Sediment Mounds Research Articles

Sediment mounds and other sedimentary features related to hydrate occurrences in a columnar seismic blanking zone of the Ulleung Basin, East Sea, Korea

A mound related to a cold vent in a columnar seismic blanking zone (CSBZ) was formed around site UBGH1-10 in the central Ulleung Basin (2077 m water depth), East Sea, Korea. The mound is 300–400 m wide and 2–3 m high according to multi-beam bathymetry, 2–7 kHz sub-bottom profiler data, and multi-channel reflection seismic data. Seafloor topography and characteristics were investigated using a remotely operated vehicle (ROV) around site UBGH1-10, which is located near the northern part of the mound. The origin of the mound was investigated through lithology, mineralogy, hydrate occurrence, and sedimentary features using dive cores, piston cores, and a deep-drilling core. The CSBZ extends to ∼265 ms two-way traveltime (TWT) below the seafloor within a mass-transport deposit (MTD) unit. Gas hydrate was entirely contained 6–141 m below the seafloor (mbsf) within hemipelagic deposits intercalated with a fine-grained turbidite (HTD) unit, characteristically associated with high resistivity values at site UBGH1-10. The hydrate is commonly characterized by veins, nodules, and massive types, and is found within muddy sediments as a fracture-filling type. Methane has been produced by microbial reduction of CO 2, as indicated by C 1/C 2+, δ 13 C CH 4 , and δ D CH 4 analyses. The bowl-shaped hydrate cap revealed at 20–45 ms TWT below the seafloor has very high resistivity and high salinity, suggesting rapid and recent gas hydrate formation. The origin of the sediment mound is interpreted as a topographic high formed by the expansion associated with the formation of the gas hydrate cap above the CSBZ. The lower sedimentation rate of the mound sediments may be due to local enhancement of bottom currents by topographic effects. In addition, no evidence of gas bubbles, chemosynthetic communities, or bacterial mats was observed in the mound, suggesting an inactive cold vent.

Paleo-redox fronts and their formation in carbonate mound sediments from the Rockall Trough

Piston cores from the summits of coral topped carbonate mounds at the south west Rockall Trough margin reveal that the sediments have undergone significant post-depositional modifications affecting the original geochemical signature and mineralogical composition of the sediments. This diagenetic imprint provides information about the geochemical processes within cold-water coral mounds. The most prominent result of diagenetic alteration of the primary sediment composition is the absence or poor preservation of aragonitic coral skeletons in certain depth intervals associated with lithification. This study focuses on the enrichment of redox-sensitive elements and the dissolution of primary magnetic ferric iron minerals in the depth interval below lithification levels. By combining the magnetic susceptibility with the XRF signal of Fe and Ti specifically, intervals with susceptibility variations related to the conversion of strongly magnetic into weakly magnetic iron species can be defined. In all three studied cores a succession is recognised with a lithified interval with aragonite dissolution and low-Mg calcite precipitates that is underlain by an interval of magnetite dissolution and of iron and manganese enrichment. For the most recent lithified interval it is demonstrated that initial lithification occurred before an erosional regime was in place, most likely near the end of interglacial or at the start of glacial periods.

The Pleistocene Cooling Built Challenger Mound, a Deep-water Coral Mound in the NE Atlantic: Synthesis from IODP Expedition 307

IODP Expedition 307 revealed the interior of a deep-water coral mound in NE Atlantic for the first time and improved our understanding of these intriguing structures. From the summit of our drilling target, Challenger Mound at ~800 m deep in the Porcupine Seabight, south west of Ireland, we recovered the entire mound section of 155 m long, which almost entirely consists of coral-bearing sediments and rests on the Miocene siliciclastics. The mound initiation is temporally correlated to the global cooling at the beginning of Pleistocene, when modern circulation was established in Atlantic.A key oceanographic feature of the mound provinces is the density gradient that developed above the saline Mediterranean Outflow Water where organic particles persist for a longer time and fuel the coral communities. Growth of the deep-water mounds reflected the glacial/interglacial change. Our age model recognized two growth stages separated by a substantial hiatus; the depositionally continuous lower mound (2.61.7 Ma) accumulated under the low-amplitude relative sea-level change, and the discontinuous upper mound (1.0 Ma to midHolocene) developed under the high-amplitude relative sea-level change. Low cellular abundances in the geochemical features of the mound sediments did not support the hypothesis that hydrocarbon seepage and associated microbial activity significantly enhanced mound initiation and development.

