Surface Sediments Off Research Articles

Contrasting vertical distributions of recent planktic foraminifera off Indonesia during the southeast monsoon: implications for paleoceanographic reconstructions

Abstract. Planktic foraminifera are widely used in paleoceanographic and paleoclimatic studies. The accuracy of such reconstructions depends on our understanding of the organisms' ecology. Here we report on field observations of planktic foraminiferal abundances (>150 µm) from five depth intervals between 0–500 m water depth at 37 sites in the eastern tropical Indian Ocean. The total planktic foraminiferal assemblage here comprises 29 morphospecies, with 11 morphospecies accounting for 90 % of the total assemblage. Both species composition and dominance in the net samples are broadly consistent with the published data from the corresponding surface sediments. The abundance and vertical distribution of planktic foraminifera are low offshore western Sumatra and increase towards offshore southern Java and the Lesser Sunda Islands (LSI). Average living depth of Trilobatus trilobus, Globigerinoides ruber, and Globigerina bulloides increases eastward, while that of Neogloboquadrina dutertrei, Pulleniatina obliquiloculata, and Globorotalia menardii remains constant. We interpret the overall zonal and vertical distribution patterns in planktic foraminiferal abundances as a response to the contrasting upper-water-column conditions during the southeast monsoon, i.e., oligotrophic and stratified offshore Sumatra (non-upwelling) vs. eutrophic and well mixed offshore Java–LSI (upwelling). Overall, the inferred habitat depths of selected planktic foraminifera species show a good agreement with those from sediment trap samples and from surface sediments off Sumatra, but not with those from surface sediments off Java–LSI. The discrepancy might stem from the different temporal coverage of these sample types. Our findings highlight the need to consider how foraminiferal assemblages and ecology vary on shorter timescales, i.e., from “snapshots” of the water column captured by a plankton net to seasonal and interannual variability as recorded in sediment traps, and how these changes are transferred and preserved in deep-sea sediments.

Silicon Isotope Signatures of Radiolaria Reveal Taxon-Specific Differences in Isotope Fractionation

The global silicon (Si) cycle plays a critical role in regulating the biological pump and the carbon cycle in the oceans. A promising tool to reconstruct past dissolved silicic acid (DSi) concentrations is the silicon isotope signature of radiolaria (δ30Sirad), siliceous zooplankton that dwells at subsurface and intermediate water depths. However, to date, only a few studies on sediment δ30Sirad records are available. To investigate its applicability as a paleo proxy, we compare the δ30Sirad of different radiolarian taxa and mixed radiolarian samples from surface sediments off Peru to the DSi distribution and its δ30Si signatures (δ30SiDSi) along the coast between the equator and 15°S. Three different radiolarian taxa were selected according to their specific habitat depths of 0–50 m (Acrosphaera murrayana), 50–100 m (Dictyocoryne profunda/truncatum), and 200–400 m (Stylochlamydium venustum). Additionally, samples containing a mix of species from the bulk assemblage covering habitat depths of 0 to 400 m have been analyzed for comparison. We find distinct δ30Sirad mean values of +0.70 ± 0.17‰ (Acro; 2 SD), +1.61 ± 0.20 ‰ (Dictyo), +1.19 ± 0.31 ‰ (Stylo) and +1.04 ± 0.19 ‰ (mixed radiolaria). The δ30Si values of all individual taxa and the mixed radiolarian samples indicate a significant (p < 0.05) inverse relationship with DSi concentrations of their corresponding habitat depths. However, only δ30Si of A. murrayana are correlated to DSi concentrations under normally prevailing upwelling conditions. The δ30Si of Dictyocoryne sp., Stylochlamydium sp., and mixed radiolaria are significantly correlated to the lower DSi concentrations either associated with nutrient depletion or shallower habitat depths. Furthermore, we calculated the apparent Si isotope fractionation between radiolaria and DSi (Δ30Si ∼ 30ε = δ 30Sirad − δ 30SiDSi) and obtained values of −1.18 ± 0.17 ‰ (Acro), −0.05 ± 0.25 ‰ (Dictyo), −0.34 ± 0.27 ‰ (Stylo), and −0.62 ± 0.26 ‰ (mixed radiolaria). The significant differences in Δ30Si between the order of Nassellaria (A. murrayana) and Spumellaria (Dictyocoryne sp. and Stylochlamydium sp.) may be explained by order-specific Si isotope fractionation during DSi uptake, similar to species-specific fractionation observed for diatoms. Overall, our study provides information on the taxon-specific fractionation factor between radiolaria and seawater and highlights the importance of taxonomic identification and separation to interpret down-core records.

