Antarctic permafrost degassing in Taylor Valley by extensive soil gas investigation

Highlights

• First extensive soil gas and flux survey in Antarctica

• Discovered zones of multigas (CO₂, CH₄, He, H₂) anomalies

• CO₂ emissions estimation in Lower Taylor Valley

• Gas rising from deep brines

Abstract

Ongoing studies conducted in northern polar regions reveal that permafrost stability plays a key role in the modern carbon cycle as it potentially stores considerable quantities of greenhouse gases. Rapid and recent warming of the Arctic permafrost is resulting in significant greenhouse gas emissions, both from physical and microbial processes. The potential impact of greenhouse gas release from the Antarctic region has not, to date, been investigated. In Antarctica, the McMurdo Dry Valleys comprise 10 % of the ice-free soil surface areas in Antarctica and like the northern polar regions are also warming albeit at a slower rate.

The work presented herein examines a comprehensive sample suite of soil gas (e.g., CO₂, CH₄ and He) concentrations and CO₂ flux measurements conducted in Taylor Valley during austral summer 2019/2020. Analytical results reveal the presence of significant concentrations of CO₂, CH₄ and He (up to 3.44 vol%, 18,447 ppmv and 6.49 ppmv, respectively) at the base of the active layer. When compared with the few previously obtained measurements, we observe increased CO₂ flux rates (estimated CO₂ emissions in the study area of 21.6 km² ≈ 15 tons day⁻¹). We suggest that the gas source is connected with the deep brines migrating from inland (potentially from beneath the Antarctic Ice Sheet) towards the coast beneath the permafrost layer. These data provide a baseline for future investigations aimed at monitoring the changing rate of greenhouse gas emissions from Antarctic permafrost, and the potential origin of gases, as the southern polar region warms.

Introduction

Permafrost is defined as any ground (soil or rock and any ice and organic material inclusions) that remains completely frozen (0 °C or colder) for at least two years (Van Everdingen, 2005). It is present in both hemispheres at high latitudes and its temperature, thickness, and continuity are controlled by the geographic setting and, to a large extent, by the surface energy balance (Schuur et al., 2008). Climate warming effects are going to impact these regions in the upcoming decades (Chapman & Walsh, 2007; Shindell & Schmidt, 2004) and all three types of permafrost (dry, ice-cemented, and massive ice) present below 1000 m elevation may be susceptible to warming-related degradation depending on future emission pathways (Hagedorn et al., 2007; Swanger & Marchant, 2007). Permafrost thawing and the microbial decomposition of previously frozen organic carbon is considered one of the most likely positive climate feedbacks from terrestrial ecosystems to the atmosphere (Schuur et al., 2015; Xue et al., 2016). Measurements of CO₂ and CH₄ soil concentrations and fluxes are essential to understand the C cycle in terrestrial ecosystems, although less is known about controls over CO₂ flux (φCO₂) in ecosystems lacking vascular vegetation, including polar deserts and some hot deserts, where autotrophic inputs are low and abiotic factors tend to dominate in determining φCO₂ (Shanhun et al., 2012).

In the Arctic and boreal regions, permafrost is found in Greenland, Alaska, Canada, Northern Europe, Russia, and China (Schuur et al., 2008), and represents about 17 % of the exposed land surface (Gruber, 2012). Studies carried out on permafrost soils in these ecosystems have shown these areas store almost twice the carbon currently present in the atmosphere (Schuur et al., 2008; Schuur et al., 2009). These regions are rich in frozen organic matter, that would lead to an increase of the production of CO₂ and CH₄ by microbial activities in case of thawing (Schuur et al., 2008). Furthermore, part of the released carbon could easily dissolve in water and, through solar radiation, produce CO₂ by the photo mineralization process (Cory et al., 2014). Large methane deposits currently stored at high latitude regions are either frozen within permafrost or trapped below impermeable buffer zones (Anthony et al., 2012). Methane has a global warming potential 28 times higher than that of CO₂ on a 100-year time horizon (Ciais et al., 2013). It is therefore imperative to provide estimates of methane and other gases released from the high-latitude regions. In remote and scarcely monitored regions soil gas release can endure for decades or even centuries before being detected and quantified.

