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Chapter 9 - Ocean Circulation

The Atmosphere and Ocean Circulation Systems Are Linked

Click on thumbnail images for a larger view.
The global atmospheric circulation system influences the movement of air masses in general "belts" that move air in rotating masses within zones around the planet (Figure 9-1). These relatively stationary wind belts impact the surface of the oceans, creating currents that circulate waters in the oceans under the influence of coriolis effect, creating five large subtropical gyres encircling the major oceans basins (Figure 9-2).

Currents in the oceans include surface currents and deep currents:
surface currents are driven horizontally by effects of the wind.
deep currents are driven horizontally and vertically by differences in density
(density changes typically start near the surface).

Ocean circulation is also influenced by seawater temperature and density.
Warm water in the tropics flows in currents to polar regions where it cools and the formation of sea ice concentrates the salt in seawater, increasing its density so that it sinks.
Cold and salty water (concentrated by surface evaporation) sinks. Elsewhere seawater rises where it is displaced by colder and saltier water.

World wind zones Gyres in the global ocean circulation system
Fig. 9-1. Global wind circulation patterns impact regional climates and drive the large surface currents in the global ocean circulation system. Fig. 9-2. Five large gyres circulate surface waters in the global oceans. These rotating subtropical gyres are influenced by the patterns of atmospheric winds and the coriolis effect.
   

Deep-Ocean "Thermohaline" Circulation

The deep ocean basins have slow moving currents (compared with the surface waters exposed to atmospheric winds (Figure 9-3). As currents move about the globe, evaporation increases salinity. Increased salinity combined with cooling increases seawater density, allowing affected seawater to sink into the deep ocean. The movement of surface waters downward supplies oxygen to the seabed, assisting in the decay of organic matter. The deep, slow-moving water picks up nutrients from the seafloor and from decaying organic particles sinking through the water column. In locations where deep-water upwells to the surface, these nutrients supply the ingredients for phytoplankton blooms, providing food for the food chain.

Animations: global perspectives of ocean currents based on salinity and temperature
Surface Salinity (annual) (NASA)
Perpetual Ocean (2005-2007) [NASA] global ocean circulation time lapse - YouTube video.
22 Years Sea Surface Temperature 1985-2007 [NOAA Polar satellite data] YouTube video
Worldwide Sea Surface Temperature simulation 2008 YouTube video

Thermohailine Ciruclation
Fig. 9-3. Thermohaline Circulation: cold and salty ocean water is dense and sinks, warm water stays at the surface until it cools and "dries"(increasing salinity) before it can sink.

Sea Ice and Halosaline Circulation

Glaciers flowing into the ocean contribute large amounts of iceberg and sea ice to the polar ocean regions. However, sea ice also forms where very cold air is in contact with the ocean surface. Currents in the upper sea (mixing zone) can inhibit the formation of sea ice. Water is most dense slightly above the freezing point and tends to sink whereas ice floats. Once sea ice starts to form the salt is either expelled back into the seawater and some is concentrated in microscopic pockets trapped in the sea ice. Antarctic sea ice is typically 1 to 2 meters (3 to 6 feet) whereas most of sea ice in the Arctic is 2 to 3 meters (6 to 9 feet) thick. However, in some Arctic regions sea ice can grow to 4 to 5 meters (12 to 15 feet) thick. The formation of sea increases the salinity of the seawater, and the combination of the increased salinity and cold water results in the formation of dense water that sinks into the deep ocean, driving the thermohaline circulation through the world’s deep ocean basins.

Arctic Sea Ice
time lapse from 1987-2009 (NOAA/NSIDC satellite data YouTube video)

Origin of glaciers, icebergs and sea ice
Fig 9-4. Origin of glaciers, icebergs, and sea ice. Sheets of sea ice form and melt back with the seasons.

Surface Currents

Surface Currents involve large masses of water moving horizontally on the surface.

The transfer of wind energy to water is not very efficient
(only about 2% energy transfer of “friction” between water and air).
Wind produces both waves and currents (more on waves in Chapter 9).

Surface currents occur in the "mixing zone" within and above the pycnocline (layer of rapidly changing density).
Effects of surface currents is to redistribute heat from equatorial to polar regions.



