Water-saturated Sediments Research Articles

Survival durations and behavioural adjustments of two freshwater bivalves (Unio terminalis, Unionida and Corbicula fluminea, Venerida) under two emersion conditions

Freshwater bivalves in shallow waters are often exposed to extended periods of drought and are highly affected due to their limited mobility. Their adaptation to emersion is a key factor for survival, particularly during human-made unnatural water regime fluctuations or short-term droughts. In the current study, survival durations of two freshwater bivalve species (Unio terminalis, Unionidae and Corbicula fluminea, Cyrenidae) were tested under two experimental emersion conditions (presence of water-saturated sediment and without sediment). U. terminalis' mean survival duration more than doubled in the water-saturated sediment treatment (992 h) compared to treatment without sediment (448 h). For C. fluminea, the mean survival duration in the water-saturated sediment treatment (278 h) was over 50% longer than that recorded in the treatment without sediment (174 h). Both species probably made behavioral adjustments according to the environmental conditions to respond to the presence of the water-saturated sediment and maximized their survival chances. In general, U. terminalis survived significantly longer than C. fluminea in both treatments. No effects of size on survival were observed for either species.

Predicting lithology using neural networks from downhole data of a gas hydrate reservoir in the Krishna–Godavari basin, eastern Indian offshore

SUMMARYWe use the unsupervised and supervised neural network methods together to predict lithology of a gas hydrate reservoir from downhole data in the Krishna–Godavari (KG) offshore basin, India. In this study, we successfully identify the host litho-units of gas hydrate and show its effects in the identification of lithology using neural network techniques, which is not reported earlier. We use well log data acquired at three holes (10A, 03A and 04A) in 2006 during the first expedition of the Indian National Gas Hydrate Program (NGHP-01). Five different logging while drilling data (e.g. density, neutron porosity, gamma ray, resistivity and sonic) are considered for the mapping of lithology and gas hydrate. In the presence of gas hydrate, the resistivity and sonic velocity of the host sediments increase significantly, whereas density, neutron porosity and gamma ray are negligibly affected. Therefore, we calculate resistivity and sonic velocity for water-saturated sediment (without gas hydrate) theoretically to remove the effects of gas hydrate. At first, we apply the seven unsupervised classification methods (i.e. elbow, dendrogram, K-means, 3-D clustering, principal component analysis, Devies–Bouldin index and self-organizing map) to the data with gas hydrate (e.g. observed) and without gas hydrate (i.e. water-saturated/theoretical) to assess the data dimensionality and the number of clusters/litho-units. Each of the unsupervised schemes has its own pros and cons, and may provide different number of cluster/litho-units; sometimes, it is difficult to interpret from only one method. However, all seven methods provide same number of clusters in our study. Then, we apply the supervised classification method (i.e. Bayesian neural networks optimized by hybrid Monte Carlo searching technique) to the training data to refine the defined litho-units and map them with depth. Our approach identifies four types of litho-units and illustrates that the lithology in this area is dominated by clay (∼64 per cent) with some amount of silty clay, silt and minor sand. Gas hydrate is found in clay, silty clay and silt and not in sand. Results also show that, if gas hydrate is not considered as a separate unit, it is distributed as lithology in its hosts (i.e. clay, silty clay and silt). The method is very stable up to ∼15 per cent of random noise added to the data and results are well matched with the analysis of recovered core data. Identified lithologies at three wells correlate very well with seismic section crossing the wells. Very low permeability (<0.1 mD) estimated at three wells also indicates the clay-dominated lithology in our study area.

