Medieval Warmth Confirmed at the Norse Eastern Settlement in Greenland G

https://doi.org/10.1130/G45833.1 Manuscript received 9 July 2018 Revised manuscript received 5 January 2019 Manuscript accepted 11 January 2019 © 2019 Geological Society of America. For permission to copy, contact [email protected]. Published online 6 February 2019 Medieval warmth confirmed at the Norse Eastern Settlement in Greenland G. Everett Lasher and Yarrow Axford Department of Earth and Planetary Sciences, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, USA ABSTRACT show pronounced cold conditions during the LIA (Axford et al., 2013), but 250 km south, temperatures are highly variable over the past 3000 yr with no clear MCA or LIA expression (D’Andrea et al., 2011). The alkenonebased record of D’Andrea et al. (2011) was found to be antiphased with a speleothem δ18O record from Ireland, suggesting contrasting climates between West Greenland and Europe. If similar antiphas ing with Europe led to cool conditions in South Greenland during Medieval times, this would contradict the hypothesis that climatic warming facilitated Norse settlement there. One mechanism proposed to explain the MCA in Europe, and now evidence for coeval antiphased cooling around Baffin Bay, is the North Atlantic Oscillation (NAO). Based upon a tree ring– and speleothembased NAO recon INTRODUCTION AND MOTIVATION reaching their maximum late Holocene extents struction, Trouet et al. (2009) hypothesized Abundant evidence exists across much of during the Little Ice Age (LIA, ca. 1250–1850 that a dominantly positive NAO (NAO+) mode northern Europe for warm conditions during CE) (Larsen et al., 2016; Miller et al., 2012). In persisted during the MCA, delivering warm air the Medieval Climate Anomaly (MCA, ca. contrast, recent investigations also found moun masses to northern Europe. Observations of NAO 950–1250 CE) (Solomina et al., 2016), however tain glacier advance comparable to that of the variability suggest that a positive mode could climate during this time was more variable else LIA between 975 and 1275 CE on Baffin Island result in cool conditions in South Greenland– where in the Northern Hemisphere (PAGES 2k and in West Greenland (Jomelli et al., 2016; West Greenland–Baffin Bay during the MCA and Consortium, 2013). This period is of interest in Young et al., 2015). Marine records around Baf perhaps explain glacial advance there (Young Greenland because favorable climate conditions fin Bay also give contrasting views of condi et al., 2015). However, other studies contra are often associated with Norse settlement of tions for the past 2000 yr. Some results indicate dict the notion of persistent NAO+ conditions the island at 985 CE (Jones, 1986). Cooling and a warm MCA relative to the LIA (e.g., Perner throughout this time period (e.g., Ortega et al., increased climate variability is conventionally et al., 2012), while others find expanded seaice 2015) (Fig. DR1 in the GSA Data Repository1). implicated in the collapse of the Norse settle cover and cool seasurface temperatures (SSTs) To test whether South Greenland experi ments at ca. 1450 CE, however nonclimatic during the MCA (e.g., Sha et al., 2017). Quan enced cold conditions during the MCA due to explanations have also been proposed (Dug titative proxy records of Greenland climate are dominant NAO+ conditions, we reconstruct more et al., 2012; Hartman et al., 2017). Data sparse, and nearly all highresolution records δ18O of precipitation over the past 3000 yr at a constraining local climate within the settlement are from ice cores documenting conditions over site within the western dipole of the NAO and areas during this period are limited (Millet et al. the Greenland Ice Sheet. Most terrestrial records within the Norse Eastern Settlement (Fig. 1). 2014), and it is unclear whether they support from Greenland are too coarse to resolve the At our study site, both temperature and mois climatic explanations for Norse settlement his MCA and LIA, and those with adequate tem ture sources are influenced by the NAO, and tory in Greenland. poral resolution suggest spatial heterogeneity both affect precipitation δ18O values in the same Many glaciers around South and West Green in the climatic expression of these periods. direction, amplifying rather than obscuring the land and Baffin Island advanced after the MCA, For example, West Greenland lake records isotopic signature of warming or cooling. 