Sat. Oct 1st, 2022
Picture of a big tree
enlarge The massive trunk of a kauri tree can remain intact for tens of thousands of years.

The Earth’s magnetic field helps protect life from energetic particles that would otherwise come from space. Mars now does not have a strong magnetic field, and conditions on its surface are considered so harmful to life that any microbes that could inhabit the planet are considered safe below the surface. On Earth, the magnetic field allows life on the surface to thrive.

Except that isn’t always true. The Earth’s magnetic field varies, with the poles moving and sometimes switching places and the field sometimes weakening or effectively disappearing. But a look at these events has revealed nothing out of the ordinary — no obvious links to extinction, no major ecological disturbances.

An article published yesterday in Science provides impressively accurate dating for a magnetic field reversal in the past using rings from trees that have been dead for tens of thousands of years. And it shows that the flip was associated with changes in climate. But the paper then tries to link the valve to everything from a minor extinction event to the explosion of cave art by our ancestors. Ultimately, the work is a mix of solid science, provocative hypotheses, and limitless speculation.

Old trees, but how old?

We’ll start with the solid science, all of which comes back to kauri trees, one of the distinctive species native to New Zealand. These trees are quite large and long-lived, regularly reaching over 1000 years of age. And the tree’s wood often survives when buried in swamps, with some samples dating back tens of thousands of years.

The team behind the new work is relying on the discovery of kauri wood that dates back to the time of the Laschamps excursion, a period when the magnetic poles briefly switched places about 40,000 years ago. Old trees tell many stories. The carbon-14 they contain can provide quite accurate data for the sample, and the individual tree rings then allow inferring conditions in individual years. Studies of other isotopes found in the wood can provide rough estimates of everything from solar activity to rainfall patterns.

The team behind the new work found that dating posted some of their material at the time of the Laschamps excursion. And there was a spike of excess carbon-14 in the tree rings that were being deposited at the time, consistent with more particles reaching Earth due to the decrease in magnetic field strength. This would normally be enough to reject the dating, and it has limited us in placing precise dates on the Laschamps excursion with previous examples.

But thanks to the details recorded in the growth rings, the research team was able to match its data with data from other sources that did have precise data attached. These included annual deposits made in a cave, which had both carbon-14 records and data provided by an isotope of thorium. The researchers could also more accurately match the data to ice core records, which also capture information from the time of the Laschamps excursion.

Once combined, these records provided accurate timing of the magnetic field reversal, as well as information about magnetic field strength over time. The combined record also provides some information about the prevailing climate and details about things like rainfall and solar activity.

Not stuck in reverse

The record suggested that the magnetic field began declining 42,350 years ago, reaching its lowest level 41,800 years ago, which is 300 years prior to the actual pole reversal. So the weakened magnetic field of that time was more of a precursor to the flip than an impact of the poles switching places. Because of the timing, centered on 42 kiloyears, the researchers decided to call this the Adams Transitional Geomagnetic Event, after author Douglas Adams.

The alignment of data also indicates that Earth wasn’t the only thing doing something unusual at the time. The isotope beryllium-10 is mostly formed by particles of cosmic rays hitting the atmosphere, so it serves as an indicator of solar activity. That’s because the sun’s magnetic field correlates with its activity level, and that magnetic field can deflect incoming particles that would otherwise travel into the solar system and potentially strike Earth. Levels of the isotope in ice cores indicate that the sun entered a long silent period that overlapped with the Adams event.

So there were two independent events that would both act to allow more high-energy particles to reach Earth’s atmosphere. Using a model of atmospheric chemistry, the researchers found that these particles would generate chemicals that destroy ozone. According to Gavin Schmidt of NASA, the ozone losses are because of this are not that big like the ones that created our current ozone hole, although they would be expected to be distributed somewhat differently, both geographically and seasonally.

The loss of ozone creates a series of relatively subtle climate effects, altering the Arctic jet stream and precipitation patterns in the southern hemisphere. These are the results of a limited number of runs of a single linked chemistry-climate model, so the researchers themselves recognize that the impact of ozone loss really needs to be studied with additional models to find out how robust these effects are.

Still, using the carbon-14 signature associated with the Adams event, the researchers identified the equivalent time periods in some sediment records. Both indicated that there were changes in atmospheric circulation patterns that occurred during the event, which is consistent with an impact on climate.

speculation time

Overall, the new accurate timing should be very useful for any study involving a sample that retains carbon-14 and dates to about this time period. In that respect, the work provides a service to the field. The prospect of a climate link and the advent of more high-energy particles is an intriguing hypothesis, and one that differs from previous attempts to link solar activity to climate change. It’s an idea that seems worth pursuing.

But for most of the rest of the paper, the researchers are looking at everything that happened about 42,000 years ago and trying to link it to the mix of altered environmental conditions they believe were caused by the Adams event. This includes the colder conditions prevailing in the Northern Hemisphere, as evidenced by glacial expansions. However, the changes in the magnetic field only last a few hundred years, while the colder climate lasts for thousands of years. So they had to imagine that the Adams event pushed the climate past a tipping point, allowing it to maintain its altered state without the original trigger. In addition, there are some climate records that show little change at the time of the Adams event.

Australia saw a major extinction of its megafauna that peaked about 42,000 years ago; this suggests a connection to the altered rainfall that seems to have caused the Adams event in the southern hemisphere. It’s an intriguing idea, although such extinction events typically extend well before and after the peak.

Other potential connections are extremely weak. Modern humans, despite having been in Central Asia for tens of thousands of years, seemed to appear in Europe around the time of the Adams event, and Neanderthals died out soon after. While it’s reasonable to suspect that those last two events are related, it’s not clear why either would be associated with the magnetic field reversal and any impact it had on climate.

The period also sees a growth in the scope and sophistication of cave art by those modern people. Again, the researchers are trying to link this growth to the Adams Event. There must have been more people in caves to escape the harsh radiation environment! And that’s why they used red ocher as a sunscreen, so they had the material for the art with them!

The reality is that humans and Neanderthals at that time had been using red ocher for artistic reasons for tens of thousands of years — and living in caves just as long. There may have been a difference in degree about 42,000 years ago, but it was not immediate.

Climate scientists and anthropologists alike have so far expressed considerable skepticism about these claims, although a number of individual claims have been found intriguing and worth heeding. The real test of some of these ideas will come when researchers use the carbon-14 signature the paper describes to look at other samples recording environmental changes, such as in sediment cores, from the same time period. This will give us a clearer picture of whether the events that happened around the same time really represent the kind of global changes that are being proposed.

Other ideas will likely be beyond our ability to come up with clean tests. It’s not clear how we would ever learn the relationship between sunscreen and art from the use of red ocher by ancient populations, or whether there were more people in caves because they somehow sensed the atmosphere was getting dangerous. So it seems that the researchers put forth provocative ideas that will not clearly affect the field.

An obvious way to monitor the work is to look more closely at other magnetic field reversals; the paper specifically mentions one that happened 35,000 years ago. But when the researchers simulated a magnetic field reversal without a major drop in solar activity, not much happened. It really seems like we need both to see the far-reaching consequences the researchers are proposing. And since the chances of both happening at the same time seem slim, it’s not clear how many other examples can tell us.

Science2021. DOI: 10.1126/science.abb8677 (About DOIs).

By akfire1

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