Fine-grained Mud Research Articles

Seismic characteristics of polygonal fault systems in the Great South Basin, New Zealand

Abstract A well-developed multi-tier polygonal fault system is located in the Great South Basin offshore New Zealand’s South Island. The system has been characterised using a high-quality three-dimensional seismic survey tied to available exploration boreholes using regional two-dimensional seismic data. In this study area, two polygonal fault intervals are identified and analysed, Tier 1 and Tier 2. Tier 1 coincides with the Tucker Cove Formation (Late Eocene) with small polygonal faults. Tier 2 is restricted to the Paleocene-to-Late Eocene interval with a great number of large faults. In map view, polygonal fault cells are outlined by a series of conjugate pairs of normal faults. The polygonal faults are demonstrated to be controlled by depositional facies, specifically offshore bathyal deposits characterised by fine-grained clays, marls and muds. Fault throw analysis is used to understand the propagation history of the polygonal faults in this area. Tier 1 and Tier 2 initiate at about Late Eocene and Early Eocene, respectively, based on their maximum fault throws. A set of three-dimensional fault throw images within Tier 2 shows that maximum fault throws of the inner polygonal fault cell occurs at the same age, while the outer polygonal fault cell exhibits maximum fault throws at shallower levels of different ages. The polygonal fault systems are believed to be related to the dewatering of sedimentary formation during the diagenesis process. Interpretation of the polygonal fault in this area is useful in assessing the migration pathway and seal ability of the Eocene mudstone sequence in the Great South Basin.

Geoacoustic Inversion for a New England Mud Patch Sediment Using the Silt-Suspension Theory of Marine Mud

This article provides an application of the silt-suspension theory to a Bayesian-inference inversion for the geo-acoustic parameters in marine mud. The theory, with consequences that have been developed recently, postulates a suspension of water and clay mineral card-houses that supports moderately dilute concentrations of silt particles. The approach is an example of a physically based model inversion, in which parameters representing physical mud-layer properties are obtained by inversion and used to produce estimates of geoacoustic properties, including their frequency dependence. The acoustic data are from a combustive source signal propagated along a track, located over several meters of fine-grained mud in the New England Mud Patch, to a single hydrophone on a receiver array during the 2017 Seabed Characterization Experiment. Data extracted from a nearby piston core inform the physical modeling, with selections of inversion parameters guided by both sensitivity analyses and bounds from archival and core measurements. Results show the feasibility of this inversion approach. The estimates of mud density and sound speed are close to values obtained independently. The frequency dependence of attenuation is estimated over the full low-frequency source band and has an approximate power exponent of 1.72.

Estimates of Low-Frequency Sound Speed and Attenuation in a Surface Mud Layer Using Low-Order Modes

Whereas there have been numerous theoretical and experimental studies on the properties of marine granular sands, there are significantly fewer studies on sediments classified as muds. The validity of geoacoustic models for muddy sediments has not been successfully tested due to the lack of inverted low-frequency sound speed and attenuation values. The geoacoustic properties of a surface fine-grained mud layer, overlaying three sand transition layers, and a half-space basement within the New England Mud Patch, were studied using explosive signals from long-range along-shelf sound propagation tracks. The sound-speed profile of the mud layer in the low-frequency band (100-500 Hz) was estimated using acoustic normal mode characteristics including the mode shapes and the modal dispersion curves of low-order modes, which mainly propagated in the water column and the surface mud layer. The ambiguity of sound speed at the top of the mud layer and sound-speed gradient was approximately removed. It was found that the resultant sound-speed ratio at the water-sediment interface was close to unity and the sound-speed gradient was 1.8 1/s with a standard deviation of 1.0 1/s. The attenuation in the mud layer was inverted using the attenuation coefficient of the first mode extracted from explosive signals at three source locations. The estimated attenuation at 150 Hz had a mean of 0.006 dB/m and a standard deviation of 0.003 dB/m.

