Final answer:
Oceans changed color from brown to blue-green primarily due to oxygen from photosynthetic organisms combining with iron, causing it to precipitate and settle on the seafloor, leading to clearer, blue-green waters.
Step-by-step explanation:
The primary cause of the oceans changing from brown to blue-green in color was oxygen combining with iron. In early Earth's history, the atmosphere had low concentrations of oxygen, which meant there was little dissolved oxygen in the oceans to cause iron to rust or oxidize. When photosynthetic organisms like cyanobacteria began to proliferate, they released oxygen through photosynthesis in the Great Oxygenation Event around three billion years ago. This excess oxygen is combined with soluble iron ions (Fe²+) in the water, eventually causing the iron to precipitate and fall to the seafloor, forming layers of sedimentary rock known as banded iron formations. As the iron settled, the remaining ocean water became more transparent, making it appear blue-green, as it is today.