Metal concentrations, fluxes, inventories and chronologies in sediments from Sepetiba and Ribeira Bays: A comparative study

Three sediment cores were sampled at Sepetiba Bay and four cores at Ribeira Bay, Rio de Janeiro State, Brazil. Sediment accumulation rates and chronologies were obtained from ²¹⁰Pb activity-depth profiles. Sediment accumulation rates in Ribera Bay ranged from 1.2 mm y⁻¹ in the inner bay to 2.6 mm y⁻¹ close to its entrance. In Sepetiba Bay two sediment accumulation rates were observed: a lower rate of 0.35 cm y⁻¹ before the 1960s and 0.76 cm y⁻¹ since then. The cause of this change is due to the construction of the Santa Cecília impoundment (1955) that brings water from the Paraíba do Sul Basin into the Guandu River, which increased its flow from the original 20 m³ s⁻¹ to 160 m³ s⁻¹. Concentration of 44 elements was obtained by ICP-MS after total dissolution of samples from two selected cores. The relative differences between the concentrations of crustal elements, such as Al, Fe and Ti are only about 20% (p < 0.05). Cd and Zn are 15 and four times larger in Sepetiba Bay than in Ribeira Bay, respectively. Other major and minor elements show lower statistically significant differences. The enrichment factors and metal inventories show that Sepetiba Bay can be considered polluted with Bi, Cd, Cr, Cu, Sb and Zn. Particularly, Cd and Zn present concentrations three and four times higher than the Brazilian existing limits.     

Efficient trapping of organic carbon in sediments on the continental margin with high fluvial sediment input off southwestern Taiwan

Accumulation rates of marine and terrigenous organic carbon in the continental margin sediments off southwestern Taiwan were estimated from the measured concentrations and isotopic compositions of total organic carbon (TOC) and previously reported sedimentation rates. Surficial sediments were collected from the study area spanning from the narrow shelf near the Kaoping River mouth to the deep slope with depths reaching almost 3000 m. The average sediment loading of Kaoping River is 17 Mt/yr, which yields high sediment accumulation rates ranging from 0.08 to 1.44 g cm⁻² yr⁻¹ in the continental margin. About half of the discharged sediments were deposited on the margin within 120 km of the river mouth. Carbon isotopic compositions of terrestrial and marine end-members of organic matter were determined, respectively, based on suspended particulate matter (SPM) collected from three major rivers in the southwestern Taiwan and from an offshore station. All samples were analyzed for the TOC content and its isotopic composition (δ¹³C_org). The SPM samples were also analyzed for the total nitrogen (TN) content. TOC content in marine sediments ranges from 0.45% to 1.35% with the highest values on the upper slope near the Kaoping River mouth. The TOC/TN ratio of the SPM samples from the offshore station is 6.8±0.6, almost identical to the Redfield ratio, indicating their predominantly marine origin; their δ¹³C_org values are also typically marine with a mean of −21.5±0.3‰. The riverine SPM samples exhibit typical terrestrial δ¹³C_org values around −25‰. The δ¹³C_org values of surficial sediments range from −24.8‰ to −21.2‰, showing a distribution pattern influenced by inputs from the Kaoping River. The relative contributions from marine and terrestrial sources to sedimentary organic carbon were determined by the isotope mixing model with end-member compositions derived from the riverine and marine SPM. High fluvial sediment inputs lead to efficient trapping of organic carbon over a wide range of water depth in this continental margin. The marine organic accumulation rate ranges from 1.6 to 70 g C m⁻² yr⁻¹ with an area weighted mean of 4.2 g C m⁻² yr⁻¹, which is on a par with the mean terrestrial contribution and accounts for 2.3% of mean primary production. The depth-dependent accumulation rate of marine organic carbon can be simulated with a function involving primary productivity and mineral accumulation rate, which may be applicable to other continental margins with high sedimentation rates. Away from the nearshore area, the content of terrigenous organic carbon in surficial sediments decreases with distance from the river mouth, indicating its degradation in marine environments.     