PCATS triaxial testing: Geomechanical properties of sediments from pressure cores recovered from the Bay of Bengal during expedition NGHP-02

Abstract The Indian National Gas Hydrate Program Expedition 02 (NGHP-02) was the most comprehensive gas hydrate drilling expedition carried out to date. From 74 pressure cores that were successfully recovered from various site locations offshore India, 19 subsamples were obtained for further detailed geotechnical analysis using the PCATS Triaxial apparatus to investigate the geomechanical behaviour of the cores, including small-strain stiffness, Gmax, undrained shear strength, Su, as well as direct-flow measurements of permeability, k. Samples tested included fine-grained muds with no appreciable hydrate through to coarse-grained sands that contained extensive hydrate within the pore space. Although the subsampling and transfer of pressure cores into the PCATS Triaxial under zero effective confining stress, σ′, remains challenging, 12 samples were successfully transferred and underwent detailed geotechnical characterization. During consolidation, after reapplication of in-situ σ′, all samples experienced axial and radial consolidation, with variations between samples seemingly dependent on the degree of sample disturbance, stress relaxation between coring and testing, hydrate saturation, Sh and grain size, D50. The largest values of Gmax were obtained from samples with high Sh in the pore space, with values much higher than that expected for similar sediments without hydrate under comparable σ’. Samples with high permeability had both high Sh and larger grain size, suggesting that in coarse-grained sediments sufficient pathways are available for reasonable fluid flow even with high hydrate saturations. Triaxial testing and unconfined compression tests highlighted the influence of hydrate on increasing sediment strength, although the size of the sediment grains appeared to have an important control on the magnitude of the strength increase and the overall stress-strain behaviour. The results from the detailed geotechnical characterization of the recovered pressure cores will be invaluable in helping India identify potential locations for future gas production wells, and assess the long-term performance of such reservoirs.

Submarine landform assemblages and sedimentary processes in front of Spitsbergen tidewater glaciers

New swath-bathymetric data from the inner parts of the three Svalbard fjords Ymerbukta, Trygghamna, and Magdalenefjorden reveal the landform assemblages deposited in front of tidewater glaciers in west and northwest Spitsbergen. Overridden moraines in Ymerbukta, a tributary of Isfjorden in central west Spitsbergen, record several re-advances of the Esmarkbreen glacier at the head of the fjord after deglaciation, and glacial lineations formed in seafloor sediments are indicative of fast ice advance during one of these events. A terminal moraine and associated debris lobe mark the maximum ice extent during the Holocene, which, implied by the presence of crevasse-squeeze ridges, is likely related to a surge of Esmarkbreen. Several De Geer moraines provide evidence for subsequent slow and step-wise retreat. In the adjacent Trygghamna and in Magdalenefjorden in northwest Spitsbergen the landforms are similar but the absence of overridden moraines and glacial lineations shows that the glaciers probably only re-advanced once during the Holocene and that ice flow was relatively slow. Terminal moraines and associated debris lobes mark the maximum extent of these advances and formed during the Little Ice Age (LIA). In Magdalenefjorden the relatively small size of the debris lobe suggests that the ice margin was at its maximum position for only a short period of time, or that sediment availability was restricted during the LIA advance. Similar to Esmarkbreen the retreat phase of the glaciers in Trygghamna and Magdalenefjorden was also characterised by periods of still-stand or small re-advances, although the comparatively small number of De Geer moraines in all three fjords shows that these landforms probably formed much less frequently than previously thought. Sub-bottom profiler data, four sediment cores and six radiocarbon dates from Magdalenefjorden further provide information about the Holocene sedimentary environments in a northwest Spitsbergen fjord. The main source of sediment is glacial meltwater entering the fjord from the surrounding coastline, which has led to the accumulation of thick sequences of fine-grained mud. Stratified and laminated muds record glacier-proximal conditions, probably related to a LIA re-advance of Waggonwaybreen around 300calaBP, where the interplay of a range of glacimarine processes led to the formation of partially rhythmic couplets of one coarser and one finer layer, accumulated at a rate of around 3cma−1. Multiple sandy layers intercalated with the glacimarine mud provide evidence for the occurrence of gravitational mass-flow events like turbidity currents. In ice-distal settings, massive to weakly stratified, occasionally bioturbated mud accumulated at a lower rate of 0.04–0.49cma−1. Occasional clasts and diamictic layers show that the depositional environment in Magdalenefjorden is also influenced by sedimentation from icebergs and sea ice, but the ubiquitous glacimarine mud underscores the predominance of meltwater-related sedimentation in the fjord.

