Isotopic signatures of organic matter in sediments of the continental shelf facing the Orinoco Delta: Possible contribution of organic carbon from savannas

The ⁸⁷Sr, ¹³C, and ¹⁵N isotopic signatures of organic matter in sediments from the continental shelf facing the Orinoco Delta were measured to determine the contribution of sediments transported from the Amazon River by the coastal Guayana current and the sediments transported by the Orinoco River. Box core samples between 60 and 300 m water depth collected along 4 transects located eastwards to the Orinoco Delta were analyzed. Carbon and nitrogen concentrations decreased with depth under water on the shelf, and were strongly correlated indicating homogeneity of organic matter composition. Phosphorus content was also associated to organic matter in most samples, but some of them revealed deposition of P-enriched sediments. The ⁸⁷Sr/⁸⁶Sr ratios showed a strong continental signature averaging 0.7117, therefore, limiting the possible carbon and nitrogen sources associated with these sediments to C3 trees, C4 grasses, or freshwater phytoplankton. The δ¹³C values were relatively high averaging −21‰, above values reported for sediments on the Amapá shelf and the Amazon River in Brazil. Average δ¹³C values did not differ significantly among transects. High δ¹³C values point to the influence of organic matter transported from the C4-plants dominated savannas in the northern fringe of the Orinoco River. δ¹⁵N values were positive and averaged 5‰, being within the range of values measured in the Marajo island (Amazon River) and the estuary of the Pará River. The δ¹⁵N values differed significantly among transects (4.9–5.2‰), lowest values corresponding to the northernmost transect near the coast of Trinidad, and the highest values corresponding to the transect located at the southernmost position.     

Pathways of clay mineral transport in the coastal zone of the Brazilian continental shelf from Ceará to the mouth of the Amazon River

The transport pathways of fine sediments (fraction <2 μm) along the Brazilian continental shelf from Ceará to the Amazon River mouth were studied by means of clay mineral analyses. On the continental shelf southeast of the Amazon mouth, fluctuations in clay mineral compositions reflect simple mixing between the suspended load of the North Brazil Current and sediment from several smaller rivers. Previously, clay mineral variations west of the Amazon mouth have been explained by variable settling velocities of different grain-size classes within the <2 μm fraction or by selective coagulation of individual clay mineral groups. By contrast, our experiments with river bank samples show that selective coagulation does not occur in Amazon River sediments. A more appropriate explanation for observed variations in clay mineral composition off the Amazon mouth seems to be, similarly to that for the shelf between Ceará and the Amazon mouth, a mixing of Amazon sediments with suspended material of the North Brazil Current. This interpretation is supported by data on clay mineral composition east and south of the Amazon mouth, showing more affinity to sediments of the North Brazil Current than to the suspended load of the Amazon River. Additionally, relatively low sedimentation rates and low concentrations of fine-grained sediments on the shelf suggest that high riverine input by the Amazon River does not overprint the sediments of the North Brazil Current in this region. The strong North Brazil Current shunts the Amazon suspended load in a north-westerly direction along the north-eastern coast of South America. Hence, stronger sedimentation of Amazon sediments would occur only west of the river mouth.     

An overview of sedimentation on the amazon continental shelf

Sedimentological, geochemical, and physical-oceanographic studies of the Brazilian continental shelf near the Amazon River help provide a broad understanding of this major sediment dispersal system. Amazon River sediment accumulates as a subaqueous delta, with the most rapid accumulation (10 cm/yr) occurring near the seaward edge of the topset beds and in the foreset beds. Amazon River sediment is dispersed northwestward along the shelf and is transported beyond the Brazilian border. Radiographic studies of sediment cores delineate three sedimentary environments: interbedded mud and sand, faintly laminated mud, and bioturbated mud. The distribution of these environments is a function of proximity to the river mouth and of sediment accumulation rate.     

