The release of carbon dioxide (CO2) during the extraction and burning of fossil fuels has contributed to major changes in the Earth’s atmosphere over the centuries since humans realized their value as an energy source. CO often accompanying2 are benign gases such as helium (He) that can be used to trace these emissions.
Scientists have long speculated that the amount of 4It, an isotope of helium, increases in the atmosphere because it is found in the same reservoirs as natural gas and other hydrocarbons. But the measures so far have been contradictory and imprecise. Now, researchers have developed a new way to measure the noble gas, shedding light on the decades-old conundrum.
“Thanks to our measurements, for the first time, we are able to demonstrate that [the theory is] actually true, that helium concentrations in the atmosphere are increasing,” said Benjamin Birneratmospheric chemist and postdoctoral researcher at the Scripps Institution of Oceanography.
The new discovery could lead scientists to better identify the sources of CO2 in the atmosphere, which could guide emission reduction policies. The raise of 4It also raises questions about its isotopic companion, 3It, and a potential undiscovered reservoir of gas, an essential resource in some research and commercial industries.
Helium pairs with fossil fuels
Some minerals naturally contain uranium and thorium. These radioactive elements decay into stable elements over millions of years, releasing 4Him in the process. Because 4It is a noble gas, it does not bond easily with other elements and slowly leaks out of its host crystal over time. Rogue helium in the earth’s crust seeps toward the surface before escaping into the atmosphere.
In some cases, the rising gas is trapped under an impermeable cap rock. Natural gas, escaping from buried source rocks, also rises through the subsoil and is trapped with helium. “If you have a geological environment conducive to containing [natural] gas, it is probably also suitable for trapping helium,” Birner said.
When humans arrive and extract the gas from these reservoirs, 4He is also freed. With the growth in the use of fossil fuels since the beginning of the industrial era, 4It should flood the atmosphere. And the scientists looked for it. Unfortunately, conflicting data has so far clouded any evidence of a long-term increase in atmospheric helium – some studies have measured an increase, while others have shown little or no change.
A precise 4It measures
Birner and his colleagues have developed a new way to calculate 4The one that claims an accuracy superior to that obtained by all previous studies.
First, they got samples. Due to the permeable nature of helium, air samples are difficult to store, and scientists have had to tap into creative sources of ancient air. A previous study extracted air from inner carburetors and sealed metal pétanque game balls. “[Helium] does not diffuse through metals. So we had to find good metal boxes,” said Bernard Marty, a geochemist at the University of Lorraine who did not participate in the study. Birner and his colleagues have used gas stored in metal tanks collected sporadically by scientists for other experiments since the 1970s.
Next, the group measured the change in the ratio of 4He and nitrogen (N2) Through time. Nitrogen levels in the atmosphere remain relatively constant over the years; therefore, any change in the ratio between the samples indicates a change in the amount of 4He. The researchers found a significant increase in 4He in air samples dating back to 1974, two orders of magnitude larger than expected from Earth’s natural processes, according to the study. The increase is also greater than the small amount released by commercial and research applications.
Because 4It can now be measured accurately and is increasing demonstrably, scientists can trace the origins of associated greenhouse gases such as carbon dioxide. 4Its concentrations are highest in natural gas compared to other fossil fuels such as coal and oil. By measuring the amount of both 4It and the carbon in an air sample scientists hope to determine how much of the total emissions come from burning natural gas as opposed to automobiles or a coal-fired power plant, Birner said.
Surprisingly, scientists still have a lot to learn about Earth’s natural carbon emissions. Having an accurate way to trace carbon with helium could help them figure out how much is being pumped into the atmosphere by nature, Marty said.
“I think we’ll learn a lot more about how the world works from helium,” Birner said.
A 3He Mystery
The new data settles the long-standing debate over 4Him in the mood. “Those are great steps,” Marty said. But, he added, they pose an interesting problem.
Previous studies, including some by Marty and colleagues, have investigated the ratio of 3He has 4He in air samples to get to the 4It concentrates in the atmosphere. 3It is a natural and stable isotope of helium. The most accurate 3He/4Available measurements have shown that the ratio does not change in the atmosphere over time. The fact that the researchers in this study independently observed an increase in 4He means that 3It must also increase.
3It is rare on Earth; it is released primarily from a remnant of the mantle reservoir from the formation of our planet. It is also produced from the bombardment of cosmic rays, solar wind and interstellar gases and in the manufacture of nuclear weapons. But none of these sources can explain the amount entering the atmosphere. “The signal is about 10 times the geological fluxes, and we don’t know how to explain the source of this extra 3He,” Birner said.
3It is used in applications such as cryogenics, nuclear fuel and medical imaging. In recent decades, as the demand on the global supply has increased, it has become a scarce resource. The prospect of an undiscovered source of 3It is therefore intriguing. “People have thought of flying to the Moon to mine 3He’s there. That’s how important this resource is,” Birner said. “This will become even more important in the future because nuclear fusion reactors are supposed to operate on 3Him,” he added.
—Jennifer Schmidt (@DrJenGEO), science writer