How can you use isotopes to calculate percentages of fresh or marine water?
Ultimately, when these clouds deliver their water vapors--rain, snow, whatever other types of precipitation--the precipitation is Oxygen-16 enriched and Oxygen-18 deplete. This precipitation of course is returned ultimately to the oceans as fresh water run off or by rivers. So what you have in coastal areas where you have this fresh water moving into the coastal environment is a mixing of marine waters and a mixing of fresh waters. And the fresh waters are much richer in Oxygen-16 in a relative sense, than marine waters are and they're much poorer in Oxygen-18.
And this is really important because we can take a body of this water, for example, this mixed water, and if we know the isotopic composition of marine water offshore, we know the composition of the precipitate or the run off, we can actually calculate the percentage or the volume of fresh water or marine water in that sample of water. So, we can go, for example, when the river used to flow into the Gulf of California, we could go to the delta region where you have marine delta California water, we have fresh Colorado river water, there's this mixing zone. We can take water from the mixing zone and we could look at it, analyze it and say, well, here's how much Colorado River water is in this sample of delta water over coastal marine water.
Of course, we can't do that for our reconstruction because there's no 2,000 year old Colorado River water sitting around anywhere. So this puts a little crimp into the process. However, we can utilize some fossils and a particular method to get at this. I'll give you a brief run down on the method first. There is a really neat and sophisticated piece of machinery that's used to examine oxygen isotopes called a mass spectrometer. I should have titled that somewhere, but mass spectrometer. If you've ever been in a mass spec lab, there are machines that used to, just five years ago, they would occupy essentially the entire back of this stage with lines and tubes going everywhere. Now they're about the combined size of this podium and this cart. And if any of you are Star Trek fans, you'll see the OA team occasionally will have their handheld mass specs. We would love to have things like that because they're undoubtedly cheaper than the ones we use.
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