Why do we care about extinction?

"Why does mass extinction occur?" is a question that scientists have been discussing for a long time, and it is also considered one of the most challenging scientific frontier issues currently.

In the more than three billion years since the birth of the original life, countless extinctions have occurred. Scholars have been exploring the secrets and laws behind these catastrophes based on clues. In 1982, Jack Sepkoski and David M. Raup used statistical methods ingeniously to study the changes in biological species in the geological history period, and thus, the "Big Five" (the famous five mass extinctions) known to everyone in the paleontological community were formed - the Ordovician-Silurian mass extinction, the Late Devonian mass extinction, the Permian-Triassic mass extinction, the Triassic-Jurassic mass extinction, and the Cretaceous-Paleogene mass extinction, which is well known to everyone.

Big five debut[1]

These events destroyed 85%, 70%, 96%, 75% and 75% of species on Earth at the time, respectively, triggering huge changes that could change the entire face of the Earth. Of course, these numbers cannot give us an intuitive understanding of what these disasters will be like, just as we are now in the midst of a huge extinction event, but this does not allow us to intuitively understand extinction itself.

So today we are not going to talk about the epic and legendary mass extinction events of the past. Let's talk about a minor extinction event in geological history - although it did not cause a doomsday disaster, it is still extremely important, and in fact, it is more closely related to us than any other extinction event.

Let's go back to 55.5 million years ago, about 10 million years after the last mass extinction event (the Cretaceous-Paleogene mass extinction). At the turn of spring and summer, the angel blew his trumpet, and a burning meteorite fell from the sky, burning everything in the world. The day was dark, and the stars and the moon disappeared. The rulers of the land, sea, and sky completely disappeared, leaving only broken walls and ruins, waiting for the later generations to slowly pick up.

The Chicxulub crater is 180 kilometers in diameter. Image credit: NASA/JPL-Caltech

However, 10 million years after the disaster, the world recovered and flourished again: mammals radiated rapidly and developed into a variety of types; reptiles still ruled many places, inheriting the family business from the Mesozoic era; ray-finned fish began to dominate the oceans, and they built the shallow sea ecological environment together with the newly emerged reef-building organisms; even the remaining birds occupied a wide range of ecological niches, and the huge crowned terror bird (Gastornis) roamed the earth, as if recalling the glory of its ancestors - everything was prosperous, including the complete rise of angiosperms - in this new world, large herbivorous mammals had not yet appeared, and although the species were rare, the dense forests still covered the continents, leaving almost no empty space.

Amidst this prosperous scene, in the ocean, an accident suddenly occurred.

To this day, there are still many different opinions about the cause of this accident, but we know its result very well: a small thing called foraminifera accidentally caused the largest extinction event in the Cenozoic Era.

Foraminifera is an ancient organism, and its history is almost as long as the entire group of metazoans. It is also the most common fossil species in our daily life. You can even find them at home - if your home is decorated with carbonate rock, then you can find this thing on it. Of course, if you don't have it at home, you can see it in the surrounding large shopping malls - almost everywhere.

Foraminifera fossils are very common in decoration materials. If you look closely at the white limestone around you, you can basically see them (Source: Paul Williams, 'Limestone country - Limestone, dolomite and marble', Te Ara - the Encyclopedia of New Zealand, http://www.TeAra.govt.nz/en/photograph/12377/limestone)

Of course, it is not just modern people, the pharaohs of ancient Egypt also saw them almost every day, and even slept with them after death. These fossil animals lying on the stones of the pyramids are called nummulites, which are the largest known protists and were first recorded by Strabo, a great geographer and historian in ancient Rome. Of course, nummulites did not become extinct in this accident, but another large group of benthic foraminifera was unlucky:

The coin worms on the pyramids were transformed from beans that Strabo attributed to the pyramid builders throwing away.[2]

During this event, the number of benthic foraminifera plummeted, and 30% to 50% of all species became extinct. At the same time, an astonishing carbon emission event occurred: according to estimates, a large amount of carbon dioxide was emitted, with an average rate of about 400,000,000 tons per year - and such rapid and large-scale carbon emissions are likely to have continued for a full 50,000 years [3]. As the concentration of atmospheric carbon dioxide continued to rise, the average temperature of the earth also rose by 5 to 8 degrees Celsius. This event is called the "Paleocene-Eocene Thermal Maximum", abbreviated as PETM.

