Australia: The Land Where Time Began

A biography of the Australian continent 

Ocean Floor Sediment

Sedimentary material from the land comprises most of the sediments deposited on the continental shelf and slope, both from rivers entering the sea and wind-blown dust. The sediment deposited on the deep ocean floor is often  more fine-grained than that deposited on the shelf and slope, and most of it is of pelagic origin deriving from life in the upper layers, 2 major sediment types in the deep ocean being "red" clay, being mainly mineral and containing less than 30% biogenic material,  and "oozes" of biogenic origin. It is comprised of fine material that has often great distances suspended in the air and volcanic material as well as the remains of meteorites. The oozes are composed of more than 30 % biogenic originating in the plankton in the surface layers. The calcium carbonate shells of zooplankton make up a high percentage in calcareous oozes. The zooplankton and phytoplankton that form siliceous shells contribute the majority of material forming the biogenic oozes. The ocean floor of the Southern Ocean and the equatorial Pacific Ocean are the main areas where there are siliceous oozes, the calcareous oozes being commonly found in areas of low nutrient levels of the surface waters, and the siliceous oozes occur mainly in areas of high nutrient content.

The average rate at which sediment is deposited on the ocean floor is 0.1-10 mm/1,000 years, apart from loads deposited by turbidity currents that occurs over a much shorter time span, and the deposits store large amounts of the history of the oceans. Corers, that are steel pipes with a heavy weight on the upper end that are forced into the sediment. The resulting cores that are retrieved from the pipes record the deposition on the sediment at the rate of 1,000 to 10 million years per metre of core. Sometimes the stages of sedimentation of different materials are indicated by the layering of the material. Layers of volcanic ash have been found in some places that correlate with historical eruptions, while in other places there were the remains of organisms that are characteristic of warm or cold waters have been found indicating by being present in different layers that indicate the temperature of the overlying surface waters had changed over the period covered by the core. The gradation from coarse to fine sediments, the coarser at the bottom, suggest the presence of turbidity currents, the coarser material brought to the region by such currents settling out first and progressively finer sediment towards the top of the sequence.

A sloping, smooth ocean floor can stretch for thousands of kilometres from the mouth of rivers bringing sediment that forms large deposits, a deep-sea sediment fan. The largest such fan occurs in the northeastern Indian Ocean, the Bengal Fan, having been formed by many rivers that include the Ganges and Brahmaputra. Other such fans are present art the mouths of rivers such as the Yangtze, Amazon and Columbia Rivers.

Water movements at the ocean floor is traced with the help of sediments. Where ripples are present on the floor of the deep sea they indicate the water above them is moving at high speed, as occurs in the backwash from waves on beaches, indicating that such places there were high-speed bottom currents.

The properties of sea water can be altered by contact with sediments, as when silicate and carbonate is dissolved  by the water in contact with them. Biological remineralisation of organic carbon, mostly in faecal pellets, to produce inorganic CO₂ in the sediment by the use of oxygen. The composition of the seawater is affected when the pore water, that is enriched in CO₂ and depleted in oxygen,  from that sediment is released to the seawater A source of inorganic nutrients is produced by the remineralisation of phosphorous and nitrogen in the sediment. Once the sediment becomes anoxic methane is produced by the action of the bacteria, that is often stored as methane hydrate, a solid form, It has been estimated that about 10¹⁹ g of methane hydrate have been formed over the history of life on Earth. When they spontaneously change from solid to gas and when this occurs they can cause submarine landslides, as well as releasing methane into the water which changes its chemistry.

Oceanography