Oceanography Chapter 4: Marine Sediments

Reservoir rock

porous rock that oil can migrates into

Lithogenous

sediments from land ~45% of ocean floor covering

Hydrogenous

chemical (inorganic) precipitate of minerals from seawater (<1%)

Diatomaceous earth, which is composed of silica, __________ above the CCD and __________ below the CCD.

does not dissolve; does not dissolve

Cosmogenous

dust from space; comet meteor, asteroid debris (<<<<1%)

Salts and Minerals What salts and minerals can be obtained from the ocean?

Magnesium Sodium chloride Manganese nodules Phosphorite Metallic sulfides and muds

Ocean sediment by origin

1. Lithogenous = sediments from land - ~45% of ocean floor covering 2. Biogenous = biochemical (organic) group - ~55% of ocean floor covering - Total volume < terrigenous 3. Hydrogenous = chemical (inorganic) precipitate of minerals from seawater (<1%) 4. Cosmogenous = dust from space; comet meteor, asteroid debris (<<<<1%)

Key Points

1. Sediments vary greatly in appearance. 2. Sediments can be classified by particle size. 3. Sediments can be classified by source. 4. Neritic sediments overlie continental margins. 5. Pelagic sediments vary in composition and thickness. 6. Scientists use sensitive tools to study ocean sediments. 7. Marine sediments are economically important.

Solid Hydrocarbons:

Asphalt

Of the following types of lithogenous sediment, which one(s) would you expect to find very far from the location of origin? Choose all that apply.

Quartz silt Volcanic ash

Petroleum and Natural Gas the Ocean's Most Valuable Resources

US consumes 25% of global oil supply • Offshore about 30% of total world production (started in 1930s) • Likely to increase in future - Esp. Arctic, Asia, W. Africa & Brazil • Deeper ocean to be exploited

Two common types of chemical compounds found in biogenous sediments are __________ and __________.

calcium carbonate; silica

Sedimentary Cycle

1. Weathering - Make particle 2. Erosion - Put particle in motion 3. Transport - Move particle 4. Deposition - Stop particle motion not necessarily continuous(rest stops) 5. Lithification

What is the approximate depth of the calcite compensation depth (CCD) in the ocean?

3 miles (4.5 kilometers)

Petroleum

A general term for all naturally occurring hydrocarbons (hydrogen + carbon)

a fine-grained, deep ocean sediment containing the skeletal remains of calcite-secreting microbes

What is calcareous ooze?

Why does calcareous material dissolve below the CCD?

Below the CCD, weak acid is formed, which dissolves calcareous material.

What would happen if the depth of the CCD were above the top of the mid-ocean ridge?

Calcareous ooze would not be found below the CCD.

Neritic sediments are most likely to be composed of __________.

sand, silt, and gravel

Pelagic sediments: biogenous

Cover the greatest area of deep ocean (62%) - Total volume < terrigenous sediments • Originate from the activity of animals and plants • Over millions of years, can form oil and natural gas • Oozes- deep-ocean sediment containing at least 30% biogenous material - Calcareous ooze - Siliceous ooze

Liquid Hydrocarbons:

Crude Oil

Studying sediments: piston corer

Cylinder of sediments allows analysis of: - Age - Earth's past climates - Environmental contaminants • JOIDES Resolution

Methane Hydrates

Methane-laced ice crystals • Occur under permafrost on land and under ocean floor • High pressure and cold temperatures trap gas in water crystal lattice • Created when bacteria breakdown organic matter in seabed sediments (creating methane & some ethane & propane)

Distribution of biogenous sediments

Most common as pelagic deposits - Calcareous oozes dominant on deep ocean floor (~48% covering) - Siliceous oozes ~14% covering of deep ocean floor • Factors controlling distribution - Productivity - Destruction (dissolution)

Gas Hydrocarbons:

Natural Gas: methane, butane, propane, etc.

Deposition

Neritic = near shore (Continental shelf & mainly lithogenous materia; Pelagic = open ocean (Continental slope, continental rise and deepocean floor &Contain more biogenous material)

Transport: Lithogenous sediments

Originate on the continents or islands • Come from rivers, glaciers, volcanic eruptions, and blown dust • Largest terrigenous deposits are on the continental margins

Oolite sands

White, rounded grains of calcium carbonate • Formed around a nucleus such as a shell fragment or a quartz grain • Abundant in many warm, shallow water - Bahama Banks

How does siliceous ooze accumulate on the seafloor if silica-based residues are dissolved slowly at all depths?

Silica tests accumulate faster than seawater can dissolve them.

