Ocean Acidification Notes

Ocean Acidification Introduction

For over 600,000 years, the ocean's pH was mildly alkaline, approximately 8.2.
Since the industrial revolution (early 1800s), the ocean's pH has dropped to 8.1, equivalent to a 30% increase in acidity.
Atmospheric carbon dioxide (CO_2) is in equilibrium with dissolved carbon dioxide in seawater.
Ocean's carbon dioxide uptake occurs at the ocean surface.
The amount of atmospheric carbon dioxide dissolved depends on the difference in carbon dioxide partial pressure between seawater and the atmosphere.
Natural gas exchange balances pressures between seawater and the atmosphere.
Surface water with lower carbon dioxide partial pressure absorbs carbon dioxide from the air.
The reverse also occurs, from water to the atmosphere.
Water temperature, salinity, wind, waves, and ocean currents influence the ocean's carbon dioxide uptake.
Warmer and saltier water dissolves less carbon dioxide and is more likely to release it.
Colder water can absorb or store more carbon dioxide.
The warm, equatorial Pacific releases carbon dioxide, while the cool North Atlantic absorbs it.
Wind and waves mix surface water, balancing carbon dioxide concentration.
Ocean currents move water masses and in upwelling zones, deep water reaches the surface and exchanges gas with the atmosphere.

The physical carbon pump is driven by ocean currents and differences in temperature and salinity.
It distributes dissolved carbon (carbon dioxide, hydrogen carbonate, carbonate) via sinking or rising water masses.
It primarily transports man-made carbon dioxide emissions into the deep ocean.
To sink, water masses must cool down to become denser.
This occurs mainly in polar regions, where carbon dioxide solubility is high at low temperatures.
Colder and saltier water sinks deeper, taking dissolved carbon and storing it in the deep sea.
Water masses travel globally on the global conveyor belt of ocean circulation.

As atmospheric CO2 increases, oceans absorb more CO2.
Scientists estimate the ocean absorbs a quarter to a third of all human-created CO_2 emissions.
When carbon dioxide dissolves in water, it forms carbonic acid (H2CO3).
- CO2 + H2O \rightarrow H2CO3
Carbonic acid dissociates into a hydrogen ion (H^+) and a bicarbonate ion (HCO_3^-).
- H2CO3 \leftrightarrow H^+ + HCO_3^-
Acidity is measured in pH, a logarithmic scale of hydrogen ion concentration; higher H^+ concentration means increased acidity and decreased pH.
Changing pH impacts chemical reactions and life processes.
The free hydrogen ion combines with a dissolved carbonate ion (CO3^{2-}) to form another bicarbonate ion (HCO3^−).
- H^+ + CO3^{2-} \leftrightarrow HCO3^−
As carbonate ions combine with hydrogen ions, the concentration of dissolved carbonate ions decreases.
Marine organisms (corals, plankton, clams, oysters, sea urchins, barnacles, crabs, lobsters) use carbonate ions with dissolved calcium ions (Ca^{2+}) to form calcium carbonate (CaCO_3), the main building block of their shells and exoskeletons.
- CO3^{2-} + Ca^{2+} \leftrightarrow CaCO3

Carbonate ions (CO3^{2-}) bond with Calcium ions (Ca^{2+}) to form carbonate (CaCO3) which is utilized by marine organisms to build shells and skeletons.
If there are excess hydrogen ions (H^+), they reunite with carbonate ions to form more bicarbonate.
This makes carbonate ions less available for shell and skeleton building.
Decreasing carbonate ion concentration reduces growth, especially during the larval form.
Extremely acidic seawater could partially dissolve shells and exoskeletons, which is unlikely even when burning all fossil fuels.
The ability of water to dissolve gases like carbon dioxide and oxygen decreases when electrolytes are added.
When ionic salts enter the water, ions from the salts naturally attract water molecules because the ions want to bond, leaving hydrogen and oxygen ions available to be captured or released.
This bond formation from the salt ions usually decreases the non-polar oxygen molecules, driving DO out of the water and decreasing the solubility of dissolved oxygen molecules.