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