How was the Vasa preserved?
Bacterial degradation of waterlogged wood
At the sea floor bacteria deplete the dissolved oxygen by decomposing organic material. Most preserved wooden shipwrecks have been buried in mud or clay where the anoxic conditions hinders wood-degrading microbial activity. How could then the large exposed wooden hull of the Vasa with its 20 m high after-castle, remain so well preserved after 333 years at a depth of only 32 m?
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| So-called erosion bacteria can slowly degrade the cellulose of waterlogged wood even under near-anoxic conditions (Blanchette 1990, Björdal 2000), leaving a weak “house of cards” of lignin-rich cell wall lamellas, filled with wet slime from the bacteria. The figure (left) shows a cross-section through wood showing fresh (lighter) and eroded (darker) cells. The lumen (grey, empty space) in the middle of the cell is much larger in the spring (cells on the left side). (right) SEM (scanning electron microscope) picture of bacteria (B, about 2µm) eroding marine-archaeological wood (Björdal 2000). Photos: Charlotte Björdal |
Only the outermost 10-20 mm of the surface layer of the Vasa's wood shows degradation by bacteria after 333 years on the seabed. This is because of several circumstances: 1. The shipworm, Teredo Navalis, does not thrive in the brackish Baltic Sea water, the salinity was only 4-6 promille at the site of the Vasa. 2. Low, fairly constant, water temperature (0 - 5 oC) at the depth of the Vasa, without strong eroding tidal currents, 3. The increasing amounts of organic matter (sewage, latrine, etc.) deposited into Stockholm's harbour. In the polluted water of Stockholm's harbour bacteria used sulfate ions, SO42-, as an oxygen source to break down the organic compounds, and then produced poisonous hydrogen sulfide dissolved in water, H2S(aq). Thus, the near oxygen-free (seasonal variations occurred) polluted water with occasionally high H2S(aq) concentration prevented attacks from fungi, rot and other wood-degrading microorganisms. This is an important special reason why the Vasa's exposed hull was so well preserved, but is also the origin of the present sulfur problems.
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