Topmost Sediment Layer Research Articles

Stable Barium Isotope Fractionation in Pore Waters of Estuarine Sediments

AbstractThe development of stable barium (Ba) isotope measurements provides a novel tool to investigate the geochemical cycling of Ba in the ocean and its sediments. In sediment pore waters, gradients of dissolved Ba concentrations result from various diagenetic processes. The distribution and fractionation of Ba isotopes in the pore waters are expected to further improve our understanding of these early diagenetic control mechanisms. Here, we present pore water profiles of dissolved stable Ba isotopic signatures (δ138Bapw) from shallow water sediments covering the entire Pearl River Estuary (PRE) in Southern China. We find pronounced depth‐dependent Ba isotope variations generally showing a shift from heavy to light δ138Bapw signatures from the sediment surface down to 15 cm depth. These gradients are well reproduced by a diffusion‐reaction model, which generates an apparent fractionation factor (138ε) of −0.60 ± 0.10‰ pointing to preferential removal of low‐mass Ba isotopes from the pore water during solution‐solid phase interactions. Consequently, the combined diagenetic processes induce the highest δ138Bapw values of +0.5 to +0.7‰ in the pore waters of the topmost sediment layer. Although the detrital fraction dominates the Ba content in the PRE surface sediments, the determined gradients of pore water Ba isotopes, together with concentration variations of Ba and other redox‐sensitive elements such as manganese (Mn), show that non‐detrital excess Ba carriers including Mn oxides and authigenic barite clearly affect the post‐depositional Ba dynamics. Stable Ba isotopes are thus a potentially powerful tracer of Ba geochemistry during early sediment diagenesis in estuarine depositional environments.

Biogeochemistry of three different shallow gas systems in continental shelf sediments of the South-Eastern Baltic Sea (Gulf of Gdańsk): Carbon cycling, origin of methane and microbial community composition

The objective of the study was to describe the biogeochemistry of three different shallow gas systems, with particular emphasis on the content of carbon forms, origin of gas and microbial community composition of sediments. Surface sediments (1–1.2 m) in the areas above geophysically different shallow gas structures located in south-eastern Baltic Sea (Gulf of Gdańsk) were investigated: MET1-MP – an active deepwater pockmark with gas seepage and upward infiltration of freshwater, MET2 – shallow gas regular accumulation with lower availability of sulphates and high input of terrestrial organic matter, and MET3 – shallow gas elongated sub-bottom furrow with sulphate presence in the whole sediment profile. For comparison purposes, a reference site (ZGG) without any gas structures in sediments was included in the study. At each station, the following parameters were examined: CH4, CORG and CINORG in sediments; DIC, DOC, H2S, SO42−, NH4+, Cl− in pore and near-bottom waters; stable isotopes in DIC (δ13C-DIC), in gases (δ13C-CH4, δ13C-CO2, δ2H-CH4), and microbial communities in sediments (16S rRNA). In addition, general physico-chemical conditions in bottom waters (dO2, T, S, pH and Eh) as well as molecular composition of gas emitted from MET1-MP pockmark were determined. In order to be able to better compare the geological background of these shallow gas systems, the sediments were investigated by non-invasive acoustic methods: multi-beam echosounder and sub-bottom profiler. Moreover, relative AOM rates were estimated. The first system (MET1-MP) was characterised by a dramatic decrease in the availability of sulphate down the sediment, acetoclastic methanogenesis taking place in surface sediments, the AOM limited to a very thin topmost sediment layer, average stable isotope values of δ13C-CH4 = −58.9‰, δ2H-CH4 = −310‰, δ13C-CO2 = −6.9‰, carbonate precipitation and both methanogenic as well as methanotrophic Archaea present in the whole sediment profile without clear zonation. In the shallow gas regular accumulation, in a close proximity to the coast (MET2), methane was virtually trapped in subsurface sediment layers, even despite not-high sulphate concentrations resulting from lower salinity (~7.5 PSU). In that area, the bacterial community was, unlike typical marine sediments, dominated by Firmicutes, and average stable isotope values were: δ13C-CH4 = −67.0‰, δ2H-CH4 = −243‰, δ13C-CO2 = −7.9‰. The third shallow gas system (MET3) was characterised by most efficient AOM, mostly due to sulphate presence in a relatively thick sediment layer, even several dozen cm below the SMT. The average stable isotope ratios for gases were as follows: δ13C-CH4 = −75.8‰, δ2H-CH4 = −241‰ and δ13C-CO2 = −11.9‰. Despite high concentrations of methane, the methanogenic Archaea were virtually absent in the sediment profile, while the methanotrophic ones showed clear zonation, with ANME-2a-2b present in shallower sediments and ANME-1 in deeper layers. From the environmental point of view, active pockmark shallow-gas systems can be considered as the least efficient in preventing methane from migration to surface sediment layers and the water column, especially when freshwater infiltration occurs, which additionally dilutes the pool of sulphates available for AOM. In our study, the MET1-MP active pockmark was the most “leaky” in terms of methane release to the water column. In contrast, the shallow gas systems being under influence of additional sources of oxidants (in our case – MET2; influence of riverine discharge introducing Fe(III), together with high concentrations of oxygen in bottom waters) or having sulphate present in a relatively thick sediment layer (e.g. due to sulphide reoxidation below the SMT; in our case – the MET3 system) can be considered efficient in stopping methane within sediments, by its almost complete oxidation before reaching the water column.

