How the Arctic Ocean became salty
Tens of millions of years ago, the Arctic Ocean was a huge freshwater lake. A land bridge separated it from the salty Atlantic Ocean. Then, around 35 million years ago, that bridge began sinking. Eventually, it fell enough that the Atlantic’s salty seawater could seep into the lake. But it hadn’t been clear precisely how and when that top-of-the-world lake became an ocean. Until now.
A new analysis describes the conditions that allowed the Atlantic’s water to overwhelm that Arctic lake, creating the world’s northernmost ocean. Its cold, south-flowing water now exchanges with warmer, north-flowing water from the Atlantic. Today, that’s what powers the Atlantic Ocean’s climate-driving currents.
Things were much different 60 million years ago. Back then, a strip of land stretched between Greenland and Scotland. This Greenland-Scotland Ridge formed a barrier that kept the salty water of the Atlantic out of the fresher water of the Arctic, explains Gregor Knorr. Knorr is a climate scientist at the Alfred Wegener Institute in Bremerhaven, Germany. He worked on the new study, published June 5 in Nature Communications.
At some point, the ridge sank far enough to let the two bodies of water mix. To find out when that was, Knorr and his Alfred Wegener colleagues ran computer models. Like time machines, these computer programs recreate or predict complex scenarios based on various conditions. Models can compress changes that took millions of years into just weeks. Earth scientists then compare them like time-lapse camera images.
To make the models as accurate as possible, Knorr’s team plugged in several factors. These included a range of carbon dioxide (CO2) levels typical of what would have been in the atmosphere at important times in the past. Those CO2 values ranged from 278 parts per million (ppm) — similar to values to just before the Industrial Revolution (when humans started adding lots of CO2 to the air) — to 840 ppm. That high is what would have existed in parts of the Eocene Epoch, 56 million to 33 million years ago.
The link between CO2 and salinity is a powerful one, explains Knorr. The more CO2 in the atmosphere, the warmer the climate. The warmer the climate, the more ice melts. And the more ice melts, the more freshwater pours into the Arctic Ocean. That, in turn, lowers its saltiness.
The team set out to simulate the period of time from 35 million years ago to 16 million years ago. First, they divided that time period into increments of 2,000 to 4,000 years. Then they let their model recreate all of those smaller time periods at once, Knorr says. They couldn’t do that with the whole 19-million-year period because it took a supercomputer running continuously for as long as four months just to run the smaller models.
Just add salt
The result that emerged from these models was crystal clear. Around 35 million years ago, Arctic water was still as fresh as a spring pond. That was true even though the ridge was already 30 meters (98 feet) underwater.
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But within the next million years or so, the ridge sunk to 50 meters (164 feet) below the surface. That’s when things really started to change. And here’s why. Freshwater is less dense than salt water. So it will floats on any denser, saltier water below it. The line between this layer of fresh and salty water is known as a halocline.
With all the freshwater being added to the Arctic from melting ice around 35 million years ago, the halocline was especially abrupt. And it happened to be some 50 meters (about 160 feet) deep.
So the salt water didn’t pour north until the Greenland-Scotland Ridge sank below that halocline. Only when that happened could the dense salt water of the Atlantic Ocean finally sweep into the Arctic.
That “simple effect” — warmer salt water pouring north and cold fresh water spreading south — forever changed the Arctic and Atlantic oceans. Along with adding salt water and heat to the Arctic, it also helped trigger the major Atlantic Ocean currents that exist today. Those currents arise from differences in water density and temperature.
Chiara Borelli is a geologist at the University of Rochester in New York. Borelli was not involved in the new study. She has, however, investigated Earth’s climate and oceans during the timeframe modeled here. Concludes Borelli, the study fits well into the long-term debate on how the Greenland-Scotland Ridge impacted oceans and climate. She says, “This adds a piece of the puzzle to how the connection started.”
Arctic A region that falls within the Arctic Circle. The edge of that circle is defined as the northernmost point at which the sun is visible on the northern winter solstice and the southernmost point at which the midnight sun can be seen on the northern summer solstice.
Atlantic One of the world’s five oceans, it is second in size only to the Pacific. It separates Europe and Africa to the east from North and South America to the west.
atmosphere The envelope of gases surrounding Earth or another planet.
carbon The chemical element having the atomic number 6. It is the physical basis of all life on Earth. Carbon exists freely as graphite and diamond. It is an important part of coal, limestone and petroleum, and is capable of self-bonding, chemically, to form an enormous number of chemically, biologically and commercially important molecules.
carbon dioxide (or CO2) A colorless, odorless gas produced by all animals when the oxygen they inhale reacts with the carbon-rich foods that they’ve eaten. Carbon dioxide also is released when organic matter burns (including fossil fuels like oil or gas). Carbon dioxide acts as a greenhouse gas, trapping heat in Earth’s atmosphere.
chemical A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds.
climate The weather conditions that typically exist in one area, in general, or over a long period.
colleague Someone who works with another; a co-worker or team member.
computer model A program that runs on a computer that creates a model, or simulation, of a real-world feature, phenomenon or event.
current A fluid — such as of water or air — that moves in a recognizable direction.
dynamic An adjective that signifies something is active, changing or moving. (noun) The change or range of variability seen or measured within something.
factor Something that plays a role in a particular condition or event; a contributor.
freshwater A noun or adjective that describes bodies of water with very low concentrations of salt. It’s the type of water used for drinking and making up most inland lakes, ponds, rivers and streams, as well as groundwater.
Greenland The world’s largest island, Greenland sits between the Arctic Ocean and North Atlantic. Although it is technically part of North America (sitting just east of Northern Canada), Greenland has been linked more politically to Europe.
halocline A sharp boundary, often found in deep coastal regions, that separates freshwater from much more dense seawater below.
Industrial Revolution A period of time around 1750 that was marked by new manufacturing processes and a switch from wood to coal and other fossil fuels as a main source of energy.
land bridge A narrow region of land linking two large masses of land.
link A connection between two people or things.
model To show or simulate a real-world event (usually using a computer).
salt A compound made by combining an acid with a base (in a reaction that also creates water). The ocean contains many different salts — collectively called “sea salt.”
scenario An imagined situation of how events or conditions might play out.
sea An ocean (or region that is part of an ocean). Unlike lakes and streams, seawater — or ocean water — is salty.
time-lapse camera A camera that takes single shots of one spot at regular intervals over a prolonged period. Later, when viewed in succession like a movie, the images show how a location changes (or something in the image changes its position) over time.
Journal: M. Stärz et al., Threshold in North Atlantic-Arctic Ocean circulation controlled by the subsidence of the Greenland-Scotland Ridge. Nature Communications. Published online June 5, 2017. doi: 10.1038/ncomms15681.