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

Although the flux of chemical elements to deep-sea sediments is affected significantly by in situ biogeochemical processes and volcanic events, a close correlation is noted between the chemical compositions of Chinese loess, marine aerosol and pelagic clays from the North Pacific Ocean. Our study strengthens the knowledge that Asian soil dust (especially from China) has an important effect on the sedimentation in the North Pacific Ocean throughout the Quaternary.

The Princeton Ocean General Circulation model (OGCM) is used to simulate the circulation in the Atlantic Ocean between 80°N and 15°S. Horizontal resolution is Δφ = Δλ = 2°, and the computations are carried out at 12 levels. The model is initialized with Levitus's (1982) data. Robust diagnostic and prognastic experiments with prescribed Mediterranean outflow are carried out to study the gross features of the North Atlantic circulation, and in particular the Mediterranean water tongue. The model simulates westward and west-southwestward flow in the area where Mediterranean water is usually observed. The positive temperature and salinity anomalies extend westward. The simulated water masses and their general features agree well with Levitus's (1982) data. Horizontal density gradients due to temperature and salinity anomalies compensate each other in the Mediterranean water tongue, and the buoyancy signal originating from the Strait of Gibraltar does not substantially affect the general circulation in the interior Atlantic Ocean.

The concept of the recurvature of the Agulhas Current has gone through a number of historical phases. The perception that a major part of the current returns to the Indian Ocean was first put forward in 1832 by Rennell. In time this idea became increasingly diluted with a bifurcation of the current being more generally accepted up until at least 1933, even though work by Dutch researchers such as Andrau in 1857 had supported the findings of Rennell. Obfuscation by various subsequent portrayals only came to an end by 1970 when Bang put forward the concept of a retroflection. All current work uses this conceptual interpretation.

Mediterranean nutrient studies differ from one another by their rates of exchange with the Atlantic Ocean and by atmospheric and terrestrial sources, which sometimes vary by six times. During the Medatlante cruises (in 1988 and 1989), increases of phosphate and nitrate concentrations were confirmed in deep western waters and may be related to increasing agricultural, industrial and urban activities around the sea since the 1960s. In a non-steady-state model, this evolution of deep water concentrations constrains uncertainties in the nutrient budgets; we propose a reduced range for atmospheric and terrestrial sources of nutrients. In the Western Basin in the late 1980s the total atmospheric and terrestrial source amounted to 8–10 × 10⁹ mol y⁻¹ of phosphate, and 190–220 × 10⁹ mol y⁻¹ of nitrate; about two-fold greater than estimates based on measurements of atmospheric and terrestrial inputs (the latter is calculated from the Rhône river concentrations over the 1971–1988 period). Consequently, phosphate budgets suggest that some of the riverine particulate input of phosphorus dissolves when entering the sea and constitutes a main source of phosphate, a scenario proposed by Froelich (1988). Likewise, dissolved inorganic nitrogen budgets require biological fixation of molecular nitrogen by plankton species and seagrasses. This process may constitute the main nitrogen source and explain the peculiar molar ratio N/P in the Mediterranean Sea (about 21–23) instead of about 15 in the global ocean.

The hypothesis is raised and tested that the formation of the seasonal pycnocline in a baroclinic field leads to a temporal increase of mesoscale spatial variability during the heating season; the reasoning is that the process of seasonal pycnocline formation sets up a necessary condition for baroclinic instability, the vertical reversal of the isopycnic potential vorticity gradient (IPVG). This hypothesis is confirmed by analysis of hydrographic data collected repeatedly in high horizontal and vertical resolution along a section running from the Azores towards Greenland; the analysis reveals a temporal increase during the heating season of (1) mesoscale horizontal variability—most prominent in the region of the North Atlantic Current eddy field; and (2) the tendency of the IPVG to change sign vertically. Comparison with altimeter sea surface height data indicates that an increase of mesoscale variability during summer is an overall feature of the mid-latitude surface-intensified western boundary currents.

An incomressible, Boussinesq fluid system on an ƒ-plane supports both the gravity and vortical modes with the vortical mode being the potential vorticity carrier of the system. In the linear limit, the gravity mode represents linear internal waves and the vortical mode zero-frequency geostrophic flows.

