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Chapter 4Marine Sediments
Essentials of Oceanography
7th Edition
Ocean sediment
Various materials settle through the water column and accumulate on the ocean floor
Layers represent a record of Earth history, including:
Movement of tectonic plates
Past changes in climate
Ancient ocean circulation patterns
Cataclysmic events
Collecting ocean sediment
Specially designed ships collect cores by rotary drilling
Cores allow scientists to analyze ocean sediment Figure 4B
The 4 main types of sediment
1. Lithogenous = composed of fragments of pre-existing rock material
2. Biogenous = composed of hard remains of once-living organisms
3. Hydrogenous = formed when dissolved materials come out of solution (precipitate)
4. Cosmogenous = derived from outer space
Origin of lithogenous sediment
Forms by:Weathering = breakup of exposed rockTransportation = movement of sedimentDeposition = settling and accumulation Figure 4-4
Sediment-transporting media
Lithogenous sediment composition
Most lithogenous sediment is composed of quartz, which is:
Abundant
Chemically stable
Durable
Figure 4-5
Lithogenous sediment texture
Texture includes:
Grain size
Sorting
Rounding
Maturity
Figure 4-7
Distribution of lithogenous sediment
Lithogenous sediment occurs as:Neritic (nearshore) deposits
Beaches
Continental shelves
Turbidites
Glacial-rafted debris
Pelagic (deep ocean floor) depositsAbyssal clay
Origin of biogenous sediment
Organisms that produce hard parts die
Material rains down on the ocean floor and accumulates as:
Macroscopic shells, bones, teeth
Microscopic tests (shells)If comprised of at least 30% test material, called biogenous ooze
Biogenous sediment composition
Microscopic biogenous tests are composed of 2 main chemical compounds:
1. Silica (SiO2) including opal (SiO2 · nH2O)
Diatoms (algae)
Radiolarians (protozoan)
2. Calcium carbonate or calcite (CaCO3)
Coccolithophores (algae)
Foraminifers (protozoan)
Examples of silica-secreting microscopic organisms
Figure 4-8
Diatom Radiolarian
Siliceous ooze
Silica-secreting organisms accumulate to form siliceous ooze (>30% siliceous test material)
Figure 4-8c
Examples of calcite-secreting microscopic organisms
Figure 4-9
Coccolithophores Foraminifers
Calcareous ooze
Calcite-secreting organisms accumulate to form calcareous ooze (>30% calcareous test material)
Figure 4-9d
Biogenous ooze turns to rock
When biogenous ooze hardens and lithifies, can form:
Diatomaceous earth (if composed of diatom-rich ooze)Chalk (if composed of coccolith-rich ooze) Figure 4-10
Chalk cliffs of southern England
Distribution of biogenous ooze
Most biogenous ooze found as pelagic depositsFactors affecting the distribution of biogenous ooze:
Productivity (amount of organisms in surface waters)Destruction (dissolving at depth)Dilution (mixing with lithogenous clays)
Distribution of siliceous ooze
Silica slowly but steadily dissolves in seawater
Siliceous ooze found where it accumulates faster than it dissolves Figure 4-11
Distribution of calcareous ooze
Calcite dissolves beneath the calcite compensation depth (CCD) at 4.5 km
Calcareous ooze can be found below the CCD if it is buried and transported to deep water
Figure 4-12
Biogenous ooze as environmental indicator
Siliceous ooze Calcareous ooze
Surface water temperature
Cool Warm
Main locations found
Sea floor beneath cool surface water in high latitudes; upwelling areas
Sea floor beneath warm surface water in low latitudes; not too deep (CCD)
Origin of hydrogenous sediment
Hydrogenous sediment forms when dissolved materials come out of solution (precipitate)Precipitation is caused by a change in conditions including:
Changes in temperatureChanges in pressureAddition of chemically active fluids
Types of hydrogenous sediment
Manganese nodules
Phosphates
Carbonates
Metal sulfides
Evaporite salts
Figure 4-25
Mining manganese
nodules
Evaporite salts
Figure 4-15
Cosmogenous sediment
Cosmogenous sediment is composed of material derived from outer spaceTwo main types:
1. Microscopic space dust2. Macroscopic meteor debris
Forms an insignificant proportion of ocean sediment
Figure 4-16
Microscopic cosmogenous
spherule
Mixtures
Most ocean sediment is a mixture of sediment typesOne type of sediment usually dominates, allowing it to be classified as primarily:
LithogenousBiogenousHydrogenousCosmogenous
Worldwide distribution of neritic and pelagic sediment
Figure 4-17
Ocean sediments as a resourceOcean sediments contain many important resources, including:
PetroleumGas hydratesSand and gravelEvaporative saltsPhosphoriteManganese nodules and crusts Figure 4-21
Offshore drilling rig
End of Chapter 4
Essentials of Oceanography
7th Edition