Microbial community composition and biogeochemical processes in cold-water coral carbonate mounds in the Gulf of Cadiz, on the Moroccan margin

This study reports on the microbial community composition of Bacteria and Archaea in a cluster of carbonate mounds located in the Gulf of Cadiz on the Moroccan margin at water depths between 500 and 600m amidst a field of mud volcanoes. Although nearly no live corals were observed on the mound surfaces, gravity cores examined from both Alpha and Beta Mound (MD 169 MiCROSYSTEMS cruise, cores MD08-3218 and MD08-3214, respectively) display a high number of cold-water coral fragments (scleractinians) throughout the cores. Molecular techniques, using taxonomic (16S rRNA gene PCR-DGGE) and functional (mcrA and dsrAB) gene markers, were applied to assess prokaryotic diversity. This was compared with the distribution of total cell counts and a novel approach of ATP-activity measurements to identify microbial activity hot spots during core sub-sampling. Despite broad similarities in the prokaryotic community at the two sites, some differences were observed at specific depth intervals, in correlation with differences in ATP activity. Alpha and Beta Mound showed evidence for the presence of a shallow sulfate–methane transition zone (SMTZ) with increased sulfate reduction rates indicating the presence of microbially mediated anaerobic oxidation of methane (AOM). The key marker gene for methanotrophy and methanogenesis (mcrA) was only found at Alpha Mound within the SMTZ where AOM is occurring. In contrast, sulfate-reducing prokaryotes (dsrAB) were identified in cores from both mounds but at different depths, correlating with increased ATP activity and SR rates. This study demonstrates that the cold-water coral carbonate mounds of the Pen Duick Escarpment host a diverse and metabolically active prokaryotic population. Although it remains unclear to what extent prokaryotes are important for the initiation of a cold-water coral carbonate mound, our results provide some evidence that microbes may play a role in the cementation of the mound sediment through microbially induced carbonate precipitation, and thereby help preserve the mound structure.

The role of glacio-eustasy in sequence formation: Mid-Atlantic Continental Margin, USA

Previous studies of the Mid-Atlantic Continental Margin had shown the lack of correlation between Pleistocene glacio-eustatic oscillations and sequence boundaries. This study documents late Pleistocene to Holocene paleoenvironments within a seismic stratigraphic sequence in order to characterize sequence formation and the development of unconformities in response to glacio-eustatic oscillations. The data, high-resolution CHIRP subbottom profiling and 28 vibracores, were collected in water depths of 38 m to 145 m as part of the R/V Endeavor cruise 370 survey to the Mid-Atlantic Continental Margin. Sediment sources, sedimentary and seismic facies associated with the last major glaciation, and oxygen isotope stages OIS 5 to OIS 1 were examined and a chronology was constructed from mollusk assemblages of intertidal origin that allowed tracking the paleoshoreline from 19 to 6 ka BP. The maximum extent of the Last Glacial Maximum (LGM) paleoshoreline at 17 ka BP was documented near the 120 m modern isobath. The shoreline was marked by sediment mounds up to 5 m high and troughs 15 m deep and 1 km wide (interpreted as shoreface and tidal inlet or lagoons) formed from 17 to 15 ka BP. Five kilometers seaward from the LGM shoreline and the head of the Hudson Canyon is a wedge-shaped feature interpreted as a lowstand delta. Sediments older than the last glacial, possibly OIS 3 or OIS 5, onlap a basal unconformity. We correlate this unconformity to a previously identified sequence boundary. Approximately 20 m to 30 m of sediment accumulated on the outer shelf NE of the Hudson Canyon during the last glacial and little deposition has occurred since ~ 11 ka BP. The modern outershelf is covered by ~ 1 m of mollusk-rich glauconitic sand deposited from latest Pleistocene to early Holocene, 15 to 11 ka BP. These results showed a close correspondence between depositional environments and glacio-eustasy but sequence development and sequence boundary formation appeared to be influenced by the incomplete filling of accommodation space due to poor preservation during OIS 2 and low accumulation during the Holocene.

