Tag: Cosmic rays

Earth’s magnetic field may have kick-started multicellular life

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Earth’s magnetic field diverts charged particles in the solar ‘wind’

The aspect of geomagnetism that is most familiar to geologists is the magnetic field’s repeated reversals of polarity. It manifests as the magnetic ‘striping’ of modern ocean floors: key to the discovery of plate tectonics. Relatively easy to measure in intricate detail using modern magnetometers, the ‘stripes’ record plate motions back to about the Middle Jurassic (~165 Ma). There are some examples of polarity reversals from Meso- and Neoproterozoic continental rocks but not a continuum. Measuring the direction of remanent magnetism in rock samples in ancient blocks of crust allow geologists to assess their former positions and orientation relative to magnetic poles of the past, and thus evidence of ancient continental drift. Measuring the intensity of past geomagnetism – its field strength – is more difficult than either. That is critically important for Earth’s biosphere because the magnetic field deflects high-energy charged particles from the Sun and cosmic rays that would otherwise reach the surface. Without a geomagnetic field it is possible that living cells could not survive. Even with the modern protective efficiency of geomagnetism, such particles are a likely source of genetic mutations in DNA: a key requirement of biological evolution.

In 2022 an extremely low magnetic field strength – about 15 times less than it is today –  was found in a 565 Ma-old anorthosite intrusion from Quebec, Canada that crystallised in the late Ediacaran Period. Samples from as far back as 3500 Ma in the Archaean showed a slow decline in field strength that culminated in its near vanishing in the Ediacaran. This fostered speculation that maybe increased genetic mutation by such an extraterrestrial bombardment of primitive eukaryote cells may have triggered the evolution of the multicelled Ediacara Fauna and perhaps even the first single-celled eukaryotes. A follow up in 2024 by a multinational group of scientists analysed minerals in igneous rocks from South Africa (2054 Ma) and Brazil (591 Ma) to check the magnetic-field strength in the Palaeoproterozoic Era and the Ediacaran Period (Wentao Huang and 16 others, 2024. Near-collapse of the geomagnetic field may have contributed to atmospheric oxygenation and animal radiation in the Ediacaran Period. Nature Communications: Earth and Environment, v. 5, article 207; DOI: 10.1038/s43247-024-01360-4). A more recent study by British and Brazilian geophysicists (Lloyd, S.J. et al. 2026. Earth’s magnetic field remained weak for 40 million years after the Cambrian radiation, Science Advances, v. 12, article eaeg2325; DOI: 10.1126/sciadv.aeg2325) suggested that field strength remained very low until about 505 Ma in the late Cambrian.

The very low magnetic field strength during the Ediacaran (pink), selenium isotope proxy for oxygen availability and radiation of bilaterian and non-bilaterian animals. (Credit: Wentao Huang et al., Fig 3b)

The slow decline in magnetic field strength during the Precambrian probably reflects slowing of turbulent motion in the early, completely molten core as it cooled. Eventually, at the time of very low magnetic field strength around 565 Ma the solid inner core slowly began to form through crystallisation of Fe-Ni alloy, reaching about half its present radius by the end of the Ordovician Period (~450 Ma). This roughly 100 Ma period encompasses the emergence of the Ediacara Fauna and the Cambrian ‘Explosion’ when the basal groups of almost all ‘modern’ eukaryote phyla evolved. Such a geologically swift burst of evolution now seems to be connected to more rapid genetic mutation when the Earth almost lost its ability to be shielded from radiation by its magnetic field.  Natural selection had a wealth of options to test, promote or snuff out. To plagiarise the Duke of Wellington, it may have been ‘the nearest run thing you ever saw in your life’. But according to Wentao Huang et al. another beneficial factor for eukaryote evolution may have been that solar-wind bombardment could have split water into its component oxygen and hydrogen. Being so light, hydrogen would have been driven into outer space leaving Earth’s atmosphere with more oxygen, which is vital for eukaryotes.

Magnetic reversal and demise of the Neanderthals?

