Benthic Storms Research Articles

Intensified Currents Associated With Benthic Storms Underneath an Eddying Jet

AbstractBenthic storms are episodes of intensified near‐bottom currents capable of sediment resuspension in the deep ocean. They typically occur under regions of high surface eddy kinetic energy (EKE), such as the Gulf Stream. Although they have long been observed, the mechanism(s) responsible for their formation and their relationships with salient features of the deep ocean, such as bottom mixed layers (BMLs) and benthic nepheloid layers (BNLs), remain poorly understood. Here we conduct idealized experiments with a primitive‐equation model to explore the impacts of the unforced instability of a surface‐intensified jet on near‐bottom flows of a deep zonal channel. Vertical resolution is increased near the bottom to represent the bottom boundary layer. We find that the unstable near‐surface jet develops meanders and evolves into alternating, deep‐reaching cyclones and anticyclones. Simultaneously, EKE increases near the bottom due to the convergence of vertical eddy pressure fluxes, leading to near‐bottom currents comparable to those observed during benthic storms. These currents in turn form BMLs with thickness of O(100 m) from enhanced velocity shears and turbulence production near the bottom. The deep cyclonic eddies transport fluid particles both laterally and vertically, from near the bottom through the entire BML and may contribute to the formation of the lower part of BNLs. A sloping bottom reduces both the intensity of the near‐bottom currents and the extent of vertical transport. Overall, our study highlights a significant response of the abyssal environment to near‐surface current instability, with potential implications for sediment transport in the deep ocean.

Late glacial to holocene sedimentary facies of the Eirik Drift, southern Greenland margin: Spatial and temporal variability and paleoceanographic implications

The Eirik Drift, off southern Greenland, is one of a series of contourite deposits in the northern North Atlantic that record changes in the strength and location of western boundary currents in the region. To date however, the sedimentary facies, and particularly the variation in facies across this drift, have received relatively little investigation. Here, we present an analysis of the sedimentary facies observed within a transect of cores from the crest to toe of the Eirik Drift from late Pleistocene to Holocene. The Holocene sequence consists of muddy contourites with high sedimentation rates at the drift toe, and a condensed sequence of sandy contourites on the upper drift flanks, consistent with winnowing under strong bottom currents on the upper drift and deposition under a low velocity, sediment-laden current at the drift toe. We interpret this to be a combined result of episodic, high-energy benthic storm events associated with the East Greenland Current (EGC) on the upper drift and more continuous, lower velocity Deep Western Boundary Current (DWBC) on the drift flanks. The deglacial interval is represented by muddy contourites across the drift, with evidence for decreasing current activity (both EGC and DWBC) and more widespread ice-rafted deposition from the Bolling-Allerod into the Younger Dryas. Palaeocurrent data from this interval show two separate current directions at the crest of the drift, suggesting temporary, local detachment of the DWBC or EGC, linked to temporal variation in current strength. The late glacial interval consists of glaciomarine hemipelagites and muddy contourites, with evidence for a higher degree of current influence at shallower depths, consistent with a moderate EGC and weak DWBC. This is the first time that the EGC is recognised as having a significant role in sedimentation on the Eirik Drift.

Faunal assemblage changes, bioturbation and benthic storms at an abyssal station in the northeastern Pacific

This study investigates the evolution of deep-sea lebensspuren assemblages before, during, and after five high-energy episodes over a storm-events period lasting 25 days. Bioturbational changes were characterized through brief benthic storm events in the northeastern Pacific at the abyssal time-series site Station M (4000 m depth). Quantification of seafloor coverage was processed to evaluate variations on the substratum type. A total of 15 lebensspuren morphotypes were identified. Most traces could be associated with feeding faecal casts, crawling tracks, dwellings and resting structures. Lebensspuren assemblages were similar before and after the storm-events period; full assemblage was re-established in one week. This high energy period was associated with an impoverishment of lebensspuren abundance and assemblage diversity. Only elpidiid holothurian feeding faecal casts recorded a notable increase in abundance during this period of high energy. In addition, this energetic stage involved the appearance of exhumed surface patches. Holothurian feeding activity appeared to have been primarily influenced by the local-scale erosion and re-suspension of unconsolidated surface sediment, which led to the reorganization of organic matter resources. We propose the mobility of elpidiid holothurians allowed them to gain a competitive advantage in obtaining these new resources. This research presents a novel relationship between lebensspuren, faunal activity, and bottom currents that broadens our understanding of benthic community responses to deep-sea bottom currents. Finally, we discuss these results as they pertain to the fossil record to assess how bioturbational structure development could be controlled by substratum type, organic matter availability and duration of energetic episodes.

