Monitoring and Mitigation Works in Unsaturated Archaeological Deposits

Monitoring of the archaeological deposits at the World Heritage Site Bryggen in Bergen has been ongoing since 2001. In latter years a large-scale project of mitigation works has been carried out, resulting in the creation of a water-management system aimed at raising groundwater-levels and increasing soil moisture content in areas with poor preservation conditions. Oxygen is a key parameter in the decay of archaeological material in the unsaturated zone, and the monitoring at Bryggen includes in situ monitoring of oxygen concentrations and comparison to soil moisture content, temperature, groundwater-level, precipitation and soil reactivity. This is used to document where and when decay takes place and to estimate “how wet is wet enough” in order to reduce the oxygen diffusion and the decay rate to an acceptable level. The results show that even small changes in the soil moisture can have a large impact on the oxygen dynamics in the ground.     

Take the Right Decision Everybody

Abstract

The archaeological deposits in medieval towns are among the most important and distinctive heritage monuments in Norway. At the same time they are among the more challenging phenomena confronting heritage management authorities, municipal planners, and property owners/developers alike, especially in relation to building and infrastructure projects. The modern settlement has developed on top of medieval and younger deposits which means that not only are they an irreplaceable depository of historical information, but they also form a significant part of the modern town’s physical foundation.

Since 2002 the Directorate for Cultural Heritage in Norway (Riksantikvaren) has been funding systematic monitoring of archaeological deposits of the World Heritage Site Bryggen in Bergen. The monitoring programme consists of several approaches: archaeological assessment of the deposits state of preservation, biochemical investigation of preservation conditions within the deposits, hydrogeological mapping of the water table, water flow, and other given parameters.

Continuous systematic monitoring by using testable, replicable methods and measures, data, and results acquire increased quality and validity. These in turn provide the cultural heritage management with a toolbox for making correct decisions and thereby allow the government’s preservation targets to be attained. But, most important, it guarantees the preservation of the ‘underground archives’ and at the same time allows the urban centres to develop.

This paper presents the knowledge developed through monitoring the Bryggen site as a basis for an official Norwegian standard covering archaeological, biochemical, and hydrogeological deposit investigations.     

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.     

The Influence of Soil Moisture,Temperature and Oxygen on the Oxic Decay of Organic Archaeological Deposits

The sensitivity of organic‐rich archaeological layers at Bryggen in Bergen, Norway, to changes in soil temperatures, water contents and oxygen concentrations is investigated. This is done by linking measurements of oxic decay at varying temperatures and water contents with on‐site monitoring data using a one‐pool decomposition model. The results show that the model can be used to elucidate the current in situ decay and to evaluate where and when the decay takes place. Future investigations need to include long‐term incubation experiments and decay studies at zero or very low oxygen contents in order to improve the robustness of predictions.     

Research on Conservation State and Preservation Conditions in Unsaturated Archaeological Deposits in Oslo

Abstract

Archaeological, biological, and geochemical investigations of soil sample series from a section and five boreholes have been studied to evaluate the state of preservation at the time of investigation and the preservation conditions of urban archaeological deposits from medieval Oslo, Norway. Focus has been put on the most fragile deposits in the unsaturated and fluctuation zones. Even with limited investigations, it is possible to assess the archaeological conservation state and the preservation conditions. Further work should focus on interpreting measured results and on mitigation strategies.     

Effects of a New Hydrological Barrier on the Temperatures in the Organic Archaeological Remains at Bryggen in Bergen,Norway

The World Heritage Site of Bryggen in Bergen, Norway, has experienced significant degradation of archaeological deposits as a consequence of changes in the soil water and groundwater balance after urban redevelopment adjacent to the heritage site. Additionally, groundwater temperatures below the heritage site were found to be significantly higher closer to the redeveloped area. One of the main mitigation measures taken to reduce the degradation of the archaeology has been the construction of a hydrological barrier along the sheet piling that divides the redeveloped area and the historic site. A shallow subsurface infiltration system was designed to achieve groundwater levels and flow conditions that are optimal for the preservation of archaeological remains directly along the sheet pile, while reducing drainage and subsidence also further upstream. Monitoring of groundwater level and temperatures after implementation of the hydrological barrier shows that groundwater levels and flow conditions have improved with respect to optimal preservation conditions, and groundwater temperatures have generally been reduced by up to 2 °C.     

