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1.
The diatom species composition of settling biogenic silica particles collected in sediment traps was compared with the underlying sediment to determine the preservation of the various diatom species and to investigate the potential of biogenic silica as an indicator for changes in paleo-upwelling intensity. During the Netherlands Indian Ocean Programme (NIOP), settling particles were collected at two sampling sites off Somalia (NW Indian Ocean) for 9 months, from June 1992 to February 1993. One sediment trap array was deployed on the Somali slope directly below one of the main upwelling gyres, and a second array, meant as a reference site to reflect pelagic sedimentation, was moored in the Somali Basin away from direct coastal upwelling influence. At both sites diatoms represented over 90% of the total opal microorganisms. On the Somali slope, total annual diatom flux was 12.6×109 valves m−2, 76% of which was collected during the 112 d of the southwest monsoon, with peak fluxes in October, the end of the upwelling season. In the Somali Basin, the total annual flux was lower, 4.8×109 valves m−2, and only 39% was collected during the SW monsoon period (98 d). At both sampling sites, a distinct seasonal diatom species succession of ‘pre-upwellers’, ‘upwellers’ and ‘oceanic species’ was apparent. Although only a small part of the diatom assemblage escaped dissolution at the sediment–water interface, two species, Thalassionema nitzschioides and Chaetoceros resting spores, were preserved in the sediment, indicating that they are resistant to dissolution at the sediment–water interface. Eighty one percent of the deposition of Thalassionema nitzschioides and 78% of the deposition of Chaetoceros occurred during the upwelling period. Since these two species are the dominant component of the diatom assemblage in the sediments, and thus determine the biogenic silica content, we conclude that this preserved biogenic silica reflects the upwelling in the surface layer of the water column. On the Somali Margin, variations in biogenic silica flux as inferred from sedimentary records can therefore be used as an indicator for changes in paleo-upwelling intensity.  相似文献   

2.
通过对南海北部夏、冬两个季节的生源要素垂直输运剖面和时间系列沉积物捕获器的测量资料进行综合研究表明:南海北部颗粒物质主要是由钙质生物壳、生物硅、海洋浮游生物的有机质以及岩源物质组成,颗粒物质通量在1 000 m处大约为90.0 mg/(m2·d);研究还表明颗粒有机碳在进入沉积物保存之前被大量溶解,南海北部来自底层顺坡搬运的有机碳远大于垂直沉降;与开阔大洋(东太平洋海域)的对比研究表明,边缘海对于季节的变化更加敏感,而东太平洋调查区位于热带赤道高生产力带,生物作用十分明显,其海洋生物呼吸和物质转移同样也较活跃.  相似文献   

3.
An array of five bottom-tethered moorings with 19 PARFLUX time-series sediment trap at three depths (1 and 2 km below the surface, and 0.7 km above the sea-floor) was deployed in the western Pacific sector of the Southern Ocean, along 170°W. The five stations were selected to sample settling particles in the main hydrological zones of the Southern Ocean. The sampling period spanned 425 days (November 28, 1996–January 23, 1998) and was divided into 13 or 21 synchronized time intervals. A total of 174 sequential samples were recovered and analyzed to estimate fluxes of total mass (TMF), organic carbon, carbonate, biogenic silica, and lithogenic particles. The fluxes of biogenic material were higher than anticipated, challenging the notion that the Southern Ocean is a low-productivity region. Organic carbon fluxes at 1 km depth within the Polar Frontal Zone and the Antarctic Zone were relatively uniform (1.7–2.3 g m−2 yr−1), and about twice the estimated ocean-wide average (ca. 1 g m−2 yr−1). Carbonate fluxes were also high and uniform between the Subantarctic Front and ca. 64°S (11–13 g m−2 yr−1). A large fraction of the carbonate flux in the Antarctic Zone was due to the presence of pteropod shells. Coccoliths were found only to the north of the Polar Front, and calcium carbonate became the dominant phase in the Subantarctic Zone. In contrast, carbonate particles were nearly absent near 64°S. Latitudinal variations in biogenic silica fluxes were substantial. The large opal flux (57 g m−2 yr−1) measured in the Antarctic Zone suggests that opal productivity in this region has been previously underestimated and helps to explain the high sedimentary opal accumulation often found south of the Polar Front. Unlike biogenic material, fluxes of lithogenic particles were among the lowest measured in the open-ocean (0.12–0.05 g m−2 yr−1), reflecting a very low dust input.  相似文献   