Abundant iron and sulfur oxidizers in the stratified sediment of a eutrophic freshwater reservoir with annual cyanobacterial blooms

The microbial community in eutrophic freshwater sediment was investigated from a 67-cm-deep sediment core collected from the Daechung Reservoir in South Korea, where cyanobacterial blooms have occurred annually for the past 30 years. The majority of core sediments were characterized by dark-grayish, fine-grained mud with abundant gas-escaped and thinly laminated layers. Intervals of summer and winter seasons were represented by periodic peaks of geochemical profiles of parameters such as grain size and relative carbon mass ratios to various nutrients such as nitrogen, sulfur, and phosphorus. In bacteria, Proteobacteria (66.6%) was the most prevalent phylum, followed by Chloroflexi (8.9%), Bacteroidetes (5.1%), and Spirochaetes (2.6%). Archaea were also abundant, representing approximately half of the total prokaryotes in the sediments. Notably, three Bacteria (Sulfuricurvum, Sideroxydans, and Gallionella) and one Archaea (Thermoplasmata) accounted for 43.4% and 38.4% of the total bacteria and archaea, respectively, implying that iron and sulfur oxidizing microorganisms dominate in this eutrophic freshwater sediment. These results indicate that 1) eutrophic freshwater lakes in monsoon climates undergo a stratified sedimentary process with seasonal and annual variations in geochemical and microbial profiles, and 2) the microbial oxidative metabolism of iron and sulfur is notably active in sediments from a eutrophic lake.

Use of Mussel Casts from Archaeological Sites as Paleoecological Indicators: An Example from CA-MRN-254, Marin County, Alta California

Archaeological investigations at prehistoric site CA-MRN-254 at the Dominican University of California in Marin County, California, revealed evidence of Native American occupation spanning the past 1,800 years. A dominant source of food for the inhabitants in the San Francisco Bay area was the intertidal, quiet-water dwelling blue mussel (Mytilus trossulus), although rare occurrences of the open coast-dwelling California mussel (Mytilus californianus) suggest that this species was also utilized sporadically. On rare occasions, cultural horizons at this site contain abundant sediment-filled casts of the smaller mussel Modiolus sp. These casts were formed soon after death when the shells filled with sediment and were roasted along with living bivalve shellfish for consumption. Thin sections of these mussel casts display sedimentological and microbiological constituents that shed light on the paleoenvironmental conditions when they were alive. Fine-grained sediment and pelletal muds comprising these casts suggest that the mussels were collected in a low energy, inner bay environment. The rare presence of the diatoms Triceratium dubium and Thalassionema nitzschioides indicate more normal marine (35 psu) and possibly warmer conditions than presently exist in San Francisco Bay. Radiocarbon dating of charcoal associated with the mussel casts containing these diatoms correlates with a 600-year period of warming from ca. A.D. 700–1300, known as the Medieval Climatic Anomaly. Results of this mussel cast study demonstrate that they have great potential for providing paleoenvironmental information at this and other archaeological sites.

Inundation duration and vertical vegetation zonation: a preliminary description of the vegetation and structuring factors in borokotóh (hummock igapó), an overlooked, high-diversity, Amazonian vegetation association

Here we present data on a study of the species composition and structure of borokotoh, a habitat associated with central Amazonian seasonally black-water flooded forests. Borokotoh appears never to have been specifically studied before, being mentioned only once or twice in the literature (and then not under this name). This is the first report on the phytosociology of this vegetation association. Borokotoh is, on average, only some 20 m wide, but very species rich (107 species in 60 genera from 34 families in one 0.25-ha plot). It consists of a series of rounded hummocks of fine-grained mud, each 6–12 m in length and 2–3 m in height, separated by narrow sloughs. Hummocks are seasonally inundated almost to their flat tops, which remained dry in the years of the survey. Species are strongly stratified within the borokotoh, with over 78% of identified species being recorded in just one of the three hummock sub-divisions: top, side or in between-hummock slough. This specificity, almost certainly linked to inundation duration and waterlogging tolerance, may explain the high species diversity recorded. The study is significant in demonstrating that undescribed habitats still exist in Amazonia, and that biodiversity there may be structured in ways that are highly localized and habitat-specific. There is clearly a need for more detailed work on borokotoh and similar vegetation associations where high biodiversity may be associated with very specific local features.

