Mid Shelf Research Articles

Dinoflagellate cysts from the Middle Eocene to ?lowermost Oligocene succession in the Kysing research borehole, central Danish basin

Rich and well‐preserved assemblages of organic walled dinoflagellate cysts in 50 samples from a 154 m thick and almost complete, Middle Eocene to ?lowermost Oligocene section from the Kysing Research Borehole in eastern Jylland, Denmark are systematically described. One genus Costacysta gen. nov., and twelve species Chiropteridium eocaenicum sp. nov., Costacysta bucina gen. et sp.nov., Distatodinium pilosum sp. nov., Echinidiniuml lucidum sp.nov., Hapsocysta kysingensis sp.nov., Horologinellal pentagonalis sp.nov., Operculodinium eisenackii sp. nov., Phthanoperidinium cornutum sp. nov., Selenopemphix septum sp. nov., Svalbardella partimtabulata sp. nov., Thalassiphora gracilis sp. nov. and Thalassiphora microperforata sp. nov. are formally described. The genus Hapsocysta is emended, and is considered to be a senior synonym of Piccoladinium. The gradual change of the dinoflagellate cyst assemblages in this section demonstrates that sedimentation was almost continuous during the time span of ca. 12 Ma. The biostratigraphic resolution based on dinoflagellate cyst events is generally finer than the calcareous nannofossil NP biozonation. A comparison with published dinoflagellate cyst data from the Norwegian‐Greenland Sea shows that several important events are mainly synchronous throughout the region, suggesting a general similarity of the water masses. An isolated occurrence of Svalbardella suggests a short cooling event during early Bartonian times. The palynofacies in most of the section is almost purely of pelagic type. A more proximal, mid shelf palynofacies and a distinctive episode of reworking coincide with the Upper Eocene Moesgaard Clay and indicate a brief, but rather large sea level fall during the deposition of this unit. The Eocene/Oligocene boundary is tentatively identified near the base of the Viborg Formation, and approximately coincides with increased reworking of dinoflagellate cysts and a strong increase in bisaccate pollen, indicating more proximal conditions and possibly also a climatic cooling.

An early Paleogene sponge fauna, Chatham Island, New Zealand

A late Paleocene‐early Eocene (c. 60–53 Ma) poriferan fauna, comprising hexactinellids (Class Hexactinellida), astrophorids (Class Demospongiae: Family Astrophorida), and lithistids ("lithistid” Demospongiae) has been identified from the Tutuiri Greensand outcropping on the north coast of Chatham Island, New Zealand. Most of the fossils are hexactinellids, comprising extremely delicate siliceous networks embedded in friable sandstone. The sediment matrix within and around these skeletons contains numerous siliceous demosponge spicules, many of which are exceptionally well preserved. The soft friable matrix of the Tutuiri Greensand has made extraction a relatively simple process, making taxonomic identification of the material, and comparison with adjacent Recent and other New Zealand Eocene faunas possible. These sponge body fossils and spicule microfossils indicate a fauna that was once dominated by hexactinellids, lithistid, and astrophorid demosponges. A qualitative comparison of the abundance and diversity of the Tutuiri Greensand sponge fauna with the present‐day Chatham Rise sponge fauna indicates that the late Paleocene‐early Eocene fauna is as diverse as the Recent fauna, for the hexactinellid sponges and astrophorid demosponges, and much more diverse for lithistid sponges. The paleoecology of the Tutuiri Greensand has been interpreted as inner to mid shelf water depths (50–250 m) but the poriferan fauna described here is more like that of the present‐day soft sediment benthic environment of the Chatham Rise at 800–1200 m.

High-resolution coral records of rare earth elements in coastal seawater: biogeochemical cycling and a new environmental proxy

