When Tianshu Kong examined fossilized foraminifera shells from the deep Pacific, she spotted two sharp peaks in nitrogen isotopes—each one aligning with the end of an ice age over the past 200,000 years. Those tiny chemical signatures, preserved in seafloor mud, are now rewriting our understanding of how the ocean breathes. A team led by Boston College’s Xingchen "Tony" Wang has uncovered evidence that falling sea levels during ice ages may have triggered increased volcanic activity along the East Pacific Rise, releasing iron-rich fluids from hydrothermal vents that traveled upward to fertilize plankton in the sunlit surface waters. This natural iron boost, the researchers report in Nature Geoscience, likely fueled phytoplankton blooms in a region where iron—not nitrogen or phosphorus—limits growth.
The eastern equatorial Pacific is one of Earth’s largest iron-limited zones, where microscopic phytoplankton, despite abundant sunlight and major nutrients, struggle to thrive without a steady iron supply. For decades, scientists believed windblown dust was the primary source. But this new research suggests a deeper, more dynamic origin: the seafloor itself. As ice sheets expanded and sea levels dropped, the reduced pressure on mid-ocean ridges may have enhanced volcanic activity and hydrothermal circulation, pumping more iron into the deep ocean. Over time, that iron rose and spread, eventually reaching surface waters where it sparked bursts of biological productivity.
By analyzing sediment cores spanning 200,000 years, the team found that spikes in phytoplankton nutrient use—recorded in nitrogen isotopes from foraminifera shells—closely matched periods of heightened hydrothermal iron release. These peaks occurred during the last two deglaciations, around 130,000 and 20,000 years ago. The researchers ruled out alternative explanations like dust input or changes in ocean circulation, strengthening the case for a volcanic-iron connection. The study, which involved collaborators from Boston University, University of Massachusetts Boston, National Taiwan University, and Princeton University, reveals a previously hidden feedback loop: sea level drops → increased seafloor volcanism → iron fertilization → plankton growth → enhanced carbon drawdown from the atmosphere.
This mechanism could help explain why atmospheric CO₂ levels consistently dropped during ice ages and rose during warm transitions. As phytoplankton bloomed, they absorbed carbon dioxide through photosynthesis, and when they died, much of that carbon sank to the deep ocean, effectively storing it away. While the study does not advocate for artificial ocean fertilization, it highlights how Earth’s natural systems are deeply interconnected—linking tectonic forces thousands of meters below the surface to climate patterns above.
As climate scientists search for ways to safely remove carbon from the atmosphere, nature may already have offered a blueprint. The ocean’s pulse, it turns out, beats in time with the planet’s volcanic heart.