Magnetic mineral diagenesis in the river‐dominated inner shelf of the East China Sea, China

AbstractThe inner shelf of the East China Sea is a river‐dominated margin characterized by fine‐grained mud deposits and a rapid sedimentation rate. Three short sediment cores (~2.7 m in length) were examined to characterize spatial variations in magnetic mineral diagenesis. The sediment cores were analyzed for sedimentation rates, magnetic properties, particle size distribution, organic carbon, and total sulfur content. The two more proximal cores with higher sedimentation rates (~2.2 cm/yr and ~0.96 cm/yr) do not exhibit obvious effects of reductive dissolution of magnetite with increasing depth, which is consistent with their lower total sulfur content. The offshore core, A12‐4, which has a lower sedimentation rate, contains clear evidence of magnetite dissolution and increasing total sulfur content with depth. The three cores have a similar sediment source and organic matter input; therefore, we suggest that a higher sedimentation rate will lead to less reductive diagenesis of magnetite, assuming that other factors are constant. The iron‐ to sulfate‐reduction boundary, i.e., revealed by the onset of a rapid decline of magnetic susceptibility, is located 1.0 m below seafloor in core A12‐4. This is much deeper than is reported in many other coastal marine environments and can be explained by the higher sedimentation rate, the presence of refractory terrestrial organic matter, and an abundant input of detrital iron oxides. This study demonstrates that analyses of the magnetic mineral zonation provide a straightforward approach to assess diagenetic organic carbon decomposition pathways in marine environments.

Frequency power-law attenuation and dispersion in marine sediments

Acoustic waves in marine sediments, including the fine-grained muds and clays, often exhibit an attenuation in the form of a frequency power law in which the exponent is close to unity. The frequency dependence of the wave speed, or dispersion, associated with such a power law has long been a subject of debate. Causality arguments, often in the form of the Kramers-Kronig dispersion relations, are usually applied to the problem, but these are not sufficient to characterize the dispersion fully. Following an alternative approach, a wave equation will be introduced which predicts the dispersion when the attenuation is of the frequency power law type. Based on this wave equation, it will be shown that only certain values of the frequency exponent are allowed, otherwise causality is violated, indicating that the associated attenuation and dispersion are not physically realizable. In effect, for these prohibited values of the frequency exponent, the Fourier components in the dispersion and attenuation cannot combine to give a zero response for negative times. Emerging from the theory is an expression for the complex wavenumber that is complete and exact, and from which all the properties of the dispersion and attenuation may be derived.

Nd and Sr isotopic evidence for provenance of clastic material of the Upper Triassic rocks of Silesia, Poland

Nd and Sr isotope data were used to characterize the sources of the Upper Triassic (Keuper) siliciclastic rocks of Silesia in southern Poland. This continental succession, consisting predominantly of fine-grained mud- stones and siltstones, yields a remarkably uniform Nd isotopic composition. Nd model ages T 2DM vary from 1.56 to 1.69 Ga and e Nd values are in the range from –8.9 to –11.2, documenting old crust contribution in the provenance. In contrast, the Sr isotopic composition ( 87 Sr/ 86 Sr) of the clastics exhibits a relatively large variation from 0.710 to 0.723. The isotopic compositions indicate that the southern part of the Germanic Basin in Silesia was supplied with clastic material from the Bohemian Massif. The axis of the drainage area must have crossed from SW to NE the Saxothuringian units of the East Sudetes and most probably also the area of the Tepla–Barrandian Unit. There is no indication of any sediment transport from the Moravo-Silesian Belt and the Fore-Sudetic Block. It seems, that the Palaeozoic rocks of the latter domain must have been buried completely during Late Triassic times.

Compositional trends through the Holocene mud succession of the southwestern Black Sea shelf: Implications for sedimentary provenance and water-level history

Cores MAR02-45 and MAR05-03 were raised from 68–69m water depth on the SW Black Sea shelf and indicate transgression and submergence beneath several tens of metres of water before 11.0calka. Those who postulate a Black Sea lowstand below −100m until the approximately 9.1calka reconnection with the world ocean have suggested that sites like MAR02-45 and MAR05-03 must have been located in perched lakes on what is now the modern shelf. However, the silt- and clay-fraction mineralogy of samples from MAR02-45 provides no evidence for a shift in provenance at 9.1calka, with earlier input from local rivers and later input from a broader region. Sedimentary abundances of Sc, Fe, Co, Ce, La, Th and Y also show no significant downcore trends. These elements likely reside in aluminosilicate mineral grains shed from terrestrial sources, so negligible downcore variation suggests long-term continuity in the composition of the detrital supply. More critically, the volume of pre-9.1calka sediment around the MAR02-45 and MAR05-03 sites is >25 times the expected yield from local rivers over a 5000year period, so other more substantial sources are required. The predominant silt size of the recovered sediments, the configuration of late Holocene currents in the western Black Sea, and analogies with dispersal systems elsewhere suggest that the bulk of the fine-grained muds at these sites likely came from the Danube and Kamchiya drainage basins where thick deposits of unconsolidated Pleistocene loess have been strongly dissected. To reach core sites on the SW Black Sea shelf, this material must have been advected from the Danube and Kamchiya deltas by unobstructed marine currents. Only thin event beds (tempestites) of fine sand and silt in the lower part of cores MAR02-45 and MAR05-03 are interpreted to have a local source in the Strandja Mountains of Thrace. Comparison of the palynology of surface samples from the Danube Delta and its associated coastal lagoons with the pre-9.1calka sediments of core MAR02-45 confirms that the hypothesis of deposition in a perched ancient lake is untenable.