Major ocean currents of the world
Fig. 9-5. Major surface currents of the oceans. See
Oceans Currents Map
(NOAA-NWS)
Mechanisms moving surface currents include:
Wind: major mechanism (result of atmospheric circulation patterns).
Solar heating: (direct heating by the sun) - a minor mechanism (influences surface waters but not water at depth).
Tides: (affect currents in coastal regions - tidal currents are discussed in Chapter 10).
Geography: locations of continents (and islands) influence direction and flow currents (acting as barriers to flow).

Subtropical gyres are large system of rotating ocean surface currents driven by global wind currents with the influence of Ekman Transport (see below) and continental geography (land masses restrict and deflect the flow of water currents).

Movement of Surface Currents

Moving water (like wind) are influenced by the coriolis effect (discussed in Chapter 7):
Moving water is deflected to the RIGHT in the Northern Hemisphere.
Moving water is deflected to the LEFT in the Southern Hemisphere.

The coriolis effect has a large influence on the movement of both surface water and deeper water. However, wind-driven currents move fastest near the ocean surface and diminish with depth. The difference in rate of movement results in a rotational process called Ekman transport.
Coriolis Effect
Fig. 9-6. Coriolis effect.

Ekman Spiral and Ekman Transport

Early sailors traveling in regions where icebergs are common noticed that the icebergs moved in a different direction than the wind (causing alarm as the icebergs were cutting across the paths of ships moving down wind).

Walfrid Ekman (1874-1954, a Swedish physicist) resolved the problem of why wind currents and water currents were not the same. The force of wind affect surface water molecules, which in turn, “drag” (by “friction”) deeper layers of water molecules below them.

The deeper below the surface, the slower the water moves compared to the water layer above it.

Surface movement ceases at a depth of about 100 meters (330 feet).

As noted above, both surface water and deeper water is deflected by the coriolis effect.
—90° to the right in the Northern Hemisphere
—90° to the left in the Southern Hemisphere.

Depth is important: Each successively deeper layer of water moves more slowly to the right (or left), creating a spiral effect (called the Ekman Spiral). Because the deeper layers of water move more slowly than the shallower layers, they tend to “twist around” and flow opposite to the surface current. Net result is that net transport in surface currents is 90° from wind.

This twisting character of ocean surface waters is called the Ekman spiral. The impact of the Ekman Spiral is enhanced where geographic features create barriers to the movement of water. Ekman transport is the net motion of a fluid (seawater) as the result of a balance between the coriolis effect and turbulent drag forces (within surface waters and geographic features (shoreline and seabed).
Iceberg and a NOAA ship near Antarctica
Fig. 9-7. Sailors of ships noticed that icebergs move in a different direction than the wind.
Ekman spiral
Fig. 9-8. The Ekman spiral

Boundary Currents

Boundary currents currents associated with gyres flow around the periphery of an ocean basin.


• Boundary currents are ocean currents with dynamics determined by the presence of a coastline.

Two distinct categories of boundary currents:
• western boundary currents
• eastern boundary currents.
Current speeds (cm/second)
Fig. 9-9. Speed of currents measured by drifting devices (annual average in cm/sec)

Western Intensification of Boundary Currents

Wind blows westward along the inter tropical convergence zone at the equator, causing:

"western intensification."
• Wind blowing across the oceans "mounds" water on the western side of ocean basins-up to 2 m.
• The mounding of water is caused by converging equatorial flow and surface winds.
• The coriolis effect is most intense in polar regions, so current flowing eastward near the poles is more dissipated than currents flowing westward at the equator.
• The higher side of a "mound" is on the western side of the ocean basins, having a steeper "slope" and therefore faster moving.

Eastern boundary currents (EBC) are slow (km/day), wide (>1000 km), and shallow (<1/2 km)
Examples: Canary, California, Benguela, Peru
EBCs form along the "cool, dry" east side of ocean basins.

Western boundary currents (WBC) are fast (km/hr), narrow (<100 km), and deep (up to 2 km)
Examples: Gulf Stream, Brazil, Kuroshio, E. Australian, Agulhas.
WBCs form along the "warm, wet" west side of ocean basins.

Pacific Circulation System
Fig. 9-10. The California Current is an eastern boundary current; part of the Northern Pacific Gyre. The Kuroshio Current near Japan is a western boundary current.
Gyres and boundary are large scale, but are also complex. Boundary currents change constantly (called meandering) producing spinning cone-shaped masses of water - spinning off of larger boundary currents.

