Submarine mountains and long-distance waves stir the deepest parts of the ocean

The ocean’s depths are far from still, with water near the seafloor constantly in motion even in the most remote regions, according to new research. A study published in the journal Ocean Science reveals the dynamic nature of the abyssal Pacific Ocean and its implications for climate, ecosystems, and understanding global ocean circulation.

Investigating the Abyss

The central and eastern Pacific Ocean contain some of Earth’s largest abyssal regions, areas where the sea is more than 3,000 meters deep. These regions feature vast abyssal plains, fracture zones, and seamounts, and are characterized by cold, dark conditions and immense pressure.

Just above the seafloor lies the bottom mixed layer, a region of uniform temperature, salinity, and density stirred by contact with the seabed. This layer can extend from tens to hundreds of meters and plays a crucial role in the movement of heat, nutrients, and sediments.

Observations of the bottom mixed layer are becoming more frequent, though deep ocean measurements remain scarce, expensive, and often decades apart. Scientists have long known that cold Antarctic waters flow northward along topographic features such as the Tonga-Kermadec Ridge and the Izu-Ogasawara and Japan Trenches.

Dynamic Ocean Floor

To investigate the Pacific abyssal ocean, researchers combined new surface-to-seafloor measurements with two decades of existing data on the ocean’s physical features. They examined temperature and pressure down to the seafloor and used machine learning to understand the factors influencing the thickness of the bottom mixed layer.

The study found that the bottom mixed layer varies dramatically, ranging from less than 100 meters to over 700 meters thick. This variability is controlled by seafloor depth and the interaction of waves generated by surface tides with the seabed’s landscape.

The deepest ocean is continually stirred by remote forces and shaped by seafloor features, dynamically connected to the waters above. Just as coastal waters are shaped by waves and currents, the abyssal ocean has its own set of drivers operating over larger distances and longer timescales.

Implications for Climate and Ecosystems

These findings have several important implications. The bottom mixed layer influences how heat is stored and redistributed in the ocean, impacting long-term climate change. Some ocean and climate models currently simplify seabed mixing, potentially leading to inaccuracies in future climate projections.

The layer also plays a role in transporting sediment and supporting seabed ecosystems. As interest grows in deep-sea mining and other activities on the high seas, understanding how the seafloor environment changes and how disturbances might spread becomes increasingly important.

Large areas of the abyssal Pacific remain largely unsampled, even as international agreements like the new UN High Seas Treaty seek to manage and protect these regions. The deep ocean is an active, connected environment, and understanding its dynamics is crucial for informed decision-making regarding the future of the high seas.