Mysterious fluctuations in oxygen caused an unusually strong mass extinction

(ORDO NEWS) — The Ordovician-Silurian event was larger in scale than the extinction of the dinosaurs 66 million years ago, but it was not preceded by known asteroid impacts or other similar events. Now his reasons are starting to become clear.

Approximately 443 million years ago, a rather strange, one might say, deeply atypical mass extinction occurred on Earth. In its first phase, 40% of known genera became extinct (obviously, there were even more extinct species), and in the second, after a million years, another 30% of genera disappeared.

Paleontologists believe that this corresponded to the extinction of 85% of all existing species of complex organisms.

Numerous more trilobites, similar to modern eels of conodonts, died out (did not survive to this day), as well as many graptolites – creatures that gave branched colonies, like corals (but not related to them). However, many species of corals that already existed at that time also died out.

For a long time, scientists believed that the start of extinction was given by global glaciation: Gondwana was then close to the South Pole, glaciers arose on it.

However, more careful dating in recent years has shown that everything was more complicated. Extinction, apparently, began before the glaciation – at least for brachiopods (brachiopods, tentacles) and graptolites.

Thallium is bound by manganese oxides, so changes in the amount of these oxides in the environment are directly reflected in thallium concentrations.

It turned out that the initial phase of the Ordovician-Silurian extinction coincided with a sharp depletion of sea waters in oxygen. It is at this point that the rapid decline of brachiopods and graptolites begins.

Mysterious fluctuations in oxygen caused an unusually strong mass extinction 2
The world of the period of the Ordovician-Silurian extinction had a completely different geography. Siberia and the Baltic were tropical mini-continents, and Gondwana was bound by ice

What is unexpected, immediately, literally hundreds of thousands of years after that, the decrease in oxygen concentration is replaced by its sharp increase. This is counterintuitive, because the number of oxygen-producing organisms usually drops sharply during extinction.

They are the basis of the food pyramid, so the decrease in the number of non-photosynthetic organisms is almost always not inferior to the decline of photosynthetics. If so, a typical mass extinction should not lead to a serious increase in oxygen concentrations. And for other extinctions, this is generally the case.

Unfortunately, it is not yet clear what could have caused the sharp drop in oxygen concentration at the beginning of the extinction. In principle, this is possible if the water temperature rose sharply (by many degrees) in a short time, but geologists do not see traces of such an event.

This is also possible with the mass death of photosynthetic organisms, but there are no traces of it in the paleontological record. However, this is possible when some photosynthetic species have not yet died, but the number of individuals in them has fallen sharply.

However, other scientific groups have previously noted that the first spores of vascular plants appear about 455 million years ago.

They were also called “higher”, and – due to the presence of a vascular system capable of lifting water and nutrients from the earth up – these are the first plants that could colonize the land to a significant extent.

Such colonization was to have two types of consequences. Firstly, terrestrial plants actively fixed carbon dioxide from the atmosphere, thereby creating the preconditions for glaciation (with a high concentration of CO2 in the air, glaciers on Earth cannot exist).

Secondly, even the most primitive land plants must have changed the structure of the land surface, creating soils of modern types.

At this point, minerals from the soil become much more prone to leaching into the ocean: plants inevitably increase the ability of water to penetrate into the soil and wash out a number of minerals from it.

From modern experience it is well known that a sharp increase in the amount of a number of minerals in water leads to its flowering. For example, cyanobacteria during water blooms can multiply in such a way that they block the light of algae at the bottom of reservoirs.

The toxins they release lead to the death of marine organisms, the decomposition of those consumes oxygen, but the algae at the bottom cannot replenish it, because they do not have enough light. Under such conditions, oxygen deficiency can occur even before glaciation.

Following the glaciation, however, the higher plants were bound to decline in numbers. After such an event, the amount of minerals carried into the ocean with rains could again decrease, leading to the resumption of normal photosynthesis in it.

We have previously detailed how fluctuations in oxygen concentration played a large role in the formation of complex life on Earth and its first paleontologically clear mass extinction.

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