Structure and composition of the oceans

Major Geochemical 

Systems/Cycles 

! Tectonics - governs cycling of rock, solid earth materials 

! Atmosphere - medium for gas-phase transport 

! Oceans - predominant water reservoir on Earth, medium for 

dissolved chemical transport, may also mediate 

continuation of tectonic system 

Oceanic Structure 

! Accumulations of water above oceanic crustal plates, also along low-lying edges of 

continental plates 

! Shallow ocean - along passive continental margins, where sediments can collect on 

continental edges 

! Deepest regions of the ocean - correspond with trenches at active subduction zones 

! Most of ocean floor - a vast plain, essentially featureless geologically - exceptions 

being mid-ocean rift volcanic features, intraplate volcanic activity (hotspots) 

Driving Energy Sources 

! Tectonics - Earth’s s interior heat, from radioactive decay 

! Oceans/Atmosphere - solar radiation 

University of Arizona 

Oceanic Structure 

! Ocean depth 

• Average depth: 3,700 meters 

• Deepest trench (Mariana Trench): 11,033 m 

• In comparison, Mt. Everest is 8,850 m above sea level 

• Deepest point in Atlantic Ocean: Puerto Rico trench, 8,648 m 

• Indian Ocean: Java trench, 7,125 m 

! Ocean water temperature 

• Lowest is about -2º C in polar waters 

• Equatorial surface waters can reach about 30º C 

! Salinity 

• 35 per mil dissolved solids (~3.5 % salts, by weight) 

• Mostly NaCl 

• Na + , Cl - , SO 2- 4 

, Ca 2+ , Mg 2+ , K + make up 99% of dissolved ocean salts

Ocean Water Composition 

! Sources of sea salts 

• Chemical weathering of oceanic crust material (basalt) 

• Minor contribution from continental crustal weathering - most 

dissolved components from continental weathering are deposited on 

seafloor as sediments, eventually subducted 

• Hydrothermal activity - at mid-ocean rifts, volcanic heating of water 

in ocean crust dissolves salts, minerals, released into oceans from 

hot springs at seafloor 

• Ancient oceans - geologic evidence suggests ocean water has been 

fairly constant in composition through geologic time, , with only minor 

variations 

• Mg-Ca ratio has changed, mainly due to increases and decreases in 

rate of mid-ocean volcanism 

– Results in changes to major sedimentary minerals formed by sea life 

– Calcite versus aragonite (both CaCO 3 ) 

Ocean Circulation 

! Surface currents: wind-driven - prevailing regional winds drive 

movement of shallow ocean water 

• Direct influence if wind only down to 100-200 m 

• Driving force imparted to water can extend down to 1 km or more 

! Upwelling currents - a type of wind-driven current that significantly 

influences ocean biological productivity 

• Occur where prevailing winds drive surface ocean water away from 

the coast 

• Deeper water rises to replace surface water 

• Deep, cold water is richer in phosphorus (P), a limiting nutrient 

• Coastal upwelling areas account for most of the oceans’ biological 

productivity, and most of commercial fish catches 

! Ocean upwelling current 

! Continental Shelves (Upwelling regions shown in circles) 

NOAA

Oceans and Climate 

! Oceans play a governing role in global climate, controlling the 

atmospheric CO 2 concentration through several mechanisms 

! Geologic timescales (millions of years) 

– Oceanic deposition of limestone (CaCO 3 ) 

– Subduction and volcanic re-release of CO 2 from limestone 

– Burial/exhumation of organic carbon on continental shelves 

! Millennial timescales (100s - 1000s of years) 

– Equilibration of CO 2 between oceans and atmosphere 

– Thermohaline oceanic circulation 

Climate Control - Millennial Timescales 

! Equilibration of CO 2 between oceans and atmosphere 

• Carbon dioxide dissolves into ocean water 

CO2(g) 

CO2(aq) 

