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We report magnetic field measurements spanning about 15 yr of four massive (7.5–15 M⊙) supergiant stars: α Per (HD 20902, F5Iab), α Lep (HD 36673A, F0Ib), η Leo (HD 87737, A0Ib) and 13 Mon (HD 46300, A1Ib). For each star, spectropolarimetric observations were collected using ESPaDOnS at the Canada–France–Hawaii Telescope. The observed spectra were coadded, normalized, and then processed using least-squares deconvolution to yield mean Stokes I and V profiles. Each spectrum was analyzed to infer the false-alarm probability of signal detection, and the longitudinal magnetic field was measured. This process yielded persistent detection of magnetic fields in all four stars. The median 1σ longitudinal field uncertainty of the Zeeman detections was 0.6 G. The maximum unsigned longitudinal magnetic fields measured from the detections are rather weak, ranging from 0.34  ±  0.19 G for α Lep to 2.61  ±  0.55 G for 13 Mon. The Zeeman signatures show different levels of complexity; those of the two hotter stars are relatively simple, while those of the two cooler stars are more complex. The stars also exhibited different levels of variability of their Zeeman signatures and longitudinal fields. We report periodic variability of the longitudinal field and (complex) Stokes V profiles of α Per with a period of either 50.75 days or 90 days. The (simple) Stokes V profiles of 13 Mon, and probably those of η Leo, show global polarity changes once during the period of observation, but the data are insufficient to place strong constraints on the variability timescales. 

Abstract In eastern Canada, Black spruce (Picea marianaMill. B.S.P.) grows in a wide variety of climates, from maritime-oceanic conditions near the Labrador Sea, to more continental climates, inland. Along this gradient, timing and provenance of heat and moisture that support growth are uncertain, weakening our capacity to predict the response of boreal ecosystems to climate variability. Here, we measured the stable oxygen isotopic composition of black spruce tree-ring cellulose at three sites in eastern Canada and provide evidence of a rapid decrease of Labrador Sea’s influence on adjacent ecosystems. Our results report a landwards decrease in the oxygen isotope composition of both tree-ring cellulose (δ¹⁸O_TRC) and precipitation water (δ¹⁸O_p). We also reveal a rapid landwards decoupling betweenδ¹⁸O_TRCvariability (1950-2013), maximum temperature and Sea Surface Temperature variations over the Northwest Atlantic. Thus, despite their apparent ecological homogeneity, eastern Canada’s black spruce ecosystems rely on heterogeneous sources of heat and moisture. 

Abstract Tree-ring chronologies underpin the majority of annually-resolved reconstructions of Common Era climate. However, they are derived using different datasets and techniques, the ramifications of which have hitherto been little explored. Here, we report the results of a double-blind experiment that yielded 15 Northern Hemisphere summer temperature reconstructions from a common network of regional tree-ring width datasets. Taken together as an ensemble, the Common Era reconstruction mean correlates with instrumental temperatures from 1794–2016 CE at 0.79 ( p  < 0.001), reveals summer cooling in the years following large volcanic eruptions, and exhibits strong warming since the 1980s. Differing in their mean, variance, amplitude, sensitivity, and persistence, the ensemble members demonstrate the influence of subjectivity in the reconstruction process. We therefore recommend the routine use of ensemble reconstruction approaches to provide a more consensual picture of past climate variability.