Surficial Layer Research Articles

Nouvelle Autoroute 30: a Champlain clay crust earthworks case study

The Champlain Sea Basin in eastern Canada is known for its sensitive clay and associated engineering problems through this much-developed area along the St Lawrence River. Geologically, the surface of the Champlain clay has been subjected to desiccation, frost action, and weathering. This has resulted in the reduction of the water content and sensitivity of the surficial crust layer of this very wet clay deposit sufficiently to enable it to be used as an earthworks material. However, the material is still much wetter than its optimum moisture content. By co-operative value engineering, the team on the Nouvelle Autoroute 30 project in Montréal, Quebec, investigated and achieved moisture content and degree of compaction derogations to the Ministry of Transport, Quebec specification. This allowed the successful construction of many low and high road embankments using an unprecedented 2.3 × 106 m3 of this weathered Champlain clay crust material. This paper describes how the design and construction issues associated with using this material for earthworks were evaluated and addressed.

Performance of a RC Building using Site Specific Ground Motion Parameter of Salt Lake Sector-V Region, Kolkata, India

This work presents the performance of a G+3 storey RC building using site specific response spectra at Maximum Considered Earthquake (MCE) condition of Salt Lake sector V, Kolkata, India, situated on the world’s largest delta island with very soft and thick soil deposit in the surficial layer. A comparison has been made with the codal response spectra [IS 1893 (Part-I):2002] and site specific response spectra of the performance of a G+3 storey building designed according to IS codes and analyzed by nonlinear static procedure. The results of simulated nonlinear behavior of the building during MCE earthquake depicts that the building without masonry infill is safe if it is designed as per IS 1893 (Part I):2002 (assuming soft soil) response spectra as well as using site specific response spectra. However, the building with masonry infill (except ground floor) may not be safe due to soft storey effect during code specified as per MCE.

Six-degree-of-freedom near-source seismic motions I: rotation-to-translation relations and synthetic examples

The paper deals with theoretical aspects of rotation-to-translation relations in six-degree-of-freedom short-period seismic records at close hypocentral distances. Rotation-to-translation ratios are introduced as the ratios relating peak amplitudes of the relevant rotational and translational components. Their frequency dependence is analyzed in simple models. The relations between translations and rotations are expressed by equations derived under the assumption of a spherical S wave radiated from a shallow point source in a homogeneous medium. A set of numerical experiments is performed to examine these relations for a double-couple source buried in simplified structure models. These experiments indicate that at local distances (up to several km), at lower frequencies (up to a few Hz), and at locations with rapid amplitude changes due to the radiation pattern (e.g., in the vicinity of nodal planes), the rotational components are a linear combination of terms proportional to translational velocity and acceleration, and none of the terms can be, in general, neglected. We also focus on the possibility to retrieve the S-wave phase velocity along the wavepath. The applicability of the equations is tested also in a layered velocity model. It has been found that, under certain conditions, the equations allow us to find correct values of wavepath velocity even in vertically inhomogeneous structures. The depth-range sensitivity is examined for a specific 1D model containing a thin surficial low-velocity layer. Based on these experiments, we have concluded that the retrieved velocity is representative down to the depth not exceeding one wavelength.

Estimating Geoacoustic Properties of Surficial Sediments in the North Mien-Hua Canyon Region With a Chirp Sonar Profiler

Acoustic reflection coefficients and attenuation rolloff rates of the seabed near North Mien-Hua Canyon, northeast of Taiwan, were measured from chirp sonar echoes during the Quantifying, Predicting, and Exploiting (QPE) Uncertainty Initiative Experiment in 2009. Using these measurements and the Biot theory with a fluid approximation, the depth-averaged sound speed, density, and medium attenuation of the surficial sediment layer were estimated. The sediment types in the chirp sonar survey area vary from fine sand to coarse silt. To capture this spatial variability, the 1-D geoacoustic parameter estimates along the chirp sonar track were interpolated onto a 2-D grid using an objective mapping technique. The 2-D maps of surficial sediment properties, along with interpolation errors, can be further applied to underwater sound propagation models in the experimental area.

