Structure and composition of the oceans
Structure and composition of the oceans
Structure and composition of the oceans
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Major Geochemical<br />
Systems/Cycles<br />
! Tectonics - governs cycling <strong>of</strong> rock, solid earth materials<br />
! Atmosphere - medium for gas-phase transport<br />
! Oceans - predominant water reservoir on Earth, medium for<br />
dissolved chemical transport, may also mediate<br />
continuation <strong>of</strong> tectonic system<br />
Oceanic <strong>Structure</strong><br />
! Accumulations <strong>of</strong> water above oceanic crustal plates, also along low-lying edges <strong>of</strong><br />
continental plates<br />
! Shallow ocean - along passive continental margins, where sediments can collect on<br />
continental edges<br />
! Deepest regions <strong>of</strong> <strong>the</strong> ocean - correspond with trenches at active subduction zones<br />
! Most <strong>of</strong> ocean floor - a vast plain, essentially featureless geologically - exceptions<br />
being mid-ocean rift volcanic features, intraplate volcanic activity (hotspots)<br />
Driving Energy Sources<br />
! Tectonics - Earth’s s interior heat, from radioactive decay<br />
! Oceans/Atmosphere - solar radiation<br />
University <strong>of</strong> Arizona<br />
Oceanic <strong>Structure</strong><br />
! Ocean depth<br />
• Average depth: 3,700 meters<br />
• Deepest trench (Mariana Trench): 11,033 m<br />
• In comparison, Mt. Everest is 8,850 m above sea level<br />
• Deepest point in Atlantic Ocean: Puerto Rico trench, 8,648 m<br />
• Indian Ocean: Java trench, 7,125 m<br />
! Ocean water temperature<br />
• Lowest is about -2º C in polar waters<br />
• Equatorial surface waters can reach about 30º C<br />
! Salinity<br />
• 35 per mil dissolved solids (~3.5 % salts, by weight)<br />
• Mostly NaCl<br />
• Na + , Cl - , SO 2- 4<br />
, Ca 2+ , Mg 2+ , K + make up 99% <strong>of</strong> dissolved ocean salts
Ocean Water Composition<br />
! Sources <strong>of</strong> sea salts<br />
• Chemical wea<strong>the</strong>ring <strong>of</strong> oceanic crust material (basalt)<br />
• Minor contribution from continental crustal wea<strong>the</strong>ring - most<br />
dissolved components from continental wea<strong>the</strong>ring are deposited on<br />
seafloor as sediments, eventually subducted<br />
• Hydro<strong>the</strong>rmal activity - at mid-ocean rifts, volcanic heating <strong>of</strong> water<br />
in ocean crust dissolves salts, minerals, released into <strong>oceans</strong> from<br />
hot springs at seafloor<br />
• Ancient <strong>oceans</strong> - geologic evidence suggests ocean water has been<br />
fairly constant in <strong>composition</strong> through geologic time, , with only minor<br />
variations<br />
• Mg-Ca ratio has changed, mainly due to increases <strong>and</strong> decreases in<br />
rate <strong>of</strong> mid-ocean volcanism<br />
– Results in changes to major sedimentary minerals formed by sea life<br />
– Calcite versus aragonite (both CaCO 3 )<br />
Ocean Circulation<br />
! Surface currents: wind-driven - prevailing regional winds drive<br />
movement <strong>of</strong> shallow ocean water<br />
• Direct influence if wind only down to 100-200 m<br />
• Driving force imparted to water can extend down to 1 km or more<br />
! Upwelling currents - a type <strong>of</strong> wind-driven current that significantly<br />
influences ocean biological productivity<br />
• Occur where prevailing winds drive surface ocean water away from<br />
<strong>the</strong> coast<br />
• Deeper water rises to replace surface water<br />
• Deep, cold water is richer in phosphorus (P), a limiting nutrient<br />
• Coastal upwelling areas account for most <strong>of</strong> <strong>the</strong> <strong>oceans</strong>’ biological<br />
productivity, <strong>and</strong> most <strong>of</strong> commercial fish catches<br />
! Ocean upwelling current<br />
! Continental Shelves (Upwelling regions shown in circles)<br />
NOAA