When the PETM was first discovered, it was discarded as an error due to its sudden insertion and large δ18O offset. It was not until 1991 that Jim Kennett of the University of California, Santa Barbara, noticed this abnormal isotopic change again in the Southern Ocean drill core: at the junction of the Paleocene and Eocene, both δ13C and δ18O showed abnormal drift [4]. The δ18O offset corresponds to a sharp rise in temperature, while the δ13C offset represents a surge in the carbon dioxide content in the atmosphere.

What’s even more interesting is that in addition to extinction, the foraminifera group also showed an even stranger response. In 2002, Deborah Thomas noticed that the foraminifera shells in this batch of drill core samples only had δ13C values ​​before the PETM and the PETM, but lacked the transition state in between [5]. In other words, at the beginning of carbon emission, the foraminifera stopped reproducing, or the speed of carbon emission was so fast that it could not be recorded in the fossil record.

Such rapid carbon emission is difficult to solve with ordinary volcanic exhaust. Although the volcanic activity in Greenland is also in the same period, researchers doubt whether it can achieve such a rapid and widespread impact. People began to turn their attention to another important carbon source - methane gas water clathrate (Methaneice). This name may not be familiar to everyone, but its other name was a household name ten years ago - combustible ice.

Since the methane in these combustible ices is produced by microbial metabolism, it naturally has a more extreme negative drift value of δ13C. Under the same circumstances, methane can cause a more obvious carbon isotope shift than carbon dioxide. But even so, it cannot explain why the negative shift of δ13C has been almost stable for 45,000 years - unless additional carbon is involved.

But where can we get all this extra carbon?

Sherlock Holmes once said, "When you have eliminated the impossible, whatever remains, no matter how unreasonable, must be the truth." Although science cannot rely on the process of elimination to explore the truth, it is indeed an extremely useful way to explore the way forward. When people rack their brains and cannot make up for the extra carbon, then this carbon may only come from outside the earth. In 2003, Dennis Kent of Rutgers University boldly judged that the large amount of carbon that caused the PETM came from the impact of a carbon-rich asteroid [6].

His evidence was single-domain magnetic nanoparticles found in the strata, which, unlike magnetic particles produced by microorganisms, can only come from outer space. However, his idea was not widely taken seriously. After all, an asteroid hit the earth only once 10 million years ago. Another one appeared now, which seemed a bit too frequent?

Single domain magnetic nanoparticles under transmission electron microscopy

Of course, the more important rebuttal comes mainly from the demand for carbon. If this event was caused by an asteroid, it would need to contribute hundreds of billions of tons of extraterrestrial carbon - this is simply unimaginable. However, if the asteroid is not the main carbon source contributor, but a triggering process, then the model may become very different. The impact of the asteroid will accelerate the release of methane hydrates on the earth, and will also induce violent volcanic activity, and the carbon it carries itself can also quickly increase the carbon content in the atmosphere in a short period of time. The result of all this is an obviously abnormal and rapid warming event.

So do we have evidence? The first piece of evidence is indirect, but very interesting. In 2013, James Wright and Morgan Schaller of Rensselaer Polytechnic Institute discovered an extremely strange clay deposit. It appeared as an extremely uniform interlaced strip, thus representing a periodic sedimentation event. According to their judgment, this sedimentation process was caused by seasonal sunlight. If their judgment is correct, then this set of sediments will become the most accurate reference for the PETM event [7].

Clay layers showing regular deposition

They measured the isotopic changes in the clay and found that the rate of δ13C decline was faster than previously estimated, falling by 0.4% in just 13 years - this is not a small number. The drift of stable carbon isotopes during the rapid warming event at the end of the Permian mass extinction was only 0.5%, and its time scale was hundreds of thousands of years. Extremely rapid carbon emissions seem to make the asteroid impact a more credible driving factor.

In 2016, Morgan Schaller reported another direct evidence, namely, impact glass found on the Atlantic coast of the United States, located at the boundary of the PETM[8]. This is a major piece of evidence of an asteroid impact, but until now, we still cannot make a final conclusion because the most important evidence - the impact crater - has not yet found any clues.

But the crater isn’t the biggest mystery—it’s the extinction. The PETM was catastrophic, dramatic, and sudden by any geochemical measure, but it didn’t trigger a mass extinction—instead, it was a major radiation event.

