Nepheloid layer dynamics in the northern Portuguese shelf

A general hydrographic and nephelometric survey of the NW Portuguese continental shelf and upper slope was undertaken, under winter and spring conditions. The nepheloid layer dynamics along the shelf during three cruises were controlled, principally by the following factors: (a) the hydrography of the shelf-slope waters, i.e. the nepheloid layers followed isopycnals and water masses; (b) prevalence of upwelling or downwelling circulation over the shelf; (c) dispersion of material by river discharge (mainly the Douro river); (d) resuspension of mid-shelf fine deposits induced by swell; (e) peculiar morphology with the presence of Porto Canyon and outer shelf rock outcrops.The general circulation controls the seaward extension of the nepheloid layers. In winter, under dominant downwelling conditions, an intense bottom nepheloid layer (BNL) was observed on the shelf, due to river borne particle supply and remobilization of mid-shelf muddy sediments (depth ≈100 m). In this period the BNL increased in thickness to the top of the slope, following the isopycnals. Near the shelf-break the BNL detached to form intermediate nepheloid layers (INL). Small INLs appear deeper in some areas of the slope. A surface nepheloid layer (SNL) appears in the surface water over the shelf and slope. In spring, biological particles mainly contribute to the SNL, which is separated from the BNL by a zone of clear water. In winter the SNL is restricted to the inner shelf where there are high inputs of fluvial particles.Downwelling circulation probably induces transport of shelf particles to deeper waters in the BNL. In spring, the predominant circulation was southward (upwelling), the water column was highly stratified, and dispersion of particles in the SNL was offshore.     

Particle flux across the mid-European continental margin

Results are presented from particle flux studies using sediment trap and current meter moorings along a transect at the European continental margin at 49°N within the EU-funded Ocean Margin Exchange (OMEX) project. Two moorings were placed, at the mid- and outer slope in water depths of 1500 and 3660 m, with traps at 600 and 1050 m and at 580, 1440 and 3220 m, respectively. Residual currents at the mid-slope follow the slope contour, whereas seasonal off-slope flow was registered at the outer slope. At 600 m on the slope fluxes are similar to those in the abyssal North Atlantic. The flux of all components (bulk dry weight, particulate organic and inorganic carbon, lithogenic matter and opal) increased with water depth. Highest fluxes were recorded at 1440 m at the outer slope, where off-slope residual currents mediate particle export. The injection of biogenic and lithogenic particles below the depth of winter mixing results in the export of particles from shallower waters. Calculated lateral fluxes of particulate organic carbon exceed the primary flux by over a factor of 2 at 1440 m on the outer slope. Estimated lateral fluxes of suspended particulate matter in the water column and intermediate nepheloid layers at the outer slope are potentially large compared to sinking fluxes measured by sediment traps. A comparison is made of particle flux at three continental margin sites and two sites in the adjacent open North Atlantic, from which it is seen that bulk and organic matter flux increases exponentially with proximity to the shelf break. The percentage contribution of particulate organic carbon to biogenic fluxes increases from a mean of 5.7% in the abyssal N. Atlantic to 13.9% at the continental margins.     

The importance of subsurface nepheloid layers in transport and delivery of sediments to the eastern Cariaco Basin,Venezuela

Optical transmissometer measurements were coupled with particulate organic matter (POM) observations to understand suspended sediment composition and distribution in the eastern Cariaco Basin during the rainy seasons of September 2003 and 2006. Our results suggest that nepheloid layers originating at the mouth of small mountainous rivers discharging into the eastern Basin are a major delivery mechanism of terrigenous sediments to the Basin interior. Intermediate nepheloid layers (INL) were observed near the shelf break (～100 m) and appear to effectively transport terrigenous material laterally from the shelf to deep waters, thereby providing a plausible supply mechanism of the terrestrial material observed in sediment traps. These findings highlight the importance of small, local rivers in the Cariaco Basin as sources of terrestrial material. In contrast, these nepheloid layers contained only limited POM. When this information is combined with published sediment trap POM data, it suggests that nepheloid layers may not be a primary mechanism for delivering terrigenous POM to the deeper waters of the basin during the rainy season. Rather, BNL may redistribute marine-derived POM from shallow waters to the Basin's interior by providing ballast materials, particularly during episodic events driven by wind and precipitation. Though we have determined that nepheloid layers play an important role in the seaward transport of particulate material in the Cariaco Basin, their composition and temporal variability have not been fully characterized. This is critical to understand lateral particle transport, since nepheloid layers constitute a significant source of sediment to the deep Cariaco Basin.     

