Lesson 15: Sediments

Geological Oceanography

Sediments are important to marine scientists

Did you know that ocean sediments hold clues about the earth?

They can provide information on earth’s past climate, the location of oil and natural gas resources, seafloor age, pollution patterns and many other processes

There are many types of sediments and we will learn different ways to classify them

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How are sediments classified?

By grain size By origin– Where do you think

sediment comes from?

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Classification Grain DiameterClay <0.004mm

Silt 0.004-0.0625mm

Sand 0.0625-2mm

Granule 2-4mm

Pebble 4-64mm

Cobble 64-256mm

Boulder >256mm

Where does sediment come from?

Lithogenous sediments come from land– Rivers, wind, ice and other geologic processes erode and

transport rocks and minerals into the sea– Also called “terrigenous,” these particles make up most of

the sediment near islands and continents

Biogenous sediments come from organisms– They come from the shells and exoskeletons of marine

organisms– They cover most of the area of the sea floor

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Where does sediment come from?

Hydrogenous sediments come from chemical reactions in the water

– Sources may include minerals or chemicals that dissolve in seawater, river runoff or hydrothermal vent water

– They make up less than 1% of seafloor sediments and tend to form slowly

Cosmogenous sediments come from space– Sources may include meteorites and cosmic dust– Least abundant of the four types of marine sediments

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Can you name some common examples?

Lithogenous

?

?

Quartz

Red Clay

Photos: USGS, USDA6

Can you name some common examples?

Biogenous

?Plankton remnants -silicasilica or calcitecalcite

Cosmogeneous

?Tektites

Photos: USGS7

Do you think sediment is uniformly distributed throughout the ocean?

No! Even though most sediment contains a mixture of

types, one type typically dominates depending where you are in the ocean

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Distribution: Biogenous sediment

The biogenic material in the ocean comes primarily from the breakdown of plankton skeletons

Deep ocean sediments are usually high in biogenic material Ooze is deep-ocean sediment that has more than 30%

biogenic material– calcareous ooze: composed mostly of calcareous ooze: composed mostly of calcium carbonate

plankton skeletons– siliceous ooze: composed mostly ofsiliceous ooze: composed mostly of silica plankton

skeletons

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Distribution: Lithogenous (terrigenous) sediment

Lithogenous sediments are generally found close to shore except for clay particles

Thus, sediments near the shore tend to be dominated by lithogenous material, while deep-ocean sediment tends to be dominated by biogenic material

Lithogenous clay particles are relatively light, so they can be carried further from shore by wind, forming pelagic red clay accumulations

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Sediment thickness in the deep ocean

Sediment layering is not the same across all of Earth’s ocean

Do you think the Atlantic or Pacific ocean basin has greater sediment thickness? Why?

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Let’s look at a global map of ocean thickness

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The Atlantic Ocean Basin has greater sediment thickness

Sediment thickness on the Pacific Ocean floor is about half that of the Atlantic Ocean

Rivers flowing into the Atlantic cover more land than those flowing into the Pacific, so they bring more sediment

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Exploring sediment distribution

In today’s activity, we will analyze maps of sediment distribution

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