Poorly oxygenated areas in the oceans could shrink, but that’s not necessarily good news

the global warming current situation impacts the land environment as much as the ocean environment. One of the consequences of this change in temperature is in particular a lower oxygenation of the oceans. This is the relentless observation made by scientists. For 50 years, the number of oxygen deficient areas has indeed increased sharply. This phenomenon, which severely impacts the ecosystems marine life, is related to the fact that warmer water absorbs less dissolved oxygen.

While it was thought that this mechanism would tend to worsen over time, a team of scientists from the Institute Max Planck and Princeton University discovered that, over the long term, these areas of low oxygenation would rather tend to resorb.

A result, published in the journal Nature, goes in the opposite direction of what was supposed until now. To arrive at this deduction, scientists base themselves on the study of past environmental conditions and on the evolution of phenomena associated with previous episodes of global warming.

Marine sediments, archives of the oceans

It is by analyzing samples of sediment sailors that the researchers were able to observe the evolution of the quantity of oxygen present in the oceans over time. A bit like the growth rings of treeswhich gradually record certain variations in terrestrial environmental conditions, marine sediments in fact record the chemical variations of the ocean environment over millions of years.

In the case of oxygen, this registration is mainly due to the presence of certain types of zooplanktonlike the foraminifera. These microorganismsconsisting of a skeleton limestone, are present in all the oceans of the world. They live close to the surface, where they absorb many elements necessary for their survival, such asnitrogen. The chemical composition of the living environment, and in particular the availability of nutrients in the ocean, will thus be part of the chemical signature of their skeleton.

When they die, the foraminifera will settle on the bottom of the ocean where their skeletons will participate in the constitution of the sedimentary layers which will accumulate over time, thus creating a veritable archive of the evolution of ocean environmental conditions. which can then be analyzed.

Oxygen deficiencies visible through the actions of zooplankton

The isotopic ratio of nitrogen present within the skeletons of foraminifera is a good marker of environmental conditions. When there is an oxygen deficit in the water, as is the case during episodes of global warming, a phenomenon of bacterial denitrification is observed. Under these conditions, in order to satisfy their needs in oxidizing elements (rich in oxygen), the micro-organisms begin to “deconstruct” the ions nitrates which are composed of a atom nitrogen for three oxygen atoms (NO₃^–). This mechanism leaves an isotopic signature at the nitrogen level in zooplankton skeletons and allows scientists to assess the extent of ancient zones by deficiency oxygen, but also to observe their evolution over time.

The results obtained are surprising. They show in particular that, during the last two episodes of climate hot, around 16 and 50 million years ago, the tropical Pacific Ocean was, against all expectations, rather well oxygenated.

A long chain of cause and effect

These observations show that some of the relationships between climate and ocean oxygenation conditions are not yet understood and that the evolutionary models currently in use are incomplete. Although the mechanisms leading to a reduction in oxygen-depleted zones during prolonged periods of warming are not yet clearly established, the scientists offer some avenues for reflection.

One of the hypotheses involves a reduction in biological productivity associated with the upwelling of deep waters, which generally provide nutrients needed by surface-dwelling organisms in tropical waters. A warm climate would have attenuated the temperature difference between theequator and the poles. The winds blowing over the tropics would thus have been weaker, inducing a weakening of the vertical current ensuring the upwelling of nutrients from the bottom of the ocean.

Since the surface waters are then less rich in nutrients, the biological productivity would have decreased with, as a consequence, a lower quantity of organic residues falling on the bottom. However, the degradation of this matter organic is usually a high oxygen consumer. In the absence of significant organic deposition during these warm periods, oxygen would have remained available in the water.

Reduction of poorly oxygenated areas: is this good news for biodiversity?

It looks like this long chain of events could get under way pretty quickly. This raises the question of our current situation. The rise in temperature of the waters of the globe currently favors a drop in the oxygenation of the oceans but if the proposed mechanism is correct, one could expect an attenuation of the phenomenon in the coming decades. And that’s not necessarily good news.

The result would be a stabilization and homogenization of the amount of oxygen at a low level. However, to function well, an ocean needs a certain heterogeneity which promotes flows and exchanges. The presence of low oxygen areas is thus essential in the open ocean domain, because it is they that ensure the chemical and biological cycles supporting ocean life. Conversely, coastal zone ecosystems require a well-oxygenated environment. A modification of these balances would lead, in the case of an increase in poorly oxygenated zones as in that of homogenization, to cascading effects on biological productivity and therefore on the biodiversity oceanic.