4.
The results of geoacoustic studies carried out by the acoustic MAK-1M system in the southeastern Ita Mai Tai Guyot (Magellan Seamounts, Pacific Ocean) are discussed. The obtained geoacoustic sections characterize the incoherent part of the sedimentary sequence with high resolution. The geoacoustic stratification is well correlative with the deep-sea drilling data: the acoustic units are reliably correlated with the lithostratigraphic units defined in the cores of DSDP holes 200 and 202. Three geoacoustic units are distinguished in the sedimentary section (from the top downward): unit I corresponding in age to the terminal Miocene (Messinian)-Quaternary; unit II correlated with the Late Miocene (Tortonian); unit III accumulated in the Early Eocene-Early Miocene. The acoustic basement is composed of reefogenic limestones that were formed at different stages of the guyot’s development through the Aptian-Turonian to Late Paleocene-Eocene. The sonar images allow the distribution of the incoherent carbonate sediments to be contoured along the periphery of the summit’s plateau.  相似文献   

5.
A δ~(44) Ca curve from shells of the planktonic foraminifera Globigerinoides sacculifer in calcareous biogenic oozes has been extracted from the Nintyeast Ridge in the Indian Ocean since 300 ka. By combining terrigenous inputs(e.g., grain size, magnetic susceptibility, and turbidite frequency) with the oceanic productivity(e.g., biogenic content and Neogloboquadrina dutertrei content), it is found that the curve's variations are closely related to the historical evolution of the oceanic calcium cycle. The δ~(44)Ca value is in lower tendency and has small oscillation during Marine Isotope Stage(MIS) 6, when the supply of terrigenous detrital is highest. In contrast, during MIS 3,5 and 7, the δ~(44) Ca values are in higher tendency, and their fluctuations are consistent with the variations of the productivity proxies. These results suggest that the calcium isotopes are mainly influenced by the input of the Himalayan erosion products to the northern Indian Ocean. In addition, the developmental stages of calcareous planktons may have a secondary impact on the fluctuations of the calcium isotope ratio of sea water.  相似文献   

6.
Dissolution experiments in batch and flow-through reactors were combined with data on sediment composition and pore water silicic acid profiles to identify processes controlling the solubility of biogenic silica and the build-up of silicic acid in marine sediments. The variability of experimentally determined biogenic silica solubilities is due, in part, to variations in specific surface area and Al content of biosiliceous materials. Preferential dissolution of delicate skeletal structures and frustules with high surface areas leads to a progressive decrease of the specific surface area. This may cause a reduction of the solubility of deposited biosiliceous debris by 10–15%, relative to fresh planktonic assemblages. Dissolution of lithogenic (detrital) minerals in sediments releases dissolved aluminum to the pore waters. This aluminum becomes structurally incorporated into deposited biogenic silica, further decreasing its solubility. Compared to Al-free biogenic silica, the solubility of diatom frustules is lowered by as much as 25% when one out of every 70 Si atoms is substituted by an Al(III) ion.The build-up of silicic acid in pore waters of sediments with variable proportions of detrital matter and biogenic silica was simulated in batch experiments using kaolinite and basalt as model detrital constituents. The steady-state silicic acid concentrations measured in the experiments decreased with increasing detrital-to-opal ratios of the mixtures. This trend is similar to the observed inverse relationship between asymptotic pore water silicic acid concentrations and detrital-to-opal ratios in Southern Ocean sediments. Flow-through reactor experiments further showed that in detrital-rich sediments, precipitation of authigenic alumino-silicates may prevent the pore waters from reaching equilibrium with the dissolving biogenic silica. This agrees with data from Southern Ocean sediments where, at sites containing more than 30 wt.% detrital material, the pore waters remain undersaturated with respect to the experimentally determined in situ solubility of biogenic silica.The results of the study show that interactions between deposited biogenic silica and detrital material cause large variations in the asymptotic silicic acid concentration of marine sediments. The production of Al(III) by the dissolution of detrital minerals affects the build-up of silicic acid by reducing the apparent silica solubility and dissolution kinetics of biosiliceous materials, and by inducing precipitation of authigenic alumino-silicate minerals.  相似文献   