1GSA Data Repository item 2019096, Figure DR1 (comparative North Atlantic Oscillation reconstructions), Figure DR2 (agedepth model for SL core 16LOWU2), Figure DR3 (isotopes of precipitation and lake water in South Greenland), and Table DR1 (radiocarbon ages from Scoop Lake), is available online at http:// www .geosociety .org /datarepository /2019/, or on request from editing@ geosociety .org. CITATION: Lasher, G.E., and Axford, Y., 2019, Medieval warmth confirmed at the Norse Eastern Settlement in Greenland: Geology, v. 47, p. 267–270, https:// doi .org /10 .1130 /G45833.1 Geological Society of America | GEOLOGY | Volume 47 | Number 3 | www.gsapubs.org 267 Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/47/3/267/4650216/267.pdf by Stephen Matthew Crabtree on 11 September 2019 80°N RESULTS Annual NAO - 2m Five 14C ages were used to develop the age Temperature Correlation depth model using the R software package Baff –0.70+0 0.55 rbacon v. 2.3.4 (Blaauw and Christen, 2018) 70°N in Ba EG (Fig. DR1), after rejection of three outliers fall C +0.55 Y1 y GISP2 ing outside the 95% confidence intervals of a model containing all dates (Table DR1). Given J Y2 60°N Davis MD99-2322 sedimentation rates, each 1cmthick sample Strait MD99-2275 18 DYE-3 integrates an average of 40 yr. δ O values mea WS sured from CHCs decrease by 2.5‰ from the beginning of the record 3000 yr ago to 900 CE WGC SL/ES IC 50°N (Fig. 2A). This trend is interrupted at 900 CE RAPiD-35-COM 18 80°W when δ O values increase by 1.5‰. The period SPG 20°E AI07-03G RAPiD-21-COM between 900 and 1400 CE is characterized by 18 NAC overall more positive δ O values than both the 60°W 0°W previous and following four centuries. A brief 40°W 20°W (<200 yr) period of lower values is centered at 1150 CE The 15th century marks the transition Figure 1. Sites, ocean currents, and atmospheric patterns in Greenland and North Atlantic to overall lower values and exhibits the highest discussed in text. NAO—North Atlantic Oscillation; SL/ES—Scoop Lake and Norse Eastern variability of the record. Low δ18O values persist Settlement; WS—Western Settlement; blue triangles are ice cores (Alley, 2004; Vinther et al., 18 2010); green diamonds are marine records (Sicre et al., 2014; Miettinen et al., 2015; Moffa- between 1500 and 1900 CE. δ O values from Sanchez and Hall, 2017); purple triangles are West Greenland and Baffin Bay moraine records surface sediments are 2‰ higher than average from Young et al. (2015; Y1 and Y2) and Jomelli et al. (2016; J); NAC—North Atlantic Current; values between 1600 and 1900 CE, indicating SPG—subpolar gyre; EGC—East Greenland Current; WGC—West Greenland Current; IC— an increase in precipitation δ18O values during Irminger Current. the last century. We estimate temperature from inferred precipitation δ18O using two relationships: METHODS AND APPROACH average lake water δ18O over the past 40 yr is the global δ18Otemperature relationship of We isolated and analyzed δ18O of subfos –11.7‰. This value is near identical to mean 0.67‰/°C (Dansgaard, 1964), and a similar sil chironomids (the aquatic larvae of Insecta: measured precipitation δ18O of summer precipi relationship observed at coastal highlatitude Diptera: Chironomidae) preserved in lake sedi tation collected over three weeks in August 2016 North Atlantic International Atomic Energy ments. Scoop Lake (60.697°N, 45.419°W) is (–11.9‰), and to the amountweighted annual Agency (IAEA) sites (0.71‰/°C; Arppe et al., a 7mdeep, ~0.05 km2, remote, nonglacial, average of historical (1961–1974 CE) precipita 2017). These relationships translate into Neogla throughflowing lake located at 477 m above tion (–12.0‰) from Kangilinnguit, Greenland cial cooling of ~2 °C between 3000 and 1500 sea level on sparsely to unvegetated grano (150 km northwest) (IAEA/WMO, 2017). Esti yr ago (Fig. 2D). This cooling trend was inter diorite bedrock. We recovered a 156cmlong mated residence time of water in Scoop Lake is rupted by warming of ~1.5 °C between 900 and gyttja core (16LOWU2), with an intact 1–3 yr. Considering these factors, we argue that 1400 C.E., punctuated by a multidecadal cool sedimentwater interface and intact horizon CHC δ18O from Scoop Lake is a strong proxy ing event. Consistent LIA cooling, 1.5 °C colder tal stratigraphy throughout (Fig. DR2), in for annual precipitation δ18O values over South than the preceding warmth, lasted between 1500 August 2016 and analyzed the upper 80 cm Greenland. and 1900 CE and was followed by a warming of the core for a highresolution late Holocene Precipitation δ18O values in the high lati of 2 °C to present. sequence. Eight aquatic plant macrofossils tudes are strongly correlated with surface air were accelerator mass spectrometry 14C dated temperatures, and in South Greenland are also DISCUSSION from this zone (Table DR1 in the Data Reposi influenced by SSTs and moisture source (Bonne The hypothesized antiphase cold expres tory). Chironomid head capsules (CHCs) were et al., 2014; Sodemann et al., 2008). In South sion over Greenland during the MCA from prepared following the methods described in Greenland, moisture source is strongly influ a dominant NAO+ configuration is not sup Lasher et al.

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