26 June 2024

Researchers document a tipping point for marine anoxia

Biogeochemical cycles

In a new study, researchers at Globe Institute show that the Earth hides a dangerous self-perpetuating feedback loop that can tip the oceans into an oxygen-depleted state

blue ocean with small waves and blue skies
Continental shelves are covered by ocean water. Oxygen depletion naturally occurs with depth inside the oceans. If the oxygen depletion becomes too severe and migrates shorewards, then this may kickstart a self-perpetuating feedback loop.

Research on the late Cambrian "SPICE" event highlights the risks of human activities pushing Earth's system beyond a critical threshold, leading to widespread ocean anoxia through a feedback loop involving phosphorus recycling in sediments under bio-productive, anoxic, and sulfidic waters.

The SPICE event demonstrates how oxygen loss in marine ecosystems can cascade towards shore, passing a tipping point that results in lasting harm to animal ecosystems and alters the carbon cycle. These findings underscore the importance of understanding marine nutrient dynamics and improving forecasting models to guide policy, ensure marine health, and prevent self-cascading oceanic damage.

Aske Sørensen, under the supervision of Tais W. Dahl, conducted a groundbreaking study as part of his master thesis project, focusing on the dynamics of global biogeochemical cycles during the SPICE event. Together, Aske and Tais developed a model for the size of the marine anoxia zones and how it affected the global Carbon and molybdenum cycles. This model was calibrated using geochemical data from drill cores through the Scandinavian Alum shale  Formation. The team’s data collection involved rigorous sampling and precise geochemical analyses, providing a robust foundation for the model’s development.

Upon integrating the data with their model, Aske and Tais discovered that the changes in the molybdenum cycle during the SPICE event were exceptionally rapid. This swift transformation could not be explained by enhanced nutrient supply from land alone. Their analysis revealed that such rapid shifts necessitated the involvement of a positive feedback loop. Specifically, this loop involved anoxia—an absence of oxygen in the water—and the recycling of phosphorus from sediments. 

In this feedback mechanism, anoxia likely increased the release of phosphorus from sediments into the water column, which in turn could have stimulated primary productivity, leading to further oxygen depletion and perpetuating the cycle. This insight provided a new understanding of the interconnectedness of biogeochemical processes and the factors driving rapid environmental changes.

The study by Aske Sørensen and Tais W. Dahl represents a significant advance in our understanding of ancient biogeochemical cycles, highlighting the importance of feedback mechanisms in driving rapid environmental changes. Their collaborative effort exemplifies the synergy between data collection and model development in unraveling complex Earth system processes.

Read the full study here


Tais W. Dahl