Microfibers are fine strands of thread used to make clothing, carpeting and household items like mops. They are found in the air we breathe, the water we drink, and throughout the world's oceans. Natural, rather than synthetic, microfibers, though, make up the majority of those found in the ocean's surface waters—despite the fact that currently two-thirds of all human-produced fibers are synthetic.
This is based on the paper: Microfibers in oceanic surface waters: A global characterization.
They seem to assume that all the "natural" fibres (wool, cotton) are "anthropogenic" (e.g. clothing, carpeting).
Do they consider the possibility that some of the "natural" fibres may come from plants or animals without human intervention? E.g., animals shedding hair or plant fibres washed into rivers.