Ancient oceans shifted long before Earth’s great warming

The oceans carry a long memory of Earth’s climate. With each pulse of carbon and each rise in temperature, the water records the story. Today, it continues to absorb huge amounts of human-generated emissions. This acidifies surface waters and places plankton – the base of the food web – at risk.

To predict what might happen next, scientists turn to the past. A striking case is the Paleocene–Eocene Thermal Maximum (PETM), a rapid warming event about 56 million years ago.

During this time, seas warmed by several degrees, and chemistry shifted quickly. Fossil records show that tiny plankton reacted strongly to these changes.

Polar oceans hold clues

Most earlier studies of the PETM focused on mid-latitudes. High-latitude oceans were less studied, even though they are more sensitive to warming. A research team at MARUM, University of Bremen, chose to close this gap. They investigated sediment cores from the Campbell Plateau in the Southern Ocean.

The focus was on calcareous nannoplankton, algae that live at the ocean surface and build chalk-like shells. Fossils of these algae reveal how ocean conditions changed before and during the PETM.

“Certain nannoplankton species prefer to live in warmer waters with less nutrients, while others can only live in colder, higher nutrient waters,” said first author Dr. Heather Jones. “Therefore, major warming events like the PETM really affect which species thrive, and which don’t.”

“This can be observed in the nannofossil record by counting how many of each species there are and how this changes through time.”

Early shift before ocean warming

The study brought a surprise. The biggest ecological change in the region did not occur during the PETM – it happened 200,000 years earlier, during what scientists now call the Pre-PETM Event (PPE).

At that time, carbon isotope levels shifted, and plankton communities reorganized. Warm-water taxa such as Discoaster and Sphenolithus expanded, while cold-water groups such as Chiasmolithus declined.

A specialist genus, Fasciculithus, dropped sharply. Meanwhile, generalists such as Hornibrookina and Zygrhablithus thrived. The ocean had already tipped into a new baseline before the PETM began.

A muted climate crisis

Because this earlier shift had destabilized the system, the PETM itself caused less disruption in the southwest Pacific. Communities had already adapted to warmer, nutrient-poor conditions. That meant the main event looked muted compared to the dramatic responses seen elsewhere.

“Most studies only focus on the PETM event itself and not the longer-term time before it,” said 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 nannoplankton 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.

Role of ocean currents

Regional ocean circulation likely played a role in shaping the PPE. Models suggest that the Proto–East Australia Current may have extended farther south at the time.

Warmer, saltier waters entered the area, stirring turbulence and mixing the upper layers. These changes helped opportunistic plankton spread while driving narrow specialists toward collapse.

This pattern shows how local currents can amplify global warming. Ecosystems do not react uniformly – regional dynamics often determine which organisms survive and which fade.

Warming signals across oceans

The PPE signal was not confined to the Southern Ocean. Similar shifts appear in records from Italy, the South Atlantic, and the Pacific.

In each case, the timing aligned: changes occurred well before the PETM, though the intensity varied. Some regions crossed ecological thresholds earlier, while others did so later.

That unevenness is important. Climate stress rarely strikes evenly across the globe. Certain regions buckle first, while others hold out longer but eventually reach a breaking point. The PPE demonstrates that this patchwork response has long been part of Earth’s climate story.

Ocean warming and species survival

The fate of Fasciculithus highlights the risk of being a specialist. Once common, it declined during the PPE and vanished soon after the PETM.

Scientists describe this as a “dead clades walking” scenario. A group survives an initial shock but cannot adapt over the long term. Generalists, with their broad tolerances, took over.

This history raises concerns for today’s oceans. Species that depend on narrow conditions may be the first to disappear under ongoing warming. Opportunistic organisms, in contrast, often seize the chance to expand.

Warnings for modern seas

The PETM has long been used as a warning for the future, but the discovery of the PPE adds an earlier stage to the story.

It shows that ecosystems can shift dramatically before the main crisis arrives. Smaller, regional changes may quietly set the stage for much larger upheavals.

By looking back, scientists uncover a sobering truth: resilience is not endless. Once thresholds are crossed, recovery may never follow. The oceans tell us that what looks minor now can reshape the future.

The study is published in the journal Communications Earth & Environment.

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