Burrowing dynamics of aquatic worms in soft sediments

We investigate the dynamics of Lumbriculus variegatus in water-saturated sediment beds to understand limbless locomotion in the benthic zone found at the bottom of lakes and oceans. These slender aquatic worms are observed to perform elongation-contraction and transverse undulatory strokes in both water-saturated sediments and water. Greater drag anisotropy in the sediment medium is observed to boost the burrowing speed of the worm compared to swimming in water with the same stroke using drag-assisted propulsion. We capture the observed speeds by combining the calculated forms based on resistive-force theory of undulatory motion in viscous fluids and a dynamic anchor model of peristaltic motion in the sediments. Peristalsis is found to be effective for burrowing in noncohesive sediments which fill in rapidly behind the moving body inside the sediment bed. Whereas the undulatory stroke is found to be effective in water and in shallow sediment layers where anchoring is not possible to achieve peristaltic motion. We show that such dual strokes occur as well in the earthworm Eisenia fetida which inhabits moist sediments that are prone to flooding. Our analysis in terms of the rheology of the medium shows that the dual strokes are exploited by organisms to negotiate sediment beds that may be packed heterogeneously and can be used by active intruders to move effectively from a fluid through the loose bed surface layer which fluidizes easily to the well-consolidated bed below.

Sediment damage caused by gas exsolution: A key mechanism for mud volcano formation

Gassy sediments are common in marine environments and are characterized by a specific mechanical behavior significantly different from that of water-saturated sediments. It is shown that gas causes damage and initiates fractures in sediments. To define the controlling parameters dominating the damage process during gas exsolution and its consequences in terms of hydro-mechanical behavior, we developed a specific consolidation apparatus to test sediments collected from the active Absheron Mud Volcano. Indeed, mud volcano formation is initiated by gas exsolution and subsequent mud generation at depth from stratified sediments.Gas was generated in the samples by circulating carbonated water through the fine-grained sediments, then decreasing the total pressure. Particular attention was given to the impact of gas saturation and associated damage and fractures on P-wave velocity, sediment compressibility, permeability and preconsolidation pressure. Results show that fracture geometry is mainly controlled by the degree of gas saturation and preconsolidation pressure. The damage level increases with the degree of gas saturation while the elastic modulus of sediments is degraded. Experimental data show that sediments do not entirely recover their original mechanical behavior when gas dissipates. Finally, the experimental data confirm that gas exsolution and expansion is a key mechanism for mud generation.

Black Point – Pyroclasts of a Surtseyan eruption show no change during edifice growth to the surface from 100 m water depth

Surtseyan eruptions have long been studied from their edifice deposits, emplaced subaqueously or subaerially. Here we characterize a Surtseyan eruption from both edifice and ash-sheet deposits using a combination of techniques. The volcano studied is Black Point, which erupted into Lake Russell about 13,000 years ago from a water depth of 105 m, producing a total of ~0.8 km3 of tephra. The techniques, applied to samples acquired by targeted sampling based on geological observations in the field, include geochemical characterization of the eruption products, grain-size analysis of the ash-size fraction at proximal and medial sites, three-dimensional characterization of vesicles in lapilli, and particle-shape analysis for ash grains using the recently introduced freeware PARTISAN©. The aim of this combined approach is to determine how the eruption evolved through its course, from before magma fragmentation to the mechanisms of formation, transport and deposition of the tephra. Field observations are consistent with the ash-sheet deposits forming from eruption-fed density currents, and/or fallout through the water column from a subaerial plume +/− umbrella cloud. Geochemical correlation links the mound deposits, as well as the tuff rings, with a stratigraphic sequence at about 2 km west from the edifice. Both show non-linear but broadly correlative geochemical variations in deposits from the subaqueous stage of the eruption through the subaerial one. Pyroclast vesicle textures indicate that volatile expansion played only a minor role in magma fragmentation, and that there was a high level of bubble coalescence and hence outgassing during the eruption. We infer that fragmentation was mostly phreatomagmatic, resulting from explosive interactions between rising magma and water-saturated sediments. Two-dimensional particle-shape analysis of ash grains shows a large variability in the shape parameters of convexity and solidity; this tells us little about fragmentation style but serves as a proxy for magma vesicularity. The most interesting aspect of this eruption is that none of the shape parameters, or qualitative observations, show any sort of systematic change during the course of the eruption, despite the eruption having begun in 100 m of water and ended up subaerial.

