Oxygen-free early oceans likely delayed rise of life on planet
Geologists at the UC Riverside have found
chemical evidence in 2.6-billion-year-old rocks that indicates that
Earth's ancient oceans were oxygen-free and, surprisingly, contained
abundant hydrogen sulfide in some areas.
"We are the first to show that ample hydrogen sulfide in the ocean was possible this early in Earth's history," said Timothy Lyons, a professor of biogeochemistry and the senior investigator in the study, which appears in the February issue of Geology.
"This surprising finding adds to growing evidence showing that ancient
ocean chemistry was far more complex than previously imagined and likely
influenced life's evolution on Earth in unexpected ways - such as, by
delaying the appearance and proliferation of some key groups of
Ordinarily, hydrogen sulfide in the ocean is tied to the presence of
oxygen in the atmosphere. Even small amounts of oxygen favor
continental weathering of rocks, resulting in sulfate, which in turn
gets transported to the ocean by rivers. Bacteria then convert this
sulfate into hydrogen sulfide.
How then did the ancient oceans contain hydrogen sulfide in the near
absence of oxygen, as the 2.6-million-year-old rocks indicate? The UC
Riverside-led team explains that sulfate delivery in an oxygen-free
environment can also occur in sufficient amounts via volcanic sources,
with bacteria processing the sulfate into hydrogen sulfide.
Specifically, Lyons and colleagues examined rocks rich in pyrite - an
iron sulfide mineral commonly known as fool's gold - that date back to
the Archean eon of geologic history (3.9 to 2.5 billion years ago) and
typify very low-oxygen environments. Found in Western Australia, these
rocks have preserved chemical signatures that constitute some of the
best records of the very early evolutionary history of life on the
The rocks formed 200 million years before oxygen amounts spiked during
the so-called "Great Oxidation Event" - an event 2.4 billion years ago
that helped set the stage for life's proliferation on Earth.
"Our previous work showed evidence for hydrogen sulfide in the ocean
more than 100 million years before the first appreciable accumulation of
oxygen in the atmosphere at the Great Oxidation Event," Lyons said.
"The data pointing to this 2.5 billion-year-old hydrogen sulfide are
fingerprints of incipient atmospheric oxygenation. Now, in contrast,
our evidence for abundant 2.6 billion-year-old hydrogen sulfide in the
ocean - that is, another 100 million years earlier - shows that oxygen
wasn't a prerequisite. The important implication is that hydrogen
sulfide was potentially common for a billion or more years before the
Great Oxidation Event, and that kind of ocean chemistry has key
implications for the evolution of early life."
Clint Scott, the first author of the research paper and a former
graduate student in Lyons's lab, said the team was also surprised to
find that the Archean rocks recorded no enrichments of the trace element
molybdenum, a key micronutrient for life that serves as a proxy for
oceanic and atmospheric oxygen amounts.
The absence of molybdenum, Scott explained, indicates the absence of
oxidative weathering of the continental rocks at this time (continents
are the primary source of molybdenum in the oceans). Moreover, the
development of early life, such as cyanobacteria, is determined by the
amount of molybdenum in the ocean; without this life-affirming
micronutrient, cyanobacteria could not become abundant enough to produce
large quantities of oxygen.
"Molybdenum is enriched in our previously studied 2.5 billion-year-old
Archean rocks, which ties to the earliest hints of atmospheric
oxygenation as a harbinger of the Great Oxidation Event," Scott said.
"The scarcity of molybdenum in rocks deposited 100 million years
earlier, however, reflects its scarcity also in the overlying water
column. Such metal deficiencies suggest that cyanobacteria were probably
struggling to produce oxygen when these rocks formed.
"Our research has important implications for the evolutionary history of
life on Earth," Scott added, "because biological evolution both
initiated and responded to changes in ocean chemistry. We are trying to
piece together the cause-and-effect relationships that resulted,
billions of years later, in the evolution of animals and, ultimately,
humans. This is really the story of how we got here."
The first animals do not appear in the fossil record until around 600
million years ago - almost two billion years after the rocks studied by
Scott and his team formed. The steady build-up of oxygen, which began
towards the end of the Archean, played a key role in the evolution of
new life forms.
"Future research needs to focus on whether sulfidic and oxygen-free
conditions were prevalent throughout the Archean, as our model
predicts," Scott said.