides and the spatial variability of recruitment, even a 180-km² sampling area may be too small.     

Anatomy of a continent-backarc transform — The Vening Meinesz Fracture Zone northwest of New Zealand

The northwestern continental margin of New Zealand offers one of the finest examples of a continent-backarc transform. This transform, part of the Vening Meinesz Fracture Zone (VMFZ), accommodated about 170 km of sea-floor spreading in the Norfolk backare basin together with eastward migration of a volcanic arc, the Three Kings Ridge, in the Mid- to Late Miocene. Before the onset of spreading, strain along the VMFZ may have been linked to a major Early Miocene obduction event — the emplacement of the Northland Allochthon. The transform is manifested by a belt up to 50 km wide of left-stepping, linear fault scarps up to 2000 m high within an approximately 100 km-wide deformed zone. A marginal ridge, the Reinga Ridge, which includes a faulted, folded and uplifted Miocene sedimentary basin, occurs within the high-standing continental side of the deformed zone, whereas a narrow strip of linear detached blocks occupies the deep backarc oceanic side. Prespreading uplift and erosion of crust in the proto-backarc region, are volcanism, and obduction of the allochthon, supplied clastic sediments to the basin on the continental side. This basin was complexly deformed as the transform evolved. The transform was initiated as a dextral strike-slip fault zone, which developed right-branching splays and left-steps along its length, uplifting and cutting the continental margin into left-hand, en echelon blocks and relays. Folds formed locally within relay blocks and at the distal ends of the splays. Only the high continental side of this zone (the Reinga Ridge) remains, the formerly adjacent crust (the Three Kings Ridge) having been displaced towards the southeast. As the Three Kings block moved and the Norfolk Basin opened, opposing rift margins of the backarc basin foundered to form terraces. The oceanic side of the transform also subsided to produce the belt of detached blocks (some laterally displaced by strike slip) and linear troughs along the main escarpment system.     

Circulation and melting under the Ross Ice Shelf: estimates from evolving CFC,salinity and temperature fields in the Ross Sea

Temperature, salinity and chlorofluorocarbons (CFCs) 11, 12 and 113 were measured on a line of stations along the front of the Ross Ice Shelf in the austral summers of 1984, 1994 and 2000. Water mass distributions were similar each year but with high variability in the cross-sectional areas. CFC concentrations increased and salinity decreased with time throughout the water column. CFC saturation levels in the shelf and surface waters also increased with time and ranged from 43% to 90%. The undersaturation was due to inflow of low-CFC modified Circumpolar Deep Water, gas exchange limited by sea ice cover and isolation of water from the atmosphere beneath the ice shelf. The residence time of dense shelf waters resulting from sea ice formation is less well constrained by the chemical data than is the strong flow into the Ross Ice Shelf cavity. Shelf waters are transformed over about 3.5 years, by net basal melting of the ice shelf, into fresher Ice Shelf Water (ISW), which emerges as a large plume near the central ice front at temperatures below the sea surface freezing point. We estimate an average ISW production rate of 0.86 Sv and an average net basal melt rate of 60 km³/year for the Ross Ice Shelf exceeding a 300 m draft (75% of the ice cavity) during recent decades from box and stream tube models fit to all of the CFC and salinity data. Model fits to the individual data sets suggest ISW production and net basal melt rate variability due to interannual changes on a shorter time scale than our observations. ISW production based on the CFC budget is better constrained than net basal melting based on thermohaline data, with a heat budget yielding a rate of only 20 km³/yr. Reconciling differences between apparent freshwater and temperature changes under the ice shelf involves considerations of mixing, freezing and the flow of meltwater across the ice shelf grounding line.     

Last glacial emplacement of methane-derived authigenic carbonates in the Sea of Japan constrained by diatom assemblage,carbon-14, and carbonate content

We studied diatom assemblages and CaCO₃ contents of methane-derived authigenic carbonates from the eastern margin of the Sea of Japan and assessed the formation time of these samples. Radioactive ¹⁴C date was determined in selected samples to obtain the maximum age of the time. The results of our study suggest mass formation of carbonate nodules in a glacial period within ∼40 ky, consistent with a published U/Th dating result of carbonate nodules in the study area. Diatom assemblages and contents in the carbonate nodules (abundance of ∼10⁶/g, dominance of neritic-littoral species, warm/cold water species ratio lower than ∼25) differ from the near-seafloor sediments in the study area, which have characteristics of Holocene sediments in the Sea of Japan, and suggest cementation of glacial sediments. Laminated sediments in some nodule samples are glacial sediments because laminations are records of a low sea level period in the semi-enclosed ocean. Similarity of diatom assemblages and contents in all carbonate samples is another evidence of glacial sediments in nodules. Glacial sediments with oceanic cold water species as low as Holocene sediments restricts the sediment age to before 20 cal. ky BP. Carbonate contents higher than 78 wt% suggest the cementation of poorly compacted sediments near the seafloor, and the date of carbonate cementation is, therefore, close to that of the cemented sediments. Most carbonate nodule samples in this study were formed in a glacial period and detection of ¹⁴C restricts this period to within ∼40 ky.     

Rapid lateral particle transport in the Argentine Basin: Molecular 14C and 230Thxs evidence

Recent studies have revealed that lateral transport and focusing of particles strongly influences the depositional patterns of organic matter in marine sediments. Transport can occur in the water column prior to initial deposition or following sediment re-suspension. In both cases, fine-grained particles and organic-rich aggregates are more susceptible to lateral transport than coarse-grained particles (e.g., foraminiferal tests) because of the slower sinking velocities of the former. This may lead to spatial and, in the case of redistribution of re-suspended sediments, temporal decoupling of organic matter from coarser sediment constituents. Prior studies from the Argentine Basin have yielded evidence that suspended particles are displaced significant distances (100–1000 km) northward and downslope by strong surface and/or bottom currents. These transport processes result in anomalously cold alkenone-derived sea-surface temperature (SST) estimates (up to 6 °C colder than measured SST) and in the presence of frustules of Antarctic diatom species in surface sediments from this area. Here we examine advective transport processes through combined measurements of compound-specific radiocarbon ages of marine phytoplankton-derived biomarkers (alkenones) from core tops and excess ²³⁰Th (²³⁰Th_xs)-derived focusing factors for late Holocene sediments from the Argentine Basin. On the continental slope, we observe ²³⁰Th_xs-based focusing factors of 1.4–3.2 at sites where alkenone-based SST estimates were 4–6 °C colder than measured values. In contrast, alkenone radiocarbon data suggest coeval deposition of marine biomarkers and planktic foraminifera, as alkenones in core tops were younger than, or similar in age to, foraminifera. We therefore infer that the transport processes leading to the lateral displacement of these sediment components are rapid, and hence probably occur in the upper water column (<1500 m).