The largest study in the southern hemisphere of the accuracy of 'bomb pulse' carbon dating following nuclear testing in Australia in the 1950s and 1960s, has found that nail samples provide the best estimate of a year of death.
Monash University's Department of Forensic Medicine, together with colleagues from the Australian National University (ANU) and The Australian Nuclear Science and Technology Organisation (ANSTO) recently conducted the study of the bomb pulse dating technique, by assessing samples where the year of birth and death were already known.
The study measured the artificial carbon 14 (14 C) content in samples of hair, nail and puparia (insect casings) collected from donors at the Australian Facility for Taphonomic Experimental Research (AFTER).
It found the radiocarbon results from nail samples provided the most accurate year of death estimation, with 91 per cent correctly predicting YOD, closely followed by hair, with a 79 per cent correct prediction rate, with both hair and nails having a lag time of 0–1 years. Puparia was found to have the highest levels of 14 C and, as such, was the least consistent with the actual YOD prediction, with only 46 per cent correct.
The findings are now published in the Journal of Forensic and Legal Medicine.
Bomb pulse dating is a scientific method that measures the levels of artificial carbon 14 (14 C) in human tissue to provide an accurate year of death (YOD) in cases where the identification of a deceased person is unknown. Widely used across Europe and the United States, but still relatively new in Australia, the method came about as an unexpected byproduct of above-ground nuclear testing in the 1950s until its ban in 1963.
Humans absorb naturally occurring radiocarbon from the atmosphere, but during the nuclear testing phase, vast amounts of artificial 14 C was absorbed into the biosphere and oceans as carbon dioxide (CO2). This CO2 was absorbed into plants via photosynthesis and consequently passed along the food chain, giving all living organisms since 1950, including humans, heightened levels of 14 C detectable in tissue.
First author Eden Johnstone-Belford, a PhD student at the Monash University Department of Forensic Medicine, said it was important to explore the method in the Australian context in more detail as everyone will have a different level of 14 C depending on when they were born, what they eat and their lifestyle.
"Carbon 14 (14 C) is a naturally occurring isotope of carbon," she said, "but levels have increased throughout the fifties and sixties when vast amounts of artificial carbon were released into the atmosphere due to nuclear bomb testing. The levels almost doubled in 10 years, creating a significant peak. Once the ban was in place, levels have been declining back to pre-nuclear testing levels, creating a bomb pulse curve measurement used by scientists. This curve has recently been updated with modern Australian values, allowing for the analysis of recent (until 2020) samples."
While the bomb curve details the environmental levels of 14 C throughout the years, the 14 C content within human tissues is dependent on a variety of factors, such as age and diet, with the most important being the rate of tissue turnover or replacement. As tissues are replaced, they undergo carbon turnover, thus incorporating new levels of 14C. Not all tissue turnover is consistent throughout the human body, and as such, different tissues will contain different 14 C levels.
When estimating YOD, tissues with a fast carbon turnover time such as soft tissues, or with a fast formation rate such as hair or nails are preferred as they contain the most recent levels of 14 C, with previous studies demonstrating levels of 14 C within three years of the actual YOD.
While soft tissues are typically the first to decompose and are often not present in forensic anthropological casework, hair and/or nails survive for longer periods, sometimes decades after death.
Lead author Adjunct Professor Soren Blau, Senior Forensic Anthropologist at the Victorian Institute of Forensic Medicine (VIFM) said: "In cases of long term unidentified human remains where methods of identification such as DNA, fingerprints or dental are not possible, then bomb pulse dating can be considered as a tool to assist in the identification process."
Read the full paper in the Journal of Forensic and Legal Medicine (2022) titled: Examining the use of different sample types following decomposition to estimate year of death using bomb pulse dating. Vol 85 DOI: 10.1016/j.jflm.2021.102275
