The role of sediments in the phosphorus cycle in Lake Lugano. II. Seasonal and spatial variability of microbiological processes at the sediment-water interface

P, Fe, Mn, and S species were analyzed in water samples from the sediment-water interface collected at four seasonally different times during the course of a year at two sampling sites in the southern basin of Lake Lugano (Lago di Lugano). The results reveal the strong influence of the biogeochemical processes in the sediment on the chemical composition of the lake water above. Consumption of oxygen and nitrate under oxic to microoxic conditions in the water column as well as sequential release of reduced manganese and iron under anoxic conditions was observed as a direct or indirect consequence of microbially mediated degradation of organic matter. The seasonal pattern observed for the release and the retainment of dissolved reduced iron and manganese correlates well with the one for dissolved phosphate. Iron, manganese and phosphorus cycling are coupled tightly in these sediments. Both sediment types act as sinks for hydrogen sulfide and sulfate. An inner-sedimentary sulfur cycle is proposed to couple iron, manganese and phosphorus cycling with the degradation of organic matter. Nutrient cycling at the sediment-water interface might thus be driven by a microbially regulated electron pumping mechanism. The results contribute to a better understanding of the role of sediment processes in the lake's internal phosphorus cycle and its seasonal dynamics.     

The role of sediments in the phosphorus cycle in Lake Lugano. I. Geochemical approach

The bottom sediment from three coring stations in Lake Lugano (Lago di Lugano) show major differences in their role in the P-cycles related to their geochemistry and characteristics of sedimentation. In the northern basin, the deepest sediment may be considered practically inactive, due to a permanently reduced condition at the sediment-water interface. In the southern basin, the sediments are active with respect to P-recycling with strong seasonal variations. One of the sites (Figino) behaves as a sink for P due to a high iron content and an important rate of detrital sedimentation.     

Intertidal mudflat properties, currents and sediment erosion in the partially mixed Tamar Estuary, UK

Results are presented from a 1-year campaign to measure the seasonal variability of some key physical and biological properties of intertidal mudflats over a section of the central Tamar Estuary and to relate these to the physical environment. Seasonal variations in physical mudflat properties, such as grain size, density and moisture content were relatively small. With the exception of the particulate organic carbon content in the upper 0.002 m of surface sediment, biological variations were large. Redox potential exhibited considerable seasonal variation and showed that the sediments were less reduced in winter and more reduced in summer. Chlorophyll a and extracellular polymeric substances (EPS) within the surface 0.002 m of sediment (due to the presence of benthic diatoms) were strongly correlated and exhibited a pronounced seasonal pattern, with smallest values during winter and greatest values during late summer and early autumn. EPS had a dominating influence on sediment erosion, as determined from annular flume measurements. Velocity measurements and velocity modelling indicated that during the flood, and for much of the time during benthic diatom bloom conditions of high chlorophyll a and EPS sediment contents, the stresses exerted by tidal currents were too small to cause significant suspension of sediments over much of the middle and upper mudflats. Suspended fine sediment in the turbidity maximum zone was transported down-estuary and deposited in the main channel at low-water (LW) slack. Some of this sediment, in the form of relatively large aggregates, was subsequently transported onto the mudflats during the flooding tide, where slack currents and fast settling velocities may have enhanced sediment deposition there.Responsible Editor: Hans Burchard     

