Methane (
Permafrost preserves large amounts of soil carbon (C) and nitrogen (N) in a frozen state (e.g. Hugelius et al., 2014; Salmon et al., 2018),
temporarily removing this frozen C and N from active global cycles. Therefore, future projections of permafrost stability are of great interest,
particularly because thawing permafrost may lead to decomposition and/or remineralisation of the buried soil C and N and their abrupt emission into
the atmosphere in the form of greenhouse gases (GHGs) such as carbon dioxide (
The gases trapped in ground ice allow unique insights into the origin of ground ice and evidence for in situ microbial aerobic and anaerobic
respiration (Boereboom et al., 2013; Kim et al., 2019; Lacelle et al., 2011). Among others, the GHGs in ground ice may provide detailed information on
in situ biogeochemical processes responsible for GHG production (i.e. methanogenesis, nitrification, and denitrification) (e.g. Boereboom et al.,
2013; Kim et al., 2019). However, the relevant analytical methods remain poorly scrutinised. Boereboom et al. (2013) utilised the conventional
melting–refreezing method (wet extraction) used in polar ice core analyses, in which the ice samples were melted under a vacuum to liberate the
enclosed gases and then refrozen to expel the dissolved gases present in the meltwater. Other studies conducted by Russian scientists used an on-site
melting method, in which a large (1–3
In this study, for the first time, we tested the reliability of both wet and dry extraction methods for
The ice-wedge samples used in this study were collected from Churapcha, Cyuie (central Yakutia), and Zyryanka (northeastern Yakutia) in Siberia, as
well as from northern Alaska (Fig. S1 in the Supplement). The Churapcha site (61.97
Zyryanka is located in the southern boreal region of the Kolyma River, at the junction of the Chersky and Yukaghir ranges, in a region affected by
thermokarst development (Fedorov et al., 1991). Site A (Zy-A) was located on a tributary of the Kolyma River,
For the Alaskan sampling locations, Bluff03 (69.40
Ice-wedge ice is different from polar ice cores in that its gas mixing ratios are not homogeneous (e.g. Kim et al., 2019), which may hinder exact
comparison with results from adjacent ice samples. We therefore randomly mixed subsamples to reduce the effect of the heterogeneous gas composition
distribution (the “random cube” method, hereafter). Approximately 100–200
For dry extraction, we used a needle-crusher system at Seoul National University (SNU, Seoul, South Korea) (Shin, 2014). In brief, 8–13
Following extraction, the sample tubes were detached from the He-CCR, warmed to room temperature (
For the control and
For biocide-treated tests, 1.84
The analytical methods described previously were used to determine the mixing ratios of
Dry soil content was measured using the leftover meltwater from the control wet extraction tests. After these were complete, the sample flasks were
shaken thoroughly and the meltwater samples were each poured into a 50
The results from the wet and dry extractions were compared using 23
Comparison of
To test the microbial production of
According to microbial sequencing studies that have shown the presence of viable microbes in permafrost and ground ice (e.g. Katayama et al., 2007),
it is likely that culturable microbes exist in the ice-wedge samples used in this study. However, considering that at least 14 d and up to 3 months
of culturing was required to identify microbe colonies extracted from ground ice (Katayama et al., 2007; Lacelle et al., 2011), our melt–refreeze time
of an hour was insufficient for microbial activity to resume production of
One limitation of our needle crushing dry extraction technique was the inability to completely extract gas from ice samples, because small ice
particles and/or flakes placed in the space between the needles were not fully crushed. The gas extraction efficiency of the SNU needle-crusher system
has been reported as
To estimate the biases arising from incomplete gas extraction, we designed a series of tests to identify the differences of the
We regarded the ratio of gas content of hit100 to that of hit5 (hit100 / hit5 ratio, hereafter) as a measure of the gas extraction efficiency of the
needle-crusher system. The results demonstrate an average hit100 / hit5 ratio of gas content of
The hardness of the ice samples may also affect the gas mixing ratio analysis in the hit5 and hit100 procedures. The hit100 / hit5 ratios of the
Although a different crushing technique might be more suitable for ice-wedge samples, none of the existing dry extraction techniques – centrifugal ice microtome (Bereiter et al., 2013), mechanical grater (Etheridge et al., 1988), or ball-mill crusher (Schaefer et al., 2011) – is more advantageous for ice-wedge analysis than the needle-crusher system used in this study. The hard portion of ice wedges (e.g. frozen soil aggregates and large soil particles) could easily damage the metal blades of the centrifugal ice microtome and mechanical grater devices, or block the space within the ball-mill chamber, limiting the movement of the milling balls.
It is worth noting that friction between stainless-steel surfaces could produce
To summarise, from the hit5 and hit100 comparison tests, we found that (1) the needle-crusher method was not able to fully crush the ice-wedge ice samples and thus was unsuitable for measuring gas content in a unit mass of ice, and (2) weak crushing (e.g. a small number of hits by the needle-crusher system) may better reflect gas mixing ratios in the soft parts of the samples (such as air bubbles) than strong crushing (e.g. a greater number of hits).
