Oceanography Lecture Outline Notes

Seafloors and Marine Sediments

 

I. Lecture Content-  Topics Covered

Introduction Earth's Hidden Surface - The Seafloor

Seafloor Topography - Undersea Bottom Relief

Seafloor Physiology - Major Features of the Sea bottom

Continental Margins - Where Ocean Meets Continent

Deep-Sea Floors - Basaltic Crust Conveyor Systems

Marine Sedimentation - Mucky Clays and Oozes

Islands, Reefs and Atolls - Interesting Places

Life and Death of an Ocean Basin - The Wilson Cycle

Sea Bottom Resources - Natural Treasure

 

II. Introduction

A. The Earth is Truly a Water Planet - "Oceania"

1. Seafloors cover nearly 3/4's of Earth's surface.

 

2. The seafloor is the final receptacle for vast amounts of terriginous and

organic material.

 

3. Ocean basins form wide barriers between the continents

 

B. The Ocean Realm is Earth's Last Frontier - Why?

1. Remote and inhospitable - Its a harsh "inner" space.

 

         Conditions at sea can be very uncomfortable

 

         Water pressure is tremendous at great depths

 

         Deep sea world is very wet, cold, and in complete darkness

 

C. Earth's Seafloors are Vast, Rugged & Resource-rich

1. A highly variable topography, which includes, rugged mountain ranges, very

deep trenches, amazing island chains, ocean plateaus, seamounts, and

deep canyons that rival anything seen on land.

 

 

III. Seafloor Topography - Underwater Bottom Relief

A. Earth has Two Distinctive Topographic Regions

 

         Continental Highlands - Continents

         Oceanic Lowlands - Ocean basins

B. The Earth's Seafloors are Rugged in Appearance and

Have Considerable Topographic Relief.

        See Figures

        Much more topographic relief than the continents

        Seafloors have distinctive topographic features

        Seafloors look much different than dry continents

 

C. Earth's Seafloor is Divided into Two Major Provinces

 

1. Continental Margins

                        Submerged shallow platforms

                        Floored mostly by granitic rock

                        Varies greatly width, depth, and topo relief

                        Vast majority of marine life concentrated there

 

2. Deep-Ocean Basins

         Starts at base of continental margins

 

         Deep seafloor consists primarily of:

1. High-standing mid-ocean ridge systems and

 

2. Low-standing sediment-covered abyssal plains

 

3. The two-province division is based upon the major inherent differences

between continental and oceanic crust.

 

        Composition (density)

        Thickness

        Isostatic equilibrium

IV. Continental Margins

A. Shallow Seafloor Rims of Ocean Basins

 

1. Continental margins - the submerged edges of continents

 

2. Continental margins are underlain by faulted blocks of granitic crust ,

overlying sediment piles, and possible accreted subduction zone material

 

B. Continental Margins are Classified into Two Types

1. Passive Margins = Atlantic Ocean style

 

         Situated within a plate

 

         Develops after continental rifting and opening of a new ocean basin opening

 

         Typically broad (avg. 100's km) with a very thick pile of

accumulating sediments

 

         Lacks much seismic or volcanic activity

 

2. Active Margins = Pacific Ocean style

 

        Situated at the leading edge of a continental plate

 

        Develops after initiation of subduction

 

        Typically narrow with rugged topography

 

        Outer edge typically forms inner wall of ocean trench

 

        Regionally unstable with much seismicity & volcanism

 

 

C. Physiological Features of a Continental Margin

         Continental Shelf

 

         Continental Slope

 

         Submarine Canyons

 

         Continental Rise

 

D. The Continental Shelf

 

        Shallow, submerged edge of continent between the shoreline and

continental slope (shelf-slope break)

 

        Has a very low sloping angle (<< 1degree)

 

        Typically shallow water depths (avg. = 75 m = 250 ft)

 

        Greatly influenced by fluctuations in sea level

 

        Shelf sediments are mainly influenced by waves and tidal currents

 

        Site of abundant mineral resources and sea life

 