Experimental and numerical study on lateral behaviour of geosynthetic-reinforced pile foundation system

ABSTRACT: This paper introduces an innovative use of polymer strips as part of a novel foundation concept, polymer strips reinforced pile foundation system, where polymer strips are used to enhance the lateral resistance of a pile foundation. The lateral static behaviour of this foundation system was evaluated using a series of reduced scale physical model tests performed on the model foundation installed in different soil beds contained in a small size soil container. These soil beds included: a soil bed representing the base case (soft clay overlying sand layer); a soil model representing the conventional ground replacement solution (the glyben layer sandwiched between an aggregate layer from top and a sand layer from bottom) and a soil model representing the proposed novel geosynthetic-reinforced backfill (the glyben layer sandwiched between a geosynthetic-reinforced aggregate layer from top and a sand layer from bottom). A one directional loading system was attached to the soil container containing a three-layer model soil deposit, which included a layer of synthetic clay (modified glyben) sandwiched between a lower and a surficial granular layer, was used. The model pile-cap system was installed through the surficial granular layer, which was reinforced using a microgrid mesh and underlying clay and sand layers. A series of lateral static pull tests have been conducted to investigate the influence of the polymer strip reinforcement on the foundation system behaviour. This study also presents the results of a three-dimensional finite-element analyses which was based on models calibrated against physical test results. The experimental results and numerical analyses showed that the lateral resistance of a pile cap system was enhanced by the microgrid mesh embedded in the surficial granular layer. The results also showed that the lateral resistance of the pile cap system was significantly influenced by the thickness of engineered backfill.

Lithological discontinuities in Podzols developed from sandstone cover beds in the Stolowe Mountains (Poland)

Podzols developed within so-called cover beds were examined on a slope built of Cretaceous sandstones in SW-Poland. Their typical feature is an easily recognizable stratification of regolith, evidenced in the dichotomy of soil texture and dense subsoil. Heterogeneity of soil profiles is manifested in vertically differentiated sequence: (1) dense, skeletal and massive layer, comprising usually Bs and BC horizons, sometimes Bhs/Bh horizons with strong platy structure is overlain by (2) the uppermost, relatively young layer, consisting of loose sandy material, bearing single coarse fragments of a less weathered sandstone, hosting AE, E and partially Bh horizons. This remarkable layering of slope sediments caused a distinctive lithological discontinuity at the boundary of E and B horizons, evidenced by the differences in: (i) rock fragments' content, (ii) degree of rock clast weathering, (iii) distribution of highly resistant minerals, and (iv) fluctuation of silt and sand fractions within profiles. Lithological discontinuity influences both the lateral podzolization, and the thickness and depth of albic and spodic horizons along the slope catena. Regarding the temporal order, the lowermost layer was influenced by periglacial conditions at before the Boreal; presumably as the surface layer (high score of pollen grains). The age of surficial sandy layer (based on pollen record) could be established at Subatlantic and correlated with forest cutting and intensive surface erosion (charcoal dating: 960+/−30calBP). Neither layers that originated from the eolian processes were distinguished nor any admixture of allochthonous material was found, therefore “typical” middle and upper periglacial cover-beds are not present in this sequence. In any case, Podzols that were formed from the slope sediments in the Stolowe Mountains are polygenetic and of the Holocene age.

Modeling the source contribution of heavy metals in surficial sediment and analysis of their historical changes in the vertical sediments of a drinking water reservoir

