Invasive species introduced by humans to new regions can also be markers, the scientists said. The inadvertent import of alien species in the ballast water of ships arriving in San Francisco from Asia transformed the San Francisco Bay. “There was a point where 98 percent of the mass of all of the animal species in the bay were actually invasive,” Waters said. Pollen from introduced plant species, such as the trees used in commercial forestry, can also record change.
Chemical and metal pollution show up in sediments too, said Turner: “The Green Revolution was based on artificial fertilizers and pesticides, and so
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