Jørgen Hollesen

Accumulation of Soil Organic Carbon Linked to Holocene Sea Level Changes in West Greenland

Abstract Changes in the amount of soil organic carbon (SOC) stored in arctic soils may influence the global carbon cycle and be an important feedback mechanism to global climate changes. In order to estimate the carbon stock and accumulation rates at Flakkerhuk on Disko Island in West Greenland, an 1800-ha study area was divided into land cover types using a satellite image. Total SOC was estimated to be on average 67.3 ton C ha−1 (6.7 kg m−2) and the fen area contributing 42% of the total SOC. Soil profiles investigated at different terraces revealed that the SOC stock was significantly age-related, increasing six fold from a terrace dated to 7000 (BP) to one dated to 10,000 (BP). This equals an average soil C accumulation rate of 0.5 kg C m−2 100 yr−1. This rate was compared to vegetation-specific accumulation rates in the last 7000 yr which were in the order of 0.05 kg C m−2 100 yr−1 in heath and sparse vegetation, 0.4 in fen areas and between 2 and 5 kg C m−2 the last 100 yr in the present salt marsh. The study shows the importance of landscape history and age when sampling and evaluating SOC stocks and provides estimates of arctic soil C accumulation rates during Holocene versus present rates.

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.

Climate change and the deteriorating archaeological and environmental archives of the Arctic

Abstract The cold, wet climate of the Arctic has led to the extraordinary preservation of archaeological sites and materials that offer important contributions to the understanding of our common cultural and ecological history. This potential, however, is quickly disappearing due to climate-related variables, including the intensification of permafrost thaw and coastal erosion, which are damaging and destroying a wide range of cultural and environmental archives around the Arctic. In providing an overview of the most important effects of climate change in this region and on archaeological sites, the authors propose the next generation of research and response strategies, and suggest how to capitalise on existing successful connections among research communities and between researchers and the public.

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.

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.

Process‐Oriented Modeling of a High Arctic Tundra Ecosystem: Long‐Term Carbon Budget and Ecosystem Responses to Interannual Variations of Climate

Terrestrial carbon (C) cycling in high Arctic tundra depends on ecosystem responses toclimatic warming and concurrent changes in environmental conditions. There are very few studies toquantify long-term C budget in high Arctic tundra due to lack of sufficient measurements. Here based onwell-established multiyear measurements, we calibrated a process-oriented model (CoupModel) to quantifyvarious components of the C budget at a Cassiope tetragona heath ecosystem in northeast Greenland. Netecosystem exchange of CO2(NEE) for 2000–2014 was estimated at 15 ± 10 g C m2yr1. Ecosystemrespiration (ER) for nongrowing seasons was estimated at 10.3 ± 5.3 g C m2yr1, representing around13% of the annual ER. Significant trends for interannual variations of aboveground and belowground C fluxesand stocks were found for the subperiods (i.e., 2000–2008 and 2008–2014) but not for the entire period.Interannual variations of NEE largely relied on the response of gross primary production (GPP) and ER toseasonal changes in climate. Moreover, the model showed that interannual variations of GPP, ER, and NEEhad a much higher linear correlation with July temperature and annual maximum thawing depth (ALDmax)than other climatic and site characteristics. ALDmaxhad the highest correlation with the decomposition rateof humus C. Overall, this modeling study suggests that a sink-source transition of the studied ecosystemdepends on ecosystem responses to interannual variations of climate and that the net C balance may besensitive to summer warmth and active layer thickness. (Less)