Distributed normal faults in the Niobrara Chalk and Pierre Shale of the central Great Plains of the United States

Normal faults occur in the Niobrara Chalk and Pierre Shale in numerous localities in South Dakota, Nebraska, and Kansas. Lakeshore outcrops provide a window into strata that otherwise outcrop poorly. Various criteria were used to distinguish mass-wasting features from faults that formed at the subsurface. Locally, these faults have variable orientations and throws that vary from centimeters to tens of meters. In the Niobrara Chalk, the fault zones typically display well-developed striae, occasional slickensides, limited to absent damage zones, and dilational jogs with coarse, microvein calcite fill. In places, the faults are associated with gentle monoclines and folds, but elsewhere they occur in horizontal strata. A weathering bias suggests these faults are more common than observed. At any one locality, these faults have been or could be attributed to local tectonism, differential compaction, and/or glacial rebound. If due to tectonism, taken in aggregate, they indicate fairly widespread (albeit low-strain) activity, which is consistent with the idea of a critically stressed continental interior with migrating sites of faulting. An alternate and preferred hypothesis is that these are polygonal faults due to diagenetically driven deformation in fine-grained mud rocks. Subsurface polygonal fault systems have been described from the Niobrara Chalk and Pierre Shale in the Denver-Julesburg Basin area in eastern Colorado. Smectitic clays, associated with polygonal faulting elsewhere, are abundant in these units. The fault kinematics (extension in multiple directions), widespread character, and other traits are consistent with an origin by diagenetically driven deformation.

A study of the contaminated banks of the Mahoning River, Northeastern Ohio, USA: characterization of the contaminated bank sediments and river water–groundwater interactions

The Mahoning River is one of the five most contaminated rivers in the U.S. This study characterized the contaminated sediments in the river banks and investigated the hydraulic interconnection between shallow aquifer in the banks with the river water. The study was conducted along the most polluted section of the river, which is 50-km long, using over 50 monitoring wells. The characterization part of the study investigated the sedimentology, hydraulic conductivity, and spatial distribution of the contaminated sediments. Results of the characterization revealed that the contaminated sediments consist of fine-grained sand, silt, mud, and clay. The spatial distribution of the contaminated sediment is heterogeneous and positively correlates with the hydraulic conductivity values, i.e., the greatest contamination occurs in high conductivity areas. Hydraulic conductivity was determined by the Hazen formula using 82 sediment samples. Bioremediation, which is one of the remedial options considered for the banks, is found to be hydraulically feasible because of sufficient hydraulic conductivity values (≥10−4 cm/s) that ensure reasonable rates of nutrient delivery. Monitoring of water levels in the river and groundwater for a 10-month period shows that flow occurs from the river to groundwater and vice versa. The exchange of flow is influenced by rainfall. Flow of groundwater to the river will continually transport the dissolved contaminants in groundwater to the river. Therefore, findings of this study show that one of the remedial options that proposes dredging of channel sediments and permits no action for bank sediments cannot be chosen due to river water–groundwater interactions.

