In the MDV, very limited data about soil gas concentrations are available (only CO2 and CH4), while for the most part they concern CO2 and CH4 flux measurements. In 2003-2005 austral summers, Gregorich et al.29 measured up to 0.55 vol% of CO2 and up to 5780 ppmv of CH4 in Garwood valley. In January 2014, MacIntyre et al.23 measured a maximum value of 0.044 vol% of CO2 in the lower Taylor Valley, near Howard Glacier (Fig. 1). Both studies found CO2 concentrations 1-2 orders of magnitude lower than the maximum value measured in this study (3.44 vol%, Table 1). The highest CH4 value reported in Gregorich et al.29 is in the same order of magnitude as found in this study, although more than three times lower than our maximum value (18,447 ppmv, Table 1). Both referenced studies, however, collected a limited number of measurements and focused on temporal variability rather than spatial variability. Worldwide, CO2 and CH4 soil gas data are numerous, and measured in different environments: in the Arctic Finnish Lapland, Voigt et al.42, measured CO2 max value about 6 vol% and CH4 max value about 300 ppmv. In Italy, CO2 mean values of about 3 vol% for Tyrrhenian basins, 1.57 vol% for Apennine Intermontane plains and 1.09 vol% for foredeep basins, based on more than 10,000 samples43.
In contrast, φCO2 has been measured in Antarctica since 1994 (see Table 3). The φCO2 measurements in this study are up to 3 orders of magnitude higher than previous studies, therefore, the highest measured in the MDV hereto. φCO2 measurements in this work in Taylor Valley are on average comparable with the values measured in the Arctic25,44, desert25,45 and alpine46 areas and lower than other areas of the globe25,47 (Table 3).
The distribution of the positive anomalies for CH4, CO2, H2 and φCO2 are consistent in the NE, NW and S sectors of the study area, while He shows a good correlation with CO2 and CH4 anomalies in the S and N sectors, respectively. He detected in shallow soil is generally indicative of deep sources32,38, and is typically associated with CO2 and CH4 emissions that act as carrier gas for trace gases (i.e., He, Rn) 35,48. The source of these gas anomalies could be linked with shallow depth hypersaline fluids31,49 that during summer periods can release the dissolved gases after permafrost thawing. These gases can then easily migrate toward the surface through permeable layers, as well as local fractures and/or buried faults. These may act as preferential migration pathways thus resulting in the linear multigas anomalous zones observed in the study area35,37,48 (Fig. 4). Figure 4 shows gas anomalies aligned in ENE-WSW direction in the S and N sectors of the study area, respectively. At the northern boundary, the anomalous zone does not appear as continuous as that occurring in the southern boundary of the area because of the presence of the Commonwealth Glacier and related wetlands and streams (Fig. 2), that most likely prevent gas upwelling.
As for the southern sector, there are also two physical factors that may increase permafrost degradation: solar radiation and soil albedo. Solar radiation is more intense on North facing slopes than South facing slopes and in summer time soil temperatures increase in this sector18,50. On the southern slope, south of Commonwealth Glacier is an area rich in dark basalt and anorthoclase phonolite51. Campbell et al.52 found the greatest heating on dark colored basalt soils. The combination of these phenomena may enhance the superficial degradation of the permafrost in the southern sector (i.e. where the major gaseous anomalies have been identified).
Regarding the origin of the atmospheric gases, the scatterplot of Fig. 5 shows the comparison between CO2, O2, and N2 concentrations. The linear trend of the samples in the graph shows the existence of a dilution process of atmospheric gases (N2 and O2) by CO2. This effect leads to exclude a biological or atmospheric contribution to the soil system, but may be linked to a different intake of CO2, for example, geological and/or abiotic contributions53. These hypotheses can be confirmed only with isotope analyses of gas samples. In the literature, the origin of CO2 in Antarctic soils is suggested to be linked to biological activity, favored by soil alkalinity and by shallow abiotic processes (CO2 solubility), soil moisture content and soil temperature variations19,21,23,29,30. Shanhun et al.9 and Risk et al.54 suggested an abiotic origin of CO2 based on the isotopic analyses, reporting very high positive φCO2 values that cannot be explained by normal microbial activity. We suggest that the measured high anomaly values, originate from the subsurface brine system31,49, and are linked to permafrost cap discontinuities at the edges of the valley (fractures or buried local faults). The spatial distribution of these anomalous zone could also be locally influenced by shallow permafrost thawing.
Although the samples were collected at shallow depths (i.e. permafrost is often reached at 30 cm depth) and are potentially affected by atmospheric gas dilution55,56, the anomalous values of He, CH4, CO2 and φCO2 cannot entirely be explained by biological activity and/or superficial physical processes. Another aspect concerns the presence of soil moisture which prevents both sampling and gas rising55,56. In Taylor Valley, the wetlands represent an area where water permeates the pores of the soil but does not emerge on the surface. Within these areas (Fig. 2) we managed to complete some sampling stations, however it should be noted that the gas concentrations are certainly underestimated compared to those conducted in dry and ventilated soil conditions. In these areas, of about 2.1 km2 (equal to 10% of the total area) 16 soil gas samples and φCO2 were collected. Statistical analyses (Table S1) confirm the low values in these areas.
Figure 6 shows the ranging and average values of φCO2 measured over time in Antarctica. It should be noted that the reported measurements were conducted using various methods and in different environments, e.g. there are measurements on dry soil, near and from lakes and from the Ross Sea. Data collected in Dec 2019 - Jan 2020 from Taylor Valley show that the average value is in the same order of magnitude of those reported from the Garwood Valley29. The maximum values measured in this work, instead, is at least one order of magnitude higher than those previously reported. Focusing on Taylor Valley, various measurements have been carried out over the years, both in the soils9,25 and around the three lakes30. The comparison of these data shows that our values are two orders of magnitude greater than those previously measured.
The total CO2 gas emission rates over the surveyed area (A = 21.6 km2) have been computed following a statistical approach (see 2.3 Statistical and geostatistical analysis) and, the calculated average CO2 output is 14.95 t d−1 (Table S2). The calculated total CO2 emission considering the three summer months, is about 1,345.5 t. This value is also considered as the total annual CO2 emitted, providing a conservative estimate for the warmest months expecting much lower emissions during the rest of the year. Then, the estimated emission factor in the study area is 62.3 t km−2 y−1. Comparing this value, for example, with those estimated for the central Apennines in Italy, ranging from 350 to 1,050 t km−2y−1(57), it is evident that the fluxes reported herein are remarkably lower, even compared with others reported worldwide58. This discrepancy is due to both climate/environmental differences and organic content and biological activity usually present in the soils of the other continents. Although our emission values are low, forecasting that ice-free regions in Antarctica are likely to expand with gradual warming, this amount will tend to increase and should be counted in the global CO2 budget estimations.
To conclude, we provide the first spatial distribution maps of soil gas concentrations and φCO2 in a large area (> 20 km2) of the Taylor Valley, Antarctica. The calculated CO2 emission output during the summer period is 1,345.5 t. Our results emphasize that extensive surveys are necessary to properly evaluate greenhouse gas (GHG) emissions in regions with permafrost. We also established the first extensive baseline maps that can be used to compare and monitor soil gas concentrations and CO2 emissions in the region. The collected data identified areas characterized by multigas anomalies where permafrost partial melting may occur during the summer period and where fluids migrate to the surface through structures/fractures aligned with the main direction of the valley. We suggest that the gases originate from the subsurface brine system that is rich in dissolved gasses. The superficial melting of the permafrost and the presence of permeable zones inside it, allowed these gases to migrate to the surface.