In the Southern Hemisphere, permafrost soils are found at high elevations, in the Sub-Antarctic islands, in the Antarctic Peninsula and in the ice-free areas of the Antarctic region. Permafrost degradation in the Antarctic continent has not been widely studied (Levy et al., 2013) due to terrain limitations (i.e., ice-free area represents only 0.35 % of the continent; (Campbell & Claridge, 2009)) and the overall limited organic content in soils (Schuur et al., 2008) making it less relevant for atmospheric carbon contributions. However, the role of microbial activity in Antarctic soils could potentially be more important than previously believed. Indeed, microbial activity affects the amount of total organic carbon and is more susceptible to weak temperature variations (Schuur et al., 2008). The McMurdo Dry Valleys (thereafter MDV) are the largest ice-free regions in Antarctica (Gilichinsky et al., 2007); and their geomorphology reveals how the landscape is strongly controlled by climate processes (Fountain et al., 2014). Attempts to quantify CO₂ emissions on the Antarctic continent have been carried out in the MDV soils, highlighting that φCO₂ is driven primarily by physical factors such as soil temperature and moisture, indicating that future climate change may alter the soil C cycle (Ball et al., 2009; Bockheim et al., 2007; Elberling et al., 2006; Gilichinsky et al., 2007; MacIntyre et al., 2019). The lack of mechanistic understanding makes it difficult to predict the contribution of soil φCO₂ to the C-cycle due to climate change in the polar deserts of Antarctica. In the MDV, φCO₂ has been used to characterize a variety of ecosystem processes and properties, including soil C turnover, the functional role of differing origins of organic matter supporting C cycling, and biotic distribution and activity (Adams et al., 2006; Barrett et al., 2006; Burkins et al., 2001; Hopkins et al., 2006; Virginia & Wall, 1999).

To date, φCO₂ studies conducted in MDV soils were performed on few measurement points. Results revealed low values that are highly uncertain and spatially variable (Burke et al., 2017; Gregorich et al., 2006). It is therefore difficult to separate the biological processes (e.g., C-fixation, heterotrophic respiration) from physical factors (e.g., carbonate dissolution). Parsons et al. (2004) suggested that in extreme desert environments, abiotic factors, like temperature gradients, parent material and soil water dynamics, may have the same magnitude of the biological processes influencing φCO₂ rates; on the contrary, in mid-latitude ecosystems the physical φCO₂ is often negligible.

Recent studies have revealed a diffuse subsurface brine system in the MDV area. This occurs preferentially near the coast and under the surface sediments of the main valleys and is sourced beneath the East Antarctic Ice Sheet (Mikucki et al., 2015). The presence of this deep fluid circulation may also favour the uprising of geogenic gases that could be in overpressure due to a permafrost cap (Cartwright & Harris, 1981). Soil gas composition in the MDV has been poorly investigated and essentially aimed to study biological process and temporal variability, respectively (Gregorich et al., 2006; MacIntyre et al., 2019). To better understand the different production and migration mechanisms of gaseous species in this environment, systematic investigations are required. To date, no studies have been completed to investigate the spatial distribution of soil gas linked to possible fault/fracture systems, or permafrost degradation, and characterize seepage for both CO₂ and CH₄ in Antarctica. Soil gas geochemistry is a useful approach that is widely used to detect diffusive/advective gas emissions and identify preferential migration pathways such as buried faults and fracture systems (Ascione et al., 2018; Baubron et al., 2002; Bigi et al., 2014; Ciotoli et al., 2016; Sciarra et al., 2018; Sciarra et al., 2019; Sciarra et al., 2021). Permafrost is generally a barrier to the migration and leakage of endogenous gaseous species. However, the presence of faults and/or fractures, the thawing of the active layer and permafrost degradation could allow the surface migration of anomalous concentrations of endogenous gaseous species.

In order to investigate the possible presence of soil gas anomalies and fill the knowledge gaps described above, a large-scale geochemical campaign was carried out in the MDV (i.e., in Taylor Valley) during the 2019/2020 austral summer. The goal of this research is to investigate (a) the composition (through soil gas survey) and (b) the potential volumes of greenhouse gasses (i.e., CO₂) released at the interface between the permafrost and active layer over a large area of Taylor Valley (Fig. 1). The emission rate obtained for the Lower Taylor Valley can be used for future monitoring surveys and for broad extrapolations across the continent's peri-coastal zone.