Sedimentation rate and organic matter dynamics shape microbiomes across a continental margin

Abstract. Marine sedimentation rate and bottom-water O2 concentration control organic carbon remineralization and sequestration across continental margins, but whether and how they shape microbiome architecture (the ultimate effector of all biogeochemical phenomena) across shelf and slope sediments is still unclear. Here we reveal distinct microbiome structures and functions, amidst comparable pore fluid chemistries, along 300 cm sediment horizons underlying the seasonal (shallow coastal; water depth: 31 m) and perennial (deep sea; water depths: 530 and 580 m) oxygen minimum zones (OMZs) of the Arabian Sea, situated across the western Indian margin. The sedimentary geomicrobiology was elucidated by analyzing metagenomes, metatranscriptomes, enrichment cultures, and depositional rates measured via radiocarbon and lead excess dating; the findings were then evaluated in light of the other geochemical data available for the cores. Along the perennial-OMZ sediment cores, microbial communities were dominated by Gammaproteobacteria and Alphaproteobacteria, but in the seasonal-OMZ core communities were dominated by Euryarchaeota and Firmicutes. As a perennial-OMZ signature, a cryptic methane production–consumption cycle was found to operate near the sediment surface, within the sulfate reduction zone; overall diversity, as well as the relative abundances of anaerobes requiring simple fatty acids (methanogens, anaerobic methane oxidizers, sulfate reducers, and acetogens), peaked in the topmost sediment layer and then declined via synchronized fluctuations until the sulfate–methane transition zone was reached. The microbiome profile was completely reversed in the seasonal-OMZ sediment horizon. In the perennial-OMZ sediments, deposited organic carbon was higher in concentration and rich in marine components that degrade readily to simple fatty acids; simultaneously, lower sedimentation rate afforded higher O2 exposure time for organic matter degradation despite perennial hypoxia in the bottom water. The resultant abundance of reduced carbon substrates eventually sustained multiple inter-competing microbial processes in the upper sediment layers. The entire geomicrobial scenario was opposite in the sediments of the seasonal OMZ. These findings create a microbiological baseline for understanding carbon–sulfur cycling in distinct depositional settings and water column oxygenation regimes across the continental margins.

