New geological evidence reveals that Earth’s climate may have been stabilized for tens of millions of years by a hidden feedback loop linking sea level, ocean oxygen, phosphate, and carbon burial, according to a study published in the Proceedings of the National Academy of Sciences. Researchers co-authored by Zunli Lu, professor of Earth and environmental sciences in the University’s College of Arts and Sciences, found that a narrow range of ocean conditions maximized carbon burial, helping regulate atmospheric carbon dioxide and keeping planetary temperatures suitable for life.
How Sea Level and Phosphate Control Earth’s Climate
At the center of the planetary thermostat is phosphate, a form of phosphorus required by marine organisms to grow. According to the study, sea level changes directly controlled how much of this nutrient reached the open ocean. When sea levels were high, wide continental shelves captured phosphate in shallow seafloor sediments before it could reach deeper waters. Marine productivity fell, organic carbon burial declined, and more carbon dioxide accumulated in the atmosphere, keeping the planet warm. Falling sea levels produced the opposite effect by contracting continental shelves and washing more phosphate into the water.
“Our co-author, Christian Bjerrum, studied the connection among sea level, ocean oxygen and phosphate with a computer model two decades ago,” Lu said regarding the collaborative effort to piece together the necessary geologic records.
The Role of Oxygen Minimum Zones in Carbon Burial
When increased phosphate stimulated marine growth, the sinking and decomposing organic material depleted dissolved oxygen in the water. According to the research team, these expanding zones of oxygen-poor water triggered a reinforcing cycle when they reached organic-rich shelf sediments. Low oxygen conditions released additional phosphate from the seafloor, fueling further marine growth and burying more organic carbon beneath the ocean floor.
Did you know? Researchers used an iodine-to-calcium proxy measured from foraminifera—microscopic marine fossils—to estimate ancient ocean oxygen levels using a mass spectrometer at Syracuse University.
The Eocene Epoch and High Sea Level Extremes
The Eocene epoch, spanning approximately 56 to 34 million years ago, demonstrates what happens when this carbon burial feedback breaks down. According to the geological data, extreme high sea levels during this period flooded continental shelves and trapped phosphate in shallow waters. The open ocean remained oxygen-rich, shutting down the organic carbon burial mechanism. Without marine sediments locking away carbon, atmospheric carbon dioxide surged and Earth remained warm.
Narrow Sea Level Sweet Spot Stabilized Carbon Sinks
The strongest carbon burial occurred within a specific window when sea levels stood about 10 to 40 meters above modern heights. Within this sweet spot, oxygen minimum zones aligned with shelf sediments to sustain the carbon sink for millions of years. Over geological time, this favorable window narrowed as oxygen minimum zones shifted into deeper waters, stabilizing atmospheric oxygen and carbon dioxide levels and increasing the resilience of Earth’s climate system.
Frequently Asked Questions
What drove Earth’s long-term climate stability?
According to the published study, a hidden feedback loop involving sea level fluctuations, marine phosphate availability, ocean oxygen levels, and carbon burial regulated atmospheric carbon dioxide for millions of years.
How do scientists measure ancient ocean oxygen?
Researchers in Zunli Lu’s laboratory at Syracuse University measured iodine-to-calcium ratios preserved in the chemistry of foraminifera, which are microscopic marine organisms found in seafloor sediments.
What happened during the Eocene epoch?
Sea levels were exceptionally high, trapping phosphate in shallow continental shelves, keeping open oceans oxygenated, and halting organic carbon burial, which led to high atmospheric carbon dioxide and warmth.
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