Glacier sliding, seismicity and sediment entrainment

ABSTRACTThe evolution of glaciers and ice sheets depends on processes in the subglacial environment. Shear seismicity along the ice–bed interface provides a window into these processes. Such seismicity requires a rapid loss of strength that is typically ascribed to rate-weakening friction, i.e., decreasing friction with sliding or sliding rate. Many friction experiments have investigated glacial materials at the temperate conditions typical of fast flowing glacier beds. To our knowledge, however, these studies have all found rate-strengthening friction. Here, we investigate the possibility that rate-weakening rock-on-rock friction between sediments frozen to the bottom of the glacier and the underlying water-saturated sediments or bedrock may be responsible for subglacial shear seismicity along temperate glacier beds. We test this ‘entrainment-seismicity hypothesis’ using targeted laboratory experiments and simple models of glacier sliding, seismicity and sediment entrainment. These models suggest that sediment entrainment may be a necessary but not sufficient condition for the occurrence of basal shear seismicity. We propose that stagnation at the Whillans Ice Stream, West Antarctica may be caused by the growth of a frozen fringe of entrained sediment in the ice stream margins. Our results suggest that basal shear seismicity may indicate geomorphic activity.

Formation of water-escape structure during shock-induced fluidization: The role of permeability contrast

Water-escape structure is considered to be a product of fluidization. The exact mechanism of formation of this structure has, however, not been adequately addressed. In the present study, laboratory experiments were carried out to observe the effect of cyclic loading on water-saturated sediment in a close system, with an objective to decipher the specific mechanism of formation of water-escape structure. Present study reveals that an optimum permeability contrast between the successive sedimentary layers of a fining upward succession is the prerequisite for formation of water-escape structures. The onset of fluidization comes into existence when at a critical minor principal stress condition during shearing, the underlying coarse-grained layer attains drained contractive state and the overlying finer-grained layer retains the undrained dilative state. In the contractive layer the sediments tend to become more compact through rearrangements of the particles and displacement of the interstitial fluid. The pore fluid is squeezed out and the sand structure suffers volume contraction. As the upwelling fluid is obstructed by the dilative layer, water-filled cavities are formed at the interface. On progressive shaking these water-filled cavities keep on enlarging and finally the expelled water ruptures through the overlying sand layer resulting water-escape structure. This study also depicts that no influx of external fluid is essential in fluidization of sediments, as was believed earlier.

Evolution of effective thermal conductivity during hydrate formation and decomposition in natural sediments

Gas hydrates are regareded as a potential alternative energy sources for sustainable development. Thermal properties of gas hydrate-bearing sediments directly govern the heat transfer process during hydrate decomposition which couples with phase transitions and multiphase flows. Latest database on the thermal properties of natural sediments containing gas hydrates still remains limited. Here we report on point heat source measurements of the effective thermal conductivity of hydrate-bearing sediments through a thermistor-based method and evaluation of existing models. Effective thermal conductivity of water saturated natural sediments and partially saturated hydrate-sediment are obtained, showing a significant effect of compaction on the effective thermal conductivity of the sediments. The evolution of thermal conductivity during hydrate formation, decomposition and reformation indicates that the variation of the components in the pores could play a crucial role in the effective thermal conductivity. A higher saturation of hydrates in the pores and a smaller porosity could contribute to a better thermal conductivity. The evolution of the effective thermal conductivity with hydrate formation and decomposition also demonstrates a temperature dependence. Moreover, the reformation process after hydrates are fully decomposed significantly impacts the variation of the effective thermal conductivity, which could be attributed to the solution migration during decomposition and the local inhomogeneity of hydrate reformation. The slightly increase in the thermal conductivity below zero degree Celsius implies the small fraction of ice generation during the cooling process. The results could provide some insights into the effect of hydrate formation and decomposition on the effective thermal conductivity of natural sediments, which would be helpful in the heat transfer evaluation and gas production simulation in the field test.