南海北部海洋雾状层特征及影响因素初步研究

在粤东上升流区和南海北部陆架陆坡区共选取7条断面,通过分析水体的温度、盐度、密度、浊度、叶绿素浓度特征和悬浮体现场粒度平均粒径、体积浓度特征等,初步研究了南海北部海洋雾状层的特征及其受控机制。研究发现粤东上升流区底部雾状层普遍发育,在个别站位存在表层雾状层,高能的近岸浪流作用和来自港湾、河流的泥沙为表层雾状层和底部雾状层提供物源;南海北部陆架陆坡区底部雾状层也普遍发育,在珠江口外的内陆架和外陆架陆坡区表层雾状层比较常见,陆坡区中层雾状层比较发育,本区雾状层的物源主要来自珠江和台湾岛西南部河流的入海泥沙、海底沉积物以及生源颗粒物。南海北部底部雾状层的强度和扩散范围主要受水动力条件、河流入海物质的扩散和底质再悬浮等控制。     

Upward transport of iron at the west shelf edge–slope of the Okinawa Trough in the East China Sea

We studied the behavior of chemical substances in the upper 300 m of the water column across the continental shelf–slope interface in the East China Sea off the Okinawa Trough. The behaviors of iron, inorganic nutrients, and humic-like fluorescent dissolved organic matter were strongly influenced by the extensive water exchange between the East China Sea and the Kuroshio Current across the shelf break and slope via upwelling and frontal processes. We attributed the high humic-like fluorescent intensity at the subsurface of the shelf break and slope regions to the lateral supply of humic-like fluorescent dissolved organic matter from the shelf sediments to the outer shelf region due to the intrusion of shelf water into Kuroshio subsurface water. We found that the behavior of iron at the continental shelf–slope was remarkably different from the conservative mixing of inorganic nutrients and humic-like fluorescent dissolved organic matter. In deep and bottom waters at the shelf–slope, high total iron concentrations, which were closely related to water transmittance, possibly resulted from the swept transport of iron-rich resuspended sediments over the shelf floor from the slope by the invading Kuroshio Intermediate Water close to the bottom.     

Slope transport of suspended particulate matter on the Aquitanian margin of the Bay of Biscay

Spatial and temporal characteristics of the water masses and the dispersion of the suspended particulate matter were investigated using current meter, hydrographic and nephelometric observations, gathered during the ECOFER experiment (1989–1991) in the Cap-Ferret Canyon on the Aquitanian margin of the Bay of Biscay. While characteristics of the deep water masses were stable from one year to another, large hydrographic change in the upper 500 m related to winter renewal induced by poleward advection of warm and saline water along the continental slope. The slope circulation and seasonal eddy activity appear as important dynamical mechanisms that control the entrainment and the dispersion of the suspended particulate matter from the neritic domain to the deep ocean. A predominantly northward along-slope current with occasional reversal characterizes this circulation. The nephelometric structures also showed seasonal changes in the overall suspended particulate matter content, but recurrent features, such as the presence of intermediate nepheloid layers at the shelf-break depth and various depths along the slope (∼500, 1000 and 2000 m), were observed. These nepheloid layers extended off the slope to about 10–30 km, but especially laterally along the slope. Their presence indicated that suspended particulate matter exchanges between the shelf and the slope occurred mainly in the head of the canyon and along the southern open slope. The intermediate nepheloid layers around 500 m depth detached from the slope particularly in regions where the bottom slope is close to critical for the M₂ internal tide.     