Time and density-dependent properties of fluid mud suspensions,NE brazilian continental shelf

Fluid mud suspensions, defined as containing between 10 to 480 g/liter, occur in numerous estuarine and nearshore continental shelf environments. The quantities of sediment incorporated in fluid mud are enormous, and they must be considered to be of major importance in the transport and deposition of fine-grained sediment in these environments.This study was performed on the NE Brazilian Continental Shelf, a muddy coastline that reaches from the Amazon to the Orinoco, a distance of 1600 km. The fluid mud forms a thick (3 to 5 m) boundary layer that extends 100-km offshore and generally coincides with the 10-m depth contour.The fluid muds appear well-adjusted to the current regime on the NE Brazilian continental shelf. Near-bottom current velocities were seldom greater than 50 cm/sec during the sampling interval. Consequently, yield stress in fluid muds of 1.20 × 10³ kg/m³ and greater was seldom exceeded, and consolidation proceeded without interruption.     

Sources and transport of lipids on Amazon continental shelf

Dissolved material and recent sediment from the Amazon continental shelf have been analyzed for hydrocarbons to study the sources and potential fate of the transported organic matter. Dissolvedn-alkanes are present at low concentrations (ppb level) and are dominated by lipids from marine phytoplankton with carbon number maxima (C_max) at C₁₈/C₂₂ and an even-to-odd carbon predominance < C₃₀ (CPI_17–27 from 0.18 to 0.54). In the sediments, bimodal distributions ofn-alkane chain length suggest a mixed input of terrestrial (C_max at C₂₇/C₂₉/C₃₁ and CPI_25–33 from 0.75 to 1.82) and phytoplanktonic/microbial (C_max at C₂₀ and CPI_15–25 from 0.38 to 0.62) organic matter. Sesquiterpenes were the most significant cyclic compounds in all the dissolved samples analyzed reflecting a contribution from resinous trees to the terrestrial organic pool. On the other hand, enhanced concentrations of these compounds in the dissolved phase on the northwest portion of the Amazon shelf, contrasting with decreased concentrations in the sediment samples, suggest that dissolved lipids are released from solid phase in the intensely stirred seabed. Structured organic matter in the sediment has been characterized as being composed of, on average: 19% plant cuticles, 25% woody tissue, 13% pollens and spores, 24% amorphous material, 7% bituminite and 12% altered organic material.     

海底沉积物孔隙度与声速的关系

给出大陆架海底沉积层中声速(压缩波)V_p依赖于沉积物孔隙度n的一个新的经验公式,该公式当n=100时,V_p=V₀,V₀为海底水声速.公式的计算值与我国周边大陆架测量值吻合较好,并将其结果与国内外成果作了对比讨论.     

Sediment distribution and transport across the continental shelf and slope under idealized wind forcing

Resuspension, transport, and deposition of sediments over the continental shelf and slope are complex processes and there is still a need to understand the underlying spatial and temporal dynamical scales. As a step towards this goal, a two-dimensional slice model (zero gradients in the alongshore direction) based on the primitive flow equations and a range of sediment classes has been developed. The circulation is forced from rest by upwelling or downwelling winds, which are spatially uniform. Results are presented for a range of wind speeds and sediment settling speeds. Upwelling flows carry fine sediments (low settling speeds) far offshore within the surface Ekman layer, and significant deposition eventually occurs beyond the shelf break. However, coarser sediments quickly settle out of the deeper onshore component of the circulation, which can lead to accumulation of bottom sediments within the coastal zone. Downwelling flows are more effective at transporting coarse sediments off the shelf. However, strong vertical mixing at the shelf break ensures that some material is also carried into the surface Ekman layer and returned onshore. The concentrations and settling fluxes of coarse sediments decrease offshore and increase with depth under both upwelling and downwelling conditions, consistent with trends observed in sediment trap data. However, finer sediments decrease with depth (upwelling) or reach a maximum around the depth of the shelf break (downwelling). It is shown that under uniform wind conditions, suspended sediment concentrations and settling fluxes decay offshore over a length scale of order τ_s/ρf|w_s|, where τ_s is the wind stress, ρ the water density, f the Coriolis parameter, and w_s is the sediment settling velocity. This scaling applies to both upwelling and downwelling conditions, provided offshore transport is dominated by wind-driven advection, rather than horizontal diffusion.     

