Chapter 6: Ocean Sediments

slumps ad turbidity currents on the continental slopes can carry lithogenous sediments...

...from the continental shelf into trenches or out over the abyssal plain where there is no trench

sediment classification by origin

1. lithogenous (derived from land) 2. biogenous (biological processes) 3. hydrogenous (chemical precipitation from seawater) 4. cosmogenous (derived from meteorites)

tectonic history in the sediments

Because the type of seafloor sediment differs depending on depth, distance from the continents, and latitude, changes on a particular piece of crust can be used to reveal various aspects of the tectonic history

dissolution of biogenous particles

Calcium carbonate particles dissolve more easily at higher pressures (depths), whereas silica particles dissolve very slowly at all depths and their dissolution rate actually decreases with increasing depth

carbonate compensation depth (CCD) and the greenhouse effect

Changes in the mean CCD over time can have a major effect on the distribution of carbon dioxide between the atmosphere, oceans, and ocean sediments and so are important for greenhouse effect studies. The higher concentration of carbon dioxide in surface ocean waters that has resulted from human use of fossil fuels will eventually reach the deep waters and may increase pH sufficiently to significantly reduce the CCD. This would cause more calcium carbonate to dissolve and the excess carbon dioxide could be released to the atmosphere when the water returns to the surface, providing a positive feedback to the enhanced greenhouse effect.

lithogenous sediment transport by winds

Dust particles can be carried very long distances through the atmosphere. Dust particles in the atmosphere are eventually deposited on the ocean surface and sink slowly to the seafloor.

volcanoes and transport of sediment by wind

Explosive volcanic eruptions of subduction zone volcanoes can inject very large quantities of fragmented rock into the atmosphere. Historical records suggest that large eruptions such as the one that created Long Valley caldera in California can inject hundreds of cubic kilometers of pulverized rock into the atmosphere. The largest particles injected into the atmosphere by volcanic eruptions rain out near the eruption site, but smaller particles can stay in the atmosphere for years and are distributed around the world before they eventually rain out onto the land or ocean

hydrogenous sediments

Hydrogenous sediments are precipitated from seawater predominantly as manganese and phosphorite nodules in certain areas near hydrothermal vents and in certain shallow tropical areas where conditions permit calcium carbonate to precipitate.

undersea volcano emissions

Hydrothermal vents have recently been found to exist on the flanks of hot-spot and magmatic-arc volcanoes where, in some instances, they discharge their fluids at much shallower depths (even sometimes within the photic zone) than at the oceanic ridges

calcareous sediments

In areas where the seafloor is shallower than the CCD and the inputs of lithogenous material are low, calcareous particles accumulate fast enough to be a major part of the sediments. These areas include oceanic plateaus, seamounts, and the flanks of the oceanic ridges

regional variations of biogenous particle production

In high latitudes diatoms are the dominant photosynthetic organisms. They have siliceous hard parts and they dominate the inputs of biogenous material to the sediments. At lower latitudes, many of the dominant photosynthetic organisms have no hard parts so inputs of biogenous material to sediments are limited except in certain regions where coccolithophores grow in abundance. Coccolithophores have calcium carbonate hard parts that can become the dominant particle in sediments beneath areas of high coccolithophore productivity if the water depth is not great enough so that the calcium carbonate is dissolved Some zooplankton, small free-floating animals that eat phytoplankton, also have hard parts that can contribute to, or dominate, sediments in some areas. Foraminifera and pteropods have calcareous hard parts, whereas radiolaria have silica hard parts. Radiolaria are abundant in those tropical waters that have high productivity and can dominate sediments in these areas.

evaporites

In marginal seas with arid climates, evaporation may increase salinity so high that salts precipitate progressively from the seawater—first calcium and magnesium carbonate, then calcium sulfate, and finally sodium chloride

siliceous red clay sediments

In the deep basins of the North and South Pacific, South Atlantic, and southern Indian Oceans there are transitional areas where sediments grade progressively between deep sea clays and diatom oozes

sediment transport, deposition, and accumulation

Large particles sink more rapidly and need higher current speeds to resuspend them than smaller particles do. Large particles are not transported far by ocean currents, but smaller particles can be carried long distances.Large particles tend to collect in sediments of the continental shelf, whereas sediments accumulating far from land are generally very fine-grained and less likely to be dominated by lithogenous particles

lithogenous sediment transport by rivers

Larger particles are generally deposited in river valleys and only small particles are carried out to the oceans. Approximately 90% of all lithogenous sediments reach the oceans through rivers and 80% of this input is derived from Asia. The largest amounts are carried by four rivers: the Ganges, Brahmaputra, and the Irrawaddy empty into the Bay of Bengal, and the Indus discharges into the Arabian Gulf. In the area where these rivers discharge to the oceans extensive deep sediment deposits extend to the deep sea floor in the form of abyssal fans. Most of the other rivers that transport large quantities of suspended sediments to the oceans empty into marginal seas.

