Buried in water, burdened by nature-Resilience carried the Iron Age people through Fimbulvinter.

Levänluhta is a unique archaeological site with the remains of nearly a hundred Iron Age individuals found from a water burial in Ostrobothnia, Finland. The strongest climatic downturn of the Common Era, resembling the great Fimbulvinter in Norse mythology, hit these people during the 6th century AD. This study establishes chronological, dietary, and livelihood synthesis on this population based on stable carbon and nitrogen isotopic and radiocarbon analyses on human remains, supported by multidisciplinary evidence. Extraordinarily broad stable isotopic distribution is observed, indicating three subgroups with distinct dietary habits spanning four centuries. This emphasizes the versatile livelihoods practiced at this boundary of marine, freshwater, and terrestrial ecosystems. While the impact of the prolonged cold darkness of the 6th century was devastating for European communities relying on cultivation, the broad range of livelihoods provided resilience for the Levänluhta people to overcome the abrupt climatic decline.

explains more than 60% of the observed JJA temperature variability [13]. Third, the studies of the AD 536 -550 climatic anomaly have extended to isotopic characterizations [29] and the recent development to construct tree-ring stable isotope chronologies for the Fennoscandia [30][31][32] has now resulted in annually resolved data of 13 C/ 12 C ratio (δ 13 C) that overlaps the mid-sixth century AD anomalies i.e. the period most topical to the underlying research questions of this study. Sensitivity of Arctic tree growth towards amount of sunlight has been well demonstrated and, particularly, photosynthetically active radiation (PAR) is considered as possibly the most important forcing factor for stable carbon isotopic ratios (δ 13 C) of northern conifers [33][34][35][36]. Compared over the instrumental period, our recent δ 13 C data show strong climatic signals related to incoming solar radiation [36,37], agreeing with the previous findings from adjacent areas [35]. Temperature reconstruction based on maximum latewood density (MXD) data [13] from northern Finland. Up: Annual MXD signal (grey) and its 5yr average (black). Down: Residual defined as ΔTJJA = TJJA -TJJA, ave. Long negative temperature anomaly spanning from AD 536 to ca. AD 710 resembles that of the Late Antique Little Ice Age (LALIA) proposed by Büntgen et al [14]. The length and the influence of this cold period are shown schematically with blue bars. This schematic representation has been used throughout the paper.
As a consequence, the δ 13 C data were recently used to reconstruct past variations in June-July irradiance between AD 519 and 610 [37] (Fig B). Particularly, during AD 541-543 the light irradiance reduced from ~190 W/m 2 to ~135 W/m 2 corresponding to nearly 30% loss. Also this reconstruction is based on tree-ring δ 13 C data produced using the RCS methods and explains more than 50% of the observed irradiance variability. Although the variations in summertime temperature and irradiance may be found interrelated [38], it was shown that the respective climatic signals are the most prominent drivers of the MXD and δ 13 C data in this region. That is, the MXD and δ 13 C data did not correlate significantly with instrumentally observed irradiance and temperature (JJA) records when these data were made statistically independent to each other. Instead, the MXD and δ 13 C proxies could be correlated significantly only with temperature and irradiance, respectively (see Figs S7 and S8 in Helama et al. 2018 [37]). The irradiance reconstruction we use in this study is available online as the supplementary material of  [37] downloadable at https://www.nature.com/articles/s41598-018-19760-w and the underlying isotope data as the NOAA contribution at https://www.ncdc.noaa.gov/paleo/study/23374.

Fig B.
Reconstructed solar irradiance based on annual δ 13 C measurements on pine (Pinus sylvestris) from northern Finland [37]. a) Solar irradiance (black line) and its estimated uncertaint (grey), and the mean irradiance of AD 519 -535. b) Difference (red line) between the reconstructed solar irradiance and the mean irradiance of AD 519 -535. The uncertainties (light red) have been estimated through Monte-Carlo simulation based on a).