ЭРОЗИОННО-АККУМУЛЯТИВНЫЕ ПРОЦЕССЫ В СЕВЕРНОМ СЕКТОРЕ КОНТУРИТОВОЙ СИСТЕМЫ КОНТИНЕНТАЛЬНОГО СКЛОНА ПАТАГОНИИ

Series of two papers are dedicated to study of erosional and depositional processes in the northern part of the previously distinguished contourite system on the Patagonian continental slope between 46-42° at a water depth range of 2100-5200 m. The first paper is focused on interpretation of high-resolution seiscmoacoustic data collected using the parametric echo-sounder SES 2000 deep with acoustic penetration of 60 mbsf. Analysis of seismic data revealed seismic facies of contourites and gravitites as well as erosional features formed under the influence of gravity flows (gravity-driven processes) moving downslope through the canyon system. It was demonstrated that gravitites deposited by gravity flows represent a source of sediment material for contourite drift formation. Domination of erosion over deposition in the study area might be explained by the intense dynamics of bottom waters, including contour currnets, internal waves, benthic storms and gravity flows.

Instability-Driven Benthic Storms below the Separated Gulf Stream and the North Atlantic Current in a High-Resolution Ocean Model

AbstractBenthic storms are important for both the energy budget of the ocean and for sediment resuspension and transport. Using 30 years of output from a high-resolution model of the North Atlantic, it is found that most of the benthic storms in the model occur near the western boundary in association with the Gulf Stream and the North Atlantic Current, in regions that are generally collocated with the peak near-bottom eddy kinetic energy. A common feature is meander troughs in the near-surface jets that are accompanied by deep low pressure anomalies spinning up deep cyclones with near-bottom velocities of up to more than 0.5 m s−1. A case study of one of these events shows the importance of both baroclinic and barotropic instability of the jet, with energy being extracted from the jet in the upstream part of the meander trough and partly returned to the jet in the downstream part of the meander trough. This motivates examining the 30-yr time mean of the energy transfer from the (annual mean) background flow into the eddy kinetic energy. This quantity is shown to be collocated well with the region in which benthic storms and large increases in deep cyclonic relative vorticity occur most frequently, suggesting an important role for mixed barotropic–baroclinic instability-driven cyclogenesis in generating benthic storms throughout the model simulation. Regions of the largest energy transfer and most frequent benthic storms are found to be the Gulf Stream west of the New England Seamounts and the North Atlantic Current near Flemish Cap.

Particle fluxes induced by benthic storms during the 2012 dense shelf water cascading and open sea convection period in the northwestern Mediterranean basin

The effects of deep dense shelf water cascading and open sea convection on the sediment dynamics of the northwestern Mediterranean basin were studied by near-bottom moored instruments recording trapped particle fluxes, suspended particle fluxes, water properties and hydrodynamics from November 2011 to July 2012. During this period, near-bottom currents induced by winter dense water formation generated benthic storms that caused resuspension at 2450 m water depth, increasing by more than one order of magnitude the ambient suspended sediment concentrations, the trapped particle fluxes and the suspended sediment fluxes. During the preconditioning phase of the open sea convection, from December 2011 to mid-February 2012, currents (1–10 cm s−1), suspended sediment concentrations (<0.1 mg l−1), Chl-a fluorescence values (<0.063 μg l−1), trapped total mass fluxes (10–50 mg m−2 d−1) and trapped organic carbon fluxes (1–4 mg m−2 d−1) were low, and organic matter was mainly undegraded and of marine origin. Open sea convection was observed at the study site in mid-February, at the beginning of the violent mixing phase, increasing current velocities up to 26 cm s−1 and Chl-a fluorescence values up to 0.074 μg l−1, supplying particles with fresh marine organic matter content. During the last fortnight of February, two major dense shelf water cascading pulses generated Chl-a fluorescence increases (up to 0.116 μg l−1) and large suspended sediment concentration peaks (up to19 mg l−1), suspended sediment fluxes (up to 6500 mg m−2 d−1) and trapped total mass flux increases (up to 22,900 mg m−2 d−1), which were associated with benthic storms resuspension. During this phase, trapped organic carbon flux increased almost two orders of magnitude (up to 260 mg m−2 d−1), with pulses of both marine and terrestrial organic matter. The sinking and spreading phase occurred from early March to mid-June. The signal of deep dense shelf water cascading lasted past early April, and the spreading of the newly formed dense water maintained maximum currents of up to 25 cm s−1 and trapped particle fluxes of up to 2000 mg m−2 d−1 until mid-June. At the beginning of this phase, organic matter was terrestrial and several turbidity peaks occurred during current speed increases generated by benthic storms. At the end of this phase, the organic matter became less terrestrial, trapped organic carbon fluxes decreased from about 190 to 10 mg m−2 d−1 and turbidity peaks occurred with low current velocities indicating the arrival of storm tails at the mooring site. The large particle fluxes of fresh or relatively undegraded organic carbon induced by deep dense water formation during winter 2012, contributed to the “fertilization” of the northwestern Mediterranean deep benthic ecosystems.

