Ocean Floor Sampling Meaning, Types, Methods, and Equipment
Ocean floor sampling is the retrieval, of both biogenic and non-biogenic marine samples, that is, both rock/sediments, and organic material; from the sea bed.
These samples usually serve as valuable sources of information, about the history and nature of the deep marine system.
During it’s development, several techniques and instruments have been employed to assess the deep marine realm, for retrieval of these samples from the sea bed, most of these techniques existing as modifications of older ones.
Herein, the terms ‘Techniques’ and ‘Instruments’, are easily used in combination as well as interchangeably, since both terms represent virtually the same concept, in this field. Our scope of interest, which is also our topic, defines the information, and the themes highlighted, in this article.
INTRODUCTION
Ocean floor sampling, may be either of sediments/mineral assemblages, or of biogenic forms. It is a practice whose immense importance transcends such disciplines as Oceanography, Biology, Paleontology, Mineralogy, Petroleum and Marine Geology.
The following amongst others, are the aims of ocean floor sampling;
-To study past climates (Paleoclimate), and trends of climatic change, for environmental prediction.
– To derive information about the Earth, and it’s environmental systems.
– It is applied in choosing sites for undersea drilling projects, seabed communication cables, and other structures.
-To estimate the impact of geologic events, on the marine systems.
-For study of evolution, of marine organic forms.
-In evaluation of impacts of offshore waste disposal.
-To learn about the habitats of marine biological communities, and the impacts of these habitats upon organic life.
-In location of offshore mineral resources.
-In study, of marine pollution patterns and development of remediation strategies.
-To find sources of dredged material, for beach replenishment.
As already has been mentioned, ocean floor sampling, may be carried out to retrieve ocean floor sediments, as well as deep water marine organisms. These two sampling cases, we would endeavor to discuss separately.
SAMPLING OF MARINE BIODIVERSITY
A vast array of varying habitats, are included in the marine environment, which is the largest ecosystem on Earth.
Over seventy percent of the planet’s surface, is covered by water from oceans, with an average depth of 3,800m. Despite the large volume(97% of total volume of Earth’s water) of water found in oceans, species diversity, surprisingly, appears to be far lower in the ocean/sea, with about 250,000 known species, than on land, which has between 1.4 to 1.7 million known species).
This, probably, is because dispersal is more wide-ranging in water, with broader geographic range, and, consequently, lower rates of speciation of many species, compared to terrestrial environments.
It implies that marine environments are less variable, physically, with time and space, than terrestrial environments. Nevertheless, the diversity of major lineages(phyla and classes), is much greater in the sea, than on land, thus proving the oceans to be the cradle of life.
All except two, of the 34 currently recognized animal phyla, occur in the marine ecosystem, where 16 are exclusively marine, and 16 both marine and fresh water.
It must be mentioned, that marine organic sampling may be of Benthic, or Pelagic organisms. Most of the exclusively marine animal phyla are relatively obscure, having a few species; that is, asides the Echinodermata phylum, with 7,000 described species.
Some other major animal phyla, such as the Cnidarians, the Sponges, as well as the brown and red algae, are largely marine, with only a small population of non-marine representation for each.
Due to the difficulty, of accessing most of the marine biosphere, the marine life remains far less well documented, than terrestrial biodiversity.
In the past few decades, knowledge of marine biodiversity has increased enormously with increase in research, although knowledge of most of the sea is based on remote sensing and sampling techniques, which remain limited and less precise.
The sampling techniques for Pelagic and Benthonic forms would herein be regarded separately.
1). PELAGIC BIODIVERSITY
The Pelagic marine organisms include the planktons and Nektons. Planktons numerically dominate the surface waters of oceans. These are drifting, or weakly-swimming organisms, which may be microscopic animals, protists, and bacteria.
The free-swimming pelagic organisms are called Nektons. Both planktons and nektons, tend to concentrate along major circulation currents, and upwelling regions, causing local variations in abundance and diversity. Vertical variations; in light, temperature, pressure, salinity, and nutrient availability, result in vertical zonation of pelagic species.
Nektons comprise the large pelagic organisms which can move independently of water currents. They include mostly fish, as well as some crustaceans (shrimps and crabs), cephalopods (e.g. squids), marine turtles and marine mammals.
SAMPLING OF PELAGIC ORGANISMS (Planktonic and Nektonic)
Planktons range vastly in size, and are passively floating, or drifting, motile organisms; comprising of bacteria, protists, tiny algae, and small animals. They may be autotrophic (producers), called phytoplakton, or heterotrophic (consumers), called zooplankton. Many are both producers and consumers, and strongly account for primary production in the marine ecosystem.