Chemical composition of short sediment cores from Thermaikos Gulf (Eastern Mediterranean): Sediment accumulation rates, trawling and winnowing effects

Four cores recovered within the framework of the INTERPOL Project have been analysed for their grain size and geochemistry; sediment accumulation rates (SARs) were also determined from ²¹⁰Pb and ¹³⁷Cs profiles. Two cores are representative of the Axios and Aliakmon Rivers depositional environment, whilst the third core represents the Pinios River province; the fourth core represents an environment of outer shelf relict sands. Apparent SARs ranged between 0.667 g cm⁻² yr⁻¹ (Axios and Aliakmon Rivers) and 0.414 g cm⁻² yr⁻¹ (Pinios River). Trawling activities and biomixing are critical processes that may be responsible for the mixing of the surface sediments, as observed from the excess ²¹⁰Pb profiles. The thickness of the surface mixed layer was 4.5 cm in the vicinity of Axios and Aliakmon Rivers and in the area of Pinios River, 3.75 cm on the outer shelf and 1 cm in the area where no trawling was observed. Sediment accumulation appeared to be regulated by variations in the riverine discharge, shelf transport pathways and winnowing processes. Major element variations, such as Si, Al, Ti, V and Ni, were dominated by terrigenous supply as aluminosilicate minerals and quartz, whereas most Ca and Sr were biogenic. Si/Al and Ca/Al ratios have been used to express changes in sediment accumulation and winnowing. Redox processes were depicted by Mn, which showed an increase in the depth of its redoxcline, from 1 cm in inshore stations to 2 cm on the outer shelf. Si/Al ratios follow the Ca/Al ratios and can be used to assess percentage winnowing in the sediment. Increases in these ratios indicate a decrease in sediment input rates and are seen in the upper parts of most of the cores. Anthropogenic or ‘excess’ metal contents have been calculated from Zn/V and Pb/V ratios. Their distributions in the cores showed that by far the highest contamination is associated with the Axios River output, whilst sediments influenced by the Pinios River were relatively uncontaminated.     

One century sedimentary record of Hg and Pb pollution in the Sagua estuary (Cuba) derived from Pb and Cs chronology

The vertical distribution of Hg and Pb were determined in a sediment core collected from the Sagua estuary (North Cuba) that receives input from the Sagua river, one of the most polluted rivers discharging into the Cuban coastal environment. Depth profiles of metal concentrations were converted to time-based profiles using the ²¹⁰Pb dating method and confirmed with the ¹³⁷Cs fallout peak. The mean mass accumulation rate was estimated to be 0.17 ± 0.04 g cm⁻² y⁻¹ (mean sediment accumulation rate 0.52 ± 0.13 cm y⁻¹) and the core bottom was estimated to date back about 130 years. The historical sedimentary record showed a strong enrichment of mercury concentrations in the past decades, caused by the incomplete treatment of industrial wastes from a chlor-alkali plant with mercury-cell technology in the Sagua river basin. Lead fluxes to sediments showed a gradual increase from the 1920s to present, which agrees with a population increase in Sagua la Grande City. Fluxes of both metals have increased the past 25 years, with values reaching a maximum of 0.5 and 3.9 μg cm⁻² y⁻¹ for Hg and Pb, respectively.     

Changes in sediment and organic carbon accumulation in a highly-disturbed ecosystem: The Sacramento-San Joaquin River Delta (California, USA)

We used the Sacramento-San Joaquin River Delta CA (Delta, hereafter) as a model system for understanding how human activities influence the delivery of sediment and total organic carbon (TOC) over the past 50–60 years. Sediment cores were collected from sites within the Delta representing the Sacramento River (SAC), the San Joaquin River (SJR), and Franks Tract (FT), a flooded agricultural tract. A variety of anthropogenic tracers including ¹³⁷Cs, total DDE (∑DDE) and brominated diphenyl ether (BDE) congeners were used to quantify sediment accumulation rates. This information was combined with total organic carbon (TOC) profiles to quantify rates of TOC accumulation. Across the three sites, sediment and TOC accumulation rates were four to eight-fold higher prior to 1972. Changes in sediment and TOC accumulation were coincident with completion of several large reservoirs and increased agriculture and urbanization in the Delta watershed. Radiocarbon content of TOC indicated that much of the carbon delivered to the Delta is “pre-aged” reflecting processing in the Delta watershed or during transport to the sites rather than an input of predominantly contemporary carbon (e.g., 900–1400 years BP in surface sediments and 2200 yrs BP and 3610 yrs BP at the base of the SJR and FT cores, respectively). Together, these data suggest that human activities have altered the amount and age of TOC accumulating in the Delta since the 1940s.     