Paleolandscape and paleoenvironmental interpretation of spring-deposited sediments in Dakhleh Oasis, Western Desert of Egypt

Abstract Iron-rich sediments in Dakhleh Oasis, Western Desert of Egypt, have been recognized as spring mounds and as archaeological sites where Paleolithic materials have been recovered. Detailed sedimentologic investigation and mapping of these features reveal that spring mound sediments were deposited in a shallow vegetated wetland formed by the discharge of iron-rich Nubian Aquifer waters along the southern margin of the oasis, controlled largely by localized faulting and the variable paleotopography of the basin floor. The spring sediments represent peri-lacustrine or lake independent features and can be differentiated from fully lacustrine deposits on the basis of their sedimentary characteristics as well as the presence of goethite and jarosite in a region where authigenic deposition during Pleistocene pluvial activity principally resulted in tufas and lacustrine marls. Spring mound formation incorporated sediment through the ponding of surface water, aeolian entrapment by local vegetation, and the formation of iron precipitates in a low-energy, oxidative and acidic environment that was not dependent upon surface water inputs into the Dakhleh basin. The potentially interpluvial nature of this water resource makes it an important sedimentary archive for archaeological investigations in the basin. The Dakhleh spring mounds record a unique groundwater controlled paleoenvironment, providing the first evidence of a bog iron in Egypt and one of the few occurrences of iron-rich wetland remnants in the modern Sahara.

Observation of bioturbation and hyporheic flux in streambeds

In the Elkhorn River, burrows, tubes, and sediment mounds created by invertebrate bioturbation were observed in the exposed streambed and commonly concentrated on the fine-sediment patches, which consist of silt, clay, and organic matter. These invertebrate activities could loosen the thin layer of clogging sediments and result in an increase of pore size in the sediments, leading to greater vertical hydraulic conductivity of the streambed (Kv). The measurements of the vertical hydraulic gradient across the submerged streambed show that vertical flux in the hyporheic zone can alter directions (upward versus downward) for two locations only a few meters apart. In situ permeameter tests show that streambed Kv in the upper sediment layer is much higher than that in the lower sediment layer, and the calculated Kv in the submerged streambed is consistently greater than that in the clogged sediments around the shorelines of the sand bars. Moreover, a phenomenon of gas bubble release at the water-sediment interface from the subsurface sediments was observed in the groundwater seepage zone where flow velocity is extremely small. The bursting of gas bubbles can potentially break the thin clogging layer of sediments and enhance the vertical hydraulic conductivity of the streambed.

The DeKalb mounds of northeastern Illinois as archives of deglacial history and postglacial environments

The “type” DeKalb mounds of northeastern Illinois, USA (42.0°N, −88.7°W), are formed of basal sand and gravel overlain by rhythmically bedded fines, and weathered sand and gravel. Generally from 2 to 7 m thick, the fines include abundant fossils of ostracodes and uncommon leaves and stems of tundra plants. Rare chironomid head capsules, pillclam shells, and aquatic plant macrofossils also have been observed. Radiocarbon ages on the tundra plant fossils from the “type” region range from 20,420 to 18,560 cal yr BP. Comparison of radiocarbon ages of terrestrial plants from type area ice-walled lake plains and adjacent kettle basins indicate that the topographic inversion to ice-free conditions occurred from 18,560 and 16,650 cal yr BP. Outside the “type” area, the oldest reliable age of tundra plant fossils in DeKalb mound sediment is 21,680 cal yr BP; the mound occurs on the northern arm of the Ransom Moraine (−88.5436°W, 41.5028°N). The youngest age, 16,250 cal yr BP, is associated with a mound on the Deerfield Moraine (−87.9102°W, 42.4260°N) located about 9 km east of Lake Michigan. The chronology of individual successions indicates the lakes persisted on the periglacial landscape for about 300 to 1500 yr.

