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

The disequilibrium between the particle-reactive tracer ²³⁴Th ($\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{= 24.1}$ days) and its soluble parent, ²³⁸U, was used to examine Th scavenging and export fluxes during the U.S. JGOFS North Atlantic Bloom Experiment (24 April–30 May 1989) at ∼47°N, 20°W. Four profiles of dissolved and particulate ²³⁴Th in the upper 300 m and a non-steady box model were used to quantify dissolved ²³⁴Th uptake and particle export rates. The highest export fluxes occured during the first half of May. From POC/²³⁴Th and PON/²³⁴Th ratios, particulate organic C and N fluxes were calculated. Results were 5–41 mmol C m⁻² day⁻¹ and 0.9–6.5 mmol N m⁻² day⁻¹ from the 0–35 m layer. The ratio of POC export flux to primary production ranged from 0.05 to 0.42, peaking in the first half of May. The estimated fluxes agree with the observed losses of total C and N from the upper ocean during the bloom, but yield significantly higher fluxes than were measured by floating traps at 150 and 300 m.

Sixteen XBT sections were occupied from December 1982 to July 1987 in and around the Bransfield Strait and the South Shetland Islands. These and other temperature data from the FDRAKE75, FDRAKE76 and DRAKE79 field programs provide a description of the temperature structure and water mass distribution of the upper 500 m in this region. Temperature distributions at 300 and 500 m show Circumpolar Deep Water (>0°C) north of the South Shetland Islands, which enters Bransfield Strait through a gap between Smith and Snow Islands. Inside the Strait this water is confined to a narrow band along the southern flank of the Islands. The fine spatial resolution of the XBT sections shows a sharp thermal gradient between the cold (<0°C) Bransfield Strait waters and the warmer (>0°C) waters of Drake Passage origin. The presence of cold Bransfield Strait waters, over the northeast portion of the Island slope region, is related to north-south motions of this temperature boundary. The Polar Slope Current, on the northern side of the South Shetland Islands, appears as a tongue of cold (<0°C) water that extends westward from Elephant Island to the gap between Smith and Snow Islands.

We describe a modified (Garside, 1982, Marine Chemistry, 11, 159–167) nitrite method that permits measurements down to subnanomolar concentrations and present datafrom Atlantic and Carribean deepwater profiles for comparison with a published Pacific section.

This important intermediate in the nitrogen cycle was detected in all samples. Concentrations were consistently lowest (0.1–0.4 nM) in oligotrophic surface waters. Below 1 km, carribean and Southwest Sargasso sea nitrite concentrations were 0.4–1 nM, decreasing with increasing depth; reported Pacific [NO₂] averages are several times higher. Profiles in the upper kilometer beneath the classical primary nitrate maximum (PNM) were qualitatively similar, exhibiting a smooth supra-exponential drop with depth to vvalues of ∼1–4 nM at 1 km.

Then nitrite inventory in this “tail” of the PNM above 1 km with 1 nM ≤[NO₂]≤50 nM roughly equals that in the classical PNM. Significant differences among profiles in the 0.1–1 km regionn are observed, consistent with nitrite pool turnover of 3–7 days estimated from Redfield stoichiometry and tritium-helium ages. Thus seasonal and/or regional variations in factors altering the nitrite production-consumption balance, rather than transport, seem to be responsible for nitrite variability.

Nitrite profiles with anomalous midwater or near-bottom fine structure, including multi-point maxima and minima, were found along the Venezuelan continental margin and at ≈ 13°N. These featurers are tentatively ascribed to boundary effects, as hydrographic and circumstantial evidence suggests that these waters interacted previously with the bottom.

Northeast of a line joining approximately 35°W, 5°S to 15°E, 25°S in the South Atlantic is the locus of a large-scale cyclonic geostrophic gyre, masked by northwestward Ekman flow at the surface and coincident with a zone of cyclonic wind stress curl. According to some descriptions, the gyre is centered near 4°E, 13°S, has a northern limb of eastward-flowing South Equatorial Countercurrernt and an eastern limb of poleward-flowing coastal Angola Current. It therefore appears to be eastern-intensified, a curious situation in view of the dynamics thought to govern motion in large-scale gyres. The northeastern corner of this ocean is also where two other eastward currents, the Equatorial Undercurrent and the South Equatorial Undercurrent, terminate and possibly feed the coastal flow.

