Henning Matthiesen

Environmental Monitoring at Nydam, a Waterlogged Site with Weapon Sacrifices from the Danish Iron Age. I: A Comparison of Methods Used and Results from Undisturbed Conditions

Abstract In situ preservation has become a common practice in the management of archaeological sites during the last decade. With this practice follows the need to have good, reliable methods for monitoring the environmental conditions at the sites. This paper describes the results from seven years of monitoring at the waterlogged site Nydam in Denmark. Results from a range of different monitoring methods are compared, and the advantages and disadvantages of each method are discussed. Measurements of water level, chemical composition, and redox potential are covered, with comparison of results from dipwells, filter probes and measurements directly in the soil. The results indicate that, with regard to methods 1) in situ measurements directly in the soil give the most reliable results, 2) analysis of water samples from dipwells and filter probes can give reasonable results except for a few species, 3) purging of dipwells before sampling improves the results when combined with a fast sample handling. With regard to the site 4) the burial environment at Nydam is permanently waterlogged, anoxic, slightly acidic, dominated by iron reduction and with a low concentration of dissolved sulphide, 5) there is a significant spatial and temporal variation of the concentrations of most dissolved species, 6) the redox potential as measured with permanently buried gold electrodes is very stable.

A Novel Method to Determine Oxidation Rates of Heritage Materials in Vitro and in Situ

Abstract A novel method for measuring oxidation rates of heritage materials is described. The method uses optical oxygen sensors, which allow measurement of the oxygen concentration through the transparent wall of a closed container. Three experimental designs are described: first, using glass containers in which the samples can be followed for extended periods; second, using bags composed of oxygen barrier film that have flexible dimensions suitable for irregularly shaped artefacts; and third, using glass dishes glued to the surface of larger artefacts, allowing measurement of oxygen consumption in situ. The flexibility of the method is especially important in studies of heritage artefacts, where it is seldom possible to take subsamples which suit fixed container dimensions, and where it may be necessary to study oxidation over extended periods (years). Ways to reduce the response time are also described.

The Use and Deployment of Modern Wood Samples as a Proxy Indicator for Biogeochemical Processes on Archaeological Sites Preserved in situ in a Variety of Environments of Differing Saturation Level

Abstract In situ preservation of archaeological sites is becoming an ever increasing trend as a means of preserving our cultural heritage. In connection with this the environmental conditions, such as water level, dissolved oxygen, temperature, pH and water chemistry, of a site are often monitored. It is generally agreed that a waterlogged and anoxic environment is essential for optimal preservation conditions but the set-up and maintenance of an environmental monitoring programme can be costly. This paper discusses the design and use of a system whereby modern samples of wood can, with a minimal disturbance of the soil, easily be deployed and retrieved from archaeological sites. The system was deployed in an unsaturated environment, an environment with fluctuating water levels and a fully saturated peat bog. The samples were assessed after two years using microscopic, physical and molecular biological methods, and the types of wood-degrading organisms seen were compared with the results of environmental measurements. Modern wood samples were used, as the microbial ecology of wood-degrading organisms in these different types of environments is relatively well documented. Preliminary conclusions show that the deterioration processes of modern wood samples in these environments act as a good proxy indicator of the environmental conditions and biogeochemical processes ongoing at a site.

Quantification and Visualization of In Situ Degradation at the World Heritage Site Bryggen in Bergen, Norway

Abstract Environmental monitoring at the World Heritage Site of Bryggen in Bergen, Norway, has shown damaging settling rates caused by degradation of underlying archaeological deposits. Measurements of piezometric head, oxygen, and soil moisture content, as well as chemical analyses of water and soil samples are key elements of the environmental monitoring. Groundwater monitoring and geochemical analyses reveal a complex and dynamic flow through the natural and anthropogenic stratigraphy. The preservation conditions within the organic archaeological deposits are strongly correlated with oxygen and soil moisture content, that are controlled by the groundwater flow conditions at the site. To quantify decay rates, it is thus essential to understand the wider hydrogeological context of the site. This paper presents recent advancements in quantifying decay rates in the saturated zone at Bryggen. The paper also shows that 3D geo-archaeological modelling can contribute to preservation management by visually combining results of geological, archaeological, geochemical, and hydrological investigations. This opens up for improved multidisciplinary understanding of preservation potential, thereby contributing to an improved protection of archaeological deposits in situ.

The Correlation between Bulk Density and Shock Resistance of Waterlogged Archaeological Wood using the Pilodyn

Abstract The correlation (R2 = 0.87) between bulk density and the shock resistance of homogeneously and heterogeneously degraded waterlogged archaeological wood, with densities ranging from 100 to 700 kg·m-3, was determined using the Pilodyn wood tester. The results showed that the energy required to fracture a given mass of wood substance was constant (R2 = 0.83) and independent of wood species. However, measurements in the three main orientations (longitudinal, radial and tangential) showed a significant variation and therefore further measurements were preferentially taken in the radial axis. A model to enable the determination of the average bulk density of waterlogged wood from the depth of penetration of the Pilodyn showed that the average bulk density of homogeneously and heterogeneously degraded waterlogged wood could be determined with an average accuracy of ± 9%. A high correlation coefficient (R2 = 0.995) for measurements taken in water and air enable the Pilodyn to assess waterlogged wood under water as well as in air.