Geomorphic assessment of the urban setting of Húsavík, North Iceland, in the context of earthquake hazard

The Húsavík–Flatey Fault Zone (HFFZ) is one of the largest system of transform faults in Iceland on which damaging earthquakes have repeatedly taken place. The town of Húsavík on the coast of the Tjörnes peninsula lies in an extensional basin formed due to a slight bend in the HFFZ. The fault movement and geological processes have over time created considerable spatial variations in both the topography and shallow subsurface materials on which the town resides. As a result, the earthquake hazard varies significantly within the town, and therefore the local seismic risk as well. In this study, we expand on previous research by mapping the geomorphology of the Húsavík area. Namely, the geomorphology identifies the secondary processes that can be activated during strong earthquake motions and cause additional damage or loss. Through field inspection and remote sensing we have examined the predominant surface geological types and actual and potential geomorphic phenomena and processes in the Húsavík area. The results are presented as a set of geological and geomorphological maps outlining the types and classes of geomorphological processes, along with the topography and slope inclinations of Húsavík. The geomorphological characteristics of the Húsavík area are shown to be largely dominated by mass gravitational motion such as landslides and rockfalls, primarily on the Húsavík mountain and along the Botnsvatn incline, but also along the coastline of Húsavík, especially along the steep slopes in the southernmost part of town. We have also identified areas with possible liquefaction potential within the town where fully water saturated Holocene sediments are located, prompting reevaluation of the local earthquake hazard, seismic risk, and urban planning.

Geochemistry of ice in frost mounds in the valley of the River Sentsa (Oka plateau, East Sayan)

Te authors examined the chemical composition of underground ice sampled from the frost mounds located in the loose (unconsolidated) sediments of the Sentsa River valley (Oka plateau, Eastern Sayan) with the purpose of reconstruction the formation history of these cryogenic creations. Numerous frost mounds of various sizes are mainly composed of alternating icy loams, sandy loams, and lenses of pure ice. Samples of underground ice taken in the outcrops of the river ledges and cores from wells together with samples of river and lake waters were analyzed by traditional hydrochemical techniques (methods) in the center "Geodynamics and geochronology" (Institute of the Earth's Crust of the Siberian branch of RAS, Irkutsk). It was found that the chemical composition of pure ice melts from lenses and layers of the frost mounds is hydrocarbonate calcium (HCO3 Ca, SO4-HCO3 Ca and NH4-HCO3 Ca) with mineralization of 6.5 – 15.6 mg/L, pH = 5.6÷6.1. Mineralization of melts of texture-forming ice, taken from icy ground (i.e. with fractions of enclosing loams) was much higher – from 50 to 792.5 mg/L. River and lake water is ultra-fresh with 99–132 mg/L salinity, and according to geochemical type it is hydrocarbonate calcium (HCO3 Ca). Te specifc features of chemical composition of the underground ice (high content of ammonium salts and sulfates) depend on a water-rock interaction, the presence of organic matter in the loose (unconsolidated) sediments and a repeated volcanic activity in the late Pleistocene–Holocene. Te frost mounds are confned to a lacustrine sediments area in the backwater zone that was formed by the Late Pleistocene terminal moraine. Teir formation in the Holocene took place as a result of segregational ice formation during freezing of water-saturated lake sediments, and, presumably, repeated injections of underground waters of the under-channel and floodplain aquifers hydraulically connected with river waters. Tus, the genesis of the studied frost mounds is probably a mixed segregation-injection process.

A story told by calcareous nannofossils—the timing and course of an Eocene meteorite impact in central Jordan

