Any ordinary person looking at the rainbow-colored map of Canberra, Australia would only see a map or maybe the weather forecast for the next day. But Patrice de Caritat sees something completely different: a detailed landscape of soils, with different colors revealing areas of land rich in elements like carbon, nitrogen and phosphorus. The geochemist has spent over 2 decades creating geochemical atlases to help other researchers track down rare minerals or understand a region’s changing environment. Now, a new group is interested in these cards: the police.

De Caritat’s first call came in 2011, from the Royal Canadian Mounted Police. They asked him if cards like his could be used to stalk the ground for criminal investigations and possibly even link suspects to crime scenes. “I never really thought it could be used for anything of this nature,” says de Caritat, who works at Australian research agency Geoscience Australia. “It kindled a spark in my head.”

Over the following years, de Caritat developed new methods to determine the origin of soil samples with the National Center for Forensic Studies in Australia. In 2018, he took a sabbatical to work with the Australian Federal Police. Now he and his colleagues have pieced together a map that uses over 100 variables to determine exactly where a soil sample in Australia’s capital Canberra most likely came from; he presented his findings at the annual Goldschmidt conference for geochemistry in July.

Police forces around the world are already using soil samples to pinpoint potential criminal sites or suspects. For example, if a person is believed to have buried an object at a crime scene, the dirt recovered from that person’s shoes or shovel is compared to the dirt at the scene. Variations in mineral and chemical components can tell officers whether samples match, says Caitlin Schmaal, forensic chemist with the Australian Federal Police.

But investigators don’t always have a benchmark sample to compare with soil taken from a suspect, Schmaal says. This is where the new cards come in. They provide investigators with a universal point of reference and a way to anticipate cases. be preemptive, ”says de Caritat.

The Caritat maps presented at the conference used variables such as color, texture, pH, and other chemical properties to differentiate soil plots. De Caritat collected the data from a mix of old data sets and new samples that he and his colleagues collected in the field. By combining these variables, his team was able to reduce the origin of a single 30% soil sample from Canberra, excluding the remaining 70%, they reported at the conference. And that’s even though Canberra has little variation in its soil.

One of the biggest challenges in determining the origin of a soil sample, according to Caritat, is the scale of variation. “We can do geochemical maps on a continental scale, but they won’t tell us if the soil taken from a particular location in your garden will be different from a sample taken at 5 centimeters. “

Nonetheless, the new maps are a valuable addition to the toolkits of forensic teams, says Schmaal. “Soil evidence is useful for making connections between people, objects and places, [and it] has the potential to provide even greater assistance to investigations. Alexander Lipp, a geochemist at Imperial College London, who was not part of the research, says he believes the application of this geochemical mapping will be on a case-by-case basis. But, “If you can bring more and more data to these cases, it won’t be a bad thing.”

In the future, de Caritat and his colleagues want to add new details to their classification system, especially on the composition of living organisms and DNA in particular soil plots. That, he says, would expand his list of parameters from a few hundred to tens of thousands, and it could help identify soil patches even more accurately. “It could be extremely, extremely powerful,” he says.