7.
The island of Menorca, one of the Balearic Islands (Spain) located in the western Mediterranean, is characterised by a contrasting geology and landscape with two major geographic domains: (1) a southern region called Migjorn, comprised of Late Miocene calcarenites and limestones, and (2) a northern region known as Tramuntana, which is composed of folded and faulted Palaeozoic, Mesozoic and Tertiary (Oligocene) siliceous and calcareous rocks. Both domains are lined by numerous pocket beaches exhibiting a high variety of surficial sediment assemblages. Grain-size and compositional analyses revealed that cliff erosion and nearshore Posidonia oceanica meadows are the main sources of sediments consisting mostly of medium- to coarse-grained carbonate sands of marine biogenic origin, with variable amounts of terrigenous rock fragments and quartz. Based on distinctly different contributions of bioclastic material, biogenic carbonates and quartz, 320 sediment samples from 64 beaches were grouped into different facies associations dominated by either (1) biogenic sands, (2) biogenic sands with terrigenous contributions or (3) terrigenous sands with quartz. Nevertheless, there is a marked regional variability in sediment texture and composition. Thus, variable mixtures of carbonate and siliciclastic sediments characterise the beaches of the northern region, whereas the beaches of the southern region are composed mostly of carbonate sands of marine biogenic origin. An exception is the central sector of the south coast, which is enriched in quartz sand (~10 %); this can be related to outcrops of quartz-rich basement rock and also to rocks exposed in some northern drainage basins captured by southern streams since the Plio-Quaternary.  相似文献   

8.
A 15-cm-thick carbonate substrate encrusted with ferromanganese oxides from the Vityaz Fracture Zone, Central Indian Ridge was analyzed to reconstruct the paleoceanography of the region. Based on the calcareous nannoplankton assemblage, an early Pliocene age has been assigned to the calcareous substrate. Among the nannoplankton, discoasters outnumber coccoliths and show signs of dissolution. The presence of certain species of benthic Foraminifera such as Uvigerina, Lenticulina, Bulimina and Bolivina, indicates the infringement of the oxygen minimum zone during the deposition of the carbonates. The occurrence of a Reticulofenestra pseudoumbilica zone of early Pliocene age suggests a change in depositional conditions coinciding with the time of formation of the large depositional hiatuses documented in sediment cores from adjacent basins of the western Indian Ocean. These hiatuses resulted from the prevalence of intermediate subsurface currents such as the Somali Current or the Western Boundary Current.  相似文献   

9.
This study investigates the stratigraphic evolution of the Late Oligocene - Early Miocene carbonate platforms of the Yadana area (offshore Myanmar). Well data, regional 2D and local 3D seismic surveys allow the identification of three shallow-water carbonate platforms (Yadana, 3DF and 3DE) showing various morphologic and stratigraphic patterns influenced by the presence of a paleohigh. The identification of seven seismic sequences in the Yadana area constrains the stratigraphic evolution in three stages: (1) development of aggrading attached and isolated platforms during the Chattian; (2) a period of platform emersion during the Oligocene - Miocene transition; (3) drowning of the smaller buildup (3DE) associated with km-scale backstepping on the large platforms (3DF and Yadana) during the Aquitanian. The Aquitanian marks the onset of renewed volcanic activity associated with the development of fringing carbonate reefs during the Burdigalian. The rapid (∼6 My) development of these wide (∼5–70 km) and thick (∼300–850 m) carbonate platforms has been mainly controlled by the subsidence. However, the results highlight a strong overprint of eustatic fluctuations on the rates of change in accommodation, and hence on the stratigraphic architecture of the carbonate platforms. Based on an alternative model for the Cenozoic geodynamic evolution of the Yadana area, our results suggest that the platforms developed on a volcanic ridge of hotspot origin located in the Indian Ocean and not on a volcanic arc. Subduction jump processes are interpreted to have played a key role in the demise of all platforms by drastically changing the paleoenvironmental conditions during the Early Miocene, and led to the present-day location of the Yadana Ridge in a back-arc setting. The carbonate platforms from the Yadana area are thus a representative example of the interplay between global mechanisms and local paleoenvironmental parameters on carbonate platform initiation, growth and demise.  相似文献   

