Volcanoes are typically linked to short-term climate disruptions, but new research indicates that their ash may also drive long-term cooling. An analysis of ancient eruptions in the Andes reveals how ash settling into the ocean can trigger biological shifts that ripple through the entire marine food web. The findings were published in the journal Communications Earth & Environment.
These transformations can pull carbon dioxide from the atmosphere and store it deep in the ocean, showcasing a powerful natural process capable of shaping the climate over millions of years.
Ash-laden sedimentary rocks and fossil remains along the margins of the Southern Ocean preserve a tightly interwoven record of volcanic eruptions, algal blooms, and large-scale changes in the marine environment.
By tracing these signals in geological deposits, Mark Clements from the University of Wyoming has directly linked recurring eruptions of the Andean volcanoes to shifts in ocean productivity and atmospheric carbon levels. During the same period, a rise in algal growth and a drop in carbon dioxide levels occurred in tandem with each major phase of volcanic activity. This coincidence points to a sustained oceanic response rather than isolated events, underscoring the need to explain how ash could repeatedly produce such large-scale biological and climatic effects.
Powerful eruptions released ash carrying iron, phosphorus, and silicon into the waters surrounding Antarctica, where even a slight deficiency can limit massive growth. Iron, phosphorus, and silicon are especially vital for diatoms—tiny algae with glass-like shells that account for about one-fifth of the world’s primary production.
Classic experiments have shown that an excess of iron can spark algal blooms in Southern Ocean waters, even when other nutrients are already present. More intense growth on the ocean floor meant more food above it, setting the stage for broader ecological shifts preserved in the fossil record.
As algal blooms expanded, more carbon moved from surface waters into the dark ocean below. Oceanographers refer to this downward transfer as the biological pump—a process that channels surface carbon downward as living organisms sink to the seafloor.
When some of this material reaches deep waters or sediment, less carbon dioxide remains in the air. A single bloom fades quickly, but repeated pulses can continue to accumulate carbon dioxide in a long-term reservoir long after the ash has settled.
Fossil remains of marine mammals show that whale life changed rapidly during the same period as ash emissions. The average length of baleen whales increased from roughly 5 meters to 12 meters as feeding grounds and shorelines shifted. Modern baleen whales transport over 3,700 tons of nitrogen annually between feeding and breeding areas. Likely, larger ancient whales enhanced nutrient cycling and stored carbon in their sinking bodies, though new models did not fully account for these effects.
To test whether this was mere coincidence, researchers recreated ash plumes and the ocean’s response. Most of the simulated ash moved eastward across South America into the South Atlantic, then further toward the southern Indian Ocean.
Some ash also fell near the Pacific coast, delivering a direct supply of nutrients to adjacent waters. The eastward-moving ash-laden currents made the oceanic ring around Antarctica the most suitable zone for repeated fertilization.
When the simulated ash reached the water’s surface, the ocean model reacted sharply and almost instantly. Within the first two years after each nutrient addition, diatom growth in surface waters more than doubled.
Over more than 300 years, four eruptions helped the ocean slightly increase its absorption of carbon dioxide from the air with each recurring cycle. Thus, short-lived eruptions can accumulate their climatic effects over time, rather than vanishing as isolated events.
Longer model runs revealed that the distance between eruption sites mattered nearly as much as eruption size in shaping long-term losses of carbon from the atmosphere. A single nutrient input briefly lowered carbon dioxide levels, after which the ocean gradually returned to its previous state.
When these pulses repeated, the drop in carbon dioxide became more pronounced and lasted much longer, especially when dust and ash accumulated simultaneously. Therefore, repeated surges of nutrient input can have a greater impact on climate over the long term than a single massive release.
Scientists refer to this period as the late Miocene—a geological timeframe spanning from roughly 11.6 to 5.3 million years ago. “Identifying the mechanisms that drove this transition is crucial, particularly for understanding how Earth’s systems may respond to current and future climate changes,” said Clements.
Ice buildup, shifting wind patterns, and reorganization of ocean currents also played a role, so ash likely acted in combination with other forces rather than alone. The paper argues that ash was an underestimated factor, not the sole cause of planetary cooling.
This study does not offer a solution to modern global warming, as today’s surge in carbon dioxide emissions is occurring much more rapidly. “By uncovering connections between volcanic activity, ocean productivity, and carbon dioxide uptake, this research provides insights into mechanisms that can influence global climate over the long term,” said Clements.
Climate is shaped not only by the air—water, food chains, and sediments also play significant roles in its transformation. Taken together, ash, algal blooms, whale migrations, and falling carbon dioxide levels are seen not as separate events but rather as one interconnected episode.
A broader perspective could improve decision-making in climate resilience, natural resources, and risks of rapid change. More accurate data on eruption sizes, ash chemistry, and ancient ocean circulation should clarify how much cooling this chain reaction caused.
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