Vegetation structure influences the retention of airfall tephra in a sub-Arctic landscape

Vegetation cover mediates a number of important geomorphological processes. However, the effect of different vegetation types on the retention of fine aeolian sediment is poorly understood. We investigated this phenomenon, using the retention of fine, pyroclastic material (tephra) from the 2011 eruption of the Grímsvötn volcano, Iceland, as a case study. We set out to quantify structural variation in different vegetation types and to relate structural metrics to the thickness of recently deposited volcanic ash layers in the sedimentary section. We utilised a combination of vegetation and soil surveys, along with photogrammetric analysis of vegetation structure. We found that indices of plant community composition were a poor proxy for vegetation structure and were largely unrelated to tephra thickness. However, structural metrics, derived from photogrammetric analysis, were clearly related to variations in tephra layer thickness at a landscape scale and tephra layers under shrub patches were significantly thicker than those outside the shrub canopy. We therefore concluded that: a) vegetation cover was a critical factor in the retention of fine aeolian sediment for deposit depths up to few centimetres; b) structural variation in vegetation cover played a major role in determining the configuration of tephra deposits in the sedimentary section. These findings have implications for the analysis of ancient volcanic eruptions and archaeological/palaeoenvironmental reconstructions based on the interpretation of tephra deposits. Furthermore, they present the possibility that the detailed form of tephra layers may be used as a proxy for palaeo vegetation structure. National Science FoundationThis is the author accepted manuscript. It is currently under an indefinite embargo pending publication by SAGE Publications

Variability in North Atlantic marine radiocarbon reservoir effects at c.1000 AD

14C age measurements made on samples from three archaeological sites located on North Atlantic coasts were used to investigate the marine reservoir effect (MRE) at c.1000 AD. This is an important period within human cultural and paleoenvironmental research as it is a time when Norse expansion to the North Atlantic islands occurred, during what appears to be a period of ameliorating climatic conditions. This makes improved chronological precision and accuracy at this time highly desirable. The data indicate a significant latitudinal variation in MRE at c.1000 AD from a ΔR of –152 ± 26 14C yr at Omey Island (530 32’ N) to 57 ± 22 14C yr at Undir Junkarinsfløtti (610 51’ N). The results are compared with modern assessments of MRE values within the context of oceanographic and climatic regimes that provide a possible driving mechanism for spatial and temporal variation in MRE

An over-optimistic pioneer fringe ? environmental perspectives on medieval settlement abandonment in Þórsmörk, South Iceland.

We assess environmental factors that may have contributed to farm abandonment in Þórsmörk, south Iceland. Here farms were established during the initial Norse colonisation of Iceland and abandoned by the 13th century AD. Soil erosion has been identified as a possible factor in this settlement change. This hypothesis is assessed using sediment profiles constrained by tephrochronology in Þórsmörk and the nearby area of Stakkholt. In Þórsmörk, there is evidence for episodes of landscape instability between the 10th and 13th centuries and localised episodes of soil erosion to bedrock that ended before 1300 AD and the onset of the Little Ice Age (LIA) climate changes. This early instability is absent from Stakkholt. Later LIA stability in Þórsmörk contrasts with episodes of instability in Stakkholt. The implication is that Þórsmörk was sensitive to early settlement impacts that lead to extensive erosion. After farm abandonment in Þórsmörk, the surviving woodland was successfully conserved as a valuable source of wood and charcoal for lowland farms where woodland resources had been depleted. Mounting pressure on woodland resources in the 11th -12th centuries could have been an important factor in determining the precise timing of abandonment

Degrees of success : evaluating the environmental impacts of long term settlement in South Iceland.