Oceans <strong>and</strong> Climate<br />
! Oceans play a governing role in global climate, controlling <strong>the</strong><br />
atmospheric CO 2 concentration through several mechanisms<br />
! Geologic timescales (millions <strong>of</strong> years)<br />
– Oceanic deposition <strong>of</strong> limestone (CaCO 3 )<br />
– Subduction <strong>and</strong> volcanic re-release <strong>of</strong> CO 2 from limestone<br />
– Burial/exhumation <strong>of</strong> organic carbon on continental shelves<br />
! Millennial timescales (100s - 1000s <strong>of</strong> years)<br />
– Equilibration <strong>of</strong> CO 2 between <strong>oceans</strong> <strong>and</strong> atmosphere<br />
– Thermohaline oceanic circulation<br />
Climate Control - Millennial Timescales<br />
! Equilibration <strong>of</strong> CO 2 between <strong>oceans</strong> <strong>and</strong> atmosphere<br />
• Carbon dioxide dissolves into ocean water<br />
CO2(g)<br />
CO2(aq)<br />
• The more CO 2<br />
in <strong>the</strong> atmosphere, <strong>the</strong> more dissolves in <strong>the</strong> <strong>oceans</strong><br />
• If CO 2<br />
is suddenly added to <strong>the</strong> atmosphere (e.g.(<br />
volcanoes), <strong>the</strong><br />
<strong>oceans</strong> will take up some <strong>of</strong> <strong>the</strong> extra to re-establish equilibrium<br />
• Governed by a chemical reaction (above) that is determinate, or<br />
predictable, given <strong>the</strong> known volumes <strong>of</strong> <strong>the</strong> atmosphere <strong>and</strong><br />
<strong>oceans</strong>, <strong>and</strong> <strong>the</strong> total mass <strong>of</strong> CO 2<br />
available to both reservoirs<br />
• Takes time - <strong>oceans</strong> circulate slowly, , <strong>and</strong> about 5,000 years are<br />
required to achieve complete equilibrium with <strong>the</strong> deep ocean<br />
Climate Control - Millennial Timescales<br />
! Map <strong>of</strong> global <strong>the</strong>rmohaline circulation system<br />
! Thermohaline oceanic circulation<br />
• A global system <strong>of</strong> surface <strong>and</strong> deep ocean currents<br />
• Driven not by wind, but by temperature-density changes in ocean<br />
water as it circulates<br />
• As surface ocean water reaches <strong>the</strong> North Atlantic, near Greenl<strong>and</strong>,<br />
it becomes colder, <strong>and</strong> <strong>the</strong>refore denser<br />
• Denser salt water sinks, , pulling a downward current to <strong>the</strong> ocean<br />
floor<br />
• Deep, cold, dense ocean water circulates until it reaches <strong>the</strong> tropics,<br />
runs up against <strong>the</strong> continental edge, warms up <strong>and</strong> rises again<br />
• The entire system forms a closed loop, , constantly circulating<br />
• Delivers CO 2<br />
to <strong>the</strong> deep ocean, also moves heat from tropics to<br />
poles (i.e.(<br />
Gulf Stream Current)<br />
• Depends on orientation <strong>of</strong> continents…. . Likely has not been a<br />
permanent feature <strong>of</strong> global oceanic circulation
Climate Control - Geologic Timescales<br />
! Burial / Exhumation <strong>of</strong> organic carbon<br />
• River deltas, , productive shallow inl<strong>and</strong> seas, , collect dead organic<br />
matter in sediments<br />
• Plankton rain down constantly onto seafloor<br />
• Dead plant material delivered by rivers from inl<strong>and</strong><br />
• Eventually can form into coal, petroleum<br />
• Carbon that is sequestered into sediments is removed from <strong>the</strong><br />
global C cycle, <strong>and</strong> thus from <strong>the</strong> atmosphere<br />
• When organic carbon in sediments is exhumed, by tectonic uplift <strong>of</strong><br />
continental shelves, it will wea<strong>the</strong>r, return to <strong>the</strong> atmosphere as CO 2<br />
• Depends on location <strong>of</strong> deposition, rate <strong>of</strong> regional tectonic uplift<br />
Climate Control - Geologic Timescales<br />
