Acid attack

日期:2019-03-08 05:09:06 作者:凌煞钩 阅读:

By Fred Pearce RISING levels of atmospheric carbon dioxide and the breath of tourists threaten to destroy records of European rainfall hidden in limestone caves. Like trees, the spectacular stalactites, stalagmites and drip curtains in caves have growth rings whose chemical makeup provides a clue to past climates. Andy Baker, a geographer at the University of Newcastle upon Tyne, has developed a technique for revealing the past moisture levels of soils by analysing light passing through the rings. Using this method, he has constructed a record of European rainfall over the past millennium, which he has submitted to the journal Nature. But this valuable record may be in danger. “I know of many caves in Britain where the stalagmites are corroding away,” says Baker. The threat comes from rising CO2 levels inside the caves. Around 40 000 people visit Poole’s Cavern in Derbyshire each year and more than half a million visit Italian caves such as the Grotta di Castellana. The CO2 in their breath can triple levels of the gas in poorly ventilated caves, reversing the process that forms the limestone structures. Stalactites and stalagmites are created when calcium carbonate precipitates from water dripping inside the cave. Water percolating through the soil above the cave dissolves CO2 from its surroundings, forming carbonic acid that strips calcite (calcium carbonate) from limestone rocks. When the water reaches the cave, however, it loses CO2 to the cave air, reducing its ability to carry the mineral. But elevated levels of CO2 in cave air can upset this process. Less calcite will be deposited if the water loses less CO2 when it reaches equilibrium with the cave air, slowing the growth of the structures. Worse still, if levels are sufficiently high, the water will dissolve CO2 from the cave air, forming more acid that erodes the limestone structures rather than building them. The risk is greatest in caves where the calcium content of the water is low. The majority of British caves, many designated sites of special scientific interest, fall into this category. But Baker refuses to name sites to avoid angering their owners, who provide him with access for research in addition to opening the caves to the public. It is not only tourist attractions that are at risk. The predicted doubling of atmospheric CO2 within the next century could be sufficient to destroy stalagmites in pristine caves, according to Baker. He notes that the stalagmites in Uamh an Tartair in northwest Scotland—some of which may be 10 000 years old—do not appear to have grown over the past hundred years or so. Changing land use can further reduce the amount of CO2 in the soil and therefore the amount of calcite in water draining into the caves. “The worst case that I have seen occurred when buildings or roads appeared above the caves,” says Baker. But construction work can cause other problems. “We have a road above our cave,” says Gordon Hanley,