Their assemblages are affected adversely by physical and chemical factors, and are abundant in the photic zone, though present at all depths. The diversity of planktons is low, in comparison to their high productivity and biomass.
Only 3,700 species of holoplankton zooplankton exist (holoplanktonic zooplanktons, are animals which remain planktonic all their lives, while meroplankton are temporarily planktonic, at specific stages of their development.
The primary means of deriving samples of plankton and nekton, is the use of towed nets, which vary in shape and mesh size. They are basically conical in shape, with a wide-mouth opening, attached to a metal ring and a narrow tapered end fastened to a collecting jar called the “cod end”.
The net can be towed vertically, horizontally or obliquely, through the desired sampling depth, to collect organisms filtered through the water.
Another instrument called the zooplankton pump, can be used. This pulls water from a selected depth, and passes it through a mesh, collecting small planktonic species. Materials collected in nets, are water washed from the mesh, and the organisms, are sorted by hand; for microscopic identification.
2). BENTHIC/DEEP SEA BIODIVERSITY
Obtaining representative benthic samples for assessing biomass and species diversity in the deep seas, is difficult , due to the patchy distribution, of deep sea benthic organisms. However, quantitative sampling of benthic forms, shows diversity of species on the ocean floor. There is also environmental diversity in the form of several microhabitats.
Ocean floor (deep sea) benthic sampling is costly and time consuming. Comprehensive surveys have however utilized trawls, bottom sledges, dredges, grabs, box samplers, and corers.
SAMPLING OF BENTHIC ORGANISMS
TRAWLS, are essentially large nets, similar to fishing nets, which can be used to take a wide variety of forms. Trawls may be applied for pelagic, as well as benthic, sampling.
They are however disadvantageous in that many organisms are capable of avoiding capture in trawls, and others can be easily damaged or destroyed (especially soft-bodied forms like jellyfish) in the process of retrieval from depth. Therefore, trawls are used often in combination with other research tools such as acoustics and layer imaging equipment.
Trawls for benthic organisms, are generally called Bottom trawls. They are designed for sampling at the surface of the sea bed, and are appropriate for collecting large and motile species.
The trawl is essentially composed of a mechanism for keeping the mouth of the net open in horizontal and vertical dimensions; a body of net which guides the organism inwards; and a cod end, where the organism is collected.
Dredges are simply instruments used for retrieving samples from the sea bed, by scooping or dragging. Types of dredges for obtaining samples of benthic fauna include the Deep Sea Anchor dredge, and Anchor-box dredge.
Dredges are usually applied when the strata is too hard, or uneven, for the use of Trawls. The Anchor dredges are pulled by a boat or ship, and are operated on a cable with a winch. The dredge digs into the ocean floor and brings the samples to the surface. A net is usually attached to the dredge for trapping the samples.
Sledges (also called benthic sledges) are similar to bottom trawls. They use nets to collect organisms, while they are towed across the sea floor.
Sledges are equally disadvantageous, due to their destructive sampling, and relative inefficiency. They are therefore mostly applied in the beginning stage of a submarine monitoring program, to derive basic preliminary data.
Box Samplers are also known as ‘Box corers’, which are used in sampling of both benthic fauna and soft sediments in oceans. It is usually deployed from a research vessel with a wire, and it may retrieve relatively undisturbed ocean floor samples.
The core is in the form of a box of area 2,500cm2 , and penetration depth of 0.5m, which is lowered vertically into the ocean until it impacts with the sea bed. The penetration into the ocean floor/sea bed, is enabled by the stem which has a weight of up to 800kg. It is effective for benthic micro and macro-fauna investigations.
Other coring instruments/techniques for ocean floor fauna sampling include gravity corer and multiple corer techniques; both of which are discussed in relation to ocean floor sediment sampling.
SAMPLING OF MARINE SEDIMENTS:
Marine sediments recovered from ocean basin depths, and from marginal seas, are valuable informative sources/evidence, used to reconstruct global paleoclimate, and changes in ocean floor properties, from as far back as the Mesozoic(150m.y.a).
The in-depth survey, of the ocean floor, began primarily during the 14th century renaissance period, in Western Europe, with extensive marine exploration. In 1872, the British corvette; H.M.S Challenger, which was the first fully equipped oceanographic research vessel, was commissioned, to explore the world’s oceans. The crew took depth soundings, by lowering a lead weight over the side, with a hemp rope. Water samples, and temperature readings, were taken.
They also dredged bottom sediments, for evidence of marine life on the deep sea floor. The nearly four-year-long exploration, led to the discovery of a number of significant deep sea and Benthic organisms, valuable minerals like copper, zinc, gold, silver, manganese, and sulfide ores, as well as important features such as the Mariana Trench(approximately 7miles below sea level), and others, provided much of the evidence for the continental drift hypothesis of the 20th century.