An experimental study of mounds formed by dumping coarse sediment in channel flow

The development of large sediment mounds formed by continuous release of coarse materials in flowing water is reported. Growths of mound height, longitudinal profile and areal extent were examined for three flow conditions. Longitudinal profiles of depositions during the development phase of the mounds were approximately similar, and they can be expressed by an exponential function if scaled with the maximum height and a width parameter. From observations, it appeared that the mound peak shifted gradually towards upstream and followed the outer part of the sediment plume. The heap growth can be expressed as a function of buoyancy flux due to sediment disposal, flow momentum and densimetric particle Froude number. The deposition height increased with normalized time until reaching close to the water surface. The width and area developed faster with normalized time than the height. The deposition area with coarse sediment was characterized by a downstream tail and fin-like extensions on the sides which differed from observations in previous experiments with fine sand.

Exploring a link between Atlantic coral mounds and Phanerozoic carbonate mudmounds: Insights from pore water fluorescent dissolved organic matter (FDOM), Pen Duick mounds, offshore Morocco

This study explores a genetic link between modern Atlantic coral mounds and ancient, sponge-rich carbonate mudmounds based on Ca-carbonate authigenesis driven by induced-and-supported organomineralization (ISOM). The potential for ISOM in Atlantic coral mounds is tracked by peak patterns of fluorescent dissolved organic matter (FDOM) present in pore waters down to 3m of sediment depth (Gamma mound, Beta mound, Pen Duick escarpment, offshore Morocco).All pore waters display a biological activity (protein-like fluorescence) with two local maxima, occurring from the surface to about 1m of sediment depth and below 2m of sediment depth. Protein-like fluorescence correlates well with the fluorescence of porphyrin-like compounds. A sample of bottom water displays a blue-shifted variety of protein-like fluorescence suggesting hydrolysis and diffusion from the benthic realm into the water column. A significant accumulation of refractory FDOM does not occur in shallower than 2m of sediment depth. It is accompanied by a late stage of biodegradation of organic matter (peak Geol.-A; ex. 224±2nm; em. 385±8nm) and the occurrence of a new fluorescence peak Q that is expressed along an emission band with peak Geol.-A (peak Q; ex. 295±5nm; em. 385±1nm). With caution, peak Q represents a product of exudation from anaerobic microorganisms and/or derives from siderophore-like pigments (pyocyanine-like; from Pseudomonas species).Permineralizing fluids that are characteristic for ISOM are locally present within the shallow subsurface and reach a maximum intensity at about 1m of sediment depth. However, no ISOM-related authigenic carbonate was observed. Both the lack of organic substrates that is due to the absence of degrading connective tissues from a cryptic community and the high amount of infiltrated mud that acts as a sorbent might explain the absence of ISOM in Gamma mound sediments. Labyrinthine deposits of coral rubble or current-swept coral mounds are prone to host a sponge-rich cryptic community and therefore, in their deeper parts, should provide a variety of substrates for ISOM during suboxic cycling of natural organic matter. In such a view, the modern coral-rich mound system, which by itself appears diverse and with a variety of controlling factors, maintains an excellent potential to drive ISOM and to share a crucial process of early diagenesis with Phanerozoic sponge-rich carbonate mudmounds.

