Piston Cores Research Articles

Slope failure and mass transport processes along the Queen Charlotte Fault Zone, western British Columbia

Abstract Multibeam echosounder (MBES) images, 3.5 kHz seismic-reflection profiles and piston cores obtained along the southern Queen Charlotte Fault Zone are used to map and date mass-wasting events at this transform margin – a seismically active boundary that separates the Pacific Plate from the North American Plate. Whereas the upper continental slope adjacent to and east (upslope) of the fault zone offshore of the Haida Gwaii is heavily gullied, few large-sized submarine landslides in this area are observed in the MBES images. However, smaller submarine seafloor slides exist locally in areas where fluid flow appears to be occurring and large seafloor slides have recently been detected at the base of the steep continental slope just above its contact with the abyssal plain on the Queen Charlotte Terrace. In addition, along the subtle slope re-entrant area offshore of the Dixon Entrance shelf bathymetric data suggest that extensive mass wasting has occurred in the vicinity of an active mud volcano venting gas. We surmise that the relative lack of submarine slides along the upper slope in close proximity to the Queen Charlotte Fault Zone may be the result of seismic strengthening (compaction and cohesion) of a sediment-starved shelf and slope through multiple seismic events.

Relative paleointensity (RPI) in the latest Pleistocene (10–45 ka) and implications for deglacial atmospheric radiocarbon

We report magnetic properties and relative paleointensity (RPI) proxies from a suite of 10 conventional piston cores and Kasten cores from the SW Iberian Margin collected during cruise JC089 of the RSS James Cook in August 2013. Mean sedimentation rates are in the 10–20 cm/kyr range. Age models were acquired by correlation of Ca/Ti and Zr/Sr XRF core-scanning data to L* reflectance from the Cariaco Basin that is, in turn, tied to the Greenland ice-core chronology. The natural remanent magnetization (NRM) is represented by a single magnetization component carried by a low-coercivity mineral (magnetite), although reflectance and bulk magnetic properties indicate the presence of a high-coercivity (hematitic) magnetic phase, possibly from eolian dust. The presence of fine-grained hematite means that the sediments are not ideal for RPI studies, however the detrital hematite does not appear to contribute to the NRM or anhysteretic remanent magnetization (ARM). In order to test the usefulness of the RPI data, we construct a stack of 12 RPI records from the SW Iberian Margin for the 0–45 ka interval and compare it with a stack of 12 globally distributed marine and lake records, chosen on the basis of mean sedimentation rates (>15 cm/kyr) and superior age models. The two stacks are similar, but different from published RPI stacks, particularly for the 10–30 ka interval, and imply a virtual axial dipole moment (VADM) high at ∼15–18 ka followed by a drop in field strength from ∼15 to 13 ka. A revised VADM estimate calculated from Greenland 10Be ice-core flux using a contemporary age model is remarkably consistent with the new overall RPI stack, based on Iberian Margin and global RPI records. The elevated atmospheric 14C levels of the last ice age cannot, however, be fully explained by this RPI stack although relative changes such as the long-term drop in atmospheric 14C from 30 to 15 ka are reproduced, supporting the hypothesis of a combined influence of production rate and ocean ventilation on 14C during the last ice age.

Dredging and canal gate technologies in Portus, the ancient harbour of Rome, reconstructed from event stratigraphy and multi-proxy sediment analysis

Ancient harbours are rich archives of human-environment interaction. However, dating harbour deposits and correlating their stratigraphy is a major challenge because of typically high sedimentation rates over short periods and possible curative dredging events. Portus, the maritime harbour of Rome at the height of the Roman Empire, was a port complex composed of basins and canals connecting the commercial harbour to Rome via the Tiber River. Sediment core CPS1 in the narrowest of these canals, Canale Traverso, is located centrally in what was the capital city's commercial hub and contains a continuous harbour depositional record with average sedimentation rates greater than 1 cm per year. Here we use piston coring, high-resolution core scanning and a multi-proxy sediment analysis including for the first time paleo- and rock-magnetism, and bulk and clay mineralogy in order to overcome the problems of dating harbour deposits and correlating their stratigraphy. The method allowed precise identification of major reworked events, including a dredged deposit and a hyperpycnal deposit, which improve the chronostratigraphy and water depth reconstruction, and sheds light on harbour technologies at the height of the Roman Empire. A debris layer with abundant ceramic fragments and rocks marks the decommissioning of Canale Traverso and provides a new chronostratigraphic marker at Portus. Multi-proxy riverine input signatures point to the possible use of canal gate technology for water flow management.