(It must be noted, that the H.M.S Challenger, during it’s exploration, sounded every ocean except the Arctic).
TECHNIQUES/METHODS OF OCEAN FLOOR SEDIMENT SAMPLING
Advances in technology from the year 1930, allowed first-hand exploration of the oceans to be carried out. The ocean covers about 70 percent of the Earth surface, to an average depth, of over 2 miles.
It is shallowest in the Atlantic basin, and deepest in the Pacific basin. One of the early methods of ocean floor sampling, was the use of a Dredge, which was dragged behind a ship, to scoop sediments from the ocean bottom; or, the use of a Snapper; a device whose jaws close automatically when it hits the bottom.
These devices, were however, handicapped; in that they sampled only topmost layers, which were not retrieved on their original order of deposition. Swedish scientists, in the 1940s, developed a Piston corer. This device retrieved an intact vertical section, when dropped to the seabed.
The piston corer consisted of a long barrow, which plunged into the bottom sediments under it’s own weight, and a piston, which fired upwards from the lower end of the barrow, generating forceful pressure to pull sediments up into the pipe, which were then brought to the surface.
Mapping of the seabed and detection of the Ocean floor geologic structures, was carried out also, using ocean-bottom seismographs, and sonar depth ranging(‘sonar’ involves the use of sound waves, incident upon the seabed, in surface evaluation and ranging).
In modern marine sediment sampling, a prerequisite for successful and appropriate site selection for sampling, is a preliminary site survey, of the target area. Acoustic survey using sonar systems, is effective in generating High resolution, 3-dimensional images, of the ocean floor. Multi-beam sonar systems, are used to accurately map the topography of an area five-and-half times, in width; of the depth below the ship’s track.
Echo-sounding systems such as Parasound, provide information on the sediments layering depositional pattern, for as deep as 200m below the seafloor, useful for sediment coring.
Also, seismic surveys are required, to derive information on the nature of ocean floor sediments at sites, before deep-drilling, to avoid accidentally drilling into sediments saturated with explosive and polluting hydrocarbons.
The techniques of Ocean floor/Marine sampling, can broadly be grouped into two, on the basis of their operation, which are;
* The Drilling techniques
*The Non-drilling techniques.
DRILLING TECHNIQUES
Such groups as the Integrated Ocean Drilling Program (I.O.D.P), launched in 2003; the ‘Deep Sea Drilling Project (D.S.D.P),(1968-1983), and the Ocean Drilling Program (O.D.P) (1985-2003), have been involved intensively, in the drilling of over 3300 ocean basin sites.
The I.O.D.P has recovered, in it’s frame, the longest and oldest marine records; these being basically from the West Pacific, and dated to 170mya.
The coring instruments/techniques, currently utilized by the I.O.D.P include:
The HYDRAULIC PISTON CORER (HPC), and the ADVANCED PISTON CORER (APC);
The HPC/APC instruments, are non-rotating, coring tools, which provide well-preserved core samples, from unconsolidated sediments.
The HPC, was first utilized by the D.S.D.P, in December 1978. It operates by a hydraulic piston principle. When fluid (from the ocean) is pumped into the drill pipe of the device, it activates the piston, which in turn drives the core barrel into the sediment, at a rate of 20 feet per second.
The core barrel assembly, is retrieved to the surface after each coring operation, using a wireline. The sediment cores derived, are most suitable for Paleo-oceanographic studies. The both techniques are quite similar in operation, but however are generally limited to the upper 200m of sediment.
The EXTENDED CORE BARREL (XCB);
This is yet another coring instrument/technique, which recovers a deeper range(>200m) of sediment.
The extended core barrel is a coring system which is used to recover 9.5 meter-long core samples, from soft to moderately hard formations. It is usually applied, where the formation is too hard for the hydraulic piston corer, or too soft for complete recovery using the Rotary core device.
It uses the same Bottom Hole Assembly (BHA) as the Advanced Piston Corer (APC), which consists of a drilling string which rotates to advance the hole, and a non-rotating, integral cutting shoe, which trims the core sample at the same time.
The cutting shoe is extended ahead of the drilling bit, in softer sediments, but retracts when the formation being drilled becomes harder; to reduce failures(i.e. breaking of the core samples). The cores are retrieved in an inner core barrel (the core liner), which is non-rotating.
III. The ROTARY CORE BARREL (RCB);
This instrument/technique, is the most efficient, and as well the oldest, method, used to retrieve core samples from hard rocks and sediments.