10.
Mass fluxes of diatom opal, planktonic foraminifera carbonate and coccolithophorid carbonate were measured with time-series sediment traps at six sites in the Arabian Sea, Bay of Bengal and Equatorial Indian Ocean (EIOT). The above fluxes were related to regional variations in salinity, temperature and nutrient distribution. Annual fluxes of diatom opal range between 3 and 28 g m−2 yr−1, while planktonic foraminifera carbonate fluxes range between 6 and 23 g m−2 yr−1 and coccolithophorid carbonate fluxes range between 4 and 24 g m−2 yr−1. Annual planktonic foraminifera carbonate to coccolithophorid carbonate ratios range between 0.8 and 2.2 and coccolithophorid carbonate to diatom opal ratios range between 0.5 and 3.3.In the western Arabian Sea, coccolithophorids are the major contributors to biogenic flux during periods of low nutrient concentrations. Coccolithophorid carbonate fluxes decrease and planktonic foraminiferal carbonate and diatom opal fluxes increase when nutrient-rich upwelled waters are advected over the trap site. In the oligotropic eastern Arabian Sea, coccolithophorid carbonate fluxes are high throughout the year. Planktonic foraminiferal carbonate fluxes are the major contributors to biogenic flux in the EIOT. In the northern and central Bay of Bengal, when surface salinity values drop sharply during the SW monsoon, there is a drastic reduction in planktonic foraminiferal carbonate fluxes, but coccolithophorid carbonate and diatom opal fluxes remain steady or continue to increase. Distinctly higher annual molar Sibio/Cinorg (>1) and Corg/Cinorg (>1.5) ratios are observed in the northern and central Bay of Bengal mainly due to lower foraminiferal carbonate production as a result of sharp salinity variations. We can thus infer that the enhanced freshwater supply from rivers should increase oceanic CO2 uptake. Its silicate supply favours the production of diatoms while the salinity drop produces conditions unfavourable for most planktonic foraminifera species.  相似文献   

11.
Hole 887B of the Ocean Drilling Program (ODP) comprises a 44 m (750 kyr) long continuous section recovered from the Patton–Murray Rise, an elevated plateau that is largely isolated from turbidite deposition. The Patton–Murray area is centered under the Alaska Gyre, a region characterized by the domal upwelling of nutrient-rich waters. Marked increases in productivity and rapid settling of biogenic matter are suggested throughout the section by the episodic accumulation of diatomaceous oozes up to ∼1 m thick that are accompanied by barium enrichments. Significant δ13Corg maxima in the major diatomaceous bands suggest that mixed-layer [CO2(aq)] may have been drawn down significantly during some of the productivity events. The episodes of enhanced productivity at Site 887 occur synchronously with short-lived minima in planktonic foram δ18O, suggesting a direct link to low salinity, or less likely, warming, events in the Gulf of Alaska. There is no obvious explanation for the events, but they may be related to seasonal incursions of meltwater from Alaska. We speculate that episodic input of meltwater- or dust-borne iron from Asian or Alaskan sources may have promoted the extraordinary diatom production events recorded in the sedimentary record.  相似文献   

12.
Data on the carbonate system of the Northwestern Indian Ocean obtained on a cruise of F.S. Meteor during SW monsoon in July/August 1995 were compared with those of George et al. [George, M.D., Kumar, M.D., Naqvi, S.W.A., Banerjee, S., Narvekar, P.V., de Sousa, S.N., Jayakumar, D.A., 1994. A study of the carbon dioxide system in the northern Indian Ocean during premonsoon. Mar. Chem. 47, 243–254] collected during intermonsoon. In general, deep water values agreed well between the two expeditions. Surface waters, however, showed a substantial increase in dissolved inorganic carbon (CT) in the coastal regions due to strong upwelling in the SW monsoon. This was also accompanied by very high CO2 partial pressures in surface waters. The north–south gradients in vertical profiles of the measured parameters in the Arabian Sea are discussed by comparing profiles from the oligotrophic equatorial region with those from the highly productive central Arabian Sea. The effect of denitrification on regenerated CT and AT is minor, with contributions of <9 and <8 μmol kg−1, respectively, to the total amount regenerated also utilizing oxygen. The dissolution of biogenic carbonates is discussed; different approaches to define the depth, where the dissolution starts (lysocline(s), carbonate critical depth (CCrD)), are compared together with the calculation of saturation depth from carbonate concentrations. It is shown, that small differences in measured CT and AT (found between our data and those measured during GEOSECS) and different calculation approaches to the CO2 system (different dissociation constants for species involved and taking into account phosphate and silicate concentrations) can produce pronounced differences in the calculated saturation depths. However, CT and AT data suggest substantial dissolution of biogenic carbonate in the water column even above the calcite lysocline, irrespective of the procedures followed to calculate this horizon.  相似文献   

