Seafloor Sediments

Ocean floor is mantled with sediment Sources

Turbidity currents Sediment that slowly settles to the

bottom from above Thickness varies

Thickest in trenches—Accumulations may approach 10 kilometers

Turbidity Currents – move sediments long distances

Submarine Canyons – Formed by Turbidity Currents

Seafloor Sediments

Thickness varies Pacific Ocean—About 600

meters or less Atlantic Ocean—From 500 to

1000 meters thick

Mud is the most common sediment on the deep-ocean floor

Sediments: Classification

• By size

• By mineralogy

• By source

Sediment Size Classification

Sediment Class Size (mm)

Boulder >256

Cobble 64-256

Pebble 4-64

Granule 2-4

Sand 0.062 - 2

Silt 0.004 - 0.062

Clay < 0.004

Sediment Thickness in the Oceans

Distribution of Marine Sediments

Region % Ocean Area

% Marine Sediment

Average Thickness

Continental Shelves 9 15 2.5 km

Continental Slopes 6 41 9 km

Continental Rises 6 31 8 km

Deep Ocean Floor

78 13 0.6 km

Sediment Classification by Source

•Terrigenous

•Biogenous

•Hydrogenous

•Cosmogenous

http://antwrp.gsfc.nasa.gov/apod/image/0003/feb26sandstorm_seawifs_big.jpg

Terrigenous Sediment

• From erosion of land, volcanic eruptions, blown dust from wind storms, glaciers and icebergs

• Dominant around continental margins and in polar oceans

• Cover ~45% of ocean floor, although they have the greatest volume of all types

• e.g. quartz sands, clays

Seafloor Sediments Terrigenous sediment

Material weathered from continental rocks

Virtually every part of the ocean receives sediment from land

Fine particles remain suspended for a long time and are carried by currents

Biogenous Sediment• Hard parts of some marine organisms –

living organisms• Covers ~55% of ocean floor and is

dominant in deep ocean• e.g. calcareous oozes from

foraminifera, pteropods, and coccolithophores; siliceous oozes from radiolarians and diatoms; phosphatic components from fish bones and teeth

Seafloor Sediments Biogenous sediment

Shells and skeletons of marine animals and plants

Most common are calcareous oozes produced from microscopic organisms

Siliceous oozes composed of skeletons of diatoms and radiolarians

Phosphate rich materials derived from the bones, teeth, and scales of fish and other marine organisms

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Foraminifera

Coccolithophores

Radiolarians

Diatoms

Hydrogenous Sediment

• From precipitation of dissolved material in seawater by bacteria or through evaporation

• Covers <1% of the ocean floor and is normally found only with other sediments

• e.g. manganese nodules

Seafloor Sediments Hydrogenous sediment

Minerals that crystallize directly from seawater

Most common types include Manganese nodules Calcium carbonates Metal sulfides Evaporites

Cosmogenous Sediment

• From space—dust and meteorite debris

• Very small proportion of sediment

• e.g. tektite spheres, glassy nodules

Distribution of Marine Sediments