Structure and subduction processes along the Oregon-Washington margin

Seismic reflection and refraction data off Washington and Oregon are used to determine the style of sediment deformation and to infer the physical properties of accreted sediments on the lower slope. Onshore-offshore seismic refraction data off Washington are used to determine the location of the trench, or where the plate bending starts.We find that off Washington the subduction zone is characterized by a trench whose physiographic expression is buried under several kilometers of sediments and is tens of kilometers landward of the lower slope, which is accreting seaward as the result of the offscraping of sediments.Seismic reflection data support previous observations that offscraping occurs along seaward and landward dipping thrust faults. Refraction data indicate that a sediment package thrust up along a seaward dipping fault (off Washington) was not measurably changed in velocity with respect to a Cascadia basin section. However a package uplifted by thrusting along a landward dipping fault (off Oregon) did have increased velocity. It is suggested that the increased velocities off Oregon could be the result of erosion and exposure of more deeply buried and compacted sediments, rather than the result of dewatering due to tectonic stress. Off Washington the sensitivity of velocity to porosity and resolution of the seismic method does not preclude dewatering due to tectonic stress, but it does limit the degree of dewatering.In the deeper parts of the lower slope section off Washington and Oregon velocities as high as 3 to 4 km/sec are found. Heat flow data indicate that the temperatures in this high velocity regime are greater than 100°C. It is hypothesized that lithification related to clay diagenesis may be partly responsible for the high velocities, rather than simply compaction. It also appears that the high velocity sediments are subducted while the unlithified low velocity sediments are offscraped.     

Trace metal remobilization in the interstitial waters of red clay and hemipelagic marine sediments

Interstitial water samples from the Guatemala Basin and the coast of Baja California have been analyzed for manganese, iron, copper, nickel and nitrate. The data provide a systematic look at changes in trace metal diagenesis proceeding from red clay to highly reducing nearshore sediments.In red clay sediments, the nitrate concentrations suggest that only aerobic respiration is occurring. Manganese and iron are below detection. Nickel concentrations remain the same as in bottom seawater but copper shows a pronounced maximum just at the sediment/water interface. Proceeding to hemipelagic sediments, denitrification becomes increasingly important and manganese and iron remobilization occur in the sediments.The linear manganese and nitrate profiles suggest regions of production or consumption separated by zones of diffusion. This differs from the conventional picture of a continuous series of reactions within the sediments. Manganese reduction always occurs before iron reduction. The pore water nickel correlates well with manganese in these sediments, suggesting that nickel is associated with MnO₂ in the solid phase. The pore water flux ratio of manganese and nickel agrees well with the ratio in solid phase authigenic oxides. Copper still displays a core top concentration maximum as well as a second maximum associated with the remobilized manganese. The calculated ratio of the Cu/C flux ratios support the argument for copper remobilization during organic carbon oxidation. Comparison of the upward and downward diffusive fluxes with the rate of copper buried by sedimentation shows that at least half of the copper buried must be of diagenetic origin and less than 25% of the copper reaching the sediments is buried.     

Magnetic Stratigraphy from Deep Clastic Turbidites: An Example from the Eocene Hecho Group (Southern Pyrenees)

A paleomagnetic study of a 2200-meter thick section of clastic turbidites from the Eocene Hecho group (southcentral Pyrenees, Spain) allows defining its magnetostratigraphic record. Natural remanent magnetization is carried by up to three components: a viscous low-temperature component; a second component unblocked between 300°C and 345°C, likely carried by iron-sulphides; and a third component which demagnetizes at temperatures higher than 345°C and is likely carried by magnetite. The second and third components may display opposite polarities at the same site. The magnetite component delineates different polarity zones and has a more consistent behavior along section if compared with the iron-sulphide component, which displays either a normal or a reverse direction without any stratigraphic consistency along section. The bulk of iron sulphides are interpreted to be secondary in origin and to carry diagenetic overprints acquired at different times after deposition and the magnetite component is taken as the characteristic primary magnetization. As supported by biostratigraphy, the section is correlated from chrons C20r to C18n.2n (Lutetian-Bartonian transition), which indicates a mean sediment accumulation rate of about 52 cm/ky for the studied section. The new chronostratigraphy allows constraining the age of the upper Hecho Group (Banastón and Jaca allogroups) to an unprecedented level and is consistent with previous magnetostratigraphic work in younger sediments from the Jaca Basin. Deep clastic sedimentary systems should not be neglected as a target for magnetostratigraphic studies despite diagenetic growth of secondary minerals may mask the primary signal.     