Methane source linked to gas hydrate system at hydrate drilling areas of the South China Sea: Porewater geochemistry and numerical model constraints

Methane of various origins is widespread in the sediments of continental margins, representing a key component of the carbon cycle in the ocean. Its occurrence in shallow marine sediments can be linked to the gas hydrate reservoir in the deep sedimentary column. However, this type of methane is difficult to track due to complex reactions occurred during its long distance migration. Porewater geochemical analyses of two ∼7 m piston cores (D17-15 and W01-16) combined with a reaction-transport model were applied to quantify methane-related biogeochemical processes and to explore the linkage of shallow methane to gas hydrate reservoir in the hydrate drilling areas of Shenhu and Dongsha in the South China Sea (SCS). The model results revealed that anaerobic oxidation of methane (AOM) is the predominant biogeochemical process in both cores, consuming approximately 85% and 74% of sulfate in the modeled domain, respectively. The crossplot of dissolve inorganic carbon (DIC) accumulation versus sulfate depletion of the system suggests that methane consumed during the AOM is mainly originated from external methane sources rather than local methanogenesis. Using a δ13CDIC mass balance calculation together with model-derived fractions of different DIC sources, we estimated that the δ13C value of the external methane in core D17-15 is −66.6‰, which is in accordance with the previous reported δ13C values of hydrate-bound methane in the area. These results suggest that methane consumed in shallow sediments in the hydrate drilling areas of the SCS is mainly originated from local gas hydrate reservoir. The proposed approach has the potential to be used to differentiate the sources of methane, which will provide constraints on dynamics of methane in gas hydrate-bearing marine sediments.

A multi-factor approach for process-based seabed characterization: example from the northeastern continental margin of the Korean peninsula (East Sea)

This study investigates sediment transport and depositional processes from a newly collected dataset comprising sub-bottom chirp profiles, multibeam bathymetry, and sediment cores from the northeastern continental margin of Korea in the East Sea (Japan Sea). Twelve echo-types and eleven sedimentary facies have been defined and interpreted as deposits formed by shallow-marine, hemipelagic sedimentation, bottom current, and mass-movement processes. Hemipelagic sedimentation, which is acoustically characterized by undisturbed layered sediments, appears to have been the primary sedimentary process throughout the study area. The inner and outer continental shelf (<150 m water depth) have been influenced by shallow-marine sedimentary processes. Two slope-parallel canyons, 0.2–2 km wide and up to 30 km long, appear to have acted as possible conduits for turbidity currents from the shallower shelf into the deep basins. Bottom current deposits, expressed as erosional moats immediately below topographic highs, are prevalent on the southern lower slope at water depths of 400–450 m. Mass-movements (i.e., slides/slumps, debris flow deposits) consisting of chaotic facies characterize the lower slope and represent one of the most important sedimentary processes in the study area. Piston cores confirm the presence of mass-transport deposits (MTDs) that are characterized by mud clasts of variable size, shape, and color. Multibeam bathymetry shows that large-scale MTDs are chiefly initiated on the lower slope (400–600 m) with gradients up to 3° and where they produce scarps on the order of 100 m in height. Sandy MTDs also occur on the upper continental slope adjacent to the seaward edge of the shelf terrace. Earthquakes associated with tectonic activity and the development of fluid overpressure is considered as the main conditioning factor for destabilizing the slope sediments. Overall, the sedimentary processes show typical characteristics of a fine-grained clastic slope apron and change down-slope and differ within each physiographic province. Furthermore, the influence of geological inheritance (i.e., structural folds and faults) on geomorphology and sediment facies development is an important additional factor on the lower slopes. Together, these factors provide a rational basis for continental margin seabed characterization.