Vertical microplastic distribution in sediments of Fuhe River estuary to Baiyangdian Wetland in Northern China

Microplastics exist widely in water environment. The microplastic distribution in sediments can better reflect the long-term microplastic pollution, especially the vertical distribution. However, the vertical microplastic distribution in sediments is diverse and unclear. This paper is the first study on vertical microplastic distribution in estuary sediment of Fuhe River, the main upstream river flowing to Baiyangdian Wetland in the northern China. The typical feature of Fuhe River is that the effluent of municipal wastewater treatment plants is its main water source. Microplastics in 15 sampling sites and different depths (0–50 cm) were examined. Results showed that the microplastic content decreased with the increase of sediment depth, and the highest content was 1049 ± 462 items/kg in the topmost sediment layer (0–5 cm). The particle size of microplastics was smaller in deeper sediment layers. The proportion of colored microplastics in deeper sediment layers was larger than that in shallower layers. Polyethylene (PE) and polypropylene (PP) were the main plastic polymer types in all sediment samples. The spatial distribution characteristics of microplastics in sediments were closely related to human activities, and the microplastic content was higher in the areas with more intense human activities. This study is helpful to understand the detailed distribution characteristics of microplastics in typical rivers in the northern China, and can provide guidance for reducing microplastic pollution.

Investigation of spatial and short-term temporal nearshore sandy sediment strength using a portable free fall penetrometer

In order to investigate the effect of wave processes on the geotechnical characteristics of topmost sediments, a Portable Free Fall Penetrometer (PFFP) was deployed in the energetic nearshore zone of the U.S. Army Corps of Engineers' Field Research Facility (FRF) in Duck, NC. A total of 335 deployments were conducted from the FRF's 560 m long pier over six non-consecutive days in September and October of 2016. During the surveying period, significant wave heights varied between Hs = 0.8–2.4 m measured at a water depth of 17.4 m. The results showed that the sediment strength is affected by the wave climate. The derived average maximum quasi-static bearing capacities (qsbc), which reflects sediment strength, during low energy wave periods (September 22–24 and 26; Hs = 0.8–1.3 m) ranged between 67 and 73 kPa with maximum qsbc values of 120–136 kPa, while the average values were 47–59 kPa with a maximum qsbc of 82–98 kPa during the high energy wave periods (October 5–6; Hs = 2.4 m). Results also showed that the sediment strength decreased in the shallow water depth regions in spite of the increase in the particle size distribution, due to the increasing wave impact on the seabed within the shallow water region. The area where seabed strength variations were affected by the wave action became wider in the cross-shore direction as the wave height increased. Results further showed that the strength of the topmost sediment layers varied spatially along the profile and temporarily with variations in significant wave height. This likely reflects the impact of waves on sediment strength.

High-resolution radiocarbon records trace episodes of Zoophycos burrowing

The unknown Zoophycos-producing organism potentially reworks marine sediments far deeper (up to >1 m) than common bioturbational mixing of the topmost sediment layer. The Zoophycos producer displaces planktonic foraminifera tests down to deep sediment levels hence, may seriously bias the faunal and isotopic composition and in particular, the radiocarbon age (14C) of the ambient host sediment. Here we present three high-resolution 14C records from sediment cores retrieved at intermediate water depths off Brazil, New Zealand, and Norway where Zoophycos burrows have biased 14C ages by 1000–5000 years. Based on closely spaced 14C ages and 14C plateau tuning we distinguish particular episodes with enhanced Zoophycos production that coincide with time spans of decreased sedimentation and thus, with probably reduced benthic nutrition. Off New Zealand, the actual burrowing activity was dated close to the Bølling/Allerød–Younger Dryas boundary. Displaced grains are traced from the ancient sediment surface of the Younger Dryas as far back as to the onset of Heinrich Stadial 1 and the terminal Last Glacial Maximum. These displaced 14C ages and similar 14C records obtained from two other sediment cores suggest that Zoophycos producers generally mount a burrowing campaign during times when environmental conditions are switching toward a starved sedimentation regime. During these tipping points Zoophycos spreiten probably serve as storage space for nutrient-rich particles foraged at the sediment surface and bioturbational mixed layer.

