The seabed is the final destination for everything from sand and mud to organic remains and pollutants. But that doesn’t mean the deep sea is a static grave. As marine biologists have now discovered, tides and currents create surprisingly dynamic and even seasonally changing conditions even at depths of thousands of meters.
At first glance, the deep sea appears to be a pitch-black graveyard. This is the final destination for many things in the sea: from gigantic dead whales to tiny carbon particles. Neither sunlight nor the water movements caused by wind and weather penetrate into these regions, thousands of meters below the water surface – or so it was thought until now. But this apparent isolation and calm is deceptive, because just as the oceans are moved by currents and waves on their surface, the same happens on the seabed – even if we know very little about these underground currents.
Dynamic like the weather
In a project that is unique to date, researchers led by Lewis Bailey from the National Oceanography Centre in Southampton have now monitored the seafloor currents off Mozambique for four years. The team anchored 34 special sensors at depths of up to 2.5 kilometres and used them as acoustic underwater cameras to monitor the flow dynamics. “These are the first measurements of deep-sea currents in such a large area, over such a long period of time and so close to the seafloor,” emphasises co-author Elda Miramontes from the University of Bremen.
Until now, it was assumed that the currents in the deep sea were continuous and uniform, but the new measurement now shows a completely different picture. Bailey and his team observed how the currents accelerated, slowed down and sometimes even changed direction completely. “The seafloor currents off the coast of Mozambique are much more variable than we expected. Just like the currents in the upper ocean, their intensity changes between seasons and can even move back and forth over the course of a few hours,” reports Bailey.
Co-author Ian Kane of the University of Manchester compares these dynamics with the weather in his home country: “Seeing how these currents behave is a bit like observing the weather in Manchester – it is constantly changing and is often surprising.”
New insights into the climate archive
Understanding the behavior of this “deep sea weather” is informative in many ways. For example, ocean floor currents can be used to track the paths of natural and man-made particles. This can then help scientists learn more about the details of the carbon cycle or the spread of human pollutants. At the same time, the accumulation of different particles such as mud, sand and organic matter is needed to reconstruct past climate and ocean conditions. Knowing more about particle movements makes this valuable deep sea archive and its information even more interpretable.
Source: MARUM – Center for Marine Environmental Sciences at the University of Bremen; Article: Nature Geoscience, doi: 10.1038/s41561-024-01494-2