Thermohaline circulation
The thermohaline circulation is a constantly moving system of deep-ocean circulation that is driven by temperature and salinity (thermo = temperature, haline = halinity = salinity of the ocean). It is also known as the global ocean conveyor. It moves heat, oxygen and nutrients around the globe.
The basic idea of the thermohaline circulation is that cold, salty water is dense and sinks to the bottom of the ocean whilst warm, less salty water is less dense and rises to the surface.
The conveyor gets its start in the Norwegian Sea (at the top of the North Atlantic Ocean). A high rate of evaporation removes heat from the water (in the process heating the atmosphere in the cold northern latitudes) and makes the water more saline (because water is evaporated but the salt is left behind). The water is now cooler and more saline. Both of these factors make the water more dense, causing it to sink to the bottom of the ocean. This sinking water is the largest waterfall on the planet.
The cold, salty water moves south, all the way down to Antarctica. Here the current turns eastward, flowing around the continent. The circular current splits into two, one part moving north into the Indian Ocean and the other moving north into the Pacific Ocean.
Eventually, the salinity gets diluted and the cold bottom waters are able to warm and rise to the surface. The warm surface current from the Pacific Ocean passes above Australia (through the Indonesian archipelago) and meets up with the warm surface current in the Indian Ocean. This warm surface water makes its way back up through the Atlantic and the cycle starts over. It takes about 1,000 years for the thermohaline circulation to complete one cycle.
So that’s the thermohaline circulation, or global ocean conveyor. But, is it really all that important?
In a nutshell, yes. Absolutely yes. Like, YES!!!
Let’s take a look at what has happened in the past when the thermohaline circulation has been slowed or stopped.
Firstly, the Younger Dryas period, which occurred at the end of the last ice age. This period lasted for 1,300 years from 12,800 years ago until 11,500 years ago. It was caused by glacial melt water from North America when an ice dam broke and allowed fresh water to flow into the North Atlantic Ocean. This diluted the salinity of the North Atlantic which either slowed or temporarily stopped the thermohaline circulation. The result was that heat stopped being moved around the globe and the northern latitudes went into another mini ice age - the Younger Dryas period.
Secondly, and this is the big one, the biggest mass extinction the Earth has yet seen. It is commonly referred to as the Great Dying, or more correctly the Permian-Triassic (P-Tr) Extinction. Let’s break down what happened.
At the end of the Permian time period, 252 million years ago, a large mantle plume started erupting in what was then northern Pangaea, and is now Siberia. Many large and continuous eruptions occurred over a period of up to a million years. This formed the Siberian Traps, an area of 2.5 million square kilometres of flood basalt with a volume of 3 million cubic kilometres.
Throughout this period of eruptions, large amounts of carbon dioxide and other material were released into the Earth’s atmosphere, leading to a drastic warming of the planet. This warming of the Earth meant there was no temperature differential to give rise to the thermohaline circulation. So, it stopped.
Remember, the thermohaline circulation moves oxygen down into the depths of the ocean. When it stopped, the oxygen already present in the ocean got used up, and the ocean became anoxic (devoid of oxygen). This resulted in the death of 97 % of marine life.
How did this affect terrestrial (land) animals?
The ocean was anoxic, which suited anaerobic bacteria just fine. They don’t want oxygen, and they thrived in the oxygen-free ocean. These bacteria released hydrogen sulfide gas, which turned the ocean pink as it bubbled up out of the ocean and into the atmosphere. The thing is, hydrogen sulfide gas is A) heavier than air, and B) toxic. So a poisonous gas hung around at low level, which resulted in the death of 70 % of terrestrial animals.
All told, about 90 % of life on Earth died when the thermohaline circulation stopped. The biggest mass extinction of them all.
So… Is this constantly moving system of deep-ocean circulation really all that important? A resounding YES.