Ancient Ocean Chemistry

The inherent assumption that certain life forms needed to be available in order to manage specific chemistries is likely suspect. It is reasonable to conjecture that some form of primitive life form emerged immediately that the rocks cooled sufficiently to allow the accumulation of water. That may not be true but obviously with additional time it becomes true.

The question is what initial ecological niche did such a biological factor occupy and what did it do?

Most probably, it liked hot spots and relied on sulphur chemistry to prosper. We actually have these critters around.

Whatever they were, it is reasonable that they took millions and perhaps billions of years to process their environment to the point that they affected the geological record. In fact as new forms of biological agent emerged, it is to be expected that they took a long time again to sufficiently alter their environment so that it became dominant. None of these events need to be sudden at all.

In the end, they produced oxygen and that took a long time to overwhelm the rest of the environment which happened about 2.4 billion years ago.

We will be making bad guesses until we are able to leave Earth and take a look at all those odd moons of the gas giants that look able top produce wet chemistry. I suspect that we will discover a proliferation of space delivered life forms already at work. If not, then an early start to a biological transformation of the Earth was unlikely.

I added an additional article on the importance of nickel in the changing chemistry. The illustration is from that article and has this comment.

Banded iron formations, such as this one in Western Australia, precipitated out of the Earth's early oceans billions of years ago, and are providing new clues to the evolution of ancient seawater and the microbes that inhabited it

Oct 30, 2009

A new wrinkle in ancient ocean chemistry

Scientists widely accept that around 2.4 billion years ago, the Earth's atmosphere underwent a dramatic change when oxygen levels rose sharply. Called the "Great Oxidation Event" (GOE), the oxygen spike marks an important milestone in Earth’s history, the transformation from an oxygen-poor atmosphere to an oxygen-rich one paving the way for complex life to develop on the planet.

Two questions that remain unresolved in studies of the early Earth are when oxygen production via photosynthesis got started and when it began to alter the chemistry of Earth's ocean and atmosphere.

Now a research team led by geoscientists at the University of California, Riverside corroborates recent evidence that oxygen production began in Earth's oceans at least 100 million years before the GOE, and goes a step further in demonstrating that even very low concentrations of oxygen can have profound effects on ocean chemistry.

To arrive at their results, the researchers analyzed 2.5 billion-year-old black shales from Western Australia. Essentially representing fossilized pieces of the ancient seafloor, the fine layers within the rocks allowed the researchers to page through ocean chemistry’s evolving history.

Specifically, the shales revealed that episodes of hydrogen sulfide accumulation in the oxygen-free deep ocean occurred nearly 100 million years before the GOE and up to 700 million years earlier than such conditions were predicted by past models for the early ocean. Scientists have long believed that the early ocean, for more than half of Earth's 4.6 billion-year history, was characterized instead by high amounts of dissolved iron under conditions of essentially no oxygen.

"The conventional wisdom has been that appreciable atmospheric oxygen is needed for sulfidic conditions to develop in the ocean," said Chris Reinhard, a Ph.D. graduate student in the Department of Earth Sciences and one of the research team members. "We found, however, that sulfidic conditions in the ocean are possible even when there is very little oxygen around, below about 1/100,000th of the oxygen in the modern atmosphere."

Reinhard explained that at even very low oxygen levels in the atmosphere, the mineral pyrite can weather on the continents, resulting in the delivery of sulfate to the ocean by rivers. Sulfate is the key ingredient in hydrogen sulfide formation in the ocean.

Timothy Lyons, a professor of biogeochemistry, whose laboratory led the research, explained that the hydrogen sulfide in the ocean is a fingerprint of photosynthetic production of oxygen 2.5 billion years ago.

"A pre-GOE emergence for oxygenic photosynthesis is a matter of intense debate, and its resolution lies at the heart of understanding the evolution of diverse forms of life," he said. "We have found an important piece of that puzzle."

Study results appear in the Oct. 30 issue of Science.

"Our data point to oxygen-producing photosynthesis long before concentrations of oxygen in the atmosphere were even a tiny fraction of what they are today, suggesting that oxygen-consuming chemical reactions were offsetting much of the production," said Reinhard, the lead author of the research paper.