中国东海陆架海域柱状沉积物对磷的吸附行为

通过研究磷在海洋柱状沉积物上的吸附动力学曲线和吸附等温线,并结合沉积物表面电荷性质以及磷形态分析,考察了我国东部陆架海域沉积物对磷的吸附特征。结果表明,所研究海域沉积物对磷的吸附过程明显分为快慢两段,48h后吸附可达到平衡;吸附后的磷主要为可交换态磷,吸附过程以物理作用为主;吸附等温线可用Langmuir交叉型模式描述,在磷初始浓度较低时,沉积物对磷存在解吸现象;沉积物对磷吸附参数的垂直分布较为复杂,受自身性质影响较大;盐度增大,沉积物对磷的吸附能力呈下降趋势。     

Benthic remineralization at the land–ocean interface: A case study of the Rhône River (NW Mediterranean Sea)

Biogeochemical processes in sediments under the influence of the Rhône River plume were studied using both in situ microelectrodes and ex situ sediment core incubations. Organic carbon (OC) and total nitrogen (TN) content as well as stable carbon isotopic composition of OC (δ¹³C_OC) were analysed in 19 surface sediments to determine the distribution and sources of organic matter in the Rhône delta system. Large spatial variations were observed in both the total O₂ uptake (5.2 to 29.3 mmol m⁻² d⁻¹) and NH₄⁺ release (−0.1 to −3.5 mmol m⁻² d⁻¹) rates at the sediment–water interface. The highest fluxes were measured near the Rhône River mouth where sedimentary OC and TN contents reached 1.81% and 0.23% respectively. Values of δ¹³C_OC ranged from −26.83‰ to −23.88‰ with a significant seawards enrichment tracing the dispersal of terrestrial organic matter on the continental shelf. The amount of terrestrial-derived OC reaches 85% in sediments close to the Rhône mouth decreasing down to 25% in continental shelf sediments. On the prodelta, high terrestrial OC accumulation rates support high oxygen uptake rates and thus indicating that a significant fraction of terrestrial OC is remineralized. A particulate organic carbon (POC) mass balance indicates that only 3% of the deposited POC is remineralized in prodelta sediments while 96% is recycled on the continental shelf. It was calculated that a large proportion of the Rhône POC input is either buried (52%) or remineralized (8%), mostly on the prodelta area. The remaining fraction (40%) is either mineralized in the water or exported outside the Rhône delta system in dissolved or particulate forms.     

Phosphorus regeneration vs. burial in sediments of the Arabian Sea

Solid-phase phosphorus (P) speciation and benthic phosphate fluxes have been determined in Arabian Sea sediments. Benthic phosphate fluxes are highest in the continental slope sediments, underlying bottom waters with low oxygen concentrations. Organic matter degradation and phosphate desorption from iron oxides do not produce sufficient phosphate to explain these high phosphate fluxes. The potentially high deposition of P associated with fish debris (P_fish) in the Arabian Sea, and a good correlation between benthic phosphate fluxes and P_fish accumulation rates suggest that benthic phosphate fluxes in these sediments are to a large extent governed by dissolution of biogenic apatite. Factors controlling dissolution and preservation of fish debris, therefore, may play an important role in the burial and regeneration of P in continental margin sediments. A sharp decrease of the reactive P accumulation rate with increasing water depth, in combination with rather constant primary productivity rates throughout the northern Arabian Sea, indicates that P burial in continental margin sediments located within the OMZ is more efficient than in deep basin sediments. The effectiveness of P burial is to a large extent regulated by P regeneration occurring in the water column and redeposition processes. Sedimentary phosphorus burial efficiencies, thus, should be interpreted with caution in terms of the environmental conditions.     