Mercury contamination history of an estuarine floodplain reconstructed from a Pb-dated sediment core (Berg River, South Africa)

Mercury deposition histories have been scarcely documented in the southern hemisphere. A sediment core was collected from the ecologically important estuarine floodplain of the Berg River (South Africa). We establish the concentration of Hg in this ²¹⁰Pb-dated sediment core at <50 ng g⁻¹ Hg_T throughout the core, but with 1.3 ng g⁻¹ methylmercury in surface sediments. The ²¹⁰Pb dating of the core provides a first record of mercury deposition to the site and reveals the onset of enhanced mercury deposition in 1970. The ratio of methylmercury to total mercury is relatively high in these sediments when compared to other wetlands.     

Nature of sediment dispersal off the Sepik River, Papua New Guinea: preliminary sediment budget and implications for margin processes

A sediment budget is constructed for the slope and narrow continental shelf off the Sepik River in order to estimate the relative importance of turbid plumes versus bottom gravity transport through a near-shore submarine canyon in the dispersal of sediment across this collision margin. ²¹⁰Pb geochronology and inventories of Kasten cores are consistent with the northwestward dispersal of sediment from the river mouth via hypopycnal and possible isopycnal plumes. Sediment accumulation rates are 5 cm yr⁻¹ on the upper slope just off of the Sepik mouth, decreasing gradually to 1 cm yr⁻¹ toward the northwest, and decreasing abruptly offshore (<0.2 cm yr⁻¹ at 1200 m water depth). A sediment budget indicates that only about 7–15% of the Sepik River sediment discharge accumulates on the adjacent open shelf and slope. The remainder presumably escapes offshore via gravity flows through a submarine canyon, the head of which extends into the river mouth. The divergent sediment pathways observed off the Sepik River (i.e., surface and subsurface plumes versus sediment gravity flows through a canyon) may be common along high-yield collision margins of the Indo–Pacific archipelago, and perhaps are analogous to most margins during Late Quaternary low sea-level conditions.     

Geochronological reconstruction of Cs transport from the Coruh river to the SE Black Sea: comparative assessment of radionuclide retention in the mountainous catchment area

The deposition record of ¹³⁷Cs was traced in the SE Black Sea sediments adjacent to the Coruh river mouth in comparison with the earlier studied chronology of ¹³⁷Cs deposition in front of the Danube delta (NW Black Sea). In both cases, the ¹³⁷Cs profiles showed two subsurface peaks attributable to maximum fallout of ‘bomb’ and Chernobyl radionuclides. The Coruh profile revealed a larger contribution of ‘bomb’ ¹³⁷Cs in comparison with the Chernobyl input, suggesting different coverage of NW and SE Black Sea regions with the Chernobyl fallout. The ¹³⁷Cs-derived dating showed that maximum deposition of particulate bound ¹³⁷Cs in sediments adjacent to the Coruh river mouth was delayed for 14 yr relative to date of Chernobyl accident, reflecting a buffer effect of the watershed soils. This transit time is 3 times longer than in the Danube catchment area, indicating a difference in retention processes in these mountainous (Coruh) and lowland (Danube) river basins. The ¹³⁷Cs profile in Coruh sediments showed penetration of ¹³⁷Cs to much greater depth than would be expected from ¹³⁷Cs fallout chronology, suggesting the sediment mixing rate of 1.3 cm² yr⁻¹. This value was used to evaluate deposition chronology of ¹³⁷Cs, applying the model developed for pulse fallout case. Comparing the measured and modelled data has allowed differentiation of the flood-induced discharge of the ¹³⁷Cs-containing suspended matter and the slower transit of eroded soil particles from the contaminated catchment areas. The obtained results may be used for the prediction of period when the pollutants, deposited over the river basins, can reach the Black Sea.     

