Research Stories

by Diane Boudreau

Let's get one thing clear from the start. The scientific method is a precise and systematic process. But real life is seldom so orderly. Samples get contaminated. Researchers are human. They make mistakes. Instruments break or produce bad data. Subjects drop out of a study or lie on a questionnaire.

But if human beings are flawed, they are also flexible. When they experience setbacks, they often turn them into opportunities. And every once in a while, a study that doesn't go at all as planned can lead to one of the biggest discoveries of the year.

In September 2007, results from a pair of studies appeared in the prestigious journal Science. Their findings showed an unexpected "whiff" of oxygen in Earth's atmosphere about 50 million years before what scientists call the Great Oxidation Event (GOE).

Ariel Anbar led the international team of scientists that conducted the studies. He says the results came as a surprise. Anbar is a biogeochemist and associate professor at Arizona State University's School of Earth and Space Exploration. The results challenge the conventional wisdom that Earth's atmosphere was devoid of oxygen before the GOE occurred about 2.45 billion years ago.

The work is an important chapter in the story of Earth's youth. The information helps us understand the relationship between biology, geology and the atmosphere. It holds important implications for today's climate issues. It can even help us figure out the prospects for life beyond our own blue-green planet.

It's very important news, but it wasn't what the researchers were looking for. A series of mishaps sent them in different directions than they originally intended. Their adaptability and perseverance led them to an incredibly important find.

Boring into the past
The project started out as a quest for eukaryotes. Eukaryotes are living things whose cells have a membrane-bound nucleus. All animals and plants are eukaryotes. Bacteria and archaea are prokaryotes—organisms without true nuclei. Earth's first known inhabitants were all prokaryotes.

"One of the big questions in evolution is when eukaryotes came on the scene. The earliest eukaryote microfossils are from about 1.8 billion years ago, after the Great Oxidation Event," Anbar explains.

However, in 1999 a group of Australian researchers found evidence that eukaryotes might have existed as early as 2.7 billion years ago. They found remains of molecules that eukaryotes use to build cell walls inside ancient rock samples.

"But there were concerns that those samples were contaminated," says Anbar. "It's very easy to contaminate samples—there are eukaryotes all over the place today."

With eukaryotes in mind, Anbar led an international group of researchers Down Under. Their goal was to analyze a kilometer-long sample of sedimentary rock excavated from the Hamersley Basin in Western Australia. The site is one of the few areas on Earth where undisturbed rock from billions of years ago can be found.

"Much of Australia is very tectonically tranquil. It's boring from a mountain building point of view, but it's nice for the preservation of ancient rocks," says Anbar.

The researchers hired a drilling rig to excavate the core. They planned to collect a clean sample of the rock that wasn't contaminated with modern-day microbes. To do this, they needed to use water instead of oil to lubricate the drill, because oil is filthy with eukaryote residue. Unfortunately, the drillers refused to use water as a lubricant because it posed too much risk to the machinery.

According to Anbar, the search for eukaryotes was to be the "sexy" piece of the project, the part that could result in stunning discoveries.

"That was a spectacular failure," he says. "Although the goal was to drill clean, for a bunch of reasons this didn't happen."

Contamination wasn't the only problem the group faced. The researchers also didn't get a sample from the location they originally wanted to drill.

"In this area the aboriginal tribes own the land. They decide where you can drill. The land and their pathways through the land are very important to them. The site where we ultimately negotiated to drill wasn't ideal. The upper part was fine but what we got at depth wasn't what we expected," says Anbar.

The younger rocks were thicker than expected at the new drill site. The scientists drilled a kilometer into the Earth. But the rocks they found at the bottom weren't as old as they had hoped.

"That turned out to be a blessing in disguise," says Anbar.

Finding a resolution
Analyzing almost a kilometer of rock—908 meters to be precise—is a gargantuan task, even with teams of researchers tackling the job.

"If we could have used the whole sample, we'd have spread out our studies over the whole kilometer," explains Anbar. "Instead, we focused on the uppermost units. This time period is well-preserved, but it has never been studied as intensively as we went at it."

The sample they used was still pretty big—100 meters long, or about the length of a football field. The researchers sampled bits of rock from regular intervals along the core's length. Anbar describes it in terms of resolution, like the resolution of a digital photo. If you take 100 samples from 1,000 meters of rock, you'll get a "picture" of the rock's makeup with a rather coarse resolution. If you take 100 samples from only 100 meters of rock, that resolution will become much finer. As a result, your "picture" will be clearer.