The 1989 submarine eruption off eastern Izu Peninsula,Japan: ejecta and eruption mechanisms

The submarine eruption of a new small knoll, which was named Teishi knoll, off eastern Izu Peninsula behind the Izu-Mariana arc occurred in the evening of 13 July 1989. This is the first historic eruption of the Higashi-Izu monogenetic volcano group. The eruption of 13 July followed an earthquake swarm near Ito city starting on 30 June. There were subsequent volcanic tremors on 11 and 12 July, and the formation of the Teishi knoll on the 100 m deep insular shelf 4 km northeast of Ito city. There were five submarine explosions, which were characterized by intermittent domelike bulges of water and black tephra-jets, which occurred within 10 min on 13 July. Ejecta of the eruption was small in volume and composed of highly crystalline basalt scoria, highly vesiculated pumice, and lithic material. Petrographical features suggest that the pumice was produced by vesiculation of reheated wet felsic tuff of an older formation. The Teishi knoll, before the eruption, was a circular dome, 450 m across and 25 m high, with steep sides and a flat summit. Considerations of submarine topographic change indicate the knoll was raised by sill-like intrusion of 10⁶ m³ of magma beneath a 30 m thick sediment blanket. This shallow intrusion is assumed to have started on 11 July when volcanic tremors were observed for the first time, but there was no indications of violent interaction between wet host sediments and intruding magma. The submarine eruption of 13 July appears to have been Friggered by a major lowering of the magma-column. The basalt scoria, having crystal-contents of more than 60%, is assumed to be derived from the cooled plastic margin of the shallow intrusive body. However, glassy scoria, which would indicate the interaction between hot fluidal magma and external water, was not observed. A scenario for the 1989 submarine eruption is as follows. When rapid subsidence of the hot interior of the intrusive magma occurred, reduced pressure caused the implosion of cooled plastic magma, adjacent pressurized, hot host material, and wet sediment. The mixing of these materials triggered the vigorous vapor explosions.     

Spreading of Acidification Front in Subsoil Water within a Waterlogged Watershed

The effect of atmospheric precipitation on the hydrogeochemical regime of subsoil water is studied in the case of Usad'e watershed in Novgorod oblast. Thermodynamic and mathematical models are used to generalize the results of long-term hydrogeological studies and predict the propagation of acidification front in groundwater flow.     

Hydrological Regime and Morphodynamics of the Yangtze River Mouth Area

The features of the hydrological regime, modern morphological structure, fluviomorphological processes, and the history of formation of the Yangtze River mouth area are discussed. The influence of the huge river water and sediment runoff and tides on the hydrological and morphological processes in the Yangtze River mouth is shown. The possible impact of construction of the large waterworks Three Gorges on the hydrological and morphological processes in the Yangtze River mouth is roughly evaluated.     

Seasonal dynamics of a cyanobacteria-dominated microbial community in surface sediments of a shallow,eutrophic lake

The seasonal variation of microbial biomass and activity in the surface sediments (0–10 cm) of the shallow, eutrophic Lake Vallentunasjön was followed during one year. OverwinteringMicrocystis colonies dominated the microbial community during all seasons, constituting 60–90% of the total microbial biomass. Expressed on an areal basis, the benthic biomass was, throughout the year, larger than or similar to the planktonic biomass during the peak of the summer bloom, indicating an ability of the colonies to survive in the sediments for extended periods. Abundance of other, non-photosynthetic bacteria varied in the range 3.0–15.5 · 10¹⁰ cells g^–1 d. w. over the year with minimum values in summer and maximum values in autumn in connection with the sedimentation of theMicrocystis bloom. A substantial part of the non-photosynthetic bacteria, up to circa 40%, was associated with the mucilage of healthyMicrocystis colonies. Bacterial production (³H-thymidine incorporation) appeared to be strongly temperature dependent and less influenced by the seasonal sedimentation pattern. Our data indicate an increasing proportion of non-growing cells in autumn and winter. Biomass-bound phosphorus constituted a significant portion, circa 10%, of the phosphorus content in Lake Vallentunasjön sediments. This pool has normally been overlooked in studies on phosphorus dynamics in lake sediments. Different mechanisms whereby organic phosphorus can be released from the sediments are discussed.     