Impacts of stormwater runoff in the Southern California Bight: Relationships among plume constituents

The effects from two winter rain storms on the coastal ocean of the Southern California Bight were examined as part of the Bight ‘03 program during February 2004 and February–March 2005. The impacts of stormwater from fecal indicator bacteria, water column toxicity, and nutrients were evaluated for five major river discharges: the Santa Clara River, Ballona Creek, the San Pedro Shelf (including the Los Angeles, San Gabriel, and Santa Ana Rivers), the San Diego River, and the Tijuana River. Exceedances of bacterial standards were observed in most of the systems. However, the areas of impact were generally spatially limited, and contaminant concentrations decreased below California Ocean Plan standards typically within 2–3 days. The largest bacterial concentrations occurred in the Tijuana River system where exceedances of fecal indicator bacteria were noted well away from the river mouth. Maximum nitrate concentrations (~40 μM) occurred in the San Pedro Shelf region near the mouth of the Los Angeles River. Based on the results of general linear models, individual sources of stormwater differ in both nutrient concentrations and the concentration and composition of fecal indicator bacteria. While nutrients appeared to decrease in plume waters due to simple mixing and dilution, the concentration of fecal indicator bacteria in plumes depends on more than loading and dilution rates. The relationships between contaminants (nutrients and fecal indicator bacteria) and plume indicators (salinity and total suspended solids) were not strong indicating the presence of other potentially important sources and/or sinks of both nutrients and fecal indicator bacteria. California Ocean Plan standards were often exceeded in waters containing greater than 10% stormwater (<28–30 salinity range). The median concentration dropped below the standard in the 32–33 salinity range (1–4% stormwater) for total coliforms and Enterococcus spp. and in the 28–30 salinity range (10–16% stormwater) for fecal coliforms. Nutrients showed a similar pattern with the highest median concentrations in water with greater than 10% stormwater. Relationships between colored dissolved organic matter (CDOM) and salinity and between total suspended solids and beam attenuation indicate that readily measurable, optically active variables can be used as proxies to provide at least a qualitative, if not quantitative, evaluation of the distribution of the dissolved, as well as the particulate, components of stormwater plumes. In this context, both CDOM absorption and the beam attenuation coefficient can be derived from satellite ocean color measurements of inherent optical properties suggesting that remote sensing of ocean color should be useful in mapping the spatial areas and durations of impacts from these contaminants.     

History of human activity in coastal southern Brazil from sediment

This study reports results of analysis of sediment cores collected from the Patos Lagoon estuary. This estuary receives materials from land runoff into Patos and Mirim lagoons and from exchange with the adjacent South Atlantic Ocean through a narrow inlet. Sediment from these sources is mostly natural, but additional contributions associated with source/activities related to the port of Rio Grande. The aim of this study was to estimate rates of accumulation of the sediments and to assess the significance of metal inputs from these activities relative to natural inputs. Our results indicate an average sedimentation rate of 0.3 cm/yr and that the transport of suspended solids from offshore sources into the estuary was enhanced after the deepening of the channel and construction of the breakwaters (in the early 1900). Results for metal accumulation in these sediments suggest that there have been only minor enrichments which can be attributed to anthropogenic sources.     

Particulate matter fluxes in the southern and central Kara Sea compared to sediments: Bulk fluxes, amino acids, stable carbon and nitrogen isotopes, sterols and fatty acids

