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Lake Sediments as an Archive of Land Use and Environmental Change in the Eastern Settlement, Southwestern Greenland
Vincent Bichet, Emilie Gauthier, Charly Massa, and Bianca B. Perre

Journal of the North Atlantic, Special Volume 6 (2014): 47–63

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Journal of the North Atlantic V. Bichet, E. Gauthier, C. Massa, and B.B. Perren 2014 Special Volume 6 47 Introduction The Norse colonization of Greenland (AD 986– ca. AD 1450) presents an excellent framework to study the relationship between a community and its environment. Norse settlers took advantage of favorable climatic conditions during the Medieval Climate Anomaly around the end of the 10th century (Mann et al. 2009, Moberg et al. 2005) to extend their pastoral activities into the subarctic of southwestern Greenland. The Eastern Settlement, which today corresponds roughly to the Municipality of South Greenland (Kommune Kujalleq), has been particularly well documented by archaeological research in terms of agro-pastoral practices (e.g., Buckland et al. 2009, Commisso and Nelson 2008, Dugmore et al. 2005), demographic pressure (Lynnerup 1996), and lifestyle and dietary habits (Arneborg et al. 1999, Perdikaris and McGovern 2007) as well as the localization of archaeological sites (e.g., Algreen-Møller and Madsen 2006, Guldager et al. 2002). After centuries of abandonment, the same areas were re-occupied by farmers using conventional agricultural methods beginning in the early 20th century. In this context, the Eastern Settlement provides palaeoenvironmental scientists with an ideal model to examine the transition from a pristine to an anthropogenic landscape. As well, studies of natural archives can provide insight into the environmental conditions surrounding known historical events and the evolution of the Norse colonies in Greenland. Over the last forty years, many palaeoenvironmental studies have been conducted in the Eastern Settlement and its surroundings. Different types of palaeoenvironmental archives have been investigated, each one advancing knowledge of environmental change and the history of the Norse in Greenland. These archive types are manifold (soils, mires or lake deposits, fjord sedimentary sequences), and their sensitivity to climate or human forcing is likewise varied. Since the work of Bent Fredskild (1973), peat deposits have been most frequently studied to evaluate the response to medieval Norse farming. Using biological proxies (pollen, coprophilous fungi spores, and diatoms) as well as abiotic proxies (e.g., LOI, geochemis- Lake Sediments as an Archive of Land Use and Environmental Change in the Eastern Settlement, Southwestern Greenland. Vincent Bichet1,*, Emilie Gauthier1, Charly Massa1, and Bianca B. Perren1 Abstract - Palaeoenvironmental studies from continental and marine sedimentary archives have been conducted over the last four decades in the archaeologically rich Norse Eastern Settlement in Greenland. Those investigations, briefly reviewed in this paper, have improved our knowledge of the history of the Norse colonization and its associated environmental changes. Although deep lakes are numerous, their deposits have been little used in the Norse context. Lakes that meet specific lake-catchment criteria, as outlined in this paper, can sequester optimal palaeoenvironmental records, which can be highly sensitive to both climate and/or human forcing. Here we present a first synthesis of results from a well-dated 2000-year lake-sediment record from Lake Igaliku, located in the center of the Eastern Settlement and close to the Norse site Garðar. A continuous, high-resolution sedimentary record from the deepest part of the lake provides an assessment of farming-related anthropogenic change in the landscape, as well as a quantitative comparison of the environmental impact of medieval colonization (AD 985–ca. AD 1450) with that of recent sheep farming (AD 1920–present). Pollen and non-pollen palynomorphs (NPPs) indicate similar magnitudes of land clearance marked mainly by a loss of tree-birch pollen, a rise in weed taxa, as well as an increase in coprophilous fungi linked to the introduction of grazing livestock. During the two phases of agriculture, soil erosion estimated by geochemical proxies and sediment-accumulation rate exceeds the natural or background erosion rate. Between AD 1010 to AD 1180, grazing activities accelerated soil erosion up to ≈8 mm century-1, twice the natural background rate. A decrease in the rate of erosion is recorded from ca. AD 1230, indicating a progressive decline of agro-pastoral activities well before the end of the Norse occupation of the Eastern Settlement. This decline could be related to possible climate instabilities and may also be indirect evidence for the shift towards a more marine-based diet shown by archaeological studies. Mechanization of agriculture in the 1980s caused unprecedented soil erosion up to ≈21 mm century-1, five times the pre-anthropogenic levels. Over the same period, diatom assemblages show that the lake has become steadily more mesotrophic, contrary to the near-stable trophic conditions of the preceding millennia. These results reinforce the potential of lake-sediment studies paired with archaeological investigations to understand the relationship between climate, environment, and human societies. In The Footsteeps of Vebæk—Vatnahverfi Studies 2005-2011 Journal of the North Atlantic 1 University of Franche-Comté, UMR CNRS 6249 Chrono-Environnement, 16 route de Gray, F-25030 Besançon cedex, France. *Corresponding author - vincent.bichet@univ-fcomte.fr. 2014 Special Volume 6:47–63 Journal of the North Atlantic V.Bichet, E.Gauthier, C. Massa, and B.B. Perren 2014 Special Volume 6 48 try), researchers from the University of Aberdeen (Edwards et al. 2008, 2011; Golding et al. 2011; Schofield and Edwards 2011; Schofield et al. 2008, 2010) have concentrated on palaeoenvironmental change during the Norse period. Studies using mire and pond deposits, soil sections, and archaeological trenches greatly advance knowledge of land use by the Norse, despite the inherent difficulties with discontinuous records and attendant dating problems (Edwards et al. 2008, 2011). Although lakes are common landscape features in South Greenland, palaeoenvironmental