Earth Science

“The Oceans”

The Oceans

Since prehistoric times, people have used Earth’s oceans for travel, recreation, and to obtain food.

In the late 1800’s the British Challenger became the 1st research ship to explore under the ocean’s surface.

The Oceans

Oceanography - The study of the Earth’s oceans.

The discipline of oceanography is usually considered to have started with the Challenger.

Modern Oceanography

The Challenger investigated: ocean currents water temperature and chemical

composition seafloor sediments and topography marine life

Modern Oceanography

1920’s German research ship Meteor used sonar to map the seafloor features

Sonar stands for sound navigation and ranging. Uses the return time of an echo, multiplied

by 1500 m/s, divided by 2 The answer gives you the distance to the

ocean floor in meters

Advanced Technology

Advanced technology has greatly expanded scientific knowledge of the oceans.

Satellites Topex/Poseidon monitor the ocean’s surface temperatures, currents, and wave conditions.

Advanced Technology

Side-scan sonar - a technique that directs sound waves to the seafloor at an angle, so that the sides of underwater hills can be mapped.

Origins of the Oceans

Has Earth always had oceans?Several geological clues indicate that

we have had oceans since the beginning of geological history.

Earth is 4.6 billion years old.

Origins of the Oceans

Scientists have found rocks made of sediments deposited in water that are nearly 4.6 billion years old.

Radioactive studies and lava flows offer evidence that there has been abundant water throughout Earth’s geologic history.

Where did the water come from?

Scientists hypothesize that water could have come from 2 sources: Comets that collide with Earth release

water on impact, over time these impacts filled the oceans

Volcanism

Volcanism

During volcanic eruptions, significant quantities of gases are emitted.

These gases consist mostly of water vapor and carbon dioxide.

At the beginning of the Earth, a massive episode of violent volcanism took place

Volcanism

This took place over hundreds of million years

This episode created massive amounts of water vapor and carbon dioxide thus creating our oceans and our atmosphere

Volcanism

Volcanoes still produce water that adds to our hydrosphere but,

Ultraviolet radiation from the Sun destroys water molecules so these two processes balance each other.

What would happen over geological time if they didn’t?

Distribution of Earth’s Water

97% salt water3% freshwaterPercentage of ice has varied on Earth

from near 0 to 10 percent of the hydrosphere

Sea level - the level of the ocean’s surface.

Distribution of Earth’s Water

Sea level has risen and fallen by hundreds of meters in response to melting ice and expanding glaciers.

Tectonic forces that lift and lower the seafloor also account for variations in sea level.

Global sea level rises at 1 to 2 mm per year because of melting glaciers.

The Blue Planet

Approximately 71% of Earth’s surface is covered by oceans.

Average depth 3800 m. Most land is in the northern hemisphere

where oceans account for 61% Southern hemisphere has 81% oceans All oceans are connected into one vast body

of water.

Major Oceans

Three major oceans: Pacific, Atlantic, and Indian

Pacific is largest - roughly half of Earth’s seawater

Atlantic - 2nd largest Indian - 3rd largest

Sea Ice

The Arctic and Antarctic Oceans are covered by vast expanses of sea ice

In the summer ice breaks up When sea-ice crystals form, it starts as

slush on the surfaceEventually solidifying into pancake iceEventually these pieces thicken and

freeze into continuous ice cover called pack ice.

Sea Ice

In the coldest part of the Arctic and Antarctic oceans there is no summer thaw and ice is generally several meters thick and can be more than 1000 km wide.

Earth Science

“Seawater”

Seawater

96.5% water3.5% saltsMost abundant salt in the sea is sodium

chloride (NaCl)Most elements on Earth are found in

seawaterBecause these substances are

dissolved they are in ion form

Chemical Properties of Seawater

Salinity - measure of the amount of dissolved salts in seawater

Oceanographers express salinity as grams of salt per kilogram of water, (ppt)

Total salt content 35ppt or 35% Nutrients and dissolved gases in seawater

greatly affect life in the oceans

Variations in Salinity

Although average salinity is 35 ppt, actual salinities vary from place to place

Subtropical regions have higher salinity (37 ppt). Why?

Equatorial where precipitation is abundant, salinities are lower.

Polar regions 32-33 ppt due to melting sea ice

Lowest salinity is found at river outlets

Sources of Sea Salt

Geological evidence indicates that the salinity of ancient seas was not much different from that of today’s oceans. Proportion of sea salts - magnesium in

calcium-carbonate shells Sources of sea salts - weathering of rocks

and minerals that wash into the rivers and carried to the ocean and seas.

Removal of Sea Salts

Salt ions are constantly being added to seawater

Salinity does not increase due to removal of salts by: Precipitate Wind carries salt from waves Marine organisms also remove ions

Removal of Sea Salts

As organisms die their solid parts accumulate on the seafloor.

