Seawater Salinity, Density, Temperature

Water molecule unique in structure and properties H 2 O is the chemical formula for water. Unique properties of water include: Higher melting and boiling point than other hydrogen compounds. High heat capacity, amount of heat needed to raise the temperature of one gram of water 1 o C Latent (hidden) heat = energy that is either absorbed or released as water changes state Can dissolve more substances than any other solvent Called universal solvent

Water molecules are asymmetrical in shape with the two hydrogen molecules at one end H+ separated by 105 o when gas or liquid phase and o when ice

Interconnections of water molecules Polarity causes water molecules to form weak (hydrogen) bonds between water molecules Water sticks to itself and to other substances Allows water to be the universal solvent Figure 5-3

Hydrogen bonds in H 2 O Figure 5-8

Pure water vs Seawater pH = 7.0 for pure water pH= 8.1 for seawater, slightly alkaline Density = g/cm3 for pure water Density = for seawater Freezing point = 0 °C or 32° F for pure water -1.9 °C or 28.6 °F for seawater Boiling point= 100°C or 212°F for pure water °C or 213°F for seawater

Snowflake geometry All snowflakes have 6-sided geometry Caused by water’s polarity and ability to form hydrogen bonds Why does ice float on water?

The formation of ice As water cools to 4°C: Molecules slow Water contracts Density increases Below 4°C: Hydrogen bonds form Water expands As water freezes: Expands by 9%

Water as a solvent Water dissolves table salt (NaCl) by attracting oppositely charged particles Pulls particles out of NaCl structure to dissolve it Figure 5-4

Salinity Salinity = total amount of solid material dissolved in water Can be determined by measuring water conductivity Typically expressed in parts per thousand ppt or (‰)

Major dissolved components in seawater: 35 g of salt in 1000 g of seawater Constituents of ocean salinity

Addition of salt modifies properties of water Pure water freezes at 0 o C. Adding salt increasingly lowers the freezing point because salt ions interfere with the formation of the hexagonal structure of ice. Vapor pressure decreases as salinity increases because salt ions reduce the evaporation of water molecules. Density of water increases as salinity increases.

Seawater density depends on temperature, salinity and pressure! Therefore, it increases with > salt content at constant temp high density in cold, salty waters –why is this important? Graph of Density of freshwater: Change in scale

Surface salinity variation Pattern of surface salinity: Lowest in high latitudes Highest in the tropics Dips at the Equator Surface processes help explain pattern

Surface salinity variation High latitudes have low surface salinity High precipitation and runoff Low evaporation Tropics have high surface salinity High evaporation Low precipitation Equator has a dip in surface salinity High precipitation partially offsets high evaporation

Why is surface Atlantic more salty than Pacific?

Salinity map showing areas of high salinity (36 o/oo) in green, medium salinity in blue (35 o/oo), and low salinity (34 o/oo) in purple. Salinity is rather stable but areas in the North Atlantic, South Atlantic, South Pacific, Indian Ocean, Arabian Sea, Red Sea, and Mediterranean Sea tend to be a little high (green). Areas near Antarctica, the Arctic Ocean, Southeast Asia, and the West Coast of North and Central America tend to be a little low (purple).

Salinity variations Location/typeSalinity Normal open ocean33-38‰ Baltic Sea10‰ (brackish) Red Sea42‰ (hypersaline) Great Salt Lake280‰ Dead Sea330‰ Tap water0.8‰ or less Premium bottled water0.3‰

Decrease Salinity by: Precipitation – rain, sleet, snow, falls directly on ocean Runoff-rivers carry fresh water to the ocean Icebergs melting – when glacial ice breaks off –this is mostly fresh water Sea ice melting with the spring thaw-this has a little salt but is mostly water

Increase salinity by: Sea ice forming in cold ocean areas as water freezes– 30% of salts in seawater are retained in ice Evaporation – removes very pure water, all salts are left behind, occurs in hot climates

Sea water consists of water with various materials dissolved within it. The solvent is the material doing the dissolving and in sea water it is the water. The solute is the material being dissolved. Solutes in seawater = organic compounds and nutrients, ionic salts, dissolved gases and trace elements Salinity = salts dissolved in seawater

Major nutrients in the sea are compounds of nitrogen, phosphorus and silicon. Nutrients are chemicals essential for life Because of usage, nutrients are scarce at the surface and their concentrations are measured in parts per million (ppm). Concentration of nutrients vary greatly over time and because of this are considered a nonconservative property Marine organic compounds occur in low concentrations consist of large complex molecules, such as fat, proteins, carbohydrates, hormones and vitamins produced by organisms or through decay Solutes in water: Nutrients and Organics

Sodium and chlorine alone comprise about 86% of the salt in the sea. major constituents of salinity display little variation over time 99% of all the salt ions in the sea are sodium (Na + ), chlorine (Cl - ), sulfate (SO 4 -2 ), Magnesium (Mg +2 ), calcium (Ca +2 ) and potassium (K + ) Solutes in water: Ionic salts

In order of decreasing abundance the major gases in the sea are nitrogen, oxygen, carbon dioxide and the noble gases, argon (Ar), neon (Ne) and helium (He). Nitrogen and the noble gases are considered to be inert because they are chemically non-reactive. Solutes in water: Gases and Trace elements Trace elements occur in minute quantities and are usually measured in parts per million (ppm) or parts per billion (ppb). Even in small quantities they are important in promoting life or killing it.

