| Summary: | The 1783 Laki eruption in Iceland was among the largest high-latitude eruptions of the last millennium, but the seasonal and regional climate response was heterogeneous in space and time. In North America, tree-ring maximum latewood density (MXD) chronologies from Alaska suggest that the summer of 1783 was extraordinarily cold, even though the eruption did not begin until early June. In order to resolve the spatial extent and seasonal timing of the climate ffects of the Laki eruption, we use a combination of high-resolution quantitative wood anatomy analyses, climate model simulations, and proxy systems modeling to examine the temperature response over northwestern North America. We measured wood anatomical characteristics of white spruce (Picea glauca) trees from two locations in northern Alaska. Cell characteristics in the earlywood of the 1783 ring are normal while latewood cell wall thickness is significantly and anomalously reduced compared to non-eruption years. Applying complementary evidence from climate model experiments and proxy systems modeling, we interpret these features as indicating an abrupt and premature cessation of cell wall thickening due to an rapid decrease in temperature toward the end of the growing season linked to both forced and internal climate variability. Reconstructions using conventional annual resolution maximum latewood density likely over-estimate total growing season cooling in this year, while ring width data fail to capture this abrupt late summer volcanic signal. Our study has implications not only for the interpretation of the climatic impacts of the Laki eruption in North America, but more broadly demonstrates the importance of considering timing and internal variability when comparing proxy temperature reconstructions and climate model simulations, and demonstrates the value of developing cellular-scale tree-ring proxy measurements for paleoclimatology. Release after 11/14/2020
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