This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Expanded excavations at White Sands National Park in New Mexico have provided researchers an opportunity to refine the age of the oldest human footprints in North America and establish a deeper understanding of the landscape and environmental conditions at the time humans were present there. The new study, published in Science Advances by a team of researchers including the University of Arizona, the United States Geological Survey and other partners, reveals a dynamic environment indicating the humans who left their tracks along an ancient lakeshore experienced abrupt changes in temperature and water availability within their lifetimes.
The research team previously discovered and confirmed the White Sands human footprints to be the oldest evidence of human presence in North America, placing people on the continent thousands of years earlier than once thought. The research details innovative extraction procedures used to isolate up to 75,000 individual pollen grains per sample and refined dating methods used at the USGS Radiocarbon Laboratory. This resulted in a more precise age of the site and reaffirmed the White Sands footprints date to between 23,500 and 21,700 years ago, or during the Last Glacial Maximum.
The new chronology was key to unlocking the rest of the story, which includes a deeper understanding of the abrupt environmental changes that early humans in North America faced. The researchers used the new chronology to investigate the climate and environmental changes at a timescale equivalent to a human generation, or approximately 50 years, which is rare for terrestrial paleoclimate archives of this time period from the southwestern U.S. "The White Sands sediments are unique in that they allow us to reconstruct environmental conditions during the last ice age at a very high temporal resolution, so that we can see fluctuations that were relevant for the humans present at that time," said co-author Jessica Tierney, a professor at the U of A Department of Geosciences.
Tierney applied her expertise in the analysis of geochemical signatures captured in sediments, which she combined with fossil pollen from the plants growing in the surrounding landscape, to reconstruct temperature, precipitation, aridity and plant communities. The results show the climate at White Sands was dynamic. Humans witnessed a complete turnover of the landscape from conifer forest to sagebrush and oak steppe and the recession of Lake Otero, a massive lake that would have covered most of what is White Sands National Park during the last ice age.
By measuring isotopes—elemental atoms that differ by their number of neutrons, in this case, hydrogen—in leaf waxes of preserved plant material, Tierney's group at the U of A discovered that there were also swings in water availability within a century that included severe aridity during an abrupt warming event also recorded in ice core samples from Greenland. "This tells us that local changes in aridity at White Sands were quite large and followed global climate shifts," Tierney said. Discover the latest in science, tech, and space with over 100,000 subscribers who rely on Phys.org for daily insights. d research that matter—daily or weekly.
During the last glacial maximum, when humans left their footprints in what likely was a muddy plain, temperatures in the Southwest were generally cooler, around 50 degrees Fahrenheit (10 degrees Celsius), as opposed to today's annual mean temperatures of around 60 degrees Fahrenheit (16 degrees Celsius) in that area, Tierney explained. During the sudden shift documented by pollen in the sedimentary layers, vegetation changed from pine forest to pinyon and juniper, all within about 100 years. Tierney said that varying amounts of pollen left behind by Ruppia, an aquatic plant that likes to grow in desert streams and springs, indicate Lake Otero's water level went up and down over the course of about 1,500 years.
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