The Kara Sea is one of the arctic marginal seas strongly influenced by fresh water and river suspension. The highly seasonal discharge by the two major rivers Yenisei and Ob induces seasonal changes in hydrography, sea surface temperature, ice cover, primary production and sedimentation. In order to obtain a seasonal pattern of sedimentation in the Kara Sea, sediment traps were deployed near the river mouth of the Yenisei (Yen) as well as in the central Kara Sea (Kara) within the framework of the German–Russian project “Siberian River run-off; SIRRO”. Two and a half years of time-series flux data were obtained between September 2000 and April 2003 and were analyzed for bulk components, amino acids, stable carbon and nitrogen isotopes as well as sterols and fatty acids.Sediment trap data show that much of the annual deposition occurred under ice cover, possibly enhanced by zooplanktonic activity and sediment resuspension. An early bloom of ice-associated algae in April/May occurred in the polynya area and may have been very important to sustain the life cycles of higher organisms after the light limitation of the winter months due to no/low insolation and ice cover. The strong river input dominated the months June–August in the southern part of the Kara Sea. The central Kara Sea had a much shorter productive period starting in August and was less affected by the river plumes. Despite different time-scales of sampling and trapping biases, total annual fluxes from traps were in the same order of magnitude as accumulation rates in surface sediments. Terrestrial organic carbon accumulation decreased from 10.7 to 0.3 g C m⁻² a⁻¹ from the riverine source to the central Kara Sea. Parallel to this, preservation of marine organic matter decreased from 10% to 2% of primary productivity which was probably related to decreasing rates of sedimentation.     

Dissolved inorganic nutrients and organic substrates in the River Warnow (North-Eastern Germany) – Utilisation by bacterioplankton

After 1990, external loads of Central European rivers with inorganic nutrients (nitrogen and phosphorus) and organic material were reduced because of changed environmental laws. However, in the eutrophic lowland River Warnow, North-Eastern Germany, nitrate concentrations remained high with 35–185 μmol l⁻¹ without a significant decrease since 1992. In contrast, phosphate concentrations, varying between 0.3 and 5.2 μmol l⁻¹ during the growth season 2002, decreased significantly over the years. However, its concentrations still exceeded 1 μmol l⁻¹ regularly in the growth seasons. This load led to a substantial accumulation of organic matter additional to high terrestrial inputs. Despite the high organic load, the remineralising bacteria were mainly inactive in River Warnow. Therefore, the composition of the dissolved organic material, especially its bioavailability, were investigated during the growth season 2002 and discussed with other potential controlling factors. River Warnow carried a high load of dissolved organic carbon (14 mg l⁻¹), especially of humic substances (5.5 mg C l⁻¹). Bacterial abundance (12×10⁶ ml⁻¹) as well as production (1.7 μg C l⁻¹ h⁻¹) depended on temperature. During late spring and summer at constantly higher temperatures, bacterial production correlated positively to readily utilisable substrates and to humic compounds. Thus, the bacterial community in River Warnow may be well adapted or contain enough species using the available amino acids and carbohydrates as well as humic matter compounds. However, calculated from protozoan biomass, grazing may control bacterial biomass and perhaps community composition profoundly, what lead to the low percentages of active bacteria.     

Deposition, mineralization, and storage of carbon and nitrogen in sediments of the far northern and northern Great Barrier Reef shelf

The flow of carbon and nitrogen in sediments of the far northern and northern sections of the Great Barrier Reef continental shelf was examined. Most of the organic carbon (81–94%) and total nitrogen (74–92%) depositing to the seabed was mineralized, with burial of carbon (6–19%) and nitrogen (8–20%) being proportionally less on this tropical shelf compared with other non-deltaic shelves. Differences in carbon and nitrogen mineralization among stations related best to water depth and proximity to river basins, with rates of mineralization based on net ∑CO₂ production ranging from 17 to 39 ( mean=23) mmol C m⁻² d⁻¹. The overall ratio of O₂:CO₂ flux was 1.3, close to the Redfield ratio, implying that most organic matter mineralized was algal. Sulfate reduction was estimated to account for ≈30% (range: 6–62%), and denitrification for ≈5% (range: 2–13%), of total C mineralization; there was no measurable CH₄ production. Discrepancies between ∑CO₂ production across the sediment–water interface and sediment incubations suggest that as much as 5 mmol m⁻² d⁻¹ (≈25% of ∑CO₂ flux) was involved in carbonate mineral formation. Most microbial activity was in the upper 20 cm of sediment. Rates of net NH₄⁺ production ranged from 1.6 to 2.7 mmol N m⁻² d⁻¹, with highly variable N₂ fixation rates contributing little to total N input. Ammonification and nitrification rates were sufficient to support rapid rates of denitrification (range: 0.1–12.4 mmol N m⁻² d⁻¹). On average, nearly 50% of total N input to the shelf sediment was denitrified. The average rates of sedimentation, mineralization, and burial of C and N were greater in the northern section of the shelf than in the far northern section, presumably due to higher rainfall and river discharge, as plankton production was similar between regions. The relative proportion of plankton primary production remineralized at the seafloor was in the range of 30–50% which is at the high end of the range found on other shelves. The highly reactive nature of these sediments is attributed to the deposition of high-quality organic material as well as to the shallowness of the shelf, warm temperatures year-round, and a variety of physical disturbances (cyclones, trawling) fostering physicochemical conditions favorable for maintaining rapid rates of microbial metabolism. The rapid and highly efficient recycling of nutrients on the inner and middle shelf may help to explain why the coral reefs on the outer shelf have remained unscathed from increased sediment delivery since European settlement.     

