ALBERT

Description: The benthic response to the sedimentation of particulate organic matter (POM) was investigated during 1985–1990 at 47°N, 20°W (BIOTRANS station). The first noticeable annual sedimentation of phytodetritus, as indicated by chlorophyll a concentrations in the sediment, occurred as early as late April-early May. Maximum amounts were found in June–July. Two different sedimentation pulses to the sea bed are described that demonstrate interannual variation: the occurrence of salp faecal pellets early in the year 1988 and the massive fall out of a plankton bloom in summer 1986, which deposited approximately 15 mmol C m−2. The benthic reaction to POM pulses was quite diverse. The mega-, macro- and meiobenthos showed no change in biomass, whereas bacterial biomass doubled between March and July. This corresponds to a seasonal maximum of total adenylate biomass. The relative abundance of Foraminifera among the meiobenthos increased during the summer. Benthic activity (ATP, ratio ATP/ETSA), as well as in situ sediment community oxygen consumption rates (SCOC), showed distinct seasonal maxima in July–August of 0.75 mmol C m−2 day−1. Based on SCOC and the carbon demand for growth, a benthic carbon consumption of 0.94 mmol C m−2 day−1 was estimated. This represents about 1.1% of spring bloom primary production and 9.6% of the export flux beneath the 150 m layer, measured during the North Atlantic Bloom Experiment. Bacteria and protozoans colonizing the epibenthic phytodetrital layer were responsible for 60–80% of the seasonal increase in SCOC. The strong reaction of the smaller benthic size groups (bacteria, protozoans) to POM pulses stresses their particular importance for sediment-water interface flux rates.

Description: Zooplankton sampling took place during cruise 5 Leg 3 of the R.V. Meteor (March-June 1987) in three hydrographically and ecologically different areas of the Arabian Sea (Indian Ocean): an upwelling area at the coast of Oman; an oligotroph area in the central Arabian Sea; and a shelf area off the coast of Pakistan. All three areas were expected to hace similar ichthyoplankton and cephalopod components and similar light conditions. These are important prerequisites for the present comparative study, which is concerned with the importance of the structure of the water column (physical stability and prey availability), compared with the influence of the light intensity (day/night) on the vertical distribution of species and size classes of fish larvae and cephalopod paralarvae in the subtropical pelagial. First results show that the vertical structure of the water column, especially the occurrence of a pynocline and the varying mixed-layer width, either directly or indirectly had important impact on the vertical distribution patterns of both fish larvae and cephalopod paralarvae. In addition, cephalopods were influenced more consistently by the diurnal change of light intensity than fish larvae. Both taxa occurred mainly below the mixed surface layer. However, cephalopod paralarvae preferred shallower depths than fish larvae in all three areas and were closer related to the pycnocline than fish larvae in most cases. In the absence of a significant pycnocline, larvae appeared close to the surface.

Description: During leg 1 of Meteor cruise 10 in March/April 1989 at 18 circ N, 30 circ W, the high spatial and temporal resolution of hydrographic CTD-stations indicated that the study site was in a hydrographically complex region in the transition zone between the Canary Current and the North Equatorial Current at the southern boundary of the subtropical gyre. Strong variability was found within the upper 120 m due to interleavings of warmer and saltier subtropical salinity maximum water with colder and less saline upper thermocline water. The interleavings caused unexpected nose-like temperature, salinity, nitrate and oxygen profiles yet not described in the literature. A second variability source was found in the Central Water area, because the study area was situated in the vicinity of the Central Water Boundary dividing North and South Atlantic Central Water. Hydrographic analysis of the study shows that interpretations of biological and chemical data can only be done in conjunction with high resolution CTD-profiling

Description: Abstract l. In Antarctica, two Adélie penguins were implanted with heart rate (HR) transmitters and released in their breeding colony where they resumed incubation. 2. HR while at rest and lying in the colony were 67 and 77 beats per min (bpm), respectively. 3. For diving experiments, the birds were introduced into a still-water canal, 21 m long, with one respiration chamber at each end. 4. The birds swam underwater for 49 and 76% of the time at speeds of 1.5 and 2.5 m/sec, respectively. 5. When floating quietly at the surface, HR in the first penguin was 89 bpm. 6. Pre-dive HR varied with duration of the inter-dive interval, being highest (250 bpm) when the bird dived in rapid succession (surface times 〈 5 see) and close to diving HR when surface time was 50 sec. 7. Mean HR while diving was constant (107 bpm) and did not vary either with surface time, or with time submerged (0–15 sec). 8. Pre-dive HR and diving HR were not correlated, 9. The extent of bradycardia upon immersion was dependent upon surface time. 10. There was a good correlation between HR and oxygen consumption in both birds, suggesting that HR might be used to determine energy expenditure.