Consistency relations among cross-spectra of horizontal velocity components and the vertical displacement are studied in projected Fourier spaces. Hypothetical models include the gravity mode, the vortical mode, the linear gravity mode, and the linear vortical mode. Consistency relations for linear internal waves (the linear gravity mode) in the frequency domain have been previously described by Fofonoff [1969, Deep-Sea Research, 16 (Suppl.), 59–71]. Here, additional consistency relations for linear internal waves are obtained in projected Fourier spaces containing the frequency, the orientation of the horizontal wavevector, and the direction of the vertical wavenumber. In addition, five independent consistency relations exist for the pure gravity mode which represents nonlinear, forced or dissipating internal waves. Consistency relations for the pure vortical mode are also obtained. Three exist in any projected Fourier space and can be applied easily to oceanic measurements.

The horizontal isotropy and vertical symmetry conditions are also investigated. They are identical for the linear gravity mode and vortical mode.

Four hydrographic sections across the North Atlantic deep western boundary current from 55°W to 70°W are analysed to distinguish the current's different water mass components. The deepest component is the Norwegian-Greenland overflow water (2–3°C) which is characterized most readily by a core of high oxygen, tritium, chlorofluorocarbons (CFCs) and low silicate anomaly. The above lying Labrador Sea Water (3–4°C) is distinguishable at this latitude only by its core of low potential vorticity. The shallowest component of the boundary current (4–5°C) is revealed by a core of high tritium, CFCs and low anomaly nut has no corresponding oxygen signal because of its proximity to the pronounced oxygen minimum layer. A careful analysis of the shallow water mass reveals that it is not dense enough to be formed in the central Labrador Sea even during warm winters. Rather, based on historical hydrography its area of formation is the southern Labrador Sea inshore of the North Atlantic current where surface layer salinities are particularly low. A simple scale analysis shows that lateral mixing with the adjacent North Atlantic current can increase the salinity of this component to the values observed in the mid-latitude data set.

In this paper we present the first data set for rare earth elements (REE) in the Arctic Ocean. The data are from unfiltered samples. The concentrations are La: 18.8–41.3 pM; Ce: 5.7–25; Pr: 1.4–9.9; Nd: 14.6–39.6; Sm: 2.7–7.3; Gd: 3.8–13.4; Dy: 3.7–12.3; Er: 3.6–9.6; and Yb: 2.9–8.1. The concentrations found in the deep water are lower than deep-water concentrations found elsewhere and are oceanographically consistent. Our data show that the distribution of REE are influenced by riverine input in the northern part of the investigated area. Transport of resuspended material into the water from the Barents Sea Shelf is suggested to influence the concentrations of REE in the southern part of the Nansen Basin. The covariation between REE and nutrients is opposite to that found in other oceans. The fresh water end member concentrations for the REE in the northern surface water have been calculated.

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.

Bulk fluorescence of filtered seawater was measured in the blue (BF) and yellow (YF) regions of the spectrum (emission centered at 452 and 550 nm respectively) at stations along a transect through the Liguro-Provençal Frontal System (northwestern Mediterranean Sea). Layers of fluorescence enhanced above the background level (peaks) were found in the 0–1100 m column at all the stations. They are attributed to fluorescent dissolved organic matter (DOM) release by various populations on the basis of coincidences between the depths of fluorescence peaks and those of some of the other parameters used (Chl a, nutrients, accoustic data and distributions of Mediterranean Winter Water and Mediterranean Intermediate Water). Several fluorescent DOM producing processes were involved at variable levels of intensity according to their position on the transect and depth in the water column. (1) Production by plankton (phyto- and zooplankton) was predominant within the main subsurface Chl a maximum in all the three delineated zones of the frontal system (marginal, frontal and peripheral). (2) Correlation between fluorescence and nutrients is visible over the whole data set for the transect. Regeneration was one of the major processes in the frontal zone: in the euphotic layer in the form of an isopycnal fluorescent layer below the main subsurface Chl a layer (rapid regeneration), as well as in the aphotic layer. (3) Significant fluorescence increases in two deep-scattering layers point to the contribution of populations of midwater organisms. (4) Inputs originating from the continent and from sediments were detected in the peripheral zone. An overall similarity between respective BF and YF intensities was found, but discrepancies occurring locally in the water column point to the particular contribution of YF to the bulk seawater fluorescence.