In this study we have used laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), to produce a high resolution coral record of rare earth elements (REE), Mn and Ba from coastal Porites corals from the Great Barrier Reef of Australia. Validation of the LA-ICP-MS technique indicated that the method provides accurate and reproducible (RSD = 13–18%) analysis of low concentration REE in corals (∼1 to 100 ppb). The REE composition in coral samples was found to closely reflect that of the surrounding seawater and distribution coefficients of ∼1–2 indicated minimal fractionation of the series during incorporation into coral carbonate. To explore the idea that coral records of REE can be used to investigate dissolved seawater composition, we analyzed two coastal corals representing a total of ∼30 yr of growth, including a 10-yr overlapping period. Comparable results were obtained from the two samples, particularly in terms of elemental ratios (Nd/Yb) and the Ce anomaly. Based on this evidence and results from the determination of distribution coefficients, we suggest that useful records of seawater REE composition can be obtained from coral carbonates. When compared to the REE composition of a mid shelf coral, coastal corals showed a significant terrestrial influence, characterized by higher REE concentrations (greater than 10 times) and light REE enrichment. The REE composition of coastal seawater inferred from the coral record was dependent on seasonal factors and the influence of flood waters. REE fractionation displayed a strong seasonal cycle that correlated closely with Mn concentration. We suggest that higher Nd/Yb ratios and higher Mn concentrations in summer result from scavenging of heavy REE by particulate organic ligands and Mn reductive dissolution respectively, both processes displaying higher rates during periods of high primary productivity. The Ce anomaly also displayed a strong seasonal cycle showing an enhanced anomaly during summer and during flood events. This is consistent with the Ce anomaly being primarily controlled by the abundance of Ce oxidizing bacteria. Based on these arguments, we suggest that the coral record of dissolved REE and Mn may be regarded as a useful proxy for biological activity in coastal seawater.

Late Quaternary core stratigraphy of the northern New South Wales continental shelf

On the southeast Australian continental margin, mixed siliciclastic and temperate carbonate sediments are presently forming along the narrow 20–35 km‐wide northern New South Wales shelf over an area of 4960 km2. Here, year‐round, highly energetic waves rework inner and mid‐shelf clastic sediments by northward longshore currents or waning storm flows. The strong East Australian Current flows south, sweeping clastic and outer shelf biogenic sands and gravels. Quaternary siliciclastic inner shelf cores consist of fine to medium, lower shoreface sand and graded storm beds of fine to coarse sand. Physically abraded, disarticulated molluscs such as Donacidae and Glycymeridae form isolated gravel lags. Highstand inner shelf clastics accumulate at 0.53 m/103 y in less than 50 m water depth. Clastic mid‐shelf cores contain well‐sorted, winnowed, medium shoreface sands, with a fine sand component. Fine sand and mud in this area is discharged mainly from New South Wales’ largest river, the Clarence. The seaward jutting of Byron Bay results in weakened East Australia Current flows through the mid‐shelf from Ballina to Yamba allowing the fine sediments to accumulate. Quaternary carbonate outer shelf cores have uniform and graded beds forming from the East Australian Current and are also influenced by less frequent storm energy. Modern clastic‐starved outer shelf hardgrounds are cemented by coralline algae and encrusting bryozoans. Clay‐sized particles are dominantly high‐Mg calcite with minor aragonite and smectite/kaolinite. Carbonate sands are rich in bryozoan fragments and sponge spicules. Distinctive (gravel‐sized) molluscs form isolated shells or shell lag deposits comprising Limopsidae and Pectinidae. The upper slope sediments are the only significant accumulation of surficial mud on the margin (18–36 wt%), filling the interstices of poorly sorted, biogenic gravels. Pectinid molluscs form a basal gravel lag. During highstand the outer shelf accumulates sediment at 0.40 m/103 y, with the upper slope accumulating a lower 0.23 m/103 y since transgression. Transgression produced a diachronous (14–10 ka) wave‐ravinement surface in all cores. Relict marine hardgrounds overlie the wave‐ravinement surface and are cemented by inorganic calcite from the shallow and warm East Australian Current. Transgressive estuarine deposits, oxygen isotope Stage 3–5 barriers or shallow bedrock underlie the wave‐ravinement surface on the inner and mid shelf. Northern New South Wales is an example of a low accommodation, wave‐ and oceanic current‐dominated margin that has produced mixed siliciclastic‐carbonate facies. Shelf ridge features that characterise many storm‐dominated margins are absent.

Late Quaternary core stratigraphy of the northern New South Wales continental shelf

