Timber species of Nanhai No. 1 shipwreck
According to the analysis on the timber species, the wood components in the Nanhai No. 1 sunken ship were mostly Pinus massoniana, Terminalia hainanensis and Fokienia hodginsii. Other types of timber species had a lower proportion. Because of the natural characteristics, P. massoniana, T. hainanensis and F. hodginsii had been used to build ships for a long time in China [23, 28]. It was found that the timbers shown in this study were the main materials of other ancient Chinese sunken ships, which were wooden ships and excavated along the Chinese coast [33,34,35,36].
In contrast, it had been proven by ancient sunken ships that all species of pines, fir and cypresses were used as the main materials to build ancient ships in other countries, especially in Europe. It might be due to the smaller sizes of these wooden ships as compared to Chinese ancient wooden ships. Some species of medium and small arbor trees could be used to build the ship components. For example, in several ancient wooden ships (with the length of 4–14 m) found in Pisa, Italy, 18 wood species were identified. Besides pines, fir and cypresses, Fraxinus excelsior, Juglans regia, Ulmus cf. minor, Quercus sp., Alnus cf. glutinosa, Fagus sp., Ficus carica, Olea europaea, Populus alba, Cornus sp. and Salix sp. were used as materials for ship building as well [37, 38]. Similar cases were also reported on another 3 ancient sunken ships from Naples, Italy [39]. Another case appeared on a sunk flat-bottomed barge of the Roman Empire period (AD 3rd Century) which was found in the Ljubljanica River of Slovenia, the major components of the barge were made from beech wood (Fagus sylvatica) [40].
The bacterial erosion degree of the wooden samples from Nanhai No. 1 shipwreck
According to Klaassen [31], different timber species presented different erosion degrees. For example, the samples from Pinus massoniana were basically in a minor erosion level, and the samples from Terminalia hainanensis had moderate to heavy erosion. A similar result was also observed in a previous study on shipwrecks [41].
Although Klaassen’s assessment system for bacterial erosion on the woods was mainly derived from spruce, pine, oak, alder and fir, the basic principle was evaluation of the erosion degree to the xylem cells of the trees. Thus, this assessment system is an important reference for this study. A few studies showed that the process of wood bacteria erosion could be affected by many factors [42,43,44,45,46]. Therefore, the physiological and biochemical reactions in the process of bacterial erosion of wood need to be further studied.
The interior changes of the shipwreck wooden components
By the observations and comparisons on the 20 wooden samples from Nanhai No. 1 shipwreck, with EM and LM, it was found that well-preserved samples (hard wood) had no traces of microbial internal erosion, with intact and tightly connected cells. However, those soft wood samples eroded by bacteria, EB and TB. The results showed, that in all severe erosion wooden samples, almost all cells were eroded by bacteria, with only the medium lamella framework in the entire cell wall left, or the S3 layer retained. The cell cavity was filled with the erosion residue, composed of bacterial mucus, bacterial fragments, and lignin of the cell wall (Fig. 6). Although not all of the erosion characteristics were clearly identified, it can be speculated that there was no fungus erosion based on the observation of the integrity of the middle lamella, the distribution of the erosion residue and the location of the shipwreck site. In many cell cavities, black and ordered stripes were visible on the stack tunnels, which is a typical feature of TB erosion [47].
In addition, the presence of bacterial erosion activity was observed in some cells, exhibiting different distribution patterns and activity characteristics from that of EB and TB, with no erosive cell composition, but only black erosive residue left around the bacteria. When bacteria erosion of pine was observed, Björdal et al. [48] found that the eroded residue density, near to the middle lamella, was lower than that in the cell cavity. They suggested that it was an indication of secondary bacterial degraders which feed on the eroded residue. This bacterium had no degradation capability on the normal plant cellular components, and the site (place or room) where they stayed could be clear. They could be called “cleaning bacteria”.
In general, the wood samples of the Nanhai No. 1 shipwreck showed absent to severe erosion by bacteria in cell structures. As compared to the hull of Xiaobaijiao No. 1 shipwreck, the wood erosion of a wooden merchant ship, sunk about 200 years ago (Qing Dynasty) in Zhejiang, was more serious [41]. Another case was from Nan’ao No. 1 shipwreck, a wooden ship sunk east of Guangdong about 600 years ago (Ming Dynasty) [19]. With EM observations, it was found that most of the wood samples were seriously corroded. The third case was from Huaguangjiao No. 1, a Song Dyansty Chinese ancient shipwreck found in Huaguangjiao Island, in the south middle of the Xi’sha Islands 1996 [49]. Shen et al. [50] found that 5 wood samples from this shipwreck were degraded between serious and total erosions. Based on the information from archaeological excavations of the Xiaobaijiao No. 1 shipwreck, a part of the ships keel and floor plates were exposed to the seabed surface, and other parts of the ship were covered by about 0–60 cm thick of cracked oyster shell and sandy sediment [11]. The Nan’ao No. 1 shipwreck sunk site was in the reefs, although the wreck was surrounded by soft sand, the hull was exposed to the sea water for a long time, and the water current there was also faster [13]. The original location of the Nanhai No. 1 sunken ship was in the silt bottom circumstance location, 24 m below the sea surface, and the whole surface of the shipwreck was covered by about 2 m thick silt. By the underwater exploration, it was confirmed that the ship’s hull was well preserved, and the ship structure under the upper and lower decks was basically complete [20]. By comparing the sunken site circumstances of different shipwrecks, it can be found that, after sinking, the former ships (Xiaobaijiao No. 1 and Nan’ao No. 1 shipwrecks) were located at the high O2 content circumstances with water flowing quickly and water exchanging frequently, facilitating the survival and reproduction of aerobic bacteria and boring animals.
On the contrary, the Nanhai No. 1 shipwreck was sunken down to a place where the seabed was relatively flat, water flowed slowly, and more suspended particles were settled. The hull of Nanhai No. 1 shipwreck was almost covered completely by a thick silt layer. The silt layer had played an objective role in isolating the wreck itself from the seawater. Between marine sediments and water, material exchanges always happen [51], while the exchanges mainly occur in the place between the water and shallow surface of the sediments [52,53,54,55,56]. Thus, after the sinking of the ship, material exchange between the sediment deposited on the ship and the overlying water happened.
The sediment pore water could also play a role in the hull degradation. With the thickening of the sediments, the material exchange between the sediment–water interface in the deep sediments were weaker and weaker. It formed a relatively stable environment for deep sediment pore water, and the influence of the sea water changes on the ship hull was relatively small as well. At the macroscopic level, the hull timber of the Nanhai No. 1 was preserved much better than that of Xiaobaijiao No. 1, and Nan’ao No. 1. At the micro level, the number of bacteria detected in the Nanhai No. 1 wood was much less than that of Xiaobaijiao No. 1 [57]. The silt-covered anaerobic environment was actually protecting the ancient sunken ships, of which was also found in European countries [38]. The cultural relics protection departments in some European countries follow the silt-covered principle to protect the wreck site and the original site of the ancient sunk ship [58].