In which of the following environments would you expect lithogenous sediment to be the dominant sediment type? Choose all that apply.

at the base of the continental slope along the mid-ocean ridge the abyssal plains a sandy beach

Biogenous

biochemical (organic) group ~55% of ocean floor covering

Sediments derived from the remains of the hard parts of once-living organisms are called __________ sediments.

biogenous

You take a sediment sample from the ocean floor at a depth of 5500 m. The area has low biological productivity and the CCD is at 4500 m depth. Your sample will probably consist of __________.

clay

From smallest to largest, which list of sediment particle sizes is in the correct order?

clay, silt, sand, pebbles

What three steps are required for calcareous ooze to exist below the CCD?

deposition of calcite shells above the CCD, cover of these shells by a non-calcareous material, and movement of the sea floor over millions of years

The two major types of microscopic, planktonic organisms that produce siliceous oozes are __________ and __________.

diatoms; radiolarians

Forams, which are composed of calcium carbonate, __________ above the CCD and __________ below the CCD.

do not dissolve; dissolve

Siliceous ooze is particularly abundant on the seafloor at __________ and at __________.

high latitudes; equatorial upwelling zones

Cap rock

impermeable rock that does not allow fluids to pass through them, traps petroleum

Of the following sediments, which one(s) is/are considered hydrogenous? Choose all that apply.

manganese nodules coatings of metal sulfides near black smokers

Sediment

particles of organic and inorganic material that settle through the water column and accumulate on the ocean floor

Marine sediments provide valuable clues about which of the following Earth history events? Choose all that apply.

previous ocean circulation patterns the ancient geographical locations of ocean basins past catastrophes, such as major extinction events movement of the ocean floor the climate on Earth millions of years ago

What is the calcite compensation depth, or CCD

the ocean depth below which calcite is unstable and will dissolve quickly

Sand and Gravel 2nd most valuable Marine Resource

• 1% of World Production is from oceans • 20% of Japan and UK needs are met this way • World's largest mining operation- oolitic sands mined from the Bahamas - - Almost pure calcium carbonate is used in Portland Cement, and sold throughout USA • Even diamonds have been found in these sediments (900,000 carats in 1998) off the coast of Africa

Studying sediments

• Age of seafloor - Deepest layers = oldest sediments - Surface sediments = youngest - Relative ages of sediment layers • Past climates - Measure 18O (stable isotope)- found in shells of foraminiferas

Example- calcareous sediment

• Chalk, limestone, sandstone, shale • Example- White Cliffs of Dover

Calcium Carbonation Depth (CCD)

• Deep water has more CO2, so it is more acidic • Below this depth, the tiny skeletons of calcium carbonate dissolve on the seafloor, so no calcareous oozes accumulate

Deposition: Pelagic sediments (lithogenous)

• Deeper water deposits (38% of deep ocean covered) • Finer-grained sediments • Deposited slowly • "Pelagic or red clay" - Volcanic ash (volcanic eruptions) - Wind-blown dust - Fine grained material transported by deep ocean currents - Turbidites • Common in deep oceans where other types are absent

Hydrogenous sediment

• Direct precipitation from seawater • Sediment-seawater reaction • Also called authigenic because the form in the place they occupy • Examples manganese nodules, evaporites, oolite sands

Deposition: Neritic sediments

• Dominantly lithogenous (coarser grains) • Typically deposited quickly • Eroded from land and carried to the sea - Beach deposits (transported by rivers) - Glacial deposits - Continental shelf deposits • Relict sediments

Oil trapped in reservoirs

• If nothing stops oil from rising, it will reach surface - Ex: The La Brea tar pits

Calcareous oozes

• Many organisms extract calcium carbonate (CaCO3) from seawater • Sea shells and corals • Marine algae: coccolithophores (phytoplankton) • Marine protozoan (single-celled animal): foraminifera (zooplankton)

Manganese nodules

• Primarily composed of manganese (29%) and iron oxides (6%) • Slow growth, ~ 1 to 10mm/million yrs Mukhopadhyag and Ghosh, 2010 • Form around nuclei such as shark's teeth, bits of bone, tiny crystals • May cover 20-50% of Pacific Ocean floor

Evaporites

• Salts precipitate as water evaporates from isolated sections of the ocean or landlocked seas • Form in arid climates - Rate of evaporations >>> rainfall - Currently form in Gulf of California, the Red Sea and the Persian Gulf

Marine sediments: economically important

• Sand and gravel - construction • Oil and gas - energy, transportation - 20-25% of US production comes from offshore areas • Diatomaceous earth - useful for everyday life - Filters, mild abrasives, chemical carriers

Formation of oil

• Sedimentary rocks rich in organic matter - 0.5 - 2% by weight • Most commonly planktonic organisms - 100-500 million years ago • Organic material broken down in anaerobic conditions - Little or no oxygen

Siliceous oozes

• Some marine organisms extract silica from seawater (SiO2) • Radiolarians: single-cell animal (protozoan) • Diatoms: single-cell marine plants • Limited to areas with high biological productivity

Ocean sediment

• Sources: - Weathering and erosion of rocks - Activity of living things - Volcanic eruptions - Chemical processes in seawater - Space • Many sizes

Cosmogenous

• Very rare • Two main sources: 1) interplanetary (cosmic) dust - Dust particles enriched with 3He 2) impacts by asteroids and comets - Tektites- melted fragments of terrestrial rocks during meteorite impacts • Impacts of extra-terrestrial bodies - Catastrophic results - Cause of "mass extinctions"?- dinosaurs