On the southeast Australian continental margin, mixed siliciclastic and temperate carbonate sediments are presently forming along the narrow 20–35 km‐wide northern New South Wales shelf over an area of 4960 km2. Here, year‐round, highly energetic waves rework inner and mid‐shelf clastic sediments by northward longshore currents or waning storm flows. The strong East Australian Current flows south, sweeping clastic and outer shelf biogenic sands and gravels. Quaternary siliciclastic inner shelf cores consist of fine to medium, lower shoreface sand and graded storm beds of fine to coarse sand. Physically abraded, disarticulated molluscs such as Donacidae and Glycymeridae form isolated gravel lags. Highstand inner shelf clastics accumulate at 0.53 m/103 y in less than 50 m water depth. Clastic mid‐shelf cores contain well‐sorted, winnowed, medium shoreface sands, with a fine sand component. Fine sand and mud in this area is discharged mainly from New South Wales' largest river, the Clarence. The seaward jutting of Byron Bay results in weakened East Australia Current flows through the mid‐shelf from Ballina to Yamba allowing the fine sediments to accumulate. Quaternary carbonate outer shelf cores have uniform and graded beds forming from the East Australian Current and are also influenced by less frequent storm energy. Modern clastic‐starved outer shelf hardgrounds are cemented by coralline algae and encrusting bryozoans. Clay‐sized particles are dominantly high‐Mg calcite with minor aragonite and smectite/kaolinite. Carbonate sands are rich in bryozoan fragments and sponge spicules. Distinctive (gravel‐sized) molluscs form isolated shells or shell lag deposits comprising Limopsidae and Pectinidae. The upper slope sediments are the only significant accumulation of surficial mud on the margin (18–36 wt%), filling the interstices of poorly sorted, biogenic gravels. Pectinid molluscs form a basal gravel lag. During highstand the outer shelf accumulates sediment at 0.40 m/103 y, with the upper slope accumulating a lower 0.23 m/103 y since transgression. Transgression produced a diachronous (10–14 ka) wave‐ravinement surface in all cores. Relict marine hardgrounds overlie the wave‐ravinement surface and are cemented by inorganic calcite from the shallow and warm East Australian Current. Transgressive estuarine deposits, oxygen isotope Stage 3–5 barriers or shallow bedrock underlie the wave‐ravinement surface on the inner and mid shelf. Northern New South Wales is an example of a low accommodation, wave‐ and oceanic current‐dominated margin that has produced mixed siliciclastic–carbonate facies. Shelf ridge features that characterise many storm‐dominated margins are absent.

Hydrography, bacteria and protist communities across the continental shelf and shelf slope of the Andaman Sea (NE Indian Ocean)

The hydrography and plankton community structure was investigated in the Andaman Sea off Phuket, Thailand. Two cruises were conducted in 1996, one representing the calm dry NE monsoon season (March) and the other representing the stormy and rainy SW monsoon season (August). Sampling was performed along 3 transects perpendicular to the shelf break, from the coast across the shelf into deep water. The water column at the nearshore stations was vertically mixed, while the water column at off shore stations was strongly stratified, hence a frontal zone was estab- lished at the mid shelf. A prominent feature of the area was the pronounced internal wave centred around the pycnocline. The wave was observed from the outermost stations to the mid-shelf front. The height of the wave reached peak values of approximately 60 m in areas of approximately 300 m bottom depth. At all stations in stratified waters the vertical distribution of the phytoplankton showed a pronounced subsurface chl a peak in association with the pycnocline. The highest chl a values and primary production was observed at the front established at the mid shelf where the pycnocline meets the bottom, and salt nutrient-rich water is mixed up in the surface layer. We did not find any relationships between hydrography and the other key components of the microbial food web. No difference in productivity or food web structure was observed between the 2 seasons despite a significant difference in climatic forcing. Pico- and nanoplankton dominated the biomass in both seasons and Synechococcus contributed 72 to 74% of the biomass. Analysis of the microbial food web and establishment of carbon-flow budgets illustrates the importance of the microbial food web for making the primary producers available to the higher trophic levels.

Observations of large-amplitude cross-shore internal bores near the shelf break, Santa Monica Bay, CA

Two sets of moorings were deployed along a cross-shelf transect in central Santa Monica bay for four months in the winter of 1998-1999. Both sites had an array of instruments attached to tripods set on the seafloor to monitor currents over the entire water column, surface waves, near-bed temperature, water clarity and suspended sediment. A companion mooring had temperature sensors spaced approximately 10 m apart to measure temperature profiles between the surface and the seafloor. One array was deployed in 70 m of water at a site adjacent to the shelf break, just northwest of a major ocean outfall. The other was deployed on the mid shelf in 35 m of water approximately 6 km from the shelf break site. The subtidal currents in the region flowed parallel to the isobaths with fluctuating time scales around 10 days, a typical coastal-ocean pattern. However, during the falling phase of the barotropic spring tide, sets of large-amplitude, sheared cross-shore current pulses with a duration of 2-5 h were observed at the shelf break site. Currents in these pulses flowed exclusively offshore in a thin layer near the bed with amplitudes reaching 30-40 cm/s. Simultaneously, currents with amplitudes around 15-20 cm/s flowed exclusively onshore in the thicker layer between the offshore flow layer and the sea surface. The net offshore transport was about half the onshore transport. Near-surface isotherms were depressed 30-40 m. These pulses were likely internal bores generated by tidal currents. Bed stresses associated with these events exceeded 3 dynes/cm(2). These amplitudes are large enough to resuspend and transport not only fine-grained material, but also medium to coarse sands from the shelf toward the slope. Consequently, the seafloor over the shelf break was swept clear of fine sediments. The data suggest that the internal bores dissipate and are reduced in amplitude as they propagate across this relatively narrow shelf. There is evidence that they reach the 35 m site, but other coastal ocean processes obscure their distinctive characteristics.

