Marine Sediments and Sedimentation

Tools used to collect and study Sediments

• How can sediment be collected from the sea floor?

• How can it be collected intact?

• How can it be collected from different depths?

Sediment surface and shallow depth sampling

Box corer: Also used for sediment surface samples

Studying box corer samples

Deeper sediment sampling

For Really Deep Cores or Drilling in Hard Bottoms…

• Drill ships are used

• Basically like oil drilling platforms that can travel easily from place to place

Another device for sampling the top meter or so of sediment

Research vessel Box corer comes aboard

Box corer safely retrieved Start looking at the sample

Seismic Profiling: Can provide info about sediment layer types and thickness

Strong sound waves can be used to penetrate into the sediments and find depths of layers

This method has to be used carefully because loud sounds can harm marine animals

ROVs can also be used to visually study the bottom

Sediment• Loose particles that accumulate on the ocean floor.

• Sources of sediments

– Terrigenous: Sediments that originate on land• Eroded bits of rock, sand, clay or silt carried to the sea by

rivers• Volcanic ash from eruptions• Dust that blows out to sea from dry areas• Ash from forest fires that blows out to sea

– Biogenous: Sediments that are formed from the remnants of living organisms

• Dead bodies, bones, skeletons, shells• Most comes from microscopic organisms known as plankton

– Hydrogenous: Sediments that form when minerals cyrstalize from seawater

• Chemical process that can occur under the right conditions at the high pressures found in deep water

• Terrigenous Sediment

• Silt, “mud” carried by rivers from land to the ocean is visible close to shore

• Most sinks to bottom quickly

• Most terrigenous sediments are found on continental shelves (close to shore)

• Terrigenous Sediment

• Wind can bring TS out to sea as well

• 2 main sources:

• Dust

• primary dust sources are deserts in Asia and North Africa

• Dust makes up much of the fine-grained sediments found far from shore (red clays)

• Volcanic Ash

• volcanic eruptions contribute ash to the atmosphere which settles within the oceans

• Sediment is considered well-sorted if most particles appear to be about the same size

• Poorly sorted sediments are made up of particles of many different sizes

•  Movement of sediment on the ocean bottom is controlled by particle size and current speed

– Fast currents and small particles faster movement

Larger sediment particles sink fastest.

So the farther you go from shore, the smaller the particles that make up sediments

• Sediment can also move from a continental shelf to the open ocean by gravity-driven turbidity currents (TCs)

• These are basically underwater mud slides or “avalanches”

• Typically started by storm activity or earthquakes 

– first identified during 1929 Grand Banks earthquake

– earthquake triggered turbidity current which cut U/W phone cables

• TCs often start in submarine canyons

Speed of Turbidity Currents?

Calculated from times at which transAtlantic phone cables broke after the quake

• boulder to clay sized particles can also be eroded and transported to oceans by glaciers

• glaciers break off when they reach the sea

• as ice (or icebergs) melt, rocks, gravel and sand they contain sinks to the bottom becoming sediment

• Biogenous Sediments:

• made mostly of marine microscopic plankton remains

• shells of one-celled plants and animals, skeletal fragments

• These constantly “rain” down to the bottom as organisms die

• There are two basic types:

• Remains made up of calcium carbonate (CaCO3) or those made of silica (SiO2)

• sediments with biogenic component less than 30% are called calcareous or siliceous clays

• They are called calcareous or siliceous 'oozes' if biogenic component is greater than 30% 

• Siliceous oozes (Made up of skeletons of single celled plants and animals called diatoms and radiolarians)

• Their silica skeletons are made of glass

• Siliceous oozes cover ~15% of the ocean floor

– Distribution matches regions of rapid plankton growth

– common at high latitudes (near poles)

– Diatoms do best in cold, turbulent water

• Used as fine abrasives and in water filtration units

• Calcareous oozes:

• Oozes made up of skeletons of plants & animals having calcium carbonate skeletons (foraminifera, coccolithophores)

• Same stuff as clam, crab or lobster shells

• Cover ~50% of the ocean floor

• But only down to 3-4000 meters. Below that, pressure causes the shells to dissolve

• depth below which CaCO3 dissolves is called the 'carbonate compensation depth'

• So these are no calcareous oozes in the really deep areas

• Form gypsum deposits used to make drywall and chalk

• high productivity in zones of upwelling and nutrient-rich high latitude waters

• calcareous oozes more common in warmer or shallower water

• siliceous oozes more common in colder or deeper water

• terrigenous sedimentation rates range from ~1 mm to 10's cm/1000 years

• biogenous sedimentation rates typically ~1 cm/1000 years

Microfossils in sediments are used to study ancient climates

Based on our knowledge of temperatures preferred by different species

• manganese nodules

– Crystals of manganese, iron, copper, cobalt, and nickel

– accumulate only in areas of low sedimentation rate (e.g., the Pacific)

– develop extremely slowly (1 to 10 mm/million years)

• Hydrogenous (or Authigenic) Sediments:

• produced by chemical processes in seawater

• essentially solid chemical precipitates of a variety of minerals and metals

Nodules are potentially rich source of metals.

Only problem is finding a way to collect them economically

Undersea mining??? Some day???

• Cosmogenous Sediments:

• sediments derived from extraterrestrial materials 

• includes micrometeorites (space dust) and tektites

• tektites result from collisions with meteorites

– fragments of earth's crust melt from the heat of impact and spray outward from impact crater

– Some falls in the oceans and forms 'glassy' tektites

• Makes up an immeasurably small fraction of total marine sediments