investigations of lacustrine sediments are scarce. Assuming that localized human impacts could overprint climatic features, regional palaeoclimatic studies and reconstructions have been preferentially conducted on marine sediments from fjords (e.g., Jensen et al. 2004, Kuijpers and Mikkelsen 1999, Lassen et al. 2004, Roncaglia and Kuijpers 2004) or on lakes beyond Medieval settlement areas (Andresen et al. 2004, Fréchette and de Vernal 2009, Kaplan et al. 2002). There are a few relatively old studies based on lacustrine sediments from within Norse settlements aimed at characterizing human impact (Sandgren and Fredskild 1991). The expansion of the Norse and the development of the Eastern Settlement, as well as its demise, were probably partially driven by climate change (Dugmore et al. 2007, 2012; Massa et al. 2012a; Patterson et al. 2010; Stuiver et al. 1995). Consequently, the establishment of high-resolution climatic reconstructions for this region is critical to understanding the Norse adaptability to climate change. Ice-core δ18O data highlight climatic fluctuations and can be compared to Norse historical records. But available glacial records are far from the settlement geographically, altitudinally, and climatically (the Dye-3 core site is located 475 km from the settlement, whereas GISP2 and GRIP are more than 1300 km away), and δ18O and snow accumulation rates record temperature and precipitation at the inland ice summit ≈3000 m above the Norse sites (Vinther et al. 2010). In order to obtain 1) a continuous high-resolution environmental archive and 2) a quantitative reconstruction of past temperatures at a decadal scale in the Eastern Settlement, we cored the deepest part of Lake Igaliku in the center of the archaeological area, at the threshold of the Vatnahverfi district. Here, we review previous palaeoenvironmental studies in the Eastern Settlement, present lake archives as a potent paleoenvironmental tool, and discuss the main results obtained from Lake Igaliku. Natural Archives and Previous Studies in the Eastern Settlement Fredskild’s pioneering studies Bent Fredskild (1973, 1978, 1992) was one of the pioneering investigators in this area. He established a biochronology for the Holocene in Greenland and, more particularly, in the Qassiarssuk area of the Eastern Settlement (Fig. 1), from ponds (Comarum Sø) and peat deposits (Comarum Mose, Galium Kaer, Qassiarsuk). Fredskild’s investigations documented Holocene vegetation history and landscape evolution, showing the immigration of taxa and the appearance of Salix, Juniperus, and then Betula. Using pollen and plant macrofossils, Fredskild also documented the arrival of Norse farmers in the 10th century and their perceptible impact on the environment through scrub clearance, the creation of hay meadows, and the introduction of non-indigenous taxa. However, due to the age of the studies, the radiocarbon dates in Fredskild’s work are few in number and imprecise, and erosion identified in the sediments (Fredskild 1973, 1978; Sandgren and Fredskild 1991) could not be quantified. Peat deposits and archaeological soils Complementing the early work of Fredskild and creating high-resolution palaeoecological records, Schofield et al. (2008, 2010), Schofield and Edwards (2011), Edwards et al. (2008, 2011), and Golding et al. (2011) have studied numerous peat deposits, ponds, and archeological trenches in the Eastern Settlement, in the Qorlortoq Valley, Tasiusaq, and Qinngua (Qassiarsuk area), in Sissarluttoq (10 km south from Igaliku, on the west side of the Igaliku fjord), and in Sandhavn (Nanortalik area) (Fig. 1). Age–depth models are not straightforward from these deposits, as peat and soil accumulation can be discontinuous (e.g., with hiatuses) and peat-columns are often truncated by local use for fuel, roofing, or bedding material. In addition, peat is often contaminated by old carbon, which complicates dating (Edwards et al. 2008). Despite these difficulties, peat, soil, and pond deposits provide sensitive records of vegetation changes during the Norse period. One such example is from a drainage-ditch soil profile in the center of the Norse Garðar settlement ruins (about 2 km from our site at Lake Igaliku), where pollen and insect remains provide compelling evidence for both manuring and irrigation practices during the Norse period (Buckland et al. 2009). Lake sediments as archives of climate changes Near the Eastern Settlement, two lacustrine sequences (Lakes Qipisarqo and N14) have been Journal of the North Atlantic V. Bichet, E. Gauthier, C. Massa, and B.B. Perren 2014 Special Volume 6 49 studied for palaeoclimatic reconstruction (Fig. 1). Lake Qipisarqo is located near a glacial tongue flowing from the Qassimiut lobe, about 80 km northwest of our investigation area. A first study based on biogenic silica and organic matter measurements documented palaeo-productivity in relation to past temperatures (Kaplan et al. 2002). A later study reconstructed climatic parameters (temperature, precipitation), from pollen assemblages using the modern analogue technique (Fréchette and de Vernal 2009). Lake N14 is located on an island near Kap Farvel. The palaeoclimatic interpretation of this sequence is based mainly on high-resolution biogenic silica measurements (interpreted as temperature/precipitation) and on sulphur flux estimates (a storminess parameter; Andresen et al. 2004). Although these two records document the Holocene period at a multicentennial to decadal scale, they are both heavily influenced by maritime climate, and do not necessarily capture the climatic changes experienced by the Norse in the more continental inner fjord area. Marine sediments Two marine sediment cores from the outer and inner part of Igaliku Fjord (cores PO 243-443 and PO 2342-451; Fig. 1) were investigated to document local palaeohydrographic conditions for the last Figure 1. Map of southwestern Greenland and the Norse Eastern Settlement showing the location of the current localities of the area (white squares), main Norse ruin groups (black dots), modern farms (black triangles), and the main sites of mentioned palaeoenvironmental studies (white circles). Journal of the North Atlantic V.Bichet, E.Gauthier, C. Massa, and B.B. Perren 2014 Special Volume 6 50 3200 years (Jensen et al. 2004, Lassen et al. 2004, Roncaglia