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81.
Two processes are generally explained as causes of temporal changes in the stoichiometric silicon/nitrogen (Si/N) ratios of
sinking particles and of nutrient consumption in the surface water during the spring diatom bloom: (1) physiological changes
of diatom under the stress of photosynthesis of diatom and (2) differences of regeneration between silicon and nitrogen. We
investigated which process plays an important role in these changes using a one-dimensional ecosystem model that explicitly
represents diatom and the other non-silicious phytoplankton. The model was applied to station A7 (41°30′ N, 145°30′ E) in
the western North Pacific, where diatom regularly blooms in spring. Model simulations show that the Si/N ratios of the flux
exported by the sinking particles at 100 m depth and of nutrient consumptions in the upper 100 m surface water have their
maxima at the end of the spring diatom bloom, the values and timings of which are significantly different from each other.
Analyses of the model results show that the differences of regeneration between silicon and nitrogen mainly cause the temporal
changes of the Si/N ratios. On the other hand, the physiological changes of diatoms under stress can hardly cause these temporal
changes, because the effect of the change in the diatom's uptake ratio of silicon to nitrogen is cancelled by that in its
sinking rate. 相似文献
82.
Several large deployments of neutrally buoyant floats took place within the Antarctic Intermediate (AAIW), North Atlantic Deep Water (NADW), and the Antarctic Bottom Water (AABW) of the South Atlantic in the 1990s and a number of hydrographic sections were occupied as well. Here we use the spatially and temporally averaged velocities measured by these floats, combined with the hydrographic section data and various estimates of regional current transports from moored current meter arrays, to determine the circulation of the three major subthermocline water masses in a zonal strip across the South Atlantic between the latitudes of 19°S and 30°S. We concentrate on this region because the historical literature suggests that it is where the Deep Western Boundary Current containing NADW bifurcates. In support of this notion, we find that a net of about 5 Sv. of the 15–20 Sv that crosses 19°S does continue zonally eastward at least as far as the Mid-Atlantic Ridge. Once across the ridge it takes a circuit to the north along the ridge flanks before returning to the south in the eastern half of the Angola Basin. The data suggest that the NADW then continues on into the Indian Ocean. This scheme is discussed in the context of distributions of dissolved oxygen, silicate and salinity. In spite of the many float-years of data that were collected in the region a surprising result is that their impact on the computed solutions is quite modest. Although the focus is on the NADW we also discuss the circulation for the AAIW and AABW layers. 相似文献
83.
With the development of high-resolution multibeam echosounder systems (MBES) for surveying shallow-water areas a new tool is available to monitor rapid changes in seabed morphology as, e.g., caused by the dumping of dredge spoil in coastal waters. In this study, four data sets of repeated bathymetric surveys with a MBES were processed and analyzed. The data were collected in a 1.94-km2 dumping site in the outer Weser Estuary (German Bight). Between June and December 1998, 2.6 million m3 of dredged sediment were deposited there. The bathymetric maps generated in the course of this study reveal features such as subaqueous dunes, scour holes, and mounds of dumped dredge spoil. The mean water depth decreased by about 1 m during the dumping period. Furthermore, difference grids showing changes in sediment volume allowed a calculation of the sediment budget for the monitored area. After a time period of only 5 months, 0.5 million m3 of the originally dumped 2.6 million m3 of dredge spoil had already been removed from the dumping site. 相似文献
84.
Jeffrey N. Cross John T. Hardy Jo Ellen Hose G.Patrick Hershelman Liam D. Antrim Richard W. Gossett Eric A. Crecelius 《Marine environmental research》1987,23(4)
Sea-surface microlayer samples were collected from six nearshore areas receiving different amounts of anthropogenic inputs. The samples were analyzed for selected trace metals, chlorinated hydrocarbons, and polycyclic aromatic hydrocarbons. The relative toxicities of the samples were determined with fish embryo bioassays.Contaminant concentrations generally increased from offshore to the inshore stations. Contaminant concentrations were several orders of magnitude higher in microlayer samples from the highly industrialized Los Angeles and Long Beach harbors compared to samples from a site 15km offshore. Microlayer samples from the inshore stations were significantly more toxic, and induced significantly more developmental abnormalities and chromosome aberrations, than samples from the offshore stations. 相似文献
85.
Don Maurer Richard T Keck Jeff C Tinsman Wayne A Leathem 《Marine environmental research》1982,6(1):49-68
The survival of benthic invertebrates from dredging and disposal activities is a major environmental concern in such projects. The purpose of the research described in this paper was to determine the ability of benthos (polychaetes, Scoloplos fragilis and Nereis succinea) to migrate vertically in natural and exotic sediments and to determine the survival of benthos when exposed to particular amounts of simulated dredged material.Mortalities generally increased with increased sediment depth, exotic sediment and burial time. Temperature also affected vertical migration and mortalities. These experiments, together with other experiments conducted by us and other workers, indicate that polychaetes in particular, and benthos in general, can survive dredging and disposal projects. Under certain conditions several major taxa (polychaetes, molluscs, crustaceans) can be expected to successfully recolonise disposal sites by vertical migration. 相似文献
86.
