Oxygen is critical for
life, but what promoted the first rise in atmospheric oxygen on Earth and
precisely when it happened have been challenging scientists for the last 70
years.
Most scientific
research suggests oxygen rose rapidly about 2.4 billion years ago and then fell
just as abruptly over the next 200 million years – this event is called the
Great Oxygenation Event (GOE).
A new international study led by a team of geologists from the
University of Tasmania in collaboration with scientists from the Carnegie
Institute in Washington and the University of Toronto offers an alternative
theory.
The researchers
propose that the rise of atmospheric oxygen was a very slow process between 2.8
and 1.8 million years ago related to the collision of continental plates during
supercontinent cycles and the evolution of cyanobacteria in our oceans.
Atmospheric oxygen
rose over a period of a billion years, with a peak close to present day levels
of 21 per cent oxygen around 1.9 billion years ago. Oxygen then declined for a
further period known as the boring billion.
The research
demonstrated that the evolution of minerals in the Earth’s crust correlate with
the rise of oxygen due to the presence of new oxidised metal species that only
became available because of the rise in oxygen.
The new theory uses
measurements on the redox chemistry of minerals that form in rocks and on the
seafloor through geological time.
University of Tasmania
geologist Professor Ross Large said their results are based on a wealth of data
from a range of minerals and isotopes.
The teams at the
University of Tasmania, Toronto and Carnegie institute have built massive
databases on the chemistry of a wide range of minerals, involving tens of
thousands of analyses collected over the last 15 years.
“The world-wide trend
toward data-driven research is increasing because our technology is rapidly changing,
enabling thousands of analyses to be acquired,” Professor Large explained.
“Much previous
research on this topic has depended on limited analyses, supported by computer
models to fill-in the data and attempt to predict outcomes. This has commonly
led to ‘straight line’ interpretations that have ignored the Earth’s up-and-down
cycles through geological time.”
Professor Large says the
first rise in oxygen was accompanied by a decline in carbon dioxide and
methane, producing ocean and atmosphere conditions more amenable to life.
“The old Archean
oceans prior to 2.6 billion years ago were enriched in toxic elements such as
arsenic and mercury and very inhospitable to life as we know it,” Professor
Large said.
“Our research shows
that with increasing oxygen the chemistry of the ocean changed, toxic elements
declined and elements important to life such as phosphorus, molybdenum and zinc
became more available to stimulate evolutionary change.”
Professor Large said
these major changes were brought on by the first development of continental drift
related to the supercontinent cycles, which describe the assembly, duration and
fragmentation of the largest land masses on Earth.
“Mountain building
during collision of plates in the first phase of each supercontinent cycle led
to erosion of nutrients to the oceans, stimulating life and release of oxygen
to the atmosphere,” Professor Large explained.
“We propose that two
phases of mountain building helped drive the rise in oxygen, production of new
minerals and evolution of early life. The first occurred around 2.8 billion
years ago with the formation of the supercontinent Kenorland, and the second
around 2.1 billion years ago which formed the supercontinent Nuna.”
The third oxygen cycle
started about a billion years ago, and from then on the cycles increased in
frequency from about 200 million years apart down to 60 million years apart. Previous research by the team has shown that each oxygen cycle
ended with a mass extinction but was rapidly followed by an explosion in
evolution.
Contrary to some
suggestions, Professor Large does not consider we are heading into another mass
extinction. He said that past mass extinctions involved carbon dioxide rising
to greater than 4000 parts per million (ppm), compared to about 300 ppm today,
and oxygen dropping well below 10 per cent and possibly as low as 5 per cent, compared
with 21 per cent today.
He suggests that, based
on Earth cycles, the next mass extinction is about 30 million years away.