and Kuijpers 2004). The authors detailed the climatic variations that occurred during the transition from the “Medieval Warm Period” to the Little Ice Age and their implications for Norse settlement. To date, these studies stand alone in providing a record of past climate change during the historical period for the Eastern Settlement. More recently, the study of ice-rafted debris deposited in the Narsaq Sound (core Ga3-2; Fig. 1) documented the dynamics of the local glacial termination in response to Holocene climate changes (Nørgaard-Pedersen and Mikkelsen 2009). As in the aforementioned lakesediment studies, these marine sequences have a temporal resolution that is too low to accurately assess the environmental changes that occurred during the historical period. Although the fjord deposits provide information about terrestrial flux (Roncaglia and Kuijpers 2004), these marine sedimentary archives cannot be used to evaluate land-use–induced changes clearly. The Opportunity Presented by Lake Sediments Complementing earlier studies, high-resolution lake-sediment archives provide insights into the long-term development of terrestrial and aquatic ecosystems in the archaeological area. Palaeoclimatic and palaeoenvironmental records established from lake sediment cores offer the opportunity to highlig-t changes that occurred during a specific time period. In particular, they may provide evidence for processes associated with the transition from pristine to human-dominated environmental conditions and may also provide perspectives on the functioning of modern ecosystems and landscapes (Battarbee and Bennion 2011, Dearing et al. 2008, Giguet-Covex et al. 2011, Ramrath et al. 2000). In order to maximize the palaeoenvironmental interpretation of the historical period, study sites must meet specific criteria, mainly related to the catchment–lake relationship. Among these conditions, the most important are: (i) Sedimentation rate. The sedimentary record must be sufficiently thick with a high age–depth ratio to allow for high-resolution, multiproxy analyses. Except for X-ray fluorescence, most non-destructive core-scanning sensors (e.g., γ-density, magnetic susceptibility) do not analyze sub-millimeter measurements, and classical destructive analyses (e.g., biotic contents, geochemistry, grain-size distribution) need at least 3–5-mm samples. If annual resolution is not feasible, decadal resolution is often realistic. For decadal resolution, sediment accumulation rate (SAR) has to be at least 0.4 mm yr-1. At low altitude (e.g., under 500 m asl) in soil- and till-covered catchments of southwestern Greenland, sequences cored in the deepest parts of lakes show varying SARs. Recent literature (Andresen et al. 2004; Fréchette and de Vernal 2009; Massa et al. 2012a, 2012b; unpublished data from the Ultimagri Project [University of Franche-Comté, Chrono-environment Laboratory]) indicates that mean values range from 0.1 mm yr-1 to 0.6 mm yr-1 for the last two millennia. Regionally, at higher altitudes, lake-sediment accumulation rates are often lower, mainly due to poor soil development and low internal biological processes, and are less amenable to high-resolution investigations. (ii) Age control. The age control of lake cores is usually based on measuring the decay of radiogenic isotopes. For dating the most recent 150 years, the short-lived radio-isotopes 137Cs and 210Pb are easily used in most cases (Appleby and Olfield 1978). However, the best way to date older deposits is using 14C in terrestrial plant remains. Accelerator mass spectrometry (AMS) dating requires a few milligrams of organic matter. In vegetated areas of southern Greenland, fragments of wood, twigs, or leaves are often present in sediments. At Lake Igaliku (see below), a 60-mm-diameter core taken in 21 m of water produced fourteen terrestrial plant macrofossils within the upper 95 cm. If terrestrial plant remains are too scarce to produce a robust age-depth model, other substances like humic acids or aquatic macrofossils can be used, taking into account a reservoir effect due to the in-lake or in-soil recycling of carbon (Abbott and Stafford 1996, Olsen et al. 2012, Wolfe et al. 2004). Where possible, the use of bulk sediment should be avoided because of the mixing of the organic fractions. (iii) Simple catchment-basin properties. Detrital inputs to the lake basin must reflect the dual control of climate and anthropogenic processes. For this consideration, there are no absolute conditions, but closed-basin lakes and those fed by runoff are preferred to lakes in which major fluvial detrital inputs may overprint the response of subtle soils erosion due to land use (Edwards and Whittington 2001). Sites with catchments containing glacial outwash plains subject to wind erosion and strong aeolian fluxes should be avoided. Otherwise, sedimentary anthropogenic imprints depend on the archaeological site density (or intensity of land-use processes) in the catchment and catchment area:lake area (or lake volume) ratio. In very large lakes, the signal from an adjacent archaeological site is likely to be diluted, and small lakes can offer a better record of anthropogenic processes. Journal of the North Atlantic V. Bichet, E. Gauthier, C. Massa, and B.B. Perren 2014 Special Volume 6 51 (iv) Continuous sedimentation. Ideal sedimentary sequences should be continuous, with no hiatuses, disturbances, or mass-wasting deposits. Catchments and lakes with gentle slopes enhance the probability of undisturbed sequences. Deep basin sequences are better than littoral deposits where the stratigraphy could be affected by ice scouring of the bed or by wind-generated turbulence and mixing of the sediment–water interface. In any case, an acoustic survey with a sub-bottom profile should be used to document the lake sediment stratigraphy and possible discontinuities and optimize the location of the coring site. Lakes which meet the above criteria are ideal and can provide knowledge of anthropogenic and palaeoenvironmental changes through time, complementing data provided by terrestrial and archaeological archives. The Example of Lake Igaliku, at the Heart of the Eastern Settlement Local and historical setting Lake Igaliku (unofficial name; 61º00'N–45º26'W, 15 m asl) is located in the low valley between the head of Igalikup Kangerlua (Igaliku fjord) and