Temperature Dependence of Apparent Respiratory Quotients and Oxygen Penetration Depth in Contrasting Lake Sediments

AbstractLake sediments constitute an important compartment in the carbon cycle of lakes, by burying carbon over geological timescales and by production and emission of greenhouse gases. The degradation of organic carbon (OC) in lake sediments is linked to both temperature and oxygen (O2), but the interactive nature of this regulation has not been studied in lake sediments in a quantitative way. We present the first systematic investigation of the effects of temperature on the apparent respiratory quotient (RQ, i.e., the molar ratio between carbon dioxide (CO2) production and O2 consumption) in two contrasting lake sediments. Laboratory incubations of sediment cores of a humic lake and an eutrophic lake across a 1–21°C temperature gradient over 157 days revealed that both CO2 production and O2 consumption were positively, exponentially, and similarly dependent on temperature. The apparent RQ differed significantly between the lake sediments (0.63 ± 0.26 and 0.99 ± 0.28 in the humic and the eutrophic lake, respectively; mean ± SD) and was significantly and positively related to temperature. The O2 penetration depth into the sediment varied by a factor of 2 over the 1–21°C temperature range and was significantly, negatively, and similarly related to temperature in both lake sediments. Accordingly, increasing temperature may influence the overall extent of OC degradation in lake sediments by limiting O2 supply to aerobic microbial respiration to the topmost sediment layer, resulting in a concomitant shift to less effective anaerobic degradation pathways. This suggests that temperature may represent a key controlling factor of the OC burial efficiency in lake sediments.

Post-depositional redistribution processes and their effects on middle rare earth element precipitation and the cerium anomaly in sediments in the South Korea Plateau, East Sea

We sampled two box-core sediments from the slope of the eastern South Korea Plateau (SKP) in the East Sea (Sea of Japan) at water depths of 1400 and 1700m. Two chemical fractions of extractable (hydroxylamine/acetic acid) and residual rare earth elements (REEs) together with Al, Ca, Fe, Mg, Mn, P, S, As, Mo, and U were analyzed to assess the post-depositional redistribution of REEs. Extractable Fe and Mn are noticeably abundant in the oxic topmost sediment layer (<3cm). However, some trace elements (e.g., S, As, Mo, U) are more abundant at depth, where redox conditions are different. Analysis of upper continental crust (UCC)-normalized (La/Gd)UCC, (La/Yb)UCC, and (Ce/Ce*)UCC revealed that the extractable REE is characterized by middle REE (MREE) enrichment and a positive cerium (Ce) anomaly, different from the case of the residual fraction which shows slight enrichment in light REEs (LREEs) with no Ce anomaly. The extractable MREEs seem to have been incorporated into high-Mg calcite during reductive dissolution of Fe oxyhydroxides. In the top sediment layer, the positive Ce anomaly is attributed to Ce oxide, which can be mobilized in deeper oxygen-poor environments and redistributed in the sediment column. In addition, differential concentrations of Ce and other LREEs in pore water appear to result in variable (Ce/Ce*)UCC ratios in the extractable fraction at depth.

MICROHABITAT SELECTION OF BENTHIC FORAMINIFERA IN SEDIMENTS OFF THE RHONE RIVER MOUTH (NW MEDITERRANEAN)

The microhabitats and the composition of living benthic foraminiferal faunas in the sediments deposited off the Rhone River mouth are directly influenced by the Rhone River input. In this shallow-water environment (20–98 m water depth), the vertical distribution of the species is not well defined, probably due to the low penetration of oxygen into the sediment. We show the existence of two types of species: “predominantly superficial” taxa showing a density maximum in the topmost sediment layer, and “potentially/ predominantly infaunal taxa” that are also frequent in the topmost sediment, but which show considerable densities in anoxic deeper sediment layers as well. In the area that is strongly influenced by river input (near the river mouth and in the southwestern direction, following the river plume), the fauna is composed mostly of “potentially/predominantly infaunal” species adapted to a higher contribution of terrestrial organic matter, generally of lower quality, and to the low penetration of oxygen into the sediment. This fauna is composed of species tolerant to strong environmental stress (e.g., Nonionella turgida , Nonion scaphum , Rectuvigerina phlegeri , and Valvulineria bradyana ). The stations less influenced by fluvial input (located south and east of the river mouth) are composed of species that colonize oxygenated interstices in the upper centimeter of sediment. The dominant species are mainly “predominantly superficial” taxa (e.g., Cassidulina carinata , Bulimina aculeata ), which are known to react quickly to provisions of labile organic matter. The correlation between these two types of faunas and the two prevalent environmental factors suggests that the vertical distribution of living foraminifera in front of the Rhone River mouth is primarily controlled by the quality of the organic matter and less by the quantity of the organic matter and depth of oxygen penetration into the sediment.