Eddie Currents

Satellite temperature data of the ocean surface reveals the spreading and mixing of surface waters as currents move from one region to another, gaining intensity and dispersing energy as they move. The temperature data reveals large spinning eddies in portions of the ocean basins along the margins of major currents.
Eddie currents in the South Atlantic Ocean
Fig. 9-11. Large eddie currents in the S. Atlantic.

Warm and Cold Core Rings

Warm core rings are rotating warm masses of water surrounded by colder water. Example: Warm water areas in the Sargasso Sea water surrounded by cool water.

Cold core rings are cold masses of water surrounded by warmer water.

These spinning rings can last for years and serve as refuges for sea life (warm and cold water) and can influence storm development (such as intensifying or reducing hurricane intensity).
Core rings typically have unique biological populations.

Warm and cold core rings Cold Core Rings in the North Atlantic Ocean
Fig. 9-12. Warm and cold core rings are created by eddies in ocean currents (Northern Hemisphere). Fig. 9-13. Cold core rings in the north Atlantic associated with the Gulf Stream current.

The Gulf Stream Current

The Gulf Stream is a fast moving ocean current.
• The North Equatorial Current moves east across the Atlantic Ocean in the Northern Hemisphere.
• This flow splits into the Antilles Current (east of the West Indies) and the Caribbean Current (around the Gulf of Mexico).
• These currents merge into the Florida Current. (~30-50 miles wide, moving 2-6 mph, a mile deep).
• Along the East Coast, the Gulf Stream experiences "western intensification."
• North of Cape Hatteras (NC) the current moves away from the coast and gradually looses much of its intensity (by meandering) producing numerous warm and cold core rings.
• The Gulf Stream gradually merges eastward with the water of the Sargasso Sea, the rotating center of the North Atlantic Gyre (named for floating marine alga (seaweed) called Sargassum that accumulates in the stagnant waters.
• For comparison, the volume of water moved by the Gulf Stream is about 100 times all the world's rivers combined!
Gulf Stream
Fig. 9-14. The Gulf Stream is the world's largest ocean current (revealed here by water temperature patterns)

Antarctic Circumpolar Current

• The Antarctic Circumpolar Current is the only current to completely encircle Earth.
• The current moves more water than any other current.
• The current is in a region of the world with intense winds and wave action.
• The region has lots of upwelling - very "rich" ocean basin (nutrients for plankton; food for higher-level feeders)

Antarctic Circumpolar Current (NOAA website); also see an animation of the changes in the mixing zone by seasons: Antarctic Circumpolar Current (NOAA)
Antarctic Circumpolar Current Mixing Zone data
Fig. 9-15. Antarctic circumpolar current revealed by mixing zone depths.

Climate Effects of Ocean Currents

• Cold water offshore results in dry condition on land (example: California).
• Warm water offshore results in more humid condition on land (example: Florida).
• Depends on seasonal wind patterns and water temperatures.
• Depends also on regional geography along coastal regions.

California climates based on geography and air flow
Fig. 9-16. California's climate and geographic factors.

Upwelling and Downwelling

Upwelling is the vertical movement of cold, nutrient-rich water from deep water to the surface, resulting in "high productivity" (plankton growth).
• Can bring cold, nutrient-rich water to the surface (photic zone) unless thermocline is strong and prevents it.
Nutrients are not food but act like a fertilizer.
• Upwelling water rich in nutrients feeds phytoplankton, the base of the food chain.

Downwelling
is the vertical movement of surface water downward in water column. Regions where downwelling is occurring typically have low biological productivity.
• Downwelling takes dissolved oxygen down where it is consumed by the decay organic matter.
Regions of coastal upwelling around the world
Fig. 9-17. Regions of the world where coastal upwelling occurs.

Where Upwelling Occurs:

Diverging surface waters occur where surface waters are moving away from an area on the ocean surface.
Equatorial upwelling occurs where SE trade wind blow across the equator (Figure 9-18); Ekman transport forces surface water movement to the south (south of the Equator), and to the north (north of the Equator). Upwelling of deep ocean waters is most intense in equatorial regions.
Coastal upwelling occurs where wind blowing along a coastline is influenced by Ekman current moving surface waters offshore, or winds blowing off the land pull surface waters away from the coast, pulling deeper water up to replace surface waters.
• Other locations where upwelling occurs include around underwater obstructions (guyots) or sharp bends in coastlines.

