The problem with aquatic plants is that lake and river water can contain dissolved carbon that is significantly older than the water itself — old carbon derived from ancient limestone or other geological sources that dissolved into the water over long periods.

If an aquatic plant is drawing some of its carbon from this old dissolved carbon rather than entirely from the atmosphere, the radiocarbon date of the plant will make it appear older than it actually is.

The organism looks ancient because some of its carbon is ancient, not because it is.

Skeptics raised this possibility explicitly after the 2021 publication.

The Ruppia seeds used for dating were aquatic plant material from a closed basin with a complex geological history.

The Tularosa Basin has abundant gypsum and other dissolved minerals.