Spatial patterns of bacterial signature biomarkers in marine sediments of the Gulf of Mexico

Sediment cores near a cold methane seep and gas hydrates in the Gulf of Mexico were analyzed for bacterial membrane phospholipid fatty acids (PLFA) and isoprenoid quinones. Nearby non-seep marine sediments were also analyzed for the purpose of comparison. Our goal was to use these biomarkers as proxies for microbial community structure and redox indicators. Total PLFA varied from core to core and were significantly less abundant in non-seep marine sediments than those near the cold seep or gas hydrates. In general, saturated straight chain, terminal branched and mid-chain branched, and monounsaturated fatty acids contributed > 98% of the total PLFA. The branched fatty acids (i.e., aC15:0, iC15:0, Me10C16:0) could be largely attributed to sulfate-reducing bacteria and the monounsaturated fatty acid (i.e., 16:1ω7c and 18:1ω7c) could be attributed to sulfide-oxidizing bacteria ( Beggiatoa/ Thioploca). Principal component analysis of PLFA data revealed that microbial communities were consistent with the spatial distributions of gas hydrates and Beggiatoa mats and with the variation in geochemical conditions. Isoprenoid quinone profiles indicated that aerobic respiration was the dominant metabolic process in the top-most sediment layers while the anaerobic respiration dominated the deeper intervals in the marine sediments. Significant amount of cyclopropyl fatty acids at all sites indicated the stress adaptation of the bacteria in the extreme environment. The consistency between the distribution of PLFA and respiratory quinones and the variation in geochemical conditions suggested that analyses of PLFA and quinone could help us to locate the existence of the coupled sulfide-oxidization and sulfate-reduction processes.

Post-depositional reactivity of the plutonium in different sediment facies from the English Channel - an experimental approach

The post-depositional reactivity of the plutonium was investigated in bottom sediments collected in the central English Channel using an experimental approach. Medium to coarse grained surficial sediments and fine-grained undisturbed sediment cores were spiked with 238 Pu. After a one-month incubation, pore waters were extracted under strictly anoxic conditions and dissolved plutonium was analysed by liquid scintillation techniques. Kd values ranged from 102 to 105 in surficial sediments undergoing oxic diagenesis, and were shown to be grain-size dependant. In fine-grained sediments undergoing anoxic diagenesis, Kd values ranged between 103 and 105 . Kd profile suggests an active recycling of the plutonium in the topmost sediment layers, and subsequent efficient uptake process at depth, tentatively acido-volatile sulphide (AVS) precipitation. This hypothesis was further confirmed by a correlation between the AVS content and the loosely bound plutonium extracted using an oxidising/complexing solution from sediment subsamples. In surficial sediments, by contrast, loosely bound plutonium was likely to be associated with the carbonate phase as surface complexes.

Spatial and temporal variability in modelled nutrient fluxes from the unpolluted Shannon estuary, Ireland, and the implications for microphytobenthic productivity

Nutrient (NO x , NH 4, PO 4) flux rates were calculated from core profiles for three regions in the intertidal area of the Shannon estuary and compared to measured fluxes in laboratory incubations. Mean annual flux rates at the three sites ranged from 1.75 μmol N m −2 h −1 to −82.3 μmol N m −2 h −1 (sediment release and uptake, respectively as NO x ), 2.82 μmol N m −2 h −1 to 26.3 μmol N m −2 h −1 (sediment release as NH 4) and 0.13 μmol P m −2 h −1 to 2.2 μmol P m −2 h −1 (sediment release as PO 4). Spatial variability was estimated within one station (5 m 2), along a transect perpendicular to the shore and among the three regions, while temporal variability was estimated from monthly samples. NO x (nitrate + nitrite) fluxes showed the greatest spatial and temporal variability. While fluxes of all nutrients at the three regions were significantly different, there was much greater homogeneity within the one station and along the transect. The results indicated considerable N consumption in the topmost sediment layers, and this is suggested as a major contribution to microphytobenthic productivity, although the sediment flux contribution to the total estuary nutrient input was negligible.

