Oxygen levels in the depths of the Antarctic Ocean are declining at a much faster pace than predicted, a new study by Australian scientists shows.
As the Antarctic Ice Sheet melts under a warming climate, the bad news is that the sinking of tons of salty, oxygen-rich water has slowed. As surface waters are rendered less salty due to melting glaciers, they grow more buoyant. This has dramatically affected a vitally important process that replenishes the deep ocean with oxygen, findings, published in Nature Climate Change, indicate.
When dense water containing oxygen sinks, it becomes what is termed by scientists as Antarctic Bottom Water (AABW). The process can be described as "pumping oxygen" into the Pacific, Atlantic, and Indian Oceans, the team from the Commonwealth Scientific and Industrial Research Organisation (CSIRO) explained.
Dr. Kathy Gunn, who led the research, along with her team, utilized a newly-developed method of measuring the deep circulation in the Antarctic Ocean, also called the Southern Ocean. They combined observational data with model simulations. Besides observations made by vessels, also taken into consideration were measurements of water speed, temperature, and salinity from instruments attached to wires anchored to the seabed. These tools would be left in place for up to two years, said the study.
As the researchers calculated the AABW’s volume and oxygen transport, they zeroed in on the dramatic changes.
“Our observations show the deep ocean circulation around Antarctica has slowed overall by about 30 per cent since the 1990s. This slowdown locks in decades of impacts," Dr. Kathy Gunn stated.
While previous modelling predicted that the AABW circulation could slow by more than 40 percent by 2050, now, new findings indicate that the projected slowdown has arrived sooner than anticipated, explained co-author of the study, Professor Matthew England, deputy director of the Australian Center for Excellence in Antarctic Science.
One of the important consequences of this process is that slowing circulation makes the bottom of oceans stagnate.
"This would trap nutrients in the deep ocean, reducing the nutrients available to support marine life near the ocean surface," Matthew England added.
Another co-author, Dr. Steve Rintoul, explained that the outcome of the research underscores more concerns than the typical ones linked to a rise in sea level due to melting of the Antarctic Ice Sheet. The registered overall decline in oxygen levels is changing the deep ocean’s structure and chemistry, the research team warned, suggesting that this could have a far-reaching impact in the coming years.