Summary Increasing water pollution in developing countries poses a significant threat to environmental health and human welfare. Understanding the spatial distribution and apportioning the sources of pollution are important for the efficient management of water resources. In this study, ten types of heavy metals were detected during 2010–2013 for all ambient samples and point sources samples. A pollution assessment based on the surficial sediment dataset by Enrichment Factor (EF) showed the surficial sediment was moderately contaminated. A comparison of the multivariate approach (principle components analysis/absolute principle component score, PCA/APCS) and the chemical mass balance model (CMB) shows that the identification of sources and calculation of source contribution based on the CMB were more objective and acceptable when source profiles were known and source composition was complex. The results of source apportionment for surficial heavy metals, both from PCA/APCS and CMB model, showed that the natural background (30%) was the most dominant contributor to the surficial heavy metals, followed by mining activities (29%). The contribution percentage of the natural background was negatively related to the degree of contamination. The peak concentrations of many heavy metals (Cu, Ba, Fe, As and Hg) were found in the middle layer of sediment, which is most likely due to the result of development of industry beginning in the 1970s. However, the highest concentration of Pb appeared in the surficial sediment layer, which was most likely due to the sharp increase in the traffic volume. The historical analysis of the sources based on the CMB showed that mining and the chemical industry are stable sources for all of the sections. The comparing of change rates of source contribution versus years indicated that the composition of the materials in estuary site (HF1) is sensitive to the input from the land, whereas center site (HF4) has a buffering effect on the materials from the land through a series of complex movements. These results provide information for the development of improved pollution control strategies for the lakes and reservoirs.

Artificial chinampas soils of Mexico City: their properties and salinization hazards .

The chinampas agriculture is a traditional land use practice in the Valley of Mexico since Pre-Hispanic time. The chinampas soils were constructed by excavation of lake sediments that resulted in the creation of a system of islands separated by channels. The agricultural productivity of these artificial soils was high; also the land use practices included forestry, fish breeding and hunting. Nowadays, the chinampas soils are affected by excessive salinity. We studied 10 representative soil profiles in the chinampas zone of Mexico City in order to characterize their properties and origin, to provide their classification, and to evaluate soil salinization, vertical distribution of the salts and their chemical composition. The soils are characterized by a layered structure, uniform dark grey colour, irregular vertical distribution of organic carbon and clay, and high percentage of carbon. Some soils show an increase in organic matter with depth, and other profiles have maximum organic matter content in the surficial layers and in the subsoil. The dynamics of sedimentation resulted in the decrease in organic matter in the upper layers of lacustrine sediments, because of recent increase in erosion rate and consequent increase in the proportion of mineral particles in the sediments. Most probably high organic matter content in surficial layers of some soils is due to excavation and accumulation of organic-rich subsoil material in the course of digging the channels. The concentration of soluble salts in superficial horizons, expressed as electric conductivity, varies in a wide range from 5 to almost 50 dS·m<sup>-1</sup>. The salts concentrate mainly in the superficial layers of soils. The abundance of the cations of soluble salts is Na<sup>+</sup>>Mg<sup>2+</sup>>Ca<sup>2+</sup>>K<sup>+</sup> and that of the anions is SO<sub>4</sub><sup>2-</sup>>Cl<sup>-</sup>>HCO<sub>3</sub><sup>-</sup>>CO<sub>3</sub><sup>2-</sup>. The alkaline reaction of soils is caused by exchangeable Na rather than by free sodium carbonates. The restoration of chinampas requires a complex approach, combining soil, water and ecosystems remediation.

The long-term denudation rate of granitic regolith in Qinhuangdao, North China determined from the in situ depth profile of the cosmogenic nuclides 26Al and 10Be

This study quantifies the surface denudation rate of granitic regolith via the application of the in situ cosmogenic 26Al and 10Be depth profile in China. The concentration ranges of 26Al and 10Be in the quartz along the ~3-m granitic regolith profile in Qinhuangdao are (4.9–23.1) × 105 and (2.3–36.6) × 104 atoms/g, respectively. With the exception of the surface sample, both 26Al and 10Be concentrations decrease exponentially with sample depth. The Chi-square best-fitting results revealed a total denudation rate of ~9 m/Ma averaged over a 103–105 a timescale, which is lower than the values observed in global granitic outcrops. Compared with global datasets, the flat terrain due to the lack of tectonic activities is most likely the dominant factor that controls the local denudation process. The surface sample offsets from the theoretical cosmogenic nuclide distribution implies that the denudation rate from river basin sediment could be overestimated because of the bioturbation in the surficial soil layer.