Analysing the impact of bottom trawls on sedimentary seabeds with sediment profile imagery

Sediment profile imagery (SPI) was evaluated for the assessment of otter trawling impacts on the seabed. This technique allows the imaging of the topmost sediment layers in profile, including the sediment–water interface. Two areas in the Aegean Sea were investigated in time series, each with control and impact areas: a commercial fishing lane with soft sediments at approximately 200 m depth and an experimentally trawled lane with harder maerly sediments at approximately 80 m depth. In total, 158 images were taken at the deep ground and 124 at the shallow ground. A number of measurements were taken from each image, leading to estimates of comparative penetration and small-scale seabed surface roughness. In addition, a large number of surface and subsurface attributes were noted in the images to form the basis of a multivariate analysis. Results indicated that penetration and roughness by themselves were not very good indicators, although roughness was a better indicator particularly in coarse sediments. The major reason for this is that the measurements alone (in particular roughness) do not distinguish between biological and anthropogenic disturbance. The multivariate analysis combining the measurements with the attributes was a good indicator in investigating trawling impacts in coarse sediments, where the lack of good penetration can be compensated by the view over the sediment surface, where more attribute-type data can be gathered. The SPI sampling window gives a relatively small imaged sample in comparison to other imaging techniques (side scan sonar, video, etc.) and in a heterogeneous environment, the more the replicates, the more reliable the method will be. A tiered imaging approach is recommended where more than one methodology is used.

Distribution of Fe, Mn, Cu and Cd in Upper Sediments and Sediment-Trap Material of Ria Formosa (Portugal)

In shallow highly productive coastal environments intense biogeochemical activity is concentrated in the upper sediment layers. However, pore water composition of permeable inter-tidal sediments change as water flows over the areas during ebb and flood tides. In order to examine metal post-mobilisation, inter-tidal sediment cores of 4 cm length and particles settled in sediment-trap s over several periods during 10 months were collected in Ria Formosa (SW Iberian Peninsula). Concentrations of Al, Fe, Mn, Cu and Cd, loss on ignition and the proportion of fine fraction were determined in sediment layers of 0.5 cm thickness and in sediment-trap material. Levels of Mn in sediments were similar to those in sediment-trap material, although enrichment in the topmost sediment layer (0.1 mm) was observed as MnO 2 is formed. Although Fe in sediments and sediment-trap material ranged within similar intervals, two relationships with Al were found showing that sediment-trap material are impoverished in Fe in comparison to sediments. This suggests that Fe is mainly cycled inside the sediment. Copper and Cd exhibited different patterns. Levels in sediment-trap material were not correlated to Al or LOI, probably due to local contamination or uptake by plankton. These values were lower than levels in the topmost sediment layer , suggesting that Cu and Cd that reach the sediment surface are rapidly released to the water column. Concentrations in sediments, including the topmost layer, exhibited two relationships to Al, for both Cu and Cd. One corresponding to background values and the other found in stations closer to the anthropogenic sources. In these cases Cu and Cd are incorporated into the sediments that is facilitated by AVS values close to the sediment surface. ADDITIONALINDEXWORDS: Surface sediments, heavy metals, geochemistr y, coastal lagoon, Portugal.

