Strong Straightforward Strontium Strategy, yes Sr!
By Fadi Hajj
One of the main objectives of the ForSEAdiscovery project is to trace the provenance of wood from Iberian shipwrecks. The project’s period of study is 16th-18th centuries; i.e. the age of discovery and European expansion. The power of the project lies in its multidisciplinarity. In other words, the same questions will be studied from different points of view: humanities (archaeology and history) and life sciences (wood science).
Furthermore, beyond the project´s overall multidisciplinarity, the wood science aspect can also be divided into different components, revealing a multilevel research approach ranging from the tree-ring level (dendrochronology), through the cellular level (anatomy), into the molecular (organic analysis) and atomic levels (inorganic and isotopic analysis). Combining all these outputs together, and with the archaeological and historical results, we hope to all reach the same conclusions through different methods.
Since I am the youngest researcher in the ForSEAdiscovery family, I will be working on the project’s smallest scale level: the atomic level! My work focuses mainly on strontium (Sr) isotopes.
What is an isotope?
All chemical components are composed of atoms. Atoms are composed of protons p+ (positive charge ‘+1’), neutrons n0 (neutral charge ‘0’) and electrons e– (negative charge ‘-1’). Non-ionic atoms are always neutral, having a number of protons equal to the number of electrons. Isotopes are also neutrally charged, but they are heavier or lighter variations of the same element. They thus have the same number of protons and electrons but differ in the number of neutrons. The difference in the number of neutrons results in a difference in total mass of two isotopes of the same element. By using this mass difference, we can measure the ratio of the two isotopes in a sample. We focus here on the ratio of Sr-87 and Sr-86 isotopes, which is one of the best tools in provenance studies.
The strontium isotopic ratios in wood reflect the signature of the soil on which trees grow, which again reflects the signature of the bedrock underneath. In other words, different forests growing on different types of bedrock will have different Sr isotopic ratios. Therefore, a reference map can be created of this variation by analyzing wood from extant forests.
Any archaeological wood sample from any shipwreck was part of a tree in the past (hundreds of years ago) and registered the signature pumped from the soil underneath – this is what provides the signature of the site of origin for the tree.
We can use this fingerprint of the archaeological wood and compare it to the reference maps created through wood from living forests to “provenance” the timber.
Shipwreck timbers are not wood conserved in the bottom of the oceans. No. If we look closely (or, very closely by counting the number of each isotope in the wood composition!), we can understand that while being waterlogged by seawater, the wood faces contamination by microorganisms, degradation by fungi, alteration by shipworms, deterioration by living animals, chemical modification by seawater elemental composition, and physical transformations by the physicochemical conditions of the seabed environment…
Therefore, in order to determine the original fingerprint of the archaeological wood, we need first to eliminate all type of seawater contamination. The precious archaeological timbers need special pre-analysis treatment.
Wood samples have to undergo a several-step procedure before being analyzed. Following the shipwreck and forest sampling campaigns, wood is cleaned, dried, cut, ground and mineralized with concentrated acids.
One part of the mineralized solution will go through 2ml plastic columns containing special resins to separate the Sr atoms from the rest of the sample. These resins are then water-washed, and the resulting solution is analyzed for the 87Sr/86Sr isotopic ratio. The rest of the sample is also analyzed: we measure the concentrations of about 50 different inorganic elements. This inorganic elemental analysis supports the 87Sr/86Sr isotopic ratio by providing additional site-specific information for wood provenance.
We work in a clean room, at constant temperature, while wearing special attire to prevent any possible type of contamination by manipulation. The analyses are destructive but only ±200mg of wood are enough to get results with a precision of 0.00001.
Samples from the same trees and same shipwrecks are collected for the different analyses. The resulting data will be combined at the end of the project to refine the answer to the original question about the provenance of archaeological wood from Iberian shipwrecks. The provenance hypothesis will contribute toward reconstructions of timber trade routes for shipbuilding and early modern forestry practices.
Fadi Hajj is a PhD student working on isotopic geochemistry and inorganic analysis at the Université de Lorraine, France. The main objective of his thesis is to trace the provenance of wood from Iberian shipwrecks.