Variation in peperite textures associated with differing host-sediment properties

Peperites formed by mixing of magma and wet sediment are well exposed along Punta China, Baja California, Mexico, where two sills intrude a section of lava flows, limestones, and volcaniclastic rocks. Irregular lobes and dikes extend from the sills several meters into host sediments, including highly comminuted flow top breccias (lithic lapilli tuff breccias) and shelly micrites, whereas intrusive contacts with lava flows are sharp and planar. Where one sill intruded both coarse-grained volcaniclastic rock and fine-grained limestone, textural differences between the hosts produced strikingly different styles of peperite. Blocky masses of the basaltic intrusions up to 1 m in size were dispersed for distances up to 3 m into host lithic lapilli tuff breccias; the blocks consequently underwent in situ fragmentation as they were rapidly quenched. The high degree of dispersion resulted from steam explosions as the magma enveloped pockets of water in the coarse-grained permeable host. Elutriation of fine-grained material from vertical pipes in tuff breccia above the lower sill provides evidence for meter-scale fluidization of the host. The contact zone between the basaltic magma and the shelly micrite host resembles a mixture of two viscous, immiscible fluids (fluidal peperite). Intrusion occurred behind a stable vapor film which entrained lime mud particles and carried them off grain by grain as magma advanced into the host. Thin-section-scale elutriation pipes formed. Microglobular peperite represents a frozen example of a fuel-coolant interaction (FCI) between basaltic magma and fluidized micrite host. The intimate intermixing of magma and host at the submillimeter level is attributed to fluid instabilities developed along the magma-vapor-host interface. Such intimate intermixing of magma and water-bearing fragmental debris is commonly a precursory step toward explosive hydrovolcanism.     

Radio Emission from Galactic Disks

This paper gives a short overview of the observational results on galactic magnetic fields. Interstellar magnetic fields, as deduced from multi-frequency polarization observations, show a well-ordered structure largely following the spiral arms. In some galaxies an axisymmetric spiral pattern dominates (the field being directed inwards), while others exhibit a dominant bisymmetric spiral field or mixed modes, as predicted from non-linear dynamo theory. As long as star formation activity is low, the magnetic fields are rather regular. Strong star formation leads to turbulent cloud motions and supernova explosions, which tangle the field, so that the radio emission is only weakly polarized. As a consequence the highest fractional polarizations and polarized intensities at centimeter wavelengths are found in interarm regions. At decimeter wavelengths, galactic disks become optically thick for polarized emission. In NGC 6946 the regular field is concentrated in narrow magnetic arms located in between the optical spiral arms. The field cannot simply be frozen into the gas and oriented by a density-wave flow. A galactic dynamo may provide a stable spiral pattern of the field, but non-axisymmetric models are still being developed.     

Controls on sonic velocity in carbonates

Compressional and shear-wave velocities (V _p andV _s) of 210 minicores of carbonates from different areas and ages were measured under variable confining and pore-fluid pressures. The lithologies of the samples range from unconsolidated carbonate mud to completely lithified limestones. The velocity measurements enable us to relate velocity variations in carbonates to factors such as mineralogy, porosity, pore types and density and to quantify the velocity effects of compaction and other diagenetic alterations.Pure carbonate rocks show, unlike siliciclastic or shaly sediments, little direct correlation between acoustic properties (V _p andV _s) with age or burial depth of the sediments so that velocity inversions with increasing depth are common. Rather, sonic velocity in carbonates is controlled by the combined effect of depositional lithology and several post-depositional processes, such as cementation or dissolution, which results in fabrics specific to carbonates. These diagenetic fabrics can be directly correlated to the sonic velocity of the rocks.At 8 MPa effective pressureV _p ranges from 1700 to 6500 m/s, andV _s ranges from 800 to 3400 m/s. This range is mainly caused by variations in the amount and type of porosity and not by variations in mineralogy. In general, the measured velocities show a positive correlation with density and an inverse correlation with porosity, but departures from the general trends of correlation can be as high as 2500 m/s. These deviations can be explained by the occurrence of different pore types that form during specific diagenetic phases. Our data set further suggests that commonly used correlations like Gardner's Law (V _p-density) or the time-average-equation (V _p-porosity) should be significantly modified towards higher velocities before being applied to carbonates.The velocity measurements of unconsolidated carbonate mud at different stages of experimental compaction show that the velocity increase due to compaction is lower than the observed velocity increase at decreasing porosities in natural rocks. This discrepancy shows that diagenetic changes that accompany compaction influence velocity more than solely compaction at increasing overburden pressure.The susceptibility of carbonates to diagenetic changes, that occur far more quickly than compaction, causes a special velocity distribution in carbonates and complicates velocity estimations. By assigning characteristic velocity patterns to the observed diagenetic processes, we are able to link sonic velocity to the diagenetic stage of the rock.     

