Ross Andersen: Insight from Lake Sediment


On a spring morning in New Hampshire, 2,000 years ago, sunlight struck a black cherry tree, opening its white-and-yellow blossoms. As the tree swayed gently in breeze, spiky, spherical pollen grains spilled out of its flowers, and floated up through the limbs and leaves of the canopy, before drifting down to the still surface of a nearby lake. Cool water stalled the pollen’s descent, but still, it kept falling, riding the currents all the way to the lake’s bottom, where it mixed with silt and slowly hardened into sediment.

Time piled new layers of mud and soil atop the pollen, pushing it deeper into the Earth. For two millennia, it continued to sink at that geologic pace, until suddenly, and with some violence, it was slurped up to the surface, through an aluminum tube.

Sitting on a floating platform, a small team of scientists pulled the pollen up as part of a cylinder of sediment, a core bored out of the lake bottom. A core looks like nothing more than a cross-section of muck, but each of its sedimentary slices is an archive, packed with fragments of sticks and leaves, charred remains of wood—and enough pollen grains to census the trees that once surrounded the lake.