A comparison of bacterial communities from OMZ sediments in the Arabian Sea and the Bay of Bengal reveals major differences in nitrogen turnover and carbon recycling potential

ABSTRACT The Northern Indian Ocean hosts two Oxygen Minimum Zones (OMZ), one in the Arabian Sea and the other in the Bay of Bengal. 16S rRNA gene amplicon sequencing was used to understand the total bacterial diversity in, the surface sediment off Goa within the OMZ of the Arabian Sea, and off Paradeep within the OMZ of the Bay of Bengal. Functional profiling was carried out to pinpoint the occurrence of specific bacterial operational taxonomic units (OTUs) which have been previously described to harbour certain genes/enzymes relevant to biogeochemical cycling of carbon, nitrogen, and sulfur compounds. The dominant phyla identified included Firmicutes (33.08%) and Proteobacteria (32.59%) from the Arabian Sea, and Proteobacteria (52.65%) and Planctomycetes (9.36%) from the Bay of Bengal. Only 30% of OTUs were shared between the sites which make up three-fourth of the Bay of Bengal OMZ bacterial community, but only one-fourth of the Arabian Sea OMZ sediment bacterial community. Statistical analysis indicated the bacterial diversity from sediments of the Bay of Bengal OMZ is ∼48% higher than the Arabian Sea OMZ. The community analysis combined with a predictive functional profiling of 16S rRNA amplicons revealed some major differences regarding sediment nitrogen fixation and carbon recycling, and identified a distinct bacterial community structure within the two shallow OMZ sites lying in the east coast and west coast of the peninsular India.

Reconstruction of radiocesium levels in sediment off Fukushima: Simulation analysis of bioavailability using parameters derived from observed 137Cs concentrations

Radiocesium was released to the North Pacific coastal waters by the accident at the Fukushima Dai-ichi Nuclear Power Plant (1FNPP) of the Tokyo Electric Power Company (TEPCO) in March 2011. Since the radiocesium in the sediment off Fukushima was suggested as a possible source for the transfer of this radionuclide through the benthic food chain, we conducted numerical simulations of 137Cs in sediments off the Fukushima coast by using a model which incorporates dynamic transfer processes between seawater and the labile and refractory fractions in sediment particles. This model reproduced the measured temporal changes of 137Cs concentration in seabed surface sediment off Fukusima coasts, by normalizing the radiocsium transfer between seawater and sediment according to the particle diameter sizes. We found that the 137Cs level in sediment decreased by desorption during the first several months after the accident, followed by a reduction in the labile fraction until the end of 2012. The apparent decrease of the total radiocesium level in surface sediment was estimated to occur at rates of approximately 0.2 y−1 within a 20 km distance from the 1FNPP. The comparison of 137Cs level decreases in the demersal fish and the simulated temporal labile fraction in fine sediment demonstrated that the consideration of radiocesium transfer via sediment is important for determining the 137Cs depuration mechanism in some demersal fish.

Tracing source–sink processes and productivity from trace metals (Ba, Zn, Pb, Cd) of the surface sediments off Mahanadi to Pennar, western Bay of Bengal

In this paper, an attempt is made to understand the provenance of sediments and the role of dissolved oxygen in the preservation of metals and the productivity variations. Sixty-one surface sediment samples collected along eight transects from western Bay of Bengal off Mahanadi, Vamsadhara, Godavari, Krishna and Pennar Rivers were analyzed for spatial variations in selected metals (Ba, Zn, Pb, Cd). A change in source of sediment from north to south was very well reflected from the distribution of metals. Ba and Pb were dominant in the northern transects off Mahanadi and Vamsadhara indicating a felsic source for the sediments as Ba and Pb are more associated with rocks rich in feldspar. Conversely, lower Ba and Pb off Godavari, Krishna and Pennar indicated mafic source for the sediments as these metals are less associated with Deccan basalts. Furthermore, the preservation and distribution pattern of the redox-sensitive metals Zn and Ba was largely controlled by the oxygen minimum zone prevalent in the region at intermediate water depths. Negative biogenic Ba values and a poor correlation of Cd with Chl-a and P indicated low Ba and Cd input from plankton productivity, further enhancing the existing knowledge regarding low productivity in western Bay of Bengal.

