Deciphering the cause of greenhouse gas (CO2, CH4) alteration in shallow ice at Larsen blue-ice area, East Antarctica

Ice cores drilled from polar ice sheets in Antarctica and Greenland contain ancient atmospheric air trapped in air bubbles. The reconstruction of past atmospheric greenhouse gas (GHG) concentrations, such as carbon dioxide (CO₂) and methane (CH₄), has enhanced our understanding of the glacial-interglacial climate cycles and their relationship to surface temperature. However, processes that alter the GHG concentrations of the trapped air poses a challenge for accurate GHG reconstruction and paleoclimate interpretation. Previous studies report excess GHG concentration related to various factors, such as ice impurities, organic carbon oxidation, methods of extracting trapped air, refrozen ice layers, and biological activity. Despite these findings, the causes and mechanisms of GHG alteration within glacial ice remain incompletely understood, for example, the alterations observed in shallow ice in blue-ice areas (BIAs). GHGs in shallow ice cores from 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. Here, we aim to decipher the cause of excess GHG (CO₂, CH₄) concentration and depleted CH₄ concentration observed in the shallow ice from Larsen BIA, East Antarctica. CO₂ concentration in the Larsen blue ice shows a gradual decrease from the surface until a depth of ~4.6 m where the concentration variation stabilizes. In contrast, CH₄ concentration records show an increasing trend from the surface to a depth of ~0.35–1.15 m. Then gradually decreases until it reaches stabilized values at ~4.6 m depth. Measurements of δ¹⁵N-N₂, ion concentrations (Ca²⁺ and Na⁺), and Pb isotopes indicate that excess GHG concentrations are not associated to the modern air/aerosol intrusion. The pronounced excess GHG concentrations in the surface ice are not related to dust content. The observed δ¹⁸O_atm depletion in the surface ice suggests that photochemical reactions have occurred within the ice. Therefore, we infer that GHG alterations observed in the surface ice from Larsen BIA are attributed to UV photochemistry. Based on δ¹³C of CO₂, we suggest that photolysis of both organic and inorganic carbon by ultraviolet light from sun is a primary source for the excess CO₂ concentration.