USGS research geologist Jeff Pigati and his colleagues (including Bennett and other co-authors of the 2021 paper) recently radiocarbon-dated conifer pollen—mostly from fir, spruce, and pine—from the same ancient ground surface as the tracks and the ditchgrass seeds. They also used another type of dating, called optically stimulated luminescence (a type of dating that measures when a grain of quartz was last exposed to sunlight) on sediment samples from between the oldest two layers of tracks. The results lined up very well with Bennett and his colleagues’ original radiocarbon dates; the tracks couldn’t be any younger than about 21,500 years old.

I had never heard of this other method with the quartz. Interesting.

Ditchgrass, as its name suggests, is an aquatic plant, exactly the kind of thing you’d expect to find along the shore of a lake. But aquatic plants tend to soak up groundwater, which can contain older carbon than the rain that waters more landlubberly plants. Seeds from aquatic plants like ditchgrass can (but don’t always) look older than they really are when radiocarbon-dated—sort of like the radiocarbon version of carrying a fake ID.

The other two methods don’t suffer such problems. So now that they have similar results from these methods the evidence is much stronger.