Supercapacitor performance of vertically aligned multiwall carbon nanotubes produced by aerosol-assisted CCVD method

Arrays of vertically aligned carbon nanotubes (CNTs) grown on n-doped silicon substrates using an aerosol-assisted catalytic chemical vapor deposition (CCVD) technique have been tested as supercapacitor electrodes. Electrochemical properties of the electrodes were shown to be significantly dependent on the array thickness. At scan rate of 20mV/s the largest specific capacitance of 124 F/g was achieved for the ∼280–μm array, while increase in the thickness to ∼ 1100μm caused a drop in electrode capacitance by four times. It was shown that in a sulfuric acid electrolyte, the redox processes with iron nanoparticles encapsulated in CNTs contribute significantly to the capacitance of array. From the Mössbauer spectroscopy, these nanoparticles are present as α-Fe, γ-Fe, and Fe3C phases. X-ray photoelectron spectroscopy revealed that during the electrode charging and discharging sulfates of Fe(II) and Fe(III) are formed in surficial layers of the nanoparticles.

Martian Low-Aspect-Ratio Layered Ejecta (LARLE) craters: Distribution, characteristics, and relationship to pedestal craters

Low-Aspect-Ratio Layered Ejecta (LARLE) craters are a unique landform found on Mars. LARLE craters are characterized by a crater and normal layered ejecta pattern surrounded by an extensive but thin outer deposit which terminates in a sinuous, almost flame-like morphology. We have conducted a survey to identify all LARLE craters ⩾1-km-diameter within the ±75° latitude zone and to determine their morphologic and morphometric characteristics. The survey reveals 140 LARLE craters, with the majority (91%) located poleward of 40°S and 35°N and all occurring within thick mantles of fine-grained deposits which are likely ice-rich. LARLE craters range in diameter from the cut-off limit of 1km up to 12.2km, with 83% being smaller than 5km. The radius of the outer LARLE deposit displays a linear trend with the crater radius and is greatest at higher polar latitudes. The LARLE deposit ranges in length between 2.56 and 14.81 crater radii in average extent, with maximum length extending up to 21.4 crater radii. The LARLE layer is very sinuous, with lobateness values ranging between 1.45 and 4.35. LARLE craters display a number of characteristics in common with pedestal craters and we propose that pedestal craters are eroded versions of LARLE craters. The distribution and characteristics of the LARLE craters lead us to propose that impact excavation into ice-rich fine-grained deposits produces a dusty base surge cloud (like those produced by explosion craters) that deposits dust and ice particles to create the LARLE layers. Salts emplaced by upward migration of water through the LARLE deposit produce a surficial duricrust layer which protects the deposit from immediate removal by eolian processes.

Aeolian sediment transport and deposition in a modern high‐latitude glacial marine environment

AbstractAeolian sand and dust in polar regions are transported offshore over sea ice and released to the ocean during summer melt. This process has long been considered an important contributor to polar sea floor sedimentation and as a source of bioavailable iron that triggers vast phytoplankton blooms. Reported here are aeolian sediment dispersal patterns and accumulation rates varying between 0·2 g m−2 yr−1 and 55 g m−2 yr−1 over 3000 km2 of sea ice in McMurdo Sound, south‐west Ross Sea, adjacent to the largest ice free area in Antarctica. Sediment distribution and the abundance of southern McMurdo Volcanic Group‐derived glass, show that most sediment originates from the McMurdo Ice Shelf and nearby coastal outcrops. Almost no sediment is derived from the extensive ice free areas of the McMurdo Dry Valleys due to winnowed surficial layers shielding sand‐sized and silt‐sized material from wind erosion and because of the imposing topographic barrier of the north‐south aligned piedmont glaciers. Southerly winds of intermediate strength (ca 20 m sec−1) are primarily responsible for transporting sediment northwards and offshore. The results presented here indicate that sand‐sized sediment does not travel more than ca 5 km offshore, but very‐fine sand and silt grains can travel >100 km from source. For sites >10 km from the coast, the mass accumulation rate is relatively uniform (1·14 ± 0·57 g m−2 yr−1), three orders of magnitude above estimated global atmospheric dust values for the region. This uniformity represents a sea floor sedimentation rate of only 0·2 cm kyr−1, well below the rates of >9 cm kyr−1 reported for biogenic‐dominated sedimentation measured over much of the Ross Sea. These results show that, even for this region of high‐windblown sediment flux, aeolian processes are only a minor contributor to sea floor sedimentation, excepting areas proximal to coastal sources.

