Bronze Age skeleton evidence contaminated by agricultural run-off
Isotope data showing ancient humans travelled long distances may be the result of farmers using lime. Andrew Masterson reports.
Research indicating that some Bronze Age people travelled far and wide may be completely wrong and the result of contaminated evidence, a pair of Danish scientists say.
Writing in the journal Science Advances, geoscientists Erik Thomsen and Rasmus Andreasen of Aarhus University suggest that isotope analyses of famous Danish ancient skeletons – such as the Egtved Girl and Skrydstrup Woman – might have been distorted by the presence of modern agricultural waste.
Previous analyses of both sets of remains indicated they had travelled long distances through their lives, including outside Denmark. The results were obtained by using a relatively common method – measuring the ratio of two isotopes of strontium in their bones.
Strontium isotope ratios derive from soils. They become bioavailable when they leach into water and are then absorbed into human and animal bones over time through the act of drinking. Because they are a reflection of soil composition, they vary by region and can therefore be used as a proxy record of travel.
In order to do this, of course, it is critically important to have an accurate reference map to enable comparisons.
Thomsen and Andreasen do not suggest that researchers looking at Egtved Girl, Skrydstrup Woman and other Bronze Age remains worked using defective soil data. However, they suggest that the ratio readings have been dramatically distorted by a chemical input not previously recognised – agricultural lime.
In Denmark – as in many other countries – lime has for a long time been added to fields to make soil more conducive to cropping.
The researchers wondered whether this common agricultural additive could affect strontium ratio levels.
To find out they collected 60 samples of surface water in areas where lime is not used, and compared it to 24 samples taken from where it is. Agricultural run-off, they found, dramatically changed the underlying strontium ratios, depending on the amount of calcium present in the soil.
The results were illuminating. Reinterpreting the Egtved and Skrydstrup data in light of the new numbers, they found that the variations in ratio levels – previously interpreted as evidence of long-distance travel – could have been achieved in just a 10-kilometre radius of the burial place of each.
The findings, they say, have implications for mobility studies based on strontium isotope ratios that have been conducted in many places around the world, given the widespread use of lime by farmers.