An increased dissolved CO2 in seawater decreases the pH value and makes seawater more corrosive to carbonate, called Ocean acidification. In short, it represents a fundamental change in the chemistry of seawater.
It is not about the process that drives surface and deep water conditions in opposite directions. Ocean acidification is the term specifically used to describe the impacts of a large and fast input of external carbon.
OCEAN ACIDIFICATION CHEMISTRY
Now, we will explore the chemistry behind the changes that are faster than the buffering action of deep-sea carbonate compensation.
Acidification of the seawater results from the gradual diffusion of CO2 into the Ocean, where it reacts with water molecules to form dissolved bicarbonate and a proton.
The external carbon source is not limited to CO2 and includes CH4 from rocks and lithosphere sediments and gas hydrate breakdown.
Seawater has a high buffer capacity. Seawater pH changes slowly with the addition of acid, but its buffer capacity has limits.
Before the Industrial Revolution, the pH of the surface ocean was 8.2. a value created by a long-term balance between the controlling processes that had been established over a more than 6000-year stable phase of the current interglacial period.
After the onset of the Industrial Revolution, it dropped to 8.1. The value 0.1 pH unit may look small, but it is a logarithmic function. So, a drop of 10.1 can create a 25 per cent increase in acidity.
According to Doney et al., 2009, the PH may fall by an additional 0.3 to 0.4 units by the end of the 21st century.
WHAT ARE THE EFFECTS OF OCEAN ACIDIFICATION?
Since all organisms are sensitive to the pH change of the environment, a drop of 0.1 to 0.2 can cause serious health problems.
If pH concentration increases, marine organisms will struggle to dissociate the bicarbonate ions they require. As a result, they have to spend more energy to form carbonate for their skeletal parts.
Acidity results in coral bleaching and includes difficulties for coral larvae in finding settling places.
Once the buffer capacity of seawater is overtaken by external carbon injection, acidity changes very fast. High atmospheric carbon dioxide and high temperatures during greenhouse conditions would help to create conditions that will favour a gradual net drawdown of the externally injected carbon levels.
Fish-blood pH is in equilibrium with the water they live. A drop in pH can create serious problems. Naturally, their body tries to restore its pH. When it drops, it costs fish more energy and, in return, slows their growth and mobility.
OCEAN ACIDIFICATION AND THE TWO VARIABLES
The ocean acidification depends on two variables
- Carbon dioxide concentration in the atmosphere
- Sea surface temperature.
HOW CAN WE RECONSTRUCT PAST SURFACE-WATER CARBON DIOXIDE LEVELS?
The Boron-based methods can be used to reconstruct detailed past surface water CO2 levels.
It falls into two categories
- Boron-Calcium (B/Ca) ratio method
- The boron isotope ratio method
Boron isotope rations are not commonly used for this because sample material availability is too limited. This method analyses the boron isotope ratio in fossil carbonate shells of surface-dwelling microzooplankton, which are typically found in seafloor sediment cores.
The boron-calcium method analyses carbonate microfossils of seafloor inhabiting unicellular organisms called benthic foraminifera. The result reflects changes in past carbonate ion concentrations in the deep sea.
CASE STUDY ON OCEAN ACIDIFICATION
PETM acidification
It happened in a Cretan-style ocean. During the end-Premian extinction, high carbon dioxide gas started to build up and peaked at levels of around 2000 ppm and 4000 ppm. This event happened between 150 and 250 million years ago.
The global temperature increased by about 6 degrees Celsius compared to conditions before the event.
As a result, Arctic went ice-free, and there was a minimal temperature difference between the equator and the poles. Rich forests and tropical plam forests can be seen at high northern latitudes.
The crocodilian fossil from Ellesmere Island supports this.
Later, the climate returned to normal levels.
Estimates of PETM’s external carbon addition range between 2000 and 7000 gigatonnes.