Description: Since large, rapidly-sinking particles account for most of the vertical flux in the ocean, mechanisms responsible for particle aggregation largely control the transport of carbon to depth. The particle flux resulting from a variety of different phytoplankton bloom conditions was simulated with a numerical model in which phytoplankton growth dynamics were combined with physical aggregation, particle size-dependent sedimentation and degradation. Model results demonstrated that particle flux to the deep ocean be generated by solely invoking physical aggregation during phytoplankton blooms. Sensitivity of the model in response to variations of both physico-chemical and biological paramters was tested. The model outcome, described as the fraction of export production leaving the upper ocean carbon pool, proved to be most sensitive to biological variables such as phytoplankton cell size, stickness, and growth characteristics (i.e. solitary vs chain-forming). Changes in these factors strongly affect the efficiency of the “biological pump” and could be explain interannual and geographic variance in deep-ocean flux.

Description: Sediment cores from the Norwegian and Greenland Seas and the Nansen Basin were studied to determine the origin of sediment pellets, centimetre-sized aggregations of clay to sandsized sediment occurring in the cores. By comparing the grain size, grain shape and composition of the pellet sediments to sediments collected directly from the surfaces of sea ice in the Nansen Basin and from icebergs in the Barents Sea, the pelleted sediment was found to be more similar to that in the icebergs than that on the sea ice. The pellets may be formed on, in or under a glacier or during transport on/in an iceberg. When icebergs overturn or melt, the pellets fall out and are consolidated enough to survive a drop of up to 4 km to the ocean bottom and to retain their integrity even after burial on the seafloor.

Description: During R.V. Polarstern expeditions ARK IV/3 and ARK VI/1, well preserved diatom assemblages were recovered from particle-laden sea ice collected from the western Barents Shelf and the Arctic Ocean between Svalbard (81°N) and the Nansen-Gakkel Ridge (86°N). Distinct variations in the abundance pattern and species composition of diatoms were found north and south of ca 83°N. Highest diatom concentrations were encountered in multi-year sea ice in the core of the Transpolar Drift Stream between 83 and 86°N. In this area diatom assemblages are dominated by marine-?brackish benthic species. Apparently, these assemblages originate in shelf waters north and east of Siberia, where they are incorporated into the sea ice as a bottom ice assemblage. During the transport of the ice floes across the Eurasian Basin within the Transpolar Drift Stream, seasonal basal freezing and surface melting processes may have led to an accumulation of diatoms at the sea ice surface. South of ca 83°N the sea ice samples contained significantly lower numbers of diatoms, dominated by freshwater taxa. Between 83 and 81°N these assemblages are dominated by planktonic freshwater taxa, but on the Barents Sea Shelf east of Svalbard significant numbers of benthic freshwater taxa and benthic marine-?brackish species also are found. This ice may originate in the Barents Sea and/or the Kara Sea, which receive a large influx of freshwater from Siberian rivers.

Description: The term Cape Verde Frontal Zone is introduced to characterize the southeastern corner of the subtropical gyre circulation in the North Atlantic Ocean far west of the upwelling area off the Mauretanean shelf. Two water mass fronts, one overlying the other, are identified with a quasi-synoptic set of CTD-OZ and nutrient data from November 1986. In the warm water sphere we encounter North and South Atlantic Central Water (NACWISACW) superimposed on extensions of Mediterranean outflow and Antarctic Intermediate Water. The Central Water Boundary, as the separator of NACW from SACW, represents the southeastern side of the Canary/North Equatorial Current system. It acts as a barrier between the well-ventilated, nutrient-poor inner part of the basin-wide circulation of the North Atlantic and the shadow zone with its lowly oxygenated and nutrified cross-equatorial influx. Year-long current meter records, having fluctuations over typical time scales of 5(1`90 days, attest to the highly variable nature of the Cape Verde Frontal Zone. Incidentally, we observe in the data an intrathermocline eddy, called Meddy BIRGIT, which has a double maximum in the vertical salinity structure. Simultaneous Lagrangian observations by RiCHAttDSON et al. (1989, Journal of Physical Oceanography, 19, 371-383) confirm the expected anticyclonic motion of this salt lens, which must have travelled without significant mixing for at least 2500 km from its likely generation region in the Gulf of Cadiz.