Nature of decadal-scale sediment accumulation on the western shelf of the Mississippi River delta

Sediment delivered to coastal systems by rivers (15×10⁹ tons) plays a key role in the global carbon and nutrient cycles, as deltas and continental shelves are considered to be the main repositories of organic matter in marine sediments. The Mississippi River, delivering more than 60% of the total dissolved and suspended materials from the conterminous US, dominates coastal and margin processes in the northern Gulf of Mexico. Draining approximately 41% of the conterminous US, the Mississippi and Atchafalaya river system deliver approximately 2×10⁸ tons of suspended matter to the northern Gulf shelf each year. Unlike previous work, this study provides a comprehensive evaluation of sediment accumulation covering majority of the shelf (<150 m water depth) west of the Mississippi Delta from 92 cores collected throughout the last 15 years. This provides a unique and invaluable data set of the spatial and modern temporal variations of the sediment accumulation in this dynamic coastal environment.Three types of ²¹⁰Pb profiles were observed from short cores (15–45 cm) collected on the shelf. Proximal to Southwest Pass in 30–100 m water depths, non-steady-state profiles were observed indicating rapid accumulation. Sediment accumulation rates in this area are typically >2.5 cm yr⁻¹ (>1.8 g cm⁻² yr⁻¹). Kasten cores (∼200 cm in length) collected near Southwest Pass also indicate rapid deposition (>4 cm yr⁻¹; >3 g cm⁻² yr⁻¹) on a longer timescale than that captured in the box cores. Near shore (<20 m), profiles are dominated by sediments reworked by waves and currents with no accumulation (the exception is an area just south of Barataria Bay where accumulation occurs). The remainder of the shelf (distal of Southwest Pass) is dominated by steady-state accumulation beneath a ∼10-cm thick mixed layer. Sediment accumulation rates for the distal shelf are typically <0.7 cm yr⁻¹ (<0.5 g cm⁻² yr⁻¹). A preliminary sediment budget based on the distribution of ²¹⁰Pb accumulation rates indicates that 40–50% of the sediment delivered by the river is transported out of the study region. Sediment is moved to distal regions of the shelf/slope through two different mechanisms. Along-isobath sediment movement occurs by normal resuspension processes west of the delta, whereas delivery of sediments south and southwest of the delta may be also be influenced by mass movement events on varying timescales.     

Rare earth elements in the pore waters of reducing nearshore sediments

The REE are mobile during early diagenesis in reducing nearshore sediments of Buzzards Bay leading to greatly enhanced concentrations in pore waters, e.g. 815 pmol kg⁻¹ Nd and 1910 pmol kg⁻¹ Ce within 30 cm of the sediment-seawater interface, about 10–50 times local seawater values. Two principal diagenetic reactions have been identified. Preferential Ce enrichment (positive Ce anomalies) and preferential heavy REE enrichment (light REE removal) in the pore waters is associated with redox cycling of Fe and Mn within the upper few centimeters of the sediment. Release of REE, without fractionation, from sediments and addition to pore waters occurs deeper within the sediment column. The impact on the bulk sediment chemistry is undetectable but the porewater gradients imply that there are significant dissolved REE fluxes, both internal to the sediment system and across the sediment-seawater interface.     