Laboratory and Field-scale Evaluation of Low-pH Fe(II) Oxidation at Hughes Borehole, Portage, Pennsylvania

The Hughes Borehole is located in the bituminous coal region of south-central Pennsylvania and drains an underground mine complex of ca. 2,950 ha. The borehole discharge averages approximately 63 L/s of acidic (pH 4, 230 mg/L acidity) mine drainage that contains elevated concentrations of Fe(II) (100 mg/L). Long-term monitoring of the site showed that biological Fe(II) oxidation occurred without human intervention and, over previous decades, has produced a 0.6 ha iron mound up to 2 m deep. On-mound channel reactors along with laboratory-scale gutter reactors were constructed to determine conditions that can exploit biological Fe(II) oxidation for passive treatment. Dissolved Fe(II) was much more efficiently oxidized from gutter reactors that contained iron mound sediment than ones without any sediment. Close to 100% of the dissolved influent Fe(II) was oxidized in 5–10 h and 75% of the total dissolved Fe was removed. In addition, the reactors performed better as the sediment aged, so that shorter residence times of 1–2 h were also capable of oxidizing substantial amounts of the influent Fe(II). The addition of surface area to the on-mound reactors improved Fe(II) oxidation at residence times of 30 min or less. The results of this study can be used to help design and build treatment systems for low-pH acid mine drainage (AMD) discharges.

Diagenetic processes in carbonate mound sediments at the south-west Rockall Trough margin

Cold water coral covered carbonate mounds at the south-west margin of the Rockall Trough form ridges several kilometres long and up to 380 m high. Piston cores obtained at three mound crests reveal the complex internal structure of the mound build up, with alternating unlithified coral-dominated intervals and lithified intervals. The most recent lithified interval is covered by corals embedded in a fine-grained matrix, comprising ca 11 000 years of continuous mound evolution. Before this time 230Th/U dating shows the presence of several hiatuses in mound build-up. Aragonitic coral material is absent or only present as mouldic porosity in the lithified intervals and coccoliths display widespread overgrowth. Downcore X-ray fluorescence scanning, computer tomography scan images and petrographic observations indicate different degrees of diagenetic alteration. The upper boundary of the most recent lithified interval shows some erosional features, but petrographic observations indicate that initial lithification of the sediments is not related to this erosive event or to long-term non-sedimentation, but to earlier sub-surface diagenesis. Organic matter oxidation and the subsequent lowering of the saturation state of the carbonate system drives dissolution of the unstable aragonitic coral skeletons. Depending on the openness of the system, this can lead to precipitation of a more stable low-magnesium carbonate. A model is presented describing the sedimentary and diagenetic processes leading to the formation of lithified intervals.

Diagenetic formation of gypsum and dolomite in a cold-water coral mound in the Porcupine Seabight, off Ireland

Authigenic gypsum was found in a gravity core, retrieved from the top of Mound Perseverance, a giant cold-water coral mound in the Porcupine Basin, off Ireland. The occurrence of gypsum in such an environment is intriguing, because gypsum, a classic evaporitic mineral, is undersaturated with respect to sea water. Sedimentological, petrographic and isotopic evidence point to diagenetic formation of the gypsum, tied to oxidation of sedimentary sulphide minerals (i.e. pyrite). This oxidation is attributed to a phase of increased bottom currents which caused erosion and enhanced inflow of oxidizing fluids into the mound sediments. The oxidation of pyrite produced acidity, causing carbonate dissolution and subsequently leading to pore-water oversaturation with respect to gypsum and dolomite. Calculations based on the isotopic compositions of gypsum and pyrite reveal that between 21·6% and 28·6% of the sulphate incorporated into the gypsum derived from pyrite oxidation. The dissolution of carbonate increased the porosity in the affected sediment layer but promoted lithification of the sediments at the sediment-water interface. Thus, authigenic gypsum can serve as a signature for diagenetic oxidation events in carbonate-rich sediments. These observations demonstrate that fluid flow, steered by environmental factors, has an important effect on the diagenesis of coral mounds.

