https://phys.org/news/2018-05-scientists-rethink-co-evolution-marine-life.html

The paper stems from a multi-year, multinational research effort led by Zunli Lu that rethinks the causes and impacts of increased oxygenation on the continental shelves during the current Phanerozoic Eon, which began more than 542 million years ago.

"Most studies of oxygen history focus on the atmosphere and deep oceans, with implications on the evolution of life," Zunli Lu says. "We believe the oceanic oxygen level in the water column above the continental shelves [i.e., the upper ocean] may have been a different beast."