accumulation rates

Sediment accumulation rates are high near continents and much lower in the deep oceans far from land. All sediments are mixtures of particles from different origins. The composition of sediment at a given location is determined by the relative rates of accumulation of each type of material at that location. Highest rates of accumulation are in nearshore areas, average rates are on the continental shelves and in marginal seas, and very low rates are in the deep oceans remote from land

ice-rafted sediments

Sediments carried to the oceans predominantly by glaciers accumulate in some areas of the Arctic Ocean, Bering Sea, and around Antarctica. They can contain pebbles and even larger particles, as icebergs originating from glaciers can carry these ice- rafted sediment particles far from land.

sediments and biogeochemical cycles

Sediments cover most of the seafloor and are important as sinks and sources in the biogeochemical cycles of elements in seawater

hydrothermal sediments

The central basin of the Red Sea is the only area where hydrothermal sediments are known to dominate. However, small areas of hydrothermal sediments also are found around hydrothermal vents.

distribution of surface sediments

The distribution of sediment types in surface sediments (currently accumulating material) reflects the proximity of lithogenous sediment sources, the productivity of the overlying waters and the type of organisms that are abundant, the seafloor depth, and the depth of the CCD

how are fine wind-blown dust particles dispersed?

They fall over all parts of the oceans at a relatively uniform rate. Although this rate is slow, atmospheric dust accumulate continuously and is a major component of sediments that are remote from land and have very slow accumulation rates of other types of particles

lithogenous sediment erosion by waves

Waves continuously erode coastlines. Sediment particles released to the oceans by coastal wave erosion are similar to riverborne suspended sediments but often have a larger proportion of unweathered mineral grains. Wave erosion creates particles of all sizes. Wave action then sorts the particles, transporting small ones offshore and leaving larger ones on or close to the shore.

two types of calcium carbonate hard parts

calcite and aragonite. Aragonite dissolves more easily than calcite

lithogenous sediment transport by landslides

can carry lithogenous materials of a wide range of particle sizes into the oceans, where they are incorporated in the sediments

phosphorite nodules

contain up to 30% phosphorus form in limited areas of the continental slope and some seamounts; grow very slowly; formation required low dissolved oxygen concentrations in the overlying bottom water and a large supply of phosphorous carried to the sediments by sinking detritus as a result of high productivity in the overlying surface waters. They grow only by accumulation on their underside, where phosphorous is released by decomposition of detritus in the sediment

manganese nodules

dark brown, rounded lumps of rock, often larger than a potato, that are found lying on the abyssal ocean floor in high abundance in some locations. form by precipitation of minerals from seawater and are usually formed initially around a large sediment particle such as a shark tooth. The minerals build in concentric layers around the nodule in a manner similar to tree rings, and occasional disturbance by marine organisms is thought to be necessary for the nodules to grow on all sides and in order that they not be buried by new sediments

cosmogenous sediments

derived from meteors and meteorites and are relatively rare, although they may amount to tens of thousands of tonnes per year spread over the entire oceans

lithogenous sediment erosion by glaciers

erode very large amounts of rock from heir valleys, producing particles that vary greatly in size. High-latitude glaciers empty directly into the oceans or end near where the glacial valley meets the sea. Icebergs that break off glaciers can carry their eroded rock long distances into the oceans to be released as the iceberg melts. Glaciers release their eroded rock at their lower ends. Much of this material then washes out to the oceans, especially the very fine-grained material called glacial flour (because it remains suspended for long periods and can give the water a milky appearance in lakes or fjords where the glacier empties)

relict sediments

found on the seafloor in areas where no new sediment accumulates on the continental shelves. They are sediments laid down when sea level was lower so that the deposition area was coastal and only covered by shallow water. Shells and other marine organism remains in relict sediment are often remains of species

saturation of calcium carbonate in the upper layer of seawater

generally saturated or supersaturated, so calcareous material does not dissolve. However, calcium carbonate solubility increases with increasing pressure (depth) and decreasing temperature. Because deep water in the oceans is cold, the calcium carbonate dissolution rate increases substantially with increasing depth

how are sediments classified?