NOAA animation: Coastal Upwelling
Equatorial Upwelling
Fig. 9-18. Equatorial upwelling involves the Trade Winds blowing across the equator and the coriolis effect taking over as diverging currents move away from the equator.

Coastal Upwelling and Downwelling

The continental margins of the world are places where coastal upwelling and downwelling are taking place (Figure 9-17). Coastal upwelling is influenced by coastal geometry, wind directions, and the influence of the coriolis effect (Ekman transport). Figures 9-19 and 9-20 illustrate how the direction of wind movement determines how coastal upwelling and downwelling takes place in the Northern Hemisphere (such as in California). Figure 9-21 shows regions of coastal upwelling along the California continental margin—revealed ocean-surface temperature imagery. Upwelling water along the coastline is colder than waters farther offshore.
Coastal upwelling Coastal downwelling
Fig. 9-19. Coastal upwelling (example of California) Fig. 9-20. Coastal downwelling (wind reversed)

Large Cycles in Ocean Climate Variability

The ocean/atmosphere systems display cyclic changes beyond annual seasonal changes. Longer-term cycles are also taking place. Changes happening in one region can gradually impact other regions on multi-year to decade cycles (example: cycles in coastal upwelling on North America's West Coast, Figure 9-22). Even longer-term cycles are influenced by extraterrestrial pattern changes in the orbit and rotation of the Earth relative to the Sun over time. These changes impact the distribution of precipitation and influence the warming or cooling of climates over multi-year periods, and changes in sealevel over time linked to the accumulation and melting of continental glaciers.
California ocean themperatures 2000 Upwelling activity by year in the California Current
Fig. 9-21. Upwelling offshore of California revealed by ocean surface temperatures. Fig. 9-22. Cycles of upwelling on North America's West Coast influenced by ENSO.

El Niño/Southern Oscillation (ENSO)

El Niño/Southern Oscillation (ENSO) in the Pacific Ocean [also called El Niño-La Niña Cycles] is associated with a band of warm ocean water that develops in the central and east-central equatorial Pacific. El Niño/Southern Oscillation (ENSO) is perhaps the most important ocean-atmosphere interaction phenomenon to cause cyclic global climate variability. Here's how the ENSO cycle works: ENSO involves the interactions of ocean currents, ocean temperatures, and atmospheric effects, over time.

Pacific Ocean currents involved with ENSO (see Figure 9-5)

• West moving winds at the Equator help to drive the two Pacific Subtropical Gyres (North and South).

• In the North Pacific Subtropical Gyre, the western-intensified Kuroshio Current moves up the Asian seaboard (warming China, Japan), flows east with the North Pacific Current, then south as the California Current along the west coast of North America.

• In the South Pacific Subtropical Gyre, the western intensified East Australian Current moves south and merges with the Antarctic Circumpolar Current, the completes the gyre as the Peru Current (flowing northward along the west coast of South America).

ENSO Ocean Temperature Effects

ENSO Cycles are influenced by ocean surface temperatures throughout the Equatorial Pacific Ocean region. During the "El Niño" periods, ocean surface temperatures are much warmer than the "La Niña" periods. This is a reflection of the amount of cloud cover (deflecting incoming solar radiation) and winds driving cold upwelling currents to the ocean surface in the equatorial region. During "El Niño" periods, the "Pacific Warm Pool" grows larger and more intense in the Eastern Pacific region (Figure 9-23).

ENSO Weather Effects

• The rising warm-moist air in the western Pacific contrasts with the cool sinking air along South America, resulting in the "Walker Cell" (an unstable equatorial air circulation pattern region in the Pacific Ocean)(Figure 9-24). The The Walker Cell operates perpendicular (East to West, not north to south like the Hadley, Farrell, and Polar circulation cells) because of temperature contrasts on opposite sides of the Pacific Basin along the equator.
Ocean temperatures during El Nino/La Nina events
Fig. 9-23. Ocean surface temperatures reveal the changing patterns and regional extent of the "Pacific warm pool" associated with El Niño-La Niña Cycles.
Enso
Fig. 9-24. Changes in the Walker Cell wind currents affect ocean surface temperatures which impact the thickness and extent of the thermocline (which impacts upwelling).
 
Under "normal year" ENSO conditions (which is rare) cool water conditions persist along the west coast of South America (Peru) (Figure 9-25):
• Trade winds blow to the west allow waters to upwell along the west coast of South America (some of the most productive waters in the world).
• West-moving winds drive surface currents westward across the Pacific Ocean where they heat up creating the "Pacific Warm Pool"- a thick thermocline in the western Pacific Ocean.