Description: After acclimation to 100, 75 and 50 % of sea water (SW) external salinities, a significant reduction in MET (Mean Epithelial Thickness) and MDR (Mean Diverticular Radius) indicates a decrease in the digestive cell volume dependant on the lowering of environmental salinity. The interstitial connective tissue seems to be unable to osmoregulate and hence stand severe changes in cell size depending on external salinity. 50 % SW acclimated periwinkles show a general pattern of general stress response (decreasing MET and MDR, and increasing ND ~Numerical Density of lysosomes- and lysosomal size). A reduction in number and size of digestive lysosomes in winkles acclimated to 75 % of Sea Water evidences the functioning of reglatory mechanisms of digestive cell volume.

Description: Geostrophic volume transports in the upper 500 m are computed from historical hydrographic data for the area off the Brazilian coast west of 30°W and between 7° and 20°S. On the basis of water mass distributions, potential density surfaces of σθ = 27.05 kg m−3 (360–670 m) and σθ = 27.6 kg m−3 (∼1200 m) are used for referencing the meridional and zonal components of the geostrophic shears, respectively. Near 15°S a northwestward flow of 8 Sv crosses 30°W. This current reaches the shelf near 10°S in February and March, the only two months for which observations are available near that latitude along the coast; of the 8 Sv, about 4 Sv continue towards the northwest into the North Brazil Current while another branch also carrying 4 Sv turns southward as the beginning of the Brazil Current. Between 10° and 20°S the Brazil Current does not appear to strengthen appreciably, but because of the likely existence of flow on the shelf these transport values represent lower limits to the actual ones. At 30°W, another westward flow of approximately 8–10 Sv enters the area near 10°S and serves to strengthen the North Brazil Current. The total transfer of 12 Sv or more from the South Equatorial Current into the North Brazil Current and later to other currents and the northern hemisphere may be an important factor contributing to the well-known weakness of the Brazil Current in its more northerly latitudes.

Description: Two stacked outflow cores of the Mediterranean Water undercurrent pass through a broad “gateway” between Cape St. Vincent and Gettysburg Bank entering the Iberian Basin. The upper core (depth ∼750 m, σ1=31.85) shows a strong tendency to follow the contours of the Portuguese continental rise. Yet, the lower core (depth ∼1250 m, σ1=32.25) primarily meanders west and northwestward forming large blobs of Mediterranean Water. The predominance of isolated Meddy structures embedded in a background field is reflected in a long-term current meter record from the deep Iberian Basin.

Description: Geostrophic computations from historical data across the Brazil Current at 23° and 24°S lead to transports of 10.2 and 9.6 Sv, respectively. Data exist from all four seasons at about 24°S, but no seasonal signal can be seen in the baroclinic transport of the Brazil Current there. At 33°S the Brazil Current transport is estimated to be 17.5 Sv. A recirculation cell of 7.5 Sv is found in the western South Atlantic south of 28°S. The major problem in computing transport of the Brazil Current is not with determining the correct reference depth, but with the Brazil Current flowing partially over the shelf and therefore not being sampled completely by deep-water hydrographic stations. As long as the vertical distribution of water masses is taken into account for choosing a reference depth, geostrophic computations lead to results consistent with previous estimates.