Chemistry of sediments in relation to trophic condition of eight Rotorua Lakes

Abstract

Chemical parameters (pH, Eh, carbon, Kjeldahl nitrogen, total phosphorus, 0.5M H₂SO₄‐extractable phosphorus, organic phosphorus, and water‐soluble phosphorus) were measured in the surface layers of sediments collected from various depths in Lakes Rotowhero, Okaro, Ngapouri, Rotokakahi, Okareka, Tikitapu, Okataina, and. Rotoma during October 1972. The sediments of the productive geothermal lake, Rotowhero, were markedly different from those of the cold‐water lakes: they had relatively low pH values, high carbon (mean 8.5%) and organic phosphorus (mean 4160 μg.g^?1) concentrations, and very high total phosphorus concentrations (mean 4770 μg.g^?1), probably as a result of enrichment by hot springs.

The mean concentrations in the sediments of the cold‐water lakes were carbon 3.2–7.9%, Kjeldahl nitrogen 3380–8310 μg.g^?1 and phosphorus 690–1780 μg.g^?1. These concentrations are within the ranges for New Zealand terrestrial topsoils, but the lake sediments appear enriched in phosphorus relative to local topsoils. Total carbon, nitrogen, and phosphorus concentrations of sediments tended to be highest in the eutrophic lakes (Okaro, Ngapouri) although the deep oligotrophic lakes (Okataina, Rotoma) had relatively high total phosphorus concentrations (means 1400, 1510 μg.g^?1). Overall, the carbon, nitrogen, and phosphorus concentrations of the sediments showed little relationship to the trophic state of the lake.

Organic phosphorus concentrations of the surface layers of sediments were similar in all the cold‐water lakes (mean 319 μg.g^?1). The proportion of the total phosphorus apparently ‘fixed’ in mineral material was minimal (0–1%) in sediments from the eutrophic and mesotrophic lakes, but in the oligotrophic lakes was similar to that in New Zealand topsoils (9–14%). Reducing conditions may cause solution of a high proportion of the ‘fixed’ phosphorus in the eutrophic lakes.

The water‐soluble phosphorus concentrations in the sediments of the five shallow cold‐water lakes (Okaro, Ngapouri, Rotokakahi, Okareka, Tikitapu) correlated positively with trophic state and with concentrations of dissolved phosphorus in the lake waters.

Carbon, nitrogen, and phosphorus concentrations in the sediments tended to vary with overlying water depth. This should be considered when comparisons are made between lakes.     

Similarity between trace-element composition of river and seabed sediments in the Amazon system

The aim of this paper was to study the elemental composition of sediments in the Amazon ecosystem by comparing bottom samples from the shelf with those from the floodplain of the Rio Amazonas. Trace elements measured by neutron activation analysis were used. The system is divided into three parts: the basin, including tributaries from the pre-Andean region to Óbidos; the delta—plain region between Óbidos and the mouth; and the continental shelf. Significant correlation was found between the shelf and floodplain sediments, especially those samples close to the mouth. In the shelf surface samples, Ta, Eu, La, and Ce (in this order) have the lowest coefficients of variation in concentration, whereas Hf, Br, and Cs have the greatest.     

Interactions between sediment chemistry and frenulate pogonophores (Annelida) in the north-east Atlantic

The small frenulate pogonophores (Annelida: Pogonophora a.k.a. Siboglinidae) typically inhabit muddy sediments on the continental slope, although a few species occur near hydrothermal vents and cold seeps. We present data on the distribution and habitat characteristics of several species on the European continental shelf and slope from 48°N to 75°N and show how the animals interact with the chemistry of the sediments. The environments inhabited include: shallow (30 m), organic-rich, fjord sediments; slope sediments (1000–2200 m) and methane seeps at 330 m depth. All the species studied obtain nutrition from endosymbiotic bacteria. They take up reduced sulphur species, or in one case, methane, through the posterior parts of their tubes buried in the anoxic sediment. We conclude that most species undertake sulphide ‘mining’, a mechanism previously demonstrated in the bivalves Lucinoma borealis and Thyasira sarsi. These pogonophores participate in the sulphur cycle and effectively lower the sulphide content of the sediments. Our results show that the abundance of frenulate pogonophores increases with increasing sedimentation and with decreasing abundance of other benthos, particularly bioturbating organisms. The maximum sustainable carrying capacity of non-seep sediments for frenulate pogonophores is limited by the rate of sulphate reduction.     

