Composition of Seawater Features of the Sea Floor Oceans – Part I Introduction Composition of Seawater Features of the Sea Floor Sediments

Introduction The “Blue Planet” Seventy-one percent of Earth’s surface is represented by oceans and marginal seas Continents and islands comprise the remaining 29

The Oceans of Earth About 7% of the size of the Pacific About half the size of the Pacific and not quite as deep The largest and has the greatest depth Slightly smaller than the Atlantic, largely a Southern Hemisphere body

Earth’s Northern & Southern Hemispheres

The Composition of Seawater

Composition of Seawater Salinity Salinity is the total amount of solid material dissolved in water Typically expressed as 0/00 or parts per thousand (ppt) 350/00 is world average

Composition of Seawater Salinity Anion + cation = a salt Salt = NaCl = Na+ + Cl- Na+ has a positive charge (cation) Cl- has a negative charge (anion)

Composition of Seawater Salinity What goes in must equal what goes out Addition of sea salts Chemical weathering of rocks on continents (cations) Volcanic eruptions / outgassing (anions)

Composition of Seawater Salinity What goes in must equal what goes out Addition of sea salts Removal of sea salts Sea spray Evaporites Biologic processes Magma at M.O.R.

Important Factors: Precipitation Evaporation Freezing Run-off

The Ocean’s Layered Structure Temperature and salinity change with depth in the oceans A three-layered structure exists in the open ocean

The Ocean’s Layered Structure Temperature and salinity change with depth in the oceans Salinity variations with depth correspond to the general three-layered structure described for temperature Thermocline = zone of rapid temperature change Halocline = zone of rapid salinity change

Features of the Ocean Floor

Mapping the Ocean Floor Bathymetry – Measurement of ocean depths and the charting of the shape or topography of the ocean floor

Mapping the Ocean Floor Echo sounder (also called sonar) Invented in the 1920s Primary instrument for measuring depth Reflects sound from ocean floor

Mapping the Ocean Floor Multibeam sonar Employs and array of sound sources and listening devices Obtains a profile of a narrow strip of seafloor

Mapping the Ocean Floor Measuring the shape of the ocean surface from space Employs satellites equipped with radar altimeters

Mapping the Ocean Floor Three major topographic units of the ocean floor

Continental Margins Classifications of ocean floor Continental Margins – the submerged outer edge of a continent Ocean Basin – the deep seafloor beyond the continental margin

Continental Margins There are two types of continental margins: Passive margins Also called Atlantic-type margins Face the edges of diverging tectonic plates. Very little volcanic or earthquake activity

Continental Margins There are two types of continental margins: Active margins AKA as Pacific-type margins Located near the edges of converging plates. Active margins are the site of volcanic and earthquake activity.

Passive Continental Margins EARTH: An Introduction to Physical Geology 7th ed / Tarbuck & Lutgens fig 18.4 Continental Shelf Shelf Break Continental Slope Submarine Canyons Continental Rise

Passive Continental Margins Continental Shelf Found at margins of continents Relatively flat Average 65-100 km in width Formed by: Thick accumulations of shallow-water sediments Some areas are mantled by extensive glacial deposits Ends at change in slope (Shelf break) back

Passive Continental Margins Continental Slope Boundary between continental crust and oceanic crust Extends from shelf break to rise Submarine canyons are major features Submarine canyons formed by: River erosion Turbidity currents

Submarine Canyons and Turbidity Currents

Graded Beds Each layer grades from coarse at its base to fine at the top. EARTH: An Introduction to Physical Geology 7th ed / Tarbuck & Lutgens fig 6.21 back

Passive Continental Margins Continental rise Found in regions where trenches are absent Located where the continental slope grades into a more gradual incline as it meets the deep ocean floor Deep-sea fans form here back

Active Continental Margins Located primarily around the Pacific Ocean Continental slope descends abruptly into a deep-ocean trench Accumulations of deformed sediment and scraps of ocean crust form accretionary wedges Some subduction zones have little or no accumulation of sediments

Active Continental Margins Deep-ocean trenches Long, relatively narrow features Deepest parts of ocean Most are located in the Pacific Ocean Sites where moving lithospheric plates plunge into the mantle Associated with volcanic activity Volcanic islands arcs Continental volcanic arcs

An Active Continental Margin

Deep-Ocean Basins Features of the deep-ocean floor Oceanic Ridges Hydrothermal Vents Abyssal Plains and Abyssal Hills Seamounts and Guyots Trenches and Island Arcs

Deep-Ocean Basins Ridges and Rises Underwater volcanic mountain chain extends for 65,000 km 1,000 km wide 1,000-2,000 m high South West Indian Ridge

Deep-Ocean Basins

Deep-Ocean Basins Ridges and Rises Ridges = steep slopes Rises = gentle slopes Contain central rift valley 15-50 km wide 500-1500m deep Offset by fractures

Deep-Ocean Basins Hydrothermal vents are sites where superheated water containing dissolved minerals and gases escapes through fissures, or vents.

Deep-Ocean Basins Abyssal hills Seamounts Flat areas of sediment-covered ocean floor found between the continental margins and oceanic ridges. Also small, extinct volcanoes or rock intrusions near the oceanic ridges. Seamounts Volcanic projections from the ocean floor that do not rise above sea level. Flat-topped seamounts eroded by wave action are called guyots. © 2002 Brooks/Cole, a division of Thomson Learning, Inc.

