Figure 3. Annual carbon turnover changes the shape of the ¹⁴C bomb curve. Carbon turnover in a macromolecular sample flattens the pulse of ¹⁴C as the rate of turnover increases. When turnover reaches 0.10 (10% annually) the pulse is almost completely flattened. If turnover is less than 0.001 (0.1% annually), molecules formed before 1955 are elevated in ¹⁴C by about 2%, but turnover is difficult to detect in molecules formed after the onset of the pulse. The differences in ¹⁴C between water-soluble and water–insoluble proteins fit a model suggesting ~0.005-0.01 turnover (0.5-1% annually) of carbon in water-soluble protein.