The researchers argue that the presence of small amounts of oxygen may have stimulated the early evolution of eukaryotes – organisms whose cells bear nuclei – millions of years prior to the GOE.

"This initial oxygen production set the stage for the development of animals almost two billion years later," Lyons said. "The evolution of eukaryotes had to take place first."

The findings also have implications for the search for life on extrasolar planets.

"Our findings add to growing evidence suggesting that biological production of oxygen is a necessary but not sufficient condition for the evolution of complex life," Reinhard said. "A planetary atmosphere with abundant oxygen would provide a very promising biosignature. But one of the lessons here is that just because spectroscopic measurements don’t detect oxygen in the atmosphere of another planet doesn’t necessarily mean that no biological oxygen production is taking place."

To analyze the shales, Reinhard first pulverized them into a fine powder in Lyons’s laboratory. Next, the powder was treated with a series of chemicals to extract different minerals. The extracts were then run on a mass-spectrometer at UC Riverside.

"One exciting thing about our discovery of sulfidic conditions occurring before the GOE is that it might shed light on ocean chemistry during other periods in the geologic record, such as a poorly understood 400 million-year interval between the GOE and around 1.8 billion years ago, a point in time when the deep oceans stopped showing signs of high iron concentrations," Reinhard said. "Now perhaps we have an explanation. If sulfidic conditions could occur with very small amounts of oxygen around, then they might have been even more common and widespread after the GOE."

Said Lyons, "This is important because oxygen-poor and sulfidic conditions almost certainly impacted the availability of nutrients essential to life, such as nitrogen and trace metals. The evolution of the ocean and atmosphere were in a cause-and-effect balance with the evolution of life."

Reinhard and Lyons were joined in the research by Clint Scott of UCR; Ariel Anbar of the Arizona State University, Tempe; and Rob Raiswell of the University of Leeds, United Kingdom.

The two-year study was supported by the National Science Foundation and NASA.

Source: University of California, Riverside

Nickel famine' caused ancient oxygen rise

09 April 2009

A crucial increase in atmospheric oxygen that occurred around 2.4 billion years ago could have been triggered by a shortage of nickel in the oceans, according to Canadian researchers.

This 'nickel famine' would have starved colonies of ancient bacteria that used nickel-based enzymes to produce methane. As a result, oxygen was able to build up in the atmosphere - giving rise to the environment we have today.

It is well understood that a 'Great Oxidation Event' occurred 2.4 billion years ago that flooded the atmosphere with oxygen, but details surrounding what caused this event are unclear. Now, researchers at the University of Alberta, Canada, believe they have the answer after analysing sedimentary rocks obtained from all over the world, including Greenland, South Africa and Australia.

The rocks, called banded iron formations, were formed by slowly precipitating from seawater over billions of years. By examining the individual layers, the concentrations of trace metals in the ancient ocean can be revealed, stretching back nearly four billion years.

'Our findings indicate that the early Earth was much more nickel-rich because of the style of higher-temperature volcanism at that time,' says Kurt Konhauser, the lead researcher. 'As the mantle became cooler, around 2.5 billion years ago, the volcanic rocks became less nickel-rich, so less of the metal was being supplied to the oceans by their weathering.'

The team's calculations estimate that nickel concentrations in the ocean fell by half during this cooling period - dropping from 400 nanoMolar (nM) to only 200nM. The process has continued ever since, leading to modern day concentrations of only around 10nM.

'Such a drop in availability would have had profound consequences for bacteria that depended on nickel,' Konhauser explains. 'Crucially, these bacteria are implicated in controlling oxygen levels on the ancient Earth because the methane they produced was reactive with oxygen and prevented oxygen from accumulating in the atmosphere.'

If correct, Konhauser's study aptly demonstrates how different systems on the Earth are intricately linked together. In this case, altering the composition of the oceans can affect which organisms live in the biosphere - in turn changing the composition of the atmosphere.

Ariel Anbar, an expert on Earth evolution at Arizona State University in Phoenix, US, thinks that this is a elegant hypothesis. 'The idea that large-scale biogeochemical cycles can be perturbed by changes in the ocean availability of key nutrient metals has been proposed before,' he told Chemistry World. 'This is the latest such idea to emerge, and is perhaps the most intriguing so far.'

Lewis Brindley

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