This paper focuses on the occupation and landscape history of Dalur and Mörk, two areas of long-term settlement in the Eyjafjallahreppur district of southern Iceland. The aim is to illustrate the importance of evaluating not only farm occupation and abandonment, but also to assess the complexities of the environmental interactions of long-term settlements. Environmental records are assessed using data from 50 sediment profiles, constrained by tephrochronology, located in the farm infields (tún) and outfields (hagi). This record indicates that despite similar outward appearances today, the environs of Dalur and Mörk have experienced different histories of environmental change over the last 1000 years. At least 14 subsidiary settlements were at one time or another established within Dalur, or were dependent on the Church farm there. Ten of these settlements were subsequently abandoned and sediment accumulation rates, a proxy indicator of erosion, remained low, indicating restricted local human impact. We conclude that this illustrates the importance of access and rights to additional resources out with the principal farm. In addition, much of the immediate environs of the main farm site was probably un-wooded at the time of settlement, so the total degree of vegetative change was limited. In contrast, palaeoenvironmental data indicates that the environs of Mörk were extensively wooded at the time of Landnám, but this woodland was rapidly cleared and this was followed by several centuries of landscape instability. In a cultural contrast, the landholdings of Mörk experienced less subdivision with a total of only five dependent farms established across the land belonging to three main farms (all of which had chapels). From the early 10th to 14th centuries there was significantly enhanced erosion within the Mörk landholdings, but this stabilised and the principal farms endured and to form sites of long-term settlement. The ownership of additional resource rights, including woodland further up-valley, may have made the crucial contribution to long term endurance

Settlement history, land holdings and landscape change, Eyjafjallahreppur, Iceland

In this paper we focus on a region in south Iceland and assess the confidence with which it is possible to determine the changing patterns of settlement history for a region and relate it to contemporaneous land boundaries. For a geographically coherent group of up to 38 possible farm sites in the northern and western part of Eyjafjallahreppur, south Iceland, the timing of occupation and abandonment is assessed. In addition, boundaries of landholdings, probable status and inter-site relationships are identified. Currently only 10 sites are occupied. There is some uncertainty over the location of the earliest settlements, but after the 11th century, successful principle farm sites such as Dalur, Seljaland and Mörk remain in the same locations. As a result, recent data on landholdings combined with a knowledge of the form of landholding sub-division, offers insights into pre-modern times, with some data relevant to medieval times. An ‘abandoned frontier’ exists in the inland area of Þórsmörk, but abandonment is not restricted to the uplands; it has occurred throughout the region. Most desertion occurred before the cold phases of the Little Ice Age in the 18th century. Overall, much farm desertion can be attributed to landscape destruction as a result of river migration, but it is also biased towards small dependent farms often established for social reasons (such as the property requirement for marriage) and evidently insufficiently resourced for long term success in a changing physical environment

Charcoal production during the Norse and early medieval periods in Eyjafjallahreppur, Southern Iceland

Timber procurement and the use of woodlands is a key issue in the Norse and Medieval period in the North Atlantic islands. This paper outlines evidence for the timing and mechanisms of Norse deforestation in an area of southern Iceland which is tracked through the mapping and analysis of charcoal production pits. Precise dating of the use of these charcoal production pits within a Bayesian framework is demonstrated through the combination of tephrochronology, sediment accumulation rates and multiple radiocarbon dates on the archaeological charcoal. The implications for using charcoal as a dating medium for radiocarbon dating in Iceland and the wider North Atlantic are then explored. Finally, the nature of the deforestation and human impact on the environment is placed into the context of the Norse landnám across the North Atlantic

How do the grain size characteristics of a tephra deposit change over time?

Financial support was provided by the National Science Foundation of America through grant 1202692 ‘Comparative Island Ecodynamics in the North Atlantic’ and grant 1249313 ‘Tephra layers and early warning signals for critical transitions’ (both to AJD).Volcanologists frequently use grain size distributions (GSDs) in tephra layers to infer eruption parameters. However, for long-past eruptions, the accuracy of the reconstruction depends upon the correspondence between the initial tephra deposit and preserved tephra layer on which inferences are based. We ask: how closely does the GSD of a decades-old tephra layer resemble the deposit from which it originated? We addressed this question with a study of the tephra layer produced by the eruption of Mount St Helens, USA, in May 1980. We compared grain size distributions from the fresh, undisturbed tephra with grain size measurements from the surviving tephra layer. We found that the overall grain size characteristics of the tephra layer were similar to the original deposit, and that distinctive features identified by earlier authors had been preserved. However, detailed analysis of our samples showed qualitative differences, specifically a loss of fine material (which we attributed to ‘winnowing’). Understanding how tephra deposits are transformed over time is critical to efforts to reconstruct past eruptions, but inherently difficult to study. We propose long-term, tephra application experiments as a potential way forward.Publisher PDFPeer reviewe

