Could limited sulfides in Earth’s ancient oceans have inhibited early life?

Earth’s oceans during the Proterozoic Eon were not as sulfidic as was expected. Image credit: NASA.

Earth’s oceans, between 2.5 billion and 541 million years ago and
coinciding with the advent of eukaryotic life, were not as sulfidic as has been
thought, according to new analysis of uranium isotopes in ancient carbonate
sea-floor sediment. The findings suggest that sulfur in the oceans may not have
played as large a role as expected in delaying the evolution of more complex
life.

This two-billion-year span of time, which immediately followed
the oxygenation of Earth’s atmosphere, is known as the Proterozoic Eon.
Eukaryotic life, which constitutes cells in which the nucleus is enclosed by a
membrane, is the basis for all multi-cellular, complex life, and such life
first appeared on Earth during the Proterozoic. Yet eukaryotic life didn’t
blossom into complex organisms until the Cambrian explosion of life, circa 541
million years ago. Evolutionary biologists don’t yet know for sure what took
life so long to evolve in more interesting ways, but the assumption is that for
most of the Proterozoic, something inhibited life’s further development.

An ancient rock, made from copper and calcite – a carbonate rock formed from ocean sediments – dating back to over a billion years ago in the Proterozoic Eon. Image credit: James St. John (Ohio State University)/CC-BY-2.0

Our attention then turns to Earth’s oceans as a place where early
life could be found. Could something in the water have held life back? As
oxygen grew more abundant in the atmosphere, oxidative weathering of rocks on
the surface increased in response. The resulting redox reactions (wherein atoms
and molecules are either ‘reduced’ by gaining electrons, or ‘oxidised’ by losing
electrons) produced sulfate, which entered into run-off that fed rivers and,
ultimately, the oceans, where it was reduced and formed sulfides.

Consequently, until now our picture of the Proterozoic has been
one with highly sulfidic oceans. Furthermore, because the atmosphere was only
just becoming enriched with oxygen, it was not thought to have had opportunity
to permeate into the oceans. Combining these two factors would lead to
conditions in mid-Proterozoic oceans as being highly ‘euxinic’ – that is,
anoxic (depleted in free oxygen) and sulfidic.

Euxinic waters would clearly be a significant constraint on the
development of life at the time. Not only would oxygen-breathing life-forms
suffocate, but sulfides are also toxic, while euxinic conditions would limit
the solubility of trace metals such as molybdenum, copper and zinc, which are
essential minerals used by life.

Uranium abundances

However, new research by a team led by Geoffrey Gilleaudeau, of George
Mason University, has found that perhaps the oceans a billion or so years ago
weren’t all that euxinic after all. They measured the concentration of
different uranium isotopes found within carbonate rocks dating back to a time
spanning from 1.8 billion to 800 million years ago.

Some of the oxidation of the surface released uranium atoms,
which subsequently made their way into the oceans. In oxygenated water, uranium
becomes soluble and finds its way into sediments on the sea floor. However, the
presence of sulfide can remove uranium from those sediments, and this removal
preferentially favours heavier isotopes. So a euxinic ocean would remove more
uranium-238 from sediments than uranium-235, leaving the abundance ratio of the
isotopes skewed more towards the lighter uranium-235 than one would expect in
today’s ‘normal’ conditions.

By analysing the relative abundance of uranium-238 to
uranium-235, Gilleaudeau’s team determined that no more than seven percent of
the global sea floor was euxinic during the Proterozoic. Although this is
higher than the level of euxinia in modern oceans, it is not as high as had
been expected. It also corroborates earlier findings of the abundance of trace
metal elements in black shale, which can record specific redox conditions that
would signify an euxinic environment. In particular, black shale can carry a
record of the abundances of those trace metals, such as molybdenum, which is
also soluble in oxygenated conditions, but in euxinic conditions it can bond to
sulfur and therefore be removed. Hence it can also act as a proxy for how
euxinic the sea floor was.

Shallow water

If the oceans of the mid-Proterozoic were not as sulfide-rich as
previously thought, then the question becomes, what inhibited the evolution of
complex life during that Eon?

“This is still an open question of debate,” says Gilleaudeau.
“Even though the oceans were probably not very sulfide-rich, euxinia may still
have been common in shallow water along continental margins, and in shallow
inland seas.”

These locations could be critical, as shorelines and shallow
water would have been an important location for the development of eukaryotic
life.

The isotope data also alludes to pulses of temporarily greater
euxinic conditions. “Its tough to say how widespread these pulses may have been
because they are only recorded by a few transient data points in our sample
set,” says Gilleaudeau. It’s likely they would have been caused by bursts of
nutrients into the oceans, which would have increased the rate at which organisms
produced organic compounds, and in anoxic environments microbes would break
down these compounds and release quantities of hydrogen sulfide with its
familiar rotten-egg odour.

The research, published in the journal Earth
and Planetary Science Letters
, shows that there is still much to
learn before we can begin to think about building a complete picture of Earth
during the Proterozoic.