Recent morphology and sedimentary processes along the western slope of Great Bahama Bank (Bahamas)

AbstractCarbonate slopes and associated resedimented deposits have recently gained renewed interest because they represent volumetrically significant parts of carbonate platforms. Carbonate slopes are highly variable compositionally, architecturally and spatially due to a spectrum of sediment sources, resedimentation processes and controlling factors. Here, new high resolution acoustic data (including EM302 multi‐beam echo‐sounder and very high resolution seismic) and piston cores document highly diverse and complex morphological features along the north‐western slope of Great Bahama Bank. The recent morphology of the slope is the result of the interplay between depositional and erosive processes that vary through time and along strike. The different sedimentary processes are recorded as a Pleistocene lowstand surface, characterized by many erosional features and a Holocene sedimentary wedge along the upper to middle slope that partially covers the underlying Pleistocene surface. Sedimentary processes during the Holocene are dominated by density cascading flows, which export muddy aragonitic sediment from the platform top towards the slope. Sedimentation rates, however, vary along strike due to platform top morphology combined with the variable strength of the basinal current. Reefs and islands in the Bimini area block off‐bank sediment export, and shoals and tidal deltas from Cat Cay to the south reduce the density cascading processes. Numerous small and large slope failure scars show the instability of the steep slopes of Great Bahama Bank. Bottom currents dominate the lower slope and the basin. Striations and moats are the morphological expressions of current directions, while areas of non‐deposition document strong current and concomitant removal of off‐bank transported sediment along parts of the slope, while the Santaren Drift and the drift on the north‐western edge of Great Bahama Bank act as the depositional locus for the fine‐grained sediments transported in the current.

Evidence for surface sediment remobilization by earthquakes in the Nankai forearc region from sedimentary records

Abstract Submarine landslides triggered by earthquakes can generate turbidity currents. Recently, several studies have reported that the remobilization of surface sediment triggered by earthquakes can also generate turbidity currents. Such sedimentary processes may be influenced by sediment characteristics, seafloor morphology and seismic motions. Here, we verify surface sediment remobilization using sedimentary records from the Nankai forearc region, SW Japan. We collected multi-core and piston core samples from a small confined basin, mainly composed of silty clay or very fine sand. Radiocaesium measurements of the multi-core show consistently high values in the upper 17 cm and low values below this depth. Rapid sediment deposition after 1950 is assumed, and the most likely cause is the 2004 off the Kii Peninsula earthquake. Based on calculations using bathymetric maps and palaeocurrent data, settlement of the upper 17 cm can be explained by redeposition of the surface ( c. 1 cm) slope sediment around the basin. Muddy turbidites are also identified in the piston core. The gap in radiocarbon age observed around 2.0 m bsf (metres below seafloor) implies similar sedimentary processes. Our study represents the first examination of surficial remobilization from sedimentary cores in the Nankai forearc region.

An eddy, a wake and a plume: controls on bathyal foraminifera around Tobago, western tropical Atlantic Ocean