The Rotary Core Barrel is a coring system which is designed to recover samples from hard sedimentary, and igneous, basements. In it’s operation, the samples are retrieved using an inner core barrel within the Bottom Hole Assembly (BHA), which is composed of the drill bit, the outer core barrel, and inner core barrel.
The inner core barrel, is supported by bearings which prevent it from rotating with the drill bit, while it holds core samples as the bit drills deeper into the formation. The inner core barrel, which contains the core sample; can be retrieved using a wireline (cores may be up to 9.5m long).
N/B: The sediment is retrieved in plastic liners, for all three techniques. The core, is then cut into 1.5m segments, which can be suitably handled, logged, and sampled on board, and onshore.
NON-DRILLING TECHNIQUES
Non-drilling systems, may be applied in ocean floor sediment sampling. They are usually deployed, from conventional research vessels. The non-drilling techniques are often applied in combination with drilling techniques.
The fundamental set up of the non-drilling coring devices, is composed, of one or more steel tubes or boxes, attached below a lead weight. This set up, is lowered using a winch, to the seafloor, and forced by the lead weight, into the sediment, to retrieve a core.
A number of types of coring devices, under the non-drilling techniques of marine sampling, exist.
The GRAVITY CORER:
This is the simplest core design, and consists of a steel tube, up to 20m long, attached below a lead weight of 1-2 tons. It is a simple, reliable, and relatively inexpensive, technique/instrument, for sampling sediments from the seafloor.
It derives its name, from the fact that the instrument operates under the force of gravity. The system consists of a metal pipe, lined internally with a removable plastic tubing of about 2-3 feet in length.
The Gravity corer is usually driven into the sediment by a heavy weight, which sits atop the pipes. The core device is lowered, using a winch, into the water, and allowed to fall freely down to the seafloor. The sediments are trapped in the tubing, and the corer is brought back to the surface.
The PISTON CORER:
Invented by Kullenberg in 1947, the piston corer, is one of the most developed, and widely applied marine coring devices.
The instrument possesses a trigger arm which causes a piston to be lifted up forcefully when the coring tube penetrates the seafloor. This operation reduces the friction within the coring tube, which allows long cores to be collected.
The KASTEN CORER is another coring device, simpler than the piston corer, and which also operates by gravity, to penetrate sediments. It consists of long, rectangular boxes, with up to 30cm edge-length. This instrument, is advantageous, due to the large volume of sediment that can be sampled.
The MULTICORER; is used to retrieve several samples of individual cores(may be up to a dozen core samples), each up to 50cm-long.
It is a sampling instrument, which is composed of a series of assembled core barrels. This instrument, is lowered into the ocean using a wireline(cables).
It has a weight attached to it’s top, which pushes the cores into the sediments as it meets the seafloor. Each of the inner tubes of the cores, contains a core sample when pulled up to the surface. This technique, is employed to retrieve undisturbed samples, from the seafloor.
SUMMARY
The evolution of ocean basins with time, has been identified, as an important concept of geologic study, as it bears relationship, to several internal and external Earth processes of interest (such as volcanism and Plate tectonics).
This study is conveniently carried out by the analysis of sediments and organic assemblages found in these ocean basins.
Studying the occurrence, and distribution of biogenic life on the ocean floor, is crucial in the understanding of organic evolution, as well as processes of ocean basin evolution, spatial differences in temperature, salinity and other important physical factors.
The floors of ocean basins, are covered by sediments of various compositions and origins (this excludes oceanic spreading-centers, where continuous generation of new oceanic crust, mitigates sediment accumulation).
The thickness of ocean floor sediments generally increases with the age of the oceanic crust in that region, and may average from approximately 500 meters, in the Pacific basin, to 1000 meters in the Atlantic.
These sediments give insight on many past processes of Earth’s geologic history; including plate tectonic movements, Sea floor spreading, Paleontological evolution, Paleoclimate, and Paleocurrent trends. Several techniques, such as are discussed briefly in this paper; have been employed for ocean floor sediment retrieval and sampling.
Mapping of ocean floor sediment thickness/topography(which is not our primary interest here) is also carried out, by such means as Sonar Ranging and Seismic Reflection, amongst others.
The equipment, spoken of; which are used by researchers as the I.O.D.P and O.D.P, are subject to subsequent modification, according to requirement, and with advancement in technology.
REFERENCES
* Ecickson Jon, 1948; Marine Geology: Exploring the New Frontiers of the Ocean. 32, 34, 41-42.
* Centre for Past Climatic Studies, Department of Geoscience, Aarhus University, Denmark.
*Ocean Drilling Program(ODP): Articles; Extended Core Barrel System; Advanced Core Barrel(2013); Rotary Core Barrel.
*Flanders Marine Institute of Research (2011): Multi-Corer.