Metalliferous and pelagic sediments are exposed within and above the extrusive successions of the Upper Cretaceous Oman ophiolite which, on the basis of mostly geochemical evidence, is believed to have formed in an incipient marginal basin setting located above a NE-dipping subduction zone. The ophiolitic extrusives document various volcano-tectonic settings which include the axial zones of a spreading ridge, fault-controlled seamounts and off-axis volcanic edifices. Most of the Fe, Mn and trace metal-enriched sediments studied are interpreted as precipitates formed by oxidation of solutions derived from high-temperature sulphide-precipitating vents. The trace element content (e.g. REE and Sr) was largely scavenged from seawater. The sediments are similar to the dispersed metalliferous sediments on the flanks of modern spreading ridges, and the ‘basal’ sediments of DSDP wells and of other ophiolite complexes (e.g. Troodos, Cyprus).Distinctive mound structures located low in the lavas are attributed to percolation of sulphide-rich solutions into already deposited metalliferous oxide sediments. The resulting iron-silica rock was probably originally precipitated as ferruginous silicates.Major massive sulphides formed off-axis at the base of intermediate-basic edifices of volcanic arc affinities. Fe, Mn and trace metal enrichment in the sediment cover of a flat-topped seamount of axial lavas is interpreted as a dispersion halo around the largest massive sulphide orebody which is situated 5 km away (Lasail). Small massive sulphide bodies are common in the axial lavas particularly along major seafloor fault zones. The metalliferous sediments, locally precipitated near these vents, are ferromanganiferous, but trace metal-depleted.The metalliferous and pelagic sediment cover of the extrusive successions, generally, documents waning hydrothermal input after volcanism ended in the area.A model is discussed in which the ophiolite was created at a spreading axis above a subduction zone dipping away from the Arabian continental margin. With progressive subduction this crust approached the margin. Initially, calcareous sediment accumulated above the calcite compensation depth (CCD), but then non-calcareous radiolarites were deposited as the ophiolitic crust approached the continental margin where the CCD was higher and marginal upwelling possibly enhanced productivity. As the edge of the Arabian continental margin entered the trench, the over-riding ophiolite was regionally uplifted allowing short-lived chalk accumulation above the CCD. This was followed by volcaniclastic deposition related to the tectonic emplacement. 相似文献
87.
88.
89.
Post-Miocene sea-level low stands allowed rivers and karst processes to incise the exposed carbonate platform along the Gulf Coast of Florida. Few Miocene to mid-Pleistocene deposits survived erosion along the present coast except within incised valleys. Since their formation, these valleys have been filled and incised multiple times in response to sea-level changes. The thick sedimentary sequences underlying the mouth of Tampa Bay have been recorded as a range of depositional environments and multiple sea-level incursions and excursions during pre-Holocene time and subsequent to the accumulation of the Miocene carbonate sequences. Sediment analysis of cores collected from a north–south transect across the mouth of Tampa Bay has enabled the identification of lithofacies, ranging from well-sorted, quartz sand to dense, fossiliferous, phosphatic grainstone. These facies were deposited in freshwater, estuarine, and shallow, open marine environments. As a result of channel development and migration within the paleovalley, and cut-and-fill associated with individual transgressions and regressions, correlation of the lithofacies does not extend across the entire transect. Fining-upward sequences truncated by tidal ravinement surfaces that extend throughout the paleovalley can, however, be identified. Age determinations based on 14-C analysis, amino-acid racemization, and strontium isotope analysis dating of numerous samples yield ages of Miocene, Pliocene, early Pleistocene, and late Pleistocene, as well as Holocene for sequences that accumulated and were preserved in this valley-fill complex. Numerous inconsistencies in the stratigraphic organization of the age determinations indicate that there are bad dates, considerable reworking of shells that were dated, or both. For this reason as well as the lack of detailed correlation among the three relatively complete cores, it is not possible to place these strata in a sequence stratigraphic framework. 相似文献
90.
T. Richard Seaborne 《Marine and Petroleum Geology》1996,13(8):963-972
The Shabwa Basin is the northeastern extension of the Marib-Al Jawf-Shabwa system of Mesozoic grabens, located onshore in the Republic of Yemen. An evaporitic sequence with an estimated maximum depositional thickness of 300 metres was deposited during the Tithonian. It is designated the Sabatayn Formation and exerts significant control on most of the play elements in the principal hydrocarbon play systems anticipated in the northeastern part of the basin. Migration of hydrocarbons from pre-salt source rocks into intra-and post-salt reservoirs is restricted by the evaporites. Localised heat flow perturbations introduced by the salt, increase the maturity of post-salt source rocks. Post and intra-salt reservoirs are structured by listric faulting on a salt detachment, salt pillowing due to post-depositional loading, by local salt dissolution and by late folding due to gravity sliding of the post-salt section on a salt detachment. Early dissolution and reprecipitation of salt is responsible for occlusion of porosity in intra-salt clastic reservoirs. 相似文献