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[ACC33-P04] Reconstruction of CH4 concentration history since the Industrial Revolution from the SE-Dome II ice core, Greenland
Keywords:CH4, ice core, Greenland, firn
Methane (CH4) is emitted from both natural and anthropogenic sources. Direct observations of atmospheric CH4 started in the 1980s; thus, we need to reconstruct the past concentrations from polar ice cores to gain the knowledge on the changes of anthropogenic and natural source strengths since the Industrial Revolution. For Antarctica, precise high-resolution CH4 data is available from the Law Dome ice core, which was drilled at a high-accumulation site (~1 m w.e. year-1) on a coastal dome. On the other hand, there have been no high-quality data from Greenland because of difficulties such as in-situ CH4 production in the ice sheet and the lack of ice cores from very high-accumulation areas. In this study, we have overcome these problems and reconstructed the CH4 concentration since the 1840s by analyzing the SE-Dome II ice core (67°19’17” N, 36°47’03” W, 3,161m a.s.l, accumulation rate: ~1 m w.e. year-1), southeastern Greenland (Iizuka et al., 2021).
We analyzed 170 ice-core samples (ice age: 1800–1969 C.E., depth: 250–78m) with an established wet-extraction technique (Oyabu et al., 2020). The length of each sample was determined to contain approximately 1 year (~1 m) of accumulation (according to Kawakami et al., 2023) to minimize possible age reversals or discontinuities of trapped gases due to inhomogeneous gas trapping in the ice sheet.
The air in open pores in firn is closed off and isolated from the atmosphere at the bottom of firn (so-called bubble close-off zone), thus the age of air is always younger than the surrounding ice. Therefore, the age difference between ice and air (Δage) is needed in addition to the age of ice to reconstruct the atmospheric history. The age of ice was taken from Kawakami et al. (2023) based on annual layer counting using the seasonal variations of H2O2 concentration. For estimating the Δage, we first compared the CH4 data of the SE-Dome II core to that of direct observation at Point Barrow, Alaska (BRW) for 1980s – early 2000s. The Δage thus estimated was 40 years. However, temporal variations in temperature and accumulation rate may change the firn densification rate, which in turn changes firn thickness and thus Δage. To investigate the possible Δage variations, we ran a numerical firn densification model (Community Firn Model, Stevens et al., 2020) with various surface temperature and accumulation scenarios to simulate the evolution of the age of ice at the density of 829 kg/m3 (approximately the level for close-off completion). Using temperature history from reanalysis data (ERA-5 and NOAA-CIRES-DOE Twentieth Century Reanalysis) and accumulation history from the ice core analysis (Kawakami et al., 2023), the modeled ice age at close-off showed a long-term decreasing trend from 1850s to 1980s, after which it increased towards the present.
The reconstructed CH4 concentration increased from ~830 ppb to ~1800 ppb between the 1840s and 2000s, with particularly rapid increase since the 1950s. In our data, we do not find anomalous values as previously reported from other ice cores, such as the Greenland NEEM core, which reported artifactual spikes of up to ~150 ppb above a 10-year moving average due to in-situ CH4 production (Rhodes et al., 2013). The lack of artifactual CH4 production in the SE-Dome II core may be due to low impurity concentrations in the core (Amino et al., 2020) because of the high accumulation rate. The reconstructed CH4 growth rate was ~10 ppb/yr since the 1840s, and rapidly increased to ~40 ppb/yr between the late 1970s and early 1980s, which is twice as large as the growth rate in the direct observation data at BRW in the late 1980s.
Iizuka et al., Bulletin of Glaciological Research., 39, 2021.
Kawakami et al., J. Geophys. Res., 128, 2023.
Oyabu et al., Atmos. Meas. Tech., 13, 2020.
Rhodes et al., Earth and Planetary Science Letters, 368, 2013.
Stevens et al., Geosci. Model Dev., 13, 2020.
Amino et al., Polar Science, 27, 2020.
We analyzed 170 ice-core samples (ice age: 1800–1969 C.E., depth: 250–78m) with an established wet-extraction technique (Oyabu et al., 2020). The length of each sample was determined to contain approximately 1 year (~1 m) of accumulation (according to Kawakami et al., 2023) to minimize possible age reversals or discontinuities of trapped gases due to inhomogeneous gas trapping in the ice sheet.
The air in open pores in firn is closed off and isolated from the atmosphere at the bottom of firn (so-called bubble close-off zone), thus the age of air is always younger than the surrounding ice. Therefore, the age difference between ice and air (Δage) is needed in addition to the age of ice to reconstruct the atmospheric history. The age of ice was taken from Kawakami et al. (2023) based on annual layer counting using the seasonal variations of H2O2 concentration. For estimating the Δage, we first compared the CH4 data of the SE-Dome II core to that of direct observation at Point Barrow, Alaska (BRW) for 1980s – early 2000s. The Δage thus estimated was 40 years. However, temporal variations in temperature and accumulation rate may change the firn densification rate, which in turn changes firn thickness and thus Δage. To investigate the possible Δage variations, we ran a numerical firn densification model (Community Firn Model, Stevens et al., 2020) with various surface temperature and accumulation scenarios to simulate the evolution of the age of ice at the density of 829 kg/m3 (approximately the level for close-off completion). Using temperature history from reanalysis data (ERA-5 and NOAA-CIRES-DOE Twentieth Century Reanalysis) and accumulation history from the ice core analysis (Kawakami et al., 2023), the modeled ice age at close-off showed a long-term decreasing trend from 1850s to 1980s, after which it increased towards the present.
The reconstructed CH4 concentration increased from ~830 ppb to ~1800 ppb between the 1840s and 2000s, with particularly rapid increase since the 1950s. In our data, we do not find anomalous values as previously reported from other ice cores, such as the Greenland NEEM core, which reported artifactual spikes of up to ~150 ppb above a 10-year moving average due to in-situ CH4 production (Rhodes et al., 2013). The lack of artifactual CH4 production in the SE-Dome II core may be due to low impurity concentrations in the core (Amino et al., 2020) because of the high accumulation rate. The reconstructed CH4 growth rate was ~10 ppb/yr since the 1840s, and rapidly increased to ~40 ppb/yr between the late 1970s and early 1980s, which is twice as large as the growth rate in the direct observation data at BRW in the late 1980s.
Iizuka et al., Bulletin of Glaciological Research., 39, 2021.
Kawakami et al., J. Geophys. Res., 128, 2023.
Oyabu et al., Atmos. Meas. Tech., 13, 2020.
Rhodes et al., Earth and Planetary Science Letters, 368, 2013.
Stevens et al., Geosci. Model Dev., 13, 2020.
Amino et al., Polar Science, 27, 2020.