The deterioration of waterlogged archaeological wood by shipworm

Since in situ preservation of archaeological material has become more widespread (EuropeanUnion, 1992, s. online) the need to stop an active attack of Teredo navalis on waterlogged archaeological wood has become more urgent. In this study the ability of two plastic materials (TERRAM4000 and a plastic membrane) was examined to stop both initial attack by T. navalis and its effects on wood with active woodborer degradation on going.

Blocks of pinewood were submerged in the Northeast waters of Denmark. After settling and attack had been confirmed, the blocks were removed and wrapped in either TERRAM4000 (polypropylene and polyethylene) or a proprietary plastic membrane (polyethylene). An optical oxygen sensor was packed together with each block to measure the amount of available oxygen around the wood. After one week the oxygen level around the test blocks wrapped in the plastic membrane had dropped drastically and lead to the death of all shipworms within the test blocks after one-four weeks. Although no new shipworm attacked the wood wrapped in TERRAM4000, the geotextile did not impede the passage of oxygenated seawater, live individuals were found in the blocks after 46 weeks of wrapping and submersion.

The shipworm receives part of its nutrition through the digesting of cellulose in the wood structure (Betcher et al., 2012, s. 1). The preservation state of the waterlogged organic material is therefore an important factor in anticipating the attack of shipworm on archaeological material in situ. A distinct selective attack pattern was seen on slices of a 6000 year old waterlogged oak wood (Quercus sp.), as only the heartwood was degraded, while the surrounding sapwood was left intact. This study investigates the state which waterlogged archaeological wood would have to be in to enable an attack of T. navalis. This is analysed physically, chemically and visually where it can be concluded, that the amount of cellulose in the sapwood is very low compared to recent oak wood.