Chapter 5c Dissolved Gases in Seawater

 

Seawater Is a Chemical Solution

 

 

 

Gases in the Oceans

 

 

An application of isotope tracers:

 

 

Helium-3 comes from the mantle

used to trace plume from the mid-ocean ridge

 

Table 5-9 in the textbook

 

Summary of factors that regulate the concentration of gases in water

 

** You gotta know these **

 

 

For the concentration of dissolved gases in water:

Fresh water     >          salt water

Cold water >    warm water

High pressure >          low pressure

 

 

Dissolved O2 and CO2 are inverse

 

Solubility of Oxygen in Seawater

 

** cold water holds more dissolved gas **

 

 

Oxygen Profile

 

      Vertical profile of dissolved oxygen in the

Atlantic Ocean

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Dissolved Oxygen in Deep Water

North-south transect in the Atlantic Ocean

 

Anthropogenic tracers in the Oceans

Examples:

  tritium   CFCs   SF6

 

CFCs in the oceans

 South Pacific near Antarctica

transect across the Atlantic

 

Time series from the Greenland Sea

 

 

Greenland Sea – downwelling history

 

 

Light absorption

 

      Photic zone about 100 m

 in the open ocean

 

 

 

 

 

 

Aphotic zone – no light penetration,

      the greatest volume of the ocean has no light

 

Light absorption in nearshore waters

 

more yellow-brown than open ocean

 

much shallower photic zone, typically 20 m, but may be 1-2 m near sources of suspended sediments (river mouth)

 

Photosynthesis and respiration

 

These are from the same chemical reaction running different directions

 

6 CO2 + 6 H20  + energy  à  C6H12O6 + 6 02

                                                            (glucose)

 

                                    photosynthesis à

                                    ß respiration

 

Carbonate buffer system in seawater

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Basic idea:  as CO2 increases in seawater, it produces carbonic acid, which dissolves CaCO3 shells, such as foraminifera

 

 

Vertical profiles in Pacific & Atlantic

 

Dissolved O2                           CO2

 

 

 

 

 

 

 

 

 

 

 

           

 

 

pH                                  CO2

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

            phosphate                               nitrate                           silica

 

 

 

 

 

 

 

 

 

 

 

 

 

 

** IMPORTANT: Understand WHY the vertical profiles have these characteristics **

(This is part of an exercise)

 

What is residence time?

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The average time that an atom or molecule remains in the ocean

 

  (or in a lake, or in the atmosphere, or in any other reservoir)

 

 

The composition of the ocean has remained constant through the last 2 billion years

 

 

Therefore, the rate of input must equal the rate of output

 

To calculate a residence time

 

 

Divide the total mass of an element (or compound) that is in the ocean

 

  by the annual rate of input (mass / year)

 

 

The answer is in years

 

 

Residence times of water

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Residence times of elements in seawater

 

 

In general, less reactive = long residence time

    more reactive = short residence time

 

 

Chloride           billions of years

Sodium            260  million

 

Potassium         11  million

Calcium               8  million

 

Zinc                 180  thousand

 

Aluminum        150  years

 

Any element used by living organisms has a relatively short residence time

** Nutrients have very short residence times **