In situ δ13C analysis of <1mm thick annual growth rings in archaeological wood samples via LA IRMS

Archaeological wood holds value not only as a source of information on how people used to live centuries or millennia ago, it is also a valuable proxy for reconstructing past climatic and environmental changes. When the sample amount available for destructive analytical methods is limited it forces the research team to judiciously prioritize what information to extract and which method to use. When it comes to light stable isotope analyses, the most widely used instrumentation requires rather intensive manual sample preparation and the prepared sample cannot be recuperated after analyses.

The elemental analyzer is currently the go-to sample introduction peripheral for stable isotope analyses of tree rings, but as any analytical method it has its draw-backs and limitations. A key limitation is that each growth ring must be individually separated mechanically and prepared for analysis; this challenge can be managed with sufficient time and manpower. However, very narrow growth rings (<1mm) are a clear limiting factor when each ring needs to be manually removed or when multiple analysis are required for each growth ring. Both issues can easily be circumvented by using a laser ablation (LA) module as sample introduction peripheral. Core segments or wood slices of up to 4.5 cm length can be analyzed in situ (including duplicates and triplicates) without further preparation. For archaeological wood, this method has the added benefits of being minimally invasive, the ablation tracks being practically invisible, and circumventing the need to sacrifice a portion of the artifact for analyses.

Our case study is a fragment of oak wood (Quercus sp.) provided by the National Museum of Denmark. The wood originates from construction timber found during an archaeological excavation of wells located near The Wadden Sea in south-west Denmark. The whole sample contains 199 growth rings and has been dendrochronologically dated to AD 407-605, covering the mid-sixth century where a global climate crisis caused a longer period of cold and wet growth seasons; this is also expressed in archaeological wood by the formation of extremely narrow growth rings. Because of growth ring widths down to 0.49 mm, it is challenging to separate and prepare wood material from each ring for stable isotope analyses using the traditional EA IRMS method.

Our LA IRMS setup comprises the isoScell Δ100 sample chamber (Terra Analitic), LSX 213 G2+ (Teledyne Photon Machines), CryoPrep and HS2022 IRMS (both Sercon). For δ¹³C a spatial resolution of 60μm is easily achievable, with precision on the QC of 0.08 ‰. Mean δ¹³C on the analyzed segment is -24.17 ‰ v. VPDB. The dataset is also in acordance with data from wider rings that could be analyzed via EA IRMS.