Enhanced particulate Hg export at the permafrost boundary, western Siberia.

Arctic permafrost soils contain large amounts of organic carbon and the pollutant mercury (Hg). Arctic warming and associated changes in hydrology, biogeochemistry and ecology risk mobilizing soil Hg to rivers and to the Arctic Ocean, yet little is known about the quantity, timing and mechanisms involved. Here we investigate seasonal particulate Hg (PHg) and organic carbon (POC) export in 32 small and medium rivers across a 1700 km latitudinal permafrost transect of the western Siberian Lowland. The PHg concentrations in suspended matter increased with decreasing watershed size. This underlines the significance of POC-rich small streams and wetlands in PHg export from watersheds. Maximum PHg concentrations and export fluxes were located in rivers at the beginning of permafrost zone (sporadic permafrost). We suggest this reflects enhanced Hg mobilization at the permafrost boundary, due to maximal depth of the thawed peat layer. Both the thickness of the active (unfrozen) peat layer and PHg run-off progressively move to the north during the summer and fall seasons, thus leading to maximal PHg export at the sporadic to discontinuous permafrost zone. The discharge-weighed PHg:POC ratio in western Siberian rivers (2.7 ± 0.5 μg Hg: g C) extrapolated to the whole Ob River basin yields a PHg flux of 1.5 ± 0.3 Mg y, consistent with previous estimates. For current climate warming and permafrost thaw scenarios in western Siberia, we predict that a northward shift of permafrost boundaries and increase of active layer depth may enhance the PHg export by small rivers to the Arctic Ocean by a factor of two over the next 10-50 years.