Application of Gibbs Energy Minimization to ModelEarly-Diagenetic Solid-Solution Aqueous-Solution EquilibriaInvolving Authigenic Rhodochrosites in Anoxic BalticSea Sediments

The natural early-diagenetic environment "anoxic porewater - authigenic mineral phases" has been characterized in sediment of the Gotland Deep, Baltic Sea, by a closed-system model. Occurrence of carbonate precipitates as thin almost pure white laminae was considered as a natural experiment for long-term equilibration between these phases and porewater. Plots of distribution coefficients indicate that metastable equilibrium exists between porewater and the authigenic Ca-rich rhodochrosite phases below 7 cm depth. A thermodynamic model of porewater geochemistry at in situP = 25 bar andT =5 C was developed using the Gibbs energy minimization (GEM) approach. The values of isobaric-isothermal potentials of Mn, Ca, Fe, Mg, Sr, Ba, C, and O, calculated from the porewater composition, were used in a new "dual thermodynamic" calculation approach to estimate solid activity coefficients of the end-members in the non-ideal solid solution (Mn, Ca, Mg, Sr, Ba, Fe)CO3, i.e., at full major and minor multi-component complexity. The regular Margules interaction parameters for the composing binaries estimated by this model wereMn-Ca =1 .9 0.5,Mn-Mg = 0.6, Ca-Mg =3 .7,Mn-Fe =0 .2,Ca-Fe =2 .8,Mn-Sr =9 .7,Ca-Sr = 2.15, Mn-Ba =4 .0,Ca-Ba = 1.4, validating the theoretical predictions given by Lippmann in his pioneering 1980's paper. The strictly thermodynamic equilibrium model is not only able to match both the measured porewater and carbonate solid-solution composition, but also to predict that the porewater pH, pe, alkalinity, and dissolved Mn, Fe, and S concentrations are controlled by the authigenic mineral buffering assemblage mackinawite-greigite-rhodochrosite. Our model is only compatible with the idea of ACR formation with typical composition (XMn between 70-75%) in the topmost sediment layer which, however, needs a major source of Mn II . This is provided by reduction of particulate Mn oxides precipitated in significant amounts in the water column upon major inflow events in the Baltic Sea. The model enables also to set up scenarios of changing environmental conditions, e.g., to predict the non-linear response of the carbonate solid-solution composition to changes in Mn loading, alkalinity and salin- ity of the sediment-water system. The results suggest that the major and especially minor element contents (Sr, Mg, Ba) in authigenic carbonates can be applied as an environmental paleoproxy.

The effect of continuous physical disturbance on mayflies of a tropical stream: an experimental approach

Field experiments to examine the effect of continuous physical disturbance on the Ephemeroptera of the Naro Moru River, Kenya, wereundertaken from June 1993 to January 1994. Continuous disturbance wasadministered on a randomly selected subsite of the sediment surface.Artificial physical disturbance within the experimental subsite involvedcontinuous local displacement, shifting and stirring of the streambedsurface substrates (about 10 cm depth) by hand every one minute for 10 or 14min. Three control samples were also taken randomly from the sedimentsurface of an undisturbed stratified area of the study riffle at the startof each disturbance occasion. All samples were collected using a Hesssampler (surface sampling area of 3.142 dm2; meshsize 80μm). Seven mayfly species were particularly abundant and these included Afronurus sp., Afroptilum sudafricanum LESTAGE, Baetis s.l., Baetis(Nigrobaetis) sp. 1, Baetis (Nigrobaetis) sp. 2, Caenis sp. and Choroterpes(Euthraulus) sp. About 83,8% of the total mayfly density and88.1% of the biomass were removed from the streambed surface withinthe first three minutes of continuous physical disturbance. A mayfly biomassof 33.7391 mg dm2 and total density of 1357.6 inddm2 were collected from the disturbed subsite during the studyduration. Further, a biomass of about 42.8335 mg dm−2and total density of 2366 ind dm−2 were collected fromthe control sites. There was a near-complete depletion of mayflyindividuals from the topmost sediment layer within 14 min of continuousdisturbance.