Demographic plasticity in tropical reef fishes

We use age-based analyses to demonstrate consistent differences in growth, mortality, and longevity of coral reef fishes from similar habitats (exposed reef crests) 20 km apart. On outer-shelf reef crests of the northern Great Barrier Reef (GBR), size in four taxa of reef fishes (Chlorurus sordidus, Scarus frenatus, and S. niger and the acanthurid Acanthurus lineatus) was systematically and significantly smaller when compared with the same taxa on adjacent mid-shelf reef crests. Differences in size could be attributed to differences in growth between habitats (shelf positions). On outer reef crests the species examined had consistently lower size at age profiles and also reduced life spans compared with populations from mid-shelf reefs. To confirm this relationship, two of the most abundant species (C. sordidus and S. frenatus) were selected for more detailed spatial analysis of demographic patterns. Sampling adults of both taxa from reef crests on three mid- and three outer-shelf reefs revealed that most of the variation in growth was explained by shelf position, although C. sordidus also displayed differences in growth among mid-shelf reefs. We conclude that differences in body sizes across the continental shelf of the northern GBR are primarily determined by these trends in growth. Strong spatial patterns also existed in the mean ages of populations and longevity estimates for C. sordidus and S. frenatus between shelf positions. Both species on outer-shelf reefs displayed less variable cohort sizes, significantly reduced mean ages, and foreshortened longevity compared with populations on mid-shelf reefs. Furthermore, differences in these parameters were rare among replicate reefs within mid- and outer-continental-shelf positions. Age-based catch curves suggested that rates of S. frenatus natural mortality on the outer shelf were nearly twice as high as on the mid shelf. Visual surveys indicated that total scarid densities on outer-shelf reef crests are on average fourfold higher than for equivalent mid-shelf habitats. This fact, coupled with significantly reduced growth rates, reduced mean ages, and increased mortality rates, suggests that density-dependent processes may be responsible for observed differences among localities.

Near-bottom suspended sediment variability caused by storms and near-inertial internal waves on the Ebro mid continental shelf (NW Mediterranean)

Near-bottom currents and suspended sediment concentrations (SSC) recorded on the Ebro continental shelf at 60 m depth during the transition between fair and stormy weather have revealed some of the major processes controlling the sediment re-suspension and transport at mid shelf depths in this NW Mediterranean area. Storm waves were the main contributor to the transport of sediment, although they only resuspended surface sediments during extreme wave conditions ( H s >4.5 m and T s >11 s ). During the recording period, the net suspended sediment flux along the shelf was one order of magnitude higher than the net suspended sediment flux across the shelf, and the resultant transport was towards the SW and seaward. Fluctuations of the near-bottom SSC during fair weather showed a periodicity around 17 h, and were related to the occurrence of near-inertial internal waves caused by wind pulses during storm events. The energy propagation of these internal waves reached critical angles at the mid and outer shelf, which could favor the development and maintenance of the bottom nepheloid layer on the Ebro continental shelf, as well as, on other Mediterranean continental shelves.

Barotropic Tides on the North Carolina Shelf*

Observations from 18 near-bottom pressure sensors and 8 current meter moorings provide a characterization of the barotropic tides over the North Carolina continental shelf between Chesapeake Bay and Cape Hatteras. The largest tidal constituents in this region are the M2 (sea level amplitude 47 cm), N2 (11 cm), and S2 (10 cm) semidiurnal tides and the K1/P1 (7 cm) and O1 (5 cm) diurnal tides. The barotropic tidal currents are small, less than 3 cm s21, except for the M2 current which reaches 10 cm s21 at mid shelf. Cross-shelf currents increase linearly from approximately zero near the coast to a maximum over the mid and outer shelf. Alongshelf currents are nonzero near the coast and increase to roughly twice the coastal value over the mid and outer shelf. While bottom friction is weak over most of the shelf, in water depths of less than 10 m bottom friction results in a rapid phase decrease toward the coast in the semidiurnal alongshelf velocities. This phase difference is not observed in the K1/P1 alongshelf flow, possibly because diurnal wind forcing causes an opposing phase increase toward the coast. Alongshelf variations in tidal amplitudes and phases near the coast are much larger than expected based on variations along the shelf break inferred from basin-scale altimeter estimates. The shelf width in this region increases from 25 to 100 km over an alongshelf distance of about 150 km. Results from an analytic, flat-bottom, two-dimensional model indicate that widening of the shelf toward the north causes the observed alongshelf increase in the semidiurnal amplitude and phase, but does not explain alongshelf variations in the weaker diurnal constituents.