Under "El Niño" (the warm phase of ENSO) wind intensity of the Walker Cell circulation is diminished (Figure 9-26). El Niño is associated with high air pressure in the western Pacific and low air pressure in the eastern Pacific.

"La Niña" (the cool phase of ENSO) is associated with below average surface water temperatures and high air pressures in the eastern Pacific and low air pressures in western Pacific (Figure 9-27). Air circulation in the Walker Cell is intensified.

Normal thermocline in the Equatorial Pacific Equatorial thermocline during
Fig. 9-25. "Normal" Thermocline and weather pattern under somewhat rare "normal" conditions in the equatorial Pacific region. Fig. 9-26. "El Niño"Thermocline and weather during strong El Niño conditions in the equatorial Pacific region; coastal upwelling near South America is diminished. Fig. 9-27. "La Niña"
Thermocline and weather during La Niña conditions in the equatorial Pacific region; coastal upwelling near South America is strengthened.
El Niño/La Niña Global Climate Impacts—NOAA videos, websites, animations

Warm El Niño Southern Oscillation (ENSO)- Episodes in the Tropical Pacific

El Niño/La Niña Explained (YouTube video).
NOAA's El Niño website.
El Niño/La Niña 1997-1998 (NOAA) - Shows sea surface temperature time lapse.
Global Sea Surface Temperature time lapse showing El Niño/La Niña (NOAA)

Impacts of ENSO Cycles

During El Niño - the "Walker Cell" circulation pattern is very week, and warm surface waters move in to and shut down upwelling in the Peru region (and causing both warm and wet conditions on land), and a collapse of fisheries offshore (associated with economic and ecological catastrophe). The warm conditions arrive around Christmas, so El Niño refers to the Christ Child in Peruvian weather.

During La Niña - the "Walker Cell" circulation intensifies, increasing greater cooling and more upwelling along the coast, enhancing ocean productivity, but drought on land in South America.

These fluctuating cycles of ocean surface water temperatures influence climate factors (warm/wet or cool/dry) conditions around the entire Pacific Basin, if not the entire world.

El Niño year
• High and Low pressures reverse
• Winds are slack or blow against the Equatorial Current
• Mounds warm water on eastern side of Pacific Basin
• Creates nutrient poor conditions. A temperate thermocline replaced with a tropical thermocline, this prevents mixing of deep cold nutrient rich water because of the buoyancy of extra warm surface water.

Monitoring for El Niño is conducted by:
• Studies of wind speed and direction on the Equatorial regions
• High and Low pressure systems on the Equatorial regions.
• Water temperature changes on Equatorial regions, mainly warming on east side of Pacific Basin
• Water heights ("mounding") along the Equator.

ENSO Impacts on Coastal California

During El Niño periods, California's coastal ocean waters are warmer, and a more well-developed thermocline hinders coastal upwelling. This reduces the nutrient supply for sea life, so marine specie either adapt and migrate elsewhere, or in many cases, loose populations due to competition for limited food resources. Southern California typically gets heavier winter rainy periods because the southern tropical jet stream move north from the Central America region. As a result, Southern California gets more tropical moisture which can translate to increased rainfall if conditions are right.

During La Niña periods, California's coastal ocean waters are cooler, only a weak thermocline can develop. As a result, there is stronger and well developed coastal upwelling. As a result, more food is available, and marine life flourishes in coastal waters. Colder waters offshore translate to drier conditions on land.

Sea Level Changes Caused by Continental Glaciation Cycles

Sea level changes caused by the melting of continental glaciers (Antarctica and Greenland) are some of the gravest concerns associated with global warming. Why we know that sea level is changing because vast amounts of data are now available. The observable effects of sea level changes are preserved everywhere around the world's ocean basins. The study of sedimentary deposits of all geologic ages has revealed that sea level has risen and fallen many times, sometimes in the range of hundreds of meters (Figure 9-28). Some of the most drastic changes in sea level are associated with mass extinctions in Earth's history. Sea levels are currently rising, and have been since the end of the last ice age of the Pleistocene Epoch that ended about 11,000 years ago).
Sea level fluxuations during Phanerozoic Era
Fig. 9-28. Sea level changes through Earth history.

