Human cadaver decomposition islands and forensic taphonomy: gravesoil δ13C and δ15N enrichment patterns in short (30 d) and extended (900 d) postmortem intervals

Soil forensics examines edaphic properties of evidentiary value in the immediate vicinity of human remains in a region called the cadaver decomposition island (CDI). As a body decomposes, soil in the CDI is permeated with liquified human nutrients and becomes gravesoil. This study is the first to specifically investigate how stable isotopes of carbon (δ¹³C) and nitrogen (δ¹⁵N) change in human gravesoils during decomposition over the postmortem interval (PMI). Gravesoil samples were analysed for %C, δ¹³C, %N, and δ¹⁵N in CDIs over the initial 30-d PMI (PMI₃₀) and over longer PMIs up to 900 d (PMI₉₀₀). Gravesoil %C and δ¹³C did not show any significant changes with decomposition progression in the PMI₃₀ study. By days 10-15, gravesoil mean %N more than doubled from 0.4% to 1.05%, while mean gravesoil δ¹⁵N increased from 3.3‰ to 8.2‰ by day 10, up to a high mean of 9.9‰ on day 20. Both %N and δ¹⁵N remained significantly elevated for the entire PMI₃₀ period. In the PMI₉₀₀ study, gravesoils increased from 0.2%-0.8% N to a high of 1.9% N after 2-3 months, then decreased to 0.5%-0.6% in the 600-900 d time frame. Gravesoil δ¹⁵N originating at 2‰-4‰ was enriched by 15‰-20‰ after the first month, eventually declining to 9‰-11‰ after 600-900 d. We conclude that δ¹⁵N in human gravesoils has promise as a forensic tool to detect a CDI and correlate it with decomposition stage that adds to existing methods in estimating PMI, even if the body is absent. <b>Key Points</b> Soil forensics examines the products of human decomposition and their impacts on underlying gravesoil.Various forms of nitrogen and carbon have proven to be a useful indicator of the presence of gravesoils.%C, δ¹³C, %N, and δ¹⁵N in CDIs were measured at different depths and distances from the remains in the short PMI (30 d) and extended PMI (900 d).When compared to control soils, δ¹⁵N and %N levels in gravesoil increased by day 10, peaked by day 20, and remained high over the 30-d study. In the longer PMI study, %N returned to control levels in the 600-900 d time frame, while δ¹⁵N levels remained elevated at the end of the experiment.We conclude that δ¹⁵N is a useful indicator of not only the presence of a CDI but also has tangible use as a PMI estimation tool for forensic taphonomists.