Spatial and temporal variations in abundance and distribution of juvenile and adult jackass morwong, Nemadactylus macropterus, in south-eastern Tasmania

Depth-stratified random demersal trawl surveys of the shelf of southern and eastern Tasmania from summer 1993 to summer 1995 showed seasonal variations in abundance of N. macropterus in all depth strata, although the trend varied between years, with abundance peaking in summer 1993 and winter 1994. This variation is attributed to the seasonal movement of fish from south-eastern Tasmanian shelf waters, although some seasonal change in size composition is also evident. Catch rates were highest on the outer shelf and lowest on the inner shelf, reflecting the distinct size-structuring, with juveniles concentrated on the inner and mid shelf and mature fish on the outer shelf. Size compositions differed among shelf regions, with few mature fish on the mid shelf of Storm Bay compared with the same stratum on the east coast, indicating that the bay is primarily a nursery area for the species. Settlement from the pelagic post-larval phase occurred in spring and early summer at 7–9 cm, with evidence of movement between inner-and mid-shelf strata by autumn. Abundance of 0+ fish was highest in 1993 and 1994 and a considerable decrease in the abundance in both regions in 1995. The influences of local hydrography and the large size at settlement are discussed.

Spatial variability in reef fish distribution, abundance, size and biomass: a multi-scale analysis

This study used underwater visual census techniques to quantify the distribution, abundance, fork lengths and biomass of scarid fishes on multiple reefs across the continental shelf of the northern Great Barrier Reef. Spatial patterns in fish distribution were examined over a cross shelf environmental gradient using a hierarchical sampling design that covered a spectrum of scales ranging from metres to tens of kilometres. The design included replicate reefs within mid and outer continental shelf positions and replicate sheltered and exposed sites within each reef. Most of the 21 species surveyed were found to be widely distributed across mid and outer shelf reefs and the number of species did not change significantly between reefs, exposures or across the shelf. Despite a similar complement of species, scarid assemblages differed markedly in the relative and absolute abundance of taxa between exposed and sheltered habitats on mid and outer shelf reefs. Principal components and cluster analyses indicated that changes in the numerical abundance of taxa resulted in consistently different assemblages from 9 sheltered mid shelf sites and 9 exposed outer shelf sites at opposite ends of the surveyed environmental gradient. Variance components calculated for scarid abundance and biomass across 4 spatial scales revealed differences in the scales at which significant variability occurred between sheltered and exposed reef habitats. On exposed reef crests significant variation occurred at both scales of tens of kilometres (shelf position) and hundreds of metres (sites), while on sheltered back reefs significant variation was attributable only to the site scale. Mean scarid abundance (±SE) for exposed outer shelf sites (3060 ± 460 ha-1) was on average 4 times higher than in all other habitat zones surveyed, while mean biomass (±SE) was only 3 times higher (920 ± 190 kg ha-1). The discrepancy between abundance and biomass estimates is explained by a 30% average decline in parrotfish fork lengths on exposed outer shelf reef crests relative to other habitats. Log-linear analysis indicated that at least 8 species of scarids displayed significant changes in body size according to their location across the continental shelf and/or the prevailing exposure regime. The observed patterns of variation in density and length frequency suggest density-dependent processes and that changes in mortality or growth rates may exist between habitats across the continental shelf.

Observations of storm and river flood-driven sediment transport on the northern California continental shelf

In the winter of 1996–1997, three bottom-boundary layer tripods were placed in an alongshelf array on the northern California continental shelf off Eureka, CA in 60–65-m water depth. During the observation period, multiple storms and river discharge events occurred, as well as the largest flood on record since 1964. Suspended-sediment concentration at all three sites fluctuated in response to both wave resuspension and advection of river-derived sediments. However, considerable spatial differences in low-frequency currents and suspended-sediment concentration were observed at the three sites. Sediment flux vectors calculated during periods of high suspended-sediment concentration suggest a convergence of sediment flux coincident with the center of recent flood deposits. Suspended-sediment concentrations observed at the two northern tripod sites following the large flood reached magnitudes typical of fluid mud (>10 g/l) in a thin near-bed layer. The net sediment flux during the single three-day event was two orders of magnitude larger than any other event during the winter, and accounted for seven times the flux observed over an entire year (1995–1996). A conceptual model for the advection of sediment to the mid shelf is proposed in which river plume sediments are trapped on the inner shelf either due to a weak front or the rapid input of sediment from a confined plume, and form a thin layer of fluid mud. The fluid mud subsequently is transported seaward due to gravitational forcing. The measured sediment concentration and velocity profiles on the shelf provide strong evidence to support this conclusion.

