impact Archives - Demons Dance Alone

Blood of birds, disintegrating butterflies, two end-Permians, innocent volcanism

So, this blog has a loooooong backlog of stuff I wanted to talk about but I didn’t. Old stuff, like, one year old or more. Alas, I cannot hope to make a complete post on all that, but I don’t want to let it rot. Let’s do a bit of quick catching up:

Bird blood on our hands. In 2019 a couple of papers pointed directly the fingers at humans for the extinction of the great auk and the Carolina parakeet, two once-widespread, iconic species of birds that went extinct in the XIX century. Both papers analyzed paleogenomes (is it right to use the ‘paleo’ prefix when it’s a couple centuries ago?) and found out that both species populations had a vibrant genetic diversity until their numbers fell abruptly to zero. Which means: no, they weren’t already fragile, declining species that we gently pushed off a cliff they would have met anyway. We systematically exterminated two robust, healthy bird species in the space of a few decades or centuries. Not exactly unexpected, but now there’s more proof.

Singapore Coney Island Butterfly - Free photo on Pixabay

Butterflies that we will never know. In Singapore, 46% of the butterfly species disappeared (locally) in a mere 160 years, according to a paper of February 2020. Interestingly enough, the study accounts for extirpations of undetected species, using a model. I’m in no position to comment on the math, but the very idea is intriguing and melancholic: about a hundred of species would have gone extinct before we ever discovered them. Of these, some could have well been endemic species: ghosts, of which now we have nothing else than numbers in a statistical analysis. “14.9% of the species discovered before 1900 also were extirpated before 1900. These high early observed extirpation rates, during a period where many species remained to be discovered, suggest that a high number of species were never detected before they were extirpated”
The Karoo Basin, in South Africa, where the best deposits on terrestrial end-Permian/early-Triassic fauna are preserved.
A tale of two end-Permians. Discerning a single event that happened 252 millions of years ago is incredibly hard; discerning a complex interplay of events even harder. Compared to the relative simplicity of the Chicxulub impact, the Permian extinction is a maddening puzzle, muddled by its remoteness in time. There were always hints of multiple extinction events or at least multiple “hits” that led to the Permian catastrophe, but now a paper of March 2020 seems to imply that the extinction on the sea was different from the extinction on the land. It seems that whatever happened on the continents, leading to the demise of most terrestrial fauna and the temporary dominance of Lystrosaurus, happened 300.000 years before the extinction in the oceans: “Instead of the currently favored paradigm of calamitous and globally synchronous turnover in ecosystems, the reported terrestrial turnover in Gondwana occurred hundreds of thousands of years before the marine one and, therefore, marine and terrestrial responses likely had different extinction mechanisms.“. We have to see if and how it will be confirmed, but if so, it seems that the end-Permian extinction is truly two extinctions, above and below water. It will be extremely interesting to grasp how did one influence the other, and how does it translate to our current situation.

Innocent volcano. In January 2020, Pincelli Hull and coworkers put another nail in the coffin of the volcanic hypothesis for the K/T extinction. The K/T event has the distinction of having two competing or possibly synergic explanations: the well known Chicxulub asteroid impact, and the Deccan traps, a major volcanic event. For decades scientists have fought on what of these events was most important, and even if the impact seemed more and more clearly the culprit, the Deccan enthusiasts didn’t lose their grip. However, if the study is correct, it seems that 1)Deccan outgassing isn’t chronologically correlated to the extinction, but the impact is, and 2)the Deccan volcanism simply wasn’t generating enough gas to trigger an extinction, since similar events didn’t alter the biosphere so much. Another paper a few months later even argued that, if the Deccan volcanism had any effect, it was mitigating the extinction effects.

Improbable ashes

The Copernican Principle states that we are not privileged observers of the Universe. It is a restatement of the mediocrity principle: there is nothing exceptional about us, about the planet, about our history and so on.

Copernicus is also the name of one of the most prominent and iconic lunar craters, testimony of a massive impact roughly 800 million years old. And while impacts are, per se, relatively common events -one needs only to look at the Moon to see how many happened in the history of the Solar Systems, not all impacts are the same. And some, maybe, are exceptional: so much to make our own history exceptional.

