SAN
FRANCISCO
— Maybe the trigger for the rise of oxygen on
Earth was nothing special. Maybe that oxidation didn’t need large tectonic
shifts or the evolution of land plants. Instead, the circulation of carbon
dioxide, oxygen and phosphorus between Earth’s atmosphere, oceans, rocks and the
simplest of photosynthesizing life forms is sufficient to produce the dramatic
shifts in atmospheric gases that occurred in Earth’s history, new research
suggests.

“The [oxygen] transitions we see are driven by
Earth’s nutrient cycles,” said Benjamin Mills, a biogeochemist and computer
modeler at the University of Leeds in England, who presented
the research
December 10 at the American Geophysical
Union’s annual meeting. The findings, led by Leeds geologist Lewis Alcott, were
also published
online
December 10 in Science.

Early Earth’s atmosphere was a steamy mix of
water vapor, CO₂, ammonia, hydrogen sulfide and methane. Then, about 2.4
billion years ago, oxygen in the atmosphere suddenly skyrocketed, a surge known
as the Great
Oxidation Event
(SN: 2/6/17). After another
billion years or so, two more large pulses of oxygen to the atmosphere followed.
One, called the Neoproterozoic Oxidation Event, which occurred from about 800
million to 540 million years ago, brought oxygen levels to within 10 to 50
percent of modern levels and oxidated the surface ocean. During a final pulse
called the Paleozoic Oxidation Event, from about 450 million to 400 million
years ago, oxygen rose to modern levels in the atmosphere and penetrated down
into the deep ocean.  

Such dramatic pulses beg an explanation.
Researchers have considered tectonics such as the formation of supercontinents,
the uplift and weathering of mountains and the eruption of vast lava fields known
as large igneous provinces. Such processes, the idea goes, might have funneled massive
amounts of nutrients into the oceans in a short period of time, fueling sudden,
world-changing blooms of algae. Other researchers propose that the three
stepwise increases correspond to three big evolutionary advances: the rise of photosynthesizing
algae, the flourishing and diversifying of those algae and the rise of land
plants.

“You have a lot of ideas about things that can
cause these stepwise events,” Mills said. “But you have no consensus, and you
don’t even know if they’re really necessary.” Instead, the new study suggests, Earth’s simple biogeochemical cycling
— the long-term recycling of
carbon,
oxygen and phosphorus —
was enough ultimately to bring up the planet’s oxygen levels (SN: 10/1/19).

The researchers first constructed a simple computer
model to consider how carbon, oxygen and phosphorus move between reservoirs on
Earth and interact with each other.

Phosphorus, present only in rocks, is a key
nutrient for creatures from microbes to algae to plants. Adding phosphorus from
weathered rocks to ocean waters can drive microbial or algal activity in the
water column. That can pull oxygen out of the water, making the water oxygen
poor, or anoxic, and pulling more oxygen out of the sediments below. Eventually,
this leads to more organic carbon getting buried in soils, and more oxygen
being produced, until the chemistry flips again with the waters turning oxygen-rich.
That oxygen can then escape from the water into the atmosphere.

The slow cooling of the planet also played a
role in allowing oxygen to begin to accumulate. Early Earth’s atmosphere wasn’t
amenable to letting oxygen stick around: Gases escaping from the mantle as the
planet cooled led to a chemical state called reducing, in which oxygen is
rapidly removed from the atmosphere through chemical reactions.

But by the time of the Great Oxidation Event,
the atmosphere had become less reducing. How isn’t clear, and is an area for
future research, Mills says. The cooling Earth may have been emitting fewer
gases, or the chemistry
of those gases
may have undergone a change that made
them less likely to remove oxygen (SN: 9/3/19).

The researchers created a simulation of these different inputs and outputs, and ran the model over billions of years to observe how atmospheric and ocean oxygen levels might change through time as a result of just these factors. The results, Mills says, were remarkably similar to the actual record of Earth’s oxygen levels reconstructed using the rock record. “Our conclusions are that no large tectonic or biological events were required,” he says. And that suggests “that complex life may not be required to build a high-oxygen world…. If you just have simple photosynthetic bacteria, like have been around [on Earth] for 3 billion years, you could reach modern [oxygen] levels.” 

Until this study, no attempts to reconstruct
Earth’s long history of oxidation included the parallel history of phosphorus —
although there have been “empirical hints that it’s linked,” says Noah
Planavsky, a geochemist at Yale University not involved in the new study. “We
know that Earth’s atmospheric oxygen is strongly tied to the biosphere,” he says,
but reconstructing how these cycles interacted through Earth’s history has been
lacking.

Still, some researchers are concerned that leaving
out the role of tectonics, for example, may result in an oversimplication.
“Certainly these biogeochemical systems are important. But in both the
Paleoproterozoic and Neoproterozoic, we see a symphony, if you like, of forces
acting to drive and sustain oxygen production,” says geologist Ashley Gumsley
of Lund University in Sweden. “These include the assembly and breakup of a
supercontinent, with … massive large igneous provinces, chemical weathering as
well as global glaciation, all acting together.”

Mills acknowledges that these events may still
have played a role in the evolution of oxygen on Earth. “The key point of all
this is that, yes, these events happened and could have caused large changes in
oxygen,” he says. But the team’s models suggest that “we didn’t need them.”

That conclusion is potentially good news for
researchers looking for life on other planets, says astrobiologist and
planetary scientist Joshua Krissansen-Totton of the University of California,
Santa Cruz who was not involved in the study. “What is most exciting about this
paper,” he says, “is that it does away with the need for [triggering] events” or
complex photosynthesizing organisms like land plants.