Surficial geology and geophysical investigations of the Capers Inlet, South Carolina (USA) 7.5-Minute Quadrangle

Surficial geology and geophysical investigations of the Capers Inlet, South Carolina (USA) 7.5-Minute Quadrangle is a compilation of geological and geophysical data collected in the Caper's Inlet Quadrangle, situated along the central coastline of South Carolina, USA. The 1:24,000 scale map area encompasses marine environments in the shallow nearshore and offshore (constituting approximately 80% of the study area) and terrestrial barrier island and Pleistocene upland environments (20% of the study area). The geologic map was compiled by overlaying existing topographic maps with data from high-resolution sidescan sonar, subbottom profiler, and GPR surveys supplemented with surficial sediment grabs and vibracore samples. These data were collected in 2009 and early 2010 for an MSc thesis project conducted through the Department of Geology and Environmental Geosciences at the College of Charleston, South Carolina. The resulting maps illustrate the spatial variability of surficial seafloor geology in the area, with sediment distribution ranging from a coarse shell hash to fine-grained mud. The shallow stratigraphy is characterized by a thin layer of modern sediment overlying the boundary between the Quaternary and Tertiary as well as the presence of several paleochannels in the backbarrier and shallow offshore. By portraying both seafloor geology and shallow stratigraphy, maps and accompanying information provide scientists and coastal managers with a better understanding of the relationship that exists between stratigraphy, sediment distribution, and geomorphology in the area.

The effects of depositional processes on benthic foraminiferal taphonomy in the Southeastern Yellow Sea of Korea

To understand the role of depositional processes on the distribution of benthic foraminifera, grain size analysis, paleontological statistics and textural analysis were conducted on three piston cores (Core Nos. 3, 7, 9) from the southeastern Yellow Sea of Korea. Two foraminiferal assemblages were defined by cluster analysis. The Cibicides lobatulus-Ammonia ketienziensis assemblage has low P.F/B.F, species diversity and evenness and high number of benthic foram. individuals/20 ml, and is interpreted as representing relict benthic foraminiferal assemblage comprising the upper layer of Core No. 7 and No. 9. The deposit of well sorted alternation layers of muddy sand and sandy mud in Core No. 9. is interpreted as being influenced by hydraulic sorting processes of higher wave and current especially during storm periods. The preservation of 13 Pseudorotalia gaimardii extracted from Core No. 7 confirmed that bed load transport was responsible for its heavily abraded and broken thick-walled tests. The Pseudoparrellan araensis assemblage is a recent benthic foraminiferal assemblage found in the modern homogeneous fine-grained mud facies corresponding to the uppermost part of HMB. It seemed that the apparent sorting phenomena and bed load transport of the benthic foraminifera are the result of turbidity currents which promulgated by rapid tidal currents formed during the last postglacial period in the southeastern Yellow Sea of Korea.

Spatial relationships between surface sedimentary facies distribution and topography using remotely sensed data: Example from the Ganghwa tidal flat, Korea

Spatial relationships between surface sedimentary facies distribution and topography, including channel networks and an intertidal digital elevation model (DEM), were estimated for the Ganghwa tidal flat, Korea. Tidal channels were extracted from high spatial resolution satellite data and used to derive maps of channel density and distance from channels. The intertidal DEM was generated by a waterline method using Landsat Enhanced Thematic Mapper Plus (ETM+) images, and the map of surface sedimentary facies distribution was produced from grain-size data on sediment samples. A GIS-based frequency ratio model was applied to quantify the relationships between the surface sediment facies and topographic factors. The result indicated that the fine-grained mud flat facies occurs mainly in areas of complex tidal channels and in elevated areas that have a high spectral reflectance in the Landsat ETM+ image. The coarse-grained sand flat facies is concentrated in areas of sparse tidal channels, and the mixed flat facies is distributed where tidal channels have an intermediate level of complexity. The results demonstrate a significant spatial relationship between the distribution of surface sediment in a tidal flat and topographic features. We conclude that tidal channels should be considered an important factor in mapping surface sedimentary facies in tidal flat environments, and that patterns of tidal topography are a useful adjunct to spectral reflectance in classifying sedimentary facies from remotely sensed data with high spatial resolution.

Enzyme activities in the water column and in shallow permeable sediments from the northeastern Gulf of Mexico

The activities of extracellular enzymes that initiate the microbial remineralization of high molecular weight organic matter were investigated in the water column and sandy surface sediments at two sites in the northeastern Gulf of Mexico. Six fluorescently labeled polysaccharides were hydrolyzed rapidly in the water column as well as in permeable sediments. This result contrasts with previous studies carried out in environments dominated by fine-grained muds, in which the spectrum of enzymes active in the water column is quite limited compared to that of the underlying sediments. Extracts of Spirulina, Isochrysis, and Thalassiosira were also used to measure hydrolysis rates in water from one of the sites. Rates of hydrolysis of the three plankton extracts were comparable to those of the purified polysaccharides. The broad spectrum and rapid rates of hydrolysis observed in the water column at both sites in the northeastern Gulf of Mexico may be due to the permeable nature of the sediments. Fluid flux through the sediments is sufficiently high that the entire 1.5 m deep water column could filter though the sediments on timescales of a few days to two weeks. Movement of water through sediments may also transport dissolved enzymes from the sediment into the water column, enhancing the spectrum as well as the rate of water column enzymatic activities. Such interaction between the sediments and water column would permit water column microbial communities to access high molecular weight substrates that might otherwise remain unavailable as substrates.

