Bacteria and fungus eat the rest? Consider the Azolla event where dead plant material sank into very salty and oxygen poor water, which left several meters of carbon on the bottom of the polar ocean after 800k years.
If fungus or aerobic bacteria eat it, then the Amazon doesn't produce surplus oxygen from it. Eating involves recombining the carbon with oxygen into CO2.
The biomass of the Amazon is not, for the most part, sinking into very salty water.
The biomass of the Amazon is not, for the most part, sinking into very salty water.