Seawater pH and CO2

Question 1

What is true of seawater?

Answer 1

• Complex mixture of compounds including salts and organic compounds
• Generally well buffered
• Surface seawater is generally at equilibrium with the atmosphere

Question 2

How many major elements are there in seawater and what are examples?

Answer 2

Eleven major elements
- including Na+, Cl-, Mg2+, K+, Ca2+, HCO3-

(CA2+ and HCO3- can vary in seawater, with the ratio of the others to salinity is constant)

Question 3

What are the minor elements in the ocean?

Answer 3

NO3-, PO4^2-, Si

(key nutrients)

Question 4

What are trace elements in the ocean?

Answer 4

Iron, copper, nickel and all the other metals, gases, etc

Question 5

What is the pH scale for?

Answer 5

It is a scale for expressing the concentration in moles per litre of the [H3O+] or [H+] ion

Question 6

What does pH measure?

Answer 6

The acidity of the solution

Question 7

What does a low pH mean?

Answer 7

A low pH may result from a concentrated solution of weak acid (e.g. ethnic acid) or a weak solution of a strong acid (e.g. sulphuric acid)

Question 8

What is true of pH as a log scale?

Answer 8

pH is a log scale - a small change in pH is a big change in free H+ ions and acidity

Question 9

What is true of pH in pure water at 25°C?

Answer 9

In pure water at 25°C one in 10 million (10^7) water molecules are dissociated - this gives a concentration of H+ ions of 10^-7

Question 10

What is the seawater CO2 cycle?

Answer 10

1. Atmospheric CO2 equilibrates with the seawater
2. It becomes hydrated
3. Carbonic acid dissociates

These equilibria are temperature and pressure dependent and transitions between the different species tend to stabilise the pH of seawater

Question 11

What is true of the effects of photosynthesis and respiration on ΣCO2?

Answer 11

• Photosynthesis – may reduce the ΣCO2 at the surface increasing the pH – making it more alkali
• Respiration – may increase the ΣCO2 at depth, reducing the pH – making it more acidic

Question 12

What is true of a buffered solution?

Answer 12

A buffered solution resists changes in pH when it is diluted or when acid or base is added
- It is composed of either a weak acid and its salt or a weak base and its salt

Question 13

What is true of buffering in the ocean?

Answer 13

Oceans are considered well buffered because you can add a relatively large amount of acid or base with little change in pH – this can also be termed a high alkalinity

Question 14

What compounds act as buffers for the ocean?

Answer 14

The complex mix of carbonates, borates, phosphates, silicates and organic compounds generally buffer seawater – many of these compounds are weak acids and bases

Question 15

What is true of buffering of freshwater?

Answer 15

Freshwater is often buffered very poorly with a low alkalinity

Question 16

What is true of the buffering capacity of the oceans?

Answer 16

The buffering capacity of oceans is not limitless as is demonstrated by ocean acidification

Question 17

What is the primary use of Calcium Carbonate? (CaCO3)

Answer 17

It is used by many planktonic organisms to form skeleton

Question 18

What is true of Calcium Carbonate solubility?

Answer 18

Calcium is a major element and so changes concentration only slightly with depth.

So the solubility of CaCo3 and the degree of saturation is determined by (CO3^2-) which varies considerably.

Question 19

What will happen if the water is unsaturated with CaCO3?

Answer 19

The shells will start to dissolve

Question 20

What are examples of organisms that utilise CaCO3?

Answer 20

• Coccolithophore phytoplankton with CaCO3 calcite skeleton
• Foraminifera zooplankton with calcite skeleton
• Pteropod – zooplankton with CaCO3 aragonite skeleton

Question 21

What is true of organisms (previous question) using CaCO3 as they die?

Answer 21

These will live in surface water but as they die and decay they sink to the sediments and the shell will dissolve if the water is undersaturated with CaCO3

Question 22

What happens when calcerous shells of plankton sinks?

Answer 22

If enough shells sink quickly they may form biogenic sediments, with new shells covering the old – this will happen especially where there is extensive growth – e.g. an upwelling area. ORIGIN OF CHALK

If shells do not sink fast enough they may dissolve either as they travel down the water column or as they lie on the ocean bottom

Question 23

What are the crystalline forms of CaCO3 used by plankton?

Answer 23

Calcite and aragonite

Question 24

What is true of calcite’s structure?

Answer 24

• Calcite is more stable
• Calcite shells will survive to greater depths or in the presence of lower carbonate concentrations

Question 25

What is true of aragonite’s structure?

Answer 25

• Aragonite is less stable
• Aragonite shells will start to dissolve at shallower depths where the carbonate concentration is higher

Question 26

What is true of CaCO3 saturation in the ocean?

Answer 26

• Surface waters are supersaturated in calcium carbonate
• Deep oceanic waters are generally unsaturated in these compounds – e.g. shells start to dissolve in the water column or on the seabed

Question 27

Why do shells dissolve at depth?

Answer 27

• The pH of seawater is inversely proportional to dissolved [CO2]
• Cold water will dissolve more CO2 at saturation
• Respiration at depth as a result of animals and bacteria will release CO2
• Cold CO2 rich water fills the deep ocean