Seamounts & Guyots Thurman Essentials of Oceanography 6/e

Deep-Ocean Basins: Trenches Mariana trench the deepest (11,020 m) Peru-Chile trench the longest ( 5900 km) EARTH: An Introduction to Physical Geology 7th ed / Tarbuck & Lutgens Distribution of the world's oceanic trenches, ridge system, fracture zones, and transform faults. Where transform faults offset ridge segments, they permit the ridge to change direction (curve) as can be seen in the Atlantic Ocean.

EARTH: An Introduction to Physical Geology 7th ed / Tarbuck & Lutgens http://agrolink.moa.my/dof/edukit/seaocean/factsheet1.html

Ocean Sediments

Seafloor Sediments Particles entering the ocean Accumulate Rapidly on continental margin (neritic) Slowly in the deep ocean (pelagic) Reflect ocean history

Seafloor Sediments Thickness varies Thickest in trenches—Accumulations may approach 10 kilometers 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

Seafloor Sediments Sediment can be classified by particle size. Waves and currents generally transport smaller particles farther than larger particles.

Seafloor Sediments Types of seafloor sediments Lithogenous sediment Material weathered from continental rocks Sands and muds from continental margins Glacial deposits Clays Fine particles remain suspended for a long time Oxidation often produces red and brown colored sediments

Sediment Sources: Terrigenous Sediments Thurman Essentials of Oceanography 6/e fig 4.06 Most clay-sized quartz in deep-sea sediments is believed to have been transported by wind. Blue arrows indicate prevailing wind. After Leinen, M., et al. 1986. Distribution of biogenic silica and quartz in recent deep-sea sediments. Geology 14:3, 199203.

Seafloor Sediments Types of seafloor sediments Lithogenous sediment Biogenous sediment Ooze = greater than 30% biogenous sediment Distribution related to sediment supply, rate of dissolution and sediment dilution Siliceous fossils at all depths, calcareous not below Calcium Carbonate Compensation depth Diatoms and radiolarians = major siliceous microfossils Calcareous = foraminifera, pteropods, coccolithophores

Biogenous Sediment Accumulation Thurman Essentials of Oceanography 6/e fig 4.15 Relationships among carbonate compensation depth, the mid-ocean ridge, sea floor spreading, productivity, and destruction allow calcareous ooze to be found below the CCD.

Seafloor Sediments Siliceous Oozes Fine-grained pelagic deposit Composition: 30% siliceous (SiO2) material of organic origin Diatoms (phytoplankton) and Radiolaria (zooplankton) Siliceous particles dissolve more slowly than calcareous particles siliceous ooze Fine-grained pelagic deposit of the deep-ocean floor with more than 30% siliceous material of organic origin. Radiolaria and diatom remains are the major constituents of the siliceous oozes, which tend to occur at depths in excess of 4500 m. A Dictionary of Earth Sciences, © Oxford University Press 1999 (www.xrefer.com)

Siliceous Oozes Thurman Essentials of Oceanography 6/e Siliceous ooze accumulates on the ocean floor beneath areas of high productivity, where the rate of accumulation of siliceous tests is greater than the rate at which silica is being dissolved.

Diatoms Composed of SiO2 Phytoplankton Base of food chain http://www.microscopy-uk.org.uk/micropolitan/marine/diatom/index.html

Seafloor Sediments Calcareous Oozes Wide-spread in relatively shallow areas of the deep sea CaCO3 particles dissolve at “Carbonate Compensation Depth” = (CCD) Atlantic: ~ 4,000 m Pacific: ~ 500 - 1,500 m

Biogenous Sediment Accumulation Thurman Essentials of Oceanography 6/e fig 4.15 Relationships among carbonate compensation depth, the mid-ocean ridge, sea floor spreading, productivity, and destruction allow calcareous ooze to be found below the CCD.

Radiolaria Composed of SiO2 Zooplankton Base of food chain http://www.microscopy-uk.org.uk/micropolitan/marine/radiolaria/index.html

Foraminifera Composed of calcium carbonate (CaCO3) Zooplankton

Seafloor Sediments Types of seafloor sediments Terrigenous sediment Biogenous sediment Hydrogenous sediment Minerals that crystallize directly from seawater Most common types include Manganese nodules Calcium carbonates Metal sulfides Evaporites

Seafloor Sediments Ferromanganese Nodules First sub-marine ferromanganese concretions were discovered in 1868 on the Kara Sea (Russia) Characteristics - small balls (lightly flattened), dark-brown, and 5 - 10 cm in diameter Found at depths of 4,000 to 6,000 m Not clear how these nodules form http://www.ifremer.fr/drogm_uk/Realisation/Miner/Nod/texte/txt1_2.html

Distribution Of Manganese Nodules Thurman Essentials of Oceanography 6/e fig 4.23 This generalized map shows the distribution of manganese nodules on the sea floor. After Cronan, D. S. 1977. Deep sea nodules: Distribution and geochemistry, in Glasby, G. P., ed., Marine Manganese Deposits, Elsevier Scientific Publishing Co.

Seafloor Sediments Evaporites are precipitates that form as water evaporates or as the conditions in the water change include many salts with economic importance. currently form in the Gulf of California, the Red Sea, and the Persian Gulf. © 2002 Brooks/Cole, a division of Thomson Learning, Inc.

Studying Sediments How do scientists study sediments? Deep-water cameras Clamshell samplers Dredges Piston Corers Core libraries Seismic profilers

Studying Sediments What can scientists learn by studying sediments? Historical information Location of natural resources, especially crude oil and natural gas

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