If you traveled back in time to before the GOE (more than ~2.4 billion years ago), you would encounter a largely anoxic (oxygen-free) environment. The organisms that thrived then were anaerobic, meaning they didn't require oxygen and relied on processes like fermentation to generate energy. Some of these organisms still exist today in extreme environments such as acidic hot springs and hydrothermal vents.
The GOE triggered one of the most profound chemical transformations in Earth's surface history. It marked the transition from a planet effectively devoid of atmospheric oxygen-and inhospitable to complex life-to one with an oxygenated atmosphere that supports the biosphere we know today.
Scientists have long been interested in pinpointing the timing and causes of major shifts in atmospheric oxygen because they are fundamental to understanding how complex life, including humans, came to be. While our understanding of this critical period is still taking shape, a team of researchers from Syracuse University and MIT is digging deep-literally-into ancient rock cores from beneath South Africa to unearth clues about the timing of the GOE. Their work provides new insight into the pace of biological evolution in response to rising oxygen levels-and the long, complex journey toward the emergence of eukaryotes (organisms whose cells contain a nucleus enclosed within a membrane).
The study, published in the journal Proceedings of the National Academy of Sciences, was led by Benjamin Uveges '18 Ph.D., who completed the project as a postdoctoral associate at MIT and collaborated with Syracuse University Earth sciences professor Christopher Junium on the chemical analyses.
To analyze the ancient sediment, Uveges worked with Junium, an associate professor of Earth and environmental sciences at Syracuse University. Junium specializes in studying how past environments evolved to better understand future global change. His state-of-the-art instruments were essential for obtaining accurate readings of trace nitrogen levels.
"The rocks that we analyzed for this study had very low nitrogen concentrations in them, too low to measure with the traditional instrumentation used for this work," says Uveges. "Chris has built one of only a handful of instruments in the world that can measure nitrogen isotope ratios in samples with 100 to 1,000 times less nitrogen in them than the typical minimum."
In Junium's lab, the team analyzed nitrogen isotope ratios from South African rock samples using an instrument called an Isotope Ratio Mass Spectrometer (IRMS). The samples were first crushed into powder, chemically treated to extract specific components, then converted into gas. This gas was ionized (turned into charged particles) and accelerated through a magnetic field, which separated the isotopes based on their mass. The IRMS then measured the ratio of 5N to 4N, which can reveal how nitrogen was processed in the past.
An essential component of the Isotope Ratio Mass Spectrometer is called the cryotrapping/capillary-focusing module. This equipment, which played a critical role in enabling the nitrogen isotope analyses presented in the paper, is housed in Junium's lab at Syracuse University. (Courtesy: Christopher Junium)
So how does this process reveal past oxygen levels? Microbes (short for microorganisms) influence the chemical makeup of sediments before they become rock, leaving behind isotopic signatures of how nitrogen was being processed and used. Tracking changes in 5N to 4N over time helps scientists understand how Earth's environment, particularly oxygen levels, evolved.
Junium notes that these results mark a critical tipping point in the nitrogen cycle, when organisms had to update their biochemical machinery to process nitrogen in a more oxidized form that was harder for them to absorb and use.
"All of this fits with the emerging idea that the GOE was a protracted ordeal where organisms had to find the balance between taking advantage of the energy gains of oxygenic photosynthesis, and the gradual adaptations to dealing with its byproduct, oxygen," says Junium.
As oxygen produced through photosynthesis began to accumulate in the atmosphere, this rise in oxygen led to the extinction of many anaerobic organisms and set the stage for the evolution of aerobic respiration-a process that uses oxygen to break down glucose and provides the energy needed for functions like muscle movement, brain activity and cellular maintenance in humans and other animals.
"For the first 2 plus billion years of Earth's history there was exceedingly little free oxygen in the oceans or atmosphere," says Uveges. "In contrast, today oxygen makes up one fifth of our atmosphere and essentially all complex multicellular life as we know it relies on it for respiration. So, in a way, studying the rise of oxygen and its chemical, geological and biological impacts is really studying how the planet and life co-evolved to arrive at the current situation."
Their findings reshape our understanding of when Earth's surface environments became oxygen-rich after the evolution of oxygen-producing photosynthesis. The research also identifies a key biogeochemical milestone that can help scientists model how different forms of life evolved before and after the GOE.
"I hope our findings will inspire more research into this fascinating time period," says Uveges. "By applying new geochemical techniques to the rock cores we studied, we can build an even more detailed picture of the GOE and its impact on life on Earth."
Research Report:Aerobic nitrogen cycle 100 My before permanent atmospheric oxygenation
Related Links
College of Arts and Sciences (A&S) at Syracuse University
Explore The Early Earth at TerraDaily.com
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