Deep‐water delivery model of Ruppia seeds to a nearshore/terrestrial setting and its chronological implications for Late Pleistocene footprints, Tularosa Basin, New Mexico

Abstract

Ruppia cirrhosa (Ruppia) seed layers have been used to constrain the age of footprints along the eastern shoreline of Paleolake Otero in southern New Mexico to around 21,000-23,000 calibrated years before the present. However, there remain two unresolved questions that can affect the reliability of the age(s) of the footprints.First, what is the nature of the geological context of the seed layers? Second, did the hard-water effect impact the accuracy of the radiocarbon dates? It has been argued that the dated Ruppia plants grew in situ in a very shallow, freshwater-infused system that minimized the hard-water effect. Many of these Ruppia seed layers contain ball-like aggregations made of Ruppia plant materials. We provide new evidence that these balls and seed layers were introduced to the discovery site by high wind seiche events during Late Pleistocene thunderstorms. In our proposed site formation model, the Ruppia plants and seeds originated in deeper water settings outside the site, thus it is very likely that the hard-water effect has impacted the accuracy of the radiocarbon dates. As such, the radiocarbon assays of Ruppia seeds previously used to date the prehistoric footprints along Paleolake Otero could be thousands of years too old.

The Pleistocene footprints are younger than we thought: correcting the radiocarbon dates of Ruppia seeds, Tularosa Basin, New Mexico

“…δ 13 C values of Ruppia seed ball aliquots To directly compare the δ 13 C values of modern plants, reported earlier, with the seed layers on the eastern shoreline of paleo-Lake Otero, a collection was made that included stems and seeds from one Ruppia seed ball. The seeds from the ball yielded several radiocarbon dates, which ranged from 22,411 to 20,834 cal yr BP (see Rachal et al, 2022). The youngest and oldest dates in this age range differ significantly at a 95% confidence level (t = 6.366141; χ 2 (0.05) = 3.84; df = 1).…”

“…The youngest and oldest dates in this age range differ significantly at a 95% confidence level (t = 6.366141; χ 2 (0.05) = 3.84; df = 1). This difference of more than 1500 yr may indicate the mixing of seeds with varying degrees of HWE (Rachal et al, 2022), particularly when considering the documented 1000 yr range among individual modern Figure 3. Linear regression demonstrating a weak correlation (r 2 = 0.18) between water depth and both calibrated and 14 C age (inset graph) offsets of modern Ruppia in the Salt Creek.…”

“…The wetland complex was a refugium during post-LLGM times for the submerged Ruppia plant. It is likely that this rooted plant once occupied the deep-water spring-fed salty embayments of paleo-Lake Otero during the LLGM (Rachal et al, 2022), and it still grows in underwater meadows in the northern, perennial portion of Salt Creek (Figure 1B and 1C). There is also a small population of Ruppia growing in the lower intermittent flow portion of the drainage (Figure 1A).…”

“…This sampled seed ball had been previously found partially buried in the same stratigraphic level as the human trackway. Ruppia seeds and stems collected from the ball yielded several radiocarbon dates that ranged from 22,411 to 20,834 cal yr BP (Rachal et al, 2022). It has been hypothesized that this range in radiocarbon dates is an indication of the mixing of seeds with differing degrees of the HWE.…”

Independent age estimates resolve the controversy of ancient human footprints at White Sands

“…The second is that Ruppia seeds are physically robust and can be preserved in sediments over geologic timescales, so it is possible that the seeds were exhumed from older deposits and reworked into the sediments at WHSA Locality 2. If so, the seeds would provide only maximum-limiting ages for the human footprints ( 6 , 7 ).…”