A new paper by the Expedition 364 to the Chicxulub crater has compared the actual geological makeup of the crater to various impact simulations and found something interesting. Most asteroid impacts are oblique, following highly inclined trajectories:

Only one quarter of impacts occur at angles between $6 0^{\circ}$ and the vertical and only 1 in 15 impacts is steeper than $7 5^{\circ}$ .

But, once again, Chicxulub is exceptional. It seems it most resembles craters from steep impacts, about 45-60 degrees. The Chicxulub impactor was steep but not quite vertical hit, zooming right on Earth from the north-east:

Comparison of these observations with our simulation results suggests that the observed configuration is most similar to the $6 0^{\circ}$ impact simulations (or possibly the $4 5^{\circ}$ impact simulation at 20 km/s; Fig. 5).

And this was very bad news for the end of the Cretaceous, because they are exactly the conditions that lead to maximum disaster:

Impacts that occur at a steep angle of incidence are more efficient at excavating material and driving open a large cavity in the crust than shallow incidence impacts^5,19. Our preferred impact angle of ca. 60° is close to the most efficient, vertical scenario[…]

Impact angle has an important influence on the mass of sedimentary target rocks vaporised by the Chicxulub impact³⁷. […] a trajectory angle of 30–60° constitutes the worst-case scenario for the high-speed ejection of CO₂ and sulfur by the Chicxulub impact³⁷. At this range of impact angles, the ejected mass of CO₂ is a factor of two-to-three times greater than in a vertical impact and approximately an order of magnitude greater than a very shallow-angle ( $1 5^{\circ}$ ) scenario³⁷. An absence of evaporites in the IODP-ICDP Expedition 364 drill core is consistent with highly efficient vaporisation of sedimentary rocks at Chicxulub²⁷. Our simulations therefore suggest that the Chicxulub impact produced a near-symmetric distribution of ejecta and was among the worst-case scenarios for the lethality of the impact by the production of climate-changing gases.

So much for the Copernican principle, this further reinforces that the Phanerozoic history is punctuated by improbable events. A giant 10-20 km asteroid such as the Chicxulub impactor hits every few hundred million years. We know of no other impact causing a mass extinction. And Chicxulub happened in the worst possible spot (a shallow sea with bottom rocks rich in sulfur and carbon), in the worst possible moment (terrestrial ecosystems dominated by very large -and thus vulnerable- vertebrates; Chicxulub would have been far less important in the Cambrian or the Silurian!) and, now we know, probably with the worst possible geometry.

Here it is, from the paper: in twenty second, a 30-km deep wound was dug, and vaporized rock was thrown beyond 20 km high in the sky. We are now on Earth because of this event. If the asteroid were a few minutes early, a few minutes late, a few hundred km somewhere else, the history of life would have been completely different. Sometimes the Copernican principle is a principle, not a law. It is a useful guidance, but after all, it would be very weird if no rare, strange events at all happened. We are all improbable ashes of a fateful day that started 66 millions of years ago and still has not ended.

The paper is: Collins, G.S., Patel, N., Davison, T.M. et al. A steeply-inclined trajectory for the Chicxulub impact. Nat Commun 11, 1480 (2020). https://doi.org/10.1038/s41467-020-15269-x

The hidden bombardment

Earth is very good at cosmetics: it quickly and efficiently covers its scars. The Chicxulub impact that wiped out the Cretaceous was one of the largest impact events in the last hundred million years in the Solar System, and yet it left no visible trace today, save for an arc of carsic structures known as cenotes in the Yucatan Peninsula. We had to dig deep and hard to find it. So it is no surprise that new impact events are still being found. Nozaki et al. now declare on Scientific Reports to have found 11 million years old ejecta in the sediments of the North Pacific. Ejecta, mind you, not the crater. That is unknown yet.

We managed to link only one mass extinction to asteroid impact. That might be the case. But impacts have been (and will be) a constant in Earth history. While globally catastrophic impacts are rare, what about locally catastrophic ones? They must have had important ecological consequences. A region-devastating impact can still alterate weather patterns and radically steer the direction of life history. A promising but localized lineage can be wiped off, a destroyed area can be re1populated and lead to speciation, and so on. Our past must have been sculpted by many more fatal days than the one at Chicxulub, and we still have almost no idea.