Recent sediment transport and accumulation on the NW Iberian margin

Five transects across the NW Iberian margin were studied in the framework of the EU-funded Ocean Margin EXchange II (OMEX II) project, to determine and establish recent sediment and organic carbon transport and accumulation processes and fluxes.On the Galician shelf and shelf edge, resuspension of sediments resulting in well-developed bottom nepheloid layers was observed at all stations, but transport of suspended sediment appears largely confined to the shelf. On the continental slope, only very dilute bottom nepheloid layers were present, and intermediate nepheloid layers were only occasionally seen. This suggests that cross-slope transfer of particles is limited by the prevailing northerly directed shelf and slope currents.Optical backscatter and ADCP current measurements by the BOBO lander, deployed at 2152 m depth on the Galician slope, indicated that particles in the bottom boundary layer were kept in suspension by tidal currents with highest speeds between 15–25 cm s⁻¹. Net currents during the recording period August 6th–September 10th 1998, were initially directed along-slope toward the NNW, but later turned off-slope toward the SW.The separation of the water masses on the slope from the sediment-laden shelf water by the along-slope current regime is reflected in the recent sedimentary deposits of the Galician shelf and slope. Apart from compositional differences, shelf deposits differ from those on the slope by their higher flux of excess ²¹⁰Pb (0.57–5.37 dpm cm⁻²y⁻¹ versus 0.11–3.00 dpm cm⁻²y⁻¹), a much higher sediment accumulation rate (315.6–2295.9 g m⁻²y⁻¹ versus 10.9–124.7 g m⁻²y⁻¹) and organic carbon burial rate (1.01–34.30 g m⁻²y⁻¹ versus 0.01–0.69 g m⁻²y⁻¹).In contrast to the observations on the Galician margin, pronounced nepheloid layers occurred in the Nazaré Canyon, which extended to considerably greater water depths. This indicates that significantly greater transport of fine-grained particles in both the INL and the BNL was occurring within the canyon, as reflected in the exceptionally high ²¹⁰Pb excess flux (up to 34.09 dpm cm⁻²y⁻¹), mass accumulation rates (maximum 9623.1 g m⁻²y⁻¹) and carbon burial fluxes (up to 180.91 g m⁻²y⁻¹) in the sediment. However, radioisotope fluxes in the lower canyon were only slightly higher than at comparable depths on the Galician margin. This suggests that transport and rapid accumulation is focused on the upper and middle part of the canyon, from where it is episodically released to the deep sea. Compared to the Galician margin, the Nazaré Canyon may be considered as an important organic carbon depocenter on short time-scales, and a major conduit for particulate matter transport to the deep sea on >100 y time-scales.     

Sedimentary matter fluxes in the contour current sedimentation system over the continental slope of the Norwegian Sea

The sedimentation system of the bottom contour current over the continental slope of Bear Island in the Norwegian Sea is considered. The nepheloid layer that provides the high horizontal flux of sedimentary material represents the main source of matter for the bottom sediments. The vertical particulate matter flux is largely formed in the nepheloid layer; the flux from higher layers of the water column is insignificant. Horizontal and vertical fluxes of sedimentary matter show a positive correlation. The flux of the matter from the bottom sediment into the nepheloid matter and the residence time of particles in the latter are estimated.     

Cross-slope zonation of erosion and deposition in the Faeroe-Shetland Channel,North Atlantic Ocean

Boundary currents and internal waves determine cross-slope zonation of erosion and deposition in the Faeroe-Shetland Channel. Currents were measured at 8 and 34–50 m above the bottom at three mooring sites (502, 595 and 708 m depth) for 14 days. The structure of the water column was evaluated from CTD sections, and included nepheloid layers and particulate matter concentrations. Indicators for recent deposition in the sediment (organic carbon, phytopigments, ²¹⁰Pb) were measured at eight stations across the slope. Strong near-bottom currents at the upper slope sustain down-slope particle transport in a benthic nepheloid layer, which is eroded under the influence of critically reflecting M₂ internal tidal waves at 350–550 m, where the major pycnocline meets the sloping bottom. Beam attenuation profiles confirmed the presence of intermediate nepheloid layers intruding into the Channel along the major pycnocline, and elevated concentrations of particulate matter and chlorophyll-a were measured at this depth. Near-bottom currents decreased with depth, thus allowing particle deposition down the slope. Inventories of excess ²¹⁰Pb activity in the sediment deeper than 600 m were higher than what was expected on the basis of atmospheric input of ²¹⁰Pb and production in the water column, thus indicating additional lateral inputs. Simple calculations showed that off-slope input of particles from areas shallower than 600 m may be responsible for the enhanced deposition at greater depths.     

