Going vertical: Gauging ocean overturn rate
We on Expedition 7 talk a lot about finding deep-sea corals, but we’re especially interested in them for what they can tell us about climate change over time. To get that information, we need to sample corals from a long vertical column of different ocean depths.
This means that while gathering a lot of samples is important, it’s evenmore important that those samples represent many different depths in the ocean,from relatively shallow to relatively deep. This is because water in the oceanhas different ages at different depths.
The age of a water mass is defined by the time since it last “saw” the surface. Because the starting point for deep ocean circulation is the North Atlantic, water is “age zero” there and gets “older” as it moves closer to the North Pacific.
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As it takes that journey, the water carries with it carbon that is undergoing radioactive decay. This decay can help us determine the age of the water it lies in, and from that we can measure how long water takes to circulate through the entire ocean and then come back to the surface. This is called the ocean’s “overturn rate”.
So how does this radioactive decay work? Carbon has three forms, or isotopes: 12C, 13C, and 14C, where the different numbers refer to different atomic weights. 14C is unstable, meaning that it undergoes radioactive decay; 12C is stable, which makes it a handy reference point when the 14C decays. They have the same chemistry, but only 14C is lost through time.
We measure the rate of decay with a term called a half-life. 14C’s half life is 5,730 years. This means that every 5,730 years, there’s half as much 14C as there was in the previous 5,730-year period. To extend this concept, in 11,460 years, there’s one-fourth the amount of 14C as there was originally.
At this point you might be asking how decaying carbon got into the ocean in the first place. The answer to that question is at the heart of what makes Expedition 7 so compelling. All radiocarbon is produced in the upper atmosphere by cosmic rays. So, all radiocarbon in the ocean must have come from the sky and entered at the surface. This occurs through the exchange of carbon dioxide (CO2) gas between the surface ocean and the atmosphere.
The radiocarbon is then distributed throughout the ocean’s depths, but unequally. At the surface of the ocean there is the same 14C/12C ratio as in the atmosphere. In the deep part of the ocean, where the water and its carbon haven’t seen the top for a while, there’s a lower ratio of 14C to 12C, or more radioactive decay.
The difference between radiocarbon levels in the deep and shallow sections of the ocean is a measurement of the time it takes for the ocean to turn over. The longer the ocean takes to overturn, the larger the difference between radiocarbon concentration at the top and the bottom.
A measurement in the modern ocean of the vertical distribution of radiocarbon lets you know how long since each layer was at the top. Today, that number is about 1,000 years for the average ocean. At the end of the circulation path in the North Pacific, there is 25 percent less radiocarbon than in the atmosphere.
Deep-sea corals, in a way no other climate archive can, record the 14C make-up of the ocean in which they lived, because they make their skeletons of calcium carbonate. This makes them ideal tools for us to measure the ocean’s rate of overturn in the past.