Circumpolar Current Research Articles

Fleshy‐fruitedness in the New Zealand flora

Summary AimIt has been claimed that the New Zealand flora has an unusually high frequency of fleshy‐fruitedness. This paper tests whether fleshy‐fruitedness is indeed more common in New Zealand than in other temperate floras, then examines the distribution of fleshy‐fruitedness among taxa and floristic elements to determine whether the flora conforms to predictions for a continental island with a relictual floristic element. Lastly, I test the extent to which fleshy‐fruitedness has influenced colonization success and subsequent speciation within New Zealand. MethodsInformation on fruit characteristics for all indigenous seed plants was extracted from the Flora of New Zealand series and analysed with χ2 tests. ResultsContrary to previous claims fleshy‐fruitedness was not unusually common in the New Zealand flora as a whole, when compared with other temperate floras. It is only more common in alpine communities and among trees. I also found no evidence for selective immigration; fleshy‐fruited New Zealand genera were not more likely, than dry‐fruited genera, to also occur in Australia. Furthermore there is no evidence that the New Zealand environment has favoured fleshy‐fruited taxa; there has been no autochthonous evolution of fleshy‐fruitedness in New Zealand, fleshy‐fruitedness has had no significant effect on speciation within New Zealand, and endemic genera are no more likely to be fleshy‐fruited than nonendemic genera. Fleshy‐fruitedness in New Zealand is, however, strongly related to floristic elements of the flora. New Zealand is a continental island and therefore, theoretically, those elements of the flora dating from a time when the landmass was less isolated, should show a more balanced representation of dispersal modes. Contrary to this, fleshy‐fruitedness is more common among species in Gondwanan taxa or in taxa with pollen records dating to before the Miocene. Main conclusionsFleshy‐fruitedness in New Zealand conforms to neither the expectations for an isolated landmasses, namely a disharmonic range of dispersal modes, nor expectations for a continental island. I suggest that this pattern may be a product of selective survival of highly vagile taxa in the low‐lying archipelago that was New Zealand during the late Cretaceous to mid‐Cenozoic, followed by an invasion by taxa with a broader range of dispersal modes facilitated by the establishment of the circumpolar current.

Influence of the Atlantic sector of the circumpolar region on the dynamics and structure of oceanic waters

By using the generalized data of measurements we determine the velocity of the Intermediary Subantarctic Current in the South and Tropical Atlantic. The mean value of this velocity is equal to 10cm/sec. According to indirect estimates, the velocity of the Bottom Antarctic Current is assumed to be equal to 4 cm/sec. The theoretical description of theT-, S-, and O2-fields on the basis of the solution of the problem of diffusion, from a periodic linear source in a moving anisotropic medium allows us to determine the amplitude of annual variations of the temperature of intermediate waters and estimate the coefficient of vertical diffusion in the layers of intermediate and bottom water masses. The basic hydrological characteristics of these waters are determined by the hydrometeorological processes running in the frontal zones of the south and north parts of the circumpolar current.

Microprotozooplankton distribution and their potential grazing impact in the antarctic circumpolar current

The abundance and composition of microphyto- and microprotozooplankton were studied in late October 1992 along a meridional transect (6°W) extending from the ice edge (56°S) in the southern Antarctic Circumpolar Current (ACC) to the Polar Front region (PFr) between 51 and 47°S. Microphytoplankton standing stocks were low in the southern ACC, but attained bloom proportions in the PFr where diatoms dominated. Microprotozooplankton standing stocks varied from 197 to 434 mgC m −2 in the southern ACC and 282 to 665 mgC m −2 in the PFr. The composition of the ciliate community and the size structure of microprotozooplankton assemblages differed significantly between the southern ACC and the PFr, whereas there was no significant difference between the ice edge and the open southern ACC water. In the southern ACC, 45% of microprotistplankton standing stocks was composed of heterotrophs (18% flagellates and 27% ciliates), with an important fraction of protozoa larger than 40 μm. In the PFr, heterotrophs r represented only 16% of microprotistplankton standing stock (12% flagellates and 4% ciliates) with the smaller size fraction (20–40 μm) largely dominating. Grazing by microprotozooplankton, estimated by converting counts into ingestion rates using literature values, ranged from 11 to 54% and 4 to 56% of primary production in the southern ACC and PFr, respectively. These grazing estimates and the importance of larger protozoa (> 40 μm) in the southern ACC suggest that they can play a significant role in controlling blooms of diatoms. Further, factors other than food supply seem to determine both the size structure and composition of microprotozooplankton assemblages.

