A team of scientists from Stanford University in the US has apparently solved the mystery surrounding the so-called Great Dying around 252 million years ago. According to a study published in the journal PNAS, ocean acidification was not the primary cause of the largest mass extinction in Earth’s history.
The paleontologist Erik Anders Sperling and the biologist Jose Andres Marquez were able to prove that a combination of rising water temperatures and massive oxygen losses wiped out around 96 percent of all marine species at the time. Extremely high volcanic activity during this era pumped huge amounts of carbon dioxide into the atmosphere and thereby greatly heated the world’s oceans.
Previous theories assumed that the carbon dioxide made the seawater so acidic that shellfish could no longer build up their protective shells. The new studies now largely refute this assumption and instead place the risk of suffocation from overheated water at the center of paleontological research.
Metabolism as a survival factor
To prove the true biological mechanism, the research team analyzed the physiological characteristics of animal groups that dominated the oceans before and after the mass extinction. The scientists placed various representative species that are still alive today in special water-filled chambers and measured their acute oxygen requirements as the water temperature systematically increased.
This revealed a striking difference between the creatures of the so-called Paleozoic fauna and the species of the modern marine fauna that later became established. The older species often consisted of animals that were fixed to the ground and had little mobility, and which naturally had a very slow metabolism.
As soon as the water temperature rose in the experiments, the oxygen requirements of these sluggish organisms rose rapidly and completely uncontrollably. The physically-related problem is that warmer water can bind significantly less oxygen at the same time, which essentially causes the animals to suffocate in their own habitat.
Winners and losers of evolution
This fatal biochemical dynamic explains why species such as the arm-footed brachiopods or the starfish-like crinoids almost completely disappeared from our planet. Their physical tolerance limit for temperature-dependent lack of oxygen was simply permanently and globally exceeded due to the volcanic heating of the earth.
The modern fauna, consisting of species such as mussels and snails, survived the catastrophe comparatively unscathed. At first glance, this seems paradoxical, as these animals move much more actively and therefore naturally have a much higher basic requirement for oxygen.
However, in contrast to their Paleozoic predecessors, they have developed more efficient metabolic and respiratory systems. These evolutionary advantages enabled them to compensate much better for the massive lack of oxygen in extreme ocean heat.
Parallels to today’s climate change
In this direct context, the research team points out the real danger that today’s man-made climate change could put the oceans in an exactly identical state in the medium term. Before the start of the industrial revolution, our current oceans were still relatively cool and well supplied with oxygen.
Sperling draws the conclusion that the initial conditions at that time were shockingly similar to today’s marine world before the massive influx of carbon dioxide began. “This study is really the final nail in the coffin of what caused the Permian-Triassic mass extinction,” explains the researcher.
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Models with natural boundaries
At the same time, with such historical data models it must always be critically considered that laboratory experiments with today’s animal species can only reproduce the extreme conditions of prehistoric times in a very simplified manner. Nevertheless, the elaborately calculated metabolic rates provide a very comprehensible scenario as to why exactly those less resilient species disappeared during the geological transition.
It therefore remains to be seen whether current global efforts to reduce greenhouse gases are sufficient to prevent widespread deoxygenation of the oceans in the future. However, the new findings clearly show where the absolute physiological stress limits of maritime ecosystems lie.
