How Does the Sediment Get to the Ocean Floor?
Sediment reaches the ocean floor through a complex interplay of processes, primarily involving erosion, transport, and deposition, driven by wind, water, ice, and gravity acting on terrestrial, atmospheric, and oceanic sources. This constant rain of material, ranging from microscopic clay particles to massive boulders, builds up over millennia, creating the vast and varied sedimentary landscapes that characterize the seabed.
From Source to Sink: The Journey of Marine Sediment
The story of marine sediment is a journey that begins high on mountainsides and ends deep within the abyssal plains. Understanding this journey requires appreciating the diverse origins and transportation mechanisms involved. Think of it as a planetary conveyor belt, constantly moving material from areas of high elevation and concentration to the ultimate repository: the ocean.
Terrestrial Sources: Rivers, Wind, and Ice
A significant portion of marine sediment originates on land. Rivers are arguably the most important agents, carrying vast quantities of weathered rock, soil, and organic matter eroded from continents. The suspended load, fine particles carried within the water column, and the bed load, larger particles that roll or bounce along the riverbed, are eventually discharged into the ocean.
Wind plays a crucial role in transporting fine-grained particles, particularly silt and clay, over long distances. Desert dust, volcanic ash, and pollen can be carried by winds across oceans and deposited directly onto the sea surface, where they eventually settle to the bottom. This aeolian transport is particularly important in regions downwind of major deserts, such as the Sahara.
Glaciers are powerful erosional forces. As glaciers grind across the land, they carve out valleys and transport massive amounts of rock and sediment. When glaciers reach the coast and calve icebergs, these icebergs can carry sediment far out to sea. As the icebergs melt, the sediment, known as glacial till, is released and deposited on the seabed.
Biogenic Sources: The Ocean’s Own Contribution
The ocean itself is a significant source of sediment. Biogenic sediment is derived from the remains of marine organisms. Microscopic plankton, such as diatoms (siliceous algae) and foraminifera (calcareous protozoans), live in the surface waters. When these organisms die, their shells and skeletons sink to the ocean floor, forming thick deposits of ooze. In some regions, these oozes can accumulate at rates of several centimeters per thousand years. Coral reefs also contribute significantly to sediment production. As corals erode and break down, they produce large quantities of carbonate sediment.
Volcanic and Hydrothermal Sources
Volcanic eruptions, both on land and underwater, can contribute significant amounts of ash and other volcanic debris to the ocean. Underwater volcanoes can erupt violently, spewing out vast plumes of ash and rock that settle to the seafloor. Over time, these deposits can form thick layers of volcaniclastic sediment.
Hydrothermal vents release chemically rich fluids into the ocean. These fluids can precipitate minerals, such as sulfides and oxides, which settle to the seabed, forming distinctive hydrothermal deposits. These deposits are often associated with unique and diverse ecosystems.
Sediment Transport Mechanisms
Once sediment enters the ocean, it is transported by a variety of mechanisms. Ocean currents play a crucial role in distributing sediment over wide areas. Surface currents can carry fine-grained particles over long distances, while deep-sea currents can erode and redistribute sediment on the ocean floor.
Turbidity currents, underwater avalanches of sediment-laden water, are particularly important for transporting large amounts of sediment to the deep ocean. These currents can travel at high speeds, carving out canyons and depositing thick layers of sediment on the abyssal plains.
Gravity itself plays a direct role. Sediment particles, regardless of their origin, eventually settle out of the water column due to gravity. The rate at which particles settle depends on their size, shape, and density.
FAQs: Delving Deeper into Marine Sedimentation
Here are some frequently asked questions to further illuminate the fascinating world of marine sediment:
FAQ 1: What is the difference between terrigenous and biogenic sediment?
Terrigenous sediment originates from the weathering and erosion of rocks on land, transported to the ocean by rivers, wind, glaciers, and other agents. Biogenic sediment is derived from the remains of marine organisms, such as plankton and coral.
FAQ 2: How do scientists study marine sediment?
Scientists use a variety of methods to study marine sediment, including sediment coring, where long tubes are inserted into the seabed to collect sediment samples. These cores are then analyzed in the laboratory to determine the composition, age, and origin of the sediment. Seismic reflection surveys are also used to image the subsurface layers of sediment.
FAQ 3: What is the significance of sediment size in marine environments?
Sediment size is a critical factor influencing the distribution of marine organisms. Grain size affects permeability and porosity, which in turn influences water flow and nutrient availability. Different organisms are adapted to different sediment types, leading to diverse benthic communities.
FAQ 4: How does climate change affect sediment transport to the ocean?
Climate change is expected to have a significant impact on sediment transport to the ocean. Increased precipitation can lead to increased river discharge and erosion, while melting glaciers can release large amounts of sediment into coastal waters. Changes in wind patterns can also affect aeolian transport.
FAQ 5: What are the different types of marine sediment?
Besides the already mentioned terrigenous and biogenic, other sediment types include cosmogenous sediment (derived from extraterrestrial sources, such as meteorites) and authigenic sediment (formed in situ through chemical precipitation).
FAQ 6: What is the role of marine sediment in carbon cycling?
Marine sediment plays a crucial role in the global carbon cycle. Organic carbon buried in sediment can be sequestered for millions of years, preventing it from re-entering the atmosphere. This process is an important long-term carbon sink.
FAQ 7: How does the depth of the ocean affect sediment deposition?
The depth of the ocean has a significant impact on sediment deposition. In shallow waters, wave action and currents can prevent fine-grained sediment from settling. In deep waters, where currents are weaker, fine-grained sediment can accumulate to form thick deposits. Also, the carbonate compensation depth (CCD) affects carbonate accumulation. Below the CCD, carbonate shells dissolve.
FAQ 8: What are turbidites?
Turbidites are sedimentary deposits formed by turbidity currents. They are characterized by graded bedding, where sediment grain size decreases from bottom to top. Turbidites are often found in deep-sea fans and abyssal plains.
FAQ 9: How is marine sediment used to reconstruct past climates?
Marine sediment contains a wealth of information about past climates. Fossil plankton preserved in sediment can provide information about past sea surface temperatures, salinity, and ocean circulation. The isotopic composition of sediment can also be used to reconstruct past climate conditions.
FAQ 10: What are the economic implications of marine sediment deposits?
Marine sediment deposits can contain valuable mineral resources, such as manganese nodules and polymetallic sulfides. These resources are of growing interest as land-based mineral deposits become depleted. Sand and gravel mining is also a significant industry in some coastal areas.
FAQ 11: How does pollution affect marine sediment?
Pollution can have a significant impact on marine sediment. Heavy metals, pesticides, and other pollutants can accumulate in sediment, contaminating the food chain and harming marine organisms. Plastic pollution is also increasingly prevalent in marine sediment.
FAQ 12: What are the long-term consequences of sediment accumulation in the ocean?
The long-term accumulation of sediment in the ocean has profound consequences for the Earth’s geological history. Sedimentary rocks formed from marine sediment record past environmental conditions and provide a record of the evolution of life on Earth. The weight of sediment can also cause the seabed to subside, creating new basins and altering coastlines.