Question: If Marie was dead when Bourne took her body out of the car in the river, why did her body sink to the bottom instead of floating after being let go? And how did Jason make sure she was dead?
Invader_Gir
21st Feb 2023
The Bourne Supremacy (2004)
Answer: We only see her sink to the bottom. We never see her surface. Also he checked her pulse before letting go. Just in a way we couldn't see. Her body would have surfaced eventually but by then we were more focused on Bourne and his escape.
Answer: The density of the human body is similar to the density of water, and what keeps us floating is the air in our lungs. Once this air is replaced with water, people will sink to the bottom. Jason probably recognized that she drowned from his training. To add to the other answer from a scientific standpoint. Bodies do resurface once putrefaction begins. Microbes will start to eat bits from inside the body that results in a lot of gas. As a result, this causes the density of the body to decrease, as the gas is light, and the body will float up to the surface of the water.
Invader_Gir