! Oceanic deposition <strong>of</strong> limestone<br />
• Marine plankton that are<br />
calcareous produce a shell<br />
<strong>of</strong> CaCO 3 mineral (calcite(<br />
or<br />
aragonite)… most<br />
calcareous plankton are<br />
single-celled algae<br />
• Dead plankton shells<br />
constantly rain down<br />
through <strong>the</strong> oceanic water<br />
column, <strong>and</strong> collect onto<br />
seafloor<br />
• Carbonate shoals -<br />
example: Bahamas<br />
• Eventually, calcite s<strong>and</strong>s are<br />
buried, compressed into<br />
limestone<br />
Climate Control - Geologic Timescales<br />
! Deposition <strong>of</strong> limestone requires…<br />
• Ca from mineral wea<strong>the</strong>ring on <strong>the</strong> continents, delivered by rivers to<br />
<strong>the</strong> ocean<br />
• CO 2<br />
from <strong>the</strong> atmosphere, drawn in as oceanic CO 2<br />
is used up by<br />
plankton growth<br />
• The faster Ca is delivered to <strong>the</strong> <strong>oceans</strong>, <strong>the</strong> faster limestone is<br />
deposited onto <strong>the</strong> ocean floor<br />
• And… <strong>the</strong> faster CO 2<br />
is drawn out <strong>of</strong> <strong>the</strong> atmosphere<br />
• Mineral wea<strong>the</strong>ring - on <strong>the</strong> continents, controls <strong>the</strong> rate <strong>of</strong><br />
limestone production in <strong>the</strong> <strong>oceans</strong>, <strong>and</strong> <strong>the</strong>refore <strong>the</strong> rate <strong>of</strong> CO 2<br />
drawdown from <strong>the</strong> atmosphere<br />
Climate-Ocean Feedback<br />
! Hot Climate (high CO 2 concentration in atmosphere)<br />
• Warmer atmosphere, more evaporation from <strong>oceans</strong>, more<br />
precipitation<br />
• With more rainfall, continental rocks are exposed to more intense<br />
wea<strong>the</strong>ring<br />
• Chemical wea<strong>the</strong>ring reactions proceed faster at higher<br />
temperatures<br />
• More Ca is delivered to <strong>oceans</strong> by river drainage from <strong>the</strong> continents<br />
• More plankton growth, thus more limestone production<br />
• CO 2<br />
is bled from <strong>the</strong> atmosphere, eventually cooling <strong>the</strong> planet…
Climate-Ocean Feedback<br />
! Cold Climate (low CO 2 concentration in atmosphere)<br />
• Colder atmosphere, less evaporation from <strong>oceans</strong>, less precipitation<br />
• Arid climate slows <strong>the</strong> rate <strong>of</strong> rock wea<strong>the</strong>ring<br />
• Arid climate means less river flow to <strong>the</strong> <strong>oceans</strong><br />
Subduction-Related Volcanism<br />
! Without subduction <strong>and</strong> volcanism, CO 2 would undergo a one-way trip to <strong>the</strong> ocean floor<br />
! No return <strong>of</strong> CO 2 to <strong>the</strong> atmosphere - eventually Earth would freeze over<br />
! Subduction destroys ocean crust, sediments laid down on ocean floor, returns volatiles<br />
(CO<br />
2 , H 2 O) ) to <strong>the</strong> atmosphere through volcanism…. . closes <strong>the</strong> cycle<br />
• Less Ca is delivered to <strong>oceans</strong> by river drainage from <strong>the</strong> continents<br />
• Less plankton growth, less limestone is produced<br />
• CO 2<br />
is allowed to build up in <strong>the</strong> atmosphere, eventually warming<br />
<strong>the</strong> planet…<br />
Origins <strong>of</strong> Life<br />
! Likely to have occurred in <strong>the</strong> <strong>oceans</strong>, but not in shallow water<br />
! The chemical conditions favoring life were most favorable in <strong>the</strong><br />
deep ocean, at hydro<strong>the</strong>rmal vents along MORs<br />
Hydro<strong>the</strong>rmal Vents<br />
! Seafloor hot springs along MORs<br />
– Complex, vigorous chemical cycling<br />
– Chimneys made <strong>of</strong> FeS 2 (pyrite), CaSO 4<br />
(gypsum)<br />
– Abundant life, based on Sulfur-using bacteria<br />
Seafloor Hydro<strong>the</strong>rmal Vent<br />
US DOE