OK, so with any luck you’ve read yesterday’s post and you’re up – sort of – on the tools oceanographers use to look into the past. So what did Tom Marchitto and friends see?
They saw evidence of two distinct, massive burps of carbon dioxide, one lasting 3,000 years and beginning about 18,000 years ago; the other following on its heels about 14,000 years ago and, like its predecessor, lasting about as long as all of Western civilization so far. The scientists calculate that the CO2 came from water that had been submerged for at least 4,000 years – 1,300 years longer than the oldest water we know about in today’s ocean.
The nice part about this finding is that it plugs a gap in our knowledge. We’ve known for some time that atmospheric CO2 levels rose – and, curiously, radiocarbon levels fell – as the glaciers retreated. We just couldn’t be sure where it all came from.
But how does water get to be “old” anyway? That’s where the radiocarbon comes in. All of us have at least a hazy understanding that we can age things like Egyptian artifacts by comparing how much radiocarbon (C-14) they contain relative to regular carbon (C-12). The reason it works is that while something’s alive, its tissues pretty closely reflect the radiocarbon levels in the atmosphere. When the tissue dies, the C-14 begins a steady decay while the C-12 remains stable: so the ratio lets us back-calculate its age. This is why you can’t use carbon dating to find out how old something is, you can only find out how long it’s been dead.
Ocean water isn’t alive, but it does move around a lot, and it mixes surprisingly poorly. So when chunks of water sink below the surface they can wander the ocean depths for centuries, the water clinging to itself like a ghost wrapped up in its own shroud.
Now, radiocarbon is only made high up in the atmosphere, where cosmic rays bash into regular carbon atoms, making C-14 that rains down on us in a sort of high-energy game of bagatelle (oops – they bash into nitrogen atoms; see comment). What this means for water is that when it sinks below the ocean surface, it’s like a dead Egyptian artifact, cut off from its source of radiocarbon. The water starts recording its age immediately.**
So putting it all together, Marchitto found evidence – in the shells of 18,000 year old protists – of 4,000 year old deep water moving around in the upper ocean. (To stretch an analogy, it’s as if the ghosts in the cellar had gotten restless and moved up to the ground floor). He and his colleagues think much of that water reached the surface and came back into contact with the atmosphere.
Like the burps of a Scribbler drinking a tamarindo-flavored Jarritos, only considerably larger, these would have raised the carbon dioxide level in the atmosphere. But because the water had been submerged so long, the burps would have been much less radioactive than the Scribbler’s (who contains only the most up-to-date radiocarbon). And because we’re talking about so much carbon dioxide, the overall effect would be an observable dip in the radiocarbon signature of the atmosphere – one that’s been puzzled over for some time in the Greenland ice cores.
In “Deglaciation Mysteries,” Ralph Keeling, of Scripps Institution of Oceanography, offers his perspective on the research, in the same issue of Science. For readers who want something more technical than this post, but less technical than the full paper.)
**This neat trick is one of the main ways oceanographers map the deep currents of the ocean, and it’s how we know that once carbon makes it below the surface waters, it can drop out of the climate picture for millennia.
Radiocarbon is phenomenally useful in other situations, too: It helps us detect manmade organic pollutants when we find them, because they’re made from petroleum, and petroleum is very, very old (so its radiocarbon ratio drops off the chart). And if you’ve ever heard someone say that atmospheric CO2 comes from forest fires rather than fossil fuel emissions? Radiocarbon lets us put a number on that claim.
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