Deep-sea research. Part A, Oceanographic research papers最新文献

Based on pyrolysis experiments and dated geological samples, kinetic parameters for the isoleucine epimerization in the foraminiferal species Neogloboquadrina pachyderma (sin) and Cibicides wuellerstorfi have been calculated. From these parameters and the down core change in the degree of isoleucine epimerization in two cores from the Norwegian Sea, it is concluded that the Norwegian Sea bottom water temperature was 2-4°C higher in oxygen isotope stages 2 and 3 than during stage 5 to 4 and stage i. Compared with epimerization data from the North Atlantic, the Norwegian Sea data suggest a smaller gradient in bottom water temperatures between the oceans during the glacial stages than during the interglacial stages. Geological data suggest a reduction in rate constant for the isoleucine epimerization in N. pachyderma at an alle/Ile ratio close to 0.28, whereas the pyrolysis experiments indicate a higher level for this inflection point. Such a temperature dependency for when the change in reaction rate occurs, could be related to different activation energies for the epimerization and hydrolysis reactions.

Steady abyssal circulation is investigated with a simple reduced-gravity model where horizontal diffusion of interfacial displacement is taken into account in addition to ordinary vertical diffusion of Newtonian cooling. The horizontal diffusion and viscosity turn out to change the structure of boundary layers and the field of vertical velocity both on ƒ- and β-planes. The dynamics of western boundary layers is classified into the viscous and diffusive regimes. In either regime, horizontal diffusion dominates the distribution of vertical velocity. Downwelling prevails in the western offshore boundary current flowing equatorward, while upwelling is always found in the poleward current. A more intense, opposite vertical motion occurs in a narrower boundary layer horizontal diffusion again plays a crucial role in determining both horizontal and vertical velocities. The present model explains this downwelling in terms of the diffusion of the thickness term in potential vorticity. It is shown that only when the horizontal diffusion is incorporated is the reduced-gravity model capable of reproducing the complicated distribution of vertical velocity in the abyssal layer which has been repeatedly reported in various three-dimensional experiments. The present model is also applicable to the surface layer, extending the Sverdrup-Stommel-Munk theory of the homogeneous ocean to that more suitable for the stratified ocean.

Euphotic zone concentrations and fluxes of nitrate, ammonium, particulate and dissolved organic nitrogen were measured over an 18 month period at the VERTEX time-series site in the oligotrophic northeast Pacific (33°N, 139°W). Variations in all N-forms were significant but not clearly linked to the temporal hydrographic cycle. Inorganic-N uptake (nitrate + ammonium) from ¹⁵N tracer experiments generally paralleled primary productivity variations, peaking in summer; ammonium accounted for most of the uptake (∼90%) and temporal variability. Comparisons of ¹⁵N results with estimates of autotropic N-uptake from ¹⁴C incorporation into protein suggest that as much as 40% of the annual inorganic-N uptake was due to microheterotrophs; peak heterotrophic N-uptake occurred in summer when heterotrophic biomass was at its maximum. Nitrate uptake (new production) was less variable than ammonium uptake and annually equivalent to particulate nitrogen export from sediment traps. Dissolved organic-N (DON) represented the largest and most variable N-pool, accounting for 80–90% of the total nitrogen in the euphotic zone. Vertical DON gradients, however, were small and temporally invariant, implying little contribution to the biogenic nitrogen export from the euphotic zone.

Three separate tracer ratio-based water mass ages (CCl₄/F11, tritium/³ He and F11/F12) are calculated for the 1987 oceanographic section by R.V. Polarstern across the Nansen Basin, Arctic Ocean. The CCl₄/F11 ratio ages are shown to be in good agreement with the apparent tritium/³He ages throughout the age range studied (52—30 years) and with F11/F12 ages in the age range for which such comparisons can be made (162–30 years). A simple box model (integrated to 1987) shows that for CC1₄/F11 and tritium/³He ages less than 12 years, the apparent ages should be close to the mean age of the water sampled. The distribution of the tracer ages is discussed with respect to the circulation of water masses in the Nansen Basin.

An acoustic Doppler current profiler (ADCP) mounted inside a towed fish was used to measure current velocity structure in the upper 400 m of the Kuroshio south of Honshu. Velocity sections were obtained along two different transects during spring 1988. The results illustrate the detailed cross-stream velocity structure of the Kuroshio, including the subsurface velocity core and the subsurface countercurrent along the shelf slope. These data, in addition to data previously collected further south near Okinawa, illustrate changes in the Kuroshio as it progresses downstream. The largest transport observed was 85 Sv, including estimated flow below the ADCP measurements down to 1000 m depth.

Hydrography and satellite-tracked drifters from the Gulf of Alaska Recirculation Study (GARS) were used to describe the regional circulation from 1986 to 1989 in the northwest Gulf of Alaska. The average baroclinic transport (0/1000 db) from six occupations of a section across the Alaska Stream near the Shumagin Islands (55°N, 160°W) was 7.4 Sv. The seasonal variation in the transport of the Alaska Stream was negligible relative to the seasonal variation of the Sverdrup transport in the Gulf of Alaska as calculated from the wind-stress curl. However, the mean transport agreed with mean annual Sverdrup transport. Anticyclonic mesoscale eddies frequently appeared in the dynamic topography and hydrography along the easternmost sections of the cruise grid (140°W).

