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

Measurement of the partial pressure of CO₂ gas in sea water (pCO₂) is usually accomplished by gas chromatography or infrared spectrometry. Both techniques require large, complex and power-demanding apparatus. In this paper we explore the possibility of developing small, low-power sensors. We have developed and tested a prototype pCO₂ sensor for seawater based upon the fluorescence of a combination of dyes encapsulated within a gas permeable silicone membrane at the tip of a single optical fiber. The optical module (Douglas Instruments) delivers 30 Hz chopped white light to a filter and is passed through a dichroic mirror. This light is then focused on to a 220 μm optical fiber. The fiber, approximately 2 m long, was terminated with a standard coupler equipped with a small silicone nipple. The internal volume of the sensor tip (about 10 μl) was filled with a combination of a fluorescent indicator and two absorbing dyes so as to achieve the required sensitivity. HPTS (hydroxypyrenetrisulfonic acid) was chosen as the fluorescent species; Neutral Red and DNPA (2-(2,4-dinitrophenylazo)-1-naphthol-3,6-disulfonic acid) were selected as absorbers. Illumination at λ_ex = 450 nm yielded fluorescence at λ_ex = 530 nm, and fluoresced light was returned through the same fiber, reflected at 90° by the dichroic mirror, passed through an interference filter and focused on to a sensitive silicon photodiode. Experiments carried out both in the laboratory on standard solutions and at sea show a precision of 3% in the range 400–500 ppm pCO₂. To our knowledge, this is the first demonstration of an optical pCO₂ sensor for detecting oceanic signals. This technology is complementary to optical detection of pH and points the way towards full characterization of the CO₂ system within this measurement framework.

An analysis of numerous meridional XBT sections near 28°W reveals that the geostrophic North Equatorial Countercurrent (NECC) continues to flow eastward throughout the year, fastest in fall and slowest in spring. Drifting buoys and historical ship drifts show that the near-surface Countercurrent reverses each spring even when systematic errors due to windage are taken into account. The seasonally fluctuating winds drive an Ekman surface current that is eastward in fall, adding to the geostrophic current, and westward in spring, countering and overwhelming the geostrophic current. The reversal of the Countercurrent in spring occurs in the near-surface layer and is driven by the Northeast Trades. Thus the near-surface velocity in the Countercurrent is determined by a competition between local wind stress and the larger field of wind stress curl, both of which have large seasonal variations in the tropical Atlantic.

The population dynamics, distribution, abundance and feeding behavior of Metridia gerlachei Giesbrecht were studied in the western Bransfield Strait in the period from mid-December 1986 through late March 1987. The greatest abundance of all copepodite stages was found in the Gerlache Strait and in the Bransfield Current, approaching an average abundance of 500 individuals (ind.) m⁻³ in the upper 200 m. However, much of the population occurred below 200 m, where it sometimes approached 1000 ind. m⁻³ at certain depths. Feeding rates on phytoplankton, determined from analysis of gut pigments and gut evacuation rate, suggest that daily rations may average between 25 and 50% body carbon day⁻¹ throughout the spring and summer. A strong diel cycle was observed in grazing rate. Cluster analysis of cephalothorax length-frequency distributions, based on measurements of approximately 10,000 individual CIV and CV copepodids, shows that the largest individuals consistently occurred in phytoplankton-rich nearshore regions, whereas the smallest individuals occurred in phytoplankton-poor offshore areas. Shipboard experiments demonstrated that Metridia gerlachei CVI females feed on the eggs of another abundant Antarctic copepod, Calanoides acutus. Spawning appears to take place before December; as many as two or three generations may be produced during the summer before the last spawning period in January.

Electrochemical and chelation/extraction trace metal techniques are used to develop an understanding of changes in copper and zinc concentrations and speciation associated with a hydrographic and atmospheric event which occurred during an 8-day station in the Gulf of Mexico. Changes in copper and zinc concentrations and speciation coincided with physical changes and biological productivity stimulation. In normally oligotrophic water masses it is possible that episodic events, such as inputs from beneath the mixed layer or aeolian transport, may impact the interaction of trace metals and marine biological processes.

Measurements of physical properties of particle aggregates, including size, density and shape, are important for understanding the dynamics of suspended particulate material in the ocean. Of these physical properties, particle density (specific gravity) is the least well known and has been considered problematic for understanding particle settling behavior. However, two types of density must be considered. The bulk density, or density of the aggregate including interstitial sea water, is a determinant of aggregate settling rate. A second density, the constituent matter density, represents the density of the aggregated mass and is essential for understanding the settling behavior of aggregates in a pycnocline. Both types of density are important in studies of the distribution and flux of particulate matter in the ocean.

We report here the development of a linear density gradient column, made from a solution of Metrizamide in sea water, that permits the direct measurement of the constituent matter density. The density gradient column has been integrated into a system that also allows independent measurements of settling velocity, and aggregate size and shape. The measurements reported here are the first direct density measurements of any type, and although the constituent matter density does not relate directly to settling velocity, these measurements will help reduce errors in estimates of other physical parameters, e.g. porosity. Aggregates produced in the laboratory from diatom culture and from particles collected in coastal waters have been used to test the system.