13.
Seasonal deposition fluxes of sinking phytoplankton, zooplankton and major mass compounds (i.e. calcium carbonate, biogenic opal and organic matter), intercepted by deep-moored sediment traps, are contrasted with their sediment accumulation rates over the 2700 m deep central Walvis Ridge in the oligotrophic SE Atlantic. These data provide the first seasonally resolved record of biogenic particle fluxes in the South Atlantic Central Gyre and serve as the oligotrophic end member of a gradient across the Benguela system to the highly productive coastal upwelling off Namibia. Maximum fluxes at the central Walvis Ridge were deposited in early austral spring, following winter deepening of the surface mixed layer and associated nutrient entrainment. Nearly 25% of the annual mass flux arrived in October, when sea surface temperature rose, deep vertical mixing halted and surface production collapsed. The annual flux of diatoms was dominated by small specimens of Nitzschia bicapitata (60%) whereas Globorotalia inflata dominated the foraminiferal fluxes (25%). Diatom diversity dropped significantly during the bloom periods, when up to 80% was composed of small N. bicapitata, but foraminiferal diversity remained about constant. The diatom flux maximum, together with those of biogenic silica and organic matter, preceded those of the foraminifera, pteropods, carbonate and total mass by 1 week. Fluxes of the left- and right-coiled shells of the deep-dwelling foraminifer Globorotalia truncatulinoides peaked in different seasons, a distinctive ecological behaviour which merits their taxonomic recognition as separate species. These findings testify to recent evidence for the existence of several genetic species within G. truncatulinoides and now suggest that such species may also have different seasonal responses.The Benguela trophic gradient showed a shoreward increase in particle fluxes, but differences were surprisingly small, testifying to only moderately enhanced export productivity and deposition at the Namibian margin relative to the oligotrophic central gyre. From the open ocean toward coastal upwelling, small and weakly silicified diatoms were substituted by other, larger and more heavily silicified species, possibly in response to decreased silica limitation. Foraminiferal deposition fluxes were increasingly dominated by G. inflata, accompanied by a change-over from many warm- to few cold-water minor species. The late winter maximum at the Namibian margin and the early spring maximum at the central Walvis Ridge were generated by the same process of collapsing surface productivity in response to the shut down of nutrient entrainment at the winter to summer transition, although delayed by up to 2 months in the Central Gyre. At the sediment-water interface, intense degradation of organic matter and biogenic silica resulted in poor preservation accompanied by pronounced changes in the species composition of siliceous phytoplankton. Of all particle groups at the central Walvis Ridge, only the export of foraminiferal shells appeared to be fully transferred into the sediment, and through their species assemblage to provide a sedimentary record of past seasonal productivity conditions of the upper ocean.  相似文献   

14.
Trawling of the bottom in the northeastern Kara Sea during cruise 125 of the R/V Professor Shtokman in 2013 recovered a block of cavernous, partly phosphatized carbonate rock consisting of biogenic carbonate material and partly crystallized diagenetic calcite. The fauna remains are mainly Oligocene–Pliocene planktonic and benthic foraminifers, with less common Oligocene–Miocene coccoliths and single wormlike organisms. Part of the phosphatized material in caverns is impregnated by manganese and iron oxides and enriched in heavy and trace metals. According to the oxygen isotopic composition, this rock formed under moderate temperature conditions. In terms of morphology, mineralogy, and the abundance of organic remains, the block is comparable to methanogenic carbonates found in other parts of the ocean, but shows no isotopically light carbon signatures typical of methane activity. This indicates the diversity of the carbon isotope composition of the Arctic carbonates.  相似文献   