Collection of intact sediment cores with overlying water to study nitrogen- and oxygen-dynamics in regions with seasonal hypoxia

Settled particles of fresh, labile organic matter may be a significant source of oxygen demand and nutrient regeneration in seasonally-hypoxic regions caused by nutrient inputs into stratified coastal zones. Studying the dynamics of this material requires sediment sampling methods that include flocculent organic materials and overlying water (OLW) at or above the sediment–water interface (SWI). A new coring device (“HYPOX” corer) was evaluated for examining nitrogen- (N) and oxygen-dynamics at the SWI and OLW in the northern Gulf of Mexico (NGOMEX). The HYPOX corer consists of a “Coring Head” with a check-valve, a weighted “Drive Unit,” and a “Lander,” constructed from inexpensive components. The corer collected undisturbed sediment cores and OLW from sediments at NGOMEX sampling sites with underlying substrates ranging from sand to dense clay. The HYPOX corer could be deployed in weather conditions similar to those needed for a multi-bottle rosette water-sampling system with 20 L bottles. As an example of corer applicability to NGOMEX issues, NH₄⁺ cycling rates were examined at hypoxic and control sites by isotope dilution experiments. The objective was to determine if N-dynamics in OLW were different from those in the water column. “Ammonium demand,” as reflected by potential NH₄⁺ uptake rates, was higher in OLW than in waters collected from a meter or more above the bottom at both sites, but the pattern was more pronounced at the hypoxia site. By contrast, NH₄⁺ regeneration rates were low in all samples. These preliminary results suggest that heterotrophic activity and oxygen consumption in OLW in the hypoxic region may be regulated by the availability of NH₄⁺, or other reduced N compounds, rather than by the lack of sufficient labile organic carbon.     

Sediment transport in distributary channels and its export to the pro-deltaic environment in a tidally dominated delta: Fly River, Papua New Guinea

Current metre deployments, suspended sediment measurements and surface sediment samples were collected from three locations within distributary channels of the tidally dominated Fly River delta in southern Papua New Guinea. Net bedload transport vectors and the occurrence of elongate tidal bars indicate that mutually evasive ebb- and flood-dominant transport zones occur in each of the distributary channels. Suspended sediment experiments at two locations show a phase relationship between tidal velocity and sediment concentration such that the net suspended sediment flux is directed seaward. Processes that control the export of fluid muds with concentrations up to 10 g l⁻¹ from the distributary channels across the delta front and onto the pro-delta are assessed in relation to the available data. Peak spring tidal current speeds (measured at 100 cm above the bed) drop off from around 100 cm s⁻¹ within the distributary channels to <50 cm s⁻¹ on the delta front. Gravity-driven, 2-m thick, fluid mud layers generated in the distributary channels are estimated to require at least 35 h to traverse the 20-km-wide, low-gradient (2×10⁻³ degrees) delta front. The velocities of such currents are well below those required for autosuspension. A 1-month time series of suspended sediment concentration and current velocity from the delta front indicates that tidal currents alone are unable to cause significant cross-delta mud transport. Wave-induced resuspension together with tides, storm surge and barotropic return-flow may play a role in maintaining the transport of fine sediment across the delta front, but insufficient data are available at present to make any reliable estimates.     

Assessment of arsenic and heavy metal pollution and ecological risk in inshore sediments of the Yellow River estuary,China

