The wrinkles on "Tollund Man's" face are still visible, even though he died more than 2,200 years ago. Moss-covered wetlands in Denmark that mummified his body are ideal for preserving organic matter, offering archaeologists an extraordinary window into our distant past. But a recent excavation at a similarly swampy site in Sweden shows that these perfect conditions are fragile, and when they decompose, so do the bodies, bones, and other organic remains that have been preserved for centuries. The finding suggests that a long-standing principle of archeology - avoiding excavation and leaving artifacts in the ground for long-term conservation - should be revised, at least for some wetland sites.
Anecdotal evidence has long suggested that the condition of wetland excavated remains such as peatlands is declining, says Benjamin Gearey, a wetland archaeologist at University College Cork who was not involved in this study. For example, bone deterioration has been documented at Star Carr, an archaeological site in northern England. But it has been difficult to know how widespread the pattern is and how fast decomposition is occurring.
Ageröd, a bog in southern Sweden that contains bones, antlers, and other artifacts from Mesolithic cultures that flourished more than 8,000 years ago, is a good place to measure the rate of decay in a bog, says Adam Boethius, an archaeologist at Lund University. Boethius and his colleagues compared freshly excavated bones in 2019 with bones that had been exhumed from the swamp in the 1940s and 1970s and stored in the Lund University Historical Museum. They rated the weather resistance of each bone, from the well preserved ones, the ones that were shiny and without cracks, to the opaque bones with worn external surfaces.
The bones from the 2019 dig were so worn out that their scoring system was broken. Some had lost more than a half centimeter from their outer shell. Other sections of the site where they hoped to find remains did not shed bones at all, suggesting that they had completely decomposed. The best-preserved bones from the 2019 excavation were in roughly the same condition as the worst-preserved bones of the 1970s, they write today in MORE ONE.
A degraded boar's heel bone from the 2019 Ageröd excavation
They found that deterioration was already underway in the 1970s. The bones from these excavations were more worn than those excavated in the 1940s. Also, the same pattern appeared with the 2019 bones - the best-preserved bones of the decade. The 1970s were in conditions similar to the worst preserved bones of the 1940s.
The culprit is oxygen, the researchers say. The organic material trapped under the mossy surface of intact peatlands is devoid of oxygen, creating an environment too hostile for fungi and bacteria that normally decompose plants or bones. But with digging comes oxygen, which reacts with buried iron sulfide to produce sulfuric acid. Another factor is agricultural activity, which has slowly drained the wetland, damaging the protective surface and allowing oxygen to enter. Groundwater throughout the area has most likely become more acidic, says Boethius. This means that bones that are submerged in deep, moist layers will be quickly destroyed. The increasing frequency of extreme climate events induced by climate change, including droughts and floods, may also contribute to the problem, he adds.
The study is "sobering" and demonstrates the "catastrophic loss of irreplaceable organic archaeological remains" in Europe's wetlands, says Gearey. It also highlights the importance of researchers better understanding conservation and decomposition and raises questions about the predetermined strategy of preserving the remains in the soil, says University of Reading archaeologist Martin Bell.
"If you can't preserve it in the ground, you have to dig it up," says Gearey. With little understanding and monitoring of the conditions of wetland sites, leaving the treasure in the ground at sites like Ageröd is not an option for Boethius. "We need to dig now," he says. "If we wait 10 or 20 years, everything will be gone."