Consequences of climatic downturns
According to Monteith [39] the efficiency of converting solar energy to photosynthetic dry mass can be estimated as a product of multiple factors, including atmospheric transmission of sunlight. Consequently, production of dry mass by photosynthesis, i.e. primary production, is inversely proportional to the absorption of sunlight by the atmospheric particles (clouds, dust, aerosols) and directly proportional to the amount of sunlight eventually available for photosynthesis (photosynthetically active radiation PAR). Recently, loss of sunlight induced by the mid-6 th century AD volcanic winter was quantified through stable carbon isotopic ratio measurements of Arctic Scotch pine [37]. The anomalic dark and cold period lasted nearly a decade and triggered a longer cold period of the Late Antique Little Ice Age (LALIA) the strongest influence lasting until ca. AD 570 [6] and milder until AD 710 (Fig A). In fact, most of the abrupt negative wood growth anomalies during the last 2000 years have been due to volcanic winters [40].
Both the MXD and δ 13 C proxies are closely related to agricultural potential in the study region. Comparing the MXD data [13,41] with rye (Secale cereale) and barley (Hordeum vulgare) yields [10], their similar positive (negative) responses to warm (cold) growing season temperatures were demonstrated over a period of non-industrialised cultivation over most of Finland (AD 1861-1913), and particularly for southern Ostrobothnia. Significantly, low temperature and significant crop loss of AD 1868 was accompanied with a low MXD signal [10] and the climatic decline led to a well-known famine and death of ca. 150 000 people in Finland. Moreover, this strong relationship enabled an MXD-based reconstruction of climate-mediated yield ratio (pertaining to harvested grain in relation to sown) in central and northern Finland over the late Holocene period [11]. Unfortunately, this reconstruction does not overlap the entire first millennium AD but was built over the timeline from AD 760 to 2000, this being due to reduced number of MXD series from middle/south boreal tree-ring archives [41] especially over the pre-AD 1000 period. More recently, however, this reconstruction was explored for climatic, agricultural and societal responses resulting from abrupt negative temperature departures in western Finland over the 17 th century AD following the explosive volcanic eruptions [12]. Additional indications of agricultural success over the earlier times can be derived from tree-ring isotopes. Given that the δ 13 C data is strongly indicate of irradiance [35][36][37] that controls the terrestrial photosynthetic production and crop yields [42], the δ 13 C variations must be directly related to potential of the contemporaries to utilise the agricultural plant products.. Collectively, the analyses performed demonstrate the value of MXD proxies to quantify the variations in climate-mediated yield ratio over the historical period and the potential of using the long MXD records to infer the first millennium yield ratios on annual resolution. Moreover, the negative δ 13 C/irradiance anomalies are explained to mean limited supply of harvested grain. These results reinforce the view of historical cultivation being highly sensitive to climate anomalies. Alternative method to assess the possible agricultural impacts of the 6 th century eruptions is to compare the impacts of latter volcanic eruptions on yields. However, it may not be reasonable to explore relationships between eruptions and crop yields with data from the era of industrial agriculture if aiming to explore the impacts on pre-industrial or prehistorical era, as agricultural practices have changed considerable during the recent past [8,43]. Prior the era of modern crop yield statistics and industrial agriculture, grain tithesapproximately one-tenth of the yield paid as a taxare commonly used as proxy data to explore pre-industrial harvest fluctuations [44][45][46]. Yet, the practices to collect the grain tithes varied over different regions and times. For the Southern Ostrobothnia, the grain tithes were based on the annual rye and barley harvest output over the seventeenth century [12]. Three large volcanic eruptions, Huaynaputina in AD 1600, Parker in AD 1641 and the unknown eruption in AD 1695, took place over this century [40].