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A rumour emerged last week that the Neanderthals met their end as one consequence of an extraterrestrial, possibly even extragalactic influence. Curiously, it stems from a recent discovery in New Zealand, where of course Neanderthals never set foot and nor did anatomically modern humans, the ancestors of Maori people, until a mere 800 years ago. It started with an ancient log from a kauri tree (Agathis australis), a species that Maoris revere. Found in excavations of boggy ground, the log weighed about 60 tons, so it was a valuable commodity, especially as it is illegal to fell living kauri trees. The wood is unaffected by burial and insect attack, has a regular grain and colour throughout, so is ideal for monumental Maori sculpture. Such swamp kauri also preserves their own life history in annual growth rings, and the log in question has 1700 of them. Using growth rings to chart climate variation gives the most detailed records of the recent past, provided the wood can be dated. Matching growth ring records from several trees of different ages is key to charting local climate with annual precision over several millennia.

An ancient kauri tree log recovered by swampland excavations in New Zealand. (Credit: Jonathan Palmer, in Voosen 2021)

Radiocarbon dating indicates that this particular kauri tree was growing around 42 thousand years ago. That is close to the upper limit for using 14C concentration in organic matter to determine age because the isotope has a short half-life (5730 years). In this case samples of the log would contain only about 0.7 % of its original complement of radioactive carbon. Cosmic rays generate 14C when they hit nitrogen atoms in the atmosphere and it enters CO2 and thus the carbon cycle. Carbon dioxide taken up by photosynthesis to contribute carbon to plants contains only about one part per trillion of 14C. Consequently wood as ancient as that in the kauri log contains almost vanishingly small amounts, yet it can still be measured using mass spectrometry to yield an accurate radiometric age.

The particularly interesting thing about the 42 ka date is that it coincides with the timing of the last reversal of the Earth’s magnetic field, known as the Laschamps event. The kauri tree bears detailed witness through its growth rings to the environmental effects of a decrease in that field to almost zero as the poles flipped. The bulk of cosmic rays are normally deflected away from the Earth by the geomagnetic field, but during a reversal a great many more pass through the atmosphere, the most energetic reaching the surface and the biosphere. The kauri growth rings record fluctuations in the generation of 14C by their passage and thereby the geomagnetic field strength, which was only 6% of normal levels from 42.3 to 41.6 ka (Cooper, A. and 32 others  2021. A global environmental crisis 42,000 years ago. Science, v. 371, p. 811-818; DOI: 10.1126/science.abb8677). This coincided with an unrelated succession of periods of low solar activity and a reduced solar ‘wind’, which also provides some cosmic-rayprotection when activity is at normal levels; a ‘double whammy’. One consequence would have been destruction of stratospheric ozone by cosmic rays and thus increased ultraviolet exposure at ground level.

Combined with the highly precise growth-ring dating, the climatic changes over the 1700 year lifetime of the kauri tree can be linked to other records of environmental change. These include glacial ice- and lake-bed cores together with stalactite layers. Apparently, the Laschamps geomagnetic reversal coincided with abrupt shifts in wind belts and precipitation, perhaps triggering major droughts in the southern continents. Highly plausible, but some of the other speculations are less certain. For instance, some time around 42 ka, but far from well-established, Australia’s marsupial megafauna experienced major extinctions, the Neanderthals disappear from the fossil record and modern humans started decorating caves in Europe (20 ka after they did in Indonesia). In fact, speculation becomes somewhat silly, with suggestions that early Europeans went to live in caves because of increased exposure to UV (they knew, did they, while Neanderthals didn’t?), their painting and, by implication, their entire culture shifting through the shock and awe of mighty displays of the aurora borealis. Just because the number 42 is (or was), according to the late Douglas Adams’s Hitchhiker’s Guide to the Galaxy, ‘the answer to life, the universe and everything’, the authors tag the episode as the ‘Adams Event’. In their summary for The Conversation they include an animation with a quintessential Stephen Fry narrative, which Earth-logs readers can judge for themselves. Perhaps ‘Lockdown Trauma’ has a lot more to answer for, other than upsurges in Zoom conferences, knitting and gourmet experimentation …

See also: Voosen, P. 2021. Kauri trees mark magnetic flip 42,000 years ago. Science, v. 371, p. 766; DOI: 10.1126/science.371.6531.766