Drifters released in the Alaska Stream offshore of Kodiak Island usually moved southwestward following the isobaths. However, the trajectories of four drifters from 1988 to 1989 described an anticyclonic meander in the Alaska Stream that propagated southwestward at about 0.022 m s⁻¹. The hydrography confirmed the existence of the meander off Kodiak Island in April 1988. The temperature and salinity characteristics of the anticyclonic meanders and eddies indicated that the water masses at the center of the features were derived from Alaska Current water.

Two mechanisms of enhanced vertical mixing in the Gulf of Alaska are suggested by the hydrography. The first one is due to wind mixing of the water column by intense winter storms and subsequent outcropping of the 26.8 δ_φ isopycnal surface in the center of the Alaska Gyre: a mechanism originally proposed by Van Scoyet al. (Journal of Geophysical Research, 96, 16,801–16,810, 1991). The second one is associated with fine structure in the temperature and salinity profiles centered at a density of 26.8 δ_ϑ, the approximate density of North Pacific Intermediate Water. These mechanisms freshen the water on the 26.8 δ_gj isopycnal surface. Subsequent lateral mixing on isopycnal surfaces by mesoscale eddy activity may contribute to the low salinity signature of the North Pacific Intermediate Water.

A detailed paleomagnetic and calcareous nannofossil study has been carried out on long sediment cores taken during the 1987 ARK IV/3 expedition of R.V. Polarstern in the eastern Arctic Ocean. The Brunhes-Matuyama boundary was not recovered in any of the cores. The magnetostratigraphies reflect the complex behaviour of the geomagnetic field during the Brunhes Chron of predominent normal polarity. Several short excursions and polarity events of the earth's magnetic field are documented in the cores. Their regional correlation was possible on the basis of the characteristic downcore magnetic susceptibility patterns and other physical properties of the sediments. Using the calcareous nannofossil chronostratigraphic framework, seven geomagnetic events of short duration were identified for the last 200 ky.

Sedimentation rates derived from the stratigraphic data are highly variable for the Nansen-Gakkel Ridge area, ranging from a few mm ky^-1 to several cm ky^-1 . More uniform rates of 2-3 cm ky^-1 were obtained for the central Fram Strait, the eastern Yermak Plateau, and the southern Nansen Basin and up to 9 cm ky^-1 at the Svalbard continental slope and the northern edge of the Yermak Plateau.

The chronostratigraphic scheme of geomagnetic events and coccolith abundance patterns developed for the Fram Strait area was applied to the sites of the ARK IV/3 expedition further north. It was then possible to relate the occurrence of coccoliths in the sediments to time. Towards the north, coccoliths occur later in the interglacial cycle, reflecting the influx of southerly currents carrying coccolith stocks to the Arctic Ocean.

Heterogeneous particles collected from seawater were analysed for physical dimensions and elemental composition using a computer-controlled image and chemical analysis system based on a scanning electron microscope and energy dispersive X-ray spectrometer. Staining the particles with the heavy metal ruthenium largely overcame the difficulties of forming an adequate image of organic particles that usually limits such automated analyses. Ruthenium staining provides an image comparable to that achieved with metal coatings, improves the contrast of organic particles, and provides a more uniform background grey level. In addition, this treatment simplifies sample preparation and improves the X-ray count rate compared with metal-coated specimens. The major X-ray energy peak of ruthenium interferes only with that of chlorine, which is usually removed during the sample preparation procedures.

In its present configuration, the system can perform analyses of particles from 0.5 to 100 μm in one pass for abundance, size, shape, and elemental composition at a rate of about 500 particles per hour. A generalized particle classification scheme based on elemental proportions and ratios is presented for suspended marine particles. The scheme is applied to samples from three depths in the photic zone for a station in the Sargasso Sea. As illustrative data, particle abundance, volumes, and size-volume distributions are presented vs chemical class for the major types of particles found, and a bulk refractive index is calculated.

Zooplankton were collected from five depth strata (0-25-50-100-200-500 m) at 12 stations along two sections across the Nansen Basin of the Arctic Ocean in July/August 1987. Vertical and horizontal distribution of biomass and abundance of the four dominant copepod species Calanus finmarchicus, C. glacialis, C. hyperboreus and Metridia longa, together with their gonad maturity state, are presented. Egg production of C. finmarchicus, C. glacialis and Metridia longa was measured from concurrent Bongo net samples in the upper 80 m.

Four biological provinces were distinguished, which closely agreed with the hydrographic regimes. In the south, the Barents Shelf slope and the Southern Transition were located in the boundary current system of Atlantic water flowing eastward along the Eurasian shelves. They were characterized by the dominance of Calanus finmarchicus. The Barents Shelf slope, representing the core of the boundary current, hosted large populations of C. hyperboreus and Metridia longa. Metridia longa, with its main occurrence in the depth of the Atlantic layer, was the biological marker of this province. The southern provinces were separated from the Central Nansen Basin and the Nansen-Gakkel Ridge by a sharp discontinuity in biomass, stage distribution and gonad maturity of the three Calanus species north of 83°N. This faunistic boundary coincided with a well defined hydrographic frontal zone. Apparently, zooplankton distribution followed closely the flow of Atlantic water below 60-200 m eastward, although the three Calanus species had their center of abundance in the surface layer consisting of polar water, which supposedly follows the Transpolar Drift westward.