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.