October 5, 2001
Changes in climate are historically linked to cultural changes, two University of New Mexico scientists have found, based on their studies of columnar stalagmites in southwestern New Mexico. The research may offer clues about the agricultural progress and migration patterns of inhabitants of the Southwest.
Victor Polyak, senior research scientist, and Yemane Asmerom, a professor of geochemistry, have applied precision uranium-thorium dating techniques to stalagmites from Carlsbad Caverns and Hidden Cave in the Guadalupe Mountains to document climate change over the last 4,000 years in the U.S. Southwest.
A stalagmite is a conical mineral deposit, typically calcite or aragonite, built up on the floor of a cavern and formed from the dripping of mineral-rich water. The annual banding, growth/no-growth behavior and mineralogy of the stalagmites indicate climate changes over the past 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.
Data drawn by Polyak and Asmerom revealed the onset of a wetter climate during the mid-to-late Holocene period (4,000 years ago), 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, including the earliest evidence for cultivation of corn and cotton, occur concurrently with 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.
The cultivation of cotton occurred at the same time 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, thus indicating dry conditions may have contributed to the population re-distribution.
Using equipment including a mass spectrometer that is capable of detecting single atoms of specific isotopes of uranium and thorium, Polyak and Asmerom were able to get accurate ages for the growth layers that were sampled from stalagmites.
“The stalagmites grow layers of calcium carbonate deposits each year, which provide an annual record of relative precipitation,” said Asmerom. “The beauty in the southwestern United States is that stalagmite growth is moisture-limited and that works to our advantage,” he said. “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.”
Polyak and Asmerom’s research, titled “Late Holocene Climate and Cultural Changes in the Southwestern United States,” was published in the Oct. 5 issue of Science Magazine.