Greenhouse gas (CO2, CH4) alteration in shallow ice at Larsen blue-ice area, Northern Victoria Land, East Antarctica

CO₂ and CH₄ records from polar ice cores have greatly enhanced our understanding of the control mechanisms of atmospheric greenhouse gas (GHG) concentrations and their relationship to surface temperature. However, multiple ice cores show offsets of 1–3 % and concerns about in situ production in trapped air were raised. Recently, GHGs in shallow ice cores from blue-ice areas (BIAs) in Antarctica show excess CO₂ and CH₄ concentration values and even extremely lower CH₄ concentration than other non-contaminated ice core records at the same gas ages. We aim to decipher the processes of GHG production and CH₄ destruction in the shallow ice at Larsen BIA. CH₄ concentration records from the Larsen BIA generally show an increasing trend from the subsurface to a depth of ~0.35–1.15 m. Then gradually decreases until it reaches the true ancient atmospheric CH₄ values at ~4.6 m depth. In contrast, CO₂ concentration in the Larsen blue ice shows a gradual decrease from the subsurface until a depth of ~4.6 m where the concentration variation stabilizes, but still has a 10–20 ppm difference with other existing non-contaminated ice core records. These alterations might be due to mixing with modern air through cracks and/or microbial activity inside the occluded air bubbles. The vertical distribution of δ¹⁵N-N₂ in several Larsen BIA ice cores indicates that alteration by modern atmospheric air is not significant at the top ~10 m. Depleted δ¹⁸O_atm in a depth of ~0.15–1.65 m might indicate in situ microbial activity consuming O₂ gas in Larsen blue ice samples, but δ¹⁸O_atm values in a depth of 1.95–10 m might indicate little microbial activity. Our future study may include analysis of Pb concentration and isotopes to investigate the effect of modern aerosol intrusion. In addition, we may measure CH₄ concentration in ice after receiving UV light in order to check whether UV photolysis is included in the mechanism for CH₄ destruction.