Final answer:
The concentration of phosphate in ocean water can be used as an estimator for the water's age due to its slow movement and long residence time. By measuring the phosphate concentration, scientists can estimate the age of the water.
Step-by-step explanation:
The concentration of phosphate in ocean water can be used as an estimator for the water's age because of its slow movement and long residence time. Phosphorus cycles between phosphate dissolved in the ocean and marine ecosystems, and the average phosphate ion can spend between 20,000 and 100,000 years in the ocean.
This means that older water will have higher phosphate concentrations, while younger water will have lower concentrations. By measuring the phosphate concentration in ocean water, scientists can estimate its age based on this relationship.