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As Permafrost Melts…

Mercury is rising.  And in many more ways than one.  As global temperatures go up, the Arctic ice is melting.  Sea level rises.  Carbon dioxide is released into the atmosphere.  But below the permafrost, another threat is lurking. 

Researchers estimated the amount of natural mercury currently stored in perennially frozen soils, or permafrost, in the Northern Hemisphere is twice as much as the rest of all soils, atmosphere and oceans combined together.

32-million gallons of mercury (Hg) are just sitting there, waiting to be potentially released into the Arctic environment – the equivalent of 50 Olympic-sized swimming pools.

The problem is that mercury, although naturally occurring, is damaging to humans and wildlife, especially in certain forms.


The Permafrost Feedback Cycle – Positive feedback from thawing permafrost amplifies the existing atmospheric global warming caused by human activities. Source: UNEP report “Policy Implications of Warming Permafrost”/

What is Permafrost exactly?

Permafrost exists at high latitudes.  It’s made of rocks or soil, which have remained at or below 0°C (32°F) – the freezing point of water – for two or more years.

Across the globe, permafrost covers large portions of Canada, Russia and other Northern countries.  Roughly 24% of exposed Earth, around 23 million km2 of land (8.8 million square miles) according to the National Snow and Ice Data Center.

The Arctic’s frozen soil, acts as a massive ice trap that keeps carbon stuck within the ground and out of the Earth’s atmosphere.  And scientists already knew that thawing Arctic permafrost would release powerful greenhouse gases.

All this naturally-trapped carbon could potentially add up to the greenhouse effect that would drive global warming if it were to be released as carbon dioxide CO2.

But not all is what it seems.

Arctic permafrost hides another threat to our planet’s ecosystems.


Total Soil Carbon and Mercury in Northern Hemisphere Permafrost Regions.  Source: Schuster et al. 2018

Soaked in Organic Mercury

Thousands of years of sedimentation buried atmospheric mercury bound to vegetable matter, and trapped it frozen in the Arctic soil.

Mercury (Hg) from the atmosphere normally deposits onto soil surfaces where it bonds with organic material in the active layer.

The organic matter is mostly plant roots.

At freezing temperatures, microbial decay ceases, thus trapping the Hg into the soil.  At the same time, sedimentation slowly increases soil depth in such a way that matter at the bottom of the active layer becomes frozen into permafrost.

But when temperatures rise…

Microbial decay then resumes.

Organic matter gets consumed, thereby releasing the mercury trapped in the Arctic soil.


What the Study Shows

The Arctic permafrost stores a massive amount of mercury (Hg).


Several variables are at work in the natural environment:

  1. Spatial variations in temperature 
  2. Variations in moisture change
  3. Microbial respiration rates


You also have to account for peaks in atmospheric mercury (Hg), resulting from the atmospheric mixing with ozone during summer, which can enhance Hg deposition 

32-million gallons of mercury (Hg)


Scientists analysed 13 soil cores from permafrost across Alaska.

The maximum depths of the cores varied between 0.98 and 2.48 metres below the land surface.  The chosen sites represented a variety of sedimentary conditions.


Maps of Mercury (Hg) jn Northern Hemisphere permafrost zones for four soil layers: 0-30 cm, 0-100 cm, 0-300 cm, and permafrost. The permafrost map represents the Hg bound to frozen organic matter below the ALD and above 300 cm depth. The relative uncertainty is 57% for all pixels. Source: Schuster et al. 2018

Some Findings

Maps of soil mercury display great spatial variability, reflecting different sedimentation histories.

Areas with high sedimentation rates show much higher soil mercury.


Over much of Siberia, for example, the slow organic decay due to ambient freezing temperatures, coupled with high sedimentation rates, has buried substantial amounts of carbon and mercury 


The geologists found little evidence of mercury deposition due to volcanic ash.

However, a large pool of mercury in the active layer, leaching into Arctic rivers, might help explain:

  • why the permafrost-dominated terrestrial environment is the dominant source of mercury in the Arctic Ocean
  • why the Arctic Ocean is a net source of mercury for the Atlantic Ocean and the Pacific Ocean.


The Danger

Changing climate in northern regions is causing permafrost to thaw with major implications for the global mercury (Hg) cycle.  Once Arctic permafrost and its associated organic material thaws, microbial decay will resume, potentially impacting the environmental balance.

A global temperature rise of 1.5 °C (2.7 °F) above current levels is all it would take to start the thawing of permafrost in Siberia, according to one group of scientists.

As permafrost thaws in the future, dangerous amounts of this mercury will inevitably get released into the global environmentaquatic resources, food chain and human health.

As yet, the magnitude of this risk is unknown.

Bit of a ticking time bomb really…



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