Relation of sortable silt grain-size to deep-sea current speeds: Calibration of the ‘Mud Current Meter’

Fine grain-size parameters have been used for inference of palaeoflow speeds of near-bottom currents in the deep-sea. The basic idea stems from observations of varying sediment size parameters on a continental margin with a gradient from slower flow speeds at shallower depths to faster at deeper. In the deep-sea, size-sorting occurs during deposition after benthic storm resuspension events. At flow speeds below 10–15cms−1 mean grain-size in the terrigenous non-cohesive ‘sortable silt’ range (denoted by SS¯, mean of 10–63µm) is controlled by selective deposition, whereas above that range removal of finer material by winnowing is also argued to play a role.A calibration of the SS¯ grain-size flow speed proxy based on sediment samples taken adjacent to sites of long-term current meters set within ~100m of the sea bed for more than a year is presented here. Grain-size has been measured by either Sedigraph or Coulter Counter, in some cases both, between which there is an excellent correlation for SS¯ (r = 0.96). Size-speed data indicate calibration relationships with an overall sensitivity of 1.36 ± 0.19cms−1/μm. A calibration line comprising 12 points including 9 from the Iceland overflow region is well defined, but at least two other smaller groups (Weddell/Scotia Sea and NW Atlantic continental rise/Rockall Trough) are fitted by sub-parallel lines with a smaller constant. This suggests a possible influence of the calibre of material supplied to the site of deposition (not the initial source supply) which, if depleted in very coarse silt (31–63µm), would limit SS¯ to smaller values for a given speed than with a broader size-spectrum supply. Local calibrations, or a core-top grain-size and local flow speed, are thus necessary to infer absolute speeds from grain-size.The trend of the calibrations diverges markedly from the slope of experimental critical erosion and deposition flow speeds versus grain-size, making it unlikely that the SS¯ (or any deposit size for that matter) is simply predicted by the deposition threshold. A more probable control is the rate of deposition of the different size fractions under changing flows over several tens of years (the typical averaging period of a centimetre of deposited sediment). This suggestion is supported by a simple depositional model for which the deposited SS¯ is calculated from measured currents with a size-varying depositional threshold. More surficial sediment samples taken near long-term current meter sites are needed to make calibrations more robust and explore regional differences.

On the Existence of Benthic Storms

We study a model for the wind-induced current field of the Pacific ocean in order to demonstrate that currents in the surface layer are carried down to the deepest regions above the abyssal sea floor, which indicates the existence of the phenomenon of comparably strong currents in bottom regions as a result of wind-stress forces at the surface, also known as benthic storms.

