Sediment Study Tools and Techniques Techniques and tools to study ocean sediments include:  Clamshell sampler – collects a large sample of the top sediment.  Piston corer – used to collect different sediment layers as deep as 25 meters (82 feet) into the ocean bottom.  Specialized vessels with drilling equipment – used to produce sediment core samples of 500 meters (1,640 feet) long.  Seismic tools – transmit sounds that travel through water and into sediment. Different sediment layers create distinct echoes. Used when looking for oil or natural gas.

Stratigraphy and Paleoceanography Scientists estimate there are no sediments on the ocean crust dated older than 200 million years ago. Study of sediment layers is called stratigraphy.  Scientists use deep-sea stratigraphy to look for clues, such as rock composition, microfossils, deposition patterns and other physical properties.  Based on these they can estimate the age of the sediment layers and draw conclusions about the past.  Ocean scientists use stratigraphy to understand changes in the ocean and atmosphere; previous circulation patterns, former sea levels, and trends in biological productivity. A new science called paleoceanography is the study of prehistoric oceans.  From sediment they have been able to estimate prehistoric ocean temperatures and climatic conditions with accurate precision. Ongoing research of the Earth’s ancient climate currently emphasizes deep ocean sediments called siliceous oozes. Oceanographers have played a key role in helping develop current theories that propose that Mars once has oceans.

Sediment Origins Sediments may be classified by origin based on four categories: 1.Lithogenous sediments- Come primarily from land erosion carrying particles into the sea or from volcanic eruptions. These are the majority of sediments.  Quartz and clay are the two most common materials along with feldspar. Volcanic islands contribute sediments of basalt and other volcanic materials. 2.Biogenous sediments- Originate from organisms and cover a large area of seafloor.  Silica and calcium carbonate are the materials that come from shells and hard skeletons of planktonic organisms. 3.Hydrogenous sediments- Result from chemical reactions within seawater accounting for less than 1% of the seafloor sediments.  Minerals come out of solution and form particles that settle on the bottom.  Produces ferromanganese and phosphorite nodules.  Sources of dissolved minerals vary – submerged rock, new crust formation, hydrothermal vent water, river runoff. 4.Cosmogenous sediments- Come from outer space consisting of cosmic dust and occasional impacts from asteroids and comets.  They settle through the air as the others do through water. About 15,000 – 30,000 metric tons of space dust settle on the Earth each year; least abundant of the sediments.

Sediment Sizes Sediments are classified on grain size – diameter of the particle. Grain size and current velocity affect the deposition and erosion of sediment.  Smallest and largest particles behave similarly with respect to transportation and erosion.  Sand in the middle of the graph takes the least amount of energy to erode.  Larger particles require energy to erode because they’re heavy. It takes a stronger current to lift them off the bottom.  Particles smaller than sand also take more energy to erode. Smaller particles (especially clay) tend to be cohesive.

Sedimentation Processes on the Continental Shelf Tides, waves, and currents strongly affect continental-shelf sedimentation.  Shoreline turbulence: waves are one of the most notable influences because it keeps particles from settling. Surf and waves carry small particles out to sea. Their affect diminishes further from shore traveled. Recent and Relict Sediments Recent sediments have accumulated since the sea level stabilized. Relict sediments accumulated and were left stranded when the sea level was lower. Overall, sedimentation on the shelf is more rapid than in the deep ocean.

Continental-Shelf Sedimentation Rates The sedimentation rate on the continental shelf varies with region. Sedimentation on the shelf is more rapid than in the deep ocean.  At the mouths of large rivers, sedimentation can occur at a rate of one meter per thousand years, but there’s a lot of variation. Continental –shelf sedimentation processes also affect the adjoining deep ocean.  Accumulating sediment on the continental shelf avalanches down the continental slopes. These are called turbidity currents and can carry sediment deposits all the way to the abyssal plain. These deposits are called turbidites.  Turbidites consist of layers of lithogenous sand embedded with the more typical, fine deep-sea sediments. The continental shelves undergo processes that produce biogenous sediments, which also affect the sedimentation rate.  Continental-shelf sediments tend to have a mix of both biogenous and lithogenous materials.

Sedimentation Processes on the Deep Ocean Bottom Like the processes that affect the continental shelf, sedimentation processes in the deep ocean vary regionally. Deep-ocean sediments tend to be high in biogenous material. Lithogenous sediments, except for clays, are generally confined near shore. Biogenetic sediments – primarily the remnants of plankton – dominate the sediments off shore waters. Because of its very small grain size, clay can remain suspended in the water for great distances and be carried by wind, allowing it to deposit in the deep sea. The variation in deep-water sedimentation causes tremendous variations in sediment accumulation. The thickness of sediments in the deep ocean also varies with topography.  Sediments are thickest on the abyssal plains and thinnest or absent on the mid-ocean ridges and seamounts.

The Carbonate Compensation Depth The carbonate compensation depth is a point at which calcium carbonate dissolves just as fast as it accumulates from above.  Above this depth calcaneous ooze dominates. Siliceous ooze dominates sediments below this siliceous remains and high diatom productivity.  The carbonate compensation depth varies with region due to temperature and water density.  In the Atlantic and Pacific, it is around 4,500 meters(14,750 feet).  In colder regions, the carbonate compensation depth is much shallower so siliceous oozes dominate biogenous sediments in polar regions.  The slow dissolution of siliceous remains and high diatom productivity allow siliceous oozes to accumulate throughout the seafloor.  Siliceous ooze are the dominant biogenous sediments below the calcium carbonate compensation depth.

Fecal Pellets Scientists find that bottom composition is usually similar to the particle composition of the water above it. This is due to fecal pellets. Large planktonic organisms, like copepods, consume the calcaneous or silicone organisms that also dominate the bottom ooze. These large organisms eliminate their waste as dense fecal pellets of multiple skeletal and shell remains compressed together. These dense pellets sink quickly and the decomposition process begins. Mineral Nodules Ferromanganese nodules consist of iron manganese found over as much as 50% of the deep Pacific floor. Phosphorite nodules consist of phosphorite and other trace minerals found on the shallow banks and continental shelves off California, Argentina and Japan.  Both forms of nodules are thought to be hydrogenous sediments produced by one of the slowest chemical reactions in nature.  Nodules grow at a rate of about 1 to 200 milimeters (.039 to 7.9 inches) per million years.  Scientists believe that biological processes – possibly involving bacteria – cause the chemical precipitation.

Petroleum and Natural Gas What is economic importance of ocean sediments and their study?  Oil and natural gas found under the ocean contribute $125 billion in annual revenues. More than a third of the world’s crude petroleum and a quarter of its natural gas come from sedimentary deposits on the continental shelf. Ferromanganese and phosphorite nodules have potential economic value. Other Sediments with Economic Importance Metal sulfide deposits found at deep- sea hydrothermal vents are rich and vast enough (especially in the Red Sea) that mining them could be economically feasible. Evaporites form at the surface and comprise the salts left behind when seawater evaporates. They are a source of calcium carbonate, calcium sulfate, gypsum and sodium chloride. Sand and gravel are an important resource for the construction industry accounting for $500 million yearly. Another sediment-based resource is diatomaceous earth.