Published: Tuesday, 10 August 2021 10:45
Any ordinary person looking at the rainbow-colored map of Canberra, Australia, would see just a map—or maybe, the following day’s weather forecast. But Patrice de Caritat sees something entirely different: a detailed landscape of soils, with different colors revealing areas of earth rich in elements like carbon, nitrogen, and phosphorus. The geochemist has spent more than 2 decades making geochemical atlases to help other researchers track down rare minerals or understand an area’s changing environment. Now, a new group is showing an interest in these maps: law enforcement agencies.
De Caritat’s first call came in 2011, from the Royal Canadian Mounted Police. They asked him whether maps like his could be used to track down soil from criminal investigations—and perhaps even tie suspects to crime scenes. “I had never really thought that this could be used for anything of that nature,” says de Caritat, who works at the Australian research agency Geoscience Australia. “It lit a spark in my head.”
Over the next several years, de Caritat developed new methods to determine the origin of soil samples with Australia’s National Centre for Forensic Studies. 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 more than 100 variables to determine exactly where a soil sample from Australia’s capital, Canberra, most likely originated; he presented his findings at the annual Goldschmidt Conference for geochemistry in July.
Police forces around the world already use soil samples to narrow down potential crime sites or suspects. For instance, if a person is thought to have buried an object at a crime scene, soil recovered from that person’s shoes or shovel is compared with soil from the scene. Variations in mineral and chemical components can tell officers whether the samples match, says Caitlin Schmaal, a forensic chemist at the Australian Federal Police.
But investigators don’t always have a reference sample to compare with soil collected from a suspect, Schmaal says. That’s where the new maps come in. They offer investigators a universal reference point—and a way to get ahead of cases.“The technique that we’re using here is not by collecting two or a dozen samples at a particular investigation, it’s actually being preemptive,” de Caritat says.
The maps de Caritat presented at the conference used variables including color, texture, pH, and other chemical properties to differentiate patches of soil. De Caritat collected the data from a mixture of old data sets and new samples he and his colleagues took in the field. By combining these variables, his team was able to narrow down the origin of a single soil sample to 30% of 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 with pinning down the origin of a soil sample, de Caritat says, is the scale of variation. “We can make geochemical maps to the continental scale, but they’re not going to tell us if soil taken at a particular location in your backyard is going to be different from a sample taken 5 centimeters away.”
Nevertheless, the new maps are a valuable addition to forensic teams’ toolkits, Schmaal says. “Soil evidence is useful for providing linkages between persons, objects, and locations, [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 thinks that application of this geochemical mapping will take place on a case-by-case basis. But, “If you can bring in more and more data to these cases, that’s not going to be a bad thing.”
Moving forward, de Caritat and his colleagues want to add new details to their classification system, specifically about the makeup of living organisms and DNA in particular patches of soil. That, he says, would expand his list of parameters from just hundreds to tens of thousands, and it could help identify patches of soil with even more accuracy. “This could be extremely, extremely powerful,” he says.
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