Long Eccentricity Pervasively Modulates Abyssal Hill Formation at Mid-Ocean Ridges

Deep beneath the waves, where the ocean floor stretches into eternal darkness, lies a landscape of rolling hills and deep valleys. These are the abyssal hills, vast underwater mountains that form along the seams where Earth's tectonic plates pull apart. For decades, scientists have known that the rhythm of our planet's climate, driven by the wobble and tilt of Earth's orbit, leaves a mark on these underwater mountains. When ice sheets grow and shrink, sea levels rise and fall, changing the weight pressing down on the ocean floor. This pressure change was thought to act like a valve, opening and closing the flow of molten rock from deep within the Earth to the surface, creating a pattern in the hills that matches the cycles of climate change.

However, this explanation has always felt incomplete. The patterns seen in the hills did not match all the known cycles of Earth's orbit, and the connection seemed to break down when looking at the oldest parts of the ocean floor. A specific, long cycle of Earth's orbit, which repeats roughly every 400,000 years, was missing from the record. This gap left researchers wondering if there was a different, more direct force at play, one that could reach deep into the Earth's crust without needing the atmosphere or the oceans to carry the signal.

A new study by Chunhui Tao and his colleagues has finally filled in this missing piece of the puzzle. By examining the shapes of the ocean floor across 22 different regions around the world, from the fastest-spreading ridges to the slowest, the team discovered that these 400,000-year cycles are everywhere. They found that the hills are not just reacting to the weight of the water above them, but are being directly nudged by the gravitational pull of the Sun itself.

To find this hidden rhythm, the researchers gathered a massive amount of data. They looked at 122 detailed maps of the ocean floor, combining measurements of the water depth with magnetic records hidden in the rocks. The magnetic data was crucial because it acts like a timeline. As new rock forms at the mid-ocean ridges, it captures the direction of Earth's magnetic field at that moment. By identifying specific, narrow bands of rock with known magnetic histories, the team could determine exactly how old the ocean floor was in each spot. This allowed them to look much further back in time than previous studies, which were often limited to the last million years. They focused on a period called the Matuyama Chron, which spans from about 780,000 years ago to the end of the period, to see if the long 400,000-year cycle was present in the topography.

The results were striking. When the team analyzed the height and spacing of the abyssal hills, a clear peak appeared at the 400,000-year mark across almost every location they studied. This signal was strongest on the slowest-spreading ridges, where the hills are larger and more widely spaced, but it was also present on the faster ridges. This finding ruled out the idea that sea-level changes were the primary driver. While sea levels do change with the shorter 100,000-year cycle, the 400,000-year cycle does not show up clearly in the history of sea levels during the time period they studied. If the hills were reacting only to the weight of the water, the 400,000-year pattern should have been absent, but it was not.

Instead, the authors propose a more direct mechanism involving gravity. Earth's orbit is not a perfect circle; it stretches and shrinks in a cycle that takes about 400,000 years. When the orbit is more stretched out, Earth gets closer to the Sun at its nearest point. This change in distance alters the strength of the Sun's gravitational pull on our planet. The researchers calculated that this variation in solar gravity is actually much stronger than the gravitational tug of the Moon, which is known to cause tides in the oceans and even in the solid Earth.

The team suggests that this stronger solar pull acts as a trigger for the magma chambers deep beneath the ocean floor. Just as the Moon's gravity can pull on the solid Earth to create tiny tides, the Sun's gravity, when it is at its strongest during the peak of the 400,000-year cycle, gives the molten rock inside the Earth a stronger nudge. This nudge makes it easier for the magma to break through the crust and erupt, creating new seafloor. When the orbit is less stretched, the pull is weaker, and the eruptions happen less frequently. This rhythmic triggering creates the pattern of hills and valleys we see today.

This discovery changes how we understand the connection between the sky and the deep Earth. It shows that the rhythm of the solar system can directly control the formation of the ocean floor, independent of the climate or the ice ages. The 400,000-year cycle acts as a global pacemaker, a steady beat that drives the creation of new crust at the bottom of the world's oceans. While the exact details of how this gravitational trigger works inside the Earth's mantle still need to be explored, the evidence is now clear: the hills of the deep ocean are keeping time with the long, slow dance of Earth's orbit around the Sun.