School โบ The ocean โบ Sources
Every fact in the three ocean lessons is listed here with where it comes from. The numbers in [brackets] on the lesson pages link to this list.
Wikipedia was not used as a source. Numbers are rounded (for example 9.8 m/sยฒ for Earth's gravity). The text was drafted with help from AI (Claude) and then checked against these sources. Corrections: hello@echoesof.science.
Where the facts are simplified or less certain: the depths of the ocean zones are conventional boundaries, not sharp lines, and how deep light reaches depends on the water: NOAA gives about 1,000 m 'under the right conditions' but rarely significant light below 200 m [5]. The share of Earth's oxygen made in the ocean is hard to measure; NOAA says 'at least half' and notes that marine life uses about the same amount [7]. The Southern Ocean is counted by most countries, but its boundaries have not yet been ratified by the International Hydrographic Organization [2]. The pressure rule (one extra atmosphere per 10.06 m) ignores the small changes in sea water density with depth, temperature and salinity, so the deep-sea pressures we calculate are approximate. Kilometre and metre values are as given by NOAA; the psi to atmosphere conversion uses NOAA's 14.7 psi per atmosphere [6].
Type: National ocean agency (fact page)
About 97 percent of Earth's water is in the ocean: about 1,335,000,000 cubic kilometres, out of a total of over 1,386,000,000 cubic kilometres of water on the planet. The ocean covers more than 70 percent of the surface of our planet.
Used in: Ages 10โ11 (quiz); Ages 10โ11 (text); Ages 15โ16 (quiz); Ages 15โ16 (text); Ages 6โ7 (quiz); Ages 6โ7 (text)
Type: National ocean agency (fact page)
There is only one global ocean, divided into named regions. Historically there are four named oceans: Atlantic, Pacific, Indian and Arctic; most countries, including the US, now also count the Southern (Antarctic) Ocean, from Antarctica to 60ยฐ South, although its boundaries have not yet been ratified by the International Hydrographic Organization.
Used in: Ages 10โ11 (text); Ages 6โ7 (quiz); Ages 6โ7 (text)
Type: National geological survey (FAQ)
Salt in the ocean comes from rocks on land. Rain picks up carbon dioxide and becomes slightly acidic, which helps break down rock; the dissolved ions are carried by streams and rivers to the ocean. Some ions are used by animals and plants; others stay for long periods and their concentrations increase over time. Salinity is about 35 parts per thousand (about 3.5% by weight); chloride and sodium make up over 90% of the dissolved ions. A cubic mile of sea water holds about 120 million tons of salt.
Used in: Ages 10โ11 (quiz); Ages 10โ11 (text); Ages 15โ16 (experiment); Ages 15โ16 (quiz); Ages 15โ16 (text); Ages 6โ7 (quiz); Ages 6โ7 (text)
Type: National ocean agency (classroom activity)
Classroom experiments on how salt changes water: freezing salt and fresh water, layering coloured fresh water over salt water, and floating an egg in fresh and salt water. Salt water is more dense than fresh water, so fresh water floats on top of salt water, and objects float more easily in salt water. As salt water freezes, the water that is not frozen becomes saltier.
Used in: Ages 10โ11 (experiment); Ages 10โ11 (quiz); Ages 10โ11 (text); Ages 15โ16 (experiment); Ages 15โ16 (quiz); Ages 15โ16 (text); Ages 6โ7 (experiment); Ages 6โ7 (quiz)
Type: National ocean agency (fact page)
Sunlight may travel about 1,000 m into the ocean under the right conditions, but there is rarely significant light beyond 200 m. The upper 200 m is the euphotic or sunlight zone; from 200 to 1,000 m (twilight zone) light fades quickly and photosynthesis is no longer possible; below 1,000 m (aphotic zone) there is no sunlight: the midnight zone (1,000โ4,000 m), the abyss (4,000โ6,000 m) and the hadal zone (6,000 m and deeper).
Used in: Ages 10โ11 (quiz); Ages 10โ11 (text); Ages 15โ16 (quiz); Ages 15โ16 (text)
Type: National ocean agency (fact page)
At sea level the air presses on us with 14.7 pounds per square inch (1 bar). For every 33 feet (10.06 m) you go down, the pressure increases by one atmosphere. Divers feel the pressure on their eardrums even a few feet down. The submersible Pisces V can work down to about 2,000 m. Whales have flexible rib cages and lungs that collapse safely under pressure; sperm whales hunt giant squid at 2,134 m or more.
Used in: Ages 10โ11 (quiz); Ages 10โ11 (text); Ages 15โ16 (quiz); Ages 15โ16 (text)
Type: National ocean agency (fact page)
At least half of the oxygen produced on Earth comes from the ocean, mostly from tiny photosynthesising plankton (drifting plants, algae and some bacteria). Prochlorococcus, the smallest photosynthetic organism on Earth, produces up to 20% of the oxygen in the biosphere. Marine life uses roughly as much oxygen as it makes. Exact percentages are hard to calculate because amounts change all the time; scientists use satellite images to estimate photosynthesis.
Used in: Ages 10โ11 (quiz); Ages 10โ11 (text); Ages 15โ16 (quiz); Ages 15โ16 (text); Ages 6โ7 (text)
Type: National weather and ocean agency (education page)
Epipelagic (sunlight) zone: surface to 200 m, where most visible light is; temperatures range from about 36 ยฐC in the Persian Gulf to โ2 ยฐC near the North Pole; wind mixes this layer. Mesopelagic (twilight) zone: 200โ1,000 m, very faint sunlight, the largest temperature change (thermocline), bioluminescence begins to appear, fish have large upward-pointing eyes. Bathypelagic (midnight) zone: 1,000โ4,000 m, always dark except for bioluminescence, about 4 ยฐC, pressure at 4,000 m over 5,850 psi; sperm whales dive here to feed. Abyssopelagic: 4,000โ6,000 m, pitch-black, near freezing, few creatures; about three-quarters of the deep-ocean floor lies here. Hadalpelagic: from 6,000 m to 10,994 m in the Mariana Trench; the water above weighs over 8 tons per square inch; single-celled foraminifera were found in the Challenger Deep in 2005; the deepest fish recorded was found at 8,372 m in the Puerto Rico Trench.
Used in: Ages 10โ11 (text); Ages 15โ16 (text); Ages 6โ7 (quiz); Ages 6โ7 (text)
Type: University science curriculum
Sea water has a slightly higher density than fresh water. Differences in temperature and salinity form water layers of different densities, and density differences decide whether a layer floats or sinks relative to other layers; fresh river water floats on top of salty ocean water.
Used in: Ages 10โ11 (experiment); Ages 10โ11 (quiz); Ages 10โ11 (text); Ages 15โ16 (experiment); Ages 15โ16 (quiz); Ages 15โ16 (text)