L5 Flashcards
How is sediment produced?
By weathering of rocks or the build-up of shells of dead organisms.
Where does most deposition in the ocean come from?
Erosion on the land and then deposition in the ocean.
What would be the net effect of erosion and deposition if it wasn’t for techtonic processes?
It would even out earth’s surface, cutting away its high points and deepening its low points.
How can sediments be clasified?
grain size and origin
Name from the largest to smallest particles common in sediment.
gravel, sand, silt and clay (colloids)
What two particle sizes are mixed to form mud?
silt and clay
What is terrigeneous sediment?
Terrigenous sediment: fine and coarse grain produced by weathering and erosion of rocks on land; typically sand and mud
what is biogenous sediment
Biogeneous sediment: fine and coarse grains derived from hard parts of organisms such as shells and bones; typically lime and siliceous muds
what is hydrogenous sediment
Hydrogenous sediment: particles that are precipitated by chemical or biochemical reactions in seawater near the seafloor; manganese and phosphate nodules are examples
what is volcanogenous sediment
Volcanogenous sediment: particles ejected from a volcano; eg ash
what is cosmogenous sediment
Cosmosgenous sediment: very tiny grains that originate from outer space and tend to be mixed into terrigenous and biogenic sediment
what are two methods of probing (sampling sections) of the seafloor, when would each be used and why
dredging - disturbs the sample and often mud is lost, this is often only used for hard rock samples
Grab samplers - spring - loaded metal jaws that bite out the bottom and close around the sample - often used over dredging to preserve layers in the sample
What is a method of deep sampling of soft sediment called?
gravity corer
what is the hollow metal tube of a gravity corer called?
core barrel
Describe briefly the process of ocean coring?
a hollow metal tube is pushed into the sediment by the force of gravity, the corer is lowered to the bottom where a heavy weight at the top of the device drives the barrel into the sediment. A plastic liner allows the oceanographers to extract the sediment core intact from the sampler and serves as a temporary storage container.
How long can a core obtained by a gravity corer be?
1-2 m
How long can a sediment core from a piston corer be?
longer than 20m
Describe briefly the process of piston coring
a piston slide up the core barrel as it penetrates the bottom, this action extrudes water from the core barrel allowing the sediment core to enter the liner with minimal disturbance and compactation. once this core is on deck the sample is extrude from the barrel and taken to a laboratory and examined.
What is at present the best the best technique for sampling the ocean bottom?
platform drilling
What are the advantages and disadvantages of platform drilling?
it is very expensive, however the scientific results are priceless, the marine geologists can recover more than 1km in length but can also drill into the hard rock of the crust.
What are the two major areas of sedimentation?
- The continental shelf which is shallow and has sedimentation from the land
- The deep sea, which is far from river supplies of sedimentation
over continental shelfs what is the most dominant process effecting the ocean bottom?
waves
drawing from swimming experience on the beach and our knowledge about waves and turbulence what can we deduce?
bottom energy induced by surface-water wave must diminish with distance off shore due to water depths increasing seaward
what kind of sediment is expected at each part of the beach and why
-coarse sand and gravel on the beach
- fine sands further offshore
into muddy san
sandy mud
mud
this is due to a decrease in energy further seaward
What has caused sea level to rise and fall over previous years, also leading to a move in sediment
galciation and deglaciation
how do glaciation and deglaciation impact sedimentation
due to the changes in water depths because of glaciation and deglaciation the level of the beaches have changed. this leads to relic sediment being mixed in with newer sediment at points on the beach
What have surveys of distribution of sediment on the continental shelf shown worldwide
that sediment types vary with latitude and depend on climate
what sediment is found on continental shelf at the equato and subtropics
deposits include:
coral reefs and grain fragment composed of calcium carbonate(shells and bones) of animals
Where is the equator and subtropic band of sediment on continental shelfs broader and why
western edges due to warm westward flowing equatiorial currents diverging at the equator and moving polewards along the western side of all basinis
What sediment is found on the continental shelf of thee middle or temperate lattitudes
- river supplied terrigenous deposits
principally sand-sized grains of quartz and feldspar
what sediment is found on the continental shelf of the polar shelves
littered with poorly sorted glacial deposits (glacial till) dumped by glaciers ore ice-rafted debris dropped from melting ice bergs
why is relic material not currently in equilibrium with present-day water depths?
because it was deposited long ago when shore lines were displaced seaward due to the holocene sea level rise being so rapid, sediments have not had sufficient time to regain equilibrium
why are dust storms so important for rainforests
because they supply lots of the essential nutrients
what is a red tide in Florida
dust-plume event which increases iron concentration and raises nitrogen levels. this devastates local ecosystems and kills millions of fish and hundreds of manatees
what is microbe transport
microorganisms transported in dust plumes that may have infected and killed coral reefs and orange froves
what human heath impacts are there from dust plumes
pesticides and hurbicides and burning of rubbish in northern africa, release toxins into air - these are inked to increase in asthma attacks in barbados
What effects the shelf bottoms on a short time scale ( thousand years to present)
- wind
- tidal currents
How does wind effect the shelf bottoms on a short time scale ( thousand years to present)
small winds create waves and currents which grow in size
in storms a lot of sediment can be moved due to the high velocity of the water (hjulstorm curve)
How does tidal currents effect the shelf bottoms on a short time scale ( thousand years to present)
tidal currents by themselves are too weak to rework bottom sediment however if there is a large tidal range and a shallow bottom this creates an increase in the waves velocity ( prime example UK)
What effects the shelf bottoms on a medium time scale ( a million years to a thousand years)
glaciation and deglaciation meant the level of water was different and this could leave the continental shelves exposed
what were important modifications when the sea levels were low
- ice caps and glaciers extended on to the shelf proper ( particularly in high and mid lattitudes) and glaciers dumped debris on the shelf floor forming blankets, mounds and ridges of glacial till
- sea level dropped meaning rivers exteded their channels across continental shelves
- deposition and erosion rates in the deep sea increased
- coral reefs were killed
What effects the shelf bottoms on a long time scale ( 100 million years to a million years)
- passive atlantic-type margins
- pacific type margins
explain Atlantic type margins
1The seafloor at the edge of the continent sinks so that the build up of sediment is in line with the rate of subsidence
2Terrigenous sediment eroded from the continent accumulates on the shelf causing the continental margin to widen and deposits thicken.
