Today I learned: scientists figured out the oxygen levels in the waters off of Central America from 40,000 years ago. Before this study, people had thought that during the last glacial period (~110,000-15,000 years ago), oxygen levels in the oceans were higher because cold water can hold onto oxygen better. But this group was able to use a type of fossilized plankton that only takes up a type of nutrient in oxygen-rich conditions to determine that the deeper waters of the eastern Pacific Ocean were actually oxygen-depleted during this time. Typically, oxygen levels in oceans increase as you go deeper, but Hoogakker et al. found that there was no increase from shallow to deeper waters. Because the Pacific Ocean is the biggest sink (or storer) of carbon, holding about 50% of the total oceanic amount, this group suggested that the Pacific Ocean played an important role in the amount of carbon dioxide in the atmosphere during the glacial period and the transition period that followed.
Reference: Hoogakker, Babette A. A., et al. “Glacial Expansion of Oxygen-Depleted Seawater in the Eastern Tropical Pacific.” Nature, vol. 562, no. 7727, 2018, pp. 410–413., doi:10.1038/s41586-018-0589-x.