Geologic framework of the 2005 Keathley Canyon gas hydrate research well, northern Gulf of Mexico

The Keathley Canyon sites drilled in 2005 by the Chevron Joint Industry Project are located along the southeastern edge of an intraslope minibasin (Casey basin) in the northern Gulf of Mexico at 1335 m water depth. Around the drill sites, a grid of 2D high-resolution multichannel seismic data designed to image depths down to at least 1000 m sub-bottom reveals 7 unconformities and disconformities that, with the seafloor, bound 7 identifiable seismic stratigraphic units. A major disconformity in the middle of the units stands out for its angular baselapping geometry. From these data, three episodes of sedimentary deposition and deformation are inferred. The oldest episode consists of fine-grained muds deposited during a period of relative stability in the basin (units e, f, and g). Both the BSR and inferred gas hydrate occur within these older units. The gas hydrate occurs in near-vertical fractures. A second episode (units c and d) involved large vertical displacements associated with infilling and ponding of sediment. This second interval corresponds to deposition of intercalated fine and coarse-grained material that was recovered in the drill hole that penetrated the thin edges of the regionally much thicker units. The final episode of deposition (units a and b) occurred during more subdued vertical motions. Hemipelagic drape (unit a) characterizes the modern seafloor. The present-day Casey basin is mostly filled. Its sill is part of a subsiding graben structure that is only 10–20 m shallower than the deepest point in the basin, indicating that gravity-driven transport would mostly bypass the basin. Contemporary faulting along the basin margins has selectively reactivated an older group of faults. The intercalated sand and mud deposits of units c and d are tentatively correlated with Late Pleistocene deposition derived from the western shelf-edge delta/depocenter of the Mississippi River, which was probably most active from 320 ka to 70 ka [Winker, C.D., Booth, J., 2000. Sedimentary dynamics of the salt-dominated continental slope, Gulf of Mexico: integration of observations from the seafloor, near-surface, and deep subsurface. In: Proceedings of the GCSSEPM Foundation 20th Annual Research Conference, Deep-water Reservoirs of the World, pp. 1059–1086]. The presence of sand within the gas hydrate stability zone (in units c and d) is not sufficient to concentrate gas hydrate even though dispersed gas hydrate occurs deeper in the fractured mud/clay-rich sections of units e and f.

Late Quaternary variability of the sedimentary record in the Sao Tome deep-sea system (south Brazilian basin)

Very few sediment records allow addressing the paleoceanographic changes that took place on orbital and/or sub-Milankovitch time scales in the South–West Atlantic. Here, a set of sediment cores located across a deep channel-levee system in the South Brazilian basin (3700–3900 m depth) is used to infer the sediment dynamics that prevailed on this system during the late Quaternary and to relate this pattern to climate changes. The spatial distribution of deposits (changes in mineralogy, texture, structure, and sedimentation rate according to the morphology) indicates bulk of sediment supplies is distributed on the Sao Tomé system by bottom advective processes. The silty–sandy layers, originate undoubtedly from turbidity currents whereas the fine-grained muds and oozes, that form the major part of the deposits, are controlled by deep geostrophic currents. In addition to variations forced by the orbital cycles, (more turbidites, less carbonates and higher sedimentation rate during glacials) the sediment records (chiefly the CaCO 3 record) document outstanding higher frequency regional changes in sedimentation. According to the location of the cores at the present day interface between NADW and AABW, we infer that changes in sedimentation are related to pulses of AABW versus NADW production. These changes, commonly marked by asymmetric carbonate pattern of about 10 kyr, suggest that the relative influence of NADW versus AABW followed, throughout the last 190 kyr, a repetitive sequence of progressive increase and sharp decrease. The rather sharp shut down observed during the late glacial could be associated to Heinrich events, when NADW production stopped, but similar events are also well expressed during MIS 5, MIS 6 and at the end of MIS 7.