Quantifying archaeo-organic degradation – A multiproxy approach to understand the accelerated deterioration of the ancient organic cultural heritage at the Swedish Mesolithic site Ageröd

Abstract

Despite a growing body of evidence concerning accelerated organic degradation at archaeological sites, there have been few follow-up investigations to examine the status of the remaining archaeological materials in the ground. To address the question of archaeoorganic preservation, we revisited the Swedish, Mesolithic key-site Agerö d and could show that the bone material had been subjected to an accelerated deterioration during the last 75 years, which had destroyed the bones in the areas where they had previously been best preserved. To understand why this has happened and to quantify and qualify the extent of the organic degradation, we here analyse the soil chemistry, bone histology, collagen preservation and palaeobotany at the site. Our results show that the soil at Agerö d is losing, or has already lost, its preservative and buffering qualities, and that pH-values in the still wet areas of the site have dropped to levels where no bone preservation is possible. Our results suggest that this acidification process is enhanced by the release of sulphuric acid as pyrite in the bones oxidizes. While we are still able to find well-preserved palaeobotanical remains, they are also starting to corrode through reintroduced oxygen into the archaeological layers. While some areas of the site have been more protected through redeposited soil on top of the archaeological layers, all areas of Agerö d are rapidly deteriorating. Lastly, while it is still possible to perform molecular analyses on the best-preserved bones from the most protected areas, this opportunity will likely be lost within a few decades. In conclusion, we find that if we, as a society, wish to keep this valuable climatic, environmental and cultural archive, both at Agerö d and elsewhere, the time to act is now and if we wait we will soon be in a situation where this record will be irretrievably lost forever.

Human encroachment, climate change and the loss of our archaeological organic cultural heritage: Accelerated bone deterioration at Ageröd, a revisited Scandinavian Mesolithic key-site in despair

“…Of these bone pieces, 61 fragments could be determined to species or family level, which means that the osseous material from Ageröd I encompass a total of 4241 determined bones (Table 1). In addition to the osseous remains, the 2019 excavation revealed additional hazelnut shells and a variety of seeds from different plants, which were recovered by flotation of soil samples from each trench [39]. Furthermore, two pieces of resin were also recovered on the excavation.…”

“…stratigraphically, and all archaeological layers (Fig 5 ) were sampled for soil analyses and macrofossils. The results from the soil and macrofossil analyses will be published separately, together with histological analyses [39]. To ensure maximum possible recovery and to avoid missing small fragments, two-step water sieving (5 mm mesh placed on top of a 2,5 mm mesh) was used.…”

“…The permission obtained from LUHM also included the sampling of six bones to conduct histological analyses (ca 3 g of bone per sample) and an additional 12 bone samples (0,5-1 g each) to investigate collagen deterioration (decision 29-08-2019). The results from these analyses will be published separately [39].…”

“…One indication of the processes involved can be gained from studying the bone histology, where the presence of oxidised pyrite was detected in bones from the trenches dug in the 2019 excavation campaign, while the histologically analysed bones from the 1940s and 1970s excavations showed only non-oxidised pyrite [39]. Pyrite is an iron sulphide and only forms under anoxic conditions due to the action of sulphate-reducing bacteria.…”

“…The released sulphuric acid will etch and damage the bone, and cause pH levels to drop in the area. Indeed, the fact that oxidised pyrite was only detected in the bones from the 2019 excavations and the bones from previous excavations show only non-oxidised pyrite, suggests that the local soil environment has gone from stable anoxic to oxic conditions [39].…”