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题目材料：
Geologists can determine the ages of rocks and fossils by using natural clocks, including the natural decay of unstable, radioactive atoms into stable forms. The element uranium occurs in minute concentrations in seawater, and certain types of organisms, particularly corals, absorb it as they grow. One isotope of uranium, 238U, decays through a series of steps to 230Th, an isotope of thorium. As a coral grows, it adds tiny amounts of 238U to its skeleton. Over time, this 238U steadily transforms into 230Th. The proportion of the two isotopes changes in a predictable way over time, allowing us to calculate the ages of fossil corals in marine terraces back as far as five hundred thousand years.
A drawback of uranium-thorium dating is that it doesn't work on most fossils. Shells of molluscs like clams and snails are common in marine terrace deposits, but molluscs don't take up uranium from Seawater. Here, though, we have another trick: amino acid racemization. The proteins of living creatures
contain amino acids in a specific molecular shape known as the L-configuration. Upon death, some of these amino acids begin shape-shifting to a new arrangement called the D-configuration-a process called racemization. Molluscs are particularly useful for amino acid dating because they live practically everywhere in shallow ocean habitats and because
their shells are bound together with matrixes of amino acid-rich proteins. By measuring the ratio of the two types of amino acids in mollusc shell proteins, we can estimate the time since death.
以上解析由 考满分老师提供。
