Oil sands mining has been linked to increasing atmospheric deposition of polycyclic aromatic hydrocarbons (PAHs) in the Athabasca oil sands region (AOSR), but known sources cannot explain the quantity of PAHs in environmental samples. PAHs were measured in living Sphagnum moss (24 sites, n=68), in sectioned peat cores (4 sites, n=161), and snow (7 sites, n=19) from ombrotrophic bogs in the AOSR. Prospective source samples were also analyzed, including petroleum coke (petcoke, from both delayed and fluid coking), fine tailings, oil sands ore and naturally exposed bitumen. Average PAH concentrations in near-field moss (199 ng/g, n=11) were significantly higher (p=0.035) than in far-field moss (118 ng/g, n=13), and increasing temporal trends were detected in 3 peat cores collected closest to industrial activity. A chemical mass balance model estimated that delayed petcoke was the major source of PAHs to living moss, and among three peat cores the contribution to PAHs from delayed petcoke increased over time, accounting for 45-95% of PAHs in contemporary layers. Petcoke was also estimated to be a major source of vanadium, nickel, and molybdenum. Scanning electron microscopy with energy dispersive X-ray spectroscopy confirmed large petcoke particles (>10 μm) in snow at near-field sites. Petcoke dust has not previously been considered in environmental impact assessments of oil sands upgrading, but improved dust control from growing stockpiles may mitigate future risks.

Floating islands mysteriously moving around on lakes were described by several Latin authors almost two millennia ago. These fascinating ecosystems, known as free-floating mires, have been extensively investigated from ecological, hydrological and management points of view, but there have been no detailed studies of their rates of accumulation of organic matter (OM), organic carbon (OC) and total nitrogen (TN). We have collected a peat core 4 m long from the free-floating island of Posta Fibreno, a relic mire in Central Italy. This is the thickest accumulation of peat ever found in a free-floating mire, yet it has formed during the past seven centuries and represents the greatest accumulation rates, at both decadal and centennial timescale, of OM (0.63 vs. 0.37 kg/m(2)/yr), OC (0.28 vs. 0.18 kg/m(2)/yr) and TN (3.7 vs. 6.1 g/m(2)/yr) ever reported for coeval peatlands. The anomalously high accretion rates, obtained using (14)C age dating, were confirmed using (210)Pb and (137)Cs: these show that the top 2 m of Sphagnum-peat has accumulated in only ~100 years. As an environmental archive, Posta Fibreno offers a temporal resolution which is 10x greater than any terrestrial peat bog, and promises to provide new insight into environmental changes occurring during the Anthropocene.

Peat cores were collected from five bogs in the vicinity of open pit mines and upgraders of the Athabasca Bituminous Sands, and from a control site (Utikuma) which is 264 km SW. Frozen cores were sectioned into 1 cm slices, and trace metals determined in the ultraclean SWAMP lab using ICP-QMS. The uppermost sections of the cores were age-dated with (210)Pb using ultralow background gamma spectrometry, and selected plant macrofossils dated using (14)C. At each site, trace metal concentrations as well as enrichment factors (calculated relative to the corresponding M/Th ratio of the Upper Continental Crust) reveal maximum values 10 to 40 cm below the surface which shows that the zenith of atmospheric contamination occurred in the past. The age-depth models show that atmospheric contamination by trace metals (Ag, Cd, Sb, Tl, but also V, Ni and Mo which are enriched in bitumen) have been declining in northern Alberta for decades. In fact, the greatest contemporary enrichments of Ag, Cd, Sb and Tl (in the top layers of the peat cores) are found at the most remote site (UTK) showing that long-range atmospheric transport from other sources must be duly considered in any source assessment.

Extreme climate events are predicted to become more frequent and intense. Their ecological impacts, particularly on carbon cycling, can differ in relation to ecosystem sensitivity. Peatlands, being characterized by peat accumulation under waterlogged soil, can be particularly sensitive to climate extremes if the climate event increases soil oxygenation. However, a mechanistic understanding of peatland responses to persistent climate extremes is still lacking, particularly in terms of aboveground-belowground feedback. Here we present the results of a transplantation experiment of peat mesocosms from high to low altitude in order to simulate, during three years, a mean annual temperature c. 5°C higher and a mean annual precipitation c. 60% lower. Specifically, we aim at understanding the intensity of changes for a set of biogeochemical processes and their feedback on carbon accumulation. In the transplanted mesocosms, plant productivity showed a species-specific response depending on plant growth forms, with a significant decrease (c. 60%) of peat moss productivity. Soil respiration almost doubled and Q10 halved in the transplanted mesocosms in combination with an increase of activity of soil enzymes. Spectroscopic characterization of peat chemistry in the transplanted mesocosms confirmed the deepening of soil oxygenation which, in turn, stimulated microbial decomposition. After three years, soil carbon stock increased only in the control mesocosms whereas a reduction of mean annual carbon accumulation of c. 30% was observed in the transplanted mesocosms. Based on the above information, a structural equation model was built to provide a mechanistic understanding of the causal connections between peat moisture, vegetation response, soil respiration and carbon accumulation. This study identify in the feedback between plant and microbial responses the primary pathways explaining the reduction of carbon accumulation in response to recurring climate extremes in peat soils. This article is protected by copyright. All rights reserved.

In his Letter to Editor, Delarue (2016) suggests that our observation of increased microbial activity cannot adequately explain the observed changes in peat chemistry (Bragazza et al., 2016). Shortly, he argues that our interpretation of the results does not account for the plant cover dynamics, and/or the preferential decomposition of labile polysaccharides. Whilst Delarue (2016) raises an interesting point, below we demonstrate that his comments cannot explain the pattern in our data. This article is protected by copyright. All rights reserved.