by Timothy Oleson Sunday, October 5, 2014
Builders and masons take note: When it comes to the durability of limestone, grain size matters. New research combining field and lab data shows that fine-grained limestone is more susceptible than its coarser-grained cousins to a one-two punch of chemical and mechanical weathering. The findings, which arose in part from observations of Jerusalem’s historic Western Wall, could have implications for Earth’s carbon cycle and landscape — as well as for architectural preservation.
Unlike weathering of granite or other hardrock, limestone weathering is often thought of as a purely chemical process in which groundwater or rain dissolve the rock, says Simon Emmanuel, a geologist at the Hebrew University of Jerusalem in Israel who led the new work, published in Geology. Now, he says, it’s “quite clear that for many [limestones], mechanical weathering is the dominant mode.”
Weathering can be challenging to study in the field because natural erosion rates of rock are typically very slow, meaning observations over long durations are necessary to get accurate, quantitative information. But in the Western Wall — part of the fortification constructed in the first century that surrounds Jerusalem’s Temple Mount — “we have this 2,000-year-old weathering experiment,” Emmanuel says.
Cut completely from local limestone, the blocks in the oldest part of the wall have been subjected to essentially the same degree of weathering. But some blocks show signs of heavy erosion, while others appear only lightly weathered. Curious about the difference, Emmanuel and Yael Levenson, also at the Hebrew University, analyzed high-resolution topographic data from lidar scans of blocks that appeared free of human-caused damage. Whereas the surfaces of well-preserved blocks appeared to have lost only about 1.5 millimeters per thousand years on average, erosion rates for portions of the damaged blocks ranged up to more than 100 millimeters per thousand years, the researchers reported.
They noted that the poorly preserved blocks were all composed of micritic limestone — lithified over time from fine-grained calcium carbonate (calcite) mud. The well-preserved blocks, meanwhile, had been cut from limestone with an average grain size (about 50 microns) roughly five times larger than the micrites. Looking more closely at tiny samples of the different rocks under an atomic force microscope, Emmanuel and Levenson observed rapid dissolution along grain boundaries at the surface of the micrite when it was exposed to water. This dissolution loosened the grains, allowing some to detach and leave pits behind.
“The particle detachment only ever occurs in these very fine-grained rocks. We never see it in the large-grain rocks,” Emmanuel says. Fine-grained rocks are usually less porous than coarse-grained rocks and are considered harder and more resistant to weathering, he notes, so the observations here may seem counterintuitive. But he likens the scenario to sugar cubes dissolving and disintegrating in coffee: “If you had a cube made of very fine crystals of sugar, it would disintegrate much quicker” than one made of large crystals.
“We can actually image this mechanical disintegration … so it’s not just chemical weathering, it’s coupled chemo-mechanical weathering,” Emmanuel says. This coupling may explain why micrite in the Western Wall and at outcrops elsewhere seems to erode faster than other limestones.
Micrites are geologically common so mechanical weathering of limestone is “potentially an important mechanism” affecting Earth’s carbon cycle, Emmanuel says. Mechanical weathering increases the surface area of rock exposed to chemical weathering, which draws down atmospheric carbon dioxide levels. “People have tried to make assessments of [carbonate] weathering rates and how they could affect the global carbon budget on short timescales. So trying to understand how much is chemical and how much is mechanical is a central issue,” he says.
“Overall, it’s a very interesting piece of work,” says Cornelius Fischer, a geochemist at the University of Bremen in Germany who was not involved with the study. Studying the weathering of rock is also “of particular interest in understanding the predictability of fluid-rock interactions,” he says, which is critical in applications like subsurface carbon sequestration or nuclear waste storage, for example.
“The important thing [in such studies] is how to bridge scales of observation … to get the right approach that provides firm predictability,” Fischer says. There are open questions, he notes, such as the precise grain size below which particles are subject to the chemo-mechanical weathering process. But, he says, the new work offers “another step forward” by combining techniques in a novel way.
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