A baseline study on the concentration of trace elements in the surface sediments off Southwest coast of Tamil Nadu, India

Forty two surface sediment samples were collected in order to document baseline elemental concentration along the Southwest coast of Tamil Nadu, India. The elements detected were Manganese (Mn), Zinc (Zn), Iron (Fe), Copper (Cu), Nickel (Ni) and Lead (Pb). The concentration of Fe and Mn was primarily controlled by the riverine input. The source of Pb and Zn is attributed to leaded petrol and anti-biofouling paints. The calculated index (EF, Igeo and CF) suggests that the sediments of the study area are significantly enriched with all elements except Pb. The contamination factor showed the order of Mn>Zn>Fe>Cu>Ni>Pb. The sediment pollution index (SPI) revealed that the sediments belonged to low polluted to dangerous category. The correlation matrix and dendrogram showed that the elemental distribution was chiefly controlled by riverine input as well as anthropogenic activity in the coast.

Processes affecting long-term changes in 137Cs concentration in surface sediments off Fukushima

Temporal changes in cesium-137 (137Cs) concentrations in the surface (0–10 cm) layer of seabed sediment were quantified from continuous observation data at 71 stations within a 150-km radius of the Fukushima Daiichi Nuclear Power Plant, and the primary processes affecting temporal changes were identified. From March 2011 to the end of 2015, about 80% of the initially deposited 137Cs in the surface sediment in the coastal region (bottom depth ≤100 m) region has dissipated (radioactive decay is not included). Such a remarkable change in the 137Cs concentration was not observed in the offshore (>100 m) region. This paper focuses on the following three processes that affected the decrease in the 137Cs concentrations, and assesses their relative importance; (1) resuspension and transport of 137Cs-bound sediment, (2) desorption of 137Cs from the sediment, and (3) dilution of 137Cs by vertical mixing of sediment. Consequently, it was estimated that the first two processes together have potentially contributed to reduce the 137Cs inventory in the top 10 cm of the coastal region by at most 35%. Furthermore, by applying a pulse input sediment mixing model to the observed vertical distribution of sedimentary 137Cs, it was also estimated that more than 43% of the 137Cs in the surface sediment was transported to deeper sediment layers by vertical mixing of the sediment. This indicates that the decrease of 137Cs concentrations in coastal sediments was mainly affected by mixing of 137Cs-bound surface sediment with less contaminated sediment in the deeper layers.

Modern muddy deposit along the Zhejiang coast in the East China Sea: Response to large-scale human projects

Grain size and clay minerals in the surface sediment off Zhejiang Province, China, of the East China Sea were analyzed to study changes in grain size, muddy deposit boundary, and major riverine and other derived matters transport paths in the Zhejiang coastal muddy deposit since the impoundment of the Three Gorges Dam and after other large-scale human projects. The results show that the sediment types are mainly silt and mud in the muddy deposit, divided based on the 10% isoline of the sand-sized component. The sources of sediment in the muddy deposit are mainly the Yangtze River and simultaneously supplies from the Qiantang Jiang, Ou Jiang, relict fine-grain matter, and hydrolyzed volcanic rocks around the Zhoushan Islands. The transport and dispersal of sediments in the study area are largely controlled by the Zhejiang–Fujian coastal current and the Taiwan Warm Current and appear seasonally. The contributions from the Ou Jiang, relict matter, local hydrolyzed matter, and the Qiantang Jiang are enlarged owing to the decline of Yangtze suspended matter and the constructions of major human projects in the Hangzhou Bay, respectively. In addition, the sediment grain size exhibits a fining trend because of the influence of the Three Gorges Dam. The boundary of the muddy deposit is relatively stable after the Three Gorges Dam impoundment north of the city of Zhoushan. In contrast, south of the city of Zhoushan the boundary of the muddy deposit lies toward the east because the sediment supply from the relict fine-grained matters resuspended by the Taiwan Warm Current east of the study area. The changes in the grain size and contributions from smaller rivers and other derived matter as well as the boundary of the muddy deposit there will probably become more pronounced in the future.

