A positive feedback loop may have driven carbon sequestration within the deep Pacific and shaped a cooling climate over millions of years.
Over the past 4.5 million years, the Earth’s climate cooled, tracing a path from the warmth of the Pliocene to the recurring ice ages of the Pleistocene. New research shows that greater amounts of carbon accumulated in the depths of the Pacific Ocean, likely driving further cooling.
Before the Pleistocene epoch began, the Pliocene was a warmer period in geological time. Some 4.5 Ma, temperatures were between 0.5°C to 6.0°C warmer than today and ice sheets much smaller, even as atmospheric levels of carbon dioxide matched present-day values. These temperatures are akin to those being predicted as the climate warms today from disruptive human activities.
Over the past few decades, researchers have found strong support for the role of the ocean as a carbon reservoir, trapping carbon dioxide that would otherwise drive warming in the atmosphere. The cold-warm climate cycles of the Quaternary period, which include the last 2.6 million years, have particularly been linked to the drawing of carbon dioxide from the atmosphere into the ocean. But whether similar processes influenced the shift from a warm Pliocene to a cyclically cooler and warmer Pleistocene is uncertain.
“Because the deep ocean is the largest active reservoir of carbon on Earth—acting like a giant ‘carbon sponge’—we wanted to understand if changes in how the ocean stored carbon helped drive these long-term global cooling trends by pulling carbon dioxide (CO2) out of the atmosphere,” writes Joseph Novak, a researcher at the University of California, Santa Cruz, in an email to Advanced Science News.
To see how carbon storage in the deep sea may have taken place millions of years ago, Novak and colleagues used a proxy captured in the shells of fossilized creatures that once lived on sea beds. They examined the chemical composition of microscopic fossils of a single-celled organism, a member of benthic foraminifera from the genus Cibicidoides. These organisms use carbon derived from sinking organic matter in the seas, acting as a record of ocean chemistry going back millennia.
Typically, the deep waters of the Pacific originate in the North Atlantic. “As this water travels, dead marine life sinks into it from above and decays, adding carbon to the water,” adds Novak. “By comparing the carbon isotope signatures between fossils in the Atlantic and the Pacific, we can measure how ‘sluggish’ the ocean circulation was and how much extra carbon accumulated in the deep Pacific over time.” If more carbon was stored in the deep seas, there would be a larger difference in the carbon levels of the two oceans. To measure this difference, the researchers looked at datasets of carbon isotope (δ¹³C) values of the shells of Cibicidoides collected at Ocean Drilling Program (ODP) sites in the North Atlantic and Pacific Oceans.
Despite similar patterns of ocean circulation, the deep North Pacific Ocean of the late Pleistocene evidently stored more carbon than that of the late Pliocene. During the Pliocene, more carbon may have stayed in the atmosphere, warming it, rather than being stored in the ocean.
While carbon isotopes are a valid measure of carbon stored in the deep ocean, the researchers wanted to further corroborate their findings and looked to a different proxy. As organic matter settles deeper into the ocean and decays, it consumes oxygen and releases carbon dioxide. Trace metals, such as uranium and manganese, respond to changes in dissolved oxygen and are preserved in marine sediments. Because lower oxygen levels generally indicate that more organic matter has decomposed, and therefore that more respired carbon has accumulated in the deep ocean, the concentrations of these trace metals provide additional evidence for how much carbon was stored over time.
Novak’s team rounded off their findings by analyzing shell chemistry to understand seawater acidity levels. Carbonate ion concentrations in the deep Pacific dropped between 4 and 2 million years ago, another indicator of rising carbon levels in the ocean.
Through multiple independent lines of investigation, the researchers found strong support that carbon sequestration in the deep Pacific steadily increased and was essential to cooling the Earth’s climate over the last 4.5 million years. When the continental ice sheets were at their largest, particularly during the late Quaternary, so too were the vast stores of carbon in the ocean. Conversely, much less carbon was stored in the ocean during the warmer Pliocene period when continental ice volume was low and ocean waters balmier.
The researchers suggest that this strong coupling between carbon storage and a cooling climate likely acted as a positive feedback loop, amplifying cooling as well as ice volume.
They propose that cooling may have shifted westerly winds from the Southern Hemisphere toward the equator over time, allowing less carbon dioxide to re-enter the atmosphere from the ocean, keeping more of it tied down. The ice-age cycles of the late Pleistocene may have come about in much the same manner, according to previous research. “Essentially, a slow but massive reorganization of ocean currents and biology progressively locked more CO2 away in the deep ocean, which reinforced the global cooling trend,” adds Novak.
While the ocean’s capacity to store carbon helped shape Earth’s climate over millions of years, that capacity was also shaped by the climate in return. “If human-caused warming significantly alters ocean circulation in the future, the ocean could become less efficient at holding onto this carbon,” writes Novak. “If some of that stored carbon leaks back into the atmosphere, it could accelerate future warming.”
For Novak, studying geology isn’t simply about understanding what happened eons ago. “The Earth’s past is the best laboratory we have for understanding its future,” Novak adds. “It provides the vital context we need to predict how our planet will respond to the rapid changes happening today.”
Reference: Joseph B. Novak et al., Evidence for Long-Term Coupling of Deep Ocean Carbon Storage and Global Climate Over the Past 4.5 Million Years, Paleoceonography and Paleoclimatology (2026). DOI: https://doi.org/10.1029/2025PA005248
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