Description: Existing information is synthesized and new data presented to describe the flow of near-bottom water from the Weddell Sea into the Scotia Sea and westward through Drake Passage along the continental slope. The water characteristics and currents along the northern margins of the South Sandwich Island Arc are examined. Long-term current measurements in the bottom waters at locations over the outer shelf and slope and along the continental rise show persistent flow from Atlantic to Pacific along isobaths at speeds of 10–20 cm s−1. Three sources for the waters in these currents are identified and discussed. At the deepest levels, Weddell Sea Deep Water enters the Scotia Sea near 40°W through a depression in the South Scotia Arc and then flows westward, constrained by the bottom topography. This cold, fresh, oxygenated bottom water then flows west to enter Drake Passage via a gap in the Shackleton Fracture Zone at the base of the continental slope northwest of Elephant Island. Mid-depth water may flow from the Weddell Sea to the Scotia Sea through the Powell Basin (sill depth approximately 2000 m) located west of South Orkney Island near 48°W. The westward flowing waters along the shelf and upper continental slope, which are denser than those immediately offshore, may be a continuation of the Polar Slope Current from the Weddell Sea or may be derived principally by convection from the shelves of the South Sandwich Island Arc. A vertical section north of Elephant Island shows downslope convection off the shelf, analogous to the observed at many locations around Antartica.

Description: Continuous current measurements at the 1000 m level were obtained in the central Canary Basin of the northeast Atlantic near 33°N, 22°W for 2398 days. Even with this very long time series no statistically significant mean current could be estimated at that level, because the energetic fluctuations are large compared to the weak mean. In the eddy scale range, i.e. at current fluctuations are scales between 47 and 455 days, a pronounced anisotropy between zonal and meridional components is apparent. For the first time in the subtropical North Atlantic gyre our data allow confirmation of the expected spectral decrease beyond the eddy scale peak in an eastern basin. With respect to future global experiments we wonder if our results from an eastern basin location are representative for the general circulation at mid-ocean sites?

Description: Helium data from the waters of the Bransfield Strait, the southern Drake Passage and the northwestern shelf of the Weddell Sea are presented. The 3He profiles from the eastern and central basins of the Bransfield Strait show maxima (δ3He ≈ 7%) below the sill depths that separate the strait from the surrounding open ocean. The 3He excess is interpreted as a local injection of a 3He-rich helium component into the deep waters of the Bransfield Strait from backarc rifting. Tritiogenic 3He and excess 3He from mixing with Circumpolar Deep Water are excluded as possible sources. The estimated 3He/4He ratio of the injected helium component (2.4–5.0 × 10−6) is less than that of pure mantle helium and may contain radiogenic helium from continental crustal material which underlies the Bransfield Strait.

Description: The microstructures and crystal fabrics associated with the development of an amphibolite facies quartzo-feldspathic mylonitic shear zone (Torridon, NW Scotland) have been investigated using SEM electron channelling. Our results illustrate a variety of microstructures and fabrics which attest to a complex shear zone deformation history. Microstructural variation is particularly pronounced at low shear strains: significant intragranular deformation occurs via a domino-faulting style process, whilst mechanical incompatibilities between individual grains result in characteristic grain boundary deformation accommodation microstructures. A sudden reduction in grain size defines the transition to medium shear strains, but many of the boundaries inherited from the original and low shear strain regions can still be recognized and define distinctive bands oriented at low angles to the shear zone margin. Grains within these bands have somewhat steeper preferred dimensional orientations. These domains persist into the high shear strain mylonitic region, where they are oriented subparallel to the shear zone margin and consist of sub-20 μm grains. The microstructures suggest that the principal deformation mechanism was intracrystalline plasticity (with contributions from grain size reduction via dynamic recrystallization, grain boundary migration and grain boundary sliding). Crystal fabrics measured from the shear zone vary with position depending on the shear strain involved, and are consistent with the operation of several crystal slip systems (e.g. prism, basal, rhomb and acute rhomb planes) in a consistent direction (probably parallel to a and/or m). They also reveal the presence of Dauphine twinning and suggest that this may be a significant process in quartz deformation. A single crystal fabric evolution path linking the shear zone margin fabric with the mylonitic fabric was not observed. Rather, the mylonitic fabric reflects the instantaneous fabric which developed at a particular location for a particular shear strain and original parental grain orientation. The mature shear zone therefore consists of a series of deformed original grains stacked on top of each other in a manner which preserves original grain boundaries and intragranular features which develop during shear zone evolution. The stability of some microstructures to higher shear strains, the exploitation of others at lower shear strains, and a continuously evolving crystal fabric, mean that the strain gradient observed across many shear zones is unlikely to be equivalent to a time gradient.