In order to investigate the pollution levels, sources and ecological risks of arsenic (As) and heavy metals (Cr, Ni, Cu, Zn, Pb and Cd) in inshore sediments of the Yellow River estuary, the surface sediment in areas of inshore coastal waters were sampled in October 2014 as the flow-sediment regulation project (FSRP) was implemented for 13 years. Results showed that the concentrations of As and heavy metals in inshore sediments of the Yellow River estuary were in the order of Zn?>?Cr?>?Cu?>?Ni?>?Pb?>?As?>?Cd. Higher levels of As, Cr, Ni, Cu, Zn and Pb generally occurred in fine-grained sediments of the Yellow River estuary and the southeast region, which was consistent with the spatial distribution of clay. In contrast, higher concentrations of Cd were generally observed in northwest area of the Yellow River estuary and near the Qingshuigou estuary, which showed similarly spatial distribution with that of sand. The sediment quality guidelines (SQG_S) and geoaccumulation indices (I_geo) indicated that the inshore sediments were polluted by Cu, Cd, As, Pb and Zn, and, among them, Cd pollution was more serious. Ecological risk indices (E _r ⁱ ) demonstrated low risks for Cr, Ni, Cu, Zn, Pb and As, and high potential toxicity by Cd. The integrated ecological risk index implied that 6.8% of stations presented moderate risk, 4.5% of stations exhibited disastrous risk, and 88.7% of stations demonstrated considerable risk. Principal component analysis indicated that Ni, Cu, Zn, Pb and As might originate from common pollution sources, while Cr and Cd might share another similar sources. With the continuous implementation of FSRP, As and heavy metal levels in inshore sediments of the Yellow River estuary could be classified as stage I (2002–2010) and stage II (2010–2014). In the stage I, As, Cr, Ni, Cu, Zn and Pb levels fluctuated but decreased significantly, whereas Cd concentrations showed little variation. In the stage II, As and heavy metal levels significantly increased although some little fluctuations occurred. The continuous accumulation of As and heavy metals (especially for Cd) in inshore sediments of the Yellow River estuary would occur again as the FSRP was implemented for 9 years (since 2010). The ecotoxicological risk of Cd, As, Ni and Cu in inshore sediments might be more serious since the accumulation of the four elements would be continuously occurred in future years. Next step, there will be long-term potential consequences for marine organism if effective measures are not taken to control the loadings of metal pollutants into estuary.     

Heavy metals in surface sediment and epibenthic macroinvertebrates from the coastal zone and continental slope of Kenya

In surface sediments, a statistically significant increase was measured in the concentration of copper and cadmium along all transects, and of zinc along only the most southern (Gazi) transect, radiating into the Indian Ocean perpendicular to the Kenyan coast. Mean copper and cadmium increased from 5 to 30 μg g⁻¹ dry wt and from 0.01 to 0.34 μg g⁻¹ dry wt in shallow coastal (±20 m depth) to deep-sea stations (±2000 m depth), respectively. These gradients were found both during the south-east monsoon and north-east monsoon period. The shallow estuarine zone of the Sabaki river mouth showed significantly enhanced levels of total organic carbon and nitrogen and all metals analysed, except cadmium.In crustaceans, the concentration of copper and particularly cadmium was significantly above baseline levels, varying from 45 to 90 μg g⁻¹ dry wt and 1.0 to 8.5 μg g⁻¹ dry wt, respectively. Zinc levels (49–102 μg g⁻¹ dry wt) were at about baseline levels or a little elevated. On the contrary, lead showed very low concentrations, varying from 0.1 to 0.6 μg g⁻¹ dry wt. Other species generally showed the same pattern.     

Transport ofBe andBe in the ocean

Beryllium isotopes (¹⁰Be and⁹Be) have been measured in suspended particles of < 1 mm size collected by mid-water sediment traps deployed in the eastern Pacific at MANOP sites H (6°32′N, 92°50′W, water depth 3600 m) and M (8°50′N, 104°00′W, 3100 m). For comparison, surface sediments from box cores taken from the two sites were also studied. The concentrations of¹⁰Be and⁹Be in sediment-trap particles are about an order of magnitude smaller than those in the bottom sediments which contain about 8 × 10⁹ and 6 × 10¹⁶ atoms g⁻¹ of¹⁰Be and⁹Be, respectively. The sediment trap samples collected from 50 m off the bottom showed significant (26–63%) contributions from resuspended bottom sediments. The¹⁰Be/⁹Be ratio in trap samples varies from 3 to 20 × 10⁻⁸. The variation may partly result from varied proportion of authigenic/detrital material. The fluxes of both isotopes exhibit a very strong seasonality. The fluxes of¹⁰Be into the traps at about 1500 m are estimated as 9 × 10⁵ and 4 × 10⁵ atoms cm⁻² a⁻¹ at sites H and M respectively. These values are to be compared with the fluxes into the sediments of 4–5 × 10⁵ atoms cm⁻² a⁻¹ at both locations. Good correlations exist between¹⁰Be,⁹Be and²⁷Al indicating that the primary carrier phase(s) for the beryllium isotopes in the water column may be aluminosilicates.