As long as there has been marine life, there has been marine snow — a ceaseless drizzle of death and waste sinking from the surface into the depths of the sea.

The snow begins as motes, which aggregate into dense, flocculent flakes that gradually sink and drift past the mouths (and mouth-like apparatuses) of scavengers farther down. But even marine snow that is devoured will most likely be snowfall once more; a squid’s guts are just a rest stop on this long passage to the deep.

Although the term may suggest wintry whites, marine snow is mostly brownish or grayish, comprising mostly dead things. For eons, the debris has contained the same things — flecks from plant and animal carcasses, feces, mucus, dust, microbes, viruses — and transported the ocean’s carbon to be stored on the seafloor. Increasingly, however, marine snowfall is being infiltrated by microplastics: fibers and fragments of polyamide, polyethylene and polyethylene […]