The debate has raged on for years in the scientific community – who or what killed the massive woolly mammoth and other mostly large North American mammals during the Pleistocene Age.
The late-Pleistocene-Holocene transition was a time of profound environmental and cultural changes in North America. Human occupation is well documented during this time with the Clovis people. Along with human occupation came the extinction of 35 genera of mostly large mammals. Arguments for overhunting have also been bandied about.
A number of causes had previously been suggested for North American mammalian extinction, including, hunting, and cometary impact leading to the Younger Dryas cold spell. Climate has also been mentioned in the debate as having an important role in extinction, although there was no agreement on the nature and timing of the responsible climatic event. Were all these events a coincidence or did they all play a part in the extinction of large mammals during the Pleistocene Age?
Researchers at the University of New Mexico, including Sr. Research Scientist Victor Polyak and Professor Yemane Asmerom in the Department of Earth and Planetary Sciences, and the University of Massachusetts and the University of Nevada Las Vegas, through a series of experiments involving small amounts of stalagmites from Fort Stanton Cave, New Mexico and supplemented with age data from pool shelfstone speleothems from the Big Room in Carlsbad Caverns in southern New Mexico, have pinpointed the most accurate climate variability period of time ever recorded in this region. The results correlate remarkably with the Northern Hemisphere Bølling-Allerød Oscillation preserved in the Greenland Ice Cores.
"Any conclusion about the role of climate on North American mammalian extinction has to be ultimately based on high-resolution data that gives a sense of the timing and magnitude of moisture variability," Polyak and Asmerom say in a paper titled, "Climatic backdrop to the terminal Pleistocene extinction of North American mammals," recently published in Geology.
Using a Thermo Neptune multi-collector inductively coupled plasma mass spectrometer, these researchers were able to produce a very accurate high-resolution chronology. This chronology was tied to a high-resolution oxygen, carbon and uranium isotope time-series. What they found correlated to a period that marks precisely the beginning of sharply drier conditions in the southwestern United States in the Pleistocene Epoch to within ±100 years.
The researchers contrasted oxygen isotope variability, which is a moisture source indicator, to carbon isotope variability, which reflects local environmental conditions responding to regional climate changes. Drier climatic conditions in the southwestern United States result in lower soil productivity, slower infiltration rates and a greater bedrock carbon deposit. A wetter climate is an indicator of more productive soil and vegetation.
Further, they measured the uranium isotope ratios in the stalagmite, with emphasis on the U-234 isotope. Using the variation of U-234 is a novel and an effective measurement of bedrock and groundwater interaction. Going from wet to dry climate in the southwestern United States results in slower water infiltration rates through the bedrock and longer water-rock interaction times. This produces more U-234 relative to the parent U-238, and vice versa.
In the bedrock, U-234 resides in damaged crystal lattice sites, so "solutions moving through the bedrock from the surface into the cave preferentially leach the U-234 from of these damaged sites, which enrich the U-234 relative to U-238 even more-so during drier climate ," said Polyak. "Carbon and uranium are independent proxies, but both recorded the drought faithfully."
The results of the research suggest a rapid transition from cool, moist Late Glacial to warm very dry Early Holocene-like climate conditions that were likely unfavorable to many Pleistocene mammals in the southwestern United States.
"This is an absolutely fantastic discovery," said Asmerom. "The Clovis people came to North America when things were drying rapidly. Flora, mega fauna - everything was in abundance before 14,500 years ago. But things got bad, real bad as this terminal Pleistocene drought progressed. It became dry, very dry. This drought was massive and unprecedented for the previous 40,000 years that we examined."
The southwestern region has two sources of moisture – winter storms and summer monsoons explained Asmerom.
"Oxygen isotope ratio tracks the source of moisture. The effect and the amount of moisture in winter storms depend on the position of the jet stream. This moisture has light oxygen isotope ratio, compared to moisture from the summer monsoons. Thus, the recorded isotope ratio in the stalagmite reflects the relative contribution of the two moisture sources, and not necessarily the amount.
"In contrast, the isotope composition of carbon in the stalagmite reflect local moisture conditions that effect the relative contribution of light isotope soil organic carbon and isotopically heavy bedrock carbon. Thus the carbon isotope data tell us what is going on locally" he said.
With the data, Polyak and Asmerom say mammal extinction started because of climate change that began over 1000 years before the Younger Dryas period. The beginning of this massive terminal Pleistocene drought coincides with the beginning of mammalian extinction, and predates the time of wide-spread human occupation of North America, and long before the presumed Younger Dryas asteroid impact. Although both human overkill and the asteroid impact may have contributed to the extinction, the new findings show that the extreme and extended arid conditions have to be a dominant factor.
"Although climate change of various forms had been previously suggested, our research nicely shows the extent and timing of extreme climate change, which coincides with timing of extinction," said Polyak.
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