In situ Preservation Solutions for Deposited Iron Age Human Bones in Alken Enge, Denmark

The rich mass deposition of Iron Age human bone material from the Danish site, Alken Enge, is extraordinary not only from an archaeological perspective but also from a preservation point of view. The main find is situated in a waterlogged, anoxic environment which provides excellent preservation conditions and therefore enables in situ preservation of the human bones. However, major differences in local environmental conditions challenge an in situ preservation of the entire site area as parts of the bone material presently deteriorate. In this paper, a multi-proxy environmental monitoring approach is used to document threats and to suggest the best preservation solution for the archaeological finds, whether in or ex situ.

The Future Preservation of a Permanently Frozen Kitchen Midden in Western Greenland

Abstract Archaeological materials may be extraordinarily well preserved in Arctic areas, where permanently frozen conditions in the ground slow down the decay of materials such as wood, bone, flesh, hair, and DNA. However, the mean annual air temperature in the Arctic is expected to increase by between 2·5 to 7·5°C by the end of the twenty-first century. This may have a significant warming effect on the soil and could lead to permafrost thaw and degradation of currently frozen archaeological remains. Here we present a four-year monitoring and research project taking place at Qajaa in the Disko Bay area in West Greenland. Qajaa is a large kitchen midden, containing frozen remains from 4000 years of inhabitation, from when the first Palaeo-Eskimos entered Greenland, until the site was abandoned in the eighteenth century. The purpose of the project is to investigate current preservation conditions through field and laboratory measurements and to evaluate possible threats to the future preservation. Preliminary results show that the archaeological material at Qajaa is still very well preserved, but some microbial decay is observed in the exposed wooden artefacts that thaw every summer. Maximum temperatures are above 0°C in the upper 40–50 cm of the midden and between 0 and −2°C down to 3 m depth. Thereby the permafrost may be vulnerable to quite small increases in air temperatures. Laboratory measurements show that the decay of the archaeological wood in the midden is temperature-dependent, with rates increasing 11–12% every time the soil temperature increases 1°C. Moreover, the soil organic material produces heat when decomposed, which could have an additional warming effect on the midden. At the moment the water or ice content within the midden is high, limiting the subsurface oxygen availability. Threats to the future preservation are related to further thawing followed by drainage, increased oxygen availability, microbial decay of the organic material, and heat production.

Climate change and the loss of organic archaeological deposits in the Arctic

The Arctic is warming twice as fast as the global average with overlooked consequences for the preservation of the rich cultural and environmental records that have been stored for millennia in archaeological deposits. In this article, we investigate the oxic degradation of different types of organic archaeological deposits located in different climatic zones in West and South Greenland. The rate of degradation is investigated based on measurements of O2 consumption, CO2 production and heat production at different temperatures and water contents. Overall, there is good consistency between the three methods. However, at one site the, O2 consumption is markedly higher than the CO2 production, highlighting the importance of combining several measures when assessing the vulnerability of organic deposits. The archaeological deposits are highly vulnerable to degradation regardless of age, depositional and environmental conditions. Degradation rates of the deposits are more sensitive to increasing temperatures than natural soils and the process is accompanied by a high microbial heat production that correlates significantly with their total carbon content. We conclude that organic archaeology in the Arctic is facing a critical challenge that requires international action.

Making Better Use of Monitoring Data

This paper addresses the knowledge gap that exists in relation to understanding and quantifying the sensitivity of organic-rich archaeological deposits with respect to changes in the soil environment. Based on two case studies we demonstrate that it is possible to quantify the current decay rate in unsaturated archaeological deposits by combining decay rates measured in the laboratory with on-site monitoring data in a simple decay model. The decay of organic archaeological deposits is highly sensitive to variations in soil temperatures and soil water content. Measurements of soil water content cannot always stand alone as a representative measurement of oxygen availability; which suggests that in situ measurements of oxygen content or redox potential are needed in order to understand the preservation conditions at a site. The results of this study emphasize the advantage of combining monitoring data with laboratory studies, in order to document in more detail where and when degradation takes place.

Impact of Roots and Rhizomes on Wetland Archaeology: A Review

The general premise for successful archaeological in situ preservation in wetlands is that raising the water table will ‘seal the grave’ by preventing oxygen from reaching the deposit. The present review reveals that this may not be the entire picture, as a change in habitat may introduce new plant species that can damage site stratigraphy and artefacts. However, reviews on the types and degree of damage caused by vegetation to archaeological remains preserved in situ in wetlands have hitherto only been sporadically treated in the literature. Thus, this paper provides an overview of the adverse effects that various plants species have on the preservation status of wetland archaeology.Disturbance, due to growth of roots and rhizomes of the surrounding soil is denoted contextual disturbance, whereas deterioration of archaeological remains per se acts by several root-related factors that may be spatially and temporally concomitant. In waterlogged anoxic environments, deterioration is mainly related to (i) preferential growth of roots/rhizomes due to nutrient uptake and lesser soil resistance, (ii) root etching due to organic acid exudates, (iii) microbial growth due to root release of oxygen and labile organic compounds, and/or (iv) precipitation of hydroxides due to root release of oxygen. For example, roots of some wetland plants, such as marsh horsetail (Equisetum palustre), have been documented to penetrate archaeological artefacts down to c. 2 m in waterlogged anoxic soils. Here, we demonstrate that cultural heritage site management may unintentionally introduce deep-rooted or exudate aggressive plants by invoking change in hydrological conditions. Moreover, the implementation of biomass energy utilization and agricultural root depth optimization on a worldwide basis stresses the need for more research within root and rhizome impact on archaeological remains in wetlands. In conclusion, the worst-case scenario may be in situ deterioration instead of preservation, and one essential threat to archaeological wetland sites is the impact of wetland vegetation.