This section is divided into 3 parts; Marine Sediments,
Gas Hydrates, Marine
Sediments II
Authigenic
(sometimes called hydrogenous) depositsAuthigenic deposits are precipitated from seawater as a result of chemical reactions, (some of which may be assisted by bacteria). They include ferromanganese nodules (often called manganese nodules) phosphorites, glauconites and evaporites.
Managanese nodules form in the deep ocean and are particularly common in the Pacific where they are estimated to cover 30-50% of the sea floor. They are made up predominantly of manganese oxide (MnO2) and iron oxide (Fe2O3) - average contents of 30% manganese and 20% iron. They are dark brown in colour, slightly flattened rough spheres, 5 to 10 cm in diameter and are generally found in water depths of 4.000 to 6.000 meters. In cross section the nodules show concentric layers, or growth rings around a core – like tree rings. The core can be a fragment of anything, a bit of basalt, skeletal material etc. The growth rate of the nodules is very slow – nodules in the Pacific Ocean are estimated to be 2 to 3 millions years old.
Nobody
is quite certain about how manganese nodules form, but it seems likely that
bacteria are involved. It is thought that the major sources of manganese in
seawater are leaching of sea floor basalts and of hydrothermal activity along
mid-ocean ridges.
>Manganese nodules on the seafloor. It has often
been proposed that manganese nodules could be mined from the deep ocean (eg
off the Cook Islands), however there is currently no shortage of easily mined
manganese on land so it seen unlikely that such an undertaking would be economically
viable in the near future.
> Cross section of a manganese nodule showing
the concentric growth rings.
< Box core showing manganese nodules. Collected as
part of the DOMES project - Deep Ocean Mining Environmental Studies carried
out by the US in response to interest in manganese mining in the ocean.
(http://www.photolib.noaa.gov/historic/c&gs/theb2827.htm)
Dissolved
phosphate (pictured left) in seawater occurs as HPO4-2 and is most abundant
at intermediate to shallow depths (500 – 1500m), corresponding with the
oxygen minimum zone. Phosphate precipitates as nodules or crust, composed principally
of carbonate phosphate (apatite). Its formation is correlated with areas of
high organic productivity. Decomposition of organic matter (mostly from shells
and bones) releases phosphate to the interstitial waters, building up the concentration
until phosphorites can form. The phosphorus concentration in phosphorites is
generally around 30%.
Glauconite is an iron rich clay mineral (from the Greek glaucos meaning blue-green, so you can guess what colour it is). It forms in shallow marine environments, maybe up to 500 m deep. It needs specific conditions where the overall environment is oxidizing but the microenvironment where the glauconite forms is reducing. So where do you find reducing conditions in an oxidising environment? Well places where organic matter is decomposing – like inside shells or fecal pellets. Large amounts of glauconite only occur where there is a low sedimentation rate. This is because the glauconites must be at the sediment water interface for a long period of time to form – in the order of 1000’s of years. This means that they can’t be continually being buried by other sediments. After they form, glauconites can be redeposited, concentrating them into deposits known as greensands.
Three things come out of volcanoes: lava, tephra and gas. We are interested
in tephra, which is all ejecta blown through the air or water by explosive volcanic
eruptions. Tephra comes in different sizes classified as -blocks, bombs, lapilli,
cinders and ash. Large-sized tephra generally falls to the close to the volcano.
Smaller fragments are carried away by the wind. Volcanic ash can travel hundreds
to thousands of kilometers downwind from a volcano.
Photo taken aboard the Endeavor Space Shuttle over Russia in 1994. The eruption
cloud from the Kliuchevskoi Volcano, shoots nearly 20 km into the atmosphere.
As the cloud of ash and gas moves away from the volcano, it loses altitude and
ash falls to the ground forming a layer of sediment.
Of interest (and with some excellent pictures) is this site the hazards presented
to aircraft by volcanic ash clouds.
http://volcanoes.usgs.gov/Hazards/Effects/Ash+Aircraft.html
Volcanic ash in deep-sea sediments may be in discrete layers or dispersed through other sediments. Size sorting by the wind may occur with distance from the source.
Eruptions into the troposphere (5-12km up) are most the most common, with residence
time of the ash measured in hours or days. Global ashfalls occur after extremely
explosive eruptions inject ash into the stratosphere. Because of wind pattern
this ash is mostly deposited in glacial and arid regions. Extremely fine-grained
ash may remain in the atmosphere for a year or more.
Sedimentation rates of volcanic ejecta range from meters per thousand years
locally to approximately 1 mm/1000 years in the deep sea. Volcanogenic sediments
react with sea water to produce clay minerals
Cosmogenous sediments are extraterrestrial in origin. As you can imagine they are the least abundant sediment type and are generally found diluted by other sediments. There are two main sources:
Research by the Ocean Drilling Program and others has revealed a thin and distinctive band of clay present in sediments around the world. This band is highly enriched in Iridium (Ir) and corresponds to the Cretaceous-Tertiary (KT) boundary. Iridium is a rare-earth element that is found at very low concentrations in the earth’s crust but is common in meteorites. The source of the iridium in this clay band is thought to be a comet that hit the earth 65 million years ago (see blast from the past). The impact produced a layer of sediment that can contain up to 20% cosmogenous material.
Comets and asteroids are also capable of producing particles called tektites. They are dark-coloured, rounded silicate glass particles that can be less than a millimetre (microtektites) to several cm in size. They are found concentrated in areas around the world that are referred to as strewn fields. The tektites are formed by impact melting of surficial sediments. Microtektites are found in deep-sea sediments within the Australasian strew field (a large area which is thought to have resulted from an impact on the Indochina Peninsula 0.77 Ma). Ocean Drilling Program researchers examining cores from the Ninety East Ridge (Eastern Indian Ocean) and the Sulu Sea (both located in the Australasian strewn field) have found increased levels of Ir. The Ir concentrations and microtektites distribution have lead them to propose that the Australasian impact could have excavated a crater between 15 and 19 km in diameter.
This exercise looks at some of the processes controlling sediment composition from the coast to the trench.
This exercise couples with the above exercise. Real examples of sediments have been used to allow you to interpret them in 'real-life' conditions.
Sediment deposition preserves more than just dirt! Conditions of deposition: temperature, depth, current, proximity to land, rate of deposition are all preserved in the sequence. With a little know-how this wealth of information can be gleaned from the rocks to give an informative 'snap shot' of million - hundreds of millions of years ago!
Vast amounts of sediment are shed off the continental slope, often settling 1000's of km from the continental rise. This exercise looks at some of the dynamics behind the process and the resulting geology.
Hung over? Procrastinating? This exercise will keep the mental cogs turning and let you take it easy for a while.
Yet to be added, this page will contain PDF's of 'real' data to allow you to nut out a few more similar topics
We realise that not everyone has had experience with the intrinsic concepts of Marine Science. This 'computer' has numerous examples which will help you understand these fundamental facets! Very helpful, very colourful!