By measuring trace levels of uranium in oceanic
It took about 50,000 years for ocean oxygen levels to return to what they were before the extinction event and it may have taken as long as 250,000 years for coral reefs around the globe to fully recover, according to the study.
The new results shed light on the state of the oceans during the end-Triassic extinction, which wiped out approximately 76 percent of all marine and terrestrial species. The end-Triassic is the fourth-largest extinction episode in Earth's history and occurred just before dinosaurs became Earth's dominant land animal.
Scientists are not sure what initiated the extinction event, but suspect a burst of volcanic activity around 200 million years ago increased carbon dioxide levels in the atmosphere. That would have made oceans more acidic and caused ocean water to become anoxic. Severe anoxia causes "dead zones" in the water where low levels of oxygen cause marine life to suffocate and die.
Anoxia has long been suspected to have played a role in the end-Triassic extinction, but the anoxia's duration and severity were not known, according to the study's authors. The new study, published in Geochemistry, Geophysics, Geosystems, a journal of the American Geophysical Union, quantifies the timing and extent of marine anoxia during and after the end-Triassic extinction, said Adam Jost, a postdoctoral fellow at MIT's Earth, Atmospheric, and Planetary Sciences department in Cambridge, Massachusetts, and lead author of the new study.
Read more at: https://phys.org/news/2017-08-global-oceanic-dead-zones-persisted.html#jCp
