Plate tectonics controlled ocean oxygenation over the last 540 million years

Ocean oxygenation paced marine biodiversity changes at the geological time scale. It is widely admitted that changes in ocean dissolved oxygen concentrations reflect in turn changes in the atmospheric oxygen concentration. In contrast, a new study published in Nature shows that continental rearrangement would have induced a decoupling between upper-ocean and deep-ocean oxygen concentrations and ultimately driven ocean oxygenation variations at the geological time scale. Changes in ocean oxygenation independent of variations in atmospheric oxygen concentrations represent a challenge for the interpretation of the geological indicators of marine oxygenation and demonstrate a previously overlooked imprint of plate tectonics on the evolution of Life on Earth.

Ocean dissolved oxygen concentrations exert a primary control on ocean habitability by marine organisms today and would have also largely constrained their evolution over the last 540 million years, since complex forms of Life appeared in the Cambrian Period. Increases in ocean oxygenation would have contributed to the largest rise in biodiversity during the Ordovician Period, around 460 million years ago, while deoxygenation (or anoxia) would have led to several mass extinctions that punctuated Earth’s history. Constraining the evolution of marine oxygenation is thus crucial to understand the trajectory of marine biodiversity.

Over the last decades, a large variety of indirect indicators was developed to reconstruct ocean oxygenation based on the geological record. These indicators – proxies – consist in chemical measurements conducted in rocks and permit tracking changes in the local oxygenation conditions several hundred million years ago. Numerical climate models then constitute an important step in the interpretation of the measured signal, by permitting to place the measurements in a broader paleo-oceanographic context. Up to now, models used to study the evolution of marine oxygenation over the last 540 million years were relatively simple. These models represent the ocean as a single reservoir (and thus feature a single value of dissolved oxygen concentration for the whole ocean). The unique reservoir does not permit representing ocean circulation, although the latter plays a major role in conveying oxygen from the ocean surface to the abyss. Therefore, in these models, ocean deoxygenation (or anoxia) necessarily implies a drop in atmospheric oxygen concentrations.

Here we used, for the first time, a model offering a spatially-resolved representation of the ocean in three dimensions, to study changes in ocean oxygenation over the last 540 million years. Our numerical simulations represent ocean currents. Results show that the global ocean circulation induces a decoupling between upper-ocean and deep-ocean oxygen concentrations. In the model, the ocean circulation in the oldest time slices, between 540 and 460 million years ago, therefore leads to deep-ocean deoxygenation under a large range of atmospheric oxyge n concentrations, including the Modern value. These results shed new light on the interpretation of the geological proxy record. There is no need to invoke an atmospheric oxygen concentration much lower than today to explain the proxy record of a poorly oxygenated ocean before 460 million years ago. By revealing a primary control of the arrangement of the continents on ocean oxygenation, this study demonstrates a previously-overlooked contribution of plate tectonics to marine biodiversity evolution.

Reference

Alexandre Pohl, Andy Ridgwell, Richard G. Stockey, Christophe Thomazo, Andrew Keane, Emmanuelle Vennin & Christopher R. Scotese. 2022. Continental configuration controls ocean oxygenation during the Phanerozoic. Nature 608, 523–527

Contact researcher

Alexandre Pohl – alexandre.pohl@u-bourgogne.fr
Biogéosciences, UMR 6282 CNRS, université Bourgogne Franche-Comté

Correspondant press office

Alexandre Pohl – alexandre.pohl@u-bourgogne.fr
Biogéosciences, UMR 6282 CNRS, université Bourgogne Franche-Comté

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Plate tectonics controlled ocean oxygenation over the last 540 million years

Ocean oxygenation paced marine biodiversity changes at the geological time scale. It is widely admitted that changes in ocean dissolved oxygen concentrations reflect in turn changes in the atmospheric oxygen concentration. In contrast, a new study published in Nature shows that continental rearrangement would have induced a decoupling between upper-ocean and deep-ocean oxygen concentrations and ultimately driven ocean oxygenation variations at the geological time scale. Changes in ocean oxygenation independent of variations in atmospheric oxygen concentrations represent a challenge for the interpretation of the geological indicators of marine oxygenation and demonstrate a previously overlooked imprint of plate tectonics on the evolution of Life on Earth.

Ocean dissolved oxygen concentrations exert a primary control on ocean habitability by marine organisms today and would have also largely constrained their evolution over the last 540 million years, since complex forms of Life appeared in the Cambrian Period. Increases in ocean oxygenation would have contributed to the largest rise in biodiversity during the Ordovician Period, around 460 million years ago, while deoxygenation (or anoxia) would have led to several mass extinctions that punctuated Earth’s history. Constraining the evolution of marine oxygenation is thus crucial to understand the trajectory of marine biodiversity.

Over the last decades, a large variety of indirect indicators was developed to reconstruct ocean oxygenation based on the geological record. These indicators – proxies – consist in chemical measurements conducted in rocks and permit tracking changes in the local oxygenation conditions several hundred million years ago. Numerical climate models then constitute an important step in the interpretation of the measured signal, by permitting to place the measurements in a broader paleo-oceanographic context. Up to now, models used to study the evolution of marine oxygenation over the last 540 million years were relatively simple. These models represent the ocean as a single reservoir (and thus feature a single value of dissolved oxygen concentration for the whole ocean). The unique reservoir does not permit representing ocean circulation, although the latter plays a major role in conveying oxygen from the ocean surface to the abyss. Therefore, in these models, ocean deoxygenation (or anoxia) necessarily implies a drop in atmospheric oxygen concentrations.

Here we used, for the first time, a model offering a spatially-resolved representation of the ocean in three dimensions, to study changes in ocean oxygenation over the last 540 million years. Our numerical simulations represent ocean currents. Results show that the global ocean circulation induces a decoupling between upper-ocean and deep-ocean oxygen concentrations. In the model, the ocean circulation in the oldest time slices, between 540 and 460 million years ago, therefore leads to deep-ocean deoxygenation under a large range of atmospheric oxyge n concentrations, including the Modern value. These results shed new light on the interpretation of the geological proxy record. There is no need to invoke an atmospheric oxygen concentration much lower than today to explain the proxy record of a poorly oxygenated ocean before 460 million years ago. By revealing a primary control of the arrangement of the continents on ocean oxygenation, this study demonstrates a previously-overlooked contribution of plate tectonics to marine biodiversity evolution.

 

Reference

Alexandre Pohl, Andy Ridgwell, Richard G. Stockey, Christophe Thomazo, Andrew Keane, Emmanuelle Vennin & Christopher R. Scotese. 2022. Continental configuration controls ocean oxygenation during the Phanerozoic. Nature 608, 523–527

 

Contact researcher

Alexandre Pohl – alexandre.pohl@u-bourgogne.fr
Biogéosciences, UMR 6282 CNRS, université Bourgogne Franche-Comté

 

Correspondant press office

Alexandre Pohl – alexandre.pohl@u-bourgogne.fr
Biogéosciences, UMR 6282 CNRS, université Bourgogne Franche-Comté