Researchers Reconstruct Changes in Ocean Chemistry During the Paleocene-Eocene Thermal Maximum (PETM)

Our team has made exciting progress in understanding how a dramatic carbon release event 56 million years ago, known as the Paleocene-Eocene Thermal Maximum (PETM), impacted ocean chemistry. Using cutting-edge data assimilation techniques, we reconstructed the decline in ocean pH, the reduction in carbonate availability, and the rise in atmospheric carbon dioxide during this period. These findings reveal how the ancient Earth experienced major ocean acidification, drawing important parallels to current trends driven by human activities.

The PETM was one of Earth's most rapid and severe natural climate change events. Triggered by a massive carbon release over thousands of years, it caused global temperatures to spike, drastically altering the climate and ecosystems. A key aspect of this event was ocean acidification, a process similar to what we observe today as rising atmospheric CO₂ leads to more acidic oceans.

Reconstructing Ocean Acidification

During the PETM, ocean acidification was characterized by a substantial decrease in ocean pH, affecting marine organisms by reducing the availability of carbonate ions crucial for building shells and skeletons. In our study, we used paleoclimate data assimilation, combining information from climate records with advanced Earth system model simulations to create a comprehensive reconstruction of ocean conditions during the PETM. By using the Earth system model cGENIE, we integrated sediment and proxy data to provide a clearer picture of changes in ocean carbonate chemistry.

Data assimilation offers several advantages in studying past climate events like the PETM. Unlike conventional models, it merges limited available proxy records with realistic climate model simulations, producing more detailed estimates of past climate conditions—especially when direct measurements are unavailable.

We reconstructed the marine carbon cycle during the PETM by assimilating data related to sea surface temperatures and calcium carbonate (CaCO₃) concentration in seafloor sediments. The results revealed a substantial increase in atmospheric CO₂ from 890 ppm before the event to 1980 ppm during the PETM. This was accompanied by a notable decline in ocean pH by an average of 0.46 units and a significant reduction in surface-water calcite saturation.

Impact on Marine Life

Carbonate undersaturation became more intense, particularly in high-latitude surface waters—similar to today’s Arctic, which is experiencing declining aragonite saturation due to human-driven CO₂ emissions. The PETM's ocean acidification led to significant disruptions in marine ecosystems, contributing to the largest extinction of benthic foraminifers in the Cenozoic era. Species losses ranged from 30% to 50%, as many marine organisms were unable to cope with changes in carbonate chemistry. The decline in pH reduced carbonate ion availability, impacting organisms such as corals, mollusks, and plankton that relied on these ions for building their shells.

Our reconstructed data showed that the average ocean surface pH decreased from 7.91 to 7.45 during the PETM, with even more severe acidification in high-latitude regions. This variability highlights the importance of understanding the regional impacts of ocean acidification, as different parts of the world may experience more drastic changes.

Lessons for Today and Future Directions

Our study serves as a clear warning for the future. The decline in ocean pH during the PETM closely resembles modern projections under high-emissions scenarios, as projected by the IPCC (Intergovernmental Panel on Climate Change) of the United Nations. If carbon emissions continue unabated, today’s oceans could face acidification rates much faster than during the PETM, leading to severe impacts on marine biodiversity.

The PETM lasted 200,000 years, and its carbon release caused enduring damage to marine life. Today’s carbon emissions are occurring at a much faster pace, making adaptation particularly challenging for marine species. The PETM serves as a natural analog for modern climate change, particularly ocean acidification driven by increased CO₂. Conditions today, especially in vulnerable regions like the Arctic, threaten marine organisms such as corals and pteropods. Lessons from the PETM underscore that accelerated emissions and acidification could have dire consequences for global biodiversity.

Understanding these historical climate changes helps us prepare for the future. The parallels between the PETM and today’s climate crisis make it clear that reducing carbon emissions is critical to safeguarding marine ecosystems from similar or worse disruptions.

Read more: https://doi.org/10.1038/s41561-024-01579-y