Ecology and biogeochemistry of the antarctic circumpolar current during austral spring: a summary of southern ocean JGOFS cruise ANT X/6 of R.V. Polarstern

The R.V. Polarstern cruise ANT X/6, part of the international Southern Ocean JGOFS programme, investigated phytoplankton spring bloom development and its biogeochemical effects in different water masses of the Atlantic sector of the Southern Ocean: the Polar Frontal region (PFr), the southern Antarctic Circumpolar Current zone (sACC), its boundary with the Weddell Gyre (AWB) and the marginal ice zone (MIZ). The relative roles of physical stability, iron limitation and grazing pressure in enhancing or constraining phytoplankton biomass accumulation were examined. Three sections were carried out between the PFr and the ice edge along the 6°W meridian from early October to late November 1992. This paper summarises the major findings of the cruise and discusses their implications for our understanding of Southern Ocean ecology and biogeochemistry. A major finding was the negligible build-up of plankton biomass and concomitant absence of CO 2 drawdown associated with seasonal retreat of the ice cover. In striking contrast to this unexpected poverty of both the MIZ and the frontal region of the AWB, distinct phytoplankton blooms, dominated by different diatom species, accumulated in the PFr. Chlorophyll stocks in the sACC remained monotonously low throughout the study. Our findings confirm those of other studies that frontal regions are the major productive sites in the Southern Ocean and that input of meltwater and associated ice algae to the surface layer from a retreating ice edge is by itself an insufficient condition for induction of phytoplankton blooms. The blooms in the PFr developed under conditions of shallow mixing layers, high iron concentrations and relatively low grazing pressure. However, in all three blooms, high biomass extended to deeper than 70 m, which cannot be explained by either in situ growth or sinking out of a part of the population from the upper euphotic zone. Subduction of adjoining, shallower layers to explain depth distribution is invoked. Despite a clear CO 2 drawdown in the Polar Frontal region, the highly variable conditions encountered render reliable estimation of annual CO 2 fluxes in the Southern Ocean difficult.

The distribution of Fe in the antarctic circumpolar current

The large-scale distributions of dissolved and total Fe in surface and deep waters of the Antarctic Circumpolar Current exhibit strong relationships with hydrography and biological processes. The mean dissolved Fe concentrations are low in surface waters of the Antarctic Circumpolar Current (0.31–0.49 nM, with a minimum of 0.17 nM) and higher (averaging 1.11.9 nM) in the Polar Frontal region. Enhanced dissolved surface water concentrations in the Polar Frontal region are attributed to input from the continental shelf and coincide with phytoplankton spring blooms of large diatoms. The effects of sea-ice melting and iceberg melting on the Fe concentrations were relatively small. Dissolved deep-water concentrations ( > 400 m) in the Antarctic Circumpolar Current ranged from 0.4 to 2.8 nM. Circumpolar Deep Water has relatively high dissolved Fe concentrations in the Polar Frontal region (0.4–2.8 nM) compared with deep waters further to the south (0.6–1.1 nM). Similarly, total dissolvable (unfiltered) Fe concentrations in the Upper Circumpolar Deep Water tend to decrease southward from the Polar Frontal region. In the Lower Circumpolar Deep Water total dissolvable Fe concentrations are higher than in the Upper Circumpolar Deep Water due to the existing nepheloid layer and sources on the Mid-Atlantic Ridge. Dissolved and total dissolvable Fe concentrations in the Antarctic Bottom Water are higher than those of other water masses in the Antarctic Circumpolar Current, consistent with the nepheloid layer as well as diagenetic input from shelf sediments. The High-Nutrient/Low-Chlorophyll areas of the Antarctic Ocean and northeast Pacific Ocean have different major Fe input sources of similar magnitude. In the Antarctic Circumpolar Current upward transport of Fe is the main input source, whereas in the North Pacific Ocean, aerosols are the dominant source.