The apparent eastern intensification of the observed geostrophic circulation is likely to be caused by the superposition of different dynamical regimes: on the one hand, a relatively weak circulation in Sverdrup balance including the South Equatorial Countercurrent and closing cyclonically within the interior, and on the other, relatively strong near-equatorial and coastal flows which, though geostrophic in the cross-stream direction, have entirely separate dynamics. Previous observations in the northeastern corner of the South Atlantic and relevant model results are examined within the framework of this hypothesis. An analysis of unpublished current measurements off Gabon and Congo (8°E-12°E, 1°S-6°S) shows a highly variable poleward undercurrent along the continental break. We refer to this current as the Gabon-Congo Undercurrent and compare it to the Peru-Chile Undercurrent in the eastern South Pacific, discussing its interpretation as a branch of the terminating Equatorial Undercurrent.

A three-dimensional time-dependent model of the circulation in the Bransfield Strait-South Shetland Islands region and a physiologically-based, temperature-dependent model of the descent-ascent behavior of the embryos and larvae of Euphausia superba were combined in a Lagrangian particle tracing model to simulate trajectories of krill embryos and larvae. The Lagrangian calculations show that: (1) surface flow is the primary factor influencing the final location of the embryo-larva particle; and (2) timing of krill spawning affects the eventual position of the feeding larvae. Seasonal changes in the wind stress field result in variability in direction and velocity of surface currents, which affects the embryo-larva trajectories. Conditions favourable for the transport of larvae to Bransfield Strait occur early in the spawning season. East of the Antarctic Peninsula larvae have a greater probability of entering Bransfield Strait if the krill embryos are released in mid-summer, January to February. Embryos released to the north of the South Shetland Islands, west of 62°W are transported into Drake Passage. Embryos released to the north of the South Shetland Islands and east of Livingston Island are transported westward where they can eventually enter Bransfield Strait. Krill larvae also are transported into Bransfield Strait from the Bellingshausen and Weddell Seas. The Lagrangian trajectories show that the western Bransfield Strait is a region of potentially high larval concentration due to transport from surrounding areas as well as local production. This is in agreement with observed krill larvae distributions, which show higher concentrations in this region.

A time- and temperature-dependent model was developed to simulate the descent-ascent behavior of the embryos and early larval stages of the Antarctic krill, Euphausia superba. This model combines laboratory measurements of temperature effects on developmental times, density and physiology of krill embryos and larvae and the observed water temperature structure in the Bransfield Strait-South Shetland Islands region. Simulations with observed vertical temperature profiles from this region show that embryos that develop at temperatures less than 0°C hatch relatively deep (≈1000 m) or hit the bottom before hatching. The presence of warm (1–2°C) Circumpolar Deep Water (CDW), between 200 and 700 m, results in hatching depths of about 700 m. The sinking rate pattern characteristic of the embryos of Euphausia superba retains the embryos in the CDW, where development is accelerated. Larval ascent rate through the CDW is rapid, so larvae reach the surface before metamorphosing into the first feeding stage, and have sufficient carbon reserves to drift at the surface for several weeks before needing to find food. These results suggest that the sinking rate pattern characteristic of embryos of Antarctic krill may be part of a reproductive strategy that evolved in response to the thermal structure of its environment. The complementary component of this reproductive strategy is the observed correlation between the distribution of krill schools containing reproducing individuals and the presence of CDW. With this reproductive strategy, the spawning regions of Antarctic krill are in areas where oceanic conditions enhance the probability of survival of its embryos and non-feeding larvae.