A circular structure, 5.5 km in diameter, in central eastern Jordan has been interpreted as a large meteorite impact structure. The age of the Waqf As Suwwan impact is poorly constrained. By examining calcareous nannofossils from the sediments exposed in this structure, an age model of the timing of the event has been obtained. A total of 81 smear slides from two cores (BH-1, BH-2) penetrating the sediments of the central structure were prepared in order to obtain a biostratigraphic age for the post-impact sediments. The calcareous nannofossils assign sediments below the breccias of the core BH-1 an age of late Campanian to late Maastrichtian and a mixture of late Maastrichtian to early Eocene ages in the breccia horizon, while core BH-2 is of early Maastrichtian to late Maastrichtian age. The upper part of the sediments, removed in from adjacent area, consists of breccia components. The presences of calcareous nannofossil marker assemblages suggest that these components were derived from two different sources: a Cretaceous and a Paleocene-Early Eocene one. The deposition of the breccia resulted from gravitational collapse of water-saturated sediments in two stages; the earlier of these was more intensive than the latter. The stratigraphic framework and the presence of reworked Cretaceous and Paleocene calcareous nannofossils within Paleogene nannofossil Zone NP12/NP13 suggests an early Eocene age for the impact. The upper part of the Cretaceous sediments was thermally altered by the impact causing partial or complete dissolution of the calcareous nannofossils. This caused overgrowth for the more resistant species, while others were dissolved.

Gas hydrate saturation and reservoir characterization at sites NGHP-02-17 and NGHP-02-19, Krishna Godavari Basin, eastern margin of India

Abstract We have characterized the gas hydrate reservoir at Sites NGHP-02-17 and NGHP-02-19 based on isotropic and anisotropic velocity modeling by utilizing both the wireline (WL) and logging-while-drilling (LWD) data independently. The isotropic study is carried out using the simplified three phase Biot equation (STPBE) with density derived porosity and calculated bulk density of sediments as inputs. The anisotropic study uses the transverse isotropic theory for laminated media by considering a layered reservoir model composed of two components: (i) fractures filled with 100% gas hydrates and (ii) isotropic medium composed of water-saturated sediments. In the anisotropic velocity modeling, the reservoir is modeled by incorporating fractures with varying dip angles from 0° to 90°. We estimated gas hydrate saturations using isotropic and anisotropic velocity analysis at both Sites NGHP-02-17 and NGHP-02-19. In both the cases the LWD data derived isotropic model yielded, what appears to be the most accurate values, an average saturation of 45.8% in the high velocity interval between 269 and 285.5 mbsf at Site NGHP-02-17 and 33.8% in the high velocity interval between 306 and 312 mbsf in NGHP-02-19.

Retention and remobilization mechanisms of environmentally aged silver nanoparticles in an artificial riverbank filtration system

Riverbank filtration systems are important structures that ensure the cleaning of infiltrating surface water for drinking water production. In our study, we investigated the potential risk for a breakthrough of environmentally aged silver nanoparticles (Ag NP) through these systems. Additionally, we identified factors leading to the remobilization of Ag NP accumulated in surficial sediment layers in order to gain insights into remobilization mechanisms.We conducted column experiments with Ag NP in an outdoor pilot plant consisting of water-saturated sediment columns mimicking a riverbank filtration system. The NP had previously been aged in river water, soil extract, and ultrapure water, respectively. We investigated the depth-dependent breakthrough and retention of NP. In subsequent batch experiments, we studied the processes responsible for a remobilization of Ag NP retained in the upper 10 cm of the sediments, induced by ionic strength reduction, natural organic matter (NOM), and mechanical forces. We determined the amount of remobilized Ag by ICP-MS and differentiated between particulate and ionic Ag after remobilization using GFAAS. The presence of Ag-containing heteroaggregates was investigated by combining filtration with single-particle ICP-MS.Single and erratic Ag breakthrough events were mainly found in 30 cm depth and Ag NP were accumulated in the upper 20 cm of the columns. Soil-aged Ag NP showed the lowest retention of only 54%. Remobilization was induced by the reduction of ionic strength and the presence of NOM in combination with mechanical forces. The presence of calcium in the aging- as well as the remobilizing media reduced the remobilization potential. Silver NP were mainly remobilized as heteroaggregates with natural colloids, while dissolution played a minor role.Our study indicates that the breakthrough potential of Ag NP in riverbank filtration systems is generally low, but the aging in soil increases their mobility. Remobilization processes are associated to co-mobilization with natural colloids.