Ice Ages of the Pleistocene Epoch

The peak of the last glaciation stage (called the Wisconsin Stage) was about 18,000 years ago. At that time large quantities of water that otherwise was in seawater was frozen in continental glaciers (on land) piled many miles thick over large portions of North America and Europe (Figure 9-29). Modern Greenland and Antarctica are a fraction of the volume of these ancient massive ice sheets. With water trapped as ice on land, sea level fell around the world by as much as 400 feet below current sea level, exposing all the regions that are now continental shelves. Current data suggests that there may have been as many as 20 glaciation cycles in the last 2 million years or so. At least 4 of them were "major" cycles well preserved in continental glacial sediments (more cycles are recorded in ocean sediments).
Continental Glaciers
Fig. 9-29. Extent of glaciers and sea ice during the peak of the last ice age and today in the Northern Hemisphere.

Glacial Cycles Interpreted From Ice Cores and Ocean Sediments

Drilling programs have collected ice cores from the Antarctic and Greenland ice sheets, and many more cores have been collected from marine sediments from around the world. Using geochemical methods and isotopic dating techniques the history of the chemistry of the oceans and atmosphere, and sea level changes through time are well documented (Figure 9-30). For instance, ice has tiny bubbles trapped in them that preserve the chemistry of the air and ice at the time it formed. Sea sediments are loaded with many organic and inorganic materials that can be studied and dated. Shell material of foraminifera contain stable isotopes of carbon and other elements that match the chemistry of seawater at the time that they lived. When glaciers form, the water that forms as ice in polar regions is enriched in light isotopes of oxygen and carbon (light isotopes evaporate from seawater faster than heavy isotopes). As a result, sea water at the peak of glaciation cycles are enriched in the heavy isotopes of carbon and oxygen. The ratios of these isotopes are preserved in microfossil shell material. As a result, scientists have been able to clearly reconstruct a "sea level curve" compared with atmospheric greenhouse gases.
Sea Level changes link to greenhouse gases.
Fig. 9-30. Comparison of concentrations of "greenhouse gases" with the sea level curve for the last 600,000 years.

World Oceans and Landmasses During the Ice Ages

Sea level change since the end of the last glaciation has had major impacts on humans and all "remaining" species alike. A major mass extinction has been on-going since the last ice age. Many will argue that it is because of "human over-consumption" but climate change and sea-level-rise have also been major contributing factors (the two factors are linked). When sea level was low, humans (and other species) were able to migrate throughout the world when what are today's "continental shelves" were "coastal plains" (Figure 9-31 and 9-32).

At the peak of the last ice age sea level about 400 feet (120 m) lower than today.What are now continental shelves were exposed land (coastal plains) that extended out to near the shelf break around continental landmasses. Rivers and streams carved canyons that have flooded as sea level rose, creating fjords, estuaries and bays we see around the world today. Most of the record of human prehistory is now submerged.
Global topgraphy (bathymetry and topography) reveals the continental shelves that were exposed as coastal plains during the Pleistocene Epoch and early Holocene times bering land bridge
Fig. 9-31. Map of the world with continental shelves shown in light blue. During the peak of the last ice age continental shelves were exposed as extensive coastal plains (allowing humans to migrate). Fig. 9-32. The continental shelf in the Bering Straits region between Siberia and Alaska was exposed during the last ice age, allowing many species (including humans) to migrate between continents.

Increasing CO2 Concentrations in the Atmosphere and Oceans

CO2 concentrations and temperature have tracked closely of the last 300,000 years (Figure 9-33).
The recent (if not alarming) increase in CO2 concentrations in the atmosphere is a result of human consumption of fossil fuel, burning forests, and other land use changes. How the Earth's ecosystems are responding to these changes is measurable, and many things are changing. Continental glaciers are melting faster (causing serious concerns about coastal flooding), and the chemistry of ocean water is slowly growing more acidic (endangering ocean species that secrete CaCO3 skeletal material).
Carbon dioxide curve and temperature curve during ice ages.
Fig. 9-33. Global temperature and CO2 concentrations over the last 300,000 years.