Figure 2. Map showing (a) the region around Lake Igaliku including the catchment area (grey dashed line), roads (black dashed lines), modern buildings (black rectangles), and current hay fields (grey shaded areas), as well as the archaeological site of Garðar, and (b) the bathymetry of the lake and the coring location. Journal of the North Atlantic V.Bichet, E.Gauthier, C. Massa, and B.B. Perren 2014 Special Volume 6 52 were central to the re-development of contemporary agriculture in southwestern Greenland. Since 1980, after the climate crisis of the 1960s/1970s (Egede and Thorsteinsson1982, Greenland Agriculture Advisory Board 2009), two sheep farms (more than 1000 sheep) were established in the catchment, and around 30 ha of hayfield were created on the shore of the lake to produce winter fodder for stabling (Figs. 2a, 3). Because of these two phases of farming in the catchment during the last millennium (Norse and modern), Lake Igaliku is an excellent site to compare and explore environmental changes and the complex relationship between climate, landscape, and human societies. Core and sediment chronology In order to obtain a continuous high-resolution environmental archive, the deepest part of Lake Igaliku was cored from a floating platform, using piston and gravity corers. A 4-m-long sandy silt Holocene composite sequence was collected, with the upper 100 cm spanning the last 2000 years. The upper 100 cm of the core is composed of very finely stratified brownish sandy silt with black horizons rich in ferrous iron oxide (Fig. 4). From ≈5 cm, the sandy silts give way to black clayey silts up to the sediment–water interface. The Xradiographs reveal continuous sedimentation with distinct lamina (≈6 mm), indicating that sediments are not bioturbated. The sequence does not contain Tunulliarfik fjord (Erik’s fjord) (Figs. 1, 2a). It is a north–south oriented lake with a surface area of 34.6 ha and a maximal depth of 26 m (Fig. 2b). The 3.55-km2catchment area is without an inlet, but has a small outlet on the northern shore that drains into the Tunulliarfik fjord. The topography of the catchment is characterized by a large, gently sloped plain (3.1 km2) surrounded in the western part by a low, rounded hill (130 m asl). The highest relief is to the northeast reaching 300 m asl. The rocks underlying most of the catchment are Proterozoic granites partly covered by arkosic sandstones and lavas that outcrop on the hills. The lake is located within 2 km of the modern village of Igaliku, which was the Norse Garðar, settled ca. AD 1000, soon after the landnám (Gad 1970, Jones 1986). Garðar rapidly became a place of prime importance for Norse society (Episcopal seat and assembly site; Krogh 1967, Nørlund and Roussell 1929, Sanmark 2009). Archaeological structures suggest that Igaliku-Garðar was a high-status farmstead with probably the largest holding of livestock in Norse Greenland (Christensen-Bojsen 1991, Mc- Govern 1991). The chronology of the abandonment of Garðar is unclear, but it is generally accepted that it must have occurred sometime in the mid- to late 15th century (Dugmore et al. 2009). The site of Garðar was settled and farmed again in the 18th century (Arneborg 2007). Modern pastoral agriculture began in 1915 (Austrheim et al. 2008). During the 20th century, the village of Igaliku and surroundings Figure 3. View of Lake Igaliku and surrounding fields (looking towards the north). Journal of the North Atlantic V. Bichet, E. Gauthier, C. Massa, and B.B. Perren 2014 Special Volume 6 53 that date. A sharp increase in the SAR, up to 1.9 mm yr-1, is noted during the 20th century. Sampling and sediment analyses The core was contiguously sampled. The top 10 cm were sampled in 0.5-cm slices, and below 10 cm, sampling intervals (≈1 cm) were chosen by using X-ray image to ensure homogenous samples according to the varying lithology. Sampling resolution is between 2 to 32 years per sample for geochemical proxies and diatoms, and 25 to 80 years for pollen and organic non-pollen palynomorphs (NPP). Core analysis was based on a multidisciplinary approach using indicators that track catchment dynamics (i.e., vegetation [Gauthier et al. 2010], sediment yield [Massa et al. 2012b]) and lake trophic changes (organic geochemistry and diatoms [Perren et al. 2012b]). A suite of geophysical (γ-density any mass wasting, rapid deposits due to slumps, or high-energy inflows which could disrupt the age– depth model. For the upper 100 cm, the chronology (Fig. 4) is based on 16 AMS radiocarbon dates (28 for the whole core) on terrestrial plant macrofossils (14 twigs and leaves) and aquatic bryophytes (2 samples) corrected for reservoir effect (Massa et al. 2012a). In addition, the last two centuries (the upper 15 cm) are dated by 210Pb and 137Cs using α spectroscopy. The age-depth model is based on the Monte-Carlo Method (Blaauw 2010), which allows for the robust estimation of the relative uncertainty and takes into account the entire probability distribution of calibrated 14C dates (for table of radiocarbon dates and details, see Massa et al. 2012b). The age-depth model is almost linear until ca. AD 1010, with a mean (SAR) of ≈0.4 mm yr-1. After that, the SAR rises to a maximum of 0.8 mm yr-1 around AD 1150, and decreases gradually after Figure 4. Stratigraphy and age-depth model of the Lake Igaliku core for the last two millennia. The probability distributions of calibrated radiocarbon dates are displayed with laboratory reference number. The chronology of the upper 15 cm is based on 210Pb and 137Cs measurements. See Massa et al. (2012) for detailed comment. Journal of the North Atlantic V.Bichet, E.Gauthier, C. Massa, and B.B. Perren 2014 Special Volume 6 54 and magnetic susceptibility with a Geotek Multi- Sensor Core Logger), geochemical (Avaatech XRF Core Logger, ICP-AES, Corg, Ntot, δ15N, δ13C), and biological proxies (pollen, non-pollen palynomorphs [NPP], and diatoms) were analyzed. Here, we synthesize the proxies that assess catchment dynamics, highlighting the impacts of farming activities during the medieval and modern periods. The parameters used are not a priori linked to human or climate-forcing individually, but their combined analysis, variations at different time scales, and comparison with the archaeological and historical data allows for human impacts to be