The stratigraphic and geographic distribution of giant craters and remobilised sediment mounds on the mid Norway margin, and their relation to long term fluid flow

Large craters associated with mounds of remobilised sediment have been recently mapped on the mid Norway margin in the Møre Basin. These craters and mounds may be linked to the long term migration of fluids upwards from the lower levels of the Møre Basin which exploit hydrothermal vent complexes emplaced in the late Paleocene and early Eocene. All of the craters are located on a regionally correlative seismic surface that is correlated with the basal shear plane of Slide W, a slide located at the base of the Plio-Pleistocene Naust Formation. The Craters are positioned in the western area of the Møre Basin at the foot of the continental slope on the crests and flanks of Miocene domes, where Oligocene biosiliceous ooze subcrops on the basal shear surface of Slide W. Not all of the craters are filled by Slide W. Mounds are emplaced above those craters which are filled by Slide W on the top surface of Slide W. Stratal relationships show that the mounds were emplaced on the paleo-seabed. We present and discuss two models that illustrate processes that may have been involved in the formation of craters and remobilisation of sediments. In one model, an eruption of fluid from beneath remobilises ooze into ooze mounds in a single event triggering slope failure, whereas in the other model the emplacement of Slide W and later slides loads low density ooze causing it to undergo liquefaction, a process which may have been facilitated by the trapping of continuous long term fluid migrating from beneath, causing the ooze to remobilise into ooze mounds in two or more events.

Changes in the stygobiont polychaete community of the Jameos del Agua, Lanzarote, as a result of bioturbation by the echiurid Bonellia viridis

The Jameos del Agua is a natural anchialine cave lagoon that has been developed as a tourist attraction. It is semi-illuminated due to a partial collapse of the ceiling (known as “jameo”). A fine carpet of benthic diatoms covers the bottom, in 0.5–2 m depth of water, of the tidally influenced and fully marine lagoon. Interstitial polychaetes inhabit this diatomean carpet and the lava rubble mixed with sand and gravel. During the last decades, the echiurid Bonellia viridis became very abundant and its bioturbation activities have changed the natural bottom conditions. The whitish fecal material from B. viridis formed sediment mounds that sharply contrast with the black volcanic rock. This new habitat have been colonized by several endemic species (e.g., Fauveliopsis jameoaquensis), but also by abundant stygoxene macro- and meiofaunal opportunistic species that compete with the endemic stygobities. The most recent colonizers are mainly deposit-feeding polychaetes that take advantage of the high amount of trophic resources available (e.g., Notomastus spp., Trichobranchus glacialis, Aphelochaeta marioni, Cirrophorus lyra). As a result of the recent colonizations, it is necessary to establish monitoring of the communities and to evaluate the viability of controlling the population of B. viridis in the lagoon, since these alterations can also affect the endemic populations living in the water column.

Cold-water coral mounds in an erosive environmental setting: TOBI side-scan sonar data and ROV video footage from the northwest Porcupine Bank, NE Atlantic

