Paleocene-Eocene thermal maximum (PETM): a potential foresight into the future of ocean life

Shallow marine ecosystem collapse and recovery during the Paleocene-Eocene Thermal Maximum

Skye Yunshu Tian, Moriaki Yasuhara, Huai-Hsuan M. Huang, Fabien L. Condamine, Marci Robinson

Summarized by Mathew Burgos, University of South Florida undergraduate geology student. Interested fields of study include solar radiation, hydrology, hydrogeology, hydroelectricity, geochemistry, and environmental sustainability.

What data were used? Rich fossil records of ostracod arthropods (the group that also includes spiders, trilobites, and insects), extracted from a Salisbury embayment (i.e., a recessed coastal body where there is a direct connection to a larger body of water) near the coast of Maryland, eastern United States. Ostracods inhabit nearly all aquatic environments on earth; their tiny shells make them look like “seed shrimp”, and they were among the only marine invertebrate fossils with a strong enough fossil record to reconstruct the group’s response to the PETM (Paleocene-Eocene thermal maximum), a time on Earth where the global temperatures skyrocketed for a geologically short period of time. A core sample was utilized to study the ostracods; a core is a cylindrical section of the Earth where the sediments, rocks, and organisms within are removed from the subsurface for analyzation. 

Methods: A sediment core was dug from the ground in the embayment, and the ostracod content within the core was analyzed for carbon-13 isotope values, to later determine the survival rate of the species during and post-PETM. Studying fossil records of creatures that existed during that time may lead to future impacts on marine life and our oceans future health. Carbon-13 isotopes can indicate periods or events of warmer temperatures when the values trend negatively, so the isotope values here helped identify the stages of the PETM alongside the fossils. The PETM (Paleocene-Eocene Thermal Maximum) is an event that occurred roughly 56 million years ago, and it was a climatic event similar to the current global warming crisis because of prolonged greenhouse climate conditions; however, the current crisis is happening at a much faster rate.

Four panel image with differences described in caption. Each panel represents a time slice to show the changes in environment that is tracked by marine species.
Top left, Pre-PETM setting: regular oceanic conditions for marine life above the oxygen minimum zone (OMZ).
Top right, During peak PETM: shallow marine migrating upslope for survival. Deep low oxygen species also moving upslope and separating from their fellow deep marine species that are adapted to low oxygen.
Bottom left, Recovery phase I: Species that survived the PETM returning to pre-event locale above OMZ.
Bottom Right, Recovery phase II: Mixing of potentially new warm adapted species and shallow marine species.

Results: Analysis of the ostracod abundance illustrated a substantial elimination of the shrimp just before the thermal maximum event, followed by a recovery and diversification of the species once the ocean temperature normalized a couple of million years later. Potential detriments of the thermal maximum are the irreversible impact that climate change had on the marine life, primarily due increased temperature and deoxygenation of the water. As deoxygenation spread (Figure 1), only species who were able to move into different areas of the water column were able to survive; those who could not went extinct. Some species nearly went extinct during the PETM but were able to recover and diversify after the event, even potentially returning to a healthy population. 

Why is this study important? This study made connections between the PETM and modern climate change that is human-driven, which is extremely harmful to marine life, as the PETM is likely the best analog to the current climate crisis. Effects of modern-day climate change are like the happenings of the Paleocene-Eocene Thermal Maximum. This is an indicator of the importance of the impact humans could possibly have on the ocean in a short period of time, relative to the Paleocene and Eocene Epochs.

The big picture: The recent global warming effects that humans have had on could prove to be detrimental to our existence. This study focuses on the PETM that occurred over a vastly longer time scale compared to the short duration of the current age of industrialization. Humans are essentially replicating an extreme thermal event, that would otherwise be relatively naturally occurring in Earth’s time, but at a rate which is exponentially smaller in timeframe. With the status of the Earth’s oceans warming, we could potentially see the ramifications of eliminated marine species within our time at an unprecedented rate.

Citation: S.Y. Tian, M. Yasuhara, H.-H.M. Huang, et al., Shallow marine ecosystem collapse and recovery during the Paleocene-Eocene Thermal Maximum, Global and Planetary Change (2018),