resolved. Vegetation changes and historical grazing indicators The current vegetation around Igaliku is a classic subarctic tundra, dominated by scrub juniper (Juniperus communis), Crowberry (Empetrum nigrum) heaths, and highly deciduous and productive grey willow (Salix glauca), dwarf birch (Betula glandulosa), and downy birch (Betula pubescens). Pollen concentration in the lake sediments is related to the pollen rain directly falling on the lake surface; however, pollen which has been brought into the lake with inflowing waters or with surface runoff may also accumulate (Hicks and Hyvärinen 1999). Therefore, variation in pollen content is a response to changing environmental conditions in plant and pollen productivity and plant density, as well as a response to increased allochthonous material from the catchment. Allochthonous material may also include NPPs and coprophilous fungi, which grow indiscriminately on herbivore dung (Bell 2005) and indicate the presence of herbivores around the lake (Davis and Shafer 2006, van Geel and Aptroot 2006). Nevertheless, the arboreal pollen content of Lake Igaliku for the last two millennia indicates relatively stable vegetation (IGA1). Since 3 ka BP, southern Greenland underwent Neoglacial cooling shown by a progressive decrease in the influx of arboreal/shrub pollen (Massa et al. 2012a). At this sampling resolution, climate changes are poorly recorded by vegetation (except for recent climate warming, which is recorded by a huge increase of Betula). However, anthropogenic forcing is easily detectable. The first sign of Norse settlement (IGA 2a; Fig. 5), is a decrease in downy birch and juniper from 1000 to 1150 cal. BC, which resulted in an overall decrease in arboreal/shrub pollen from 60 to 45%. Nevertheless, these values remain higher than the birch percentages recorded in most of the peat records adjacent to the Norse ruins. Meanwhile, coprophilous fungi spores synchronously increase indicating the occurrence of herbivores in the catchment. Small amounts of coprophilous fungi appear before the medieval period, suggesting that wild herbivores, probably caribou, may have been grazing in the area (Davis and Shafer 2006, Gauthier et al. 2010, Schofield and Edwards 2011, van Geel and Aptroot 2006). Another impact associated with the landnám is the rise in moss and fern spores, probably associated with erosion, as suggested by the denudation rate. The loss of arboreal taxa, and the concomitant rise in coprophilous fungi, moss, and fern spores are a likely response to grazing pressure, erosion of soil, and the development of agro-pastoralism in the catchment during the landnám. From the mid-12th century (IGA2b), percentages of coprophilous fungi as well as Rumex acetosatype pollen (a combination of pollen from Rumex acetosa, R. acetosella, and Oxyria dygina) increase. The Norse agricultural “weeds” R. acetosa and R. acetosella are common in grazed environments and are traditionally regarded as evidence of Norse settlement in south Greenland (Edwards et al. 2008, Fredskild 1973). The first occurrences of this pollen type, before the Norse period, probably correspond to local production of Oxyria pollen or long-distance transport. The Igaliku sequence shows a certain lag between the first clearance, introduction of cattle and the development of R. acetosa-type (Bichet et al. 2013, Gauthier et al. 2010). However, palynological data from peat deposits rarely record such a lag except in Tasiusaq, where there is a very precise chronology for vegetation changes between ≈AD 1000–AD 1100 (Edwards et al. 2008). At the beginning of the 14th century (IGA 2c), pollen and NPP document the steady decrease of all grazing pressure and erosion indicators. These changes are likely a response to a reduction in grazing herbivores and could be chronologically related to the end of the development of plaggen (i.e., man-made soil) at Igaliku (Buckland et al. 2009). Coprophilous fungi disappear almost completely around AD 1400, and the tundra vegetation returns to almost pristine conditions. However, agricultural weeds and some native ruderal herbs (e.g., Rumex acetosa-type and Plantago maritime; Edwards et al. 2008) remain present. For almost five centuries, the vegetation remained stable (IGA2d). After ca. 1950, trees and shrubs decrease, weeds, apophytes, and coprophilous fungi increase, all in response to the new phase of grazing pressure and the re-establishment of farming activities (IGA2e). Estimating soil erosion in the Igaliku Lake Journal of the North Atlantic V. Bichet, E. Gauthier, C. Massa, and B.B. Perren 2014 Special Volume 6 55 Figure 5. Percentage pollen diagram for trees, shrubs and heaths, herbs, aquatics, coprophilous fungi, and pteridophytes. Exaggeration curves x2 for Thalictrum and Juniperus, and x10 for all other taxa except Betula pubescens, B. glandulosa, Salix, Poaceae, and Cyperaceae. Journal of the North Atlantic V.Bichet, E.Gauthier, C. Massa, and B.B. Perren 2014 Special Volume 6 56 (BMC) of 420 mg cm-3 was measured using 25 samples of soil surface horizon collected in the lake catchment. For details, see Massa et al. (2012b). Calculated values of DR are a first approximation of real land erosion. Use of this method assumes that SAR is representative of the entire lake bottom, and the values of DR are given as an average erosion yield on the global surface of the catchment. The first millennium and the pre-landnám period show relatively stable soil erosion that ranges between 2.5 and 5 mm century-1 in response to natural climate variability. The high detritic inputs (Ti) and low productivity (C:N) around AD 250 suggests a cooling which corresponds to relatively low temperatures in the Arctic between AD 165–AD 345 (Kaufman et al. 2009). The low C:N and Ti values at ca. AD 400 indicate low erosion and/or high algal productivity which could be linked to a warmer time interval (AD 375–AD 415) recorded in the Arctic (Kaufman et al. 2009). The most detritic period at AD 470–AD 550 roughly corresponds to a cooling in the nearby Igaliku Fjord (Jensen et al. 2004, Lassen et al. 2004), and to cold atmospheric conditions and enhanced advection of EGC water masses in the Narsaq Sound (Nørgaard-Pedersen and Mikkelsen 2009), as well as a decrease in biological productivity in Lake N14 (Andresen et al. 2004). Extensive sand horizons in lakes