Les scientifiques ont identifié la potentielle cause du réchauffement climatique ayant causé les premières extinctions de masse



Un ours polaire sur un iceberg au Canada *** Local Caption *** TITRE/ Préserver les océans pour sauver la terre LEG/ La fonte des glaciers se traduit par une montée inexorable du niveau des océans. Les conséquences seraient redoutables si le réchauffement climatique n’était pas maîtrisé.

 

L’Histoire de la Terre est riche et chaque période géologique apporte sont lot d’informations, permettant ainsi d’améliorer nos modèles et de mieux comprendre l’évolution des espèces qui l’ont peuplée. Certains cataclysmes ou événements particuliers sont liés à des extinctions de masse, et une nouvelle étude suggère que les premières de ces extinctions pourraient avoir été provoquées par les animaux eux-mêmes via le réchauffement climatique. 

Il y a environ 3 milliards d’années, les premières bactéries, aidées par la suite dans leur effort par les algues et le phytoplancton, ont commencé à convertir le dioxyde de carbone atmosphérique en oxygène, ouvrant la voie à l’apparition des animaux.

Il y a 540 millions d’années, au Paléozoïque, plus précisément au début du Cambrien, les premiers animaux benthiques apparaissent et, en creusant le plancher marin, déclenchent un nouveau changement atmosphérique conduisant aux premières extinctions de masse. C’est ce que suggère une étude anglo-belge publiée dans la revue Nature Communications.

Lors de l’explosion cambrienne, les fonds marins se retrouvent colonisés par de très nombreux petits animaux benthiques creusant et retournant le plancher à la recherche de nourriture ou en quête d’un terrier. « Comme les vers de terre dans les jardins, les petites créatures des fonds marins mélangent et recyclent le matériel organique — un processus connu sous le nom de bioturbation » explique Tim Lenton, climatologue à l’université d’Exeter (Angleterre). « Puisque l’effet de ce creusage est extrêmement important, il est normal de s’attendre à de grands changements environnementaux lorsque l’ensemble des fonds océaniques passent d’un état non-perturbé à un état bioturbé ».

Grâce à des analyses expérimentales et des modélisations mathématiques, les chercheurs ont pu relier l’émergence des animaux à la période de réchauffement climatique apparue il y a environ 520 millions d’années. Avant l’explosion cambrienne, le tapis marin microbien était stable et très peu dynamique.

Puis l’apparition soudaine des animaux benthiques a tout changé ; bien que ces derniers aient laissé très peu de fossiles de leur corps, de nombreux fossiles secondaires (sillons, terriers, etc) ont été retrouvés. Ces animaux ne creusaient en moyenne qu’entre 1 et 3 cm, contrairement à aujourd’hui où cette moyenne tourne autour de 10 cm.

La photographe allemande qui a pris le cliché accuse le réchauffement climatique de menacer la population d’ours polaires en Norvège.



En commençant à creuser les fonds marins, ces animaux ont fortement modifié la géochimie de l’environnement et atmosphérique, notamment le cycle de l’oxygène, du carbone, du phosphore et du soufre. Ces mélanges, échanges et brassages incessants des sédiments et de la matière organique ont conduit à une sévère anoxie océanique et atmosphérique (diminution de l’oxygène biodisponible), tout en relarguant des quantités massives de dioxyde de carbone.

« L’évolution de ces petits animaux a en effet diminué la quantité d’oxygène dans les océans et l’atmosphère, mais a également augmenté les taux de dioxyde de carbone atmosphérique jusqu’à provoquer un réchauffement climatique global. Nous savions qu’un réchauffement s’était opéré à cette époque de l’histoire terrestre, mais n’avions pas réalisé que les animaux pouvaient en être la cause » explique Benjamin Mills, paléo-climatologue à l’université de Leeds.

Cependant, l’arrivée soudaine des animaux benthiques avec l’explosion cambrienne, la bioturbation massive des fonds marins, l’anoxie océanique, l’augmentation brutale du dioxyde de carbone et le réchauffement climatique s’enchaînent logiquement, et correspondent aux extinctions récurrentes constatées entre le milieu du Cambrien et le début de l’Ordovicien par déprivation de l’oxygène océanique.


Pour Lenton, cette situation n’est pas différente de celle que nous vivons actuellement et nous devons en tirer une leçon, car contrairement aux vers marins, nous pouvons volontairement agir. « Il y a un intéressant parallèle à faire entre les premiers animaux changeant leur monde de façon menaçante, et ce que nous, les animaux humains, faisons de notre planète aujourd’hui. Nous créons un monde plus chaud avec une anoxie océanique de plus en plus importante, ce qui n’est pas simplement mauvais pour nous, mais pour tous les autres êtres vivants » conclut-il.

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Source : Nature Communications / Traduction : Trust My Science

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