On the origins of deep and bottom waters of the Indian Ocean

The characteristics of the deep and bottom waters of the Indian Ocean, when illustrated on potential‐density anomaly surfaces, indicate that the waters enter from both the Atlantic and Pacific Oceans. The paths of spreading are constrained by the complex topography, and characteristics are seen to be altered by exchange with the overlying and underlying water and with the sediments, especially in the northern Indian Ocean. The Weddell Sea contributes to the densest waters found in the western basins and the Ross Sea and Adelie coast to the densest waters found in the eastern basins. Both dense water varieties are altered by and incorporated in the less dense water above; initially, water carried by the circumpolar current, then water from the north Atlantic, and finally by deep water whose characteristics are derived in the northern Indian Ocean. Contact with the sediments increases the silica content of the bottom water in the Southern Ocean. In the northern Indian Ocean the sediments alter the silica of the water at the bottom and, together with enhanced salinity from diffusion of saline overflows from the marginal seas above, imprint unique markers to the deep water that flows back to the south. At middepths the series of ridges between Madagascar and Australia confine the flow to a series of gyres that carry characteristics from the circumpolar current equatorward and the northern Indian Ocean characteristics southward. Within the circumpolar current, low‐oxygen deep water from the Pacific is carried across the Atlantic and into the Indian Ocean south of Africa. Part flows around the cyclonic Weddell Sea Gyre, and part extends across the Southern Ocean. Water from another Pacific source can be seen near 2000 m extending westward from the Tasman Sea, south of Australia and across the Indian Ocean, and perhaps to the Aghulas Current region southeast of Africa.

The South Atlantic in the Fine-Resolution Antarctic Model

Abstract. The geographical area covered by the Fine-Resolution Antarctic Model (FRAM) includes that part of the South Atlantic south of 24°S. A description of the dynamics and thermodynamics of this region of the model is presented. Both the mean and eddy fields in the model are in good agreement with reality, although the magnitude of the transients is somewhat reduced. The heat flux is northward and in broad agreement with many other estimates. Agulhas eddies are formed by the model and propagate westward into the Atlantic providing a mechanism for fluxing heat from the Indian Ocean. The confluence of the Brazil and Falkland currents produces a strong front and a large amount of mesoscale activity. In the less stratified regions to the south, topographic steering of the Antarctic circumpolar current is important.

Modeling of topographic effects on Antarctic Sea ice using multivariate adaptive regression splines

The role of seafloor topography in the spatial variations of the southern ocean sea ice cover as observed (every other day) by the Nimbus 7 scanning multichannel microwave radiometer satellite in the years 1980, 1983, and 1984 is studied. Bottom bathymetry can affect sea ice surface characteristics because of the basically barotropic circulation of the ocean south of the Antarctic Circumpolar current. The main statistical tool used to quantify this effect is a local nonparametric regression model of sea ice concentration as a function of the depth and its first two derivatives in both meridional and zonal directions. First, we model the relationship of bathymetry to sea ice concentration in two study areas, one over the Maud Rise and the other over the Ross Sea shelf region. The multiple correlation coefficient is found to average 44% in the Maud Rise study area and 62% in the Ross Sea study area over the years 1980, 1983, and 1984. Second, a strategy of dividing the entire Antarctic region into an overlapping mosaic of small areas, or windows, is considered. Keeping the windows small reduces the correlation of bathymetry with other factors such as wind, sea temperature, and distance to the continent. We find that although the form of the model varies from window to window due to the changing role of other relevant environmental variables, we are left with a spatially consistent ordering of the relative importance of the topographic predictors. For a set of three representative days in the Austral winter of 1980, the analysis shows that an average of 54% of the spatial variation in sea ice concentration over the entire ice cover can be attributed to topographic variables. The results thus support the hypothesis that there is a sea ice to bottom bathymetry link. However, this should not undermine the considerable influence of wind, current, and temperature which affect the ice distribution directly and are partly responsible for the observed bathymetric effects.