A study of Cs-137 spatial distribution in soil thin sections by digital autoradiography and microscopy

Digital autoradiography with imaging plates and microprobe analysis was used to study 137Cs micro-distribution in thin sections of the podzolic sandy soil typical for the Chernobyl remote impact zone 25years after the accident. The zone is notable for contamination of the so-called condensation type where the contribution of the “hot” fuel particles was comparatively low. The initial 137Cs contamination level of the study plot located on the Iput river terrace ca. 20km to the east of the town of Novozybkov, Bryansk region, was approx. 40Ci/km2. According to the modern soil core data the main portion of cesium radioisotopes is still concentrated in the 10–20cm thick surface layer. Thin sections were prepared from the top 0–10cm layer of the soil profile located on the shoulder of the relatively steep northern slope of the forested hill. Autoradiography of the thin sections has clearly shown both cluster and dispersed patterns of 137Cs distribution presumably reflecting initial radiocesium accumulation in the surficial organic layer with mineral admixture and subsequent lateral and vertical propagation due to water movement and filtration. The propagation was accompanied by fixation of 137Cs ions on surfaces of soil particles. High sensitivity (0.03Bq/cm2) and resolution (ca 40μm) of the applied technique enable us to reveal concentration and dispersion zones in soil thin sections on microscale level. Soil micro-morphology and microprobe analysis have shown to be helpful in deciphering soil chemical and mineral composition assumed to be responsible for 137Cs retention in the soil top layer.

Sedimentation, bioturbation, and sedimentary fabric evolution on a modern mesotidal mudflat: A multi-tracer study of processes, rates, and scales

A study of muddy tidal-flat sedimentation and bioturbation was undertaken in the Waitetuna Arm of Raglan Harbor, New Zealand, to evaluate the physical and biological processes that control cycling of sediment between the intertidal seabed and sediment–water interface, and also the formation of tidal flat sedimentary fabric and fine-scale stratigraphy. Cores were collected along an intertidal transect, and analyzed for sedimentary fabric, 210Pb and 7Be radiochemical distributions, and grain size. At the same locations, a new approach for time-series core-X-radiography study was undertaken (spanning 191 days), using magnetite-rich sand as a tracer for sedimentation and bioturbation processes in shallow tidal flat sediments. Sedimentary fabric consists of a shallow stratified layer overlying a deeper zone of intensely bioturbated shelly mud. Bioadvection mixes the deeper zone and contributes fine sediment to the surface stratified layer, via biodeposition. Physical resuspension and deposition of surface muds by wave and tidal flow are also likely contributors to formation of the surficial stratified layer, but physical stratification is not observed below this depth. The deliberate tracer study allowed calculation of bioadvection rates that control strata formation, and can be used to model diagenetic processes. Results suggest that the upper ∼15 cm of seabed can be fully mixed over timescales <1.75 y. Such mixing will erase pre-existing sedimentary fabric and transport buried sediment and chemical compounds back to the tidal-flat surface. Shallow biodiffusion also exists, but produces much slower and shallower mass transport. Best fits for 210Pb profiles using a diagenetic bioadvection/sedimentation model and independently measured tiered bioadvection rates suggest that sediment accumulation rates (SARs) on the tidal flat are ∼0.25 cm/y, near the low end of contemporary New Zealand muddy intertidal SARs. Frequent deposition and erosion of the surface layer demonstrates that long-term sediment accumulation captures only a small fraction of sediment deposited at any one time. Model results also suggest that our magnetite tracer method may slightly underestimate short-term shallow mixing rates (demonstrated by 7Be profiles), and slightly overestimate longer-term, deeper bioturbation rates (demonstrated by 210Pb profiles).

