Oceanography 100
P Anderson
Chapter 4
Marine Sediments
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
The 4 main types of sediment
• Lithogenous = composed of fragments of pre-existing rock material
• Biogenous = composed of hard remains of once-living organisms
• Hydrogenous = formed when dissolved materials come out of solution (precipitate)
• Cosmogenous = derived from outer space
Origin of lithogenous sediment
• Forms by:
– Weathering = breakup of exposed rock
– Transportation = movement of sediment
– Deposition = settling and accumulation
Lithogenous sediment composition
• Most lithogenous sediment is composed of quartz, which is:
– Abundant
– Chemically stable
– Durable
Lithogenous sediment texture
• Texture includes:
– Grain size
– Sorting
– Rounding
– Maturity
Distribution of lithogenous sediment
• Lithogenous sediment occurs as:
– Neritic (nearshore) deposits
• Beaches
• Continental shelves
• Turbidites
• Glacial-rafted debris
– Pelagic (deep ocean floor) deposits
• Abyssal 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:
•
Silica (SiO2) including opal (SiO2 · nH2O)
• Diatoms (algae)
• Radiolarians (protozoan)
•
Calcium carbonate or calcite (CaCO3)
• Coccolithophores (algae)
• Foraminifers (protozoan)
Examples of silica-secreting microscopic organisms
Siliceous ooze
• Silica-secreting organisms accumulate to form siliceous ooze (>30% siliceous test
material)
Examples of calcite-secreting microscopic organisms
Calcareous ooze
• Calcite-secreting organisms accumulate to form calcareous ooze (>30% calcareous test
material)
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)
Distribution of biogenous ooze
• Most biogenous ooze found as pelagic deposits
• Factors 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
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
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 temperature
– Changes in pressure
– Addition of chemically active fluids
Types of hydrogenous sediment
• Manganese nodules
• Phosphates
• Carbonates
• Metal sulfides
• Evaporite salts
Cosmogenous sediment
• Cosmogenous sediment is composed of material derived from outer space
• Two main types:
•
Microscopic space dust
•
Macroscopic meteor debris
• Forms an insignificant proportion of ocean sediment
Mixtures
• Most ocean sediment is a mixture of sediment types
• One type of sediment usually dominates, allowing it to be classified as primarily:
– Lithogenous
– Biogenous
– Hydrogenous
– Cosmogenous
Worldwide distribution of neritic and pelagic sediment
Ocean sediments as a resource
• Ocean sediments contain many important resources, including:
– Petroleum
– Gas hydrates
– Sand and gravel
– Evaporative salts
– Phosphorite
– Manganese nodules and crusts