Seawater pH and CO2 Flashcards
What is true of seawater?
- Complex mixture of compounds including salts and organic compounds
- Generally well buffered
- Surface seawater is generally at equilibrium with the atmosphere
How many major elements are there in seawater and what are examples?
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)
What are the minor elements in the ocean?
NO3-, PO4^2-, Si
(key nutrients)
What are trace elements in the ocean?
Iron, copper, nickel and all the other metals, gases, etc
What is the pH scale for?
It is a scale for expressing the concentration in moles per litre of the [H3O+] or [H+] ion
What does pH measure?
The acidity of the solution
What does a low pH mean?
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)
What is true of pH as a log scale?
pH is a log scale - a small change in pH is a big change in free H+ ions and acidity
What is true of pH in pure water at 25°C?
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
What is the seawater CO2 cycle?
- Atmospheric CO2 equilibrates with the seawater
- It becomes hydrated
- Carbonic acid dissociates
These equilibria are temperature and pressure dependent and transitions between the different species tend to stabilise the pH of seawater
What is true of the effects of photosynthesis and respiration on ΣCO2?
- 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
What is true of a buffered solution?
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
What is true of buffering in the ocean?
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
What compounds act as buffers for the ocean?
The complex mix of carbonates, borates, phosphates, silicates and organic compounds generally buffer seawater – many of these compounds are weak acids and bases
What is true of buffering of freshwater?
Freshwater is often buffered very poorly with a low alkalinity
What is true of the buffering capacity of the oceans?
The buffering capacity of oceans is not limitless as is demonstrated by ocean acidification
What is the primary use of Calcium Carbonate? (CaCO3)
It is used by many planktonic organisms to form skeleton
What is true of Calcium Carbonate solubility?
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.
What will happen if the water is unsaturated with CaCO3?
The shells will start to dissolve
What are examples of organisms that utilise CaCO3?
- Coccolithophore phytoplankton with CaCO3 calcite skeleton
- Foraminifera zooplankton with calcite skeleton
- Pteropod – zooplankton with CaCO3 aragonite skeleton
What is true of organisms (previous question) using CaCO3 as they die?
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
What happens when calcerous shells of plankton sinks?
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
What are the crystalline forms of CaCO3 used by plankton?
Calcite and aragonite
What is true of calcite’s structure?
- Calcite is more stable
- Calcite shells will survive to greater depths or in the presence of lower carbonate concentrations
What is true of aragonite’s structure?
- Aragonite is less stable
- Aragonite shells will start to dissolve at shallower depths where the carbonate concentration is higher
What is true of CaCO3 saturation in the ocean?
- 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
Why do shells dissolve at depth?
- 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