Oceanic islands in the paths of currents induce the development of wakes and stationary eddies. The situation to the lee of Tobago, western tropical Atlantic Ocean, is further complicated by the occurrence of the seasonally variable, hypopycnal Orinoco plume. Here we investigate the impact of the combined plume, wake and eddy on bathyal benthic foraminifera to the NW of Tobago. Three surface sediment samples were recovered from around each of five well-sites to the NW of Tobago, three of the sites (Warap-A, Cassra-A and Cassra-CC) being at upper bathyal depths and two (Bene-1, Sancoche-1) at middle bathyal depths. Warap-A, Cassra-A, Cassra-CC and Bene-1 form a transect along the northern side of the leeward wake, while the other two sites are in the vicinity of the stationary eddy. The samples obtained around Sancoche-1 were taken north of the wake. These were supplemented by samples from four 80-cm piston cores from upper bathyal and outer neritic depths sampled at ~ 10 cm intervals. Benthic foraminifera reveal different biofacies at upper (Warap-A, Cassra-A, Cassra-CC) and middle (Bene-1, Sancoche-1) bathyal depths. The upper bathyal biofacies is dominated by Cassidulina curvata and the middle bathyal biofacies contains abundant Uvigerina hispidocostata, both of which are indicative of a high nutrient flux. The presence of Martinottiella communis and M. pallida at Warap-A indicate that pore waters are low in dissolved oxygen in the immediate lee of the island. Percentages of the fauna as serial tests indicated decreasing current velocities with increasing depth, as confirmed by the high abundance of Cibicides ex gr. aknerianus in the shallowest water core. Upper bathyal bottom-current strength was at its lowest in the immediate lee of the island. Species indicative of a perennial nutrient flux were more abundant to the NW, where the interaction of the plume and eddy appears to concentrate nutrients. The short cores, each from a different biofacies, indicate that these environmental conditions have been in place for at least the later Holocene. The most northerly, upper bathyal core presented a stable community structure with low assemblage turnover, while two cores taken farther south (upper bathyal and outer neritic) had an expansive structure with high assemblage turnover. These data raise the possibility of using benthic foraminifera to track the positions of the plume, core and eddy throughout the later Neogene.

Late Quaternary palaeoenvironmental reconstruction of sediment drift accumulation in the Malta Graben (central Mediterranean Sea)

The Malta Graben is a deep tectonic depression in the Sicily Channel, bounded by NW–SE normal faults and filled by thick Pliocene–Quaternary deposits. A previous analysis of a giant piston core (LC09) from the Malta Graben had revealed a wide range of sedimentary features (carbonate turbidites, bioturbated mud and scours), although the chronostratigraphic constraint of the stacking pattern has remained elusive. After establishing a reliable chronological framework based on seven radiocarbon dates for a shorter core from the Malta Graben (ANSIC03-735), a down-core analysis of planktonic foraminifer and coccolith abundance, stable isotopes and sediment grain size was carried out. Since the last glacial maximum, palaeoenvironmental conditions (surface fertility and deep chlorophyll maximum during the last glacial and the Younger Dryas; warm and oligotrophic water masses, with a deep nutricline and intense winter mixing during the Holocene) as well as selected calcareous plankton taxa trends and peaks seem to be similar to those reported for other central and western Mediterranean sites, possibly in spite of a unique response of these areas to late Quaternary climatic fluctuations. Four distinct layers, each tens of centimetres thick, are barren of foraminifers but not of coccoliths. Morphobathymetric data as well as new high-resolution and high-penetration seismic profiles show that prolonged contouritic activity has persisted on the western side of the Malta Graben. It is thus likely that layers barren of foraminifers are due to the overflow of fine-grained (clayey) material beyond drift channel dikes.

Methane seepage inferred from the porewater geochemistry of shallow sediments in the Beikang Basin of the southern South China Sea

Several tens of methane (CH4) seepage sites have been discovered in the northern South China Sea (SCS) during the investigation and exploration of gas hydrates and cold seeps over the past decade. However, our knowledge about methane seepage in the southern SCS remains scarce, especially in the Beikang Basin on the southern slope of the SCS. Four piston cores (BH-CL18, BH-CL23, BH-CL29 and BH-CL35) were collected in the Beikang Basin to understand CH4 cycling in shallow sediments. The extrapolated depths of the sulfate–methane transition zones (SMTZ) of the four cores are between 5.3 and 8.8 mbsf. By simulating the porewater profiles of SO42−, CH4, DIC and PO43− using the reaction-transport model, we estimated that the dissolved SO42− was predominantly consumed by the anaerobic oxidation of methane (AOM) at rates of 27.5 mmol m−2 yr−1 in BH-CL18, 28.1 mmol m−2 yr−1 in BH-CL23, 43.1 mmol m−2 yr−1 in BH-CL29 and 39.4 mmol m−2 yr−1 in BH-CL35, respectively. Due to intense AOM, almost all the methane sourced from subsurface sediments was depleted within the SMTZ, resulting in negligible benthic methane fluxes. The kink-type depth SO42− profile of BH-CL23 was apparently attributable to an enhanced methane flux, which was not captured by the steady-state modelling. Furthermore, highly depleted δ13C values of the dissolved inorganic carbon (DIC) in BH-CL18 (−43.6‰), BH-CL23 (−44‰) and BH-CL29 (−51.2‰) also indicated a methane-derived DIC. In contrast, relatively higher δ13C values in BH-CL35 suggested a mixture of 13C-enriched DIC produced by methanogenesis, as shown by the reversal toward more 13C-enriched DIC below 4.2 mbsf in BH-CL29. To our knowledge, this is the first quantitative study dealing with methane cycling in the shallow sediments of the southern SCS and may have considerable implications for understanding the poorly investigated cold seep and gas hydrate systems on the continental slope of the southern SCS.

