We can actually sample something that represents more into five days of growth.
The reason why we want to sample so closely is that we can now, with our cruder sample, take a sample of carbonate that represents about ten days of growth, and then our finest, our highest resolution, we can actually sample something that represents more into five days of growth. That gets us really good resolution on the growth environment of the specimen. The samples are then analyzed with the mass spectrometer. This is the picture that we get. Here's the distance from the umbo, so this would be the growing edge of the shell. And this is the isotopic profile of a specimen that was collected alive in 1994. What you'll notice are these very nice cycles. That's exactly what we're looking for. Each cycle represents a single year of variation or single year of growth. So this cycle represents 1993. This is February 94 here. And this represents the year 1992.
Remember that all of the variation in the isotopes that we see in these shells is due to temperature variation. The isotopic composition of the water doesn't vary anymore because it's all just marine water; there's no fresh water flowing into it anymore. So what we're looking at here, here's February 1992 which is the coldest month of the year in the gulf, and what we're looking at is the isotopic composition changing as the temperature warms up. We go into spring and into summer and then the year begins to cool down again. We also get the slowing down of growth as the year cools down, these values move up. Here's spring and summer for 93. We move into February of 1994.
These two cycles are very different in length, by the way, because like most other animals, clams slow down their growth or their growth slows down as they get older. Eventually, in very old bivalves, these become so compressed you can't distinguish them. A little factoid, by the way, there's a species of bivalve from the North Atlantic that have been aged using this kind of isotopic chronology and they are the oldest, as far as we know, the longest lived animals in the world. They range, they have life spans of 250 to 300 years. That's pretty old clams. Not doing too much, either.