Fig C.
Collected barley tithes (in barrels, c. 146.5 litres) on the year of peak volcanic forcing (year zero) and seven succeeding years [11]. The dashed lines (red for AD 1601 and black for AD 1641 and AD 1695) indicate the mean barley tithes on three data years preceding the volcanic events. In year AD 1601 tithes were not collected at all due to total harvest failure. The location of the Levänluhta has been marked with a star. Map was made with Natural Earth data (https://www.naturalearthdata.com/) and created by using QGIS 3.4. (https://www.qgis.org/).
The tithe data suggest that the harvest was half of the century mean on the year of the peak global volcanic forcing (AD 1601, AD 1641 and AD 1695, respectively [12]). Moreover, following the year of first crop failure, the harvest remained notably below the mean over one or two subsequent years. The agricultural impact of the 17 th century eruptions appears to be especially profound with barley ( Fig C). These harvest failures had, in turn, considerable impact on the wealth and wellbeing of the 17 th century agricultural population, as their subsistence was largely dependent on harvest success. Administrative written records indicate that sudden impoverishment followed each volcanic event [12]. Furthermore, the human resilience to cope with adverse climate and resulting harvest failures was low in the 17 th century due to man-made factorssuch as political instability, population growth, increased social inequality and poor trade networks [12,47,48]. Consequently, the crises that were triggered by the AD 1601 and AD 1695 anomalies accelerated to catastrophic famines and demographic crisis (Fig D). Overall, the comparative approach suggests that these 17 th century eruptions, which were likely lesser volcanic events than the AD 536 and AD 540 eruptions, and related anomalies in temperatures and irradiance caused sharp decline in grain yields and considerable socio-economic consequences in the Southern Ostrobothnia [12]. Importance of seal hunting at ca. AD 300 is demonstrated by the ancient Roman price list Edict of Diocletian [49][50][51]. Seal skin was, together with leopard skin, the most valued skin quality and its value corresponded to 25 times the daily income of an artisan. Moreover, seal oil has been traditionally used within Europe in oil lamps for lighting. Similarly than cultivation, seal hunting within the Baltic Sea [50] have also faced climatic consequences [52]. During the 16 th and 17 th century AD seal hunting was practicedparticularly in Ostrobothniaby seal hunters and local peasants through long-distance expeditions lasting several months. Historical records reveal the large preys obtained: during the peak year in AD 1558 the exported amount of seal oil from Finland was 307 000 litres [50]. At the end of the 16 th century AD the climate cooled when the Little Ice Age started to set in. The coldness extended the annual ice cover of the Gulf of Bothnia and the breeding grounds of grey seal (Halichoerus grypus) moved southwards. Consequently, the seal preys of Ostrobothnia declined drastically after ca. AD 1570 [50,52]. It is reasonable to think that the most significant climatic disturbance during the last 2000 years would have caused even larger effect.
Written and archaeological evidence suggests that the human consequences of the AD 536-550 eventextended to the Late Antique Little Ice Age (LALIA) [14] -were diverse and widespread, stretching from the British Isles to the Far East and from North Africa to Scandinavia [53]. Contemporary written records from the Mediterranean region document a mystery cloud dimming the sun in AD 536 for more than a year. The same sources record persistent cold and drought and one to two years of bad harvests being associated with the anomaly [54]. Sources from British Isles likewise mention crop failure and dearth in AD 536-539 [15]. Furthermore, Chinese sources notion signs of dimming of the sky, unusual cold events and crop failure in AD 536-537 [53]. In Northern Europe, particularly in Scandinavia and Estonia, agriculture suffered and settlements were abandoned after the cold years of AD 540s [55,56]. In many locations of these regions, the harvest shortfalls caused food shortageeven famineduring the following years. Moreover, the outbreak of the Justinian plague in AD 542 contributed to the hardship following the climatic deterioration and crop failures in Europe at the turn of the AD 530/540s [53,57]. The spread of the pandemic may have been accelerated due to the impacts of decreasing temperatures and irradiance on food availability and human immune system [14,37,58,59].
There are well-documented negative climatic impacts on cultivation and seal hunting during the 16 th and 17 th centuries AD within Ostrobothnia induced by volcanic eruptions and by the onset of the Little Ice Age. As the period of AD 536-550 has been the coldest decade observed during the last two millennia [40] and was extended to longer climatic disturbance of LALIA [14], it is reasonable to assume that this climatic downturn had similar, if even worse, negative influence in Ostrobothnia, particularly through losses of cultivated crops and receding seal breeding grounds. This assumption is quantitatively supported by nearly similar averages of reconstructed temperatures for pre-anomalic periods AD 500-535 (TJJA, ave 500-535 = 13.8(9) °C) and AD 1530-1570 (TJJA, ave 1530-1570 = 13.8(8) °C) based on Matskovsky and Helama (2014) [13]. The decadal light intensity loss inevitably reduced the primary production accordingly and eventually this would have led to severe consequences for societies dependent on cultivationparticularly at the northern latitudes where plant growth correlates strongly with light intensity and where long volcanic winter has multitude of chances to reduce the summer-night temperatures even below zero inducing frost damage on crops.