Slow Motion, Big Impact: How a Slowdown in Seafloor Spreading Reshaped Sea Levels
A new study uncovers how pre-ice age tectonic shifts influenced the planet’s water levels and climate
Breaking the Ice:
Seafloor spreading—the process by which new oceanic crust forms at mid-ocean ridges—slowed down significantly during the Miocene epoch period, and scientists are now uncovering its effects. A recent study published in Geochemistry, Geophysics, Geosystems by Colleen Dalton and colleagues investigated the historical impact of a 35% reduction in ocean crust production between 15 and 6 million years ago. Their findings suggest that this slowdown led to a sea-level drop of 26 to 32 meters by altering the balance of young, shallow seafloor and older, deeper seafloor. Moreover, they propose that this tectonic shift could have contributed to subsequent global cooling by reducing volcanic carbon dioxide emissions—potentially setting the stage for increased ice sheet formation and further sea-level decline.
Quick Melt:
The study’s findings carry major implications for our understanding of historical sea-level changes. In addition to ice melt and thermal expansion, the study highlights another significant factor in past climate variability: the shifting volume of ocean basins themselves. A slowdown in seafloor spreading reduces the amount of young, buoyant crust, increasing the proportion of older, denser seafloor that sits deeper in the ocean. This has the effect of lowering global sea levels by deepening the ocean basins.
Beyond sea levels, the study also examines mantle heat loss. As spreading slows, less heat escapes from the Earth's interior, which could influence broader climatic trends. If this tectonic shift reduced the emission of greenhouse gases from mid-ocean ridges, it may have contributed to pre-ice age global cooling. The researchers estimate that such climate-driven effects could have caused an additional sea-level drop of over 60 meters, as cooling ocean waters contract and expanding ice sheets lock away more water.
The Thaw:
How Does Slower Seafloor Spreading Lower Sea Levels? AccumulationZone Explains.
Seafloor spreading is a fundamental mechanism of plate tectonics, where new oceanic crust forms at mid-ocean ridges and gradually moves outward, driven by convection currents in the Earth's mantle. As magma rises to the surface, it cools and solidifies, creating fresh crust that pushes older crust further away from the ridge. Over time, this older crust cools, contracts, and becomes denser, eventually subducting back into the mantle at convergent boundaries. This continuous cycle regulates the structure of the ocean floor and over time influences global sea levels by controlling the balance between buoyant, young crust and denser, older crust. The rate of seafloor spreading directly impacts ocean basin volume, which in turn affects long-term changes in sea level and climate.
When spreading rates are high, the newly formed oceanic crust is hot and buoyant, leading to elevated seafloor topography. The result is a shallower ocean basin, displacing water and raising global sea levels. Conversely, when spreading slows, the crust cools and becomes denser over time, sinking deeper into the mantle. This causes a relative increase in ocean basin volume, effectively lowering global sea levels.
Beyond just seafloor topography, spreading rates can also influence the Earth’s climate through volcanic degassing. As magma rises and solidifies at mid-ocean ridges, it releases significant amounts of carbon dioxide, which contributes to atmospheric greenhouse gas levels. The rate of seafloor spreading thus directly affects the volume of these emissions, with faster spreading periods typically correlating with higher atmospheric CO2 concentrations and warmer global climates, while slower spreading leads to reduced emissions and potential cooling trends. Mid-ocean ridges are significant sources of carbon dioxide, as mantle-derived gases are released during magma upwelling. A reduction in spreading rates means less volcanic activity at ridges, leading to a decrease in atmospheric CO2 over geologic time scales. This decline in greenhouse gases can contribute to long-term global cooling, promoting ice sheet expansion and further sea-level decline.
Final Thoughts
While these historical processes unfolded over millions of years, they offer crucial insights into Earth's climate sensitivity to internal geodynamic forces. By studying past variations in tectonic activity, scientists can gain further insight into long-term sea-level dynamics.