Detecting winter biogeochemical connectivity in polar environments with silicon isotopes

The seasonally thawing active layer in permafrost environments is no longer fully frozen during winter. Here, we demonstrate that active layer areas are biogeochemically connected during periods when they were previously locked, creating pathways for organic carbon export from permafrost soils.
Published in Earth & Environment
Detecting winter biogeochemical connectivity in polar environments with silicon isotopes

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In April-May 2018, we were three female scientists leading an expedition to Alaska at the transition period between Winter and Spring. Late Winter snowfall had covered the tundra with a large white coat. The timing was perfect. We were poised to capture the transition from frozen soils and ice-covered rivers to soil thaw and soil to river connectivity. Moreover – and key to our papers’ findings – we were set to reveal that permafrost degradation can play havoc with the accustomed order of events (soil thaw and snowmelt) during Spring. 

We collaborate with the Northern Arizona University who has been monitoring the site of Eight Mile Lake for more than ten years. This long-term monitoring of thaw depth, water table depth and air temperature is key to the findings in this paper. This firmly states that long term permafrost degradation (increase in thaw depth) is occurring at this site. With increasing ground temperature, the seasonally thawing upper part of the ground called the "active layer" is thickening. This leads to the thawing of the permafrost, or perennially frozen ground. This degradation of the permafrost has multiple consequences for the ecosystem such as ground subsidence, change in water flow paths and changes in vegetation.

Our aim was to sample soils and soil pore waters during Spring along a gradient of well-monitored soils, with a contrast in ground temperature. 

We set up our sampling sites to cover "control sites" considered as poorly degraded permafrost (with a maximum active layer depth below 60 cm), and "degraded sites" with soils presenting deeper maximum active layer depth and warmer ground temperature in the winter. Before snowmelt, we went to the field with shovels to remove the snow. Below the snow, with a hammer and a chisel, we sampled the soils by slices of 5 cm, like a slice of cake (Figure 1). Eventually, we reached 20 cm depth. And this is when in some of the degraded profile, we saw water seeping through the side of the hole (Figure 2a). We were sampling before snowmelt, so supposedly before the onset of soil thawing. Yet, we witnessed pools of liquid water adjacent to ice crystals (Figure 2b) in the ground.


Figure 1. Hammering and chiseling the ground below snow to sample 5 cm thick slices of soil.

We were very puzzled by this observation. The co-existence of liquid water and ice in the ground is evidence for a system which is progressively warming. We reasoned that this is evidence for a system where biogeochemical processes can occur within the soil before snowmelt. The open questions in our head were: is this water locked in closed pockets or can it move laterally through the ground? Are there local patches of nutrient release or is there a lateral transfer of nutrients through the ground below snow? If the former, then local biogeochemical cycling at a time when it should be shut off – could trigger the vertical release of greenhouse gases. If the latter, then a new pathway for organic carbon transfer and degradation is revealed. Both locked and laterally connected unfrozen water pockets during winter months could be crucial sources of permafrost carbon emissions. 


Figure 2. Water seeping from the side of the hole in the frozen ground (a) surrounded by ice lenses (b) within the first 20 cm of the active layer.

We decided to use our expertise in isotope geochemistry to test whether the unfrozen and frozen soil pore water sampled in "each slice of cake" represented a locked or laterally connected system. The beauty of silicon stable isotopes is that they respond to freezing. We had made recent progress to be able to determine the silicon isotope composition of water samples with complex matrices such as those highly concentrated in dissolved organic carbon. The stage was set to apply silicon isotopes to the organic rich soils at Eight Mile Lake. The numbers surprised us. We found heavier Si isotope compositions than ever expected in the soil pore water from “controlled soil”, the one which remained frozen and not biochemically connected. In contrast, the soil pore water from the “degraded soils” showed much lower values of Si isotope composition demonstrating a lateral connectivity and the contribution from processes such as mineral weathering. 

This was a first, and we decided to set up another field campaign to collect soils and soil pore waters by slice at the end of the thawing season, when the active layer was thawed at it's maximum. In August-September 2019, a team of four scientists from the same project went back to Alaska and reproduced the careful sampling. By doing this, we demonstrated that portions of the ground in the active layer or at the base of the active layer are less biogeochemically connected – and retain a locked system from winter months – compared to the rest of the ground.  

Our Spring measurements are evidence for the occurrence of unfrozen portions of the active layer during the winter which are biogeochemically connected below the soil surface before the snowmelt. With growing evidence from the scientific community for the occurrence of taliks, i.e., perennially unfrozen portion of the ground surrounded by frozen ground during the winter, this highlights the importance of accounting for the transition period (Winter-Spring) and hidden processes below ground. At this stage, there is a growing presence of taliks surrounded by permafrost. But sooner or later, this will become isolated permafrost surrounded by taliks.  

We consider the presence of these unfrozen portions of the ground as key locations for interactions between water and soil constituents, and therefore for mineral weathering, nutrients release, microbial activity, and organic carbon decomposition. These are key processes which should not occur in the winter in a system that is frozen. Biogeochemical connectivity during winter is opening new avenues for the exchange of heat, water and nutrients within soils, and new belowground pathways from soil to rivers. These are essential processes to identify, and which cannot be seen from a frozen surface. 

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