Cold-water coral mounds are common features in certain regions along the Atlantic margin. They occur in mound provinces in various settings, characterised by specific environmental conditions that steer and influence coral mound initiation, growth and demise (e.g. bottom current intensity, sediment input and food supply). In order to add detail to the diversity of environmental conditions described in relation to these structures, this study investigates mounds in a generally erosive setting on the northwest Porcupine Bank (NE Atlantic)—in contrast to previous studies in less hydrodynamically active settings. TOBI (Towed Ocean Bottom Instrument) side-scan sonar data revealed abundant erosive features in the study area. They occur in the form of erosional scarps, erosional pits and locally scoured seabed. Furthermore, two large (several hundred metres high) and 101 small (tens of metres high) cold-water coral mounds were identified on the TOBI images. Most of the mounds are located on top of erosional scarps pointing to the cold-water corals' preference for areas with enhanced current intensities. Within the study area, a general trend of northward decreasing mound heights and increasing abundance of erosive features can be observed. High-resolution ROV (Remotely Operated Vehicle) video observations at four sites within the TOBI coverage illustrate that only the southernmost mound shows signs of active mound growth while the other mounds represent relict structures with limited coral cover opportunistically taking advantage of available hard substrate in the form of consolidated/lithified sediments. The distribution of erosion features, seabed features and seabed facies on the northwest Porcupine Bank indicates an environmental window in which the interplay of coral growth, sediment input and sediment preservation leads to the formation of mound sediments and therefore mound growth. Whereas too weak bottom currents do not support thriving coral thickets, too strong bottom currents hamper any deposition and, thus, mound growth. As this environmental window varies over time, mound sizes on the Porcupine Bank (and probably also in other mound settings) most likely reflect the duration in time of such an environmental window of optimal growth conditions around a given mound, rather than an indication of the overall age of the mound.

A giant (5.3 × 10 7 m 3) middle Miocene (c. 15 Ma) sediment mound (M1) above the Siri Canyon, Norwegian–Danish Basin: Origin and significance

A large-scale enigmatic mound structure (M1) has been discovered in middle Miocene strata of the Norwegian–Danish Basin, c. 10 km east and updip of the Central Graben. It is located about 1 km beneath the seabed and clearly resolved by a 3D seismic data set focused on the deeper, remobilised, sand-filled Siri Canyon. M1 comprises two culminations, up to 80 m high and up to 1400 m long, constituting a sediment volume of some 5.3 × 10 7 m 3. It is characterized by a hard reflection at the top, a soft reflection at the base, differential compaction relative to the surrounding sediments, and 10 ms TWT velocity pull up of underlying reflections, indicating a relatively fast mound fill, attributed to the presence of sand within the mound. Internal seismic reflections are arranged in an asymmetric concentric pattern, suggesting a progressive aggradation to the NW, downstream to a mid-Miocene contour current system. Numerous elongated pockmarks occur in the upper Miocene succession close to the mound and indicate that the study area was influenced by gas expulsion in the mid- and late Miocene. The reflection configuration, velocity, dimensions, regional setting, and isolated location can best be explained by interpreting the mound as a giant sand volcano extruded >1 km upward from the Siri Canyon during the middle Miocene (c. 15 Ma). The likely causes of this remarkable structure include gas charge and lateral pressure transfer from the Central Graben along the Siri Canyon reservoir. While this is the first such structure described from this part of the North Sea, similar-aged sand extrudites have recently been inferred from seismic observations in the North Viking Graben, thus suggesting that the mid-Miocene was a time of widespread and intense sediment remobilization and fluid expulsion in the North Sea.

Sediment dynamics modulated by burrowing crab activities in contrasting SW Atlantic intertidal habitats

Biogenic bottom features, animal burrows and biological activities interact with the hydrodynamics of the sediment–water interface to produce altered patterns of sediment erosion, transport and deposition which have consequences for large-scale geomorphologic features. It has been suggested that depending on the hydrodynamic status of the habitat, the biological activity on the bottom may have a variety of effects. In some cases, different bioturbation activities by the same organism can result in different consequences. The burrowing crab Neohelice granulata is the most important bioturbator at SW Atlantic saltmarshes and tidal plains. Because of the great variety of habitats that this species may inhabit, it is possible to compare its bioturbation effects between zones dominated by different hydrodynamic conditions. Internal marsh microhabitats, tidal creeks bottoms and basins, and open mudflats were selected as contrasting zones for the comparison on a large saltmarsh at Bahía Blanca Estuary (Argentina). Crab burrows act as passive traps of sediment in all zones, because their entrances remain open during inundation periods at high tide. Mounds are generated when crabs remove sediments from the burrows to the surface and become distinctive features in all the zones. Two different mechanisms of sediment transport utilizing mounds as sediment sources were registered. In the first one, parts of fresh mound sediments were transported when exposed to water flow during flooding and ebbing tide, with higher mound erosion where currents were higher as compared to internal marsh habitats and open mudflats. In the second mechanism, mounds exposed to atmospheric influence during low tide became desiccated and cracked forming ellipsoidal blocks, which were then transported by currents in zones of intense water flow in the saltmarsh edge. Sedimentary dynamics varied between zones; crabs were promoting trapping of sediments in the internal saltmarsh (380 g m −2 day −1) and open mudflats (1.2 kg m −2 day −1), but were enhancing sediment removal in the saltmarsh edge (between 10 and 500 g m −2 day −1 in summer). The implication is that biologically mediated sedimentological changes could be different among microhabitats, potentially leading to contrasting geomorphologic effects within a particular ecosystem.

