Sources of riverine mercury across the Mackenzie River Basin; inferences from a combined Hg[sbnd]C isotopes and optical properties approach

Audrey Campeau,Karin Eklöf,Anne L Soerensen,Staffan Åkerblom,Shengliu Yuan,Holger Hintelmann,Magdalena Bieroza,Stephan Köhler,Christian Zdanowicz

doi:10.1016/j.scitotenv.2021.150808

Abstract

The Arctic environment harbors a complex mosaic of mercury (Hg) and carbon (C) reservoirs, some of which are rapidly destabilizing in response to climate warming. The sources of riverine Hg across the Mackenzie River basin (MRB) are uncertain, which leads to a poor understanding of potential future release. Measurements of dissolved and particulate mercury (DHg, PHg) and carbon (DOC, POC) concentration were performed, along with analyses of Hg stable isotope ratios (incl. ∆199Hg, δ202Hg), radiocarbon content (∆14C) and optical properties of DOC of river water. Isotopic ratios of Hg revealed a closer association to terrestrial Hg reservoirs for the particulate fraction, while the dissolved fraction was more closely associated with atmospheric deposition sources of shorter turnover time. There was a positive correlation between the ∆14C-OC and riverine Hg concentration for both particulate and dissolved fractions, indicating that waters transporting older-OC (14C-depleted) also contained higher levels of Hg. In the dissolved fraction, older DOC was also associated with higher molecular weight, aromaticity and humic content, which are likely associated with higher Hg-binding potential. Riverine PHg concentration increased with turbidity and SO4 concentration. There were large contrasts in Hg concentration and OC age and quality among the mountain and lowland sectors of the MRB, which likely reflect the spatial distribution of various terrestrial Hg and OC reservoirs, including weathering of sulfate minerals, erosion and extraction of coal deposits, thawing permafrost, forest fires, peatlands, and forests. Results revealed major differences in the sources of particulate and dissolved riverine Hg, but nonetheless a common positive association with older riverine OC. These findings reveal that a complex mixture of Hg sources, supplied across the MRB, will contribute to future trends in Hg export to the Arctic Ocean under rapid environmental changes.

Highlights

Mercury (Hg) is a neurotoxin that accumulates in the Arctic Ocean due to long-range atmospheric transport of anthropogenic emissions from lower latitudes ((AMAP), 2001; Driscoll et al, 2013)
Identifying the terrestrial sources of riverine Hg among Arctic rivers is key to understanding future contamination levels in the Arctic Ocean, and how these may evolve under rapid environmental changes
Data collected in this study suggest that Hg source tracing through isotope ratios (IR) in the Mackenzie River Basin (MRB) is complicated by mixing of different sources

Summary

Introduction

Mercury (Hg) is a neurotoxin that accumulates in the Arctic Ocean due to long-range atmospheric transport of anthropogenic emissions from lower latitudes ((AMAP), 2001; Driscoll et al, 2013). Permafrost thaw and increasing land-water connectivity due to rapid high-latitude warming in the Arctic may mobilize previously stored terrestrial Hg into aquatic environments (Bishop et al, 2020; Coquery et al, 1995; St Pierre et al, 2018). This could undermine the efficacy of policy efforts imposing limits on global anthropogenic Hg emissions under the Minamata Convention (2017) (Wang et al, 2019), and lead to continued or even increased Hg input to the Arctic Ocean (Schuster et al, 2011; Stern et al, 2012). Identifying the terrestrial sources of riverine Hg among Arctic rivers is key to understanding future contamination levels in the Arctic Ocean, and how these may evolve under rapid environmental changes

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