Distribution of natural disturbance due to wave and tidal bed currents around the UK

The UK continental shelf experiences large tidal ranges and winter storm events, which can both generate strong near-bed currents. The regular tidal bottom currents from tides plus wind driven ‘benthic storms’ (dominated by wave-driven oscillatory currents in shallow water) are a major source of disturbance to benthic communities, particularly in shallow waters. We aim to identify and map the relative impact of the tides and storm events on the shallower parts of the North West European continental shelf.A 10-year simulation of waves, tides and surges on the continental shelf was performed. The shelf model was validated against current meter observations and the Centre for Environmental, Fisheries and Aquaculture Science (CEFAS) network of SmartBuoys. Next, the model performance was assessed against seabed lander data from two sites in the Southern North Sea; one in deep water and another shallow water site at Sea Palling, and a third in Liverpool Bay. Both waves and currents are well simulated at the offshore Southern North Sea site. A large storm event was also well captured, though the model tends to underpredict bottom orbital velocity. Poorer results were achieved at the Sea Palling site, thought to be due to an overly deep model water depth, and missing wave–current interactions. In Liverpool Bay tides were well modelled and good correlations (average R2=0.89) are observed for significant wave height, with acceptable values (average R2=0.79) for bottom orbital velocity.Using the full 10-year dataset, return periods can be calculated for extreme waves and currents. Mapping these return periods presents a spatial picture of extreme bed disturbance, highlighting the importance of rare wave disturbances (e.g. with a return period of 1 in 10 years). Annual maximum currents change little in their magnitude and distribution from year to year, with mean speeds around 0.04ms−1, and maximums exceeding 3ms−1. Wave conditions however are widely variable throughout the year, depending largely on storm events. Typical significant wave heights (Hs) lie between 0.5 and 2m, but storm events in shallow water can bring with them large waves of 5m and above and up to 18m in North West Approaches/North West Scotland (Sterl and Caires, 2005).The benthic disturbance generated by waves and currents is then estimated by calculating the combined force on an idealised object at the bed. The patterns of this disturbance reflect both regular tidal disturbance and rare wave events. Mean forces are typically 0.05–0.1N, and are seen largely in areas of fast currents (>1ms−1). The pattern of maximum force however is more dependent on water depth and exposure to long-fetches (>1000km) suggesting that it is dominated by wave events.

Shelf and deep-sea sedimentary environments and physical benthic disturbance regimes: A review and synthesis

Physical disturbances of the seafloor play a key role in ecosystem function and are postulated to exert control over spatial patterns of biodiversity. This review investigates the role of natural physical sedimentological processes that occur in shelf, slope and abyssal environments that also act as disturbances to benthic ecosystems and which, under certain circumstances, give rise to benthic disturbance regimes. Physical sedimentological processes can cause both press (process that causes a disturbance by acting over a timespan that is intolerable to benthos) and pulse (process that causes a disturbance by exceeding a threshold above which benthos are unable to remain attached to the seabed or are buried under rapidly deposited sediment) types of disturbance. On the continental shelf, pulse-type disturbances are due to temperate and tropical storm events, and press-type of disturbances identified here are due to the migration of bedforms and other sand bodies, and sustained periods of elevated turbidity caused by seasonally reversing wind patterns. On the continental slope and at abyssal depths, pulse-type disturbances are due to slumps, turbidity currents; benthic storms may cause either press or pulse type disturbances. A possible press-type of disturbance identified here is inter-annual changes in abyssal bottom current speed and/or direction. It is concluded that: 1) physical sedimentary disturbance regimes may characterize as much as 10% of the global ocean floor; 2) multidisciplinary research programs that integrate oceanography, sedimentology and benthic ecology to collect time series observational data sets are needed to study disturbance regimes; and 3) predictive habitat suitability modeling must include disturbance regime concepts, along with other biophysical variables that define the fundamental niches of marine species, in order to advance.

Prevalence of strong bottom currents in the greater Agulhas system

Deep current meter data and output from two high‐resolution global ocean circulation models are used to determine the prevalence and location of strong bottom currents in the greater Agulhas Current system. The two models and current meter data are remarkably consistent, showing that benthic storms, with bottom currents greater than 0.2 m s−1, occur throughout the Agulhas retroflection region south of Africa more than 20% of the time. Furthermore, beneath the mean Agulhas Current core and the retroflection front, bottom currents exceed 0.2 m s−1 more than 50% of the time, while away from strong surface currents, bottom currents rarely exceed 0.2 m s−1. Implications for sediment transport are discussed and the results are compared to atmospheric storms. Benthic storms of this strength (0.2 m s−1) are comparable to a 9 m s−1 (Beaufort 5) windstorm, but scaling shows that benthic storms may be less effective at lifting and transporting sediment than dust storms.