and soil profiles in the area of Sondre Igaliku have been linked to Norse erosion (Fredskild 1978, Jakobsen 1991, Sandgren and Fredskild 1991). Some authors suggested however that these deposits could be partly the result of enhanced wind activity between the 9th and 14th centuries (Kuijpers and Mikkelsen 2009, Lassen et al. 2004), which would have produced a large amount of clastic material transported from the nearby well-developed sandur. However, Lake Igaliku is not located in a glacial valley, and an increase in wind strength during Norse period may have played only a minor role in the high sediment supply into Lake Igaliku compared with farming activities. Soon after the Norse arrival, land clearance (indicated in pollen data by a decrease in woody taxa) and the introduction of sheep led to a rapid increase in soil erosion to a maximum of 8 mm century-1, twice the background level, around AD 1180 (Fig. 6). The maximum values in C:N ratio and Ti between ca. AD 1030 and ca. AD 1230 suggest a strong impact of Norse farming during this period. Compared with historical data, maximum erosion appears a few decades after the appointment of the first bishop, when Garðar was probably close to its maximum development and activity. First evidence of erosion caused by land catchment The hypothesis of overexploitation of the environment has been frequently proposed as a major cause of the collapse of the medieval Norse society in Greenland (e.g., Diamond 2005; Edwards et al. 2008; Fredskild 1973, 1988; Gad 1970; Jacobsen 1987; Jakobsen 1991; McGovern et al. 1988). Estimating the chronology of the terrigenous flux into the Lake Igaliku may help to further evaluate the validity of this explanation. Among the geochemical proxies studied, titanium (Ti) is a conservative lithogenic element that participates in very few biogeochemical processes (Kauppila and Salonen 1997, Koinig et al. 2003). Higher Ti concentrations in the sediment point to enhanced physical weathering of alumino-silicates in the watershed, which can be due to climatic changes or to erosion from land use (Kylander et al. 2011). The Ti profile (Fig. 6) remains relatively stable before AD 1010, with an average value of 3200 ppm. The concentration then increases, with peak values of 4400 ppm between AD 1010 and AD 1335. After that time, Ti concentrations fall to an average of 3650 ppm, which is 14% more than the pre-landnám baseline. Beginning ca. AD 1960, the titanium content increases sharply to reach the maximum values of the profile at around 4600 ppm during the 1980s. Bulk sediment C:N ratios are widely used in palaeolimnology for assessing the abundance of terrestrial and aquatic components of organic matter (e.g., Kaushal and Binford 1999). For Igaliku, C:N ratios between 11.5 and 16 indicate a mixture of lacustrine and terrestrial contribution to the organic matter (OM) pool (algae: ≈4–10, lacustrine plants: ≈6–9, land plants: ≥20; Meyers and Ishiwatari 1993). Values above 14 during the period AD 1010–AD 1335 and after ca. 1960 indicate increases in terrestrial OM input resulting from soil erosion. The covariance of Ti and C:N ratio demonstrates that they constitute two robust, but related, proxies of soil erosion. Soil erosion can be quantified by calculating the mean denudation rate (DR; Fig. 6). DR, expressed in mm century-1, is calculated as follows: ([MARmin*Lake area] / catchment area) DR = [BMC*1000] Mass accumulation rates of minerogenic matter (MARmin) were calculated according to Enters et al. (2008) using wet bulk density, water content, minerogenic matter content (deduced from organic carbon measurement), and SAR derived from the age–depth model. A mean bulk mineral content Journal of the North Atlantic V. Bichet, E. Gauthier, C. Massa, and B.B. Perren 2014 Special Volume 6 57 windiness during the Little Ice Age (Willemse et al. 2003) but may also reflect four centuries of Norse farming, which could have decreased the resilience of the soils for a long time. From the beginning of the 20th century to ≈1960 (± 5 yr), corresponding to the re-establishment of farming at Igaliku, none of the sedimentary parameters reveals any significant increase of erosion around the lake of Igaliku (Fig. 6). Since ≈1960, the grazing pressure (shown by a decline in woody taxa and a rise in coprophilous fungi [Fig. 5]) caused a progressive increase in detrital parameters. Soil erosion accelerated beginning in 1969 (± 4 yr), with a sharp increase in Ti and C:N ratio, reaching ≈11 mm century-1 in 1988 (± 2.5 yr), slightly more than during the medieval period. Around 1988, major earthworks and digging of drainage ditches were carried out in both hayfields (Fig. 3), which caused soil erosion to increase dramatically up to 21 mm century- 1. After 1997 (± 2 yr), the erosion rate decreased continuously, whch may mark the stabilization of the material remobilized by the drainage work. Farming activities and their consequences on water quality: The diatom signal In a lake catchment, detrital organic matter and nutrient inflows due to animal dung and fertilizers may produce an excessive load of nitrogen (N) and phosphorus (P) and induce an increase in lake primause around Lake Igaliku also pre-dates the formation of an anthropogenic soil horizon at Garðar between by 100 years (ca. AD 1110–AD 1370; Buckland et al. 2009) and constitutes one of the oldest evidence of human impact in the area. Soil erosion is perfectly synchronized with the grazing pressure recorded by the amount of coprophilous fungal spores (grazing indicators; Gauthier et al. 2010). The grazing pressure and associated soil erosion remained high until ≈AD 1335, with high values and large amplitude fluctuations of the C:N ratio indicating a destabilization of soils in the watershed. Interestingly, our data also indicate a substantial decline of agro-pastoral practices around Lake Igaliku and reduced soil erosion beginning ≈AD 1230, well before the end of the Norse Eastern Settlement. After ≈AD 1335, grazing pressure decreased as indicated by the return of coprophilous fungal spores to pre-landnám background values. At the same time, the dwarf-shrub community recovered and soil erosion decreased to reach presettlement values. After the demise of the Norse colony and during the following four centuries of cool and dry Little Ice Age conditions (Jackson et al. 