Biological and physical controls on dissolved dimethylsulfide over the north-eastern continental shelf of New Zealand

Data presented in this paper are part of an extensive investigation of the physics of cross-shelf water mass exchange in the north-east of New Zealand and its effect on biological processes. Levels of dissolved dimethylsulfide (DMS) were quantified in relation to physical processes and phytoplankton biomass. Measurements were made at three main sites over the north-east continental shelf of New Zealand's North Island during a current-driven upwelling event in late spring 1996 (October) and an oceanic surface water intrusion event in summer 1997 (January). DMS concentrations in the euphotic zone ranged between 0.4 and 12.9nmoldm−3. Integrated water column DMS concentrations ranged from 33 to 173μmolm−2 in late spring during the higher biomass (15–62 Chl-a mgm−2) month of October, and from 25 to 38μmolm−2 in summer during the generally lower biomass (16–42 Chl-a mgm−2) month of January. We observed high levels of DMS in the surface waters at an Inner Shelf site in association with a Noctiluca scintillans bloom which is likely to have enhanced lysis of DMSP-producing algal cells during phagotrophy. Integrated DMS concentrations increased three-fold at a Mid Shelf site over a period of a week in conjunction with a doubling of algal biomass. A high correlation (r2=0.911, significant <0.001) of integrated DMS and chlorophyll-a concentrations for compiled data from all stations indicated that chlorophyll-a biomass may be a reasonable predictor of DMS in this region, even under highly variable hydrographic conditions. Integrated bacterial production was inversely correlated to DMS production, indicating active bacterial consumption of DMS and/or its precursor.

The palaeogeographic and palaeoenvironmental evolution of a Palaeogene mixed carbonate–siliciclastic cool-water succession in the Otway Basin, Southeast Australia

The Otway Basin in southeast Australia contains a thick sequence of Cenozoic shelfal carbonates and siliciclastics that preserve signals relating to the progressive opening of the Southern Ocean since the Paleogene. This multidisciplinary study integrates outcrop and subsurface well data from over 100 wells and bores throughout the Otway Basin with micropalaeontological analyses to constrain the age and palaeoenvironments of the Nirranda Group (Late Eocene to Middle Oligocene) and the Heytesbury Group (Late Oligocene to mid Miocene). These data were used to deduce the Late Eocene to Late Oligocene palaeogeographical evolution of the area. During the Late Eocene paralic high energy siliciclastic shoreline to shelf facies dominated the region, deepening southwards where mid to outer shelf conditions preserved high energy sandy carbonate facies. Above the Eocene–Oligocene boundary low energy inner to mid shelfal silt and muddy sand persisted to the north, deepening southward to carbonate-dominated low energy outer shelf to bathyal marls. The change from siliciclastics to carbonates at the Eocene–Oligocene boundary in the Otway Basin may relate to regional tectonics. In the Early Oligocene, high energy inner to outer shelf sand bodies formed in front of marine to inner shelf mudstone facies; the sand units are likely to have been influenced by strong local longshore drift and ocean swells that increased as the Southern Ocean widened to create a larger fetch. In the eastern half of the basin, later in the Early Oligocene, mixed paralic to inner shelf siliciclastic and carbonate facies were deposited passing to inner to mid shelf marl and mudstone and outer shelf to bathyal marls basinward. During this time, low to high energy shelfal calcarenite, chalk and marl dominated the westerly edge of the basin. The contrast in facies from west and east in the basin is inferred to be due to contrasting terrigenous input, environmental energy and ramp/shelf geometry. By Late Oligocene times (the Clifton Formation) the Otway Basin was dominated by high energy carbonate facies deposited in mid to outer shelf palaeoenvironments. The base of the Clifton Formation preserves a shift in facies and foraminiferal faunas that correlates to the major sea level fall at the Early–Late Oligocene boundary. This sea level fall is related to a major ice advance in Antarctica that corresponds to mid-Oligocene unconformities globally. The switch from low to high energy facies across the Early–Late Oligocene boundary in the Otway Basin suggests that the Southern Ocean swells experienced by the modern Otway coast were well established by Middle Oligocene time, evidence of the strengthening ‘proto‘ Antarctic swell regime. By Early Miocene times, with final deepening of the Drake Passage, the Antarctic Circum-Polar Current formed and predominantly inner to outer shelf marls and limestones were deposited in the Otway Basin.