Assessment of trace element accumulation in surface sediments off Chennai coast after a major flood event

The present study was conducted to assess the trace element concentration in marine surface sediments after major flood event of Chennai metropolis, India. Thirty surface samples were collected from off Chennai coast. Trace elements, organic matter, CaCO3, sand-silt-clay and C/N ratios were studied to understand the accumulation dynamics on sediments. The elemental concentration, calcium carbonate and OM distribution suggest that they are derived from urban runoff and transported through Adyar and Cooum Rivers. The enrichment factor reveals that the sediments are enriched by Pb, Cu, Zn, Cr, Co, Ni followed by Fe. The observed Igeo value shows that the samples are contaminated by Pb, Cu and Zn. The elemental concentration of the surface sediments is low when compared to other coastal region except Pb. The elevated level of Pb in the surface sediments is probably due to migration of contaminated urban soil from industrial and transportation sectors into marine environment.

Metals bioavailability in surface sediments off Nile delta, Egypt: Application of acid leachable metals and sequential extraction techniques.

Surface sediments from Nile Delta coast were analyzed for texture, CaCO3, organic matter, fractionation, and acid leachable metals (Cr, Fe, Mn, Ni, Pb, and Zn). The distribution pattern of acid leachable heavy metals in the sediment follows the sequence: Fe>Mn>Pb>Zn>Ni>Cr. All the acid leachable metals didn't exceed the sediment quality guidelines values (effects range low (ERL) and effects range medium (ERM)) and therefore doesn't represent a danger to marine organisms. The correlation of acid leachable Fe, Ni, and Mn indicates a similarity in the association of metals of similar origin. The negative correlation of sand with acid leachable Cr, Fe, Ni, Pb, and Zn indicates that these elements can be easily released by ion exchange processes due to the electrostatic interaction of trace metals as they are weakly bound and is bioavailable to the liquid phase. The acid leachable Cr, Pb, and Zn indicate their association with the CaCO3, while acid leachable Fe, Mn, and Ni are hardly combined with carbonates. All the contents of acid leachable metals are negatively correlated or uncorrelated with OM, which indicates that the studied heavy metals are hardly combined with OM. The results of the partitioning study showed that the residual form was the dominant fraction of the Cr, Fe, and Ni among most of the studied locations. Among the non-lithogenic fractions, the Fe-Mn oxy-hydroxide is the main scavenger for all metals. In terms of risk assessment code (RAC) value, a decrease order in environmental risk by heavy metals was Pb>Mn>Zn>Ni>Cr>Fe. Although the results of the two techniques were not consistent with each other in terms of predicting the metals bioavailability, a combination of total metal concentrations, acid leachable metals, and sequential extraction analysis is necessary to acquire the comprehensive information on the baseline, anthropogenic input, and bioavailability of heavy metals.

Submarine weathering of silicate minerals and the extent of pore water freshening at active continental margins

In order to investigate how submarine weathering processes may affect the water balance of sediments at convergent plate margins, six sediment cores were retrieved off Central Chile at water depth between ∼800 and 4000m. The sediment solid phase was analyzed for its major element composition and the pore fluids were analyzed for dissolved sulfate, sulfide, total alkalinity, major cations, chloride, bromide, iodide, hydrocarbons as well as the carbon isotopic composition of methane.Because of negligible weathering on land, surface sediments off Central Chile are rich in reactive silicate minerals and have a bulk composition similar to volcanic rocks in the adjacent Andes. Deep-sourced fluxes of alkalinity, cations and chloride indicate that silicate minerals are subject to weathering in the forearc during burial. Comparison of deep-sourced signals with data from nearby Ocean Drilling Program Sites reveals two different types of weathering processes: In shallow (tens of meters), methanic sediments of slope basins with high organic carbon burial rates, reactive silicate minerals undergo incongruent dissolution through reaction with CO2 from methanogenesis. At greater burial depth (hundreds of meters), silicate weathering is dominated by authigenic smectite formation. This process is accompanied by uptake of water into the clay interlayers thus leading to elevated salinities in the surrounding pore water. Deep-seated smectite formation is more widespread than shallow silicate dissolution, as it is independent from the availability of CO2 from methanogenesis. Although solute transport is not focused enough to form cold seeps in the proper sense, tectonically induced, diffuse fluid flow transfers the deep-seated signal of smectite formation into the shallow sediments.The temperature-controlled conversion of smectite to illite is considered the most important dehydration process in marine forearc environments (depth of kilometers). However, in agreement with other studies at active margins (e.g. Aleutians, Cascadia, Nankai Trough) and despite ubiquitous evidence for smectite formation, little evidence for seafloor seepage of dehydration fluids could be found off Central Chile. We argue that the circular process of pore water uptake during smectite formation and release upon illitization implies a balanced freshwater budget and therefore a rather limited potential for net pore water freshening on a margin-wide scale. According to this rationale, pore water freshening at seafloor seeps preferentially occurs at lower latitudes (Central America, Barbados, Mediterranean Ridge) where terrestrial weathering is more intense thus leading to external (i.e. detrital) smectite and thus freshwater inputs to the subduction system.