Cosmopolitanism and Biogeography of the Genus Manganonema (Nematoda: Monhysterida) in the Deep Sea

Simple SummaryThe deep sea comprises more than 60% of the Earth surface, and likely represents the largest reservoir of as yet undiscovered biodiversity. Nematodes are the most abundant taxon on Earth and are particularly abundant and diverse in the deep sea. Nevertheless, knowledge of their biogeography especially in the deep sea is still at its infancy. This article explores the distribution of the genus Manganonema in the deep Atlantic Ocean and Mediterranean Sea providing new insights about this apparently rare deep-sea genus.Spatial patterns of species diversity provide information about the mechanisms that regulate biodiversity and are important for setting conservation priorities. Present knowledge of the biogeography of meiofauna in the deep sea is scarce. This investigation focuses on the distribution of the deep-sea nematode genus Manganonema, which is typically extremely rare in deep-sea sediment samples. Forty-four specimens of eight different species of this genus were recorded from different Atlantic and Mediterranean regions. Four out of the eight species encountered are new to science. We report here that this genus is widespread both in the Atlantic and in the Mediterranean Sea. These new findings together with literature information indicate that Manganonema is a cosmopolitan genus, inhabiting a variety of deep-sea habitats and oceans. Manganonema shows the highest diversity at water depths >4,000 m. Our data, therefore, indicate that this is preferentially an abyssal genus that is able, at the same time, to colonize specific habitats at depths shallower than 1,000 m. The analysis of the distribution of the genus Manganonema indicates the presence of large differences in dispersal strategies among different species, ranging from locally endemic to cosmopolitan. Lacking meroplanktonic larvae and having limited dispersal ability due to their small size, it has been hypothesized that nematodes have limited dispersal potential. However, the investigated deep-sea nematodes were present across different oceans covering macro-scale distances. Among the possible explanations (hydrological conditions, geographical and geological pathways, long-term processes, specific historical events), their apparent preference of colonizing highly hydrodynamic systems, could suggest that these infaunal organisms are transported by means of deep-sea benthic storms and turbidity currents over long distances.

Ecosystem effects of dense water formation on deep Mediterranean Sea ecosystems: an overview

Natural episodic events, such as gravity flows, submarine landslides, and benthic storms can determine severe modifications in the structure and functioning of deep-sea ecosystems. Here, we report and compare the ecosystem effects produced by dense water formation events that occurred in the Gulf of Lions (NW Mediterranean) and the Aegean Sea (NE Mediterranean). In both regions, the rapid sinking of cold dense waters, driven by regional meteorological forcings, results in important immediate modifications that can be summarised in: (i) increased organic matter content in the deep basin; (ii) diminished benthic abundance; and (iii) changes of benthic biodiversity. At longer time scale the analysis reveals, however, different resilience times in the two regions. The Gulf of Lions is characterized by a very fast (months) recovery whereas the Aegean Sea shows much longer (45 years) resilience time. New long-term studies are further needed to identify the potential effects that changes in the duration, intensity and frequency of episodic events could have on the structure, biodiversity and functioning of the deep Mediterranean Sea under environmental and climate change scenarios.

Ecosystem effects of dense water formation on deep Mediterranean Sea ecosystems: an overview

Natural episodic events, such as gravity flows, submarine landslides, and benthic storms can determine severe modifications in the structure and functioning of deep-sea ecosystems. Here, we report and compare the ecosystem effects produced by dense water formation events that occurred in the Gulf of Lions (NW Mediterranean) and the Aegean Sea (NE Mediterranean). In both regions, the rapid sinking of cold dense waters, driven by regional meteorological forcings, results in important immediate modifications that can be summarised in: (i) increased organic matter content in the deep basin; (ii) diminished benthic abundance; and (iii) changes of benthic biodiversity. At longer time scale the analysis reveals, however, different resilience times in the two regions. The Gulf of Lions is characterized by a very fast (months) recovery whereas the Aegean Sea shows much longer (45 years) resilience time. New long-term studies are further needed to identify the potential effects that changes in the duration, intensity and frequency of episodic events could have on the structure, biodiversity and functioning of the deep Mediterranean Sea under environmental and climate change scenarios.