2005, Kaufman et al. 2009, Willemse et al. 2003), the C:N ratio returned to natural values, and soil erosion decreased to 3 mm century-1. Higher Ti concentrations are likely due to eolian dust inputs linked to the enhanced Figure 6. Sediment accumulation rate (SAR), minerogenic mass accumulation rate (MARmin—translated to soil-denudation rate), organic mass accumulation rate (MARorg), titanium concentration measured by ICP-AES (points) and calibrated from XRF scan results (curve),and C:N atomic ratio from the last 2000 years of the sediment archive of lake Igaliku. The grey shaded areas highlight the periods of Norse and modern farming. Journal of the North Atlantic V.Bichet, E.Gauthier, C. Massa, and B.B. Perren 2014 Special Volume 6 58 ry productivity. Lake-water trophic status reflects the nutrient balance between the lake and its catchment, and palaeoecological records of this balance help to illustrate the agricultural pressure on these systems (Anderson et al. 1995, Bradshaw et al. 2005). Among the studied parameters, diatoms provide insight into lake trophic changes over time and document nutrient enrichment in response to local agriculture. Over 140 species of diatoms from 25 genera were identified from the sediments of Lake Igaliku (see Perren et al. 2012a for details), and the flora is typical of dimictic oligotrophic West Greenlandic lakes (Perren et al. 2012b). During the last two millennia, diatom assemblages in the lake have been remarkably stable and poorly linked with climate changes. The Norse period has slightly higher relative frequencies of Cyclotella stelligera constrained to the period ca AD 1280–1350. However, the abundance of Fragilaria tenera, the main mesotrophic planktonic taxon stays under 5% of the total population (Fig. 7). During that time, despite grazing activities that used manure fertilization to increase soil nutrient levels (Buckland et al. 2009; Commisso and Nelson 2007, 2008; Ross and Zutter 2007), the diatom assemblages were almost undisturbed and the lake remained at a low trophic level. For the same period, the pastoral land use inferred by diatom changes appears to be better recorded at a pond site at Sissarluttoq (Edwards et al. 2011) than at the Lake Igaliku, where the nutrient input is probably diluted due to the lake size. In contrast, modern farming practices have produced a large ecological response from the diatom community, which reflect increased nutrient inputs and a change in the lake’s trophic status. Although diatoms appear to be stimulated by recent global warming (Box et al. 2009), the sharp increase of Fragilaria tenera since 1980, which recently reached more than 35% of the diatoms assemblage, strongly indicates nutrient enrichment of lake water within the last 30 years (Perren et al. 2012). This trophic shift is likely a response to the ≈200–250 kg.ha-1.yr-1 of N fertilizers that are deployed for hayfield production around Lake Igaliku (Miki Egede, farmer, Igaliku, Greenland, pers. comm.) as well as the effluent from the sheep stables that currently drains into the lake. A history of Norse and modern agriculture As shown above, selected proxies from the sediments of Lake Igaliku provide a detailed account of farming history and its impact on the local environment. During medieval times, events in the lake record are consistent with the archaeological documentation of the Norse at Igaliku. The first evidence of human impact appears ≈AD 1000, a few years Figure 7. Comparison of environmental changes recorded by the Igaliku Lake system during the last 2000 years. (A) Vegetation change (grazing indicators correspond to the sum of coprophilous, microrrhizal fungi, and Norse apophytes (e.g., Rumex acetosa-type and Ranunculus acris-type)); (B) Catchment soil-denudation rate and main historical cultural events; (C) Lake trophic status evaluated by mesotrophic diatoms (e.g., Fragilaria tenera); and (D) Climate change illustrated by Arctic temperature anomaly (from Kaufman et al. 2009) and Dye 3 winter δ18O (from Vinther et al. 2010). The grey shaded areas highlight the periods of Norse and modern farming. Journal of the North Atlantic V. Bichet, E. Gauthier, C. Massa, and B.B. Perren 2014 Special Volume 6 59 after the landnám (Fig. 7). Then, the increase of anthropogenic signals (pollen, NPP, and soil erosion parameters) indicates a progressive development of agro-pastoral activities until ≈AD 1230. Maximum erosion appears a few decades after the appointment of the first bishop (AD 1126) when Garðar was probably close to its maximum development and activity. However, compared with modern farming standards (Montgomery 2007), the highest level of Norse soil erosion recorded at Igaliku (8 mm. century-1) falls within the framework of sustainable agricultural practice. After ≈AD 1230, soil-erosion indicators began to decrease but remained at a high level, with largeamplitude fluctuations until ≈AD 1335. Climate records are too poorly resolved or too far removed on the inland ice to describe the weather conditions of this period in detail and their effects on the growing season. Since the mid-12th century, a regime of more extreme climatic fluctuations was inferred from outer Igaliku Fjord, with more influence of the cold East Greenland Current resulting in more sea ice and lower summer temperatures after ≈AD 1245 (Jensen et al. 2004). A succession of harsh winters is also noted in the Dye-3 winter δ18O record toward the end of the 12th century (Vinther et al. 2010). This drop in temperatures may have led to a significant reduction in numbers of sheep and grazing pressure because cold years in such marginal agricultural areas as southern Greenland can have dramatic consequences for livestock. After ≈AD 1335, our data indicate a substantial decline of agro-pastoral practices around Lake Igaliku. This last decline occurred a few decades before the death of the last bishop known to have lived at Garðar. This finding suggests that farming activity was already in decline before this historical event and well before the end of the Norse Eastern Settlement. The palaeoenvironmental evidence for a decrease in anthropogenic impacts is consistent with archaeological evidence of Norse adaptation to worsening climate conditions. During this period, the reduction of agricultural dependence is demonstrated by archaeofauna from several