3With continuing drifting the crust and underlying mantle at the passive edge of the continent cool, contract and are weighed down by sediment load. the edge of the continent continually providing room for incoming sediment
4 the sand fractions of the sediments are reworked by waves and currents and dispersed across the shelf but fine mud is kept in suspension and moved seaward where it will settle on the floor of the continental slope or reach the abyssal depths
5 this results in a long history of deposition and a broad smooth continental shelf.
explain pacific type margins
they receive sediment from the land but are also effected by subduction zones:
1 the strong techtonic forces squeeze beds of sediment between colliding plates, folding and faulting sedimentary layers
2 the sedimentary layers and basalt are being forced downward by the upper plate creating an accretionary prism
3 along with volcanic activity sedimentary debris is derived from the erosion of the volcanic flows and deposited mostly underwater
4 eventually these materials get crushed against and are added into the accretionary prism
5 this results in the continental shelf tends to be narrow and have an irregular surface
What conditions are needed for carbonate shelves or platforms to form?
low supply of river sediment
warm waters
What kind of water is needed for carbonate secreting organisms to survive?
clear, warm and shallow
why are carbonate shelves situated away from large river outputs
the terrigenous sand and mud make the water muddy and this interferes with the growth of reef organisms and dilutes carbonate contributions to the bottom sediments
what does carbonate rocks eg limestone in the rocky mountains, alps, Himalayas and andes show?
how changable environments are over long periods of times and the seas have repeatedly drowned the land and retreated
what are two sources of sediment to the deep ocean floor?
terrigenous mud and sand that have bypassed the continental shelf
the hard parts of surface-water microorganisms that settle to the deep-sea bottom
what are the three classifications of deep-sea deposits
bulk emplacement
pelagic sediment
hydrogenous sediment
What is bulk emplacement
when sea levels are low rivers deposit at the outer continental shelf and upper continental slople
what are slumps
slumps are sediment piles that slide downslope intact, there is little internal disturbance of the sedimentary package
what is a slurrie
a mud flow or debris flow, they destroy any previous bedding that has existed
what are turbidity currents
powerfull bottom currents that transport sediment to the deep sea (sediment laden slurries)
what happens to turbidity currents when the sea bottom flattens
the current slows and deposits sediment rapidly on the deep sea bottom
how are deep submarine canyons formed
for a long time it was not known
now - combination of sediment slumping and turbidity currents that have deepened a gully or depression in the sea bottom
how do deep submarine canyons affect how deposition is distributed
serves as a chute and channels large quantaties of mud and sand from the shelf and slope to the deep sea
what is graded bedding
as a current slows the suspension is dropped according to size with largest first and smallest last, leading to a sorted deposit
what are beds of sediment laid down by turbidity currents called
TURBIDITES
what is a turbidites
beds of sediment laid down by turbidity currents
what are deep sea fans
turbidities deposit at the mouth of a submarine canyon and form a thick sequence of cone-shaped deposits
what is ice rafting
ice bergs carry sediment, then melt further out at sea depositing the sediment
what is glacial marine sediment
produced by ice rafting, poor sorted and heterogenous composition of tock and mineral fragments
what is hemipelagic sediment
a type of marine sediment that consists of fine-grained biogenic and terrigenous material
what is the inorganic type of pelagic deposit
red clay - oceanographers call this brown clay or pelagic clay
where does pelagic clay originate from
derived from several sources weathering of granitic and volcanic rocks, dust plumes fallout from extraterrestrial space dust and precipitation from seawater
what is pelagic clay made from
kaolinite, chlorite, felspar and quarts
what is a biogenouse ooze
30% or more of skeletal debris of microscopic organisms
70% or less of inorganic mud particles
what are the two major types of biogenous deposits
calcareous ooze and siliceous ooze
what are calcareous oozes made of
mainly tiny shells of zooplankton such as formanifera, pteropods and phytoplankton such as coccolithophores
what is the carbonate compensation depth
the level below which the preservation of CACO3 shells in surface sediment in negligible
what does CCD stand for
carbonate compensation depth
what does the CCD depend on
acidity, temperature and pressure
what do siliceous oozes contain
remains of diatoms and radiolaria
what are authigenic deposits and and example
chemical precipitates that form in places witi an ocean basin - ferromanganese nodules
typically where do carbonate oozes form
the seafloor that is shallower than CCD
typically where do siliceous oozes accumulate
in the deep water below the CCD
what do ferromaganese consist of
concentric layers of various metal oxides - mainly oxides and iron
what is phosphorite
another mineral deposite
what normally happens when sediment is burried and compacted
slowly transformed into sedimentary rock
what two deposits were found in the Mediterranean that lead to the theory of it drying up
anhydrite - forms in hot arid desert conditions
stromatolites - also unrelated to sea environments