The scientists pummeled the rock samples into powders. These were distributed to different members of the team for chemical analysis. Yun Duan is an ASU doctoral student. He dissolved some of the powders in acid. He then vaporized the acid solutions for analysis using an inductively coupled plasma mass spectrometer. The tool allowed Duan to figure out the chemical makeup of the rock.

The researchers were looking for the elements rhenium and molybdenum. The amount of these elements in the rock depends on the amounts that were dissolved in the oceans at the time. In turn, the amounts dissolved in the oceans depend on the amount of oxygen (O₂) present in the environment.

Another part of the team was led by Alan Jay Kaufman of the University of Maryland in College Park. They studied sulfur isotopes, which are also affected by O₂.

The researchers expected the analysis to be rather boring. The history of O₂ in the environment at that time was thought to be well understood.

"Our expectation was that we'd see very little molybdenum and rhenium in these rocks because there was little or no O₂ around at that time," says Anbar. "The very bottom part of the sample confirmed our expectations. But then as you go up there's this spike in molybdenum and rhenium, before the oxidation event. That was a surprise."

The group studying the sulfur isotopes also found unexpected variations in the same area of rock, supporting the evidence for O₂ in the atmosphere. The amount of O₂ wasn't anywhere close to the amount found in today's atmosphere, but it was there, and it was a surprise.

The story of O₂
Until now, scientists assumed that our atmosphere was virtually oxygen-free before the GOE. We know that oxygen levels rose abruptly during the GOE, but no one is entirely sure why. One long-accepted explanation is that the evolution of photosynthetic bacteria led to the increase. This makes sense because O₂ is a by-product of photosynthesis, released when water molecules are broken down using energy from the sun.

This explanation is not widely accepted anymore. Scientists now have evidence that photosynthetic bacteria lived at least 300 million years before the GOE. So why didn't O₂ start building up in the atmosphere at that time?

In 2007, scientists from Pennsylvania State University and The University of Western Australia fingered volcanoes as the culprit. At the time of the GOE, the Earth was undergoing massive geological changes. One of these changes was a shift in the type of volcanoes dominating the Earth. Before this period, most volcanoes were under the oceans. But around the time of the GOE, terrestrial volcanoes started to gain the upper hand.

Undersea volcanoes produce a mixture of gases and lavas that "scrub" O₂ from the atmosphere and bind it into minerals. Terrestrial volcanoes, however, remove far less of the gas from the atmosphere. As terrestrial volcanoes gained primacy, oxygen could finally build up in the atmosphere.

As O₂ levels rose, weathering of the Earth's crust increased. We tend to think of oxygen as a benign, life-giving element, and in many ways it is. But oxygen has a dark side, too. It is highly reactive, changing materials in ways that aren't always desirable. For example, it causes rust in metals, and cellular damage in humans. Oxygen also breaks down rocks in the Earth's crust, releasing their component elements. This is how most molybdenum and rhenium gets into the ocean.

This all provides a good explanation for the massive rise in oxygen levels starting around 2.45 billion years ago. Now, thanks to a study that didn't go as planned, we also know that O₂ existed in the atmosphere even before the GOE. The work provides yet another chapter in the story of this life-giving gas. But Anbar's research into the connections between the atmosphere, oceans, elements and life is not only about understanding ancient history.

"When we try to understand something like the carbon cycle today, we know that it's not enough to study the oceans, atmosphere and biology in isolation from each other," he explains. "These pieces all fit together as part of what we call Earth systems science, which we use to make predictions about the environment of the future.

"But how can we test our ideas of how those pieces fit together? There are two ways," he says. "We can wait for the future to come and see what happens. Or, we can find ‘experiments' nature has done in the past and see if our ideas can explain those events. If it works, that gives us confidence in our predictions for the future. We are testing our limits of understanding Earth as a system."

This research was supported by NASA's Deep Time Drilling Project (DTDP) and the National Science Foundation, with logistical support from the Geological Survey of Western Australia. The DTDP is part of NASA's Astrobiology Institute. For more information, contact Ariel Anbar, Ph.D., School of Earth and Space Exploration, College of Liberal Arts and Sciences, 480.965.0767. Send e-mail to: anbar@asu.edu