A paper last week in Science reached back 38,000 years to trace how the ocean dumped heaps of carbon dioxide into the atmosphere just as the last glaciation was starting its decline. Tom Marchitto and colleagues discovered that around 18,000 years ago, atmospheric carbon dioxide began its steady rise from 180 ppm to the oft-quoted 280 ppm before the start of the industrial revolution. They think the CO2 came from very old, very deep ocean water that burst to the surface in two prolonged belches.
You could be forgiven for wondering how we’re so sure what the molecular composition of air and ocean water were 14,000 years before the pyramids had been built. Paleoceanographic research is a scavenger hunt of bizarre techniques on unlikely objects: sea mud; old ice; corals.
First you bring up some seafloor mud in what is essentially a very long soda straw. Put it under a microscope and pull out the shells of tiny dead creatures called forams (Not plants, not animals; they’re protists.). During their brief but happy lives, some of these floated in the surface water while others lived on the seafloor. Learn how to tell them apart, and you can compare their radiocarbon ages – along with oxygen isotopes – to surmise how the deep water was different from the surface water way back then.
If that sounds shaky, there are at least supplementary techniques that scientists use to make sure they’re in the right ballpark. Some 3-km deep holes in the Greenland ice sheet (and Antarctica) provide similar information from gases caught in the annual snow layers. People actually count, layer by layer, 100,000 years into the past. When they see a series of spikes in ice-core isotopes mirrored in seafloor mud isotopes, they can be reasonably sure they’re looking at the same time in prehistory. In the pages of Science, these are called “tie points.” In the bar after work, it’s called “wiggle matching.”
Other people pull up coral from the seafloor and look at heavy elements trapped in its layers. A neat trick with the way uranium transmogrifies into thorium and protactinium as it decays – and how those elements tend to sink differently – lets them figure out the volume of ocean currents in the past.
With me so far? Me neither. This is one reason why it’s so hard to find good articles about paleoceanography in People or Reader’s Digest: too much background.
Tomorrow: We’ll step back and just think about radiocarbon. Everybody knows what carbon dating is. But how does it work? And what does it tell us about the ocean?
Photo: Francis Frith, 1862