Strontium Ratios May be Affected by Agriculture
By measuring the ratios of strontium (Sr) isotopes, specifically 87Sr and 86Sr, archaeologists have been able to make some fascinating strides in understanding the patterns of human migration in various regions of the world.
Once it was figured out that 87Sr/86Sr ratios in the soil vary by geographic location, researchers started looking at the strontium values present in human remains then comparing with the values cataloged by geographic location. If the skeletal remains of an individual show a value of xyx and the region the remains were found in have a value of yxy, then researchers know this person migrated from somewhere else.
The researchers then look at a list of data that shows all the collected reference values and finds the value of xyx, which reveals where the individual originated before migrating. As methods go, this one is pretty solid given the wide variation of strontium ratios in soils from one geographic locality to the next. But the method is only as good as the data in the reference list.
In a recent study by Erik Thomsen and Rasmus Andreasen in the journal Science Advances, the authors show that strontium ratios in low to non-calcareous soils may need to be re-assessed. This is because of the use of agricultural lime, which is introduced to some soils in order to allow crops to take in more nutrients or to increase the pH of otherwise acidic soils.
The unfortunate drawback is that the lime-treated farmland can change strontium ratios and may be throwing off our reference maps and lists since they were created using modern isotope values. Luckily, the misleading values can be avoided by taking reference data at pristine (unaffected by human agriculture) locations when building reference maps.
Thomsen, Erik and Rasmus Andreason (2019). Agricultural lime disturbs natural strontium isotope variations: Implications for provenance and migration of studies. Science Advances, 5(3), DOI: 10.1126/sciadv.aav8084 (http://advances.sciencemag.org/content/5/3/eaav8083)
Using DNA from Historic Artifacts to Determine Sex and Ancestry
One of the most challenging aspects of historic archaeology is using an artifact to derive information about an individual. Occasionally this can be done by associating gender to an item like a shoe, which can also provide a tentative stature of the individual. But there are limits.
Recently, archaeologists working at an historic site in Maryland, specifically the Belvoir plantation site near Annapolis, used DNA obtained from two clay pipe stems. Recovered from an African American slave quarter on the plantation, the DNA results suggest that both stems belonged to–or were at least used primarily by–women.
A couple of mathematical algorithms were applied to the samples in order to narrow down ancestry. Using the ADMIXTURE method (this estimates population allele frequencies simultaneously with ancestry proportions) on one of the stems showed that one of the women was of African ancestry. The haplogroup identified was L3e, a subclade of one of the major African haplogroups.
When the PCA (principal component analysis) method was applied, the results showed the same woman to be related to the Mende people living in present-day West Africa–specifically Sierra Leone.
This small bit of genetic data says something specific about an individual. Somebody who once had a name, an identity, and people that loved her. But it also provides a hint at a connection between the Belvoir slave population and West Africa, something supported by various historic documents of the slave trade.
The archaeologists working at the Belvoir plantation were hoping to apply DNA analysis to one or more artifacts and, in their planning, they kept a sterile collection kit on hand in order to retrieve and store artifacts that might potentially contain historic DNA. The future of DNA collection at similar archaeological sites will depend on similar planing and preparation, but could provide valuable data linking artifact assemblages to ancestry.
Schablitsky, Julie M.; K. Witt; J. Madrigal; M. Ellegaard; R. Malhi; and H. Schroeder (2019). Ancient DNA analysis of a nineteenth century tobacco pipe from a Maryland slave quarter. Journal of Archaeological Science, 105, 11-18. https://doi.org/10.1016/j.jas.2019.02.006