A landslide in China in 2009 may have been caused by acid rain, suggests new research.
Earth Magazine has reported that a landslide, which killed 74 people in southwestern China, may have been caused by acid rain weakening the limestone and shale slope in an unexpected manner.
According to the magazine a lack of recent earthquake activity or heavy rainfall left geologists questioning what had triggered the slide.
The Jiweishan Mountain landslide happened near Chongqing, in the southwest China’s limestone karst region, where rainwater has gradually been dissolving limestone and creating potholes and caverns.
The study was published in Earth and Planetary Science Letters.
China’s air pollution has led to some of the most acidic rain on earth, according to the magazine.
The study’s authors Ming Zhang and Mauri McSaveney of GNS Science in New Zealand set about testing the effects of acid rainwater percolating down through the limestone to the shale layers.
They discovered when they immersed samples of the shale in dilute hydrochloric acid, the microscopic shell fragments in the shale fizzed and dissolved, leaving behind a weakened rock resembling a porous black sponge.
“This shale is described geologically as a bituminous shale, because it contains so much organic matter,” McSaveney told the magazine. Once the calcium and magnesium shell fragments were gone, the organic matter was left behind.
“We reasoned that if some process could remove the [organic] component, what would be left would be mostly talcum powder, which is soft and slippery” and could be the weak trigger point for the slide.
Zhang and McSaveney then looked for evidence of microbial activity in the shale, McSaveney says, because “we knew that organic matter could be attacked by microbial decay.” DNA tests revealed hundreds of genera of microorganisms, including ones capable of decomposing organic material within the shale and weakening it further.
“Without calcite and organic material, the basal shale at the Jiweishan rock avalanche is composed largely of detrital talc,” the team wrote.
Colgate University in New York geochemist Richard April was not involved in the new research, and told the magazine that link between acid rain and landslides is new. “This is a plausible mechanism for how the effects of acid rain may have accelerated the pace of the decomposition of that shale layer and possibly triggered this slide. It’s a whole new way of thinking about what might start a landslide.”
However, April does have some reservations about the hypothesised mechanism related to the timescale of acid rain erosion, he told the magazine. “In China, the acid rain problem has only peaked in the last 30 years. Can 30 years of acid rain reach the thin shale layer and decompose it to the point that it initiated a landslide?”
“I would like to see a lot more fieldwork — collecting rainwater, groundwater and rock samples,” April says. Additionally, “more modelling and calculations about how much acid rain has fallen on this region will give a much better idea of whether this is a feasible means of initiating a landslide.”