Did Sea Oxygen Levels Play Role In Earth’s First Mass Extinction?

By James Anderson •  Updated: 11/21/22 •  3 min read

According to a recent study led by Florida State University scientists, the first mass extinction on Earth may have been caused by abrupt changes in marine oxygen levels.

The Late Ordovician mass extinction, or LOME, which wiped out about 85% of marine species, occurred 443 million years ago. Scientists have long studied this mass extinction and are still looking into possible causes, such as reduced habitat loss in a rapidly cooling world or persistently low-oxygen conditions in the oceans.

Graptolites and brachiopods are two examples of groups of organisms that started to decline very early during this mass extinction interval, but paleontologists didn’t really have any solid evidence of an environmental or climate signature to link that early decline of these groups to a specific mechanism.

“This paper can directly link that early phase of extinction to changes in oxygen. We see a marked change in thallium isotopes at the same time these organisms start their steady decline into the main phase of the mass extinction event,”

said co-author Seth Young, an associate professor in Florida State’s Department of Earth, Ocean and Atmospheric Science.

Icehouse Environment

Ordovician flora and fauna

Ordovician flora and fauna as shown in a diorama at the US National Museum of Natural History. Credit: Fritz Geller-Grimm CC-BY

The oxygen level dropped and was immediately followed by a rise. This sudden change in oxygen occurred simultaneously with the traditional first die-off of a mass extinction and a significant expansion of the ice sheet over the former South Pole.

“Turbulence in oxygen levels in oceanic waters is really what seems to have been pretty problematic for organisms that were living in the Late Ordovician at that time, which might have been adapted to cope with low oxygen conditions initially or vice versa,”

Young said.

One of the five significant mass extinctions in Earth’s history, the Late Ordovician extinction is the only one that experts are certain occurred in an environment known as an “icehouse” where ice sheets cover much of the planet’s surface.

Earth is currently experiencing icehouse conditions and biodiversity loss, making this ancient mass extinction an important analog for current conditions, as well as attempting to understand Earth’s future as our climate warms and ice sheets recede.

Low Oxygen Levels

Previous studies of the environment around the Late Ordovician mass extinction used evidence from places with more oxygen. This study, on the other hand, used shales that were laid down in deeper water with less oxygen. These shales have different geochemical signatures, so the researchers were able to draw conclusions about global marine conditions rather than local ones.

“The discovery of the initial expansion of low-oxygen conditions on a global level and the coincidence with the early phases of decline in marine animals helps paint a clearer picture of what was happening with this extinction event,”

said lead author Nevin Kozik.

The other big five mass extinctions are:

Inadequate data hampered the study of mass extinctions for much of the twentieth century. Though acknowledged, mass extinctions were regarded as mysterious exceptions to the dominant gradualistic view of prehistory, in which slow evolutionary trends define faunal changes. Many other authors have reevaluated geological events in the context of their impacts on life as a result of the resurgence of interest in mass extinctions.

Reference: Nevin P. Kozik et al. Rapid marine oxygen variability: Driver of the Late Ordovician mass extinction. Science Advances, 18 Nov 2022, Vol 8, Issue 46 DOI: 10.1126/sciadv.abn8345

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