Final answer:
Remineralization is the main process that contributes to the presence of CO2 in the deep ocean, where the decomposition of organic matter releases carbon dioxide.
Step-by-step explanation:
The main source of CO2 in the deep ocean is remineralization. This process involves the decomposition of organic matter by microorganisms, which results in the release of carbon dioxide.
In the deep ocean, this carbon dioxide is produced from the breakdown of sinking dead organisms and fecal matter, collectively known as 'marine snow', as well as the dissolution of carbonate shells and the release of carbon from sediments.
While hydrothermal vents also release gases, including CO2, the quantity is much smaller than the amount generated by remineralization processes. On the other hand, fossil fuels and rivers are not direct sources of CO2 in the deep ocean waters.