Spatial and temporal heterogeneity of Lake Lugano sediment composition

The concentrations of eighteen elements (Al, C, Ca, Cd, Cr, Cu, Fe, K, Mg, Mn, N, Ni, P, Pb, S, Si, Ti, and Zn) in 76 sediment samples collected in Lake Lugano in 1973 and in 83 samples collected in 1985 were measured and used to draw distribution maps. The chemical compositions of sediments from the three lake subbasins were statistically compared, and used to describe the state of the lake at two time-points twelve years apart. The results obtained seem to be consistent with the available information about the recent evolution of Lake Lugano; both phosphorus and nitrogen are statistically higher in the 1985 samples than in those of 1973, confirming the reported tendency of the lake to evolve towards higher trophic conditions in this period of time. For both metals and mobile elements, the differences between the two sampling years and the literature data reflect fluctuations in the normal geochemical supply from the catchment basin, rather than significant contamination phenomena. However, elements like Ni, Cr, Zn, Pb and Cu tend to be more abundant in the most recent layers of the sediments, so further studies are needed to assess whether potentially hazardous conditions will evolve in the future.The work was performed at the EEC — Environment Institute, Joint Research Centre, I-21020 Ispra (Varese) Italy     

The information content of high-frequency seismograms and the near-surface geologic structure of “hard rock” recording sites

Due to hardware developments in the last decade, the high-frequency end of the frequency band of seismic waves analyzed for source mechanisms has been extended into the audio-frequency range (>20 Hz). In principle, the short wavelengths corresponding to these frequencies can provide information about the details of seismic sources, but in fact, much of the signal is the site response of the nearsurface. Several examples of waveform data recorded at hard rock sites, which are generally assumed to have a flat transfer function, are presented to demonstrate the severe signal distortions, includingf _max, produced by near-surface structures. Analysis of the geology of a number of sites indicates that the overall attenuation of high-frequency (>1 Hz) seismic waves is controlled by the whole-path-Q between source and receiver but the presence of distinctf _max site resonance peaks is controlled by the nature of the surface layer and the underlying near-surface structure. Models of vertical decoupling of the surface and nearsurface and horizontal decoupling of adjacent sites on hard rock outcrops are proposed and their behaviour is compared to the observations of hard rock site response. The upper bound to the frequency band of the seismic waves that contain significant source information which can be deconvolved from a site response or an array response is discussed in terms off _max and the correlation of waveform distortion with the outcrop-scale geologic structure of hard rock sites. It is concluded that although the velocity structures of hard rock sites, unlike those of alluvium sites, allow some audio-frequency seismic energy to propagate to the surface, the resulting signals are a highly distorted, limited subset of the source spectra.     