Randomness or order in the occurrence and preservation of shallow-marine carbonate facies? Holocene, South Florida

This paper re-examines unsolved problems concerning the relationships between skeletal benthic communities, the skeletal carbonate sediments they produce and how these are preserved in the subsurface. Recent work based on shelf-wide datasets of modern shallow-marine carbonate sediments of South Florida suggest that the boundaries between facies occur randomly and that facies occurrence bears little relation to water depth. This is at variance with earlier work from the region that indicated facies occurrence related to different environments and which helped establish the basis for palaeoenvironmental analysis of ancient limestones. A windward-facing depositional margin of a carbonate mound in the back-reef area of the Florida Keys is used as a small-scale, case study to examine whether surface peritidal facies occur in an ordered or random fashion and whether they are depth related. Lateral facies transition analysis along transects from the shoreline to the shallow subtidal indicates that peritidal facies occur in a very well-ordered (i.e. non-random) arrangement of zones and patches. Surface facies are generally well-preserved and recognisable in the shallow subsurface and in cores through the Holocene carbonates and shoreline mangrove peats. Analysis of upward facies transitions in cores also indicates common facies trends reflecting the evolution of the sediment mound in response to rising Holocene sea level. However, even though the modern facies occur in an ordered and depth-related pattern, subsurface facies do not show a simple relation to the known sea-level curve in the area. Rather, they relate to a complex of different rates of sea-level rise, sea-floor topography, carbonate production rates, wave/storm energy input, and bioturbation.

The Intertidal Burrowing Crab Neohelice (=Chasmagnathus) granulata Positively Affects Foraging of Rodents in South Western Atlantic Salt Marshes

The role of positive and indirect interactions is often crucial in communities with intense abiotic stress such as salt marshes. The burrowing crab, Neohelice (=Chasmagnathus) granulata, is the dominant benthic macroinvertebrate of southwest Atlantic marshes (southern Brazil to Northern Argentinean Patagonia), having strong direct and indirect effects on marsh soil and, in consequence, on marsh vegetation and primary consumers. In this work, we investigate if this crab indirectly modifies habitat use by the granivorous rodents, Akodon azarae and Oligoryzomys flavescens, by increasing nutrient availability and thus enhancing seed production by the marsh plant Spartina densiflora. The study was conducted at the Mar Chiquita Coastal Lagoon, Argentina (37°32′ S). Rodent frequencies in S. densiflora were positively correlated with crab densities throughout the low and middle marsh. Additionally, the highest quality of S. densiflora and inflorescence density was recorded at the highest crab densities. Experimental manipulation of crab densities shows that N. granulata indirectly enhances the performance of S. densiflora (e.g., decreased fiber content and C/N ratios) and increases density of seeds. Moreover, N. granulata also facilitates S. densiflora seed availability to rodents by concentrating them in sediment mound at their burrows entrances. Experimental rodent exclusions showed that rodent species used S. densiflora seeds, a variable positively related to crab burrow density. Thus, our results show that N. granulata drives the granivorous rodent distribution and the intensity of seeds–rodent interaction trough facilitative and indirect interactions in marsh community.