CHAPTER 23:MARINE

GEOLOGY

Earth’s Water

• Earth's oceans are unique in the Solar System and are the largest single feature on the planet.

• 70% of the Earth’s surface is ocean water.

Earth’s Water

• With an average depth of 3,800 metres, the oceans contain ~97% of Earth's water.

Marine Geology is the study of geologic processes within ocean basins

• The five recognized oceans and the three largest seas

Oceans

Salinity

• Chemical weathering and hydrothermal processes dissolve ions and compounds.

• >90% of all dissolved ions are chloride and sodium. Average global

salinity ~35 parts per

1,000

Can you explain the distributions of some of the areas of very high and very low salinity?

Why are the oceans salty?

Oceanic Gyres

• There are numerous surface-ocean currents, and in several areas they have circular patterns known as oceanic gyres

Why are ocean gyres clockwise in the northern hemisphere and counter-clockwise in the southern hemisphere?

Currents at Depth

• In addition to the surface currents ocean waters (and the heat energy they contain) are also redistributed by currents at depth

Ekamn Transport

• As the Coriolis Effect deflects a current, water in lower layers deflects more slowly than surface water.

• The resulting movement of water is known as Ekman Transport

Shelf, Slope and Rise

• The continental margin consists of the shelf, slope, and rise.

Continental Shelf

• A continental shelf is the submerged border of a continent

What type of crust (oceanic or continental) would you expect to find beneath a continental shelf?

Continental Shelves

• Continental shelves are also rich in marine life

Canada’s Atlantic shelf is rich with oil and gas deposits, which are exploited by almost 330 active wells (red dots).

By comparison, the U.S. Gulf of Mexico has almost 4,000 active wells.

Margins

• Passive Margins lack active plate boundaries

• Active Margins, located at plate boundaries, may have Accreted Terrane.

Canada has 3 ocean margins. Which are active and which are passive?

Submarine Canyons

• Submarine Canyons are valleys that cut into the shelf and slope.

Turbidity Currents

• Turbidity Currents result from the collapse of an unconsolidated sedimentary deposit on the slope or shelf edge

Continental Margin

• Most ocean sediment is deposited on the continental margin

U.S. Continental Shelf, Gulf of Mexico

Neritic and Hemopelagic Sediments

• Neritic Sediments (between the shoreline and the shelf edge) can be of several different types:– Detrital– Biogenic– Eolian– Residual– Authigenic– Relict

• Hemipelagic Sediments - draped on the upper and middle slopes - grade from terrigenous muds into biogenic sediments

In this context what is the difference between “detrital” and “authigenic”?

Detrital Sediment from a stream

Wind blown (eolian) detrital sediment

Deltas

• Some deltas contain so much detrital sediment that the topography of the continental shelf is hidden by the submarine fan.

Why does the Ganges Brahmaputra delta and submarine fan have so

much sediment?

Pelagic Sediment

• Pelagic sediment covers the abyssal plains

Why do the Atlantic ocean margins have so much more sediment than the Pacific ones?

Calcareous Ooze

• Calcite is stable in shallow ocean water but below the Carbonate Compensation Depth (CCD) (which is typically around 4000 m) CaCO3 dissolves.

foraminifera

coccolithophores

Diatoms and Radiolaria

• Diatoms and radiolaria make their tests out of silica, and they accumulate on the sea floor to form siliceous mud (as opposed to calcareous mud)

radiolariadiatoms

Pelagic Sediment

• Pelagic sediment distribution reflects water depth, biological productivity and terrigenous inputs

Mid-Ocean Ridge

• The mid-ocean ridge is the site of seafloor spreading.

• The water depth near to the ridge is typically about half of what it is elsewhere.

Oceanic crust is formed at mid-ocean ridges

Marine Stratigraphy across the Mid-Atlantic Ridge

Why does calcareous sediment accumulate on the mid-Atlantic ridge, but not on the sea-floor away from the ridge?

Marine Stratigraphy along a N - S section through the Pacific Ocean

North South

Abyssal hills are seamounts extending above the sediments of the seafloor

Why does the source and composition of sediment change between the coast, shelf, rise, abyssal plain, and oceanic ridge?

Oceanic trenches

• Oceanic trenches occur at subduction zones

Why are some of the trenches the deepest parts of the oceans?

The major world trenches

Human impacts to reefs include pollution, over fishing, and climate change

What are two climate-related impacts to tropical reefs?

Human impacts to the oceans are global in extent

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