Dynamics of benthic copepods and other meiofauna in the benthic boundary layer of the deep NW Mediterranean Sea

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 396:181-195 (2009) - DOI: https://doi.org/10.3354/meps08408 Dynamics of benthic copepods and other meiofauna in the benthic boundary layer of the deep NW Mediterranean Sea L. D. Guidi-Guilvard1,2,*, D. Thistle3, A. Khripounoff4, S. Gasparini1,2 1CNRS, and 2Université Pierre et Marie Curie, Laboratoire d’Océanographie de Villefranche, 06234 Villefranche/Mer, France 3Department of Oceanography, Florida State University, Tallahassee, Florida 32306-4320, USA 4IFREMER centre de Brest, Laboratoire Environnement Profond, 29280 Plouzané, France *Email: laurence.guidi@obs-vlfr.fr ABSTRACT: A continuous high-resolution time-series survey of the hyperbenthic community and local environmental conditions was conducted in the benthic boundary layer (BBL) of the DYFAMED-BENTHOS station (43°24.61’N, 7°51.67’E at 2347 m depth in the NW Mediterranean) between January 1996 and April 1998 using bottom-moored sediment traps and a current meter. Sediment traps were set 4 m above the bottom. Hyperbenthos was collected as ‘swimmers’, i.e. those organisms that are alive when they enter the traps but are not part of the particle flux. Identification of these organisms showed that ~90% were meiobenthic. Copepods dominated and comprised on average 75% of total swimmers. They were followed by nauplii (12%), annelids (7.8%), nematodes and bivalves (1.8% each), ostracods, isopods, and amphipods (1.2% altogether). Of the 3930 copepods examined, 4% were calanoids, 15% were harpacticoids and 81% were cyclopoids. Among the non-calanoid copepods, 25 species or groups of species were distinguished. Two benthic copepod species outnumbered all others: the cyclopinid genus Barathricola represented 90% of the cyclopoids, and the tisbid genus Tisbe represented 57% of the harpacticoids. Temporal variations, both intra- and interannual, in swimmer fluxes were high (26 to 361 ind. m–2 d–1), but not all groups/taxa/species were equally affected. Statistical analyses showed that these variations were the result of variability in both physical (near-bottom current) and trophic (particle flux) environmental factors. Organisms had both immediate and delayed responses, which involved passive (i.e. erosion, suspension) and active (i.e. emergence) reactions, as well as population growth. Most of the dispersal mechanisms previously reported for shallow-water benthic organisms were encountered, denoting the remarkable similarities in the general processes between coastal and deep-sea environments. KEY WORDS: Deep sea · Swimmers · Hyperbenthos · Benthic storms · Resuspension · Emergence · Population growth · Biodiversity Full text in pdf format PreviousNextCite this article as: Guidi-Guilvard LD, Thistle D, Khripounoff A, Gasparini S (2009) Dynamics of benthic copepods and other meiofauna in the benthic boundary layer of the deep NW Mediterranean Sea. Mar Ecol Prog Ser 396:181-195. https://doi.org/10.3354/meps08408Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 396. Online publication date: December 09, 2009 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2009 Inter-Research.

Size sorting in marine muds: Processes, pitfalls, and prospects for paleoflow‐speed proxies

The basis for, and use of, fine grain size parameters for inference of paleoflow speeds is reviewed here. The basis resides in data on deposited sediment taken in conjunction with flow speed measurements in the field, experimental data on suspended sediment transport and deposition, and theoretical treatments of the generation of size distributions of deposits from suspension controlled by particle settling velocity and flow speed. In the deep sea, sorting events occur under resuspension/deposition events in benthic storms. At flow speeds below 10–15 cm s−1, size in the noncohesive “sortable silt” (10–63μm) range is controlled by selective deposition, whereas above that range, removal of finer material by winnowing also plays a role. The best particle size instruments to measure a flow speed–related grain size employ the settling velocity method, while laser diffraction sizers can yield misleading results because of particle shape effects. Potential problems, including source effects, downslope supply on continental margins, spatial variability of flow over bedforms, and influence of ice‐rafted detritus, are examined. A number of studies using the sortable silt flow speed proxy are reviewed, and inverse modeling of grain size distributions is examined. Outstanding problems are that corroboration is sparse because almost no studies have yet used the full range of proxies for flow rate and water mass identification and that the sortable silt mean size is not yet properly calibrated in terms of flow speed.

