| http://www.w3.org/ns/prov#value | - They determined how lead levels had changed over time by carbon-dating seashells found in the core.Clear pulses of lead contamination occurred between 2686 and 2181 BC and then again from 1000 to 800 BC. The researchers conclude that these peaks were associated with human activities such as plumbing, fishing, building and ship-building.
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