Formation and distribution of manganese nodule deposit in the northwestern margin of Clarion-Clipperton fracture zones, northeast equatorial Pacific

In the northwestern margin of the Clarion and Clipperton fracture zones, manganese nodules are latitudinally variable in character, resulting from the progressive (re)generation of nodules since the Oligocene as the Pacific Plate migrates northwestward beneath the equatorial production zone. Complemented by the surface water productivity, bottom current activities have concentrated nodules in the Oligocene to Miocene siliceous clay. Resedimentation processes on nodules form three types of nodules with concentric structures and 10 A-manganaterich bottoms, by periodically causing the diagenetic growth of nodules within the topmost sediment layer.

Sedimentary processes and manganese nodule formation in the Korea Deep Ocean Study (KODOS) area, western part of Clarion-Clipperton fracture zones, northeast equatorial Pacific

In the southern part of the Korea Deep Ocean Study (KODOS) area, the western part of the Clarion-Clipperton fracture zones, abundant nodules occur in the Oligocene to Miocene radiolarian-bearing smectitic clay. The nodules are generally concentrated in patchy pavements (more than 5 kg/m2) on the hilly seafloor with sediment waves. They are either rare or abundant on the seamounts and are generally rare in the abyssal plain. The nodules can be classified into three types based on distinct surface texture. The S-type nodules occur in intensely bioturbated sediment of the abyssal hill and seamount areas. In the abyssal plain, the R-type nodules are dominant, embedded within the topmost Quaternary sediment layer. In the southeastern part where Pliocene sediment occurs, they are in an incipient stage of growth. The S-type nodules are rich in Fe and Co in columnar structures of δ-MnO2, whereas the R-types contain abundant Mn, Ni, Cu, and Zn in dendritic and massive 10Å-manganate layers. The T-type nodules occur together with either the R- or S-type, showing intermediate composition. Regardless of the type and shape, the bottoms of nodules contain more abundant Mn, Ni, Cu, Zn, and 10Å-manganate than the tops. As the MnFe ratio increases in the nodules, the abundance of Ni, Cu, and Zn increases up to a constant value (about 11% of Mn) southward. In the underlying sediment, however, it decreases northward. Manganese and these minor metals are concentrated in the lowermost Tertiary sediment as much as 2 to 3 times than those in the topmost sediment layer which is intensely bioturbated. Iron and Co are relatively uniform downcore. Manganese, Ni, and Cu are abundant in micronodules, whereas Fe is concentrated in smectite.Redistributional processes of bottom currents seem to be most responsible for the (re)generation and concentration of nodules in patchy pavements, by excavating buried nodules. These processes result in an inverse relation of nodule abundance to the thickness of uppermost sedimentary layer which comprises the radiolarian ooze of Clipperton Formation. In the seamount areas, sediment slumping has also contributed to the concentration of nodules. The fragmentation of old nodules also enhances the nodule (re)generation. Activities of benthic animals cause the nodules to be lifted upward onto the seafloor mostly during the quiescent periods of bottom currents. The growth of nodules by resedimentation processes results in the large discrepancy in composition not only between the types but also the tops and bottoms, controlled by the burial rates of nodules within the topmost sediment layer where the diagenesis takes place. Diagenetic growth of nodules occurs by utilizing Mn and minor metals remobilized from micronodules during biomixing and decomposition of organic materials within Tertiary sediment. Nodule growth is more active southward toward the equator. Latitudinal variations of nodule size, distribution, abundance, and composition result from the change in surface water productivity and the timing of nodule nucleation as the Pacific plate migrates northwestward.

Estimation of the elastic properties of seafloor sediments by inversion of precritical reflection data

A method has been presented for estimating the elastic properties of sea-floor sediment by inverting the amplitude-range information of precritical reflection data. The method was tested using synthetic data for reflection from a half-space sediment model and the estimates values for the P- and S-wave speeds and the density were in good agreement with the geoacoustic model parameters. The synthetic data were also used to determine the sensitivity of the method to interference from reflections from subbottom layers. In most practical cases the interference is not likely to be a problem for precritical reflection data. The method was then applied to measurements of seafloor reflection obtained in an experiment carried out off the west coast of Canada. The estimated values of the elastic properties of the topmost sediment layer were consistent with values available from the literature, and with estimates from other experiments carried out in the same region.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>