Spatial heterogeneity of diatom stratigraphy in varved and non-varved sediments of a small,boreal-forest lake

Spatial and temporal heterogeneity of recent diatom stratigraphy were assessed for a small (23 ha) dimictic lake in Northern Sweden (Kassjön). Varves are confined to water depths > 10 m. Six freeze cores were taken along a transect covering a range of water depths (3–12 m) and both varved and non-varved sediments. Core profiles were compared for dry mass accumulation, loss-on-ignition (LOI), and diatom relative frequency stratigraphy and accumulation rate. Excluding the 3.2 m water depth, non-varved core, all parameters showed good repeatability between cores, apart from diatom accumulation rates which were more variable. The 3.2 m core was atypical and had lower LOI values, low planktonic diatom percentages and high values of benthic taxa that were not abundant in the deep-water sediments. Detrended Correspondence Analysis (DCA) and Canonical Correspondence Analysis (CCA) ordinations were compared; both methods clearly differentiated the shallow water core, and showed the general similar ecological trends of the deeper-water cores. CCA axes constrained by environmental (i. e. core location) data resulted in slightly lower eigenvalues than that obtained by Correspondence Analysis (CA), but the axes were significantly non-random. A Partial-CCA of the four varved cores alone (with effects of sediment depth, i.e. age, partialled out), indicated that there was no significant difference between their diatom assemblages.     

Investigation of the instability of graveyard orbits due to the earth's gravitational perturbations

Summary The Banach theorem is applied to the Lagrange planetary equation for the semimajor axis of a geostationary satellite orbit to estimate the stability of near-geostationary satellite orbits. To achieve a graveyard (disposal) orbit, which will not interfere (=cross) the initial geostationary orbit, the geostationary semi-major axis a_g have to be increased at least by 50 km. Numerical results for a variety of graveyard orbits show that the increase of a_g by about 100 km will yield sufficiently stable orbits (accounting for the Earth's gravitational perturbations only) during the next 150 years.Dedicated to the 75th Birthday of Professor Academician Tibor Kolbenheyer     

Do trench sediments affect great earthquake occurrence in subduction zones?

Seismic energy release is dominated by the underthrusting earthquakes in subduction zones, and this energy release is further concentrated in a few subduction zones. While some subduction zones are characterized by the occurrence of great earthquakes, others are relatively aseismic. This variation in maximum earthquake size between subduction zones is one of the most important features of global seismicity. Previous work has shown that the variation in maximum earthquake size is correlated with the variation in two other subduction zone properties: age of the subducting lithosphere and convergence rate. These two properties do not explain all the variance in maximum earthquake size. I propose that a third subduction zone property, trench sediments, explains part of the remaining variance in maximum earthquake size. Subduction zones are divided into two groups: (1) those with excess trench sediments, and (2) those with horst and graben structure at the trench. Thirteen of the 19 largest subduction zone events, including the three largest, occur in zones with excess trench sediments. About half the zones with excess trench sediments are characterized by great earthquake occurrence. Most of the other zones with excess trench sediments but without great earthquakes are predicted to have small earthquakes by the age-rate correlation. Two notable exceptions are the Oregon-Washington and Middle America zones. Overall, the presence of excess trench sediments appears to enhance great earthquake occurrence. One speculative physical mechanism that connects trench sediments and earthquake size is that excess trench sediments are associated with the subduction of a coherent sedimentary layer, which at elevated temperature and pressure, forms a homogeneous and strong contact zone between the plates.     

Ion composition measurements and modelling at altitudes from 140 to 350 km using EISCAT measurements

This work aims at processing the data of CP1 and CP2 programs of EISCAT ionospheric radar from 1987 to 1994 using the –full profile method which allows to solve the –temperature-composition ambiguity problem in the lower F region. The program of data analysis was developed in the CEPHAG in 1995–1996. To improve this program, we implemented another analytical function to model the ion composition profile. This new function better reflects the real profile of the composition. Secondly, we chose the best method to select the initial conditions for the –full profile procedure. A statistical analysis of the results was made to obtain the averages of various parameters: electron concentration and temperature, ion temperature, composition and bulk velocity. The aim is to obtain models of the parameter behaviour defining the ion composition profiles: z₅₀ (transition altitude between atomic and molecular ions) and dz (width of the profile), for various seasons and for high and low solar activities. These models are then compared to other models. To explain the principal features of parameters z₅₀ and dz, we made an analysis of the processes leading to composition changes and related them to production and electron density profile. A new experimental model of ion composition is now available.