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Forensic Geochemistry

Whodunit, and where did he do it?

Earth science is gaining importance in forensics and it is not just about the microfossils embedded in the soil that got trapped in the soles of the murderer’s shoes of which traces were then left behind at the crime scene. Geochemistry is a useful forensic tool as well.

The journal Environmental Chemistry Letters already published a review on the use of stable isotopes in environmental and forensic geochemistry in 2006. The article discusses how the past decade has seen a huge increase in the application of geochemistry in litigation. It mentions the use of biomarkers to link spilled crude oil and oil products to their origins. It also explains that, although the use of isotope compensation is not new, the ability to determine the isotopic composition of individual compounds in mixtures has not been around yet very long.

Excavation site De Balije II, Vleuten-De Meern (near Utrecht, The Netherlands)
with ship De Meern 4 and revetment.

Inorganic geochemistry also features in forensics albeit often of a somewhat different nature. In February of 2002, the Dutch weekly “Intermediair” published an article partly based on an interview with Dutch geochemist Gerard Klaver of NITG-TNO. It discusses the rule of geochemistry as, basically, a decision-making tool in cases of lead pollution.

Lead-polluted soils can be found at sites that were previously occupied by gas stations - because of the lead that was formerly added to gasoline. If that is the case, attempts to recover the remediation costs from the polluters make sense. The first instances of lead pollution in the Netherlands, however, can be attributed to the Romans and some later occurrences of lead pollution are related to mills that produced the paint for painters like Rembrandt.

So how do you determine who to (pur)sue in such cases, if anyone at all? This is where geochemistry steps in. The isotope composition of the pollution often reveals the lead’s source region. The Romans’ lead came from Cornwall, but lead from ores mined in Broken Hill, Australia has a different isotopic signature and the latter is the lead that is generally associated with gas stations.

One of the blocks from the excavation site.

Archaeologists also conduct a type of forensic investigation, although in a broader sense as it does not relate to crimes (which is what the word “forensic” refers to, to the Roman practice of judging criminals before a forum). There definitely is an aspect of whodunit and where did he do it to some of these investigations, however.

A Dutch paper will soon be published which describes a study of basalt blocks used by the Romans around 100 A.C. near the Dutch city of Utrecht. The Netherlands is a large delta and has a very few rock occurrences, let alone any igneous rocks. The Romans, therefore, must have mined these blocks somewhere else and transported them to Holland on ships along the Rhine River.

The Dutch archaeologists had nineteen blocks, from a revetment along a fossil branch of the Rhine and from aboard the remnants of a Roman ship, and also included some blocks from a Roman watchtower. Thinking that these rocks must surely originate from the Eiffel region, the archaeologists contacted hard-rock geologists in the Netherlands and asked them to help determine from where exactly in the Eiffel these rocks originated.

All blocks were alkali olivine basalts with peridotite xenoliths and quartz xenocrysts. All but one also contained resorbed plagioclase xenocrysts. Archaeological constraints made it likely that Romans acquired these rocks from only the German part of what is known as the Cenozoic European Volcanic Province. This translates into a fifty-kilometer-wide belt with a length of three hundred and fifty kilometers.

To the surprise of the archeologists, the geologists first ruled out the Eiffel as the source region on the basis of petrography alone. Any further distinction had to be made on the basis of geochemistry: a comparison against available analysis data for rocks from various areas and quarries. The actual paper by Linthout et al. will reveal the details.

The probable site where these blocks came

This type of research is not as straightforward as it may sound. After all, what may appear to be a good candidate quarry nowadays may well have been located at a too large distance from the course of the River Rhine in the past, and make it a logistically unlikely possibility. Also, currently still lacking is actually simple database-building: more sampling and analysis of rocks from potential source areas, needed to pinpoint the Roman mining sites more precisely.

Now, going back to our murderer at the beginning... If you happen to be contemplating bashing in someone’s head, of course, what you should do is call the person and invite him over for a beer and a BBQ and work it out amicably. But if you must, then it would be best not to use a rock you picked up somewhere locally or, eh, during one of your recent field trips.

Angelina Souren
geologist/marine biogeochemist
Associate Editor for The Geochemical News
Southampton, U.K.