Chemical speciation and contamination assessment of Pb and V by sequential extraction in surface sediment off Nile Delta, Egypt

Environmental pollution caused by lead and vanadium is almost entirely due to industrial activities, such as the production of steel, pigments, photographic materials and insecticides.The concentration and chemical speciation of Pb and V were studied in surface sediments from 11 stations off Nile River Delta. Sequential extraction technique was applied to assess the four (exchangeable, acid-reducible, oxidizable-organic and residual) fractions in surface sediment, also to obtain an overall classification of lead and vanadium pollution in this area through its spatial distribution. This investigation was the first study on the basis of the chemical speciation of Pb and V in surface sediments off Nile Delta.The total concentrations of metals were ranged (22.8–41.3μgg−1) for Pb and (66.6–142.5μgg−1) for V. The chemical speciation in most sampling stations was in the order of Residual>acid-reducible>oxidizable-organic>exchangeable for Pb and in the order of Residual>oxidizable-organic>exchangeable>acid-reducible for V. The results showed that the Pb in surface sediments off Nile River Delta existed in the nonresistant fractions while vanadium existed in the resistant fractions. The degree of surface sediments contamination was determined for individual contamination factors (ICF) and global contamination factor (GCF). The result of ICF and GCF values showed that those stations located in the vicinity of municipal area (especially Lake Burullus outlet) had high potential risk to fauna and flora of study area. Risk assessment code (RAC) analysis indicated that although Pb presented a moderate overall risk to the aquatic environment, vanadium showed a low risk (RAC<10%) at six sites.

Spatial Distribution of Δ14C Values of Organic Matter in Surface Sediments Off Saru River in Northern Japan, One Year After a Flood Event in 2006

Dispersion and deposition of terrestrial organic matter by flooding on the inner shelf were studied using C/N ratios, δ13C, and Δ14C values of sedimentary organic matter. Surface sediment samples (top 2 cm) were collected from coastal areas near the Saru River in southwestern Hokkaido, northern Japan, 1 yr after a flood event in 2006. Riverine suspended solids were also collected at a fixed station downstream during 2006–2008. Sandy sediments were located at the front of the river mouth and the western part of the sampling area, with the δ13C of organic matter ranging from −23.8‰ to −22.0‰, Δ14C of –655‰ to –388‰, and an organic carbon/total nitrogen (C/N) ratio of 5.9–7.7. On the other hand, silt and clay sediments were distributed in a restricted area 11–16 km from the river mouth, with lighter δ13C (–26.7‰ to −24.1‰) and higher Δ14C (–240‰ to –77‰) of organic matter and C/N ratio (7.8–13.3). From end-member analysis, the apparently younger and less degraded organic matter in the silt and clay sediments consists mainly of terrestrial organic matter released by flood events. They remain in the depression, although most flood deposits were moved to deep-sea environments.

