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A source of sea salts appears to be minerals and chemicals eroding and dissolving into fresh water flowing into the ocean.

Waves and surf contribute by eroding coastal rock. Hydrothermal vents change

seawater by adding some materials while removing others.

Scientists think these processes all counterbalance so the average salinity of seawater remains constant.

The ocean is said to be in chemical equilibrium.

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Why the Seas Are Salty

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Salinity, Temperature, and Water Density

Most of the ocean surface has average salinity, about 35‰. Waves, tides, and currents mix waters to make them more uniform.

Precipitation and evaporation have opposite effects on salinity. Rainfall decreases salinity by adding fresh water. Evaporation increases salinity by removing fresh water. Freshwater input from rivers lowers salinity. Abundant river input and low evaporation results in salinities

well below average.

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Salinity, Temperature, and Water Density

Salinity and temperature also vary with depth.Density differences causes water to separate into

layers.High-density water lies beneath low-density water.

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Salinity, Temperature, and Water Density

Water’s density is the result of its temperature and salinity characteristics: Low temperature and high salinity are features of high-

density water. Relatively warm, low-density surface waters are separated

from cool, high-density deep waters by the thermocline, the zone in which temperature changes rapidly with depth.

Salinity differences overlap temperature differences and the transition from low-salinity surface waters to high-salinity deep waters is known as the halocline.

The thermocline and halocline together make the pycnocline, the zone in which density increases with increasing depth.

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Salinity, Temperature, and Water Density

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Global Salinity

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Acidity and Alkalinity

pH measures acidity or alkalinity. Seawater is affected by solutes. The relative

concentration of positively charged hydrogen ions and negatively charged hydroxyl ions determines the water’s acidity or alkalinity. It can be written like this:

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Acidity and Alkalinity

Acidic solutions have a lot of hydrogen ions (H+), it is considered an acid with a pH value of 0 to lessthan 7.

Solutions that have a lot of hydroxyl ions (OH-) are considered alkaline. They are also called basic solutions. The pH is higher than 7, with anything over 9 considered a concentrated alkaline solution.

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Acidity and Alkalinity

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Acidity and Alkalinity

pH can be measured chemically or electronically. Pure water has a pH of 7 - neutral pH. Seawater pH ranges from 7.8 to 8.3 - mildly alkaline. Ocean’s pH remains relatively stable due to buffering.

A buffer is a substance that reduces the tendency of a solution to become too acidic or alkaline.

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Acidity and Alkalinity

Seawater is fairly stable, but pH changes withdepth because the amount of carbon dioxidetends to vary with depth.

Shallow depths have less carbon dioxidewith a pH around 8.5.

Shallow depths have the greatest density of photosynthetic organisms which use the carbon dioxide, making the water slightly less acidic.

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Acidity and Alkalinity

Middle depths have more carbon dioxide andthe water is slightly more acidic with a lower pH.More carbon dioxide present from the respiration

of marine animals and other organisms, which makes water somewhat more acidic with a lower pH.

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Acidity and Alkalinity

Deep water is more acidic with no photosynthesisto remove the carbon dioxide.At this depth there is less organic activity, which

results in a decrease in respiration and carbon dioxide. Mid-level seawater tends to be more alkaline.

At 3,000 meters (9,843 feet) and deeper, the water becomes more acidic again.This is because the decay of sinking organic

material produces carbon dioxide, and there are no photosynthetic organisms to remove it.

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Acidity and Alkalinity

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The variation of pH and total dissolved inorganic carbon with depth.

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The Organic Chemistryof Water

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Biogeochemical Cycles

Organic chemistry deals mainly with chemical compounds consisting primarily of carbon and hydrogen.

Inorganic compounds such as dissolved sea salts account for the majority of dissolved solids in seawater.

Dissolved organic elements also interact with organisms on a significant scale.These elements are crucial to life and differ from

the sea salts in several ways.

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Biogeochemical Cycles

Proportions of organic elements in seawater differ from the proportions of sea salts because:The principle of constant proportions does not

apply to these elements.These nonconservative constituents have

concentrations and proportions that vary independently of salinity due to biological and geological activity.

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Biogeochemical Cycles

All life depends on material from the nonliving part of the Earth.The continuous flow of elements and compounds

between organisms (biological form) and the Earth (geological form) is the biogeochemical cycle.

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Biogeochemical Cycles

Organisms require specific elements and compounds to stay alive.Aside from gases used in respiration or

photosynthesis, those substances required for life are called nutrients. The primary nutrient elements related to seawater

chemistry are carbon, nitrogen, phosphorus, silicon, iron, and a few other trace metals.

Not all elements and compounds cycle at the same rate. The biogeochemical cycle of the various nutrients affects

the nature of organisms and where they live in the sea.

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Carbon

Carbon is the fundamental element of life. Carbon compounds form the basis for chemical

energy and for building tissues. The seas have plenty of carbon in several forms. It

comes from:Carbon dioxide in the air.Natural mineral sources - such as carbonate

rocks.Organisms - excretion and decomposition.

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