Linking the Ukinrek 1977 maar-eruption observations to the tephra deposits: New insights into maar depositional processes

The Ukinrek Maars erupted 30 March to 9 April 1977, forming two maars, a line of small pit craters and a tephra blanket extending to ~2 km from the vents. We combine photographic and written observations with stratigraphic analysis to reconstruct the eruption. The eruption began with very low (a few meters high) fountaining from small craters above an inferred east-west-trending dike, creating local scoria/spatter agglomerate ramparts with a sandy matrix. The eruption very quickly (in minutes to hours) centered on the West Maar. The West Maar eruption lasted 1–2 days, starting and ending with phreatomagmatic explosions with weak phreato-Strombolian activity in between. Initial explosions formed a 30-m-wide crater, enlarged by crater-wall collapse, and columns as high as 6500 m. Phreato-Strombolian activity produced ~72% of the erupted volume, including a small spatter cone and a scoria blanket around the vent. A final explosion series emplaced a lithic-rich breccia as ballistic blocks, possibly as the northern half of the final crater collapsed into the southern vent area. The East Maar formed over the last nine days of the eruption and represents ~93% of the total volume (4.6 × 106 m3) of the Ukinrek eruption. Initial explosions were probably shallower than 10–20 m but most of the eruption occurred from explosions at 50–60 m below the pre-eruptive surface, with evidence of explosions to 90 m depth only at the very end of the eruption. The East Maar eruption mostly produced columns of lapilli, ash, and steam and the deposits are mostly fallout. Winds blew fallout mostly to the north for the first 5–6 days and to the south for the last three days of the eruption. Wind-directed pyroclastic density currents collapsed from the column, producing fines-rich layers within the coarser fallout. Sporadic explosions produced weak density currents in the first few days and lithic-and juvenile-block-rich breccias in the last few days of the eruption. We interpret that collapse of the crater walls made a slurry that in part provided the water for phreatomagmatic interaction. Explosions came from depths <90 m below the pre-eruptive surface except for a few explosions at the end of the eruption, with most occurring at <70 m depth. The East Maar crater was open to 40–60 m depth throughout most of the eruption, so the explosions were rarely, if ever, deeper than 30 m below the crater floor. Thus, we infer there is no classic, well-formed diatreme structure below the maar. Collapse of the East Maar crater walls provided a supply of water-saturated sediment for much of the phreatomagmatic activity, which came from two vents that did not migrate much, if at all, during the eruption. The Ukinrek Maars deposits were nearly entirely emplaced by fallout, rather than density currents, from explosions and low columns.

Laboratory Strategies for Hydrate Formation in Fine‐Grained Sediments

AbstractFine‐grained sediments limit hydrate nucleation, shift the phase boundary, and hinder gas supply. Laboratory experiments in this study explore different strategies to overcome these challenges, including the use of a more soluble guest molecule rather than methane, grain‐scale gas‐storage within porous diatoms, ice‐to‐hydrate transformation to grow lenses at predefined locations, forced gas injection into water saturated sediments, and long‐term guest molecule transport. Tomographic images and thermal and pressure data provide rich information on hydrate formation and morphology. Results show that hydrate formation is inherently displacive in fine‐grained sediments; lenses are thicker and closer to each other in compressible, high specific surface area sediments subjected to low effective stress. Temperature and pressure trajectories follow a shifted phase boundary that is consistent with capillary effects. Exo‐pore growth results in freshly formed hydrate with a striped and porous structure; this open structure becomes an effective pathway for gas transport to the growing hydrate front. Ice‐to‐hydrate transformation goes through a liquid stage at premelt temperatures; then, capillarity and cryogenic suction compete, and some water becomes imbibed into the sediment faster than hydrate reformation. The geometry of hydrate lenses and the internal hydrate structure continue evolving long after the exothermal response to hydrate formation has completely decayed. Multiple time‐dependent processes occur during hydrate formation, including gas, water and heat transport, sediment compressibility, reaction rate, and the stochastic nucleation process. Hydrate formation strategies conceived for this study highlight the inherent difficulties in emulating hydrate formation in fine‐grained sediments within the relatively short time scale available for laboratory experiments.

