8 Million Years of Climate History Found Beneath the South-East Pacific

An extraordinary archive of ocean mud collected near the Drake Passage has opened a window onto 8 million years of climate history, revealing how shifts in the Antarctic Circumpolar Current (ACC) and sea surface temperatures reverberated through the global carbon cycle. The results help explain past swings in atmospheric CO2 and offer critical clues for how today’s warming could reshape ocean circulation.

A climate time machine beneath the waves

The Drake Passage is the narrowest gateway for the ACC, the planet’s most powerful current, which rings Antarctica and links the Pacific, Atlantic, and Indian oceans. Because the ACC controls the movement of heat, salt, nutrients, and dissolved carbon between ocean basins, even subtle changes in its strength can have outsized effects on climate.

From 3,800 meters below the surface in the South-East Pacific, researchers recovered a 380-meter-long sediment core—layer upon layer of tiny particles that settled to the seafloor over millions of years. Each layer holds chemical fingerprints of the ocean conditions at the time it formed, turning the core into a high-resolution climate ledger.

Reading ancient temperatures from microscopic molecules

To reconstruct past sea surface temperatures, the team used alkenone paleothermometry. Alkenones are waxy compounds produced by certain single-celled algae; their molecular structure shifts with water temperature. By analyzing these molecules in 300 samples spaced along the core, scientists built a continuous temperature record with an average spacing of about 25,000 years per data point—dense enough to detect long cycles and regional anomalies.

When cold seas sped up the Southern Ocean

The most unexpected signal emerged between roughly 5.3 and 2.2 million years ago: during prolonged cool phases lasting around 400,000 years, the ACC appears to have intensified. That pattern contrasts with behavior inferred for the most recent million years, when the current tended to strengthen during briefer warm intervals of roughly 10,000 years. The finding underscores that the coupling between the ocean and atmosphere isn’t fixed; it can flip depending on the timescale and background climate state.

A warm pulse in a cooling world

Another surprise came around 2.7 million years ago, a period often flagged as the onset of major Northern Hemisphere glaciations. While the global picture is commonly framed as a general cooling, the South-East Pacific bucks the trend: the record shows a remarkable ~5°C warming that persisted for roughly 700,000 years. This regional heat pulse likely weakened the ACC’s barrier effect, allowing carbon-rich deep waters to become more isolated and accumulate CO2. Eventually, that stored CO2 ventilated to the atmosphere, followed by a gradual drawdown—changes that may have reconfigured deep Atlantic circulation and contributed to the buildup of ice sheets in the north.

Why the ACC matters for the planet’s thermostat

The ACC acts like a planetary flywheel for climate. A stronger current can restrict the exchange between surface and deep waters, locking away carbon and altering how heat is moved around the globe. A weaker current can do the opposite, releasing stored carbon and reshuffling ocean heat. The new record captures both modes in action, showing that the Southern Ocean’s role in climate hinges on how winds, currents, and temperature interact over different timeframes.

Implications for today’s warming

These ancient shifts provide a stress test for climate models. They suggest that regional warming in the Southern Ocean can trigger far-reaching consequences, from atmospheric CO2 swings to changes in the Atlantic’s deep overturning circulation. At the same time, the record reveals that the system’s response depends on slow rhythms—hundreds of thousands of years—as well as shorter pulses, reminding us that modern changes will play out on multiple, overlapping timescales.

As scientists extend these reconstructions with finer resolution and additional chemical tracers, they aim to pin down how wind patterns, sea ice, and ocean mixing co-evolve. The goal is not only to match the past but to sharpen projections of the ACC’s future under continued greenhouse warming—an outcome tied to how much carbon the oceans can absorb and how heat is redistributed across hemispheres.

The seafloor’s enduring message

From molecules preserved in ancient mud, a coherent story emerges: the Southern Ocean is a crucial lever in Earth’s climate system. Over millions of years it has alternately locked away and released carbon, circulated heat differently between basins, and nudged the planet toward or away from ice. Understanding those levers is essential for navigating the century ahead, when the same ocean corridors may again decide how quickly the climate changes—and how far it goes.