Physical model of bathymetric effects on the Antarctic circumpolar current

Laboratory experiments were conducted to simulate some of the effects of the bathymetry of the southern ocean on the physical characteristics of the Antarctic Circumpolar Current (ACC). An idealized zonal wind stress, which varied inversely with the distance from the model Antarctic continent, was simulated in the laboratory model by a radially inward sink‐source flow in a thin layer along the surface of the circular test cell. The experiments demonstrated the feasibility of simulating the surface wind stress in physical models by source‐sink forcing. The present model, however, has the limitation of not accounting for such factors as the longitudinal variations in the wind shear and the decrease in wind stress on approaching the Antarctic continent from the north. Planetary beta effects were neglected because the topographic beta term can be shown to dominate over large portions of the model area. The neglect of beta effects is also a limitation of the model. In spite of these limitations, however, the simulations of the physical model for both the homogeneous and linearly stratified cases were shown to be in good agreement with observations of the ACC. These include well‐defined strong currents along the mid‐ocean ridge; strong perturbations in the vicinity of the Macquarie Ridge, Campbell Plateau, and Kerguelen Gaussberg Plateau; strong meridional transport to the east of the Drake Passage; and anomalies to the south (wave troughs) and to the north (wave ridges) of the main circumpolar current over ocean basins and mountain ridges, respectively. It was shown that the Eltanin and Udintsev fracture zones in the vicinity of 135°W are important factors in directing the ACC eastward across the Southeast Pacific Basin to the Drake Passage. The model results suggest that these fracture zones can influence the nature of the flow in the western Atlantic because of their control of the flow through the Drake Passage. The estimated volume transports through the Drake Passage based on the model results are in fair agreement with oceanic observations. Estimates of the spin‐up time of the system for homogeneous and stratified cases have been provided.

Managing Southern Ocean krill and fish stocks in a changing environment

Management of Antarctic krill,Euphausia superba, and the mackerel icefish,Champsocephalus gunnariunder the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR) is discussed in relation to changes in their distribution and abundance arising from variation in circulation of the circumpolar current. It is concluded that on a Southern Ocean scale it is currently not possible to detect change but on a local scale, such as at South Georgia, major changes are detectable. These changes affect the krill fishery directly in terms of total catch and the way the fleets are deployed. Major local reductions in krill are thought to have a significant effect on natural mortality of the icefish.

Proton flares in the variation of the auroral electrojet in solar cycle no 21

The effects of the solar proton flare product on ionospheric circumpolar currents (Auroral Electrojet Index) for the interval 1978–1986 (solar cycle No 21) are investigated. A significant increase of the AE two days after the zero-day of appearance of the proton flare was displayed.

The South Indian Ocean Current

In this paper, the historical hydrographic database for the south Indian Ocean is used to investigate (i) the hydrographic boundary between the subtropical gyre and the Antarctic Circumpolar Current (ACC), the sub-tropical front (STF), and especially (ii) the southern current band of the gyre. A current band of increased zonal speeds in the upper 1000 m is found just north of the STF in the west near South Africa and at the surface STF in the open Indian Ocean until the waters off the coast of Australia are reached. As neither any other investigation of this current nor a name for it are known, the flow has been called the South Indian Ocean Current (SIOC). This name is anologous to the same current band in the South Atlantic Ocean, the South Atlantic Current. The STF is located in the entire south Indian Ocean near 40°S. The associated current band of increased zonal speeds is the SIOC, which is found at or north of the STF. East of 100°E the SIOC separates from the STF and continues to the northeast. The zonal flow south of the STF is normally weak and serves to separate the South Indian Ocean and Circumpolar currents. New Africa the SIOC has a typical volume transport of 60 Sv (1 Sv ≡ 106 m33 s−1) in the upper 1000 m relative to deep potential density surfaces of σ4 = 45.87 kg m−3 (2800–3500 m) or σ2 = 36.94 kg m−3 (1500–2500 m). Near western Australia the SIOC is reduced to about 10 Sv as it turns to the northeast.