E. Continental Slope and Rise (See Figs )

 

        Deeper, steeper, outermost edge of continent between the continental shelf and the deep ocean floor

 

        A continental rise may separate the continental slopefrom the deep ocean basin along passive margins

 

      A continental rise forms a thick pile of sediments that have accumulated at the base of the continental slope

 

        The shelf-slope break marks the abrupt transition between the slope and the shelf

 

        Location of Earth's greatest depository of sediments

      Roughly 70% of Earth's sediments

 

        Slope and Rise sediments are mainly influenced by gravity, and are transported down-slope via strong turbidity currents and deposit as submarine fans.

 

         Submarine canyons and fan deposits are present on all continental slopes and rises, and on some continental shelves

 

         Submarine fan deposits grade into deep-ocean deposits

 

 

V. Deep-Ocean Basins - True Oceanic Seafloor

A. Ocean Basins are Classified by Size and Extent

 

         Oceans - broad, large, and globally extensive

Examples: Pacific, Atlantic and Indian

 

         Seas - narrow, smaller, and regionally limited

Example: Mediterranean, South China, & Red

 

B. Deep-ocean basins are underlain by basaltic crust

1. Ocean Crust - A typical cross section (See Figs .)

 

        Layered basaltic crust covered by sediments

 

        Rugged volcanic surface covered by layers upon layers of very fine pelagic sediment

 

      Pelagic clays

 

      Silica and carbonates Oozes

 

        Oceanic igneous crustal column is also layered

 

      Pillow lava basalt

 

      Sheeted gabbroic dikes

 

      Massive gabbro (intrusions)

 

      Layered gabbro (intrusions)

 

      Layered Peridotite

 

        Oceanic crustal sections found on land are termed an ophiolite suite

 

C. Ocean Basins are Relatively Young Earth Features

 

         Oldest part of ocean basins is 180 million years old

 

         Average age of deep ocean seafloor is 60 million y.o.

 

         Age distribution pattern of deep-ocean crust is striking

 

         Study Text Figure - Ocean crust age map

 

 

D. Deep-ocean basins are rugged with variable relief, andhave a wide variety of

distinctive physiological features

      See Figure 4.30 - Seafloor Topographic Map

 

1. Mid-ocean ridges

 

2. Mid-ocean ridge fractures

 

3. Hydrothermal vents

 

4. Abyssal plains and Abyssal hills

 

5. Seamounts and Guyouts

 

6. Oceanic island chains

 

7. Oceanic plateaus

 

8. Trenches and Island arcs

 

E. Most Deep Ocean Features are the Result of Seafloor Spreading Processes

Occurring at Mid Ocean Ridges

1. Seafloor spreading processes create:

      Mid-ocean rift valleys and ridge flanks

      Vast expanses of ocean crust (abyssal plains)

      Chains of volcanoes (seamounts and islands)

      Transform fracture systems

      Hydrothermal systems (black smokers)

 

 

VI. Marine Sedimentation - Pelagic Clays and Oozes

A. Sediments - Defined

1. Sediments are particles that accumulate on the sea floor. Sediments have three major sources: lithogenous, biogenous, and hydrogenous.

B. Nearly All Deep Ocean Sediment is Very Fine Grained

 

         No mechanisms to transport coarse-grained material

      Exception is icebergs

 

         Vast majority of deep seafloor sediment is deposited via vertical settling of suspended material

 

         Sediment deposited from suspension is termed pelagic

 

C. Sediments Sources

1. Sediments have four major sources:

        Lithogenous

        Biogenous

        Hydrogenous

        Cosmogenous

 

2. Classification Chart of Marine Sediments See Table 5.2

D. Lithogenous Sediments

1. Lithogenous Sediments come from Rocks.

         Lithogenous sediments are produced by weathering processes, which break up rocks into smaller fragments and, sometimes, change their chemical composition.

         Clays and quartz are major end products of weathering.

         Most lithogenous sediments are found near land, since nearly all lithogenous sediment comes from the continents.