NotesBenthic foraminiferal distribution in surface sediments along continental slope of the southern Okinawa Trough: dependance on water masses and food supply

Benthic foraminiferal analysis of 29 samples in surface sediments from the southern Okinawa Trough is carried out. The results indicate that benthic foraminiferal abundance decreases rapidly with increasing water depth. Percentage frequencies of agglutinated foraminifera further confirm the modern shallow carbonate lysocline in the southern Okinawa Trough. From continental shelf edge to the bottom of Okinawa Trough, benthic foraminiferal fauna in the surface sediments can be divided into 5 assemblages: (1) Continental shelf break assemblage, dominated by Cibicides pseudoungerianus, corresponds to subsurface water mass of the Kuroshio Current; (2) upper continental slope assemblage, dominated by Cassidulina carinata , Globocassidulina subglobosa, corresponds to intermediate water mass of the Kuroshio Current; (3) intermediate continental slope assemblage, dominated by Uvigerina hispi-da, corresponds to the Okinawa Trough deep water mass above the carbonate lysocline; (4) lower continental slope- trough b     

Processes influencing dissolved iron distributions below the surface at the Atlantic Ocean–Celtic Sea shelf edge

Shelf break systems are highly dynamic environments. However little is known about the influence that benthic interactions and water mass mixing may have on vertical distributions of iron in these systems. Dissolved Fe (< 0.4 μm) concentrations were measured in samples from nine vertical profiles across the upper slope (150–2950 m water depth) at the Atlantic Ocean–Celtic Sea shelf break. Dissolved iron concentrations varied between less than 0.2 and 5.4 nM, and the resulting detailed section showed evidence of a range of processes influencing the Fe distributions. The near sea floor data were interpreted in terms of release and removal processes. The concentrations of dissolved Fe present in near seabed waters were consistent with release of Fe from in situ remineralisation of particulate organic matter at two upper slope stations, and possibly release from pore water upon resuspension on shelf. Lateral transport of dissolved iron was evident from elevated Fe concentrations in an intermediate nepheloid layer and its advection along isopycnals. Surface waters at the shelf break also showed evidence of vertical mixing of deeper iron-rich waters. These waters contained macronutrients that sustained primary productivity in these otherwise nutrient-depleted surface waters. The data also suggest some degree of stabilisation of relatively high concentrations of iron, presumably through ligand association or as colloids. This study supports the view that lateral export of dissolved iron to the interior of the ocean from shelf and coastal zones and may have important implications for the global budget of oceanic iron.     

Bulk particulate organic carbon in the East China Sea: Tidal influence and bottom transport

Five oceanographic surveys were carried out in the East China Sea (ECS) and Yellow Sea from 1999 to 2003. In all, seven different sections were surveyed, but one section (the PN section) was observed on every cruise. Two time-series stations were also surveyed, one located at the Changjiang River mouth, the other over the continental shelf in the PN section.We identified biogeochemical characteristics for waters close to the Changjiang Estuary and in the Kuroshio waters (KW), respectively. Resuspension is a strong feature near bottom over the ECS continental shelf, with suspended matter values 13 times higher than that for the surface. A model of particulate organic carbon (POC) dynamics based on a rectangle equation reveals that POC concentration close to the Changjiang Estuary varies with a semidiurnal period of ∼13 h, coinciding with the tidal period. The upper limit for POC residence times in the seasons we covered over the shelf are estimated to be on the order of weeks and generally increase seaward from near the Changjiang Estuary to the KW. Short POC residence times suggest that POC in the ECS is rapidly exported from euphotic waters.A nepheloid layer, observed as elevated suspended matter in near the bottom of the water column, is important in particle transfer over the shelf, especially in winter when the residual current flows mainly eastward. Cross-shelf transport of POC via the nepheloid layer is estimated to be 0.22 × 10¹² g yr⁻¹. Comparison with other work indicates that POC transport is ∼2% of the Changjiang POC input.     

Suspended particulate matter and nepheloid layers over the southern margin of the Cretan Sea (N.E. Mediterranean): seasonal distribution and dynamics

Seawater along the southern margin of the Cretan Sea (May 1994–September 1995) has been found to have light transmission values ranging from 79% to 94%, corresponding to SPM values ranging from 1.5 mg l⁻¹ to 0.2 mg l⁻¹. The highest SPM concentrations (mostly of terrigenous origin) were found close to the sea-bed over the shelf-break and upper slope. The origins of SPM in the surface waters (<150 m) is principally biogenic. The occurrence of nepheloid layers at intermediate depths within the upper water column is mostly a result of density stratification. The dynamics of SPM distributions are governed by the 2-gyre system which induces a general onslope flow; and so inhibits the seaward dispersion of the relatively more turbid coastal/shelf waters. This is in agreement with the virtual absence of suspensates of terrigenous origin offshore of the shelf-break. Near bottom nepheloid layers (BNL) and detached intermediate nepheloid layers occur in the vicinity of the shelf-break and over upper slope region; these may be explained by resuspension induced by near-bed current activity and breaking of internal waves. High concentrations of SPM near the seabed may be caused by anthropogenic (trawling) activity. Occasionally, the formation of BNL may result from local seismic activity resulting in gravity-driven mass movements.     

