Does the Earth really have a pulse?

The pulse

The pulse of the Earth is a central theme in Yara's campaign for the ambition of Growing a Nature-Positive Food Future. To learn about the science behind the pulse, Yara spoke with Lars Eivind Augland, associate professor in the Department of Geosciences at the University of Oslo. Augland works with geochronology and studies the timing of various events throughout geological time, ranging from how the Earth's continents have formed to how climate has changed over time.

Augland finds the phenomenon of a 26-second pulse fascinating and exciting.

​– "Yes, you may call it a kind of pulse. The Earth’s crust has regular tremors. They are so small that they do not pose a threat as real earthquakes can."

Augland explains that every 26 seconds, the pulse from Earth is captured by seismic stations around the world. The signals are most evident in West Africa, North America, and Europe. The pulse is one of the few signals being generated regularly, clearly, and accurately. It is unclear what the cause may be, but there are various possible explanations, including ocean waves, volcanoes and pressure build-up and release within water-filled cracks in sedimentary layers below the seabed.

​– "Originally, the micro-quakes, or the pulse detected at intervals of 26 seconds, were explained by wave activity in the Gulf of Guinea in West Africa. Special depth conditions, the geometry of the ocean floor and the coast have been pointed out as possible causes. Due to how the waves hit and create a resonance on the seabed, they could in turn propagate as earthquake waves in the Earth's crust," says Augland.

He continues to explain that volcanic activity has been cited as another possible explanation, but no traces of active volcanoes have been found in the sea in the area.

​– "A third explanation can be found in the latest study published in the renowned journal Earth and Planetary Science Letters, which states that fluid flowing through fractal fissure networks in sediments under the seafloor is the cause of the tremors," says Augland.

The regularity of the pulse may be due to special conditions on the Gulf of Guinea seabed, consisting of water-rich sediment layers that are under pressure. Due to sediment loading from the Niger River, water pressure increases in the seabed below. The pressure differences lead to water flows in cracks in the seabed, like in a hydraulic pump, where the pressure increases to a certain point before being released as a trigger. The regularity of pressure build-up and release is what produces tremors that can be recorded as a pulse on seismometers globally. The pressure difference can be amplified by wave activity over the Gulf of Guinea.

​– "In this sense, the new study unites previous explanations for wave activity and movements in the upper part of the Earth's crust," says Augland.

Augland emphasizes that none of the three explanations have been tested well enough. This would require thorough underwater surveys in relevant areas of the Gulf of Guinea, as well as measurements to identify the exact source of the quakes.

Discovered in the 60s

The fact that the Earth has a pulse every 26 seconds was discovered in the early 1960s. The pulse was first recorded by American seismologist Jack Oliver, who, among other things, did important work in the development of the theory of plate tectonics and worked on recording atomic bomb explosions using seismic waves. Since then, scientists have gathered enough data to determine that the regular tremors have persisted beyond its first recording and form a kind of rhythmic pulse.

​– "It is remarkable that these tremors occur in such a regular way, and have done so for so many decades. This is yet another transient phenomenon in a geological context. If we go back a few thousand years, the sea level was different. The last ice age, which ended about 10,000 years ago, led to major changes in sea level when the ice on land melted. Such changes in sea level probably play a role,” Augland explains.

Many "pulses"

According to Augland, the Earth has many pulses. One is this short pulse occurring every 26 seconds, while other sustained pulses are those controlled by astronomical parameters and solar radiation.

​– "We have to look at the variations in Earth's orbit around the Sun and the tilt of the Earth's axis, defining the so-called Milankovitch cycles. These pulses with different predictable periodicities of between about 10,000 and 400,000 years are actively used to study climate. Other pulses that have been proposed, but are currently more speculative, are linked to the heat exchange between the Earth's deep mantle and its crust, which could give rise to supervolcanoes, the formation of continents, and plate tectonic cycles that affect the climate by absorbing or releasing CO₂ into the atmosphere. These cycles have pulses of tens to hundreds of millions of years,” explains Augland.

​– "When I first read about the [26-second pulse] phenomenon, it immediately piqued my interest because I am working to understand some of these other cycles to say something about the history of the Earth. Obviously, it is exciting that we have regular and predictable processes locally that can be felt globally. This pulse may not say anything about the major processes on Earth and the conditions for life, but it is undoubtedly interesting because it shows how the world is connected. And then there is something mysterious when you discover phenomena that are not so easy to explain, and that you have been working on for over 50 years without knowing for sure what it comes from.”

Sources

Chen, Y., Xie, J. and Ni, S., 2022. Generation mechanism of the 26 s and 28 s tremors in the Gulf of Guinea from statistical analysis of magnitudes and event intervals. Earth and Planetary Science Letters, 578, p.117334.

Lantink, M.L., Davies, J.H., Ovtcharova, M. and Hilgen, F.J., 2022. Milankovitch cycles in banded iron formations constrain the Earth–Moon system 2.46 billion years ago. Proceedings of the National Academy of Sciences, 119(40), p.e2117146119.

Meyers, S.R. and Malinverno, A., 2018. Proterozoic Milankovitch cycles and the history of the solar system. Proceedings of the National Academy of Sciences, 115(25), pp.6363-6368.

Michael R.Rampino, KenCaldeira & Yuhong Zhuc, 2021: «A pulse of the Earth: A 27.5-Myr underlying cycle in coordinated geological events over the last 260 Myr», Geoscience Frontiers.

Müller, R.D. and Dutkiewicz, A., 2018. Oceanic crustal carbon cycle drives 26-million-year atmospheric carbon dioxide periodicities. Science advances, 4(2), p.eaaq0500.

Oliver, J., 1963. Additional evidence relating to “a worldwide storm of microseisms with periods of about 27 seconds”. Bulletin of the Seismological Society of America, 53(3), pp.681-685.

Shapiro, N.M., Ritzwoller, M.H. and Bensen, G.D., 2006. Source location of the 26 sec microseism from cross‐correlations of ambient seismic noise. Geophysical research letters, 33(18).

Wu, Y., Fang, X., Jiang, L., Song, B., Han, B., Li, M. and Ji, J., 2022. Very long-term periodicity of episodic zircon production and Earth system evolution. Earth-Science Reviews, p.104164.