This poster was presented at A Century of Design in the Parks Symposium, Santa Fe, New Mexico, June 21-23, 2016.

By Henry J. Sun

Surface weathering on stone monuments is generally blamed on acid rain or thermal stress. In this talk I will present evidence that this is not the case at least in Athens (Greece), Beijing (China), Chicago, Washington DC, and Las Vegas. Instead photosynthetic cyanobacteria growing under the translucent stone surfaces using carbonate minerals as a carbon source are the culprits. Weathering appears to accelerate because new, polished stones are resistant to colonization. Over time corners and edges are inadvertently damaged, creating openings for attachment and entry. Once a colony is established in a microscopic crack, it enlarges laterally under the protective stone surface using a yet-to-be-elucidated chemical dissolution mechanism. This happens while the center of the colony recedes as a result of exfoliation the organisms are forced deeper into the stone. This hypothesis explains why stone deteriorations occur in regions that far away from pollution sources and in desert regions where the atmosphere is buffered against acid rain by carbonate-rich aeolian dust. It also explains why deteriorations on flat stones are often localized to corners and edges. This new stone decay theory has far-reaching implications for conservation. For instance, some of the winter covers being used to protect statues from air pollution not only may not help. They may in fact make the problem worse, by warming the statues and stimulating the biological weathering activity.

Dr. Henry J Sun is an associate research professor at the Desert Research Institute in Las Vegas Nevada where he conducts federally-funded research in geomicrobiology. He earned his Bachelor’s degree in botany and Master’s degree in phycology from University of Nanjing (China) and his doctorate in microbiology from Florida State University. His prior professional appointments include six years at the NASA Center Jet Propulsion Laboratory as research scientist. He has served on editorial boards or as reviewer for several scholarly journals including Astrobiology and Microbial Ecology. He has authored more than 30 peer-reviewed research articles.

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