A millennial summer temperature reconstruction for northeastern Canada using oxygen isotopes in subfossil trees

Abstract

Climatic reconstructions for northeastern Canada are scarce such that this area is under-represented in global temperature reconstructions. To fill this lack of knowledge and identify the most important processes influencing climate variability, this study presents the first summer temperature reconstruction for eastern Canada based on a millennial oxygen isotopic series (δ18O) from tree rings. For this purpose, we selected 230 well-preserved subfossil stems from the bottom of a boreal lake and five living trees on the lakeshore. The sampling method permitted an annually resolved δ18O series with a replication of five trees per year. The June to August maximal temperature of the last millennium has been reconstructed using the statistical relation between Climatic Research Unit (CRU TS3.1) and δ18O data. The resulting millennial series is marked by the well-defined Medieval Climate Anomaly (MCA; AD 1000–1250), the Little Ice Age (AD 1450–1880) and the modern period (AD 1950–2010), and an overall average cooling trend of −0.6 °C millennium−1. These climatic periods and climatic low-frequency trends are in agreement with the only reconstruction available for northeastern Canada and others from nearby regions (Arctic, Baffin Bay) as well as some remote regions like the Canadian Rockies or Fennoscandia. Our temperature reconstruction indicates that the Medieval Climate Anomaly was characterized by a temperature range similar to the one of the modern period in the study region. However, the temperature increase during the last 3 decades is one of the fastest warming observed over the last millennium (+1.9 °C between 1970–2000). An additional key finding of this research is that the coldest episodes mainly coincide with low solar activities and the extremely cold period of the early 19th century has occurred when a solar minimum was in phase with successive intense volcanic eruptions. Our study provides a new perspective unraveling key mechanisms that controlled the past climate shifts in northeastern Canada

Bayesian multiproxy temperature reconstruction with black spruce ring widths and stable isotopes from the northern Quebec taiga

“…Thus, this variable was retained for the climate reconstruction. This is consistent with previous findings showing that summer temperatures control the growth and isotopic values of these trees (Gennaretti et al 2014;Naulier et al 2014Naulier et al , 2015. Low and high temperature frequencies were treated separately and obtained with the same method as for the ring width chronology such that the LF chronology was comparable with that of the stable…”

“…The R 2 of the LF 3P-STREC versus the temperature of the last century was 0.81. This is a substantial improvement relative to previous reconstructions with the same ring width data (STREC; R 2 = 0.64; Gennaretti et al 2014) or with the same δ 18 O data (i-STREC; R 2 = 0.58 or 0.64 if considering mean maximal temperature values such as in Naulier et al 2015). Note that here and hereafter, the comparisons with previous data were performed under the same conditions (using the same climate variable, frequency components, smoothing algorithm and 1905-2006 or 997-2006 periods).…”

“…From one of the six sites from the northern Quebec taiga, 60 subfossil logs and 5 living trees were further analyzed at the Delta-lab of the Geological Survey of Canada to extract two millennium-long chronologies of stable isotope ratios in tree-ring cellulose. The δ 18 O chronology is already in the public domain (Naulier et al 2015), whereas the δ 13 C chronology is detailed here and is accessible in the supplementary material (Dataset S1). These chronologies were built with the "offset-pool plus join-point" method (Gagen et al 2012) to obtain an annual resolution from five tree replicates (this replication was proven to be adequate to obtain robust site chronologies; Naulier et al 2014) and successive tree cohorts.…”

“…Assuming that the five time series of each cohort are realizations of the same stochastic process, this method produces chronologies equivalent to time series smoothed by the aforementioned 9-year triangular filter. Indeed, within every cohort of five trees, the rings of each tree were divided into 5-year blocks for the isotopic measurements with an offset of 1 year among trees (Naulier et al 2015). The δ 13 C values of the modern part of the chronology were also mathematically corrected for atmospheric δ 13 C CO 2 changes due to fossil fuel combustion (Suess effect), and for plant response to increasing isotopes.…”

“…B.S.P] subfossil trees preserved in six lakes. Samples from one of these sites were also used to obtain a millennium-long record of oxygen isotopic ratios (δ 18 O) in tree-ring cellulose (Naulier et al 2015). Until now, these ring width and δ 18 O data have only been used separately in independent summer temperature reconstructions.…”