Chapter 5b Chemistry of Seawater

Seawater Is a Chemical Solution

Solvent – the water is the dissolving agent

Solute – the salt ions (and other compounds) dissolved in the water

Concentrations of dissolved salts are

measured in parts per thousand (^o/_oo)

Major ions in seawater

Six most abundant ions account for over 99% of the solutes in seawater

chloride Cl^–

sodium Na⁺

sulfate SO₄^–2

magnesium Mg⁺²

calcium Ca⁺²

potassium K⁺

bicarbonate HCO₃^–

Conservative Ions

The major ions in seawater vary little over time and location in the ocean

They are not affected by biological activity

Linear mixing between water masses with different salinities

Other Dissolved Constituents

Nutrients – needed for phytoplankton growth

N (as nitrate) P (as phosphate) Si

concentrations in the ocean in

parts per million (ppm) or milligrams per liter (mg / L)

Gases – surface water in equilibrium with the atmosphere

N₂ O₂ CO₂ H₂ noble gases: Ar Ne He

Trace elements – practically all natural elements are dissolved in seawater

but in parts per billion (ppb) or parts per trillion

** life is adapted to extremely low concentrations of these elements,

even slight contamination may be toxic **

Organic compounds – large, complex molecules produced by organisms

Principle of Constant Proportion

The ratio between the major ions is constant, regardless of the salinity

So, the ratio of Na⁺ / K⁺ is the same

at 5 ‰ at the head of an estuary

at 25 ‰ near the mouth of an estuary

at 30 ‰ on the continental shelf

at 35 ‰ in the open ocean

Salinity from Chlorinity

Salinity is the total weight in grams of salts dissolved in 1 kilogram of seawater

Salinity can be calculated by measuring chlorinity (all of the halides)

salinity (‰) = 1.80655 x chlorinity (‰)

Salinity analyses in the lab

Titration – chemical reaction of chloride with silver nitrate

Hygrometer – measures density

Salinometer – measures conductivity

Practical Measurements of Salinity

In practice, salinity is measured with a precisely calibrated salinometer

which measures the electrical conductivity of seawater, and corrects for the temperature

practical salinity units (psu)

Water sampling at sea

CTD: conductivity temperature depth

Effect of Salinity

Salinity depresses freezing point and temperature of maximum density

The Clines

Thermocline – a sharp change in temperature with water depth

Halocline – a sharp change in salinity with depth

Pycnocline – a sharp change in density with depth

Thermocline by latitude

Near the poles, almost uniform temperatures through the water column,

allows sinking of cold surface water

Global sea-surface temperature

Several abrupt changes in surface temperature – for example, the polar front

Latitudinal temperature section – Pacific

Most pronounced thermocline in low latitudes

Seasonal thermocline – warming

Stratification of the water column as solar heating increases in summer

Seasonal thermocline – cooling

Destratification and mixing (turn-over) as surface water cools in the fall

Balance of Evaporation – Precipitation

Sea-surface salinity

Highest salinity in open ocean occurs between 20-30 degrees N and S latitude

Greatest evaporation

Coincides with deserts on land

Global sea-surface salinity

Seawater density

Isopycnals – lines of constant density

Remember, density is controlled by temperature & salinity

Density structure of the oceans

Dominated by temperature differences,

because salinity differences are relatively minor

Atlantic Ocean

sea-surface salinity

latitudinal salinity transect

Main water masses:

NADW – North Atlantic Deep Water

AABW – Antarctic Bottom Water

AIW – Antarctic Intermediate Water

The Rock Cycle and Seawater

Main point: the primary salts in the ocean have been derived from the weathering of igneous rocks