15.
The Apennine Chain provides the first example of stratigraphic (time) and synsedimentary tectonic (space) distribution of the calcari aLucina Miocene equivalents of modern cold-vent carbonates. Chemosynthetic faunal assemblages and related carbonate deposits are found at different stratigraphic levels, with peaks during Langhian-Serravallian and late Tortonian-early Messinian times. A general increase in frequency and volume occurs with time. A genetic link between venting and the Messinian Evaporite event is difficult to demonstrate. However,Lucina limestones are limited to preevaporitic times, and their maximum abundance is reached just before the onset of the Messinian Evaporite accumulation.Lucina limestones occur in almost all tectofacies of the orogen, from backland to foreland.  相似文献   

16.
A new approach to predict biogenic particle fluxes to the seafloor is presented, which is based on diffusive oxygen uptake and, in particular, opal fluxes to the seafloor. For this purpose, we used a recently published empirical equation coupling benthic silica to oxygen fluxes, and showing a clear negative correlation between Si and O2 fluxes. Dissolution of biogenic silica mediated by aerobic microbial activity has been inferred at 24 sites along the African and South American continental margins. Based on the assumption that these findings hold essentially for the entire Southern Atlantic Ocean, we applied the silica to oxygen flux ratio to a basin-wide grid of diffusive oxygen uptake extracted from the literature. Assuming that the silica release across the sediment-water interface equals the particulate flux of biogenic opal to the seafloor, we estimated minimum opal rain rates. We combined these calculations with published relationships between aerobic organic carbon mineralization and dissolution rates of calcite above the hydrographical lysocline, thereby assessing the calcite rain rate and particulate organic carbon flux to the seafloor. The addition of the buried fraction completes our budget of biogenic particulate rain fluxes. The combination of such empirical equations provides a powerful and convenient tool which greatly facilitates future investigations of biogenic particle fluxes to the seafloor.  相似文献   

17.
M. S. Barash 《Oceanology》2011,51(2):306-314
Among the abiotic factors that determined via the paleoceanographic processes development and evolution of the oceanic biota in the Neogene, noteworthy are the tectonic, volcanic, climatic and extraterrestrial events. The most important tectonic events of such kind include the subsidence of the Faroe-Iceland Threshold 14–13 Ma ago, the closure of the Tethys Ocean in the east 19–12 Ma ago, the orogenesis in the western Mediterranean region and closure of the Mediterranean Sea (Messinian Crisis) 5.59–5.33 Ma ago, the formation of the Central American Isthmus 6.0–3.5 Ma ago, and the opening of the Bering Strait that occurred (according to different data) in the period of 7.4 to 3.1 Ma ago. The most significant climatic consequence resulted from the formation of the Circum-Antarctic Current, the irregular growth of the Antarctic ice shield, the cooling in the Arctic region 3.2–3.1 Ma ago, and the development of continental glaciations in the Northern Hemisphere approximately 2.5 Ma ago. The variations in the atmospheric CO2 content are correlative with the climatic fluctuations. The entire Cenozoic climatic record reflects the influence of the orbital parameters of the Earth. The Neogene was marked by several significant extraterrestrial events: the fireball falling in southwestern Germany in the middle Miocene 14.8–14.5 Ma ago probably accompanied by enhanced volcanic activity particularly in the rift valley of eastern Africa; the drastically increased influx of interplanetary dust due to the disruption caused by a large asteroid in the late Miocene 8.3 ± 0.5 Ma ago, the fall of a large (>1 km in diameter) asteroid in the Eltanin Fault zone of the Southern Ocean in the terminal Pliocene 2.15 Ma ago; and the explosion of a supernova star, which was probably responsible for the partial extinction of marine organisms at the Pliocene-Pleistocene transition approximately 2 Ma ago.  相似文献   