Accumulation of prokaryotic remains during organic matter diagenesis in surface sediments off Peru

Bacterial biomarkers (D‐amino acids and muramic acid) were investigated in surface sediments (0–1 cm) at 22 stations in the Peru margin. Concentrations were used to quantitatively estimate the relative importance of peptidoglycan in the preservation of prokaryotic remains during diagenesis. The bacterial imprint in organic matter was also evident from low molar ratios (average of 1.6) of glucosamine and galactosamine. Estimates of the fraction of biomarkers associated with intact peptidoglycan showed that peptidoglycan became a progressively less important component of prokaryotic remains with ongoing diagenesis, whereas other prokaryotic biomolecules increased in importance. The exact nature of these biomolecules remains unknown, but constant ratios between individual D‐amino acids and amino sugars observed in this study provide novel information on their molecular composition. Examination of total hydrolyzable amino acids revealed systematic compositional changes with increasing water depth and age of the sediment, reflecting increased diagenetic alteration of the organic matter. The time scale integrated in the upper 1 cm of the sediment was ca. 2 yr at the shallow sites and up to 20 yr at the deeper stations. The alteration stages of organic matter ranged from coastal and ocean margin sediments at shallow water depth to organic matter as degraded as hemipelagic surface sediments at greater water depths.

Time-series sediment trap record of alkenones from the western Sea of Okhotsk

C 37–C 39 alkenones were measured in time-series sediment trap samples collected from August 1998 to June 2000 at two depths in the seasonal sea ice region of the western Sea of Okhotsk, off Sakhalin, in order to investigate alkenone production and water-column processes in the region. Measurable export fluxes of alkenones are ranged from < 0.1 to 5.8 μg/m 2/day and clearly showed that the alkenone production was restricted to autumn. In 1998, maximum export flux of alkenones occurred in September when surface water column was well stratified with low nutrients in the surface mixing layer. In the next year, the maximum flux is observed in October. Comparison between alkenone temperature and satellite based sea surface temperature (SST) shows that the estimated alkenone temperatures in August 1998 were found to be ∼ 10 °C lower than the temporal satellite SST, suggesting that alkenones are produced in surface to subsurface thermocline layers during the period. Annual mean flux of alkenones is lower in the lower traps than that of the upper traps, suggesting rapid degradation of alkenones in water column, but the U K 37′ value is not significantly altered. This study indicates that U K 37′ values preserved in the surface sediments off Sakhalin reflect the seasonal temperature signal of near surface water, rather than annual mean surface temperature.

Recent distribution and accumulation of organic carbon on the continental margin west off Spitsbergen

The study compiles the controlling factors for organic matter sedimentation patterns from a suite of organogeochemical parameters in surface sediments off Spitsbergen and direct seabed observations using a Remotely Operated Vehicle (ROV). In addition we assess its storage rates as well as the potential of carbon sinks on the northwestern margin of the Barents Sea with short sediment cores from a selected fjord environment (Storfjord). While sedimentation in the fjords is mainly controlled by river/meltwater discharge and coastal erosion by sea ice/glaciers resulting in high supply of terrigenous organic matter, Atlantic water inflow, and thus enhanced marine organic matter supply, characterizes the environment on the outer shelf and slope. Local deviations from this pattern, particularly on the shelf, are due to erosion and out washing of fine‐grained material by bottom currents. Spots dominated by marine productivity close to the island have been found at the outer Isfjord and west off Prins Karls Forland as well as off the Kongsfjord/Krossfjord area and probably reflect local upwelling of nutrient‐rich Atlantic water–derived water masses. Accumulation rates of marine organic carbon as well as reconstructed primary productivities decreased since the middle of the last century. Negative correlation of the Isfjord temperature record with reconstructed productivities in the Storfjord could be explained by a reduced annual duration of the marginal ice zone in the area due to global warming. Extremely high accumulation rates of marine organic carbon between 5.4 and 17.2 g m−2 yr−1 mark the Storfjord area, and probably high‐latitude fjord environments in general, as a sink for carbon dioxide.