2. River Sediment via Turbidity Currents

        Turbidites and Submarine Canyons

 

3. Windblown dust and volcanic ash

         Continentally derived

         Deep-sea clays are termed pelagic clay

 

4. Accumulation Rates of Lithogenous Sediments

        In river deltas, sediments can accumulate at >800 cm/year. Tidewater glacial fjords also have extremely high sedimentation rates.

        In estuaries and bays, such sediments accumulate at about 0.5 cm/year. These often "trap" the sediment carried by rivers and streams.

        On the continental shelf and slope, sediments accumulate at about 10-40 cm/1000 years. Some shelves presently accumulate no sediment at all.

        Large areas of the deep sea, deeper than about 4500 m, are also covered with lithogenous sediment, called red or brown clay. Most of this sediment reaches the mid-ocean regions where it is found via wind transport.

        The deep ocean has extremely low sedimentation rates, <1 cm/1000 years.

5. Lithogenous Sediment is Sorted by Size

        Lithogenous sediments tend to be made up of progressively smaller particles with increasing distance from shore. This is because the larger particles sink more quickly, and so can't be transported as far by ocean currents.

        An exception to this pattern is rocks and gravel rafted by glaciers.

 

E. Biogenous Sediments

1. Defined: Sediments consisting of at least 30% of the remains of marine plankton (small marine plants and animals) are termed biogenous oozes.

2. Two Categories of microscopic marine organism skeletons

 

        Carbonate hard parts = calcareous ooze

 

        Silica hard parts = siliceous ooze

3.      Calcareous oozes are made up of the remains of tiny shells or tests consisting of calcium carbonate.

        Coccolith oozes consist of plates from coccolithophorids (tiny, single-celled plants).

        Foraminiferal oozes consist of shells of ameba-like animals (foraminifera).

        Both coccolith and foram oozes are found mostly along the upper part of ridges and rises, and are more common in the Atlantic than in the Pacific. This is because calcium carbonate dissolves where pressure is high and temperature low, i.e., in waters of the deep sea.

        Also, carbon dioxide dissolves calcium carbonate. There is more carbon dioxide in deep Pacific water because it is older than deep Atlantic water.

        The depth at which calcium carbonate becomes totally dissolved is termed the CCCD

        Pteropod oozes consist of shells of tiny, snail-like animals. Because they dissolve more easily than forams or coccoliths, they are found only in relatively shallow water.

4.      Siliceous oozes are composed of plankton shells made of silica (opal).

        Radiolarian oozes are made up of the remains of radiolarians, which are also ameba-like animals. Radiolarian oozes are found under the equatorial Pacific.

        Diatomaceous oozes are made up of the remains of diatoms, single-celled plants. Diatomaceous oozes are found in highly productive regions, upwelling areas. The ocean surrounding Antarctica and the western coastal oceans of Africa and South America are upwelling areas.

        Siliceous oozes are only found where the amount of silica reaching the sediments is high. Elsewhere, silica dissolves.

5.      Accumulation Rates of Biogenous Oozes

        Accumulate faster than most lithogenous deep-sea sediments, at rates of about 1 cm/1000 years.

        Diatomaceous oozes on continental shelves accumulate at up to 1 cm/year.

F. Hydrogenous Sediments

1. Form by precipitation from seawater or pore water (the water between the mineral grains of sediments).

        Manganese nodules consist of iron and manganese oxides, and can be rich in some other metals like cobalt, nickel, and copper.

        Phosphorites, found in sediments of upwelling areas, and carbonates, found beneath very warm, saline waters, are two other types of hydrogenous sediment.

2. Accumulation Rates of Hydrogenous Sediments

        Accumulation rates are extremely slow. Manganese nodules grow very slowly, as little as 1 mm/million years.