Norse farms and human isotopic data showing an increasing proportion of subsistence from hunting, fishing, and sealing sources (Arneborg et al. 1999, Dugmore et al. 2012, Enghoff 2003, Mc Govern et al. 1996, Mc- Govern and Pálsdóttir 2006). During the first step of the modern re-establishment of agriculture, from the beginning of the 20th century to ≈1960, the lake record suggests that the effect of sheep grazing around the lake was minimal. Erosion yield increased sharply after 1960 and became marked after 1988. These two periods of soil erosion, 1960–1988 and 1988–2007, are consistent with the two modern agricultural phases of south Greenland (Egede and Thorsteinsson 1982). The former corresponds to the first phase of free-ranging sheep, whereas the latter is the expression of intensified practices and hay-field management that followed the agrarian reform of 1982. That reform was related to the successive harsh winters of 1966/1967, 1971/1972, and 1976/1977 which starved and killed nearly 60% of the sheep in Greenland (Greenland Agriculture Advisory Board, 2009). As a result of current agricultural practices, the Igaliku Lake system is currently undergoing the most important environmental change of the last 2000 years, with soil erosion three times greater than during the Norse tenure. At the same time, nutrient impacts from industrial fertilizers have outpaced the geochemical and biological resilience of the lake, which is becoming mesotrophic like its more southerly European counterparts. Faced with a climate crisis and the resulting decrease in agricultural productivity, the Norse adapted their dietary habits and lifestyle, whereas modern society has used intensive practices and industrial processes to guard against failure. Towards a local climate reconstruction In the Eastern Settlement area, climate variability and its effects on the growing season drove adaptation or failure as much for Norse settlers as for modern Greenlandic farmers. A detailed reconstruction of short-lived climatic events during the medieval period and a reliable reconstruction of temperature variations at the local scale would greatly help assess the role of climate change on Norse population habits and migrations. While lake sediments have the temporal resolution necessary to capture the full range of climate variability, their suitability for palaeotemperature reconstructions is usually limited due to the lack of direct quantitative proxies. In suitable hard-water lakes (when the evaporative effect on lake-water δ18O is low), authigenic calcite may reliably record the lake-water δ18O, which is controlled by the δ18O of precipitation, itself strongly correlated to the mean annual temperature at high latitudes (Masson Delmotte et al. 2012). However, lakes in southern Greenland are poorly buffered, and calcite is largely absent there. So, our ongoing studies at Lake Igaliku are now focused on two ways of extracting palaeotemperatures records. The first involves chironomid larvae for both the analysis of chitin δ18O and species assemblage-based temperature inference. The latter uses alkenone biomarkers, which are highly resistant organic compounds produced by phytoplankton, where molecular long-chain saturation depends Journal of the North Atlantic V.Bichet, E.Gauthier, C. Massa, and B.B. Perren 2014 Special Volume 6 60 Acknowledgments The authors are grateful to M. Campy, H. Grisey, C. Petit, and B. Vannière, for technical help during the coring campaign in Greenland. This research is supported by the University of Franche-Comté, the University of Burgundy, the Burgundy Regional Council, the French Polar Institute (IPEV), and the ANR CEPS “Green Greenland” project. We acknowledge the two anonymous reviewers for their helpful and constructive remarks to improve the manuscript. Literature Cited Abbott, M.B., and T.W. Stafford. 1996. Radiocarbon Geochemistry of Modern and Ancient Arctic Lake Systems, Baffin Island, Canada. Quaternary Research 45:300–311. Algreen-Møller, N., and C.K. Madsen. 2006. The Norse in Vatnahverfi. Report on the reconnaissance and survey of Norse ruins in Vatnahverfi, summer 2005. SILA Arctic Centre of the National Museum of Denmark, field report 24. 38 pp. Copenhagen, Denmark Anderson, N.J., I. Renberg, and U. Segerstrom. 1995. Diatom and lake productivity responses to agricultural development in a Northern Swedish, boreal forest catchment. 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Using these complementary approaches, a quantified reconstruction of historical temperatures at decadal scale in the settlement may be feasible. Conclusion Among the palaeoenvironmental records studied in the Norse Eastern Settlement in southern Greenland, high-resolution lake records provide insight into local environmental history from the Norse colonization and medieval agro-pastoralism to the modern farming activities. The ongoing study of Lake Igaliku and its catchment demonstrates that the lake system is very sensitive to land use and conducive to a high-resolution reconstruction of past and modern agricultural impacts on the landscape. Pollen and non-pollen palynomorph studies demonstrate vegetation change due to climatic and anthropogenic forcing. The estimation of sediment yield from an accurate core chronology as well as the use of geochemical parameters are two valuable and independent methods to investigate past soil erosion. Diatoms record the lake-water response to the nutrient inputs resulting from agricultural practices. The environmental footprint of the Norse appears subtle. Vegetation was slightly impacted, and the diatom flora suggests that the lake trophic status was unmodified. Soil erosion was the main impact of medieval agro-pastoralism. However, even if the sediment yield increased during the Norse period, our data do not support the hypothesis of overgrazing for the Norse collapse at Igaliku. This scenario is contradicted by the progressive decrease in grazing pressure and agro-pastoral impacts, especially when compared with the high sediment yield of the modern agriculture. By modern environmental standards of soil erosion (Montgomery 2007), Norse agricultural impacts can be considered sustainable, though perhaps unsuited to subarctic climatic conditions for an agriculture-dependent economy. 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