Dynamics of particle export on the Northwest Atlantic margin

The Northwest Atlantic margin is characterized by high biological productivity in shelf and slope surface waters. In addition to carbon supply to underlying sediments, the persistent, intermediate depth nepheloid layers emanating from the continental shelves, and bottom nepheloid layers maintained by strong bottom currents associated with the southward flowing Deep Western Boundary Current (DWBC), provide conduits for export of organic carbon over the margin and/or to the interior ocean. As a part of a project to understand dynamics of particulate organic carbon (POC) cycling in this region, we examined the bulk and molecular properties of time-series sediment trap samples obtained at 968 m, 1976 m, and 2938 m depths from a bottom-tethered mooring on the New England slope (water depth, 2988 m). Frequent occurrences of higher fluxes in deep relative to shallower sediment traps and low Δ¹⁴C values of sinking POC together provide strong evidence for significant lateral transport of aged organic matter over the margin. Comparison of biogeochemical properties such as aluminum concentration and flux, and iron concentration between samples intercepted at different depths shows that particles collected by the deepest trap had more complex sources than the shallower ones. These data also suggest that at least two modes of lateral transport exist over the New England margin. Based on radiocarbon mass balance, about 30% (±10%) of sinking POC in all sediment traps is estimated to be derived from lateral transport of resuspended sediment. A strong correlation between Δ¹⁴C values and aluminum concentrations suggests that the aged organic matter is associated with lithogenic particles. Our results suggest that lateral transport of organic matter, particularly that resulting from sediment resuspension, should be considered in addition to vertical supply of organic matter derived from primary production, in order to understand carbon cycling and export over continental margins.     

The nepheloid layer and horizontal sedimentary matter fluxes in the Norwegian Sea

In this paper, the results of long-term studies of the nepheloid layer in the Norwegian Sea in the Komsomolets test area are considered. Its principal characteristics such as its thickness, the particulate matter concentration in the nepheloid layer, and its standing crop are presented. The nepheloid layer is formed by the near-bottom current, which, together with the particulate standing crop, determines the horizontal flux of the sedimentary matter over the continental slope. Also presented are data about the currents in different years of observations as well as estimates of the horizontal fluxes.     

海洋雾状层的成因及其对海洋碳循环过程的影响

海洋雾状层既是陆源物质进入海底的输送通道,又是海洋水体中沉降颗粒及底部再悬浮颗粒物的停留场所。雾状层物质来源主要有陆源、生源以及海底表层沉积物的再悬浮,不同海区、不同层位的雾状层的物质来源有所差异;雾状层的成因具有复杂性,既有物理作用,又有生物及化学作用,大量研究表明,海底洋流、内波(潮)等物理作用是雾状层形成的主要控制因素。雾状层中碳的存在形态主要有颗粒有机碳(POC)、溶解有机碳(DOC)、胶体有机碳(COC)以及无机碳,雾状层与其上下海水之间、雾状层与海底表层沉积物之间不同形态碳在生物-化学-物理动力系统作用下不断发生物质交换与迁移,对海洋碳循环生物地球化学过程起重要的控制作用,是整个海洋碳循环的一个不可忽视的环节。     

Summer distribution and geochemical composition of suspended-particulate matter in the east china sea

The distribution and geochemical composition of suspended-particulate matter (SPM) in the East China Sea (ECS) were investigated during the summer period of high continental runoff to elucidate SPM sources, distribution and cross-shelf transport. The spatial variability of SPM distribution (0.3–6.5 mg l⁻¹) and geochemical composition (POC, Al, Si, Fe, Mn, Ca, Mg and K) in the ECS was pronounced during summer when the continental fluxes of freshwater and terrestrial materials were highest during the year. Under the influences of Changjiang runoff, Kuroshio intrusion, surface production and bottom resuspension, the distribution generally showed strong gradients decreasing seaward for both biogenic and lithogenic materials. Particulate organic carbon was enriched in surface water (mean ∼18%) due to the influence of biological productivity, and was diluted by resuspended and/or laterally-transported materials in bottom water (mean 9.4%). The abundance of lithogenic elements (Al, Si, Fe, Mn) increased toward the bottom, and the distribution correlations were highly significant. Particulate CaCO₃ distribution provided evidence that the SPM of the bottom water in the northern part of the study area was likely mixed with sediments originally derived from Huanghe. A distinct benthic nepheloid layer (BNL) was present in all seaward transects of the ECS shelf. Sediment resuspension may be caused by tidal fluctuation and other forcing and be regarded as the principal agent in the formation of BNL. This BNL was likely responsible for the transport of biogenic and lithogenic particles across or along the ECS shelf. Total inventories of SPM, POC and PN are 46, 2.8 and 0.4 Tg, respectively, measured over the total area of 0.45 × 10⁶ km² of the ECS shelf. Their mean residence times are about 27, 13 and 11 days, respectively. The inventory of SPM in the water column was higher in the northernmost and southernmost transects and lower in the middle transects, reflecting the influences of terrestrial inputs from Changjiang and/or resuspended materials from Huanghe deposits in the north and perhaps from Minjiang and/or Taiwan’s rivers in the south. The distribution and transport patterns of SPM and geochemical elements strongly indicate that continental sources and cross-shelf transport modulate ECS particulate matter in summer.     