Discovery of nanoscale reduced surfaces and interfaces in VO2 thin films as a unique case of prewetting

VO2 thin films grown on SiOx/Si substrates have been characterized at the sub-nanometer level by Cs-corrected scanning transmission electron microscopy along with electron energy loss spectroscopy. Reduced transitional regions of 2–3 nm thick were found at both the surface and the interface, where the vanadium valence progressively changes from +4 to +2. The formation of these nanometer-thick surficial and interfacial layers can be interpreted as a unique case of prewetting, and it explains the degradation of metal-to-insulator transition properties in VO2 thin films.

Combining Seismic Noise Techniques for Landslide Characterization

A strong topographic relief and the presence of weakly consolidated sediments create favorable conditions for the development of landslides around the eastern rim of the Fergana Basin (Central Asia). In summer 2012, a field experiment employing small aperture seismic arrays was carried out on an unstable slope, using ambient vibration recordings. The aim of the study was to constrain the seismic response of a potential future landslide and to map lateral and vertical changes in the shear-wave velocity of the surficial soil layers. Strong variations of horizontal-to-vertical spectral ratios in terms of amplitude and directionality indicated clear differences in local site effects, probably reflecting the stability of different sections of the slope. Results further showed resonant frequencies of both the entire unstable block, as well as for smaller, individual parts. The use of an ad hoc, passive seismic tomography approach based on noise correlograms allowed for the mapping of the shear-wave velocities of the sliding material, even in cases of significant topography relief. Based on the recording of seismic noise only, we clearly identified a low-velocity body of weakly consolidated claystone and limestone material, which can be interpreted as the landslide body, with laterally varying thickness.

Water budgets of martian recurring slope lineae

Flowing water, possibly brine, has been suggested to cause seasonally reappearing, incrementally growing, dark streaks on steep, warm slopes on Mars. We modeled these Recurring Slope Lineae (RSL) as isothermal water flows in thin surficial layers driven by gravity and capillary suction, with input from sources in the headwall and loss to evaporation. The principal observables are flow duration and length. At 40% porosity, we find that flow thicknesses reaching saturation can be just 50mm or so and freshwater RSL seasonally require 2–10m3 of H2O per m of source headwall. Modeled water budgets are larger for brines because they are active for a longer part of each day, but this could be partly offset by lower evaporation rates. Most of the discharged water is lost to evaporation even while RSL are actively lengthening. The derived water volumes, while small, exceed those that can be supplied by annual melting of near-surface ice (0.2–2m3/m for a 200-mm melt depth over 1–10m height). RSL either tap a liquid reservoir startlingly close to the surface, or the actual water budget is several times smaller. The latter is possible if water never fully saturates RSL along their length. Instead, they would advance like raindrops on a window, as intermittent slugs of water that overrun prior parts of the flow at residual saturation. Annual recharge by vapor cold trapping might then be supplied from the atmosphere or subsurface.

Assessment of the distribution and risks of organochlorine pesticides in core sediments from areas of different human activity on Lake Baiyangdian, China

Burial characteristics and risks associated with 13 organochlorine pesticides (OCPs; α-HCH, β-HCH, γ-HCH, δ-HCH, p,p′-DDE, p,p′-DDD, p,p′-DDT, heptachlor, aldrin, dieldrin, endrin, α-endosulfan and β-endosulfan) in core sediments of Lake Baiyangdian were investigated. The core sediments were taken from areas of different anthropogenic activity along the lake. Concentrations of hexachlorocyclohexane (HCH), dichlorodiphenyltrichloroethane (DDT), heptachlor, aldrin, and endosulfan ranged from 0.64 to 2.72, nd (undetectable levels) to 2.79, 0.29 to 1.37, nd to 2.62, and nd to 1.76 ng g−1, respectively. Concentrations of OCPs in the core sediments exhibited the following spatial trend: Dongtianzhuang > Shaochedian > Zaolinzhuang. Concentrations in the surficial layer were also found to be greater than those in the bottom layer. OCPs were dominated by HCH. β-HCH was found in the greatest concentrations, indicating that the majority of HCH came from older inputs to the area. The ratio of DDT and its metabolites (DDD and DDE) also suggested a lack of recent inputs to the lake environment. The residues of heptachlor, aldrin, and endosulfan in core sediments indicated that cyclodienes were historically applied in the area. The OCP risk assessment, which was based on effect range low and threshold effect level values, suggested that the top layer of sediment in Dongtianzhuang cores had a relatively high toxicity to human health and the environment.