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, ?esh, 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-?rst century. This may have a signi?cant 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 ?rst 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 ?eld 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.     

Research and Monitoring on Conservation State and Preservation Conditions in Unsaturated Archaeological Deposits of a Medieval Farm Mound in Troms and a Late Stone Age Midden in Finnmark,Northern Norway

This paper presents archaeological observations and results of palaeoecological and geo-chemical analyses of archaeological deposits from two rural sites in northernmost Norway. These are combined with climate data and the first period of continuous monitoring of soil temperature, moisture, and redox potential in sections. This data constitutes the basic research material for evaluations of conservation state and preservation conditions. The data has been collected in collaboration with the partners of a cross-disciplinary project: ‘Archaeological Deposits in a Changing Climate. In situ Preservation of Farm Mounds in Northern Norway’ funded by the Norwegian Council for Research (http://www.niku.no/en/archaeology/environmental_monitoring/archaeological_deposits_in_a_changing_climate_in_situ_preservation_of_farm_mounds/). This is an important Norwegian research initiative on monitoring of rural archaeological deposits, and the results have consequences for heritage management of a large number of sites from all periods. Palaeoecological analyses and redox measurements have revealed ongoing decay that might not otherwise have been detected. Decay studies indicate that both site types may be at risk with the predicted climate change. Some mitigating acts are suggested.     

A Qualitative Approach for Assessment of the Burial Environment by Interpreting Soil Characteristics; A Necessity for Archaeological Monitoring

Abstract

Modern-day archaeological monitoring is often hampered by lack of money, lack of time, inadequate measuring equipment, and lack of insight in the conservation potential of a site.

Although in modern archaeological excavations soil characteristics are noted (colour, texture, groundwater level, and sometimes mineralogy), these characteristics are mainly used for the interpretation of a site. However, by looking to these characteristics from a conservational view eventually combined with the conservation status of the archaeological objects, much can be learned about the burial environment. This is essential for optimizing archaeological monitoring.

Degradation processes result from the change of reactive phases in the soil or the site. Reactive phases are soil components such as organic matter, sulfides, iron(hydr)oxides and carbonates (chalk, shells), and, if present, components in the ground or interstitial water such as hydrogen ions and sulphate. The presence of these phases can easily be established by the archaeologist or soil scientist in the field. We propose a simple field-based method for assessing degradation processes essential for in situ preservation and monitoring.     

In situ Measurements of Oxygen Dynamics in Unsaturated Archaeological Deposits

Oxygen is a key parameter in the degradation of archaeological material, but little is known of its dynamics in situ. In this study, 10 optical oxygen sensors placed in a 2 m deep test pit in the cultural deposits at Bryggen in Bergen have monitored oxygen concentrations every half hour for more than a year. It is shown that there is a significant spatial and temporal variation in the oxygen concentration, which is correlated to measured soil characteristics, precipitation, soil water content and degradation of organic material. In these deposits oxygen typically occurs when the air content of the soil exceeds 10–15% vol, while oxygen dissolved in infiltrating rainwater is of less importance for the supply of oxygen in the unsaturated zone.     

Monitoring and Management Options in the Preservation of Urban Waterlogged Deposits,Nantwich, UK

A baseline survey in 2007 characterized the physical and chemical conditions for preservation within waterlogged remains at Nantwich. Installation of eighteen dipwells has allowed a five-year monitoring programme to be conducted from 2011. Two add-on projects supplemented this monitoring programme by examining the different methods for redox measurement, and for soil moisture measurement using Time Domain Reflectometry (TDR).

Initial results show two main zones of preservation, with reducing conditions attributed to location of the deposits in the floodplain, and a second zone uphill with more variable conditions for preservation. This study shows that sediment-coring programmes combined with dipwell installations can provide useful data to assess and monitor in situ preservation conditions, which can help to formulate management strategies for conservation of waterlogged archaeological deposits. This paper focuses on the efficacy of the methodology, referencing in particular comparative monitoring techniques and lists a series of recommendations for future studies.     