Depth profiles of marine snow aggregate abundance were acquired using a photographic technique at several stations in the Panama Basin in order to look at regional and temporal variations in aggregate abundances. Concentrations were generally highest in the surface water and adjacent to the sea floor, with maximum abundances varying from 4 to 6 aggregates l⁻¹. In areas adjacent to the basin margin, subsurface maxima of aggregate concentrations were observed. Time-series samples from sediment traps show that sediment flux (mg m⁻² day⁻¹) varied by a factor of 6 over the 28 day deployment. Aggregate abundance, in contrast, varied only by a factor of 3 with depth and little over time scales of hours to weeks. Mass fluxes and suspended mass concentrations estimated from aggregate abundances using published conversion parameters were orders of magnitude larger than those measured by sediment traps and submersible pumps. These results suggest that aggregates detected by the survey technique do not directly produce the flux of particles collected by the sediment traps.

In the proposed particle transport model, suspended aggregates (produced in situ or resuspended from the sea floor and transported laterally) settle at negligible rates and produce the depth-varying signal in the abundance profiles. Fast-sinking (and consequently rare) aggregates fall from the surface, collide with suspended aggregates, increase their sinking speed and scavenge them from the water column. The flux signal observed in the sediment trap collections results from a combination of the time-varying direct surface-derived flux of fast-sinking aggregates and the depth-increasing concentration of suspended (scavenged) aggregates. These results indicate that aggregate size, as determined by the photographic technique, may be a poor indicator of sinking speed and that fluxes calculated from profiles of suspended aggregates may show poor relationships to actual fluxes.

An eddy-kinetic energy model for simulations of the oceanic vertical mixing (Gasparet al., 1990, Journal of Geophysical Research, 95, 16179–16193) is used to investigate the seasonal cycle of dissolved total CO₂ and CO₂ partial pressure at Station P (Gulf of Alaska) during the years 1971 and 1972.

The model simulates relatively weak seasonal variations of surface seawater pCO₂. All processes appear to contribute to the same order of magnitude: interaction of gas exchange and biology tends to compensate for the thermodynamical effect on pCO₂. The influence of various model parameters (total annual production of O₂, P, depth of photic zone, z₀, gas transfer velocity, K) on the seasonal variability of seawater pCO₂ and air-sea CO₂ flux is examined. Total annual production, P, seems to be the most influential.

The model also suggests a strong short-term variability of pCO₂ during episodic events. Model results show that the area around Station P behaves as a weak or neutral CO₂ source during summer and as a net sink during other seasons. The net annual CO₂ flux is an invasion flux, with values of 0.88 and 0.70 mol m⁻² y⁻¹ for 1971 and 1972, respectively, in agreement with the flux of 0.7 mol m⁻² y⁻¹ derived from observations during 1973–1978 (Wong and Chan, 1991, Tellus, 43B, 206–223).

Ostracod faunas from Pacific hydrothermal vents include eucytherurine and pontocypridid Podocopa and are very similar to those previously recorded from experimental wood islands. Both vent and wood-island ostracods may belong to a single deep-sea faunal association that is adapted to a eutrophic regime. This regime, which is quite distinct from normal deep-sea conditions, is probably widespread in the world's ocean, although individual manifestations tent to be localized and ephemeral.

An idealized box model of the Indian Ocean forced by steady winds and Haney-type surface heat fluxes is used to examine the importance of the warm, fresh throughflow from the equatorial Pacific on the climate of the Indian Ocean. In particular, the hypothesis proposed by Godfrey and Weaver (1991, Progress in Oceanography, 27, 225–272), that the anomalous poleward buoyancy-forced Leeuwin current off the west coast of Australia is a manifestation of a basinwide thermohaline circulation driven by the Indonesian throughflow, is examined.

The stronger Sverdrup circulation dominates the thermohaline circulation in most of the model ocean except near the eastern boundary. The effects of the throughflow, however, can be determined by comparing two runs forced by a Pacific Ocean with either the warm, fresh profile of the western equatorial Pacific or a cooler, more saline profile more typical of the eastern equatorial Pacific (where a more usual equatorward wind-driven boundary current is found). It is found that heat imported from the Pacific is transported zonally across the Indian Ocean to the western boundary by the South Equatorial Current. The enhanced meridional steric height gradient south of the SEC drives an eastward return flow back to the eastern boundary, where it turns south to form the poleward Leeuwin Current. The reverse path is traced out by the waters immediately below the thermocline. None of these features are observed when the model Pacific has the cooler profile typical of the eastern boundaries of other oceans.

Indonesian throughflow significantly affects the surface heat fluxes and the meridional heat transport in the Indian Ocean. The importance of the throughflow in maintaining the very warm climate of the Indian Ocean (a net exporter of heat) is noted.

In the model, the poleward western boundary current along the coast of Africa south of about 27°S appears to play only a very minor role in the basinwide thermohaline circulation described above. This differs from the “warm water route” proposed by Gordon (1986, Journal of Geophysical Research, 91, 5037–5046) where heat is returned to the South Atlantic past the Agulhas Retroflection.

Large-scale, fairly long period (>100 days) barotropic eddies are found in the western portion of the basin for some solutions. The generation mechanism for these eddies appears to be barotropic instability in the model South Equatorial Current.