GEOSECS Pacific and Indian Ocean 32Si profiles

Results of measurements of twelve 32Si vertical profiles, nine from the Pacific Ocean at latitudes 45°N–58°S, and three from the Indian Ocean between the Equator and 38°S are presented. The amounts of in-situ extracted SiO 2 range from ∼ 1 to 25 g. The volumes of water from which dissolved silicon was extracted range from 200 to 9 × 10 5 kg. The net 32P activities range from 0.7 to 3.8 cph. It is possible to measure accurately 32Si ( 32P) activities as low as 2 × 10 −2 dpm from 25 g SiO 2 with the present techniques. The 32Si concentrations in water range from 0.1 dpm/10 6 kg seawater to 178 dpm/10 6 kg seawater. The overall pattern of 32Si increase with depth in the oceans resembles that of Si but the two differ appreciably; the enrichment of the former is controlled by its relatively short half-life. The 32Si/SiO 2 ratios vary from ∼ 1 in deeper waters ( > 1000 m) to 81.5 dpm/kg SiO 2 in the surface waters. Three depth variation patterns are observed in the Pacific and Indian oceans: (i) a low ratio, ∼ 10 dpm/kg SiO 2 varying within a factor of two between the surface and ∼ 5000 m depth; (ii) a monotonic increase from a high surface value (> 10 dpm/kg SiO 2) to a low value ( ∼ 5 dpm/kg SiO 2) at ∼ 3000 m beyond which the value either remains constant or increases slightly with depth; and (iii) in half of the cases, the ratios increase with depth in the depth interval 1–3 km. The above patterns do not show any latitudinal variation. The column inventories of 32Si and Si in the north Pacific show a rather similar latitudinal variation, which is a coincidence. The column inventories of 32Si show a pronounced peak at ∼ 35°N/S in the Pacific and Indian oceans that must be attributed to an increased tropospheric fallout in the spring injections in mid-latitudes. This is also expected in view of the short half-life of 32Si. A similar feature was observed in the south Atlantic Ocean. Now that the 32Si measurements are available for the three major oceans, the oceanic budget of 32Si can be estimated. There are ∼ 920 × 10 14 dpm of 32Si in the major oceans which corresponds to a global-average-production rate of ≥ 4.3 × 10 −4 atoms 32Si/cm 2 s. Furthermore, since the atmospheric fall out of 32Si has been determined, we can obtain a fairly accurate half-life of 32Si. The geochemical half-life is estimated to be ≥ 120 yrs, in good agreement with the best value of 140 yrs based on other recent estimates. The lower limit reflects on the present uncertainty in the size and lifetime of the transient silicon pool at the water-sediment interface. From the observed decrease in the water column-averaged 32Si/SiO 2 ratios from the Circumpolar Atlantic waters to their Pacific counterparts, we deduce an apparent velocity of 55 km/yr for the Circumpolar current. Using a one-dimensional model, the vertical advection velocity ( w) and turbulent diffusion coefficient ( K) have been deduced using both the 32Si and 14C data. The values of w range from 1.3 to 15 m/yr and of K from 0.5 to 8 cm 2/s, except for the Somali basin where we obtain: w = 53m/yr and K = 16cm 2/s . The 32Si-based values are about 3 times higher than those based on 14C.

Spatial and temporal variability of global surface solar irradiance

A fast scheme for computing surface solar irradiance using data from the International Satellite Cloud Climatology Project (ISCCP) is described. Daily mean solar irradiances from the fast scheme reproduce the detailed global results from full radiative transfer model calculations to within 6 and 10 W m−2 over the ocean and land, respectively. In particular, the fast scheme reproduces the same dependence of surface irradiance on solar zenith angle which is critical for proper calculation of daily, seasonal, and latitudinal variability. Validation of both model results is limited because globally distributed data sets of high quality are lacking, particularly over the oceans. However, comparison of calculated monthly mean results using 5 months of ISCCP data (July 1983 to July 1984) with climatology from the 1970s at six temperate latitude ocean weather stations shows agreement within published estimates of interannual variability of monthly means at the individual stations. A further test against a 17‐day time series at a continental site (43°N, 90°W, October–November 1986; 13–170 W m−2 range of irradiance), where ground and satellite data were spatially and temporally coincident, showed an accuracy of better than 9 W m−2 on a daily basis and less than 4% bias in the 17‐day mean. Frequently used bulk formulae for solar irradiance were also evaluated in each of these tests. All suffered in comparison because they did not include a parameterization of the effects of the global variability in mean cloud optical thickness. Data from July (1983 and 1984) and January (1984 and 1985) were used to examine the spatial and temporal variability of surface irradiance and its potential impact on biospheric processes. Results show that the oceans and land experience fundamentally different light regimes, with continents receiving significantly greater irradiance. In summer, major interocean differences in zonally averaged irradiance are found in the northern hemisphere with the Atlantic greater than Pacific by up to 80 W m−2; in the southern hemisphere, interocean differences are small. Regional interannual variability (July 1983 versus 1984) ranged between +100 and −100 W m−2. The variability, perhaps due to the 1982–1983 El Niño event, occurred mostly in the Pacific but extended beyond the tropics over the entire north Pacific basin. The nutrient‐rich northern and southern ocean waters are almost perpetually cloud covered; however, there is a correspondence between higher than average surface irradiance and productivity in nutrient‐rich areas of the southwest Atlantic and Weddell Sea sector of the circumpolar current. This suggests that solar irradiance must be considered as an important factor governing the productivity of these waters.