A Ticking Clock? Preservation and Management of Greenland’s Archaeological Heritage in the Twenty-First Century

ABSTRACT

Evaluating the rate of deterioration at archaeological sites in the Arctic presents several challenges. In West Greenland, for example, increasing soil temperatures, perennial thaws, coastal erosion, storm surges, changing microbial communities, and pioneer plant species are observed as increasingly detrimental to the survival of organic archaeological deposits found scattered along the country’s littoral zones and extensive inner fjord systems. This article discusses recent efforts by the REMAINS of Greenland project for developing a standardised protocol that defines the archaeological state of preservation, the preservation conditions, and asset value of organic deposits. Special emphasis is given to the degradation of materials such as bone and wood that are historically observed to be well-preserved in Greenland but now currently at risk. The protocol provides a baseline for monitoring future changes and will assist archaeologists in Greenland with a procedure for documenting and predicting areas of increasing vulnerability due to a warming climate.     

Quebrada Tacahuay,Southern Peru: A Late Pleistocene Site Preserved by a Debris Flow

Abstract

Here we describe the properties of a debris flow apparently generated by a warm phase El Niño event that buried an intermittently occupied Late Pleistocene forager site located in the southern coastal desert of Peru. Although the event deposited roughly one meter of sediment over the initial occupation, our analyses of debris flow dynamics and data from large scale archaeological investigations indicate that the earliest anthropogenic deposits at the site of Quebrada Tacahuay were preserved intact as a result of the burial episode; there is no evidence that the debris flow scoured or disturbed the cultural deposits. The event that buried the oldest archaeological contexts at Quebrada Tacahuay differs from other flood events that are characterized by turbulent, fast-moving floodwaters. Our data on debris flow dynamics and our results from excavation have implications for identifying, investigating, and interpreting other deeply buried archaeological sites both in the central Andes and in other geographic regions.     

Monitoring the burial environment of terrestrial wet archaeological sites in The Netherlands

Abstract

Monitoring the burial environment of archaeological sites is necessary to assess the success of their preservation in situ. Also, monitoring the state of preservation of actual archaeological remains together with that of their burial environment will further our understanding of the degradation processes acting on archaeological remains in situ. These remains consist not only of objects made from wood, metal, stone, etc., but also of pollen, soil features and even micromorphological features. Although, to date, the precise degradation mechanisms of archaeological remains in situ are not yet fully understood, general agreement exists on which parameters should be monitored in wet terrestrial environments. Also, it has been established that in situ measurements are preferable to laboratory analyses of soil (water) samples. In practice, it is difficult to find suitable monitoring equipment for in situ measurement as it must meet many requirements: an in situ measuring principle; stable for a period of at least several months; robust for use in the field; and equipped with a datalogger. A suitable principle exists for measuring the redox potential, however a simple, robust field instrument with datalogger is not yet available. Monitoring of the water table level, temperature and oxygen content is possible with recently developed, commercially available instruments. Monitoring of acidity is less complex as it does not vary as rapidly as, for example, the redox potential; however, the recommended method is still based on analysing soil samples, which is not acceptable in the long term at archaeological sites.     

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.     

Relax,Don’t Do It: A Future for Archaeological Monitoring

Preservation in situ and the monitoring of archaeological sites have become important themes since the acceptance and implementation of the Valletta Treaty. In the last few decades, our knowledge of degradation processes has increased manyfold, and a range of techniques have been tested and applied for use in both assessment and monitoring. Despite these successes, all is not well. First, we have little notion of the speed of the decay processes involved. This makes it difficult to distinguish between acute and protracted degradation. Apart from that, many assessments and subsequent monitoring projects rely (too?) heavily on complex and costly specialist technology. For any future preservation — in situ — projects low-tech observations together with best estimates of decay rates and archaeological site information should be combined to make an accurate assessment of the effects of decay on the archaeological record. Monitoring for preservation purposes is only appropriate if (1) decay processes occur within a relevant and measurable time scale, and (2) if mitigating actions can be taken or preservation ex situ can be performed (i.e. a rescue excavation) if significant degradation takes place.     

