These are rock drill cores removed from the drill hole at the FAR DEEP site in northwestern Russia. Photo: FAR DEEP |
The appearance of oxygen in the Earth’s atmosphere probably
did not occur as a single event, but as a long series of starts and stops,
according to an international team of researchers who investigated rock cores
from the FAR DEEP project.
The Fennoscandia Arctic Russia
– Drilling Early Earth Project—FAR DEEP—took place during the summer of 2007
near Murmansk in the northwest region of Russia. The
project, part of the International Continental Scientific Drilling Program, drilled
a series of shallow, 2-in diameter cores and, by overlapping them, created a
record representing stone deposited during the Proterozoic Eon—2,500 million to
542 million years ago.
“We’ve always thought that oxygen came into the
atmosphere really quickly during an event,” says Lee R. Kump, professor
and head of geosciences, Penn
State University.
“We are no longer looking for an event. Now we are looking for when and
why oxygen became a stable part of the Earth’s atmosphere.”
The researchers report in Science Express that
evaluation of these cores and comparison with cores from Gabon
previously analyzed by others, supports the conclusion that the Great Oxidation
Event played out over hundreds of millions of years. Oxygen levels gradually
crossed the low atmospheric oxygen threshold for pyrite—an iron sulfur mineral—oxidation
by 2,500 million years ago and the loss of any mass-independently fractionated
sulfur by 2,400 million years ago. Then oxygen levels rose at an
ever-increasing rate through the Paleoproterozoic, achieving about 1% of the
present atmospheric level.
“The definition of when an oxygen atmosphere occurred
depends on which threshold you are looking for,” says Kump. “It could
be when pyrite becomes oxidized, when sulfur MIF disappears or when deep
crustal oxidation occurs.”
When the mass-independent fractionated sulfur disappeared,
the air on Earth was still not breathable by animal standards. When red rocks
containing iron oxides appeared 2,300 million years ago, the air was still
unbreathable.
“At about 1% oxygen, the groundwater became strongly
oxidized, making it possible for groundwater seeping through rocks to oxidize
organic materials,” says Kump.
Initially, any oxygen in the atmosphere, produced by the
photosynthesis of single-celled organisms, was used up when sulfur, iron, and
other elements oxidized. When sufficient oxygen accumulated in the atmosphere,
it permeated the groundwater and began oxidizing buried organic material,
oxidizing carbon to create carbon dioxide.
The cores from the FAR-DEEP project were compared with the
Francevillian samples from Gabon
using the ratio of carbon isotopes 13 and 12 to see if the evidence for high
rates of oxygen accumulation existed worldwide. Both the FAR-DEEP project’s
cores and the Francevillian cores show large deposits of carbon in the form of
fossilized petroleum. Both sets of cores also show similar changes in carbon 13
through time, indicating that the changes in carbon isotopes occurred worldwide
and oxygen levels throughout the atmosphere were high.
“Although others have documented huge carbon isotope
variations at later times in Earth history associated with stepwise increases in
atmospheric oxygen, our results are less equivocal because we have many lines
of data all pointing to the same thing,” says Kump. “These
indications include not only carbon13 isotope profiles in organic mater from
two widely separated locations, but also supporting profiles in limestones and
no indication that processes occurring since that time have altered the signal.”
SOURCE – Pennsylvania State University