Hypoxia and Eutrophication

Hypoxia is oxygen deficiency in a biotic environment. Eutrophication is caused by excessive amounts of nutrients in a body of water (lake, sea, or parts of an ocean) which causes a dense growth of plant life and death of animal life from lack of oxygen (hypoxia). Excessive amount of nutrients come from runoff from land, with agriculture and sewage being primary contributors. Hypoxia has become a major problem in many parts of the world where whole regions of the seabed are dead or dying because of lack of oxygen. Eutrophication is a serious problem in the northern Gulf of Mexico around the mouth of the Mississippi River delta (Figures 9-34 and 9-35). Density stratification in isolated ocean basins can lead to depletion of oxygen at depth at as microbial decay consumes free oxygen and then starts to break down sulfate ions (-HSO4) to hydrogen sulfide (H2S). The Black Sea is an example where halosaline density stratification has lead to anoxic conditions at depth (Figure 9-36).
Hypoxia in the Gulf of Mexico Fish killed by hypoxia
Fig. 9-34. Region impacted by eutrophication in the northern Gulf of Mexico caused by high-nutrient runoff in the Mississippi River system. Fig. 9-35. Example of a fish kill caused by hypoxia in a coastal marine environment. Hypoxia increases in the Gulf as water warms up during the summer season.

Could the Oceans become “anoxic?”

Large portions of the world’s ocean basins have gone “anoxic” in the past. During a million year interval of the Late Cretaceous Period the world’s ocean basins became density stratified. This period called the Cenomanian-Turonian ‘Oceanic Anoxic Event‘ (OAE). This happened between about 90.5 and 91.5 million years ago (the Cenomanian and Turonian are named epochs of the Cretaceous Period). The world was much warmer in the Cretaceous Period, and there were no continental glaciers. The oceans were warmer, and a thick thermocline and intense pycnocline blocked oxygen-rich surface waters from penetrating deep water. Organic-rich deposits preserved in ocean sediments of the OAE show that there is no bioturbation, suggesting that plankton in the grew in the shallow mixing zone was not consumed if their remains sank into the anoxic condition that existed at the seabed.

Density stratification can cut of oxygen supply to deep water in restricted basins (including isolated lake basins and inland sea basins). The Black Sea is an inland sea that has anoxic conditions. Marine surface waters flow into the Black Sea from the Aegean Sea through the shallow Bosphorus Straight (Figure 9-36). Denser saline water trapped in the basin are unable to circulate out of the basin. A strong pycnocline prevents oxygen from reaching depths below about 100 meters.
Anoxic deep waters of the Black Sea
Fig. 9-36. Halosaline density stratification cuts off oxygen supply to deep water in the Black Sea, causing anoxic conditions below ~100 meters. Normal seawater exists above a halocline in the basin.

Ocean Acidification

Ocean acidification is the reduction in the pH of the ocean over an extended period, typically taking decades or longer. The primarily cause is the uptake of carbon dioxide from the atmosphere into seawater, but can also be caused by other chemical additions or subtractions from the ocean. Examples of ocean acidification are recorded in the geologic record associated with major periods of geologic eruptions and massive extraterrestrial impacts (such as the event that wiped out the dinosaurs along with many groups of marine organisms with shells about 66 million years ago).

Anthropogenic ocean acidification refers to the component of pH reduction that is caused by human activity.  In the last 250 years, the concentration of CO2 in the atmosphere has increased from 280 parts per million to over 394 parts per million. Most of this is due to the burning of fossil fuels ( coal, gas, and oil) and also by CO2 and other acid-forming compounds released by land use changes (such as burning off forests to be replaced by agriculture). Ocean acidification has potentially devastating ramifications for all forms of ocean life, from microscopic plankton to the largest animals at the top of the food chain. See Environmental consequences of ocean acidification (United Nations).
Beached coral
Fig. 9-37. Bleaching of coral results death of symbiotic algae living within the corals. This also kills the coral, and is resulting in the collapse of local ecosystems. Elevated water temperatures and acidification are contributing factors.
What is a Garbage Patch?

The "garbage patch" is a popular name for concentrations of marine debris (mostly small pieces of plastic) that accumulate across the more stagnant central parts of the large gyres in the ocean basins. The central regions of ocean basins are areas of convergence and downwelling, so trash from sources on land and sea are carried long distances by currents, much of it ending up in a convergence zone "garbage patch." The largest garbage patch is in the north Pacific Ocean (Figure 9-38). Garbage is generally quite hazardous to sea life.
(See NOAA's Great Pacific Garbage Patch website.)
Great Pacific Garbage Patch
Fig. 9-38. Garbage patches in the Pacific Ocean basin.
Chapter 9 quiz questions
http://geologycafe.com/oceans/chapter9.html
1/1/2016