Trace metal in sediment from a deep-sea floor of Makassar Strait

Makassar Strait is located in the entrance of Indonesian Through Flow (ITF). However, the geochemistry of metals in sediment within Makassar Strait remains unexplored. The aim of this study was to measure the concentration of metals in sediment and to assess the sediment quality based on those metals concentrations. The sediment was collected from 632-4730 m in depth using giant piston corer on R/V Baruna Jaya VIII in December 2014. In each observation point, three layers of sediment were sub-sampled from the core i.e. surface layer (0-5 cm), middle layer (45-55 cm) and bottom layer. The metals were analyzed using acid digestion procedure followed by Atomic Absorption Spectrophotometer. The result indicated that the metal has spatially insignificant differences in sediment and the increase of metal concentration by depth was noticed. The Enrichment factor presented as no enrichment to minor enrichment of metal in sediment.

Geochemical sedimentary evidence from core 973-2 for methane activity near the Jiulong Methane Reef in the northern South China Sea

The Jiulong Methane Reef, located on the northern slope of the South China Sea, is characterized by several features, such as bottom-simulating reflections and authigenic carbonates, which are indicative of methane seeps that are currently occurring as well as those that occurred in the past. However, to date, the effect that these methane seeps have on the sedimentary environment is not completely clear. To provide further insights into the biogeochemical processes involved in methane seeps, a 6.73 m piston core (973-2) was retrieved from this area in 2011 to perform an in-depth analysis. The chronology of the core has been established by [Formula: see text] dating, and sedimentary events since the last glacial period have been recorded. The results indicate abnormally low abundance and diversity, as well as high infaunal percentages of benthic foraminiferal assemblages in the lower part of the core from 673 to 350 centimeters below the sea floor (cmbsf), indicating impacts from methane activities. Major elemental barium, bromine, and titanium, which serve as proxies of the paleoproductivity, organic contents, and terrestrial supply, respectively, are found to make little contribution in [Formula: see text] content. However, regardless of the factors mentioned above, the [Formula: see text] content is still higher in the lower part of core, indicating the presence of authigenic carbonates. In addition, the total organic carbon/total sulfur ratios of less than 400 cmbsf are found to be beyond the normal range, which is [Formula: see text], reflecting the effects of methane oxidation. In one of the sections, the trace metals and molybdenum profiles show enrichments that are closely related to those of the methane seeps. The results indicate that this area experienced methane seep events in the last glacial period.

Iceberg Alley, East Antarctic Margin: Continuously laminated diatomaceous sediments from the late Holocene

A 24-meter jumbo piston core (NBP0101 JPC41) collected from an inner shelf basin in Iceberg Alley reveals an approximately 2000-year history of unusually high primary productivity. Iceberg Alley, an ~ 85 km long and 10–20 km wide cross-shelf trough on the Mac.Robertson Shelf, East Antarctica, reaches depths of 850 m and is bounded on either side by shallow banks that are lined with grounded icebergs. The sediments are laminated on a mm- to cm-scale throughout and are highly biosiliceous. Microscopic examination of smear slides, quantitative diatom slides, and sediment thin sections reveals that the sediments are visually dominated by the diatom Corethron pennatum, a large and lightly silicified species notable for its long and narrow shape; the valves, girdle bands and spines are all exceptionally well-preserved, suggesting rapid sedimentation. Other common species include sea ice-related Fragilariopsis, such as F. curta and F. cylindrus, with lesser contribution from other large diatoms, including Rhizosolenia spp. and Chaetoceros Ehrenberg subg. Chaetoceros. Chaetoceros Ehrenberg subg. Hyalochaete Gran resting spores, typically associated with large early-season blooms and common in many laminated sedimentary sections around the Antarctic margin, are surprisingly rare. Laminae with any significant terrigenous component are also very rare. Individual laminations appear to represent blooms, and in some cases sub-seasonal events are likely preserved. We suggest that this productive system is associated with the continuous presence of low-salinity meltwater derived from a combination of sea ice melt and grounded icebergs, which may be a source for a steady supply of micronutrients such as iron to the surface mixed layer.