Coastal upwelling in the Canary Island region: spatial variability reflected by the surface sediment diatom record

Modern diatom and C org distribution patterns were investigated in 39 surface sediment samples of the Canary region in order to quantitatively determine the influence of coastal upwelling and Saharan dust on the flux of these sedimentary components. High diatom abundance in surface sediments (>2×10 6 valves g −1 dry sediment) as well as the dominance of the assemblages by spores of Chaetoceros, which is a genus related to upwelling conditions, reflect the stronger coastal upwelling conditions occurring at Cape Ghir off the NW African coast. Further offshore, total diatom abundance decreases one order of magnitude (±1×10 5 valves g −1 dry sediment) and the assemblages are dominated by oceanic species of the genus Nitzschia and fresh water diatoms, mainly the genera Aulacoseira, which is known to be a component of Saharan dust. This association suggest that offshore diatom concentrations may be generated by dust input from desert regions. Even though the sediments are expected to preserve only a mixed and averaged oceanographic signal, characteristic biogeographic patterns of a phytoplankton community occurring across a gradient of turbulence and nutrient availability, are recorded in surface sediments off Cape Ghir. This is seen in the subsequent maximum abundances of the genera Thalassiosira, Chaetoceros and Rhizosolenia.

Diatom assemblages as upwelling indicators in surface sediments off Portugal

Diatom abundance and assemblage composition have been determined for 169 surface sediment samples from the Portuguese margin, an area where seasonal upwelling occurs each year from April to October. Absolute diatom abundance distribution patterns reflect the more intense and/or persistent and more homogeneous upwelling north of the Nazare Canyon, the position of the “inner” upwelling front south of Lisbon, and the eastern continuation of the coastal upwelling jet along the Algarve coast. Distribution patterns of species and of ecologically defined groups, meroplanktonic, oceanic, freshwater and marine benthic, support the hypothesis that diatom species and assemblages are good indicators of the changing character of the upwelling system. Small species of the genus Thalassiosira occur in areas of persistent upwelling where availability of nutrients is more constant and/or higher. Paralia sulcata (Ehr.) Cleve is an abundant species whose mean cell diameter increases as nutrient availability decreases. Resting spores, mainly of Chaetoceros Ehrenberg, record the position of the zone where higher variability occurs, the “inner” upwelling front.

Distribution of quartz in late Quaternary Atlantic sediments in relation to climate

The distribution of quartz in the surface sediments of the Atlantic Ocean reflects derivation from continents by means of rivers, wind, ice, and coastal erosion. Enrichment of quartz thus supplied has occurred in some deep basins of especially the southern high latitudes from winnowing of finegrained clays by bottom currents. Although similar modes of quartz transport may have operated both during the Holocene and the last glacial maximum (18,000 yr B.P.), significant differences in the intensity of transport and in the locii of deposition, which are attributable to climatic variations during these times, exist in some areas of the Atlantic. In Holocene sediments of the eastern equatorial Atlantic, a band of high percent quartz exists directly off the present Saharan Desert and Sahel region and reflects the trade-wind transport of dusts from these arid and semiarid regions. During the last glacial maximum (18,000 yr B.P.), this high quartz band expanded southward by about 8° of latitude. This expansion was caused not only by the southward expansion of aridity and desert dunes but also by the southward migration of the northerly belt of trade winds during the last glaciation. Relatively high abundances and accumulation rates of quartz during the last glaciation suggest higher intensities of trade winds during that time compared to the Holocene. In the North Atlantic, the abundances of quartz in Holocene sediments are high adjacent to Greenland-Iceland and in the areas off Newfoundland-Labrador, and gradually decrease toward the central areas. The polar front and limit of sea-ice melting are at present confined to the northern part of the North Atlantic. The ice-rafting of quartz grains is, therefore, effective in the areas adjacent to Greenland and to some extent off Labrador causing high abundances in these areas. In contrast to this, during the last glaciation, the quartz abundances and accumulation rates are high in the central areas of North Atlantic around 45°N and decrease toward Greenland-Newfoundland. The migration of the polar front to as far south as 45°N and the consequent southward migration of sea-ice melting and ice-rafting during the last glaciation apparently caused this change in distribution. In addition to ice-rafting at present, wave or current reworking of relict glacial-marine detritus may have caused the high abundance of quartz in the surface sediments off Newfoundland-Labrador. In 18,000 yr B.P. sediments of the Norwegian Sea, the area of high percent quartz (>10%) is more extensive than that in Holocene sediments. This reflects the greater influence of ice-rafting or glacier activity in the sediment dispersal in the Norwegian Sea during the last glacial times.

Density and Porosity of Sea-floor Surface Sediments off San Diego, California

Density and Porosity of Sea-floor Surface Sediments off San Diego, California