 

G. Distribution of Deep-Sea Sediments - A Global Pattern

 

1. Pelagic clays carpet the deepest parts of the oceans

 

      Cover about 38% of world's deep-sea bottoms

 

      Deposition rate is roughly 2 mm/1000 years

 

2. Pelagic calcareous oozes cover a good portion of the Atlantic, Indian and

Southern Pacific Ocean seafloors

 

      Cover about 48% of world's deep-sea bottoms

 

      Accumulation rate is 1 to 6 cm per 1000 years

 

3. Pelagic siliceous oozes cover equatorial Pacific and Indian Ocean and high

latitude ocean seafloors

      Cover about 14% of world's deep-sea bottoms

 

      Accumulation rate is 1 to 6 cm per 1000 years

 

4. Hydrogenous sediments like manganese nodules are found mainly in the Pacific Basin

 

5. Study - Global ocean floor sediment distribution map

 

VII. Origin of Islands, Atolls, Guyouts and Reefs

A. Islands and Seamounts are formed by volcanism

 

1. Formed on or near mid-ocean ridges

 

2. Basaltic shield volcanoes

 

3. Migrate away from mid-ocean ridges over time

 

B. Atolls and Guyouts are Modified Oceanic Islands

 

1. Circular coral reef systems develop around islands.

 

2. Oceanic crust cools and subsides with increasing age, causing the attached

islands to also subside over time.

 

3. Islands slowly wear down to sea level by wave erosion.

 

4. Upwards reef growth keep ups with sinking island.

 

5. Island eventually worn down to below sea level, with only the growing reef

able to maintain at sea level.

                          This stage of an island is termed an atoll.

 

6. Eventually reef growth lags behind rate of atoll subsidence, and entire atoll

structure becomes permanently submerged - this is termed a guyout.

 

VIII. Birth, Growth and Death of an Ocean Basin

---- The Wilson Cycle ----

 

A. Initiation of New Ocean Basin via Continental Rifting

1. Initial stages of plate divergence

 

2. Rift valley floored by new basaltic (oceanic) crust.

 

3. Further widening of rift, marine waters begin filling valley

B. Young Ocean Basin is Born - A True Sea

 

1. Continued plate divergence now in full swing

 

2.True seafloor spreading in operation = Mini ocean basin

 

3. Matching set of opposing coastlines frame the sea

 

C. Full Maturation of Ocean Basin -

 

1. Divergence begins to stall - spreading rate slows

 

2. Continental margins, abyssal seafloors, and mid-ocean ridge

 

3. Fully-developed ocean has emerged with an age 200-400 Ma

 

D. Mature Ocean Basin Starts to Collapse near Its Margins

 

1. Old, dense ocean lithosphere becomes isostatically unstable

2. Subduction initiated; ocean basin lithosphere dives into upper mantle forming

ocean trenches and island arcs.

 

3. Beginning of plate convergence of sides of ocean basin

 

E. Collapsing Ocean Basin Becomes Narrow and Irregular

 

1. Plate convergence in full swing

 

2. Subduction zones established along continental margins

 

3. Extensive volcanic and uplifted mountain chains result from continued subduction

and intense collision forces

 

F. Total Collapse of Ocean Basin - Suturing of Continents

 

1. Plate convergence reaches an apex - subduction wanes

 

2. Last of oceanic lithosphere subducted - Ocean basin gone

 

3. Massive thrusted and uplifted mountain ranges form a complex continental suture

zone marking the site of the now totally collapsed ocean basin

 

 

 

IX. Sea Bottom Resources - Natural Treasure

A. Continental Margins

 

1. Oil and Gas

 

2. Sand and Gravel

 

3. Plankton, Fish and Shellfish

 

B. Deep-Sea Bottom

 

1. Manganese nodules

 

2. Massive sulphide deposits

 

3. Migratory fish

 

X. Seafloor Vocabulary -

Abyssal plain

Active continental margin

Atoll

Continental margin

Continental rise

Continental slope

Guyout

Isostatic equilibrium

Mid-oceanic ridge

Oceanic trench

Ooze

Ophiolite

Passive continental margin

Pelagic clay

Reef

Ridge fracture zones

Seamount

Submarine canyon

Submarine hydrothermal vent

Submarine fan

Turbidity current

Wilson cycle