Patterns of bottom environments and macrobenthos communities along the depth gradient in the bathyal zone off sanriku,Northwestern Pacific

Quantitative samples of sediments and macrobenthos were collected with a spade corer to reveal the relationships between macrobenthos communities and bottom environments. Twenty-five stations were established along a transect in the lower sublittoral and bathyal zones between the depths of 120 m and 2,600 m off Sanriku, the northeastern coast of the Japanese mainland, Northwestern Pacific.These stations were clustered into two groups on the basis of the principal component analysis of environmental factors and topographic features of the study area. The clustering of stations corresponded to that based on polychaete species composition.The biomass of macrobenthos followed two different patterns in two oceanographic environments; the first trend is found from the shelf to the seaward margin of the deep-sea terrace where macrobenthos biomass did not depend on depth, and the second trend on the lower continental slope, where biomass decreased exponentially with depth as Rowe's formula predicted. The density of macrobenthos showed a simple exponential decrease with depth over the two ecological zones. The data of stable carbon isotope ratio of organic matter in the sediments and topographic features suggested that the pattern of the biomass of macrobenthos corresponded with two different modes of food supply to the deep-sea bottom communities,i.e., vertical transport of the surface products and horizontal transport of food materials.     

冲绳海槽中南部不同环境表层沉积物质来源

对冲绳海槽中南部3种不同沉积环境(东海外陆架、东海陆坡和冲绳海槽)表层沉积物进行了稀土等元素地球化学分析,结果显示,冲绳海槽和陆坡表层沉积物具有与台湾物质来源类似的稀土元素配分模式,La/Sm-Gd/Yb散点图也显示海槽和陆坡沉积物主要分布在台湾物源端元区,表明冲绳海槽中南部海槽和陆坡表层沉积物主要来源于台湾,而外陆架...     

Sedimentary carbon-isotope systematics on the Amazon shelf

Sedimentary carbon systematics on the Amazon continental shelf were investigated using¹⁴C and ¹³C measurements of sedimentary organic materials. Mass balance calculations indicate that the¹⁴C content of modern shelf muds results from burial of old (~ 5000 yr BP) terrestrial soil carbon and bomb-¹⁴C-enriched marine carbon, implying that most of the bomb-labeled riverine particulate carbon is not buried on the shelf. The¹⁴C signature of Amazon shelf deposits records the effects of both biogeochemical and sedimentary processes active in this dynamic environment.     

Cenozoic tectonics of Amazon Mouth Basin

 The Cenozoic shelf margin of the Amazon Mouth Basin is characterized by a thick prograding prism of siliciclastic sediments. This prism, composed mainly of Upper Miocene and younger sediments, overlies a Lower Tertiary carbonate shelf. Two tectonic–sedimentary models for the area were developed with the aid of new deep-reflection seismic data. Gravitational tectonics dominate the regional geological framework. Tensional stresses are created near the shelf margin, and compressional features dominate at the base of the slope. The morphology of this compressional zone is closely associated with the St. Paul Fracture Zone and the boundary between continental and oceanic crusts. Received: 20 August 1996 / Revision received: 11 June 1998     

Seasonal and spatial patterns of sedimentary denitrification rates in the Chukchi sea

Recent constructions of the global nitrogen budget estimate that at least half of the ocean's fixed nitrogen is lost by sedimentary denitrification, the majority of which occurs in continental shelves. The Arctic contains approximately 20% of the world's continental shelf, suggesting it is a substantial contributor to the global sedimentary denitrification rate. During two cruises in the summer and spring of 2002 and 2004, respectively, denitrification rates were calculated from the downward diffusive flux of nitrate in the shelf and slope sediments of the Chukchi Sea in the western Arctic. Additionally, in the spring of 2004, denitrification rates were determined by whole-core incubations in which the flux of nitrogen gas out of the sediments was measured. Measurements were made along three transects crossing the shelf and slope (50–3000 m), each transect having different overlying water characteristics. Denitrification rates generally decreased with increasing water depth: rates varied from about 1.6 mmol N m⁻² d⁻¹ for the shallow-water sediments to undetectable in deep-water sediments. Rates showed little variation between the two seasons. However, rates were found to correspond with differences in annual overlying primary productivities and particulate organic carbon export fluxes. An extrapolation to the whole Arctic yielded an average Arctic sedimentary denitrification rate of 13 Tg N yr⁻¹. Taken in the context of the global nitrogen budget, it is about 4–13% of the total sink of fixed nitrogen in the ocean.     

