Solving the mystery of a metal-oxide coating threatening to cover Mi’kmaq Petroglyphs in Nova Scotia

Laboriously carved into the rocky shores of lakes and rivers in central Nova Scotia are approximately 400 historic petroglyphs created by the people of the Mi’kmaq First Nation. These beautiful and elaborate images record intricate aspects of daily life, such as hunting and fishing, as well as the arrival of Europeans in the 18th and 19th centuries.

The preservation of these petroglyphs is under constant threat, whether from natural erosion, vandalism, or changing water levels caused by hydroelectric dams.

Since the 1990’s another threat has emerged: several of the petroglyphs on the shores of Kejimkujik Lake, in Kejimkujik National Park, have been overgrown by an unusual metal-oxide rock coating.

Now, Parks Canada conservation scientist Despoina Kavousanaki, Ph.D. and I are using some of the most advanced analytical techniques to determine the origin of this mysterious coating, and hopefully find ways to prevent it.

Our research began with using a state-of-the-art field-emission scanning electron microscope to map the distribution of metal elements on the surface of a small subsample of metal-oxide covered rock.

In a scanning electron microscope, electrons are bounced-off the surface of a sample to construct an image that provides data about the sample’s shape and chemical composition. The rock images revealed a distribution of the metals manganese and iron on the rock’s surface.

Did these elements originate from inside the rock, the lake water, or elsewhere?

A collage of four images showing rock samples as described in the caption.

Images taken of a sample of rock cut from near one of the Kejimkujuk Lake petroglyphs. (a) The original rock type (light colour) and the metal-oxide coating (dark colour). The average thickness of the coating is less than 1 mm. (b) A scanning electron micrograph of the coated region. Brighter colours reveal metal-rich regions. (c-d) A series of chemical maps showing the distribution of the elements on the rock surface, iron (Fe, blue) and manganese (Mn, green). More intensely-coloured regions are those with the highest metal concentration. Image: Aaron Lussier, © Canadian Museum of Nature.

To answer this question, we used a focused ion beam to remove a tiny section of the metallic coating so that we could image it with a transmission electron microscope. The focused ion beam uses a beam of gallium ions to cut a slice of rock just 65-nanometers thick, approximately 1,500 times thinner than the width of a human hair. In a transmission electron microscope, a powerful electron beam is shot right through the sample material and then used to construct an image, similar to how a normal microscope uses light waves. The transmission electron microscope is so powerful it can magnify objects more than one million times, almost imaging single atoms.

The images we collected in this way show an intricate array of extremely small mineral fragments. The shapes and textures of these hold important clues about the complex processes, whether natural or human, that may have instigated the movement and deposition of metal oxides on the rock surface. The shape and aggregation of the mineral fragments suggest the involvement of bacterial processes.

Samples of electron microscope data

Transmission electron microscope data collected on a subsample of the metal-oxide coating found on a Kejimkujuk petroglyph. Note the very small scale. General overview of the sample showing (a) the mineralogical/textural and (b) compositional complexity of coating. Each colour corresponds to a different mineral composition. (c) A high-resolution transmission electron microscope (TEM) image of a single grain of clay found in the sample, the observed parallel lines (e) result from the underlying atomic structure in the sample. (d) A higher-resolution TEM image of manganese-oxide nanoparticles. Image: Aaron Lussier, © Canadian Museum of Nature.

Our next step will be to make a detailed geological map of the areas of interest in order to assess the extent of the coating and correlate it with rock composition.

These analyses will enable us to better understand how the coating-related metals move in the local environment and how to make the best possible choices to ensure the preservation of this important First Nations heritage. Stay tuned!

 

This entry was posted in Art, Research, Rocks and minerals and tagged , , , , . Bookmark the permalink.

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