The origin of tree-ring reconstructed summer cooling in Northern Europe during the 18th century eruption of Laki

Julie Edwards (University of Arizona, julieedwards@email.arizona.edu), Kevin J. Anchukaitis (University of Arizona, kanchukaitis@arizona.edu), Björn E. Gunnarson (Stockholm University, bjorn.gunnarson@natgeo.su.se) , Charlotte Pearson (University of Arizona, c.pearson@ltrr.arizona.edu), Kristina Seftigen (University of Gothenburg, Swiss Federal Institute for Forest Snow and Landscape Research WSL, kristina.seftigen@gvc.gu.se), Georg von Arx (Swiss Federal Institute for Forest Snow and Landscape Research WSL, Oeschger Centre for Climate Change Research, University of Bern, georg.vonarx@wsl.ch), Hans W. Linderholm (University of Gothenburg, hansl@gvc.gu.se)


Introduction
More than 800 million people live within 100 km of an active volcano and may therefore 27 potentially be exposed to health and environmental hazards from eruptions [Hansell and 28 Oppenheimer , 2004;Brown et al., 2015]. Basaltic fissure eruptions, which are characeach curve were estimated from the residuals of the lowess fit. Although the eruption 146 and its environmental consequences did continue into 1784 [D'Arrigo et al., 2011;Zambri 147 et al., 2019b], for this study we consider 1783 the 'Laki year' and we use the remaining 148 30 years of wood anatomical data as a 'control' to provide context for growth anomalies 149 in 1783. 150 We calculated anatomical MXD (aMXD) as the maximum ratio between the cell wall for example, the raw cellular measurements are assigned 10 µm wide bands parallel to 156 the ring borders [Björklund et al., 2020]. Then, the median value of all cells within each X -10 EDWARDS ET AL.: APPARENT COOLING AFTER LAKI each ring. The average aMXD of the 9 cores were used to create an ensemble of multiple 161 resolution aMXD chronologies. A number of studies have now shown that detrending 162 may not be necessary for some anatomical data [Liang et al., 2013;Carrer et al., 2018;163 Björklund et al., 2020]. We calculated the Pearson correlation coefficient between the 164 original CSCAN2019 MXD chronology and each aMXD chronology from 1768-1798. We 165 also applied a Mann-Kendall test (α = 0.05) to the TRW, MXD, and aMXD chronologies 166 to identify any significant trends. For automatic detection of IADFs, we leveraged the increase in lumen area in latewood 174 that is caused by IADFs. We first created tracheidograms [Vaganov , 1990], standardized 175 to 100 cells, of lumen area for each core from 1768-1798. Then, we applied a 10-cell 176 Gaussian smoothing filter to the standardized tracheidograms. Years with an IADF were 177 then defined by a peak in the smoothed lumen area tracheidogram in the last 20% of the 178 ring.

Climate data
In order to evaluate the climate signal contained in wood anatomy and wood density 180 series from the 18th century, as well as the potential for short-term weather fluctua-

X-ray fluorescence
We conducted X-ray fluorescence (XRF) analysis on three tree-ring core samples in 216 order to test for the presence of a potential dendrochemical response from the impact of 217 the acidic haze. Two trees were young (< 33 years old) and one tree was mature (> 100 218 years old) at the time of the eruption. These three cores were also used for QWA analysis.

219
Tree rings can be promising targets for detecting sulfur pollution and may be used to

Carbon isotope data
The ∆ 13 C series has a large negative excursion at 1783-1785, with the largest decrease 301 in isotopic discrimination in 1784 ( Figure 10). If we consider the entirety of the 1768-302 1798 period, there is no significant correlation between any of the MXD chronologies 303 and the ∆ 13 C series (Table 1) with an acid deposition hypothesis, were inconclusive, and cannot be strongly argued to 322 support the direct impact of the acidic Laki haze on wood anatomical anomalies in 1783.   ing anomalous years associated with rare disturbances or extreme environmental events.

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However, it is not yet clear that it is possible to recover the 'true' temperature signature 521 from rings where anatomical anomalies reflect a non-climate cause. Further work is nec-522 essary to determine how best to use aMXD records to analyze and quantify these extreme  d.