Researcher Yousong Cao samples at the Great Wall. Credit: Bo Xiao
A small international team of specialists in the conservation of soil and water ecosystems has discovered that biocrusts clinging to parts of the Great Wall of China served to protect the famous structure from erosion. In their article published in the journal Scientists progressthe group describes their study and analysis of the materials growing on the wall.
The Great Wall of China was built over several centuries starting around 221 BC. Its function was to protect the people living behind it from enemies trying to invade the other side. Previous research has shown that different parts of the wall were made with different materials, mainly rammed earth or stone.
Rammed earth is made by mixing organic materials with inorganic materials. By their nature, these materials are more sensitive to erosion. This led to questions about how sections of the wall made from this material survived for so many years. In this new effort, the researchers wondered whether biocrusts might have played a role.
For many years, scientists have assumed that these biocrusts, generally made up of cyanobacteria, lichens and moss, accelerate the erosion process. To find out if this is the case, the research team collected biocrust samples from several points along the wall and took them back to a laboratory for study.
The researchers measured the mechanical strength and soil stability of the samples. They also tested parts of the wall by directly comparing those covered in biocrusts and those directly exposed to the elements.
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Close-up view of biocrusts on the Great Wall. Credit: Bo Xiao
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Researcher Yousong Cao samples at the Great Wall. Credit: Bo Xiao
They found that the biocrusts were stronger than the rammed earth they grew on – in some cases, three times stronger. The researchers also found that the strength of the biocrusts was due to the secretion of tightly bound polymers.
The research team concluded that rather than speeding up erosion, the biocrusts slowed the process, helping to preserve the famous structure. A University of Granada team working in Honduras has discovered that organic plant matter added to plasters by the early Maya served to reduce weathering of the stone structures they built.
More information:
Yousong Cao et al, Biocrusts protect the Great Wall of China from erosion, Scientists progress (2023). DOI: 10.1126/sciadv.adk5892
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