Tikalon Blog by Dev Gualtieri

Climate Clues from Caves

June 8, 2015

The common perception of early man is as a caveman. However, as I wrote in a previous article (Lunar Caves, August 4, 2014), it doesn't appear that there would have been enough caves to house everyone. While there's fossil evidence that Neanderthals and Cro-Magnons took occasional refuge in caves, our ancestors more likely took shelter from most wild animals by staying in trees.

Caves protect human artifacts from environmental degradation, so the presence of humans is more likely to be found in caves. Early art has been preserved along with the fossils, as exemplified by the photograph, below. As experience has shown with the Lascaux Caves, we need to be careful that such art, surviving after thousands of years, isn't destroyed in our lifetime.

Cave painting of a bison in the caves of Altamira, Spain

Painting of a bison in the caves of Altamira, near Santander, Spain.

(Photo by Baperukamo, via Wikimedia Commons.)

Many caves are formed by the dissolution of limestone by acidic groundwater. Water processes continuing after cave formation can lead to stalagmite speleothems, which are secondary mineral deposits. The starting chemicals for this mineral formation are rainwater and carbon dioxide from the air. These produce weak carbonic acid (H₂CO₃), which dissolves more limestone (CaCO₃) that's transported to the cave ceiling and is deposited as a stalagmite.

Since the presence of water and temperature are important factors in the chemical process, analysis of such speleothems can reveal temperature cycles and periods of drought in the world above the cave. A team of scientists from Vanderbilt University (Nashville, Tennessee), the Berkeley Geochronology Center (Berkeley, California), the National Museum of Natural History of the Smithsonian Institute (Washington, DC), and the University of Cambridge (Cambridge, UK), has used this principle to study moisture transport and drought in India caused by the Pacific Decadal Oscillation, a climatic feature that affects the Indian summer monsoon.[1-2]

Today, scientists have available a remarkable variety of instruments for recording climate change. However, indirect measurement methods are needed to ferret out historical climate data in the times before such instruments were common.[2] Historical records suggest that reduced monsoon rainfall in central India has occurred during recurrent sea surface temperature anomalies known as the El Niño Modoki.[2] In an attempt to quantify the historical record, the research team analyzed fifty years of growth of a stalagmite formed in Mawmluh Cave in the northeastern Indian state of Meghalaya. The cave region, known as the East Khasi Hills district, is known as the rainiest place on Earth.[2]

The team was aided in their study by several factors. A magnitude 8.6 earthquake that hit Assam, Tibet, in 1950 broke off a number of stalagmites in the cave, and new mineral growth had appeared on the broken bases. Since a large amount of water seeps into the cave, the regrowth on the stalagmite was 2.5 centimeters in 50 years, compared with just a few millimeters per thousand years in some arid climates. The average growth of 0.4 millimeters per year allowed measurements at about two-month intervals.[2]

Analysis of a Mawmluh cave stalagmite. Left, the stalagmite in its natural state. Right, cross section annotated with discovered dates and their uncertainty.(Vanderbilt University images by Jessica Oster.)[2]

Another study aid was the finding that the mineral deposits had a high concentration of uranium. This allowed dating of the layers using the decay of uranium into thorium. Also, by an analysis of the ratios of heavy to light isotopes of oxygen, it was possible to track temperature changes and whether the rainwater originated locally, or from a monsoon.[2] The results of the analysis were consistent with the historical record, so the method appears to be applicable for looking into climate in the distant past.[1-2]

Says Jessica Oster, a team member from Vanderbilt University,

"Now that we have shown that the Mawmluh cave record agrees with the instrumental record for the last 50 years, we hope to use it to investigate relationships between the Indian monsoon and El Niño during prehistoric times such as the Holocene."[2]

The technique might be a way to probe the details of the Holocene Climate Optimum, a period of global warming occurring from nine to six thousand years ago. This was not a mild warming, since the average global temperature was 4-6 °C higher than today. This is about the same degree of warming expected from our present build up of atmospheric greenhouse gases, and information about the monsoon during the Holocene would be important information. The monsoon provides the Indian subcontinent with 75% of its annual rainfall.[2]

Such research in using mineral deposits in caves as an indirect technique to examine climate started in the 1990s. However, each cave is unique, and things such as its water transport pathways must be studied for years before it's useful as a climate gauge.[2] This research was supported by the National Science Foundation, the Cave Research Foundation, the Geological Society of America and the Swiss National Science Foundation.[2]

Collection of stalactite liquid runoff.

The particular mechanisms of water flow into a cave must be quantified to make it a useful climate proxy.

(Still image from a Vanderbilt University YouTube Video.)[3]

Climate Clues from Caves

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