Tracking nuclear generating station emissions and fossil fuel contributions using tree rings in southern Ontario, Canada

Abstract

Changing atmospheric concentrations of stable (12C, 13C) and radioactive (14C) isotopes of carbon are an important indicator of the Anthropocene. Carbon from anthropogenic sources can be released to the atmosphere as radiocarbon (14C) from nuclear power and above-ground nuclear weapons testing. Carbon-12 and carbon-13 (12,13C) are derived from fossil fuel combustion in the urban, industrial, and transportation sectors. In Ontario (Canada), ~half of the province’s power generation is derived from three nuclear generating stations (NGS), including Bruce Nuclear Generating Station (BNGS), one of the world’s largest NGS. Thus, understanding the contributions of NGS emissions and fossil fuel use in urban, industrial, and transportation sectors is essential for developing accurate atmospheric carbon inventories in southern Ontario. This study analyzes tree-ring Δ14C (the amount of 14C in a particular sample) as a proxy for atmospheric Δ14C by isolating tree-ring alpha cellulose and using accelerator mass spectrometry (AMS). Results indicate that Δ14C tree-ring signatures from white spruce (Picea glauca) and blue spruce (Picea pungens) can track 14C-enrichment from NGS 14C emissions when comparing peaks in tree-ring Δ14C offset data and 14C emissions during the trees’ growing season. Tree-ring Δ14C signatures are also observed in Canada’s largest urban centre the City of Toronto, which is nearby to Pickering Nuclear Generating Station (PNGS). Conversely, Toronto also experiences fossil fuel contributions and thus is susceptible to the Suess Effect. We observe fossil fuel-derived 12,13C from urban activities via tree-ring Δ14C-depletion. Peaks were also observed in the tree-ring Δ14C offset and PNGS 14C emissions. To determine 14C-enrichment or -depletion, sites are compared to a regional clean-air site (Egbert, Ontario, Canada) and northern hemisphere atmospheric background sites (Northern Hemisphere Zone 1; Jungfraujoch, Switzerland). Tree growing season and meteorology analyses strengthen the validity of this study’s results. Similar Δ14C levels from white spruce tree rings and an Environment and Climate Change Canada (ECCC) air sampler (ρ = 0.9273) were also observed. Thus, tree-ring Δ14C signatures can be used to measure local influences from anthropogenic sources of NGS 14C emissions and fossil fuel contributions to help build an atmospheric carbon inventory.