The South Atlantic Current

In this paper we use the historical hydrographic data base for the South Atlantic Ocean to investigate (i) the hydrographic boundary between the subtropical gyre and the Antarctic Circumpolar Current (ACC), the Sub-tropical Front (STF), and (ii) the southern current band of the gyre, which is called the South Atlantic Current (SAC). The STF begins in the west in the Brazil-Falkland (Malvinas) confluence zone, but at locations at and west of 45°W this front is often coincident with the Brazil Current front. East of 45°W the STF appears to be a distinct feature to at least the region south of Africa, whereupon it continues into the Indian Ocean. The associated current band of increased zonal speed is the SAC, which, except for one instance, is found at or north of the surface STF until Indian Ocean water from the Agulhas retroflection is reached. A reversal of baroclinicity in the STF is observed south of a highly saline Agulhas ring, causing the SAC to separate from the STF and turn north into the Benguela Current. Zonal flow south of the STF is generally weak and serves to separate the South Atlantic and circumpolar currents. In the Argentine Basin, the SAC has a typical volume transport of 30 Sv (1 Sv = 106m3s−1) in the upper 1000 m relative to a deep potential density surface (σ4 = 45.87 kg m−3), and can be as high as 37 Sv. It is thus comparable to, or stronger than, the Brazil Current. In the Cape Basin, the transport of the SAC is reduced to about 15 SY before it turns north to feed the Benguela Current. In late 1983 this flow was joined by about 8 Sv of water from the Agulhas Current.

Simulations of radiocarbon in a coarse‐resolution world ocean model: 2. Distributions of bomb‐produced carbon 14

Part 1 of this study examined the ability of the Geophysical Fluid Dynamics Laboratory (GFDL) primitive equation ocean general circulation model to simulate the steady state distribution of naturally produced 14C in the ocean prior to the nuclear bomb tests of the 1950s and early 1960s. In part 2 we begin with the steady state distributions of part 1 and subject the model to the pulse of elevated atmospheric 14C concentrations observed since the 1950s. This study focuses on the processes and time scales which govern the transient distributions of bomb 14C in the upper kilometer of the ocean. Model projections through 1990 are compared with observations compiled by the Geochemical Ocean Sections Study (GEOSECS) in 1972, 1974, and 1978; the Transient Tracers in the Ocean (TTO) expedition in 1981, and the French INDIGO expeditions in 1985–1987. In their analysis of the GEOSECS 14C observations, Broecker et al. (1985) noted that much of the bomb 14C which entered the ocean's equatorial belts prior to GEOSECS accumulated in the adjacent subtropical zones. Broecker et al. argued that this displacement of bomb 14C inventories was caused by the wind‐driven upwelling and surface divergence in the tropics combined with convergent flow and downwelling in the subtropics. Similar displacements were invoked to shift bomb 14C from the Antarctic circumpolar region into the southern temperate zone. The GFDL model successfully reproduces the observed GEOSECS inventories, but then predicts a significantly different pattern of bomb 14C uptake in the decade following GEOSECS. The post‐GEOSECS buildup of bomb 14C inventories is largely confined to the subthermocline layers of the North Atlantic, the lower thermocline of the southern hemisphere, and down to 2000 m in the circumpolar region. A great deal of attention is devoted to detailed comparisons between the model and the available radiocarbon data. A number of flaws in the model are highlighted by this analysis. The Subantarctic Mode Waters forming along the northern edge of the circumpolar current are identified as a very important process for carrying bomb 14C into the thermoclines of the southern hemisphere. The model concentrates its mode water formation in a single sector of the circumpolar region and consequently fails to form its mode waters with the correct T‐S properties. The model also moves bomb 14C into the deep North Atlantic and deep circumpolar region much too slowly.

