Mercury Export from Arctic Great Rivers

Land–ocean linkages are strong
across the circumpolar north,
where the Arctic Ocean accounts for 1% of the global ocean volume
and receives more than 10% of the global river discharge. Yet estimates
of Arctic riverine mercury (Hg) export constrained from direct Hg
measurements remain sparse. Here, we report results from a coordinated,
year-round sampling program that focused on the six major Arctic rivers
to establish a contemporary (2012–2017) benchmark of riverine
Hg export. We determine that the six major Arctic rivers exported
an average of 20 000 kg y–1 of total Hg (THg,
all forms of Hg). Upscaled to the pan-Arctic, we estimate THg flux
of 37 000 kg y–1. More than 90% of THg flux
occurred during peak river discharge in spring and summer. Normalizing
fluxes to watershed area (yield) reveals higher THg yields in regions
where greater denudation likely enhances Hg mobilization. River discharge,
suspended sediment, and dissolved organic carbon predicted THg concentration
with moderate fidelity, while suspended sediment and water yields
predicted THg yield with high fidelity. These findings establish a
benchmark in the face of rapid Arctic warming and an intensifying
hydrologic cycle, which will likely accelerate Hg cycling in tandem
with changing inputs from thawing permafrost and industrial activity.