grain size and origin

what drives hydrothermal vents that discharge hot water?

heat flowing up through thin mantle, especially at the oceanic ridges, thought to be because of the convection of water heated within the rocks and sediments and replacement of this water by movement of seawater into the rocks and sediments from areas surrounding the vents

where do biogenous particles dominate sediments?

in areas where the productivity is high in the overlying water, but not where the hard parts are dissolved before they can accumulate in the sediments

sediment classification by grain size

in descending order of predominant grain size: gravel, sand, or mud (silt and clay)

two types of cosmogenous sediments

iron-rich and silicate-rich; derived from different types of meteorite

how does increased carbon dioxide affect pH? how does this affect the solubility of calcium carbonate?

it lowers pH (makes it more acidic) and thus increases the solubility of calcium carbonate

carbonates

limestone rocks. calcium carbonate precipitated directly from seawater. both high temperature and high productivity appear to be necessary for calcium carbonate precipitation to occur, as these conditions cause the pH to rise. form rounded grains called ooliths

phytoplankton

microscopic organism that performs photosynthesis; almost all ocean life depends on this

describe the temperature and composition of the water discharged by hydrothermal vents in most locations (because it varies)

most vent plumes have no oxygen, substantial sulfide concentrations, and high concentrations of iron, manganese, and other metals (including copper, cobalt, lead, nickel, silver, zinc) that have soluble sulfides

composition of manganese nodules

mostly of manganese oxide and iron oxide but also have high concentrations of other metals, including copper, nickel and zinc. The source of these minerals is not known but may be particles transported from hydrothermal vents

orbital velocities

much higher in waves than ocean current speeds. waves resuspend sand-sized particles and move them long distances along the coast. However, when these particles are carried offshore to deeper water where wave motion does not extend to the seafloor, the particles are deposited and only the smallest particles are transported further by ocean currents

where are hydrothermal vents?

on oceanic ridges

lithogenous sediments

particles are produced by wind and water erosion and weathering of terrestrial rocks. During weathering, easily dissolved minerals are removed, leaving mostly siliceous minerals including quartz, feldspars, and clay minerals. Clay minerals are layered structures of silicon, aluminum, and oxygen atoms, some containing iron and other elements. They are carried to the oceans by rivers, glaciers, and winds, or eroded from coastlines by waves

diagenesis

physical and chemical changes that occur in sediments over time as they are progressively buried; important because it can recycle nutrients from detritus in the sediments back to the water column

why do most ocean sediments contain a little organic matter?

phytoplankton are consumed by zooplankton (and other larger marine species) which poop. the poop is decomposed by bacteria and other organisms, or is consumed by detritus feeders as it sinks through the water column or after it has been deposited on the seafloor

turbidite layer

sediment grain size decreases upward toward the surface; finer-grained sediment layers of locally deposited material appear above and below turbidite layers

what predominantly composes biogenous particles?

solution-resistant silica or calcium carbonate hard parts of microscopic phytoplankton and zooplankton

turbidity currents

swift-moving slumps of sediment similar to avalanches. They can carry coarse grain-sized sediments to and across the deep ocean floor adjacent to the continental shelves. They leave layers of sediment called turbidite layers

carbonate compensation depth, CCD

the depth below which all calcium hard parts are dissolved before they can accumulate in sediments; varies by location, between oceans, and over time

what affects the accumulation rate of biogenous particles in sediments?

the rate of production of the particles in the overlying water column, and the rate of decomposition or dissolution of the particles

what happens to metal sulfides once they are discharged from hydrothermal vents?

they are oxidized and precipitate as a rain of fine particles of their hydrous oxides. Some particles sink to the seafloor to form metal-rich sediments in areas surrounding the vents and others are transported and deposited far from the vent to contribute to sediments elsewhere

why are hydrothermal vents important?

they support communities of unique organisms that depend on the chemosynthetic primary production by microbial organisms in the vent water as the ultimate source of their food