Water masses and currents of the Southern Ocean at the Greenwich Meridian

The frontal structure of the Antarctic Circumpolar Current (ACC) at the Greenwich Meridian is similar to that at Drake Passage even though the current is not confined to flow between two continents: There are sharp horizontal gradients in all properties throughout the water column, the fronts are narrow relative to the total width of the current, and most of the transport occurs within the frontal zones. East of Drake Passage, saline North Atlantic Deep Water (NADW) is incorporated into the Circumpolar current, and at the Greenwich Meridian it influences the water characteristics as far south as the Polar Front. Although the transport between Antarctica and Africa is close to that between Antarctica and South America, the transport within the ACC at our section is about 20% greater than at Drake Passage, probably due in part to the addition of NADW. Separating the ACC from the Weddell Gyre is a sharp front, south of which the signature of all but the densest Circumpolar Deep Water (CDW) is lost by mixing with the surface waters. The intermediate water of the central Weddell Gyre is formed from this dense CDW, which is modified by biochemical processes to become oxygen poor and nutrient rich. Warm, salty, less dense CDW from the southern edge of the ACC rounds the eastern end of the gyre and appears in the southern limb, which meanders around Maud Rise.

Generation of small-scale turbulence in the Antarctic circumpolar current

A functional connection between probability of turbulence generation by shear instability in current velocity and Vaisala-Brunt frequency is obtained. Examples are cited of sections with isoclines of probable turbulence generation in the 30–500-m layer in the Antarctic circumpolar current north of Kergelen island.

Paleoceanographic implications of terrigenous deposits on the Maurice Ewing Bank, southwest Atlantic Ocean

Grain size distributions of sediment samples from the Maurice Ewing Bank (MEB) were utilized to identify styles of sediment transport and to estimate current speeds within the benthic boundary layer. These data allow for examination of the relationship between sedimentation on the bank and circumpolar currents. Causal relationships between glacial episodes, current intensification, and bottom current scour were previously postulated. The intent of this study was to test the validity of these suggested relationships. Modern sediment distribution on the MEB is controlled by water column winnowing, bottom currents, and variations in the flux of ice-rafted detritus and biogenic material, and is modified by mass-flow processes. Indirect determinations of current speeds, using well established curves relating current velocity to sediment transport, suggest uniform, near-bottom current speeds of approximately 6–15 cm s −1 over the bank. This velocity range is considered relative, not absolute. Nonetheless, these moderate velocities are supported by geostrophic velocity calculations. Grain size distributions of down-core samples from Plio-Pleistocene sandy units were examined in order to investigate the history of bottom current flow over the bank during this time. The lack of significant variability of grain size distributions down-core indicates that no major fluctuations in circumpolar current intensification have occurred since Plio-Pleistocene time, and, therefore, argue against a causal relationship between glacial conditions and deep-sea (geostrophic) current intensification. In addition, mass movement, mainly in the form of debris flows and turbidites, has resulted in interruptions in the sedimentary record, which in some cases could have been misinterpreted as unconformities. Our scenario for sedimentation over the bank differs from previous ones in that it calls for less dramatic variations in bottom current speeds during Pliocene—Holocene time.

Evidence of tertiary forest in the Falkland Islands (Islas Malvinas)

Recent examination of the sediments and microfossils of a wood-bearing deposit in West Falklands (Lat. 51° 21′S, Long. 60° 42′W) indicates that it represents an in-situ forest environment and not driftwood, as originally assumed following its description in 1912. Palynomorphs in the organic matrix are consistent with wood identifications, and record a local palaeoenvironment of podocarp-type forest with some Nothofagus and tree ferns. Physical characteristics of the sediment matrix also indicate a terrestrial rather than coastal origin. The immense contrast of this forest with the treeless oceanic heath of the Falklands at the present time makes a Quaternary date extremely unlikely, despite the excellent preservation of micro- and macrofossils. In the context of present knowledge of the Southern Hemisphere palaeoclimate, a date of deposition prior to Antarctic cooling and the initiation of the circumpolar current is suggested. Palaeobotanical characteristics of the palynomorph assemblage also suggest pre-Late Tertiary date.