The particulate fluxes of A1 are generally greater in the western North Pacific than in the central and eastern North Pacific, Atlantic and Antarctic oceans. For instance, sediment trap data reported in this paper show the Al flux in the northern part of the Japan Trench is 12.7 mg m⁻² day⁻¹ at 5.2 km depth, 130 times greater than that in the deep Antarctic, even though total particulate fluxes are similar. The particulate fluxes of A1 extrapolated to the ocean surface layer roughly equals the observed A1 flux occurring at the ocean-atmosphere interface, suggesting that particulate A1 is atmospheric in origin. Excess A1 fluxes in the subsurface water probably indicate horizontal transport from the continental margin. This is indicated by the different Mg/K ratios of settling particles between the western and eastern North Pacific.

We develop a generalized approach for the objective analysis of nonstationary, heterogeneous fields. An algorithm is presented that uses an anisotropic, time-dependent correlation function with correlation parameters that vary in space/time and a time-dependent trend surface for efficient objective analysis of dynamically heterogeneous and nonstationary fields. The algorithm, which we term the “parameter matrix algorithm”, is applied to two data sets. The first is tropical Pacific sea surface temperature (SST) derived from satellite AVHRR data and Pan-Toga drifting buoys. The SST appliclication illustrates how the parameter matrix is used for the computationally efficient objective analysis of the tropical Pacific SST from 30°S to 30°N at 0.2° resolution (over 290,000 grid points) using approximately 350,000 data points from 12 2-day satellite SST composites. The second example uses data from the Anatomy of a Meander/ BIOSYNOP experiment in the Gulf Stream ring and meander region and illustrates that an objective analysis using the parameter matrix can yield a more accurate representation of oceanic features than typical objective analysis techniques.

Gymnodinium sanguineum dinoflagellate cultures were subjected to constant staining flows to quantify the threshold level for growth inhibition by viscous streses for this organisms and to compare its response to that of a red tide species Gonyaulax polyedra previously studied. Growth inhibition of Gymnodinium was detected at rates-of-strain γ as small as 1 rad s⁻¹ and was independent of γ in the range 1–40 rad s⁻¹. Growth was delayed for 2–5 days, but not prevented entirely. For Gonyaulax, growth inhibition began for γ ≈ 3 rad s⁻, and growth was prevented entirely above 8 rads s⁻¹. The fishery implication is that the apparent necessity of calm seas (Lasker Events) for the growth of larval northern anchovies may not be due to turbulence inhibition of their Gymnodinium food supply.

Jet-like extensions of the slope current off northern Spain and France in the southern Bay of Biscay in the winter develop into anticyclonic eddies with an upper core of slope water. Three eddies were observed to develop in the winter of 1989/1990, subsequently named F90a, F90b and O90. F90a was an anticclonic eddy of radius 50–60 km, with a central core of mixed water of slope origin (from the slopes in the vicinity of Cap Ferret Canyon) and of volume ∼400 k³. In the eddt centre, the mixed core extended from ∼70 to ∼280 m. Measurements at sea showed minimal changes in the core characteristics of F90a (potential temperature, 12.95°C, salinity, 35.74 psu) with time, and remote sensing studies demonstrated that these slope water core eddies can maintain their identities for about a year. Rotation rates in the central core of F90a were about 3 days and values of normalized relative vorticity were about −0.5. Maximummean azimuthal velocities were about 30 cm s⁻¹ at a radius of 30 km. Hydrographic data showed that the presence of the core was felt to a depth of ∼1500m, resulting in an azimuthal transport of about 8 Sv. Both F90a and O90 moved westward across the deep (4800 m) Bay of Biscay, and the westward migration speed (2 cm s⁻¹) seems in excess of simple theoretical estimates for the β-induced westward propagation speed (0.4 cm s⁻¹) of anticyclonic eddies. If the latter figure is used for self-advection and the effects of topography and mutual influence are neglected, the observed westward movement suggests a clockwise mean circulation −2 cm s⁻¹) for the oceanic water in the Bay of Biscay. By contrast, F90b remained nearly stationary near 4°W. Remote sensing studies indicate that the occurence of a 4°W eddy in the summer of any year miight be attributed to warm slope water inflow along the northern Spanish slope in the previous winter.