Results of dry extraction tests with 5 and an additional 100 hits to ice-wedge samples, denoted as “hit5” and “hit100”,
respectively
Comparison of wet-extracted gas and residual gas for
To examine how well the gas was extracted by wet extraction, we applied the dry extraction method to refrozen ice-wedge samples after wet
extraction. We first prepared degassed ice-wedge samples that had undergone repetitive wet extractions (wet-degassed ice, hereafter). Once the wet
extraction experiments were completed, we repeated two cycles of melting–refreezing and evacuation procedures to degas the ice melt. After degassing
by a total of three cycles of wet extraction and evacuation, the outermost surfaces (
These tests using the wet-degassed ice showed an additional gas extraction of
Figure 3 and Table A2 show the mixing ratios and contents of
In summary, we found that a certain amount of gas remained in ice wedges, even after three cycles of wet extraction, and that it was extractable
instead by needle crushing. This implies that, unlike polar ice cores, wet extraction of ice wedges does not guarantee near-complete gas extraction,
and therefore precise measurements of the gas content of ice wedges are difficult to obtain. This difficulty in measuring gas content imposes a large
uncertainty when estimating
In this study we carried out comparisons between (1) wet and dry extraction, (2) untreated and biocide-treated wet extraction, and (3) gas extraction
from the easy-to-extract and difficult-to-extract parts of ice-wedge ice in order to better understand the characteristics of each extraction method
and adequately analyse Existing wet and dry extraction methods allow gas extraction from the soft parts of ice (e.g. ice bubbles) and show insignificant differences
in Wet extraction results are unlikely to be affected by microbial production of Both dry and wet extraction methods are not able to fully extract gas from ice-wedge samples, presumably due to gas adsorbed on soil particles
or enclosed within soil aggregates, which may have different gas mixing ratios compared to the gas in bubbles. Further research is required to
develop a proper method to quantify and extract adsorbed and enclosed gases. In the meantime, we propose that both existing techniques may be
suitable for gas mixing ratio measurements for bubbles in relatively soft ice wedges (i.e. easily crushed ice wedges by hit5 extraction, e.g.
Cyuie ice wedges in this study). Although the Previous estimates of ground ice Saturated
Systematic blank of the needle crushing (dry extraction) and melting–refreezing (wet extraction) methods for
Since the SNU dry extraction systems, including the sample tubes, were originally designed for
The systematic blanks were tested with bubble-free ice (BFI) and standard air in a cylinder calibrated by NOAA. The BFI was prepared as described in
Yang et al. (2017), other than cutting the BFI block into small pieces of 3–4
For the wet extraction, a total of
The results of the blank experiments are shown in Fig. A1. The systematic blanks appeared to be inversely correlated with the gas pressure in the sample tube. The systematic blank test results were fitted using exponential regression curves (dashed lines in Fig. A1), and these regression curves were then used for systematic blank correction in our ice-wedge sample analyses.
To calculate uncertainties of the blank corrections, the blank test data were fitted with exponential regression curves (Fig. A1). The
root-mean-square deviations (RMSDs) of the data from the regression curves were taken as the uncertainties of blank corrections (Fig. 1). Since the
ice-wedge data used in this study showed pressure in the GC sample loop of about 8–50
Influence of different amounts of hitting on the systematic blank of the needle crushing (dry extraction) system for
Comparison between control and BES-treated wet extraction results for
Results of dry extraction tests with 5 and an additional 100 hits to the ice-wedge samples, denoted as “hit5” and “hit100”,
respectively. “hit100
Comparison of results from extracted gas from the conventional wet extraction method and the residual gas in ice after three wet extractions. The residual gas was extracted by a needle crusher (see Sect. 3.4 for details of the methods).
All data used in this study are available at the Zenodo repository:
The supplement related to this article is available online at:
JWY and JA conceived the research and designed the experiments. GI, JA, KK, and AF drilled the ice-wedge ice samples from Alaska and Siberia. JWY, JA, SH, and KK conducted the laboratory experiments. JWY and JA led the manuscript preparation with inputs from all other co-authors.
The authors declare that they have no conflict of interest.
The authors greatly acknowledge those who contributed to collect ice-wedge ice samples. We thank Gwangjin Lim and Jaeyoung Park for their help in sample preparations and gas extraction experiments, and Min Sub Sim for kind advice on inhibition experiments for methanogen.
This project was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) (grant nos. NRF-2018R1A2B3003256 and NRF-2018R1A5A1024958) and the NASA ABoVE (Arctic Boreal and Vulnerability Experiment (grant no. NNX17AC57A)).
This paper was edited by Ylva Sjöberg and reviewed by two anonymous referees.