How thermal plumes affect the Southern Ocean
The Antarctic Circumpolar Current is the strongest ocean current in the world moving 140 million tons of water every second. It flows clockwise around the Antarctic continent distributing dissolved minerals, biological specimens and perhaps most importantly heat. However, as things stand, we don’t know nearly as much as we should about this current especially considering the importance of the Antarctic region to climate science.
One scientist who is working to put that right is Dr Stephanie Downes of the ANU Research School of Earth Sciences. “Given how important the Southern Ocean is to climate models, we’re especially interested to learn as much as we can about ocean currents in the region,” Dr Downes says, “And we’re starting to uncover some very significant things such as hydrothermal plumes that many current climate models don’t take into account.”
Hydrothermal plumes are jets of hot water created by volcanic processes along tectonic plate boundaries, and are found all over the global ocean. Such plumes spew out vast quantities of superheated water laced with many minerals dissolved from rocks deep in the mantle. It’s not unusual for the highly pressurized water from such plumes to be at over 200°C. Once this hits the freezing cold water at the bottom of the ocean it mixes and cools but the quantity of plume water is so large and the temperature so high that it has a very significant heating effect.
“Collectively the deep thermal plumes around Antarctica are contributing heat in the same general ball park as solar radiation mixed down to the ocean floor, so it’s not something we can simply ignore when modelling ocean circulation.”
To study the effects of these plumes scientists like Dr Downes need to know where the water from them goes and what it does. But identifying which water comes from where in an entire ocean is a tricky business. “For these particular plumes originating in the south Pacific, we can look at what’s called stratification to follow the movement of water, that’s the profile of how density changes with depth,” Dr Downes explains, “The plumes create a very specific stratification and the immensely strong current carries that warmer water thousands of miles east to the southern tip of South America, and south to the coast of Antarctica.”
Whilst stratification measures give scientists a good clue to the origins of a particular mass of water, it’s not absolute proof like a DNA sample. To be sure that the stratification measures are really signatures of a plume and not just a freak mixing of other water, scientists turn to those dissolved materials.
“The water from the vents contains lots of minerals like iron, magnesium and even gold,” Dr Downes says, “But that doesn’t give us certainty because after hundreds of years of venting, those elements are quite abundant in the sea at large. But what is rather unique are elevated levels of helium.”
Helium is a common element in the universe but rare on Earth because our planet’s gravity isn’t strong enough to prevent this lightweight molecule from simply drifting off into space. However radioactive decay of heavy elements deep within the Earth create large quantities of helium in the form of alpha particles, which become trapped below ground. Water surging through thermal vents can pick up this helium and bring it to the surface.
“We correlate our stratification estimates with helium data so we can be far more confident that the flows we’re mapping are really warmer water leaving those vents.” Dr Downes says, “And it’s fascinating to see how far this warm water travels because of the strong Antarctic Circumpolar Current.”
“We know from recent studies that climate change is beginning to have an impact on deep ocean temperatures, so the behaviour of ocean currents around Antarctica is a really important thing to study. It’s not just a case of what direct effects a warmer ocean floor has, those temperature changes also influence most of the great circulatory currents on the planet which in turn have a huge effect on local climate, and the global ocean storage of heat and carbon.”