The highest oxygen levels on Earth are believed to have occurred during the Paleozoic era, specifically in the Carboniferous and Permian periods, around 300 to 350 million years ago. During this time, oxygen levels in the atmosphere were estimated to be significantly higher than they are today, possibly reaching levels of 30% or even higher. This was due to several factors, including the extensive growth of land plants and the lack of terrestrial vertebrate animals, which meant that there were fewer organisms consuming oxygen. Additionally, geological and climatic conditions during the Paleozoic era may have contributed to higher oxygen production and retention.

Deep ocean water generally contains more dissolved carbon dioxide (CO2) and less oxygen (O2) compared to surface waters due to a combination of physical and biological processes:

It's important to note that while deep ocean waters may have lower oxygen levels and higher CO2 concentrations compared to surface waters, these variations are part of the natural oceanic carbon and oxygen cycles. Human activities, such as the burning of fossil fuels, deforestation, and land use changes, have led to an increase in atmospheric CO2 levels and changes in ocean chemistry, which can impact marine ecosystems and contribute to ocean acidification.

Dr Gaurav H Tandon thank you for your contribution to the discussion

Oxygen made up 20 percent of the atmosphere about today's level around 350 million years ago, and it rose to as much as 35 percent over the next 50 million years. The layer of the atmosphere that has the highest level of oxygen is the troposphere. The troposphere is the largest layer of the atmosphere that is located closest to the Earth's surface. However, over the long history of Earth's oxygenation, researchers now realize that atmospheric oxygen levels have fluctuated significantly. Case in point, some 300 million years ago, during Earth's Carboniferous period, researchers know that Earth's oxygen levels peaked at some 31 percent. At first, the oxygen produced by cyanobacteria was sequestered in minerals and seawater. But between 2.4 and 2.5 billion years ago, cyanobacteria were producing enough oxygen to be stored in Earth's atmosphere. As plants became firmly established on land, life once again had a major effect on Earth's atmosphere during the Carboniferous Period. Oxygen made up 20 percent of the atmosphere about today's level around 350 million years ago, and it rose to as much as 35 percent over the next 50 million years. The air the dinosaurs breathed was in fact much richer in oxygen than now, and is the reason why winged reptiles of those days had pinions too small to work in today's atmosphere. Cold water is better at dissolving and absorbing gasses like CO2 compared to warmer water, which is why a large amount of it gets dissolved in the ocean's chilliest waters, according to the report. When that heavy water sinks to the deep sea, large portions of that CO2 can be stored for a long time.Oxygen gets into the sea in two ways: through photosynthesis, which takes place only near the top where light penetrates, or through the mixing of air and water at the surface by wind and waves. Deep ocean waters hold far less oxygen than surface waters because they haven't been in contact with air for centuries. The downward flux of organic matter decreases sharply with depth, with 80–90% being consumed in the top 1,000 m (3,300 ft). The deep ocean thus has higher oxygen because rates of oxygen consumption are low compared with the supply of cold, oxygen-rich deep waters from Polar Regions.Molecular weight of CO2 is 44u while the molecular weight of O2 is 32u. Hence, carbon dioxide has a higher density or is heavier than oxygen. The researchers identified an adaptation that helps these loriciferans to survive in their environment. Instead of mitochondria, which rely on oxygen, the creatures have organelles that resemble hydrogenosomes, which some single-celled organisms use to produce energy-storing molecules anaerobically.