Salinity in the ocean: steady-state condition because - amount of salt added to the ocean (input from source) equals the amount removed (output into sinks) Salt sources include weathering of rocks on land and the reaction of lava with sea water. Salt sinks include the following: Evaporation removes only water molecules. Remaining water becomes increasingly saline, producing a salty brine If enough water evaporates, the brine becomes supersaturated and salt deposits precipitate forming evaporite minerals Wind-blown spray carries minute droplets of inland Adsorption of ions onto clays Shell formation by organisms

Cycling of dissolved components in seawater Oceans’ salinity may have increased over time

The solubility and saturation value for gases in sea water increase as temperature and salinity decrease and as pressure increases. The surface layer is usually saturated in atmospheric gases because of direct exchange with the atmosphere. Below the surface layer, gas content reflects relative importance of respiration, photosynthesis, decay and gases released from volcanic vents. Gases in Seawater

Gases in Seawater: O 2 Surface layer is rich in oxygen because of photosynthesis and contact with the atmosphere. Oxygen minimum layer occurs at about 150 to 1500m below the surface and coincides with the pycnocline. Sinking food particles settle into this layer and become suspended in place because of the greater density of the water below food draws large numbers of organisms which respire, consuming oxygen. Decay of uneaten material consumes additional oxygen

Gases in Seawater: O 2 Density difference prevents mixing downward of oxygen-rich water from the surface or upwards from the deep layer Deep layer is rich in oxygen because its water is derived from the cold surface waters which sank to the bottom Consumption is low because there are fewer organisms and less decay consuming oxygen Anoxic waters contain no oxygen and are inhabited by anaerobic organisms (bacteria) Oxygen tends to be abundant in the surface layer and deep layer bottom, but lowest in the pycnocline.

Gases in Seawater: CO 2 Major sources of carbon dioxide are respiration and decay Major sinks are photosynthesis and construction of carbonate shells Carbon dioxide controls the acidity of sea water Dissolved CO 2 in water acts as a buffer, a substance that prevents large shifts in pH

Gases in Seawater: CO 2 Dissolution of carbonate shells in deep water results because cold water under great pressure has a high saturation value for CO 2 and the additional CO 2 releases more H + ions making the water acid. Warm, shallow water is under low pressure, contains less dissolved CO 2 and is less acidic. Carbonate sediments are stable and do not dissolve.

pH is related to the amount of CO 2 dissolved in water because it combines with the water to produce carbonic acid which releases H + ions. CO 2 + H 2 O  H 2 CO 3  H + + HCO 3 -  H + + CO 3 -2 H 2 CO 3 is carbonic acid, HCO 3 - is the bicarbonate ion and CO 3 -2 is the carbonate ion. Adding CO 2 shifts the reaction to the right and produces more H + ions making the water more acid. Removing CO 2 shifts the reaction to the left, combining H + ions with carbonate and bicarbonate ions reducing the acidity. Gases in Seawater: CO 2

Ocean buffering Ocean pH = 8.1 (slightly basic) Buffering protects the ocean from experiencing large pH changes

Water samples must be collected in inert containers and isolated during recovery to prevent contamination Nisken bottle has valves at each end which are automatically closed when a weight is sent down the cable causing the bottle to flip over and seal itself Sample depth can be determined from cable inclination and length or with a pulsating sound source

Collection of water samples

The relationship between temperature, salinity and density of seawater. Temperature, Salinity, and Water Density

Temperature vs Heat Temperature is the measure of how fast the molecules in a substance are moving Temperature measured in Kelvin, Celsius, Fahrenheit Heat is a measure of how much energy has to be put into a system or removed from a system to change its temperature or state (solid, liquid, or gas) Heat has units of Energy (1 calorie, calor = heat; the amount of heat required to raise the temp. of 1 gram of water by 1 C°)

Ocean moderates coastal temperatures Water has high heat capacity it can absorb or release large quantities of heat without changing temperature Hydrogen bonds cause thermal inertia Moderates coastal temperatures

But it requires more energy to do so Evaporation occurs at temp < 100° C

Atmospheric transport of surplus heat from low latitudes into heat deficient high latitudes areas:

Seawater density Factors affecting seawater density: Temperature ↑, Density ↓ (inverse relationship) Salinity ↑, Density ↑ Pressure ↑, Density ↑ Temperature has the greatest influence on surface seawater density

Density and temperature variations with depth

Pycnocline and thermocline Pycnocline = layer of rapidly changing density Thermocline = layer of rapidly changing temperature Present only in low latitude regions Barrier to vertical mixing of water and migration of marine life

Pyncnocline graphics From Scripps about 11 min

Thermocline graphics Swimming through a thermocline 40 sec Thermocline explained related to hurricanes 15 minutes on oceans and motions

Thermocline Thermocline circulation at risk

Ocean layering based on density Mixed surface layer (surface to 300 meters) Low density; well mixed by waves, currents, tides Upper water (300 to 1000 meters) Intermediate density water containing thermocline, pycnocline, and halocline (if present) Deep water (below 1000 meters) Cold, high density water involved in deep current movement

Salinity variation with depth Curves for high and low latitudes begin at different surface salinities Halocline = layer of rapidly changing salinity At depth, salinity is uniform Figure 5-22

Halocline graphics Diving through a halocline 45 sec BbC halocline 1 min scuba diving in florida caves

Arctic sea ice has grown to a record breaking amount