Assemblage turnover among Upper Quaternary planktonic foraminifera in the subtropical North Atlantic Ocean

The ATI (assemblage turnover index), which is inversely related to the concept of complementarity, detects points of enhanced assemblage change.Whereas complementarity calculates the permeability of boundaries to a community’s continuance, the ATI quantifies boundary impermeability. Sample-wise ATI values for planktonic foraminifera (ATIsp) are here reported for the Late Quaternary at the ODP(Ocean Drilling Program) Hole 994C, Blake Ridge, western subtropical Atlantic Ocean, and the pattern compared with that for a piston core from a comparable latitude south of the Azores archipelago. There were no statistically significant correlations in Hole 994C between ATIsp and either interpolated d18O data or benthic foraminiferal ATIs. Thus, turnover in the planktonic and benthic foraminiferal assemblagesmust have been driven by different factors. Assemblage turnover peaks inATIsp occurred at someGlacial Terminations (specifically, T-II and T-IV) and within some interglacial MISs (marine isotope stages). This reflects rapid restructuring of the planktonic foraminiferal community at those times. Few comparable peaks inATIsp were recorded during the commencement of glacials, the protracted onset of which apparently induces only low levels of assemblage turnover. There were no ATIsp peaks in the interglacial MIS 3,which was a muted event. The conditioned-on-boundary index (CoBI), which is derived from theATI, showed that different tropical to subtropical species contributed to each of these peaks in turnover, such that there was no distinct cycling between glacial and interglacial planktonic foraminiferal assemblages. The migrations of zonal foraminiferal provinces is unlikely to be the dominant process, as thatwould have induced repetitive changes in assemblage composition between provincial types thatwere not observed. The piston core taken south of the Azores archipelago contained peaks in assemblage turnover that occurred shortly after Terminations. This might reflect that that authors for that core reported only the eight most abundant species. Alternatively, it might be inferred that the situation of the Azores core, taken north of the tropical-subtropical provincial boundary, was such that it does not reflect provincial migration.

A Direct‐Push Freezing Core Barrel for Sampling Unconsolidated Subsurface Sediments and Adjacent Pore Fluids

Core Ideas A direct‐push freezing core barrel (DPFCB) was developed and field tested. The DPFCB is suitable for sampling in coarse, unconsolidated sediments. The DPFCB freezes a 10‐ to 15‐cm plug that retains fluid and sediment in core sleeves. The DPFCB is interchangeable with unsaturated zone sampling tool strings. Contaminants passing through the unsaturated zone can undergo changes in narrow reaction zones upon reaching saturated sediments. Understanding these reactions requires sampling of sediment together with adjacent water and microbes in a manner that preserves in situ redox conditions. Use of a basket‐type core catcher for saturated, noncohesive sediments results in redistribution or loss of fluids during sample retrieval. Previously developed sample‐freezing drive shoes for hollow‐stem auger drilling rigs lessened fluid redistribution and retained all material that entered the core barrel in noncohesive sediment cores by freezing the base of the core with liquid CO2. This technology has not previously been compatible with direct‐push rigs that are commonly used for contaminated site assessments. Here, we describe a freezing core barrel designed for direct‐push rigs that is compatible with commercially available tool strings. The device can be used interchangeably with unsaturated‐zone direct‐push tool strings, enabling core collection for studies of contaminant transport and transformation spanning unsaturated to saturated profiles. In all 10 attempts during testing near Bemidji, MN, the device froze a 10‐ to 15‐cm (4–6‐in) plug that retained fluids and sediments in a 1.2‐m (4‐ft)‐long, 5.0‐cm (2.0‐in)‐diameter polyvinyl chloride (PVC) sleeve. Cores were collected from variably saturated sediments spanning the capillary fringe through the upper 2 m of the saturated zone in sandy glacial outwash sediments. The median recovery was 81% of the drive length, similar to a sample‐freezing drive shoe developed for a wire‐line piston core sampler operated with a hollow‐stem auger drill rig.