New discoveries in dynamics of an M8 earthquake-phenomena and their implications from the 2003 Tokachi-oki earthquake using a long term monitoring cabled observatory

At the 2003 Tokachi-oki earthquake of M8, seafloor phenomena such as a generation process of tsunami, seafloor uplifts, turbidity current, etc., were observed using a cabled observatory installed on the seafloor. The turbidity current was observed as a benthic storm caused presumably by the mainshock. The seafloor uplifts were observed at the mainshock and continuously after the mainshock. The uplifts were 0.35, 0.37, and 0.12 m for epicentral distances of 25.5, 31.4, and 81.7 km, respectively. After the mainshock, a continuous uplift of the seafloor is observed at all three pressure gauge locations indicating that there was a change in the state of friction on the plate boundary interface by the mainshock. In this paper, we first show what was observed using the cabled observatory installed right above the focal area of the earthquake, and then we discuss to summarize these phenomena associated with the earthquake, its possible causes, and future directions in long term monitoring of seismogenic processes.

First Phase Monitoring Studies of Simulated Benthic Disturbance Delineating Movement of Fine Particles in the Central Indian Basin

Benthic disturbance due to future deep-sea polymetallic nodule mining would involve expensive sediment plume generation and resedimentation on the sea floor. In order to evaluate the effects of resedimentation on benthic environment, the Indian Deep-sea Environment Experiment (INDEX) was conducted in 1997, and pre-, and post-disturbance studies on grain size were carried out. The initial increase in clay content after the experiment, continued to increase further as measured in the first monitoring phase samples, 44 months later. Increase in clay-sized particles during monitoring-l (M-l) was highest within the simulated (disturbed) zone and to the north of it, which is attributed to the combined effects of disaggregation, abrasion, and powderization of sediments during transportation. Due to this fractionation (breaking up), the particles appear to have remained in suspension over a prolonged period of time after they were discharged in the water column 5 m above the seabed during INDEX. The travel effects of INDEX plume appears to be localized and confined within and around the disturbed zone (DZ) as resettlement of fine particles from the benthic plume was traced up to 2 km south and 12 to 18 km north of the DZ. The evidence does not suggest the existence of strong currents and benthic storms in the CIB

Turbulent diffusion and transport from a CO2 lake in the deep ocean

If liquid CO2 is stored as a dense “lake” on the deep ocean floor, it is expected to dissolve in seawater. Ocean currents and turbulence can increase the net rate of CO2 release by several orders of magnitude compared to molecular diffusion. However, density stratification in the seawater created by dissolved CO2 will tend to reduce vertical mixing. A two‐dimensional numerical study with a high‐resolution advection‐diffusion model, coupled with a general turbulence model, reveals significant modifications of the boundary layer structure above a generic CO2 lake taken to be 500 m in length and placed in a 10 km domain that is subject to specified far‐field currents in the range of 0.05–0.20 m s−1. Dissolution rates of order 0.07 μmol cm−2 s−1, reaching approximately 0.5 μmol cm−2 s−1 during a benthic storm, are predicted. The friction velocity is reduced above the CO2 lake, and CO2 concentrations corresponding to excess water densities of up to 0.5 kg m−3 occur in the lower 10 m of the water column. The persistency of this low‐pH, CO2‐enriched water layer in weakly stratified and neutral background conditions in the model suggests that gravity current dynamics are important over considerable distances and should be considered in future larger‐scale models and impact studies.

Cyclogenesis in the deep ocean beneath Western Boundary Currents: A process‐oriented numerical study

A two‐layer shallow‐water equation model is applied to a flat‐bottom ocean on the f plane to explore instability mechanisms in Western Boundary Current (WBC) that lead to the formation of strong cyclones in the deep ocean underneath. Findings reveal a tight coupling of surface meandering and deep cyclogenesis, in agreement with observational evidence. Barotropic cyclones develop in timescales of 5–10 days and attain swirl speeds of &gt;50 cm/s (depends on initial strength of WBC) on a diameter of ∼100 km. Cyclogenesis is driven by advection of relative vorticity in the surface ocean and failure of the thermocline to respond rapidly enough to the associated sea level variations. Findings suggest that cyclogenesis and the associated strong abyssal flows (benthic storms) are ubiquitous features of WBCs and other frontal flows.