Evaluation of in situ preservation method applied at a terrestrial archaeological shipwreck site by use of sacrificial wood samples installed for 25 years

In 1995 remains of a 13th century shipwreck, “the Kronholmen cog”, was discovered in sandy soil at a golf green on the island of Gotland, Sweden. Prior to backfilling and in situ preservation, four sacrificial wood samples (SWS) of sound pine sapwood were installed for long-term monitoring of wood decaying processes. In year 2020, the site was re-opened to evaluate the protective nature of the site. All SWS and six archaeological samples were taken for a comparative analysis. Observations by light- and scanning electron microscopy showed severe microbial decomposition by fungi and bacteria. Based on occurrence of decay forms in the wood fibres, soft rot (SR) was identified as the main degrader of both SWS and in the first 2 cm of the heavily eroded archaeological material, accompanied by bacteria (type tunnelling bacteria (TB)). Moderate decay by bacteria (type erosion bacteria, EB) in archaeological samples indicated longer period during the last 700 years under waterlogged and anaerobic condition – e.g., protective. Only initial decay of erosion bacteria (EB) was observed in SWS, strongly indicating a non-protective environment during the last 25 years. Severe brown rot decay was established in SWS and archaeological wood located in the front of the wreck where burial conditions were most critical (0.5 m above groundwater level, and 0.4 below soil/air interface). Here, a total of 0.50 cm surface layer of SWS was lost, yielding an initial decay rate of 20% in 25 years. In general, surfaces were heavily attacked especially for archaeological samples, and plant roots penetrated the soft and degraded surface layer. We conclude that the site since backfilling in 1995 has been semi-oxygenated and has promoted more severe fungal decomposition of cultural heritage. Thus, the site is not able to protect and preserve the precious archaeological remains for future generations and actions must be taken. SWS are concluded to be an important technique for long term monitoring of archaeological sites preserved in situ.