Yemane Asmerom, (505) 277-4434
Victor Polyak, (505) 277-1640
Steve Carr, (505) 277-1821

October 5, 2001


Two University of New Mexico Earth and Planetary scientists, Victor Polyak and Yemane Asmerom, have applied high precision uranium-thorium dating to columnar stalagmites from caves in the Guadalupe Mountains in southwestern New Mexico to document climate change over the last 4,000 years in this region of the United States.

The research, titled “Late Holocene Climate and Cultural Changes in the Southwestern United States” and published in the October 5 issue of Science Magazine, establishes a link between cultural and climate changes.

Polyak, senior research scientist, and Asmerom, a professor of Geochemistry, have studied stalagmites from Carlsbad Caverns and Hidden Cave, and using uranium-thorium dating, they were able to establish that growth bands preserved in these stalagmites were annually deposited. A stalagmite is a conical mineral deposit, usually calcite or aragonite, built up on the floor of a cavern and formed from the dripping of mineral-rich water.

Using the state-of-the-art analytical facility, including a mass spectrometer that is capable of detecting single atoms of specific isotopes of uranium and thorium, that was built by Professor Asmerom here at UNM, Polyak and Asmerom were able to get accurate ages for the growth layers that were sampled from stalagmites.

This is the most precise method of dating,” said Asmerom. “The stalagmites grow layers of calcium carbonate deposits each year, which provide an annual record of relative precipitation. The beauty in the southwestern United States is that stalagmite growth is moisture – limited and that works to our advantage. Thus, based on the thickness of the bands, we can determine how wet or dry a given period is, and in addition, trace that to the cultural changes of the time.”

“What motivates our work is that we want to document what the climate was doing in the past – especially whether it was dry or wet, relative to today’s climate. Obviously, the ultimate aim of all climate research is to develop the capability for predicting climate,” said Polyak.

The annual banding, growth-no growth behavior, and mineralogy of these tell the story of climate changes over the past 4,000 years. This is the most complete and precise climate record for this part of the Southwestern United States. Moreover they show that changes in climate correlate cultural changes such as dwelling abandonment and population redistribution for the last 4,000 years.

“The scientifically exciting aspect of the research is that we have a complete climate record for the last 4,000 years. When compared, the interpreted changes in climate match the cultural changes” said Asmerom.

The overall interpretation of the data drawn by Polyak and Asmerom reveals the onset of a wetter climate during the mid-to-late Holocene period (4,000 years ago) and comparable to or slightly wetter than the present climate until 3,000 years ago. The most significant period of increased moisture occurred between 3,000 and 1,700 years ago. The greater than present day wetness persisted until about 800 years ago. Afterward, conditions became as dry as or drier than present-day conditions.
Cultural changes during this period such as the earliest evidence for cultivation of corn and cotton occur concurrently with interpreted changes in available moisture. For instance, the earliest evidence for the growth of corn by ancestral Americans in the southwestern United States begins with the late Holocene wet period defined by the stalagmite data.

Another crop, cotton, appeared at the same time where the data shows a decrease in moisture (around 1,700 to 1,300 years ago). An increase in relative moisture about 1,250 years ago occurred about the same time that cultures moved from pithouses to above-ground dwellings. The stalagmites quit growing around 700 to 800 years ago when large-scale abandonment of many pueblos took place.

“We are positioning ourselves in the middle of an argument of what caused various culture changes,” said Asmerom. “Climatic change to drier conditions was the very first reason presented near the turn of the century to explain puebloan abandonment. Since that time, other reasons have been introduced.”

“In our data, we see close comparisons between climate and cultural changes as the underlying factor,” added Polyak.

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