Description: The spatial distribution of the subtropical salinity maximum is identified using historical and recent data from the eastern North Atlantic. In the regions with high frequency of occurrence of the salinity maximum, the relative contributions of advection, eddy diffusion and double diffusion to the salt balance below the maximum salinity layer are determined. McDougall's (1984, Journal of Physical Oceanography, 14, 1577–1589) salt balance equation for neutral surfaces is used in this analysis. The data base consists of two meridional CTD sections along 33° and 27°W between 10° and 35°N, mean temperature-salinity profiles in 5° × 5° squares presented by Emery and Dewar (1982), and mean velocity profiles in 3° × 3° squares evaluated by Stramma (1984, Journal of Marine Research, 42, 537–558). The tropical salinity maximum tongue is found to be quite persistent in its salinity value and its geographic distribution, but less clearly in its vertical or isopycnal position. Double diffusion due to salt-fingering appears to be an important process for the salt balance below the salinity maximum layer. An approximate estimate of the double-diffusive salt flux is obtained. Near the subtropical source region, the double-diffusive salt flux is balanced primarily by isopycnal advection; further to the south it is also balanced by isopycnal eddy diffusion. Maximum double-diffusive fluxes correspond in magnitude to the mean salt flux caused by the excess in evaporation at the surface in the central subtropics. The resulting isopycnal and diapycnal eddy-mixing coefficients derived by a linear inversion technique have the reasonable values of Ki = (11 ± 5) × 102 m2 s−1 and Kd = (4 ± 2) × 10−5 m2 s−1. Considering the intermittency of the double-diffusive process, limiting values for the mean eddy-mixing coefficients are determined by neglecting the contribution of the double-diffusive salt fluxes. This leads to Ki = (5 ± 2) × 102 m2 s−1 and Kd = (5 ± 1) × 10−5 m2 s−1 for the isopycnal and diapycnal mixing coefficients, respectively.

Description: An analysis of published results on the dispersion behavior of SOFAR floats indicates a systematic depth dependence of the mixing length in the North Atlantic subtropical gyre. In contrast to the integral time scale, the length scale appears to be independent of eddy intensity in the thermocline (Lx, Ly ∼ 80, 45 km) and in the deep ocean (Lx ∼ Ly : 20 – 30 km). A similar decrease with depth is revealed by particle dispersion in an eddy-resolving circulation model and interpreted as an enhanced effect of wave behavior in the weaker, subthermocline flow. The only weak anisotropy of deep float dispersion suggests an influence of bottom roughness on the structure of eddy variability.

Description: The orientation of shear bands relative to foliations defined by elongated mineral aggregates, is often used to determine the large-scale sense of displacement in ductile faults. Data from contact strain zones at the bases of large overthrust complexes in the Eastern Alps and the Betic Cordilleras demonstrate that there is not always a simple geometrical relation between shear band orientation and sense of vorticity in bulk non-coaxial flow. In addition to single shear band sets that show displacements synthetic with the large-scale displacement, there are single sets with antithetic displacements, and conjugate sets. The last two observations are at variance with published data and interpretations, and cast doubts on the general applicability of shear bands as indicators of large scale flow kinematics.

Description: In a regional study of the eastern North Atlantic Ocean east of 35°W between 41°N and 8°N the mean meridional ocean temperature flux was computed from oceanographic and meteorological measurements using the direct method. In the area of the permanent subtropical gyre between 36°N and 22°N, a southward geostrophic temperature flux dominates. The Ekman temperature flux is weak and changes from a southward flux north of 32°N to a northward flux south of 32°N. In the area of the North Equatorial Current and in the tropics the Ekman temperature flux is comparable in magnitude to the geostrophic temperature flux. Therefore, the total temperature flux changes to a northward direction at 20°N, where the geostrophic transport is still to the south, and becomes large in the tropics, where both components show northward temperature fluxes. The heat flux divergence for the area investigated leads to an ocean heat gain of 0.19 PW. A comparison of annual mean temperature fluxes with temperature fluxes of east-west CTD sections from the winter half-year shows a small seasonal signal in the geostrophic temperature flux in the subtropical gyre but large differences in the tropics. The seasonal changes for the Ekman temperature fluxes are weak.