SEDIMENTS Text Book – Chapter 5 Why do we care about oceanic sediments?

SEDIMENTSText Book – Chapter 5

Why do we care about oceanic

sediments?- Continents are sites of erosion;- Oceans are sites of depositions;- Therefore oceans retain a more-nearly

complete and organized record of Earth history.

1. Understand the origin and classification of marine sediments.

2. Explain the factors controlling origin and deposition of sediment on the continental shelf and in the deep ocean.

Questions:

1. Why don't the oceans have more sediment in them? Where does it all go? Earth is 4.6 billion years old and the oceans should have more sediment in them.

2. Salt composition of the oceans has not changed for the last 1.5 billion years. Explain why?

Learning Objectives:Ocean Sediments

Marine Sedimentation

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Classification of marine sediments can be based upon size or origin.

•Size classification divides sediment by grain size into gravel, sand, silt and clay.

– Mud is a mixture of silt and clay.

•Origin classification divides sediment into four major categories:

Sediments in the Sea

The 4 main types of sediment

•Lithogenous/terrigenous = 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

Factors that control sedimentation include particle size and the turbulence of the depositional environment.

•Terrigenous sediments strongly reflect their source and are transported to the sea by wind, rivers and glaciers.

Sediments in the Sea

Streams

Rivers

Glaciers

Landslide

(Gravity)

ocean environment can be divided into:

the shelf – neritic deposits(shallow areas and near a terrigenous source)

the deep ocean basin – pelagic deposits

(deep areas and far from a terrigenous source)

Sedimentation in the Ocean

Shelf Versus Basin

Shelf sedimentation is strongly controlled by tides, waves and currents, but their influence decreases with water depth.

•Shoreline turbulence prevents small particles from settling and transports them seaward where they are deposited in deeper water.

•Particle size decreases seaward for recent sediments.

Sedimentation in the Ocean

Worldwide distribution of recent shelf sediments by composition is strongly related to latitude and climate.

•Calcareous biogenic sediments dominate tropical shelves.

•River-supplied sands and muds dominate temperate shelves.

•Glacial till and ice-rafted sediments dominate polar shelves.

Sedimentation in the Ocean

Shelf Sedimentation Model

Geologic controls of continental shelf sedimentation must be considered in terms of a time frame.

•For a time frame up to 1000 years, waves, currents and tides control sedimentation.

•For a time frame up to 1,000,000 years, sea level lowered by glaciation controlled sedimentation and caused rivers to deposit their sediments at the shelf edge and onto the upper continental slope.

•For a time frame up to 100,000,000 years, plate tectonics has determined the type of margin that developed and controlled sedimentation.

Influence of Past Sea Level

Case study:The Atlantic Passive Margin

Case study:Pacific Destructive/Subduction Margin

Carbonate Shelves

If influx of terrigenous sediment is low and the water is warm, carbonate sediments and reefs will dominate.

Ice Rafting

Deep-sea Sedimentation has two main sources of sediment: external- terrigenous material from the land and internal-biogenic and authigenic from the sea.

Sedimentation in the Ocean

Sedimentation in the Deep Sea

•Major pelagic sediments in the ocean are red clay and biogenic oozes.

•Authigenic deposits are formed in situ (at place) by precipitation of submarine weathering (e.g. hydrothermal areas, ridges); metal rich sediments and manganese nodules;

Sedimentation in the Ocean

3CaCO2SiO

Calcium Carbonate(e.g. mollusks, corals, foraminifers, some algae)

Silica(e.g. diaotms, radiolaria)

Biogenic Deposits

Tiny animals with calcite shells (CaCO3)

Coccoliths

Tiny plants with a calcite skeleton (CaCO3)

Diatoms

SiO2

Radiolaria

Silica Sedimentation

Siliceous ooze found where it accumulates faster than it dissolves

2SiO

Calcium carbonate Sedimentation

CCD – Calcite dissolves beneath the calcite compensation depth (CCD) at ~ 4.5 km

3CaCO

Nodules Deposits – hydrogenous sediment

from precipitation of metal oxides

1) latitude2) distance from landmasses3) CCD (carbonate compensation depth)

• Glacial marine sediments occur in the high latitudes.

• Pelagic clays occur far from land and in the deepest water.

• Calcareous oozes occur above the calcium carbonate depth (CCD); while siliceous ooze occur below;

• The rate of sedimentation depends on the type of sediment in deep sea.

Distribution of sediments in the deep ocean

•Deep-sea stratigraphy refers to the broad-scale layering of sediments that cover the basaltic crust. The stratigraphy of the deep sea is strongly influenced by sea-floor spreading.

Sedimentation in the Ocean

The Atlantic basin contains a “two-layer-cake” stratigraphy–a thick basal layer of carbonate ooze overlain by a layer of mud.

Sedimentation in the Ocean

Stratigraphy of the Atlantic Basin

The Pacific basin contains a “four-layer-cake” stratigraphy, because unlike the Atlantic its sea floor as it spreads crosses the equator where the CCD is lowered to the ocean bottom.

Sedimentation in the Ocean

Stratigraphy and Model of Pacific Basin

•Bottom dredges scrape the sediment and collect material in a wire or canvas bag.

•Grab samplers take a “bite” out of the sediment covering the bottom.

•Gravity and piston corers use a weight to drive a core barrel into a soft bottom. A piston corer takes a much longer core than a gravity corer because of the piston in the core barrel.

Collecting Marine Sediment

There are a number of sampling techniques for obtaining sediment from the ocean bottom.

1243 sites

•The Mediterranean basin is located where plates are colliding as Africa moves northward relative to Europe.

•Anhydrite and stromatolites of Miocene age indicate that the Mediterranean sea “dried” out between 5 and 25 million years ago.

•Two models have been suggested to account for this emptying of the Mediterranean Sea of its water.

The Drying Up of the Mediterranean Sea

Refilling the Mediterranean Sea

•After drying out, seawater from the Atlantic Ocean cascaded down the face of the Gibraltar Sill, refilling it in about 100 years.

04_T04

1. Understand the origin and classification of marine sediments.

2. Explain the factors controlling origin and deposition of sediment on the continental shelf and in the deep ocean.

Learning Objectives