Benthic foraminifera suffered a huge blow, while planktonic foraminifera evolved rapidly and occupied the shallow ocean. Fish continued to flourish, and even reached a small peak in tropical regions. Insects expanded rapidly, and also ushered in a period of growth in number and species. Mammals also ushered in a glorious dawn: even-toed ungulates, odd-toed ungulates, and primates emerged in this event - it can even be said that humans originated in such high temperatures, accompanied by the largest snake in history and the smallest horse.

The largest snake, Titanoboa, may have been more than 12 meters long. It is generally believed that its appearance was closely related to the high temperature environment at that time. [9]

Everything seemed to be fine, except for the unfortunate benthic foraminifera.

It’s not a dramatic, dramatic change, but the rapid carbon emissions feel familiar—the PETM is perhaps the closest climate change event to the present in Earth’s history. But in reality, the asteroids, volcanism, and methane hydrates of the PETM pale in comparison to the Industrial Revolution—we’re dealing with an unprecedented carbon emissions process, 10 times more extreme than a natural side-effect like the PETM. At the current rate of human carbon dioxide equivalent emissions of about 370 billion tons per year, we’ll catch up to the PETM in 150 years. At this scale, no one can predict what will happen next.

A: The number of mass extinction events in the Phanerozoic Eon and the extinction rate of marine life; B, C: Estimated global extinction events. If carbon emissions cannot be curbed, we are likely to encounter an extinction event in the short term that is comparable to past mass extinction losses. [10]

"Life will find a way out" - we cannot predict what we will see when we excavate the strata of our era tens of millions of years later. Even if we count from the Industrial Revolution, the rapid growth of human carbon emissions has only been more than 200 years. In the scale of geological history, what is 200 years? It is not even enough to draw a clear line or provide even a small data point.

But if we are truly at the beginning of an extinction event, then it doesn't matter whether we are recorded or not. There will be thousands, tens of thousands, hundreds of thousands of years of strata behind us recording everything we have done. It will become a slow recovery curve, and the speed of recovery will depend on the severity of the extinction and completely unexpected positive and negative feedback events.

No other creature has ever held the pen to write about nature like humans do today, and what to write down depends on the choice we make.

The only regret is that the prologue has already begun and we can never throw away this pen.

References:

[1] Raup DM, Sepkoski JJ, Mass Extinctions in the Marine Fossil Record[J]. Science, 1982, 215(4539):1501-1503.

[2] Hohenegger J, Kinoshita S, Briguglio A, et al. Lunar cycles and rainy seasons drive growth and reproduction innummulitid foraminifera, important producers of carbonate buildups[J]. Scientific Reports, 2019, 9(1). https://doi.org/10.1038/s41598-019-44646-w

[3] Gutjahr M, Ridgwell A, Sexton PF, et al. Very large release of mostly volcanic carbonduring the Palaeocene-Eocene Thermal Maximum[J]. Nature. 2017;548(7669):573-577.

[4] Kennett JP, Stott LD. Abrupt deep-seawarming, palaeoceanographic changes and benthic extinctions at the end of thePalaeocene[J]. Nature, 1991, 353(6341):225-229.

[5] Thomas DJ, Zachos JC, Bralower, et al. Warming the fuel for the fire: Evidence for the thermal dissociation of methanehydrate during the Paleocene-Eocene thermal maximum[J], Geology, 2002,30(12):1067-1070.

[6] Kent DV, Cramer BS, Lanci L, et al. Acase for a comet impact trigger for the Paleocene/Eocene thermal maximum andcarbon isotope excursion[J]. Earth & Planetary Science Letters, 2003,211:13-26.

[7] Wright JD, Schaller MF. Evidence for arapid release of carbon at the Paleocene-Eocene thermal maximum[J]. Proceedings of the National Academy of Sciences of the United States of America, 2013,110(40):15908-15913.

[8] Schaller MF, Fung MK, Wright JD, et al. Impact ejecta at the Paleocene-Eocene boundary[J]. Science, 2016, 354(6309):225-229.

[9] Head JJ, Bloch JI, Hastings AK, et al. Giant boid snake from the Palaeocene neotropics reveals hotter past equatorialtemperatures[J]. Nature, 2009, 457(7230): 715-717.

[10] Penn JL, Deutsch C. Avoiding oceanmass extinction from climate warming[J]. Science, 2022, 376(6592): 524-526.

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Author: Sunny the Broken, Excellent Answerer of Paleontology on Zhihu

The article only represents the author's views and does not represent the position of China Science Expo

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