• The more CO 2 

in the atmosphere, the more dissolves in the oceans 

• If CO 2 

is suddenly added to the atmosphere (e.g.( 

volcanoes), the 

oceans will take up some of the extra to re-establish equilibrium 

• Governed by a chemical reaction (above) that is determinate, or 

predictable, given the known volumes of the atmosphere and 

oceans, and the total mass of CO 2 

available to both reservoirs 

• Takes time - oceans circulate slowly, , and about 5,000 years are 

required to achieve complete equilibrium with the deep ocean 

Climate Control - Millennial Timescales 

! Map of global thermohaline circulation system 

! Thermohaline oceanic circulation 

• A global system of surface and deep ocean currents 

• Driven not by wind, but by temperature-density changes in ocean 

water as it circulates 

• As surface ocean water reaches the North Atlantic, near Greenland, 

it becomes colder, and therefore denser 

• Denser salt water sinks, , pulling a downward current to the ocean 

floor 

• Deep, cold, dense ocean water circulates until it reaches the tropics, 

runs up against the continental edge, warms up and rises again 

• The entire system forms a closed loop, , constantly circulating 

• Delivers CO 2 

to the deep ocean, also moves heat from tropics to 

poles (i.e.( 

Gulf Stream Current) 

• Depends on orientation of continents…. . Likely has not been a 

permanent feature of global oceanic circulation

Climate Control - Geologic Timescales 

! Burial / Exhumation of organic carbon 

• River deltas, , productive shallow inland seas, , collect dead organic 

matter in sediments 

• Plankton rain down constantly onto seafloor 

• Dead plant material delivered by rivers from inland 

• Eventually can form into coal, petroleum 

• Carbon that is sequestered into sediments is removed from the 

global C cycle, and thus from the atmosphere 

• When organic carbon in sediments is exhumed, by tectonic uplift of 

continental shelves, it will weather, return to the atmosphere as CO 2 

• Depends on location of deposition, rate of regional tectonic uplift 


! Oceanic deposition of limestone 

• Marine plankton that are 

calcareous produce a shell 

of CaCO 3 mineral (calcite( 

or 

aragonite)… most 

calcareous plankton are 

single-celled algae 

• Dead plankton shells 

constantly rain down 

through the oceanic water 

column, and collect onto 

seafloor 

• Carbonate shoals - 

example: Bahamas 

• Eventually, calcite sands are 

buried, compressed into 

limestone 


! Deposition of limestone requires… 

• Ca from mineral weathering on the continents, delivered by rivers to 

the ocean 

• CO 2 

from the atmosphere, drawn in as oceanic CO 2 

is used up by 

plankton growth 

• The faster Ca is delivered to the oceans, the faster limestone is 

deposited onto the ocean floor 

• And… the faster CO 2 

is drawn out of the atmosphere 

• Mineral weathering - on the continents, controls the rate of 

limestone production in the oceans, and therefore the rate of CO 2 

drawdown from the atmosphere 

Climate-Ocean Feedback 

! Hot Climate (high CO 2 concentration in atmosphere) 

• Warmer atmosphere, more evaporation from oceans, more 

precipitation 

• With more rainfall, continental rocks are exposed to more intense 

weathering 

• Chemical weathering reactions proceed faster at higher 

temperatures 

• More Ca is delivered to oceans by river drainage from the continents 

• More plankton growth, thus more limestone production 

• CO 2 

is bled from the atmosphere, eventually cooling the planet…

Climate-Ocean Feedback 

! Cold Climate (low CO 2 concentration in atmosphere) 

• Colder atmosphere, less evaporation from oceans, less precipitation 

• Arid climate slows the rate of rock weathering 

• Arid climate means less river flow to the oceans 

Subduction-Related Volcanism 

! Without subduction and volcanism, CO 2 would undergo a one-way trip to the ocean floor 

! No return of CO 2 to the atmosphere - eventually Earth would freeze over 

! Subduction destroys ocean crust, sediments laid down on ocean floor, returns volatiles 

(CO 

2 , H 2 O) ) to the atmosphere through volcanism…. . closes the cycle 

• Less Ca is delivered to oceans by river drainage from the continents 

• Less plankton growth, less limestone is produced 

• CO 2 

is allowed to build up in the atmosphere, eventually warming 

the planet… 

Origins of Life 

! Likely to have occurred in the oceans, but not in shallow water 

! The chemical conditions favoring life were most favorable in the 

deep ocean, at hydrothermal vents along MORs 

Hydrothermal Vents 

! Seafloor hot springs along MORs 

– Complex, vigorous chemical cycling 

– Chimneys made of FeS 2 (pyrite), CaSO 4 

(gypsum) 

– Abundant life, based on Sulfur-using bacteria 

Seafloor Hydrothermal Vent 

US DOE

Structure and composition of the oceans

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