The Oceans Ch. 15 & 16

Intro. The oceans are majestic. A lot is known about them, but we still know only a fraction of the secrets they carry. The study of the Earth’s oceans is known as Oceanography.

How Do We Study Them? We investigate ocean currents, water temperature and chemical composition, seafloor sediments and topography, and marine life. To do this, oceanographers use nets, dredges. sonar, and satellites

Where’d They Come From? Earth is approximately 4.6 billion years old and oceans have been here for pretty much the entire time. 2 possible sources for the water: Comets Volcanism

Water from Comets: Comets and meteorites have a lot of water in their composition. Meteorites have up to .5% water and comets have even more. This would be a high enough percentage to create the oceans if enough of them collided with the Earth. Upon collision between Earth and comets, the water most likely rose to the surface through volcanism. Gases, mostly water vapor and carbon dioxide, get released into the atmosphere. For hundreds of millions of years, Earth was extremely volcanic. Water vapor began to condense into oceans very gradually.

Distribution of Earth’s Water The level of the ocean’s surfaces, or sea level, is affected by the amount of ice on Earth and by tectonic forces. Ice amounts have varied from 0 to 10% of all water on the planet. This means that sea levels have risen and fallen by hundreds of meters. Tectonic forces lift and lower portions of the seafloor. Raises also cause sea levels to rise.

How Much Water Is There? Approximately 71% of the surface is covered in water. Average depth of the ocean is 3800 m.

Seawater

Chemical Properties of Seawater Seawater is a solution of roughly 96.5% water and 3.5% dissolved salts. Most of the salt is sodium chloride, but there are chlorides and sulfates of magnesium, potassium, and calcium present as well. Salinity is the measure of the amount of dissolved salts in seawater. In addition to salt ions, seawater also contains dissolved gases and nutrients that are essential to life in the oceans.

Variations in Salinity Salinity varies from place to place and is most concentrated in areas where rates of evaporation exceed those of precipitation.

Sources of Sea Salt Earth’s ocean salinity hasn’t really changed from ancient times to now. We know this by testing the concentrations of magnesium in fossil shells and today’s shells. Volcanic gases contain chlorine and sulfur dioxide, which dissolve in water and become salt ions. The others come from the weathering of rocks.

Why Doesn’t Sea Salinity Increase Some salts are removed as precipitate in arid areas. Salt spray from waves are carried away by winds. Many ions are absorbed by animals to form bones, shells, and teeth. When the creatures die, these ions sink to the bottom and are buried with the bones.

Physical Properties of Seawater Seawater is denser than freshwater because of the salt Because of this, it has a lower freezing point. We’ll come back to this later! Light really only penetrates the upper 100 m of seawater.

Ocean Layering Temperature Profiles are diagrams that show how water temperatures change with depth. 1st Layer: Surface Layer (Surface Zone) is relatively warm and sunlit. Roughly 100 m thick. 2nd Layer: The thermocline (Twilight Zone) is a transitional layer characterized by rapidly decreasing temperatures with depth. 3rd Layer: Bottom layer (Deep Ocean) is cold and dark with temperatures near freezing. Ocean layering is caused by density differences in the water. Cold water is more dense so it sinks to the bottom of the ocean.

Water Masses The cold water in the bottom layer comes primarily from polar regions, where water becomes cold and sinks, then disperses toward the equator. Formation of sea ice leads to higher concentrations of salt in these areas, causing denser waters.

Ocean Movements The most obvious movement of water is a wave. Wave: A rhythmic motion that carries energy through space and matter. Ocean waves are typically generated by wind. Only the energy moves forward in an ocean wave. The water moves in circles until the energy passes, but does not move forward.

Wave Height depends on: Wind Speed Wind Duration Fetch (Expanse of water that the wind blows across) Breaking Waves Friction from shallow waters cause the waves to slow down, causing them to become higher, steeper, and more unstable, which causes them to collapse forward. A collapsing wave is known as a breaker.

Tides Tides are periodic rises and falls in sea of sea level. What causes them? Gravitational attraction between the Earth, Moon, and Sun. Unbalanced forces caused by gravitational and centrifugal forces create tidal bulges on opposite sides of the Earth. The sun’s gravity influences the tides, but the moon’s influence is even greater because of its close proximity to the earth. Tidal bulges are always aligned with the moon. When the moon is new or full, the tides are at their highest. (Spring Tides) When the moon is in 1st or 3rd quarter, the tide highs and lows are not as extreme (Neap Tides) These spring and neap tides alternate every 2 weeks.

Ocean Currents Density currents are caused by differences in the temperature and salinity of ocean water, which in turn affect density. These move slowly in deep ocean waters. Surface currents affect the upper layer of the ocean and move up to 100 km a day. These are driven by the wind. In our area, the prevailing westerlies result in ocean currents that move from west to east.

Gyres Because of continents blocking ocean currents from just moving in east-west rotational movement, closed circular current systems, called gyres are formed. There are 5 major gyres in the world. The parts of all 5 gyres closest to the equator move west. They are deflected toward the poles when they hit a landmass These carry warm water toward the poles. When they cools, the gyres sink and move back toward the equator.

Upwelling In addition to moving horizontally, they also move vertically. The upward motion of ocean water is called upwelling. These areas originate from the bottom of the ocean, making them cold. These waters are rich in nutrients, making them great for fishing. Peru and California are examples of this. Areas of upwelling exist off western coasts of continents in the trade-wind belts.