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Abstract: The stable carbon isotopic signature of carbon dioxide (δ13CO2) measured in the air occlusions of polar ice provides important constraints on the carbon cycle in past climates. In order to exploit this information for previous glacial periods, one must use deep, clathrated ice, where the occluded air is preserved not in bubbles but in the form of air hydrates. Therefore, it must be established whether the original atmospheric δ13CO2 signature can be reconstructed from clathrated ice. We present a comparative study using coeval bubbly ice from Berkner Island and ice from the bubble–clathrate transformation zone (BCTZ) of EPICA Dome C (EDC). In the EDC samples the gas is partitioned into clathrates and remaining bubbles as shown by erroneously low and scattered CO2 concentration values, presenting a worst-case test for δ13CO2 reconstructions. Even so, the reconstructed atmospheric δ13CO2 values show only slightly larger scatter. The difference to data from coeval bubbly ice is statistically significant. However, the 0.16‰ magnitude of the offset is small for practical purposes, especially in light of uncertainty from non-uniform corrections for diffusion related fractionation that could contribute to the discrepancy. Our results are promising for palaeo-atmospheric studies of δ13CO2 using a ball mill dry extraction technique below the BCTZ of ice cores, where gas is not subject to fractionation into microfractures and between clathrate and bubble reservoirs. [Copyright &y& Elsevier]

Abstract: An ice core air content record that was recovered from the refrozen-recrystallization ice formation zone in the Dasuopu Glacier was investigated in this work, which showed that the air content in ice performed significant fluctuations both in the seasonal and long-time series. The air content was low in summer and high in winter, and fluctuated around the mean value of 5.025 cm3 per 100 g ice from AD 1571 to AD 1927. The correlation of the air content in ice with the climatic and environmental factors was discussed combining with the dating results, which showed that over about 400 yrs from AD 1570 to AD 1927 the air content in ice from the refrozen-recrystallization ice formation zone in the Dasuopu Glacier was mainly dominated by the insolation intensity rather than the temperature and other environmental factors in the Southern Tibetan Plateau. [Copyright &y& Elsevier]

Changes in temperature and carbon dioxide during glacial cycles recorded in Antarctic ice cores are tightly coupled. However, this relationship does not hold for interglacials. While climate cooled towards the end of both the last (Eemian) and present (Holocene) interglacials, CO 2 remained stable during the Eemian while rising in the Holocene. We identify and review twelve biogeochemical mechanisms of terrestrial (vegetation dynamics and CO 2 fertilization, land use, wildfire, accumulation of peat, changes in permafrost carbon, subaerial volcanic outgassing) and marine origin (changes in sea surface temperature, carbonate compensation to deglaciation and terrestrial biosphere regrowth, shallow-water carbonate sedimentation, changes in the soft tissue pump, and methane hydrates), which potentially may have contributed to the CO 2 dynamics during interglacials but which remain not well quantified. We use three Earth System Models (ESMs) of intermediate complexity to compare effects of selected mechanisms on the interglacial CO 2 and δ 13 CO 2 changes, focusing on those with substantial potential impacts: namely carbonate sedimentation in shallow waters, peat growth, and (in the case of the Holocene) human land use. A set of specified carbon cycle forcings could qualitatively explain atmospheric CO 2 dynamics from 8 ka BP to the pre-industrial. However, when applied to Eemian boundary conditions from 126 to 115 ka BP, the same set of forcings led to disagreement with the observed direction of CO 2 changes after 122 ka BP. This failure to simulate late-Eemian CO 2 dynamics could be a result of the imposed forcings such as prescribed CaCO 3 accumulation and/or an incorrect response of simulated terrestrial carbon to the surface cooling at the end of the interglacial. These experiments also reveal that key natural processes of interglacial CO 2 dynamics – shallow water CaCO 3 accumulation, peat and permafrost carbon dynamics - are not well represented in the current ESMs. Global-scale modeling of these long-term carbon cycle components started only in the last decade, and uncertainty in parameterization of these mechanisms is a main limitation in the successful modeling of interglacial CO 2 dynamics. [ABSTRACT FROM AUTHOR]