Continental-Shelf Sediments Deep-Ocean Sediments Choose to view chapter section with a click on the section heading. The Study of Sediments Types of Sediment Continental-Shelf Sediments Deep-Ocean Sediments Sediments as Economic Resources Chapter Topic Menu

Sediment Study Tools and Techniques The Study of Sediments Chapter 12 Pages 12-3 to 12-6

Stratigraphy and Paleoceanography Scientists estimate there are no sediments on the ocean crust dated older than 200 million years. – Why???????? 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. Siliceous ooze is a soft siliceous pelagic sediment that covers large areas of the deep ocean floor. Siliceous oozes consist predominately of either diatoms or radiolarians The Study of Sediments Chapter 12 Pages 12-6 to 12-9

Sediment Origins Types of Sediment Chapter 12 Pages 12-10 to 12-13

Sediment Sizes Sediments are classified on grain size – the diameter of the particle. Grain size and current velocity affect the deposition and erosion of sediment. Sediment is transported based on the strength of the flow that carries it and its own size, volume, density, and shape. Stronger flows will increase the lift and drag on the particle, causing it to rise, while larger or denser particles will be more likely to fall through the flow. Types of Sediment Chapter 12 Pages 12-13 to 12-15

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 Sediments Chapter 12 Pages 12-16 & 12-17

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. Continental-Shelf Sediments Chapter 12 Pages 12-17 to 12-19

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. Deep-Ocean Sediments Chapter 12 Pages 12-20 & 12-21

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 calcareous ooze dominates. Siliceous ooze dominates sediments below this depth due to the slow deep-sea dissolution of 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. Deep-Ocean Sediments Chapter 12 Pages 12-22 & 12-23

Fecal Pellets Mineral Nodules 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 calcareous 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 and 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 millimeters (.039 to 7.9 inches) per million years. Scientists believe that biological processes – possibly involving bacteria – cause the chemical precipitation. Deep-Ocean Sediments Chapter 12 Pages 12-23 & 12-24

Petroleum and Natural Gas What is the 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. Sediments as Economic Resources Chapter 12 Pages 12-25 to 12-27