After collecting the shells, the first thing that we do is we age them.
We use a process called amino acid racemization and this essentially relies on the fact that as the animal is assembling that skeleton, that calcium carbonate skeleton, it's placing the calcium carbonate in an organic proteinaceous framework made up of amino acids. When the animal dies, amino acids begin to decay. We can take a shell and we can measure the ratio of pristine or original amino acids to the decay products. Each amino acid in this case is a very fixed and predictable decay product. We then calibrate the whole thing by Carbon-14 dating some of the shells.
The reason we use amino acid racemization instead of Carbon-14 aging is that it's much faster and significantly less expensive. We are utilizing a lot of specimens for this work. So, of course, the isotopic analysis is also very expensive. The cost is significant.
The error of the amino acid effort is + or - 50 years so when we obtain an age on a shell, we then place it within an appropriate 50 year time block. So we've essentially divided the last 2,000 years into these 50 year time blocks. We then take the specimen, we section it as I showed you earlier. This is a much coarser photograph. We section it along the axis of the growth. So here's a growing edge and here's the umbo. We then proceed to extract samples of carbonate. This is really nice sampling for a bivalve relative to what we were able to do a few years ago. But we've actually taken this further, more recently. To extract samples now, we use a drill bit that's less than a half a millimeter in diameter. We take a photo, an electronic photo micrograph or micro image of the shell. We then digitize it and what we do is we run a transect along those growth increments that I showed you earlier and we identify the points that we want to sample. We then give the image and the shell to this cool little robotic machine that then proceeds to drill the samples out. Again, we're taking samples that are less than 20 micrograms in mass.
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