Contrasting chemical and isotopic compositions of organic matter in Changjiang (Yangtze River) estuarine and East China Sea shelf sediments

To examine the source and preservation of organic matter in the shelf sediments of the East China Sea (ECS), we measured bulk C/N and isotopes, organic biomarkers (n-alkanes and fatty acids) and compound-specific (fatty acids) stable carbon isotope ratios in three sediment cores collected from two sites near the Changjiang Estuary and one in the ECS shelf. Contrasting chemical and isotopic compositions of organic matter were observed between the estuarine and shelf sediments. The concentrations of total n-alkanes and fatty acids in the shelf surface sediments (0–2 cm) were 5–10 times higher than those in estuarine surface sediments but they all decreased rapidly to comparable levels below the surface layer. The compositions of n-alkanes in the estuarine sediments were dominated by C₂₆-C₃₃ long-chain n-alkanes with a strong odd-to-even carbon number predominance. In contrast, the composition of n-alkanes in the shelf sediment was dominated by nC₁₅ to nC₂₂ compounds. Long-chain (>C₂₀) fatty acids (terrestrial biomarkers) accounted for a significantly higher fraction in the estuarine sediments compared to that in the shelf sediment, while short-chain (<C₂₀) saturated and unsaturated fatty acids were more abundant in the shelf surface sediments than in the estuarine sediments. Stable carbon isotopic ratios of individual fatty acids showed a general positive shift from estuarine to shelf sediments, consistent with the variations in bulk δ ¹³C_TOCTOC. These contrasts between the estuarine and shelf sediments indicate that terrestrial organic matter was mainly deposited within the Changjiang Estuary and inner shelf of ECS. Post-depositional diagenetic processes in the surface sediments rapidly altered the chemical compositions and control the preservation of organic matter in the region.     

Trace metal distributions across the continental shelf near Otago Peninsula, New Zealand

The distributions of the trace metals iron (Fe), copper (Cu) and cadmium (Cd) along with hydrological parameters (salinity, temperature and reactive phosphate) across the New Zealand continental shelf near Otago Peninsula have been studied. This is a region in which the Subtropical Convergence (STC), a major oceanic front separating subtropical and subantarctic waters, is uniquely located close to land, permitting an examination of the influence of terrestrial sources of Fe and Cu on oceanic waters containing excess micronutrients. Acid-soluble (110 nmol kg⁻¹) and dissolved (6.3 nmol kg⁻¹) Fe concentrations were highest over the central shelf, and decreased rapidly across the mixing zone of the STC to about 5 nmol kg⁻¹ for both forms. The distribution of acid-soluble and dissolved Cu were similar to their counterparts for Fe. Depth-concentration profiles for acid-soluble Fe and Cu suggest resuspension of shelf sediments is the main source. The ratio of oxine-labile to acid-soluble Fe varied from 0.03 to 0.26, with the highest values found in the near surface waters. Oxine-labile Fe and Cu also decreased in concentration in a seawards direction, and with depth, indicating the influence of near surface processes on the reactivity of these elements. Cd concentrations across the continental shelf were very low (<200 pmol kg⁻¹) and exhibited no clear spatial trend and no correlation with phosphate. Comparison of the Cd/P ratio across the shelf indicated that the waters in this region were strongly depleted in Cd relative to P. Phosphate concentrations were lowest in neritic water and increased in the seawards direction because of mixing with nutrient-rich Subantarctic Surface Water.