Distinguishing upper bathyal benthic foraminifera beneath the seasonally variable edge of the Orinoco Plume

The Orinoco is the world’s fourth largest river, outflow from it forming an extensive hypopycnal plume around Trinidad and Tobago and over much of the eastern Caribbean Sea. The plume’s areal extent fluctuates through the year, being at its greatest during the northern South American, July-December rainy season. The area perennially-impacted by the plume is thus surrounded by a rim that is only seasonally-impacted. The plume is transported NW across the Caribbean Sea by the Caribbean Current. Two upper bathyal piston cores were used in this study. Core BGT-031 (water depth 260 m) was taken from the perennially-impacted area east of Trinidad, while JPC-ASC-001 (water depth 428 m) was collected from the seasonally-impacted area northwest of Tobago. Both cores are ~80 cm long. They were sampled at 10 cm intervals, four replicates being collected from each of nine horizons. Qualitative comparisons are made with a previously-reported core (BGT-086) collected from the seasonally-impacted area east of Trinidad. Distinct differences in foraminiferal assemblages were found between JPC-ASC-001 and BGT-036. These differences are ascribed to the cores’ differing relationships with the Orinoco Plume. The assemblage in the perennially-impacted Core BGT-031 was dominated by Uvigerina parvula with subdominant Cibicidoides umbonatus and Cibicides pachyderma. That in the seasonally-impacted JPC-ASC-001 was dominated by Sphaeroidina bulloides, with (in rank order) subdominant Globocassidulina subglobosa subcalifornica, C. umbonatus and C. pachyderma. ANOVA indicated that U. parvula and S. bulloides are both specialist species with relatively narrow niches, the mean proportional abundances differing between horizons in their respective cores. Cibicidoides umbonatus is in contrast shown to be a generalist species. The mean Shannon’s index H for replicates from JPC-ASC-001 was significantly greater than that for replicates from BGT-031, but qualitatively comparable to that for BGT-086. Meanwhile, mean dominance, measured using max(pi), was significantly lower in JPC-ASC-001 than in BGT-031, that in the former being comparable to that in BGT-086. Epistominella exigua, which is indicative of a seasonal phytodetrital flux, was recovered from BGT-086, but not from JPC-ASC-001, even though both cores were recovered from the seasonally-impacted rim. This difference is ascribed to the development of a wake and an eddy in the Caribbean Current to the lee of Tobago that locally change the nature of surface productivity.

KISAP: A new In situ seafloor velocity measurement tool

ABSTRACTThe KISAP (Korea Institute of Geoscience and Mineral Resources Seafloor Acoustic Probe) was developed to obtain in situ compressional wave velocity and attenuation profiles for seafloor sediments. The instrument comprises recording channel receivers, and an acoustic source, and can be attached to a corer or a probe. Here, we report experiments comparing KISAP in situ velocity to laboratory velocity of colocated piston cores. KISAP data matched laboratory data (corrected for temperature and pressure) reasonably. KISAP can be used to collect in situ acoustic data and can be effectively used to calibrate previous laboratory data to in situ data.

