Trevor J. Porter

Climatic Signals in δ13C and δ18O of Tree-rings from White Spruce in the Mackenzie Delta Region, Northern Canada

Abstract Here we present the first tree-ring series (1850–2003) of stable carbon (δ13C) and oxygen (δ18O) isotope ratios from a high-latitude treeline site in northwestern Canada. Both δ13C and δ18O were measured at annual resolution from whole-ring α-cellulose of three white spruce trees (Picea glauca [Moench] Voss) growing in the Mackenzie Delta. There is a strong positive association between δ13C and maximum summer temperatures. This relation likely results from the influence of temperature-induced drought stress on stomatal conductance. Mean summer relative humidity is also significantly correlated, inversely, with δ13C reflecting its direct influence on stomatal conductance. The δ18O record is strongly and positively correlated with early-spring to mid-summer minimum temperatures likely owing to the temperature dependence of δ18O in precipitation and uptake of this water during the growing season. Mean summer relative humidity is also significantly and inversely correlated with δ18O due to leaf water evaporative enrichment. Our δ13C and δ18O records contain a large amount of climate-driven variability indicating their considerable potential to infer past climate changes in the Mackenzie Delta region.

A global multiproxy database for temperature reconstructions of the Common Era

Reproducible climate reconstructions of the Common Era (1 CE to present) are key to placing industrial-era warming into the context of natural climatic variability. Here we present a community-sourced database of temperature-sensitive proxy records from the PAGES2k initiative. The database gathers 692 records from 648 locations, including all continental regions and major ocean basins. The records are from trees, ice, sediment, corals, speleothems, documentary evidence, and other archives. They range in length from 50 to 2000 years, with a median of 547 years, while temporal resolution ranges from biweekly to centennial. Nearly half of the proxy time series are significantly correlated with HadCRUT4.2 surface temperature over the period 1850–2014. Global temperature composites show a remarkable degree of coherence between high- and low-resolution archives, with broadly similar patterns across archive types, terrestrial versus marine locations, and screening criteria. The database is suited to investigations of global and regional temperature variability over the Common Era, and is shared in the Linked Paleo Data (LiPD) format, including serializations in Matlab, R and Python.

400-Year Record of Atmospheric Mercury from Tree-Rings in Northwestern Canada

Tree-rings are a promising high-resolution archive for gaseous atmospheric mercury (composed primarily of Hg0) reconstruction, but the influence of cambial age (ring number from pith) and tree-specific differences are uncertainties with potential implications for interpreting tree-ring Hg signals. We address these uncertainties and reconstruct the last 400 years of Hg0 change using a tree-ring Hg data set from 20 white spruce ( Picea glauca) trees from a pristine site in central Yukon. Cambial age has no significant influence on tree-ring Hg concentration, but tree-specific differences in mean concentration are prevalent and must be normalized to a common mean to accurately constrain long-term trends in the mean tree-ring Hg record. Our record shows stable, low Hg0 concentrations prior to ∼1750 CE, a persistent rise from ∼1750-1950 (increasing more rapidly post-1850), a pause from ∼1951-1975, and then a resumed increase to record-high levels at present. This general pattern is reflected in other proxy-based Hg reconstructions worldwide. This study provides a novel long-term Hg0 reconstruction in the Western subarctic from one of the most widely distributed boreal tree species in North America and, therefore this proxy may also hold potential for investigating broader spatial patterns in Hg0 cycling across the subarctic and northern boreal forest.

Data Descriptor: A global multiproxy database for temperature reconstructions of the Common Era

PAGES, a core project of Future Earth, is supported by the U.S. and Swiss National Science Foundations. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Some of this work was conducted as part of the North America 2k Working Group supported by the John Wesley Powell Center for Analysis and Synthesis, funded by the U.S. Geological Survey. B. Bauer, W. Gross, and E. Gille (NOAA National Centers for Environmental Information) are gratefully acknowledged for helping assemble the data citations and creating the NCEI versions of the PAGES 2k data records. We thank all the investigators whose commitment to data sharing enables the open science ethos embodied by this project.