Which is heavier ¹⁸O or ¹⁶O?

Right, ¹⁸O has more stuff in it, so it is heavier. Now why is that important? Because, the fact that 18 is heavier is going to have ramifications when you try to move oxygen from one state to another.

Let’s talk briefly about evaporation. As water evaporates, water molecules go from a liquid to a gaseous state. Suppose you have two water molecules, one made with ¹⁶O the other made with ¹⁸O, which is going to be more likely to evaporate?

Right, the lighter one!

Now, suppose you have a cloud that has formed from evaporation over a lake. Which will be isotopically heavier, the lake or the cloud?

The lake

Now, suppose it starts to rain out of that cloud. Which will be isotopically heavier, the cloud or the rain?

The rain

So as you move inland, the precipitation that falls from a cloud should decrease in isotopic mass as the cloud becomes depleted in O-18.

How about latitude? Where does most evaporation take place? Right, near the equator, where it is warmest. What you might not be aware of is the fact that the atmosphere (just like the mantle) convects. There is a general circulation pattern that tends to take the atmosphere from the tropics to the poles. As clouds move towards the poles, they will also lose their O-18 preferentially.

Now, let’s talk about how you could possibly use this information to measure temperature. Suppose I have two pools of water, one at 10° and one at 20°, which is going to be isotopically heavier and why?

Right, the 20° water has more energy in it. The more energy in a system, the heavier the things you can move. Therefore, more 18-rich water will evaporate out of the warmer water.

PUNCHLINE HERE: All other things being equal (see above) d¹⁸O increases as temperature goes down.

So now, all we need is some ancient water, and we can go to work measuring ancient temperatures. To figure out where we can get samples of ancient water, we need to talk for a moment about one of those great asinine projects that could only have been birthed in the 1950’s.

Somewhere in between, can we split the atom and can we put a man on the moon, American scientists asked themselves another question. Being of that gung-ho we-can-do-anything age, we came up with a great way to answer the question. We decided to drill a hole all the way through the crust.

Where should we have drilled this hole to improve our chances of getting there?

The Oceans!

Eventually, (read as after we spent WAY too much money on this idiotic idea) we realized this would never work for several reasons. However, in the process, we collected tons (literally) of sediment cores from the deep ocean. Deep marine sediment is almost entirely composed of the skeletons of microscopic organisms called foraminifera (or forams for short).

Forams, like most marine critters make their skeleton from calcium carbonate (CaCO₃). Calcium carbonate contains oxygen, and that oxygen comes from seawater. Now, as these creatures make their skeletons, they do not make skeletons in perfect equilibrium with seawater.

Will foram shells be heavier or lighter than the surrounding seawater?

Right, foram shells will tend to be absolutely heavier than the surrounding seawater, but we can look at relative changes in their isotopic contents to track relative changes in the isotopic content of seawater through time.

One more useful thing about forams: There are two types, benthic and planktic.

Benthic = lives on the bottom of the ocean

Planktic = lives floating in the near surface water

Water has a very high “specific heat.” That means that the surface will change in temperature much more quickly than the deep ocean will. As a result, planktic forams are good recorders of local conditions, while benthic forams do a much better job of telling you about global conditions.

By using our knowledge of how oxygen isotope fractionates, and combining it with the nearly perfect record of forams in sediment cores, we’ve got a pretty good idea of how global temperature has changed over the past 200 million years.