The Seno Otway pockmark field and its relationship to thermogenic gas occurrence at the western margin of the Magallanes Basin (Chile)

Pockmarks are variably sized crater-like structures that occur in young continental margin sediments. They are formed by gas eruptions and/or long-term release of fluid or gas. So far no pockmarks were known from the Pacific coast of South America between 51°S and 55°S. This article documents an extensive and previously unknown pockmark field in the Seno Otway (Otway Sound, 52°S) with multibeam bathymetry and parametric echosounding as well as sediment drill cores. Up to 31 pockmarks per square kilometer occur in water depths of 50 to >100 m in late glacial and Holocene sediments. They are up to 150 m wide and 10 m deep. Below and near the pockmarks, echosounder profiles image acoustic blanking as well as gas chimneys often crosscutting the 20 to >30 m thick glacial sediments above the acoustic basement, in particular along fault zones. Upward-migrating gas is trapped within the sediment strata, forming dome-like features. Two 5 m long piston cores from inside and outside a typical pockmark give no evidence for gas storage within the uppermost sediments. The inside core recovered poorly sorted glacial sediment, indicating reworking and re-deposition after several explosive events. The outside core documents an undisturbed stratigraphic sequence since ~15 ka. Many buried paleo-pockmarks occur directly below a prominent seismic reflector marking the mega-outflow event of the Seno Otway at 14.3 ka, lowering the proglacial lake level by about 80 m. This decompression would have led to frequent eruptions of gas trapped in reservoirs below the glacial sediments. However, the sediment fill of pockmarks formed after this event suggests recurrent events throughout the Holocene until today. Most pockmarks occur above folded hydrocarbon-bearing Upper Cretaceous and Paleogene rocks near the western margin of the Magallanes Basin, constraining them as likely source rocks for thermogenic gas.

Influences of Climate and Tectonic on the Middle to Late Holocene Deltaic Sedimentation in Lake Hazar, Eastern Turkey

Analyses of two piston cores from Lake Hazar together with high-resolution seismic data provide unique evidence of complex interaction of tectonics and lake level changes during the mid-to-late Holocene. This period is associated with stacked shelf-edge deltaic units at the mouth of the Kurkcayi River at the western extreme of the lake. Despite the general dry climate during the middle Holocene (between 8.2 and 5.1 cal ka BP), transgressive and landward retreating deltaic successions developed due to the deepening of the lake that induced by transtentional activity of the East Anatolian Fault Zone. The arid climate during 5.1–2.6 cal ka BP is supported by the multi-proxy analyses of the cores. This period coincides with the late Bronze age deforestation, which probably provided the sediment supply by soil erosion for the deltaic deposition at the mouth of the paleo-Kurkcayi River. On the other hand, the flooding of the youngest deltaic unit in the lake took place under a general wet climate after 1.4 cal ka BP. The seismic architecture and the core sedimentology indicate that the existing hydrological regime with the latest deltaic deposition started 2.6 cal ka BP was accompanied by a hyperpycnal mixing.

The Morphometry of the Deep-Water Sinuous Mendocino Channel and the Immediate Environs, Northeastern Pacific Ocean

Mendocino Channel, a deep-water sinuous channel located along the base of Gorda Escarpment, was for the first time completely mapped with a multibeam echosounder. This study uses newly acquired multibeam bathymetry and backscatter, together with supporting multichannel seismic and sediment core data to quantitatively describe the morphometry of the entire Mendocino Channel and to explore the age and possible causes that may have contributed to the formation and maintenance of the channel. The first 42 km of the channel is a linear reach followed for the next 83.8 km by a sinuous reach. The sinuous reach has a sinuosity index of 1.66 before it changes back to a linear reach for the next 22.2 km. A second sinuous reach is 40.2 km long and the two reaches are separated by a crevasse splay and a large landslide that deflected the channel northwest towards Gorda Basin. Both sinuous reaches have oxbow bends, cut-off meanders, interior and exterior terraces and extensive levee systems. The lower sinuous reach becomes more linear for the next 22.2 km before the channel relief falls below the resolution of the data. Levees suddenly decrease in height above the channel floor mid-way along the lower linear reach close to where the channel makes a 90° turn to the southwest. The entire channel floor is smooth at the resolution of the data and only two large mounds and one large sediment pile were found on the channel floor. The bathymetry and acoustic backscatter, together with previously collected seismic data and box and piston cores provide details to suggest Mendocino Channel may be no older than early Quaternary. A combination of significant and numerous earthquakes and wave-loading resuspension by storms are the most likely processes that generated turbidity currents that have formed and modified Mendocino Channel.