23 k year old footprints at Lake Otero, New Mexico, Round 2

Back in October, I commented on the fairly amazing find of human footprints at Lake Otero in New Mexico, which Bennett et al (2021) argue are 23k years old. As I said at the time, whether the footprints are 23k, or 15k, or 12k years old, it is still an amazing find. However, if they do turn out to be 23k years old, it would not only be amazing, it would be redefining for the archaeology of the Americas.

Now Madsen et al (2021), who have their own contested early date of 16k at Cooper’s Ferry, Idaho, have published a short but pointed critique of the original Lake Otero claim in Science, to which Pigati et al (2022) have replied. Not surprisingly, Madsen et al focus on the carbon dating of the Ruppia cirrhosa seeds on which Bennett et al primarily rely for their conclusion that there were people at Lake Otero 23k years ago.

Madsen et al. worry that Ruppia cirrhosa, an aquatic plant, “use dissolved inorganic carbon (DIC) in their photosynthesis and can date thousands of years older than their true age”, and that there is “ancient Ruppia seeds from shallow lakes, 1 to 3 m deep, dating thousands of years too old.” This would create a site-wide systematic bias in the carbon dates, and the footprints would therefore be much younger than 23k years, perhaps by as much as 5-10k years, which would put them comfortably in the range of ages for well dated early sites in the Americas.

To this, Pigati et al respond that “the distinct seed layers we sampled were positioned in gypsum-rich alluvial sediments that overlie the lake sediments”, and paint a scenario in which lake margins are periodically exposed by varying lake levels, and in which people and animals walk in much shallower water, perhaps a few centimetres, trampling Ruppia plants and seeds in the process.

In other words, they suggest that the seeds were deposited when the water was much shallower, and that those conditions would not have favoured a reservoir effect that would lead the seeds to give deceptively older ages. They rely again on the fact that some of the seeds are found in some of the human footprints to place the creation of the footprints and the deposition of the seeds as part of the same event. Incidentally, they still don’t categorically state that some of the seeds found in the footprints are among the ones actually dated. Still a minor problem, but still an irritant.

One of the other possible explanations that should be ruled out of course, is that the footprints were formed and preserved in a shallow water period, and that the seeds were deposited in and around them much later, in a deeper water period, during one of the periodic inundations of the lake margin. The seeds could then give dates that are too old.

Pigati et al. sort of address that indirectly by pointing to “discontinuous, thin (<1 cm) lenses of clay interbedded with gypsum-rich silt and sand” left by the periodic inundations, which we suppose would then lie between the footprints and any seeds found directly above them. But there are a number of scenarios that would still accord with the evidence, which involve erosion events in between inundations.  In pre-emptively ruling this out as unlikely, Pigati et al. seem to have more confidence in the fine grained nature of the paleo record than I do, whether archaeological or environmental.

To the broader critique that “paleogenetic estimates suggest that the Americas south of the ice sheets were not occupied by humans until after ~20 ka” (in support of which Madsen et al. not surprisingly cite their own contested date of 16k at Cooper’s Ferry), Pigati et al. reasonably reply that the new findings at Lake Otero “must be judged on their own scientific merit rather than whether they agree with prior findings.” To which I say, fair enough.

In general, Pigati et al. don’t really address Madsen et al.s critique, beyond saying, that they already accounted for all that in their original paper. I am still left with some definite questions. The risk of site-wide systematic bias in carbon ages is still there, as far as I can tell. Madsen et al. do a good job of articulating it, and Pigati et al. do no more than they did in the original paper to lay it to rest.

Regardless, a fantastic find from which we can learn a great deal, and which will be significant to many for many different reasons. But as far as I am concerned, the age of the footprints at Lake Otero is still uncertain.

References

M. R. Bennett, D. Bustos, J. S. Pigati, K. B. Springer, T. M. Urban, V. T. Holliday, S. C. Reynolds, M. Budka, J. S. Honke, A. M. Hudson, B. Fenerty, C. Connelly, P. J. Martinez, V. L. Santucci, D. Odess, Evidence of humans in North America during the Last Glacial Maximum. Science 373, 1528–1531 (2021). https://www.science.org/doi/full/10.1126/science.abg7586

J. S. Pigati, K. B. Springer, M. R. Bennett, D. Bustos, T. M. Urban, V. T. Holliday, S. C. Reynolds, D. Odess, Response to Comment on “Evidence of humans in North America during the Last Glacial Maximum”. Science 375 (2022). https://www.science.org/doi/10.1126/science.abm6987

D. B. Madsen, L. G. Davis, D. Rhode, C. G. Oviatt, Comment on “Evidence of humans in North America during the Last Glacial Maximum”. Science 374 (2021). https://www.science.org/doi/10.1126/science.abm4678