Dispersal and deposition of suspended sediment on the shelf off the Tagus and Sado estuaries, S.W. Portugal

The Portuguese margin in front of the Tagus and Sado rivers is characterized by a narrow shelf incised by numerous canyons and by a large mud deposit. The two estuaries that feed this continental margin have distinct impact. The suspended particulate matter concentration values in the mouth of the Tagus are four times higher than in the Sado. During the summer the surface nepheloid layer is always larger than during the winter when it is restricted near the mouth of the estuary. This nepheloid layer may reach 30 km in length extending westward. The bottom nepheloid layer usually shows higher nephelometer values, and has a typical distribution: it is usually diverted southward in the direction of the Lisbon Submarine Canyon. We estimate the amount of suspended matter being discharged annually from the Tagus estuary to be between 0.4 and 1×10⁶ t. The area covered by fine deposits is about 560 km². Hence the thickness of sediments deposited annually should be between 0.07 and 0.18 cm. The sedimentation rates calculated from the ²¹⁰Pb excess vary between 0.16 and 2.13 cm y⁻¹ which correspond to the maximum rate. For a layer of 1 cm thick, 81,000 t of particulate organic carbon (POC) should be trapped. That would represent, with a minimum sedimentation rate between 0.07 and 0.18 cm y⁻¹, an entrapment of 6000–15,000 t POC y⁻¹. The trace metals content of box core samples clearly shows the anthropogenic impact in the uppermost level (5 cm thick) in the Tagus estuary and in all the sedimentary deposits (15 cm thick) on the shelf muddy area. Despite the narrowness of the shelf, a significant part of continental fluxes fails to reach the deep ocean.     

The pattern of cross-slope depositional fluxes

A simple model with horizontal and vertical diffusivities and settling velocity is used to calculate expected distributions of suspended particulate matter in a section across the continental shelf and slope. Dependencies on the shelf and slope profile, diffusivities, settling velocity, cross-slope advection and boundary sources/sinks are explored. It is found that the strongest factors are relative values of diffusivities and settling velocity, and the distribution of sources and sinks – including bottom deposition or resuspension. The latter is the principal means whereby an increased concentration near the bottom is likely, and is suggested as the usual reason for increased deposition recorded by sediment traps nearer the bottom. Observed thin, near-horizontal intermediate nepheloid layers put bounds on the vertical diffusivity and settling velocity, e.g. O(10^-4 m² s^-1, 10^-5 m s^-1) over Goban Spur in OMEX.     

Benthic foraminiferal distribution in surface sediments along continental slope of the southern Okinawa Trough: dependance on water masses and food supply

Benthic foraminiferal analysis of 29 samples in surface sediments from the southern Okinawa Trough is carried out.The results indicate that benthic foraminiferal abundance decreases rapidly with increasing water depth.Percentage frequencies of agglutinated foraminifera further confirm the modern shallow carbonate lysocline in the southern Okinawa Trough.From continental shelf edge to the bottom of Okinawa Trough,benthic foraminiferal fauna in the surface sediments can be divided into 5 assemblages:(1) Continental shelf break assemblage,dominated by Cibicides pseudoungerianus,corresponds to subsurface water mass of the Kuroshio Current;(2) upper continental slope assemblage,dominated by Cassidulina carinata,Globocassidulina subglobosa,corresponds to intermediate water mass of the Kuroshio Current;(3) intermediate continental slope assemblage,dominated by Uvigerina hispida,corresponds to the Okinawa Trough deep water mass above the carbonate lysocline.