New developments in ambient noise analysis to characterise the seismic response of landslide-prone slopes

Abstract. We report on new developments in the application of ambient noise analysis applied to investigate the dynamic response of landslide-prone slopes to seismic shaking, with special attention to the directional resonance phenomena recognised in previous studies. These phenomena can be relevant for seismic slope susceptibility, especially when maximum resonance orientation is close to potential sliding directions. Therefore, the implementation of an effective technique for site response directivity detection is of general interest. In this regard methods based on the calculation of horizontal-to-vertical noise spectral ratio (HVNR) are promising. The applicability of such methods is investigated in the area of Caramanico Terme (central Italy), where ongoing accelerometer monitoring of slopes with different characteristics offers the possibility of validation of HVNR analysis. The noise measurements, carried out in different times to test the result repeatability, revealed that sites affected by response directivity persistently show major peaks with a common orientation, consistent with the resonance direction inferred from accelerometer data. In some cases such a directivity turned out parallel to maximum slope direction, but this cannot be considered a systematic feature of slope dynamic response. At sites where directivity is absent, the HVNR peaks do not generally show a preferential orientation, with rare exceptions that could be linked to the presence of temporarily active sources of polarised noise. The observed variations of spectral ratio amplitude can be related to temporal changes in site conditions (e.g. groundwater level/soil water content variations affecting P wave velocity and Poisson's ratio of surficial layer), which can hinder the recognition of main resonance frequencies. Therefore, we recommend conducting simultaneous measurements at nearby sites within the same study area and repeating measurements at different times in order to distinguish significant systematic polarisation caused by site-specific response directivity from polarisation controlled by properties of noise sources. Furthermore, an analysis of persistence in noise recordings of signals with systematic directivity showed that only a portion of recordings contains wave trains having a clear polarisation representative of site directional resonance. Thus a careful selection of signals for HVNR analysis is needed for a correct characterisation of site directional properties.

Bacteria contribute to sediment nutrient release and reflect progressed eutrophication-driven hypoxia in an organic-rich continental sea.

In the sedimental organic matter of eutrophic continental seas, such as the largest dead zone in the world, the Baltic Sea, bacteria may directly participate in nutrient release by mineralizing organic matter or indirectly by altering the sediment’s ability to retain nutrients. Here, we present a case study of a hypoxic sea, which receives riverine nutrient loading and in which microbe-mediated vicious cycles of nutrients prevail. We showed that bacterial communities changed along the horizontal loading and vertical mineralization gradients in the Gulf of Finland of the Baltic Sea, using multivariate statistics of terminal restriction fragments and sediment chemical, spatial and other properties of the sampling sites. The change was mainly explained by concentrations of organic carbon, nitrogen and phosphorus, which showed strong positive correlation with Flavobacteria, Sphingobacteria, Alphaproteobacteria and Gammaproteobacteria. These bacteria predominated in the most organic-rich coastal surface sediments overlain by oxic bottom water, whereas sulphate-reducing bacteria, particularly the genus Desulfobacula, prevailed in the reduced organic-rich surface sediments in the open sea. They correlated positively with organic nitrogen and phosphorus, as well as manganese oxides. These relationships suggest that the bacterial groups participated in the aerobic and anaerobic degradation of organic matter and contributed to nutrient cycling. The high abundance of sulphate reducers in the surficial sediment layers reflects the persistence of eutrophication-induced hypoxia causing ecosystem-level changes in the Baltic Sea. The sulphate reducers began to decrease below depths of 20 cm, where members of the family Anaerolineaceae (phylum Chloroflexi) increased, possibly taking part in terminal mineralization processes. Our study provides valuable information on how organic loading affects sediment bacterial community compositions, which consequently may maintain active nutrient recycling. This information is needed to improve our understanding on nutrient cycling in shallow seas where the dead zones are continuously spreading worldwide.