All processes remove immense quantities of salt ions from the ocean

The existing salinity of seawater represents a balance between the processes that remove and add salts

Physical Properties of Seawater

The presence of various salts causes the physical properties of seawater to be different than freshwater. Seawater is denser than freshwater Temperature affects density, colder water

is more dense Range of seawater is 1.02 to 1.03 grams

per centimeter cubed

Physical Properties of Seawater

These variations are small but vary significant.

They affect many oceanic processesDifference in salinity also changes

causes freezing point to lower compared with freshwater

Freshwater is 0°C, seawater is -2°C

Absorption of Light

Intensity of light decreases with depthWater absorbs light, so oceans are darkRed light penetrates less than blue lightLight sufficient for photosynthesis only

exists in the top 100 m of the ocean

Ocean Layering

Temperatures range from -2°C to 30°CAverage temp. 15°CTemperature dramatically decreases

with depthTemperature profile - plots changing

water temp with depthDark waters below 1km have fairly

uniform temp of less than 4ºC

Ocean Layering

Three layers: Surface layers - first 100 m Thermocline - 100 - 1000 m, characterized

by rapidly decreasing temp with depth Bottom layer - 1000m and below, cold and

dark temps near freezing Cold water sinks so the bottom because it

is more dense

Water Masses

High salinity and and cold temps cause seawater to become more dense

Seawater freezes during arctic and Antarctic winter

Salt ions aren’t incorporated into the growing ice crystals and accumulate beneath the ice.

Cold water beneath the ice becomes saltier and denser so it sinks

This salty water migrates toward the equator as a cold, deep water mass along the ocean floor.

Three water masses account for most of the deep water in the Atlantic ocean. Antarctic Bottom Water North Atlantic Deep Water Antarctic Intermediate Water

While the Atlantic contains all three major deep water masses

Indian and Pacific contain only two deep water masses

Earth Science

“Ocean Movements”

Ocean Movements

Wave - a rhythmic movement that carries energy through space or matter - in this case, ocean water

Generated by wind over the water’s surface.

Energy moves forwardWater itself moves in circles until the

energy passes, but it does not move forward.

Wave Characteristics

Highest point of wave is the crest.Lowest point of wave is the trough.Vertical distance between crest and

trough is the wave heightCrest to crest is the wavelength

Wave Characteristics

The wavelength determines the depth to which the wave disturbs the water.

The depth, called the wave base, is equal to half the wavelength

Wavelength also determines speed; wave speed increases with wavelength

Wave Height

Wave height depends on three things: Wind speed Wind duration Fetch - refers to the expanse of water that

the wind blows across

Largest wave ever recorded was more than 30 meters - in the North Pacific

Breaking Waves

Waves lose energy as they approach the shore due to friction with the ocean bottom

Waves slow downCrest-to-crest wavelengths decreaseIncoming waves become higher,

steeper, and unstable collapsing their crest

Breakers - Collapsing wavesMajor role in shaping shorelines

Tides

Tides are the periodic rise and fall of sea level; cycles every 24h and 50 min

Difference in high and low tide varies from place to place Gulf of Mexico tidal range is less than 1 m New England can be as high as 6 m Highest range 15 m is in the Bay of Fundy

off of Nova Scotia

Problem - Solving Lab

Analyze a tidal recordPage 401Individually create your graphComplete “Thinking Critically”

Causes of Tides

Basic cause of tides are the gravitational attraction among the Earth, the Moon, and the Sun

Gravitational attraction decreases with distance

Earth and Moon experience differing gravitational forces

Unbalanced forces generate tidal bulges on opposite sides of the Earth.

The gravitational effect on Earth’s oceans is similar to coffee in a cup going around a corner

Spring tides occur when the Moon is either full or new; when Sun, Moon and Earth are aligned; large tidal ranges

Small tidal ranges caused by neap tides, moon is either first or third quarter

Spring and neap tides alternate every 2 weeks; spring tides 3x higher than neap.

Ocean Currents

Density current - caused by differences in the temperature and salinity of ocean water, which in turn affect density

Density currents move slowly in deep ocean waters

Surface currents - affect mainly the upper few hundred meters of the ocean

Surface currents move fast, 100km/day

Gyres

Continents deflect ocean currents to the north and south so that closed circular current systems, called gyres, develop.

Five major gyres: North Pacific South Pacific North Atlantic South Atlantic Indian Ocean

Gyres in the northern hemisphere circulate clockwise, southern hemisphere counter-clockwise.

Upwelling

The upward motion of ocean water is upwelling.

Originate from the bottom of the ocean, so they are cold.

Rich in nutrients