High-resolution swath sonar investigation of sand ridge, dune and ribbon morphology in the offshore environment of the New Jersey margin

Sand ridges on the northeast US Atlantic shelf form in the near-shore environment, most likely in response to storm-driven flows. As the Holocene transgression has continued, the ridges have been transferred to an offshore hydrodynamic regime, where currents are not constrained by the coast and storms do not influence bottom currents as frequently or as strongly. Here, we qualitatively and quantitatively investigate the morphology of offshore sand ridges and smaller-scale features in an effort to place constraints on bedform development in these deeper waters. A recent high-resolution swath sonar survey mapped a portion of the New Jersey shelf from ∼20 m water depth to the shelf break (∼120 m), imaging both sand ridges and smaller-scaled dunes and ribbons in far greater detail than has been previously possible. Using a robust statistical analysis, we find that the gross morphology of ridges (height, width, length) does not change with depth beyond ∼20 m water depth, and changes in ridge orientation generally mirror changes in regional contour orientation. Hence, ridges have not continued to grow since transgression has brought them into the offshore hydrodynamic regime. However, on the inner shelf (∼20 m water depth to the Mid-Shelf shore), we do find evidence in the ridge shape, which has an asymmetry opposite to that seen near shore, and in the complex backscatter response that some important modifications to ridges are taking place at these water depths. In contrast, on the mid-shelf (from the Mid-Shelf shore to the Franklin shore), ridges tend to have higher backscatter at the crests, implying that these are largely winnowed, relict features. Lineated, smaller-scale (∼100–500 m wavelength, <1 m rms height) features are also present in the seafloor morphology. On the inner shelf these are ∼North-trending, and are evidently transverse to an onshore-directed flow (dunes), whereas on the mid shelf these are ∼NE/SW-trending and are parallel to a SW-directed flow (ribbons). The inner shelf dunes frequently exist on the seaward flanks of ridges, whereas the mid-shelf ribbons only inhabit large NE/SW-oriented swales between clusters of ∼ENE-trending ridges. Morphologic and seismic evidence suggests that the ribbon-floored swales represent erosional intrusions into the otherwise relict ridge morphology. Also present on the mid-shelf are asymmetric blow-out pits and transverse dune packets, both of which are consistent with a SW-directed flow. Relict iceberg scours are present on the outer shelf Hudson Apron, evidently preserved by outcropping stiff clays, and are overlain in some places by ∼ENE-trending ridges.

Mesoscale estimations of primary production in shelf waters: a case study in the Golfo Artabro (NW Spain)

Primary production was estimated in an area of ∼500 nm 2 (1645 km 2) in the Golfo Artabro (Galicia, NW Spain) during March 1993. Estimations were made either using vertical profiles of simulated `in situ' incubations of water samples, determination of photosynthetic parameters at the surface and the bottom of the euphotic zone (100% and 1% of surface irradiance, respectively) and estimations made entirely from variables measured at the sea surface. Complementary measurements included vertical profiles of temperature, salinity, irradiance and chlorophyll- a concentration at 38 stations regularly distributed over the continental shelf, and phytoplankton species composition at 10 stations. The study area was divided in six zones according to the hydrography and chlorophyll concentration, each zone including at least one station in which vertical profiles of primary production and photosynthetic parameters were measured. A synoptic map of water column integrated estimations of daily primary production was computed using average values of photosynthetic parameters in each zone and the measured chlorophyll concentrations. The observed situation corresponded to a period between spring blooms, in which a previous bloom dominated by the diatom Chaetoceros socialis was disrupted by a change in the meteorological conditions and the intrusion of a saline water body into the shelf. The saline intrusion caused the formation of a thermohaline front in mid shelf, separating slightly stratified outer shelf waters from mixed inner shelf waters. A new bloom started in the outer shelf side of the front and also in some locations near the coast, while the mid shelf area had the lowest chlorophyll concentrations and primary production rates. Maximum values of primary production were equivalent to those measured during upwelling events although non-upwelling conditions prevailed during the study. Meanwhile phytoplankton species composition was relatively uniform over the shelf. This study illustrates the feasibility of mapping primary production in relatively large shelf areas using photosynthetic parameters and vertical profiles of irradiance and chlorophyll in selected subareas. The employed method can be easily combined with remotely sensed data to produce large scale estimations. However, parallel estimations of primary production with other methods are needed to obtain appropriate interconversion factors and allow for comparison of production values between different datasets or computation models.

