Uncovering the secrets behind Earth's first major mass extinction
A team of researchers publish a new study exploring the cause of the Late Ordovician mass extinction.

Date:
November 1, 2021
Source:
Syracuse University
Summary:
New research reveals more information about the first and oldest of
the 'big five' extinctions. Around 85% of marine species, most of
which lived in shallow oceans near continents, disappeared during
that time.



FULL STORY ==========================================================================
We all know that the dinosaurs died in a mass extinction. But did you know
that there were other mass extinctions? There are five most significant
mass extinctions, known as the "big five," where at least three-quarters
of all species in existence across the entire Earth faced extinction
during a particular geological period of time. With current trends of
global warming and climate change, many researchers now believe we may
be in a sixth.


========================================================================== Discovering the root cause of Earth's mass extinctions has long been a
hot topic for scientists, as understanding the environmental conditions
that led to the elimination of the majority of species in the past could potentially help prevent a similar event from occurring in the future.

A team of scientists from Syracuse University's Department of Earth and Environmental Sciences, the University of California, Berkeley and the University of California, Riverside, Universite' Bourgogne Franche-Comte',
the University of New Mexico, the University of Ottawa, the University
of Science and Technology of China and Stanford University recently
co-authored a paper exploring the Late Ordovician mass extinction
(LOME), which is the first, or oldest of the "big five (~445 million
years ago)." Around 85% of marine species, most of which lived in shallow oceans near continents, disappeared during that time.

Lead author Alexandre Pohl, from UC Riverside (now a postdoctoral research fellow at Universite' Bourgogne Franche-Comte' in Dijon, France) and
his co- authors investigated the ocean environment before, during, and
after the extinction in order to determine how the event was brewed and triggered. The results from their study will be published in the journal
Nature Geoscience on Nov. 1.

To paint a picture of the oceanic ecosystem during the Ordovician Period,
mass extinction expert Seth Finnegan, associate professor at UC Berkeley,
says that seas were full of biodiversity. Oceans contained some of the
first reefs made by animals, but lacked an abundance of vertebrates.

"If you had gone snorkeling in an Ordovician sea you would have seen
some familiar groups like clams and snails and sponges, but also many
other groups that are now very reduced in diversity or entirely extinct
like trilobites, brachiopods and crinoids" says Finnegan.



========================================================================== Unlike with rapid mass extinctions, like the Cretaceous-Tertiary
extinction event where dinosaurs and other species died off suddenly some
65.5 million years ago, Finnegan says LOME played out over a substantial
period of time, with estimates between less than half a million to almost
two million years.

One of the major debates surrounding LOME is whether lack of oxygen in
seawater caused that period's mass extinction. To investigate this
question, the team integrated geochemical testing with numerical
simulations and computer modeling.

Zunli Lu, professor of Earth and environmental sciences at Syracuse
University, and his students took measurements of iodine concentration
in carbonate rocks from that period, contributing important findings
about oxygen levels at various ocean depths. The concentration of the
element iodine in carbonate rocks serves as an indicator for changes in
oceanic oxygen level in Earth's history.

Their data, combined with computer modeling simulations, suggested that
there was no evidence of anoxia - or lack of oxygen - strengthening
during the extinction event in the shallow ocean animal habitat where
most organisms lived, meaning that climate cooling that occurred during
the Late Ordovician period combined with additional factors likely was responsible for LOME.

On the other hand, there is evidence that anoxia in deep oceans expanded
during that same time, a mystery that cannot be explained by the classic
model of ocean oxygen, climate modeling expert Alexandre Pohl says.



========================================================================== "Upper-ocean oxygenation in response to cooling was anticipated, because atmospheric oxygen preferentially dissolves in cold waters," Pohl says.

"However, we were surprised to see expanded anoxia in the lower
ocean since anoxia in Earth's history is generally associated with volcanism-induced global warming." They attribute the deep-sea anoxia
to the circulation of seawater through global oceans. Pohl says that a
key point to keep in mind is that ocean circulation is a very important component of the climatic system.

He was part of a team led by senior modeler Andy Ridgwell, professor at
UC Riverside, whose computer modeling results show that climate cooling
likely altered ocean circulation pattern, halting the flow of oxygen-rich
water in shallow seas to the deeper ocean.

According to Lu, recognizing that climate cooling can also lead to
lower oxygen levels in some parts of the ocean is a key takeaway from
their study.

"For decades, the prevailing school of thoughts in our field is
that global warming causes the oceans to lose oxygen and thus impact
marine habitability, potentially destabilizing the entire ecosystem,"
Lu says. "In recent years, mounting evidence point to several episodes
in Earth's history when oxygen levels also dropped in cooling climates."
While the causes of Late Ordovician extinction have not been fully agreed
upon, nor will they for some time, the team's study rules out changes
in oxygenation as a single explanation for this extinction and adds new
data favoring temperature change being the killing mechanism for LOME.

Pohl is hopeful that as better climate data and more sophisticated
numerical models become available, they will be able to offer a more
robust representation of the factors that may have led to the Late
Ordovician mass extinction.

========================================================================== Story Source: Materials provided by Syracuse_University. Original written
by Dan Bernardi.

Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Alexandre Pohl, Zunli Lu, Wanyi Lu, Richard G. Stockey, Maya Elrick,
Menghan Li, Andre' Desrochers, Yanan Shen, Ruliang He, Seth
Finnegan, Andy Ridgwell. Vertical decoupling in Late Ordovician
anoxia due to reorganization of ocean circulation. Nature
Geoscience, 2021; DOI: 10.1038/s41561-021-00843-9 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/11/211101141526.htm

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