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A large proportion of the carbon dioxide emissions that are currently being released into the atmosphere by human activities are absorbed by the surface ocean, making it more acidic. As a result, the tiny organisms (plankton), which lie at the base of the marine food web and make the surface ocean their home, are at risk. The fossil record can tell us how these plankton responded during ancient intervals of climatic change that were similarly associated with increased carbon dioxide emissions.

One such event is the Paleocene-Eocene Thermal Maximum (PETM), about 56 million years ago, which can be used as a case study for near-future climate change if carbon dioxide emissions continue to increase (the so-called worst-case scenario). Hundreds of deep-sea sediment archives that span the PETM reveal global turnover in plankton communities due to sea surface warming and ocean acidification.

High-latitude phytoplankton particularly sensitive to climate change

A team of researchers from MARUM, University of Bremen have now investigated how high-latitude phytoplankton communities responded to PETM warming. Examining high-latitude communities is especially important because they are historically understudied and are likely to be particularly sensitive to human-driven climate change.

The focus of the study, in Communications Earth & Environment, was on deep-sea sediment cores from the Campbell Plateau in the Southern Ocean, which were recovered during International Ocean Discovery Program (IODP) Expedition 378.

In their study, based on the fossilized remains of calcareous nanoplankton—microscopically small, single-celled algae that photosynthesize in the surface ocean and produce calcium carbonate (e.g., chalk) shells—the team was able to reconstruct changes in their community composition both before and during the PETM.

"Certain nanoplankton species prefer to live in warmer waters with fewer nutrients, while others can only live in colder, higher-nutrient waters. Therefore, major warming events like the PETM really affect which species thrive, and which don't. This can be observed in the nanofossil record by counting how many of each species there are and how this changes through time," explains first author Dr. Heather L. Jones.

Even small changes can have dramatic impacts on marine ecosystems

Somewhat surprisingly, the results from the research team's study show that the PETM did not seem to affect nanoplankton communities as much as anticipated. They attribute this to a preceding, smaller warming event, which they propose had already destabilized nanoplankton communities approximately 200 thousand years before the PETM.

"Most studies only focus on the PETM event itself and not the longer-term time before it," explains Dr. Jones. "However, examining these background intervals is absolutely critical in determining the extent to which warming events actually drove ecosystem change.

"In the case of our study, pre-event environmental conditions seem not to have been completely stable, which had a direct influence on how nanoplankton proceeded to respond to the PETM. It also highlights that even relatively small environmental changes can have dramatic impacts on marine ecosystems in certain locations, which has important implications for the current, highly regional effects of modern climate change."

As the current study is the first to formally document this pre-PETM event, its global significance is uncertain. It therefore sets the stage for future studies to use the expansive archive of legacy deep-sea sediment cores—such as those housed at the Bremen Core Repository (BCR) in the MARUM—to identify this newly-described event in different ocean basins.