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Stable Isotope Constraints on Carbon Dioxide and Nitrous Oxide Variations at the Onset of the Last Glacial Period: New Ice Cores from Taylor Glacier, Antarctica

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  • New ice cores retrieved from the Taylor Glacier (Antarctica) blue ice area contain ice and air spanning the Marine Isotope Stage (MIS) 5/4 transition, a period of global cooling and ice sheet expansion. Chronologies were determined for the ice and air bubbles in the new ice cores by visually matching variations in gas and ice phase tracers to preexisting ice core records. The chronologies reveal an ice age gas age difference (Δage) approaching 10 ka during MIS 4, implying very low snow accumulation in the Taylor Glacier accumulation zone. A revised chronology for the analogous section of the Taylor Dome ice core (84 to 55 ka), located to the south of the Taylor Glacier accumulation zone, shows that Δage did not exceed 3 ka. The difference in Δ age between the two records during MIS 4 is similar magnitude but opposite direction of what is observed at the last glacial maximum. This relationship implies that a spatial gradient in snow accumulation existed across the Taylor Dome region during MIS 4 that was oriented in the opposite direction of the accumulation gradient during the last glacial maximum. An enduring problem in paleoclimatology is to mechanistically explain the full 80 ppm change in atmospheric CO₂ concentration that accompanied glacial-interglacial climate cycles in the past. A record of the stable isotopic composition of atmospheric CO₂ (δ¹³ C-CO₂) was developed that spans the interval 74- 59.5 ka, including the period of global cooling and ice sheet growth known as the marine isotope stage (MIS) 5/4 transition. The interval contains small (~ 10 ppm) variability in CO₂ concentration associated with Dangaard-Oeschger (DO) event 19, the 40 ppm drop in CO₂ across the MIS 5/4 transition, and the 35 ppm rise in CO₂ associated with the MIS 4/3 transition. The δ¹³ C-CO₂ record reveals large and fast changes at specific time intervals, implying that different carbon cycle mechanisms controlled the atmospheric CO₂ concentration at distinct times. The isotope data are generally consistent with proxy data in suggesting that the marine carbon cycle evolved toward more efficient nutrient utilization and carbon storage in the deep ocean during MIS 4. Model experiments suggest that the biological pump may not be the only mechanism responsible for lowering atmospheric CO₂, possibly implicating changes in Southern Ocean air-sea gas exchange and/or the extent of Antarctic sea ice. Large depletions in δ¹³ C-CO₂ observed in the new data, including an enigmatic decrease in δ¹³ C-CCO₂ of - 0.8 ‰ at the MIS 4/3 transition, may highlight times when carbon was transferred from the terrestrial biosphere to the atmospheric and oceanic reservoirs. Glacial-interglacial cycles in N₂O concentration have received far less scientific attention than CO₂, but they signify a strong coupling between climate and natural N₂O emissions that is presently poorly understood. High-resolution records of the stable isotopic composition of N₂O (δ¹⁵ N-N₂O and δ¹⁸ O-N₂O) were developed from a new Taylor Glacier ice core spanning the interval 74-59.5 ka. The interval includes a 40 ppb rise in N₂O concentration associated with DO 19, a 60 ppb decrease across the MIS 5/4 transition, and a 50 ppb increase at the onset of the MIS 4/3 transition. The new isotope data reveal large changes (1.65 ‰ for δ¹⁵ N-N₂O and 3.4 ‰ for δ¹⁸ O-N₂O) associated with the N₂O concentration variations that occurred at DO 19. The isotopic composition of N₂O also changed significantly across the MIS 4/3 transition (1.4 ‰ for δ¹⁵ N-N₂O and 2.3 ‰ for δ¹⁸ O-N₂O). The new data were used to reconstruct marine versus terrestrial N₂O emissions histories. Both terrestrial and marine sources were approximately equally responsible for controlling atmospheric N₂O through most of the onset of the last glacial period, and marine emissions dominated the rapid N₂O increase observed at the MIS 4/3 transition. The relative dominance of marine versus terrestrial N₂O sources provides insight into (and fuels speculation about) how global climate changes impacted nitrogen cycling in the past.
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  • Menking, J.A., 2019. Stable isotope constraints on carbon dioxide and nitrous oxide variations at the onset of the last glacial period: New ice cores from Taylor Glacier, Antarctica, CEOAS. Oregon State University.
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