The deep ocean surrounding Antarctica, known as the Southern Ocean, is a remote region that acts as the primary engine for the planet’s deep-sea circulation. By connecting the world’s major ocean basins and facilitating the exchange between surface and deep layers, the Southern Ocean controls the global distribution of heat, carbon, and nutrients. The abyssal plains are supplied by the coldest, densest water masses on Earth, which ventilate the deep sea. The stability of this region is a major influence on the planet’s climate and ocean health, and the changes now being observed demonstrate a fundamental shift in this global system.
The Slowdown of Antarctic Bottom Water Formation
The deep-ocean circulation is powered by the formation of Antarctic Bottom Water (AABW), the densest water in the world’s oceans. AABW is created through brine rejection: as sea ice forms in coastal areas, salt is expelled, increasing the salinity and density of the remaining supercooled water. This dense water then sinks to depths below 4,000 meters.
This sinking process ventilates the abyssal plains, supplying approximately 40% of the world’s deep ocean with oxygen and nutrients. However, AABW formation is now slowing measurably due to an influx of freshwater from melting ice shelves. Freshwater reduces the overall salinity and density of the surface water, inhibiting the deep sinking process that creates AABW.
Observations indicate that the volume of AABW flowing northward has decreased significantly, with studies showing reductions ranging from 12% to nearly 30% in certain regions since the 1990s. This reduction is projected to continue, potentially leading to a near-complete shutdown of AABW formation within 50 years under high meltwater scenarios. The slowdown has large-scale implications for the Atlantic Meridional Overturning Circulation (AMOC), which distributes heat and carbon globally.
Warming and Deoxygenation of the Deep Zones
The slowing of the AABW-driven circulation has direct consequences for the temperature and dissolved gas content of the deep ocean. The abyssal waters of the Southern Ocean are now warming. This warming is driven partly by a feedback loop: the reduction in cold, dense AABW allows warmer intermediate waters to penetrate deeper and closer to the Antarctic continental shelf.
The intrusion of these warmer waters increases the basal melting of ice shelves, which releases more freshwater and further accelerates the AABW slowdown. The reduction in AABW formation also means less oxygen-rich surface water is delivered to the abyssal depths. Because cold water holds more dissolved oxygen than warmer water, this loss of ventilation significantly contributes to deep ocean deoxygenation over time.
The Southern Ocean below 1,200 meters has shown a measurable decrease in oxygen, accounting for more than 10% of the global ocean’s oxygen loss. This deoxygenation results from water masses becoming “older,” meaning they spend more time isolated from the surface. This allows oxygen to be consumed by deep-sea organisms without being replenished, creating a cycle that intensifies surface warming.
The Intensification of Deep Ocean Acidification
The Southern Ocean is a major global carbon sink, absorbing a large amount of atmospheric carbon dioxide (\(\text{CO}_2\)). Cold water absorbs \(\text{CO}_2\) more efficiently than warmer water, which drives this high uptake. When \(\text{CO}_2\) dissolves into seawater, it forms carbonic acid, lowering the \(\text{pH}\) and causing ocean acidification.
This process is intensified in the Antarctic region because the sinking deep water carries this \(\text{CO}_2\)-rich, acidic surface water directly into the abyssal depths. As \(\text{pH}\) decreases, the concentration of carbonate ions—the building blocks for shells and skeletons—also falls.
This chemical change leads to the shoaling of the aragonite saturation horizon. This horizon marks the depth below which water is corrosive to aragonite, a form of calcium carbonate used by many marine organisms. In the Southern Ocean, this corrosive water is rising closer to the surface, reaching vulnerable deep-sea habitats earlier and more severely than in most other parts of the world.
Impacts on Unique Deep-Sea Ecosystems
The combined effects of circulation slowdown, warming, deoxygenation, and acidification threaten the unique ecosystems of the Antarctic deep sea. These biological communities are highly endemic, meaning the species exist nowhere else, and are adapted to stable, cold conditions. The slow growth rates and long lifespans of Antarctic deep-sea fauna make them sensitive to rapid environmental shifts.
Acidification directly threatens calcifying organisms, which rely on calcium carbonate to build their shells and skeletons. The rising aragonite saturation horizon exposes these organisms to water that makes shell formation difficult or causes existing structures to dissolve. Changes in oxygen levels and temperature also disrupt physiological processes, including reproduction and metabolism, across the food web.
These changes risk biodiversity loss and a shift in the entire ecosystem structure. The disruption of deep-sea currents affects the transport of organic matter and nutrients, altering the fundamental resource base supporting life in the abyssal zone. The interconnected nature of these physical and chemical shifts suggests a cascading impact that could fundamentally alter this globally important biome.