The ocean's depths are far from the serene, static realm we often imagine. Beneath the surface, a dynamic world unfolds, where submarine mountains and long-distance waves stir the ocean's deepest parts. Our new research, published in the journal Ocean Science, reveals that water near the seafloor is in constant motion, even in the abyssal plains of the Pacific Ocean. This discovery has significant implications for our understanding of climate, ecosystems, and the ocean as an interconnected system.
The central and eastern Pacific Ocean boasts some of Earth's largest abyssal regions, where the sea plunges more than 3,000 meters deep. Here, the seafloor lies four to six kilometers below the surface, shaped by vast abyssal plains, fracture zones, and seamounts. It's a cold, dark realm, where water and ecosystems endure immense pressure from the ocean above.
Just above the seafloor, regardless of depth, lies the bottom mixed layer. This part of the ocean is relatively uniform in temperature, salinity, and density due to its constant stirring by the seafloor. This layer can extend from tens to hundreds of meters above the seabed, playing a crucial role in the movement of heat, nutrients, and sediments between the pelagic ocean and the seabed. It also marks the beginning of the slow return of water from the ocean's depths towards the surface as part of global ocean circulation.
Observations focused on the bottom mixed layer are rare, but this is beginning to change. Most ocean measurements focus on the upper few kilometers, and deep observations are scarce, expensive, and often decades apart. In the Pacific, especially, scientists have long known that cold Antarctic waters flow northward along topographic features like the Tonga-Kermadec Ridge and the Izu-Ogasawara and Japan Trenches.
However, the finer details of how these waters interact with seafloor features, intermittently stirring and reshaping the bottom layer of the ocean, have remained largely unknown. To investigate the Pacific abyssal ocean, my colleagues and I combined new surface-to-seafloor measurements collected during a trans-Pacific expedition with high-quality repeat data about the physical features of the ocean gathered over the past two decades.
These observations allowed us to examine temperature and pressure all the way down to the seafloor over a wide range of latitudes and longitudes. We then compared multiple scientific methods for identifying the bottom mixed layer and used machine learning techniques to understand what factors best explain the variations in its thickness.
We found that the bottom mixed layer in the abyssal Pacific is not a uniform layer. Instead, it varies dramatically, with some regions less than 100 meters thick and others exceeding 700 meters. This variability is not random; it's controlled by the seafloor depth and the interactions between waves generated by surface tides and rough landscapes on the seabed.
In other words, the deepest ocean is not quietly stagnant as is often imagined. It is continually stirred by remote forces, shaped by seafloor features, and dynamically connected to the rest of the ocean above. Just as coastal waters are shaped by waves, currents, and sediment movement, the abyssal ocean is shaped by its own set of drivers, operating over larger distances and longer timescales.
This matters for several reasons. First, the bottom mixed layer influences how heat is stored and redistributed in the ocean, affecting long-term climate change. Some ocean and climate models still simplify seabed mixing, which can lead to errors in how future climate is projected. Second, it plays a role in transporting sediment and seabed ecosystems. As interest grows in deep-sea mining and other activities on the high seas, understanding how the seafloor environment changes, and importantly how seafloor disturbances might spread, becomes increasingly important.
Our results highlight how little of the deep ocean we actually observe. Large areas of the abyssal Pacific remain effectively unsampled, even as international agreements such as the new UN High Seas Treaty seek to manage and protect these regions. The deep ocean is not a silent, static place; it is active, connected to the oceans above, and changing. If we want to make informed decisions about the future of the high seas, we need to understand what's happening at the very bottom in space and time.