Cyclostratigraphy of middle Pliocene mid shelf to upper slope strata, eastern Wanganui Basin (New Zealand): correlations to the deep sea isotope record

Rangitikei Valley in eastern Wanganui Basin contains a well exposed 750-m thick middle Pliocene succession that accumulated during the Gauss Chron. Traditional facies analysis involving laboratory grain size determinations and benthic foraminiferal census on a suite of closely spaced (∼5 m) samples, show that the Utiku Group (lower 350 m) accumulated predominantly in a mid-shelf environment; the overlying Mangaweka Mudstone (400 m thick) accumulated in an outer shelf to upper slope environment. Combined with the identification of sequence stratigraphic surfaces (sequence boundary, transgressive surface of erosion and downlap surface) four sequences have been identified in the Utiku Group. In the outwardly massive Mangaweka Mudstone there is no outcrop evidence for sequences; seven sedimentary cycles are nevertheless defined by changes in grain size and by variations in foraminiferal paleobathymetry. The Utiku Group sequences show mostly asymmetric sequence architecture, with comparatively thin TSTs. Rapid deepening across the Utiku Group-Mangaweka Mudstone boundary at 3.0 Ma, involving an increase in water depth of at least 100 m that persisted until 2.55 Ma, orignated tectonically, probably as a result of plate boundary interactions. This is the second of two known rapid deepening tectono-sedimentary events, each separated by about 1.5 My. Higher order sea-level cyclicity, involving 11 sedimentary cycles as presently defined, are superimposed upon this second tectono-sedimentary cycle. Independent magnetostratigraphies allow the sedimentary cycles to be correlated to the astronomically tuned oxygen isotope stratigraphy for ODP Site 846. For the interval 3.55-3.0 Ma the sedimentary cyclicity reflects the periodicities of the more major enriched δ 18O peaks, which range from 41 ka (MG2 to M2), to 210 ka (MG6 to MG2). Thus the duration of cyclicty in the Utiku Group closely corresponds to the duration between significant sea-level lowering events, which are marked by glacial δ 18O shifts of > 0.4 ‰ on the ODP Site 846 benthic δ 18O curve.

Seasonal surface ocean variability in the South Atlantic Bight derived from CZCS and AVHRR imagery

Spatial and temporal variability of sea-surface temperature (SST) and surface chlorophyll derived from the AVHRR and CZCS sensors was examined using a 5-year (1981–1986) time series of 866 SST and 372 CZCS images averaged into monthly mean fields. Five-year mean CZCS-Chl and SST mean fields showed similar patterns, which closely followed the shelf and slope water bathymetry. Along-shelf surface pigment variability is greatest in the near-shore waters and decreases offshore. Monthly mean CZCS-Chl and SST showed consistent cross-shelf gradients with near-shore surface waters cooler and more pigment-laden than offshore waters and with summer months warmer and with lower CZCS-Chl concentrations compared to winter. Harmonic regression analysis of monthly (59 months) CZCS-Chl and SST vs time showed that the seasonal cycle explains the highest proportion (> 80%) of CZCS-Chl variability in shelf waters from Cape Fear, S.C. to Cape Lookout, N.C., with highest concentrations observed during colder months of the year (ca November through March). The seasonal signal also dominates SST in this region explaining approximately 80–90% of the variability in the 5-year time series. Seasonal cycles of CZCS-Chl and SST are inversely related with the highest correlation observed south of Cape Hatteras and relatively poor correlation north of Cape Hatteras. In general, the seasonal cycle explained more of the variability in near-shore and mid shelf waters than in waters near the shelf-break. © 1997 Elsevier Science Ltd

Patterns of zonation of assemblages of the Lutjanidae, Lethrinidae and Serranidae (Epinephelinae) within and among mid-shelf and outer-shelf reefs in the central Great Barrier Reef

Visual censuses were used to quantify the distribution and abundance of the Lutjanidae, Lethrinidae and Serranidae in three reef zones (windward reef slope, lagoon, back reef) of three reefs on the mid shelf and three reefs on the outer continental shelf in the central region of the Great Barrier Reef. Significant spatial variability was identified in the abundances of many species between locations (outer-shelf and mid-shelf communities), between reefs (high-abundance v. low-abundance reefs), and within reefs (characteristic communities within zones on individual reefs). Species of the Lutjanidae, Lethrinidae and Serranidae were found to occur in assemblages that were characteristic of major zones (windward reef slopes, lagoons and leeward back reefs), this pattern being consistent within and among shelf locations. Care will need to be taken in determining the appropriate spatial scales of sampling in any future experiments (e.g. manipulations of fishing pressure) to ensure that the effects of smaller-scale spatial differences are not confounded when larger-scale comparisons are made.