Geophysical characterization of a fine-grained gas hydrate reservoir in the Shenhu area, northern South China Sea: Integration of seismic data and downhole logs

Abstract High concentrated gas hydrate deposits in fine-grained sediment are proved to be eligible for exploitation with the current technology. Nevertheless, new insights and more understandings of the accumulation of gas hydrate and characterization of hydrate reservoir in fine-grained sediment are required, which can facilitate the commercial production of gas hydrate in the future. Combining seismic data and downhole logs, here we investigate a fine-grained gas hydrate reservoir in the Shenhu area, northern South China Sea (SCS). Gas hydrate with saturations of 10–45% occurs in fine-grained sediment. Besides the typical bottom-simulating reflectors (BSRs) and high amplitude reflection below BSR, a high amplitude positive reflector (referred to as the top of gas hydrate reservoir reflector; TGHR) exists at the top of high concentrated fine-grained gas hydrate reservoir. According to the result of acoustic impedance inversion, the TGHR is considered to represent a transitional zone from the overlying water-saturated sediment to the underlying high concentrated gas hydrate occurrence zone (GHOZ). Discontinuous BSRs beneath the traditional BSRs are also discovered in the study area, which are interpreted as the seismic evidence of the SII gas hydrate. In the Shenhu area, fluid flow features (such as chimney structures and faults) are found on the ridges where gas hydrate occurs, indicating that gas migration plays a critical role in the formation of gas hydrate in the fine-grained sediment. Our results provide valuable geophysical indications for detecting the presence of gas hydrate in fine-grained sediments.

Effects of Inundation, Nutrient Availability and Plant Species Diversity on Fine Root Mass and Morphology Across a Saltmarsh Flooding Gradient.

Saltmarsh plants are exposed to multiple stresses including tidal inundation, salinity, wave action and sediment anoxia, which require specific root system adaptations to secure sufficient resource capture and firm anchorage in a temporary toxic environment. It is well known that many saltmarsh species develop large below-ground biomass (roots and rhizomes) but relations between fine roots, in particular, and the abiotic conditions in salt marshes are widely unknown. We studied fine root mass (<2 mm in diameter), fine root depth distribution and fine root morphology in three typical communities (Spartina anglica-dominated pioneer zone, Atriplex portulacoides-dominated lower marsh, Elytrigia atherica-dominated upper marsh) across elevational gradients in two tidal salt marshes of the German North Sea coast [a mostly sandy marsh on a barrier island (Spiekeroog), and a silty-clayey marsh on the mainland coast (Westerhever)]. Fine root mass in the 0–40 cm profile ranged between 750 and 2,500 g m−2 in all plots with maxima at both sites in the lower marsh with intermediate inundation frequency and highest plant species richness indicating an effect of biodiversity on fine root mass. Fine root mass and, even more, total fine root surface area (maximum 340 m2 m−2) were high compared to terrestrial grasslands, and were greater in the nutrient-poorer Spiekeroog marsh. Fine root density showed only a slight or no decrease toward 40 cm depth. We conclude that the standing fine root mass and morphology of these salt marshes is mainly under control of species identity and nutrient availability, but species richness is especially influential. The plants of the pioneer zone and lower marsh possess well adapted fine roots and large standing root masses despite the often water-saturated sediment.

Microscale evidence of liquefaction and its potential triggers during soft-bed deformation within subglacial traction tills

