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Isotopic profiles for fossil specimens...

Here are the isotopic profiles for two fossil specimens, aged 1652 and 1667. Here we see a nice annual cycle. These are sampled somewhat more crudely. This is our push to get results early in the project so that we could renew our funding to demonstrate that the method works. The green lines, the horizontal green lines represent the mean isotopic value for our modern live collected specimens such as the specimen I just showed you, and these lines represent the maximum and minimum of the isotopic range. And the thing that you should note immediately is that the isotopic range of the fossils is significantly lower than it is for the recent specimens. And these are two sort of moderate specimens. I'll show you a more dramatic one in a moment.

But this decrease, this additional loss of Oxygen-18 is due to the presence of the Colorado River. So not only do the isotopes decrease in value, or the Oxygen-18 decrease in values because of temperature each year but superimposed on that is the arrival each year of this Oxygen-16 rich, Oxygen-18 poor freshwater from the Rocky Mountains. This is exactly what we're looking for here.

What we do next is we standardize these isotopic profiles. We remove differences in growth rate and here is our mean modern isotopic profile. This is what the average Chione fluctifraga from the northern Gulf of California looks like. Here is a specimen from the 15th century. Look at that isotopic profile. The Del 018 values are down in -7. It never gets lower than -2 in the recent. So essentially, what we're looking at is superimposed on the temperature variation, this variation due to the arrival of fresh water.


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