18.
Thorium-234 and particle composition (organic matter, biogenic silica, carbonate and lithogenic component) were examined in the East China Sea (ECS) and the northern South China Sea (NSCS) in order to constrain the particle types scavenging thorium isotopes. Good positive correlations between particulate organic matter (POM) or carbonate and thorium-234 in suspended particulate matter (SPM) indicates that POM and carbonate are efficient to scavenge thorium-234. No relationship between biogenic silica and thorium-234 suggests that geochemical behavior of thorium-234 may be not influenced by biogenic silica. A simple model was used to evaluate the affinity of thorium-234 to different particle components. The results show that POM is the most efficient scavenger for thorium-234 in the ECS and the NSCS, followed by carbonate. The authors'' results lend support to the utility of thorium-234 as a proxy of POC and carbonate in the upper layer. However, the strong dependence of thorium scavenging on particle composition challenges thorium-230 as a constant flux proxy.  相似文献   

19.
A 1-D coupled physical-biogeochemical model is used to study the seasonal cycles of silicon and nitrogen in two High Nutrient Low Chlorophyll (HNLC) systems, the Antarctic Circumpolar Current (ACC) and the North Pacific Ocean, and a mesotrophic system, the North Atlantic Ocean. The biological model consists of nine compartments (diatoms, nano-flagellates, microzooplankton, mesozooplankton, two types of detritus, nitrate, ammonium and silicic acid) forced by irradiance, temperature, mixing and deep nitrate and silicic acid concentrations. At all sites, nanophytoplankton standing crop variations are low, in spite of variations in primary production, because of a “top–down” control by microzooplankton. Although nanophytoplankton sustain more than 60% of the annual primary production in these areas, their contribution to the export production does not exceed 1% of the total. The differences in the seasonal plankton cycle among these regions come mainly from differences in the dynamics of large phytoplankton (here diatoms). In the ACC, the chlorophyll maximum remains <1.5 mg m−3, as an unfavourable light/mixing regime and a likely trace-metal limitation keep diatoms from blooming. In the northeast Pacific, trace-metal limitation seems to keep diatoms from blooming throughout the year. In both these systems, light or iron limitations induce high Si/N uptake ratios. Incidentally these high Si/N uptake ratios lead to a net excess of silicic acid utilization over nitrate, and to a subsequent silicic acid limitation during the summertime. In the North Atlantic, under favourable light/mixing regime and nutrient-replete conditions at the onset of the growing period, diatoms outburst and sustain a bloom >3.5 mg Chl-a m−3. Thereafter, mesozooplankton grazing pressure and silicic acid limitation induce the collapse of the chlorophyll maximum and the persistence of lower chlorophyll concentrations in summer. Although the ACC and the North Pacific show HNLC features, they support a high biogenic silica production (1.9 and 1.07 mol Si m−2 yr−1) and export flux (0.79 and 0.61 mol Si m−2 yr−1), compared to the North Atlantic (production: 0.23 mol Si m−2 yr−1, export: 0.12 mol Si m−2 yr−1). The differences in Si production and export between the HNLC systems and the mesotrophic North Atlantic come from both higher Si concentrations and Si/N uptake ratios in the HNLC areas compared to the North Atlantic. Also, the low dissolution rate of biogenic silica compared to nitrogen degradation rate, and the inhibition of nitrate uptake by ammonium, reinforce the net excess of silicic acid utilization over nitrate. As a result, the model also illustrates the efficiency of the silica pump for the three sites: about 50% of the biogenic silica synthesized in the euphotic layer is exported out of the first 100 m, while only 4–11% of the particulate organic nitrogen escapes recycling in the surface layer.  相似文献   

20.
南大洋普里兹湾沉积物中生物硅含量与分布   总被引：1，自引：2，他引：1  
利用中国第18,21次南极考察获得的沉积物样品,对南大洋普里兹湾沉积物中生物硅(BSiO2)的含量以及分布特征进行了初步研究.结果表明:普里兹湾表层沉积物中生物硅含量丰富,生物硅含量在4.89%~85.41%之间变化,平均为30.90%.最高值出现在湾内的IV-10站.生物硅的垂向分布与间隙水中硅酸盐呈现相反的变化趋势.表层沉积物中生物硅和有机碳分布趋势与表层海水中叶绿素a、初级生产力的分布趋势密切相关,最大值均出现在普里兹湾环流中心区域,较好地反映了上层水体中初级生产力的变化状况.  相似文献   

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