The interpretative value of transformed tephra sequences

Financial support was provided by the National Science Foundation of America through grants 1202692 and 1249313, the Carnegie Trust for the Universities of Scotland through a grant to RTS, and the NERC Doctoral Training Partnership PhD studentship NE/L002558/1 to PT.We explore developments in tephra science that consider more than chronology, using case studies. Volcanic processes and prevailing weather conditions determine the distribution of tephra deposits immediately after an eruption, but as these freshly fallen tephra become part of the stratigraphic record, the thickness, morphology and definition of the layers they form changes, reflecting the interplay of the tephra, Earth surface processes, topography and vegetation structure, plus direct or indirect modification caused by people and animals. Once part of the stratigraphic record, further diagnostic changes can happen to the morphology of tephra layers, such as the creation of over folds by cryoturbation. Thus, tephra layers may contain proxy evidence of both past surface environments and subsurface processes. Transformations of tephra deposits can complicate the reconstruction of past volcanic processes and make the application of classical tephrochronology as pioneered by Thorarinsson (Sigurður Þórarinsson in Icelandic) challenging. However, as Thorarinsson also noted, novel sources of environmental data can exist within transformed tephra sequences that include the spread or removal of tephra, variations in layer thickness and internal structures, the nature of contact surfaces and the orientation of layers.PostprintPeer reviewe

Disequilibrium, adaptation and the Norse settlement of Greenland

This research was supported by the University of Edinburgh ExEDE Doctoral Training Studentship and NSF grant numbers 1202692 and 1140106.There is increasing evidence to suggest that arctic cultures and ecosystems have followed non-linear responses to climate change. Norse Scandinavian farmers introduced agriculture to sub-arctic Greenland in the late tenth century, creating synanthropic landscapes and utilising seasonally abundant marine and terrestrial resources. Using a niche-construction framework and data from recent survey work, studies of diet, and regional-scale climate proxies we examine the potential mismatch between this imported agricultural niche and the constraints of the environment from the tenth to the fifteenth centuries. We argue that landscape modification conformed the Norse to a Scandinavian style of agriculture throughout settlement, structuring and limiting the efficacy of seasonal hunting strategies. Recent climate data provide evidence of sustained cooling from the mid thirteenth century and climate variation from the early fifteenth century. Archaeological evidence suggests that the Norse made incremental adjustments to the changing sub-arctic environment, but were limited by cultural adaptations made in past environments.Publisher PDFPeer reviewe

A Bayesian approach to linking archaeological, paleoenvironmental and documentary datasets relating to the settlement of Iceland (Landnám)

YesIcelandic settlement (Landnám) period farmsteads offer opportunities to explore the nature and timing of anthropogenic activities and environmental impacts of the first Holocene farming communities. We employ Bayesian statistical modelling of archaeological, paleoenvironmental and documentary datasets to present a framework for improving chronological robustness of archaeological events. Specifically, we discuss events relevant to the farm Hrísbrú, an initial and complex settlement site in southwest Iceland. We demonstrate that tephra layers are key in constraining reliable chronologies, especially when combined with related datasets and treated in a Bayesian framework. The work presented here confirms earlier interpretations of the chronology of the site while providing increased confidence in the robustness of the chronology. Most importantly, integrated modelling of AMS radiocarbon dates on Hordeum vulgare grains, palynological data, documented evidence from textual records and typologically diagnostic artefacts yield increased dating reliability. The analysis has also shown that AMS radiocarbon dates on bone collagen need further scrutiny. Specifically for the Hrísbrú farm, first anthropogenic footprint palynomorph taxa are estimated to around AD 830–881 (at 95.4% confidence level), most likely before the tephra fall out of AD 877 ± 1 (the Landnám tephra layer), demonstrating the use of arable fields before the first known structures were built at Hrísbrú (AD 874–951) and prior to the conventionally accepted date of the settlement of Iceland. Finally, we highlight the importance of considering multidisciplinary factors for other archaeological and paleoecological studies of early farming communities of previously uninhabited island areas