On the challenges and benefits of indoor archaeology: 15 years at the Archeodome (Mitchell Prehistoric Indian Village,South Dakota)

Abstract

Field archaeology is normally associated with outdoor excavation and exposure to the natural environment. Archaeological excavations have adapted to a wide spectrum of these conditions, but the recent prominence of archaeological sites as tourist attractions and educational facilities has occasionally led to dramatically different environments for the archaeological recovery, interpretation, and preservation of evidence, including facilities that permit indoor excavation. This article explores 15 years of experience at the Mitchell Prehistoric Indian Village in South Dakota. An “Archeodome” covering part of the site represents a non-traditional excavation and preservation environment that presents considerable benefits and challenges for archaeologists. The structure provides the basis for evaluating the nature of indoor excavation within its archaeological and educational context and provides a cautionary note for archaeologists, heritage groups, tourist boards, and others interested in the preservation of archaeological sites. Though this article focuses on the Mitchell site, the information reported has broad implications for sites where structures cover archaeological deposits.     

An Assessment of the Status and Condition of Archaeological Remains Preserved In Situ in the Medieval Town of Trondheim Based on Archeochemical Investigations Conducted During the Period 2007–2010

Abstract

In recent years systematic archeochemical investigations in the medieval town of Trondheim have provided the heritage management authorities with a rich and complex set of data concerning the status and condition of the cultural deposits in the anthropogenic material. The collected data raises important questions for the long-time management of in situ preservation for archaeological material in non-saturated zones. In this paper we present the standardized scientific methods used in these archeochemical investiga- tions. We examine the results from several sites in the town and discuss the challenges facing modern heritage management in its efforts to protect a complex body of archaeological material in the non-saturated zone. We show that sediments with a thickness of 1–2 m have low levels of moisture and organic matter, and most of the inorganic parameters analysed are found in oxidized form with low preservations. The 2–3 m thick anthropogenic sediments showed better preservation conditions.     

The use of geosynthetics for archaeological site reburial

Abstract

Applications of geosynthetic materials in reburial practice include geotextiles for separation, filtration and protection (cushioning); geomembranes and geosynthetic clay liners for infiltration control; geonets and geocomposites for sub-surface drainage; and geocells for erosion control. Mechanically stabilized earth reinforcement using geocells, geogrids and geotextiles can also provide substantial benefits for reburial projects by reducing lateral earth pressure against backfilled structures. Other aspects of modern geotechnology that may be useful for reburial projects include micro-piles and soil nailing for foundation and excavation support and evapotranspirative capping technology to establish the depth of soil cover required to isolate a structure or artefact from moisture and temperature fluctuations. Optimal application of these geotechnologies requires an understanding of the basic engineering principles associated with their implementation, as well as knowledge of the factors influencing archaeological site preservation.     

The Never-Ending Story? The Lessons of Fifteen Years of Archaeological Monitoring at the Former Island of Schokland

Abstract

The former island of Schokland became part of the mainland of the Netherlands when the Noord-Oost polder was drained in the 1940s. Mesolithic camp sites, Neolithic and Bronze age settlements, and medieval dwelling mounds (terps) on Schokland and in its immediate surroundings now form a UNESCO World Heritage site.

The main threat to the former island and the archaeology is drying out of the soil profiles, causing degradation of organic remains. Because of this, on the island and in its immediate surroundings the groundwater table is kept high in a specially created hydrological zone.

Schokland was one of the first sites in the Netherlands to be monitored in order to assess threats to the archaeological record and ongoing degradation processes. Monitoring started in 1999, and subsequent measurements were taken in 2001, 2006, and 2009/10. This included measurements of groundwater tables, water composition, redox, soil moisture and soil chemistry, micromorphology, and the degradation of botanical remains and bone. This time series of measurements makes it possible — first and foremost — to study long-term effects and changes in the Schokland burial environment, and their effects on the archaeology. In addition, the development of monitoring techniques around Schokland illustrates how the field of archaeological monitoring has evolved over the years.

Since the first monitoring round in 1999, developments in monitoring have included (1) technological developments enabling monitoring of high-frequency variations in groundwater, redox, and moisture contents, for example; (2) growing knowledge of degradation processes and the relevant characteristics of the burial environment; and (3) an increase in easily accessible datasets from third parties. Worries have arisen about the long-term storage and availability of monitoring data.

For future monitoring rounds, the value of the various monitoring techniques need to be critically evaluated, and the purpose of monitoring specific sites need to be reconsidered.