Published conceptual models argue that much of the forward motion of modern and ancient glaciers is accommodated by deformation of soft-sediments within the underlying bed. At a microscale this deformation results in the development of a range of ductile and brittle structures in water-saturated sediments as they accommodate the stresses being applied by the overriding glacier. Detailed micromorphological studies of subglacial traction tills reveal that these polydeformed sediments may also contain evidence of having undergone repeated phases of liquefaction followed by solid-state shear deformation. This spatially and temporally restricted liquefaction of subglacial traction tills lowers the shear strength of the sediment and promotes the formation of “transient mobile zones” within the bed, which accommodate the shear imposed by the overriding ice. This process of soft-bed sliding, alternating with bed deformation, facilitates glacier movement by way of ‘stick-slip’ events. The various controls on the slip events have previously been identified as: (i) the introduction of pressurised meltwater into the bed, a process limited by the porosity and permeability of the till; and (ii) pressurisation of porewater as a result of subglacial deformation; to which we include (iii) episodic liquefaction of water-saturated subglacial traction tills in response to glacier seismic activity (icequakes), which are increasingly being recognized as significant processes in modern glaciers and ice sheets. As liquefaction operates only in materials already at very low values of effective stress, its process-form signatures are likely indicative of glacier sub-marginal tills.

Acoustic wave attenuation in the gas hydrate-bearing sediments of Well GC955H, Gulf of Mexico

A better understanding of wave attenuation in hydrate-bearing sediments is necessary for the improved geophysical quantification of marine gas hydrates. Here we compare the attenuation behavior of hydrate-saturated vs water-saturated sediments at site GC955H, in the Gulf of Mexico, which was surveyed during the JIP Leg II expedition. We compute the P-wave attenuation of the gas hydrate bearing sediments using the median frequency shift method on the monopole waveforms. The results show that P-wave attenuation due to low saturation ( 0.4) in the hydrate-filled pores of coarse-grained sediments can be up to as much as three times more than that of the water-saturated case. The correlation analysis shows that the P-wave attenuation increases with the increasing gas hydrate saturation for the highly saturated gas hydrate-bearing sand interval while the correlation of the P-wave attenuation and hydrate saturation is weak for low saturated gas hydrate-bearing shale interval. The results show that P-wave attenuation is more likely to be used as a geophysical proxy for gas hydrate quantification of highly concentrated coarse-grained sediment rather than for that of fine-grained sediment. To examine the P-wave behavior in sand, we use the improved LCAM model, which accounts for physical factors such as grain boundary roughness and squirt flow to explain the observed differences in P-wave attenuation between hydrate and water-saturated coarse-grained sediment. Our results provide further geophysical evidences for P-wave behavior in the gas hydrate-bearing sediments in the field.

First report of Ignotornidae (Aves) from the Lower Cretaceous Gates Formation (Albian) of western Canada, with description of a new ichnospecies of Ignotornis, Ignotornis canadensis ichnosp. nov.

Ignotornidae is an avian vertebrate ichnofamily known from the Lower to middle Cretaceous (Aptian – Cenomanian) of North America (Colorado) and South Korea. The ichnogenus Ignotornis was first described from North America; however, in the eight decades since no Ignotornidae have been described from North America, and hitherto none have been reported from Canada. Here we describe the first ichnospecies of Ignotornis, Ignotornis canadensis ichnosp. nov. (Gates Formation, Lower Cretaceous, Albian), the first new ichnospecies of Ignotornis to be described from North America since the original description of Ignotornis mcconnelli, and the first report of Ignotornidae from Canada. Ignotornis canadensis ichnosp. nov., while morphologically similar to Ignotornis mcconnelli, is larger and has a relatively longer digit I impression than Ignotornis mcconnelli, and has digit I impressions with a similar orientation to those of the ichnogenus Goseongornipes. Discriminant analyses show that Ignotornis canadensis ichnosp. nov. is most similar to the ichnogenera Goseongornipes and Hwangsanipes due to the divarication of digits I–II and II–III. The apparent sinusoidal digit III impression in Ignotornis canadensis and other Ignotornidae is due to water saturated sediment not completely preserving digital pad impressions, rather than a morphologic feature of digit III. While the majority of ichnospecies of Ignotornidae are from South Korea, and while Asia currently contains the largest diversity of avian ichnotaxa from the Cretaceous Period, the apparent lower diversity of Cretaceous avian ichnotaxa from North America may be preservationally controlled, and will be resolved with a greater focus on Early Cretaceous ichnofauna in North America.