The evolution of Earth's atmosphere is a major area of study. It's primal atmosphere consisted of hydrogen and helium accumulated from the solar nebula. That atmosphere was lost to space, replaced by the products of volcanic outgassing and asteroid and comet impacts. Eventually, free oxygen accumulated and the planet's atmosphere began to appear more like the current atmosphere. That took billions of years.
How exactly this happened, and how atmospheres evolve on exoplanets, are critical questions in space science. Atmospheric evolution is intimately connected to the evolution of life, and scientists want to understand these process in greater detail. One of the ways they can study atmospheric evolution is by studying rocks of different geological ages, especially their isotopic composition.
An atom of any given element always has the same number of protons. For example, oxygen has 8 protons. But the number of neutrons can vary in different isotopes of oxygen. There are three stable oxygen isotopes. Oxygen-16 with 8 neutrons is the most abundant, making up 99.76% of oxygen. Oxygen-17 and oxygen-18 are rarer and have 9 and 10 neutrons respectively. The ratio of these three has changed throughout Earth's atmospheric history.
When one type of metallic micrometeorites fall through Earth's atmosphere, the heat from the friction melts them. They're called I-type cosmic spherules, and they form when extraterrestrial iron and nickel in the micrometeorites melt and oxidize, meaning they combine chemically with oxygen in the atmosphere.
These micrometeorites can become deposited in Earth rock and fossilized. Earth's oxygen isotope ratio has changed over time, and micrometeorites retain a record of the isotope ratio in the atmosphere at the time they fell, recorded in the oxidized iron and nickel.
Researchers at Göttingen University’s Geoscience Centre and the Leibniz University in Hannover have developed a way to determine the composition of oxygen and iron isotopes in these tiny fossil micrometeorites from Earth's different geological periods. Their results are in a paper titled "Traces of the oxygen isotope composition of ancient air in fossilized cosmic dust" published in Nature Communications Earth and Environment. The lead author is Dr. Fabian Zahnow, formerly a Doctoral Researcher at Göttingen University.
"As a sub-type of micrometeorites, I-type cosmic spherules form by complete melting and oxidation of extraterrestrial Fe, Ni metal particles during their atmospheric entry," the authors write. "All oxygen in the resulting Fe, Ni oxides sources from the Earth’s atmosphere and hence makes them probes for the composition of atmospheric oxygen."
The researchers explain that these I-type cosmic spherules can be recovered from sedimentary rocks of different ages, and that they act as probes of Earth's atmospheric oxygen composition over the ages. The rocks can reveal the triple-oxygen isotope (O-16, O-17, and O-18) ratio and also put constraints on CO2 levels from the past. CO2 levels are important because it plays an important role in the carbon cycle and moderating the climate.
"Here we establish using fossil I-type cosmic spherules as an archive of Earth’s atmospheric composition with the potential for a unique record of paleo-atmospheric conditions dating back billions of years," the authors explain.
Their research is based on sediments from the Phanerozoic eon, the latest of Earth's four geological eons. It spans from the Cambrian explosion 538.8 million years ago to the present. During the Phanerozoic, life has rapidly evolved and multiplied and into an abundance of forms across ecological niches. Earth's continents were also combined into the single supercontinent Pangaea at the beginning of the Phanerozoic.
"We reconstruct the triple oxygen isotope anomalies of past atmospheric O2 and quantify moderate ancient CO2 levels during the Miocene (~8.5 million years) and late Cretaceous (~87 million years)," the authors write.
The researchers extracted a total of 92 micrometeorites from six sediments in different locales. The samples spanned from the Carboniferous to Cretaceous periods. Another eight micrometeorites from existing collections were included, which extended the time period under study from the Silurian to Quaternary periods.
The micrometeorites are tiny. They have diameters between 18 and 429 µm and range from 0.02 to 103 µg with a median mass of 0.2 µg. They show characteristic dendritic surface textures and no signs of terrestrial alteration.
The triple oxygen isotope ratio is the critical part of this work, and the researchers were able to measure that in one modern and 20 fossil I-type cosmic spherules large enough to be analyzed individually. They show two distinct patterns of triple isotope ratios. Most of them are in what the researchers call the low δ18O population. A second group of five spherules have a wider scattered range with elevated O-18.
The researchers also determined the triple-iron isotope ratio for one modern and 12 fossilized micrometeorites. This ratio can tell scientists about when oxygen first appeared in the atmosphere, the chemistry of ancient oceans, and other facets of ancient Earth.
“Our analyses show that intact micrometeorites can preserve reliable traces of isotopes over millions of years despite their microscopic size” said lead author Zahnow in a press release.
While some of the micrometeorites have been altered by their time on Earth, four of the fossil micrometeorites are entirely preserved. They're from the late Miocene and the Cretaceous.
The end result of the work is a reconstruction of the pCO2 (partial pressure of CO2) in Earth's atmosphere over time. pCO2 represents how much CO2 is concentrated in the atmosphere, and is important because it's the primary greenhouse gas scientists can track through time. Even small changes in pCO2 can drive major shifts in the climate. The micrometeorite data, combined with modelling and data from proxies like C-13 in liverworts, indicate "moderate CO2 levels in modern times (<2 Ma) as well as 10 and 90 million years ago," the authors explain.
The team's research shows how powerful their method can be. A deeper modelling of the climate over geological time will require more unaltered micrometeorites from across Earth's history. Their work supports the existing idea that carbonate rocks are the sole source of unaltered I-type cosmic spherules.
Finding more of them will allow the researchers to expand on their work. "Targeting carbonate host rocks can yield larger populations of pristine spherules, notably reducing uncertainty in pCO2 reconstructions, as larger sample quantities allow for considerably improved precision," the researchers write in their conclusion.
The goal is to find these unaltered cosmic spherules in ancient rocks.
"The extraction of unaltered I-type cosmic spherules from 2.7 Ga old carbonates is promising in this context," the researchers write. The 2.7 Ga figure alludes to I-type cosmic spherules extracted from Australia's Pilbara region. The Pilbara craton is an ancient part of Earth's lithosphere that's important in paleoclimate studies. Since the Great Oxygenation Event created a dramatic rise in Earth's atmospheric oxygen about 2.4 billion years ago, rocks from just before that time are of great interest to researchers if they can reveal the atmospheric oxygen isotope composition.
"The oxygen isotope composition of unaltered I-type cosmic spherules has the potential to trace paleo-atmospheric processes further back in time than any other existing proxy," the researchers conclude.